Morphology, Structure and Pretreatment of Anti-Corrosion

Home , Heavy water, Silane

UNIVERSITY OF CINCINNATI

Date:______

I, ______, hereby submit this work as part of the requirements for the degree of: in:

It is entitled:

This work and its defense approved by:

Chair: ______

Pretreatment, Morphology and Properties of Organosilane Anti-corrosion Coatings

A dissertation submitted to the Division of Research and Advanced

Studies of the University of Cincinnati

In partial fulfillment of the

requirements for the degree of

DOCTOR OF THE PHILOSOPHY

In the department of Chemical and Materials Engineering of the College of Engineering

2007

Yimin Wang

B.S. Tianjin University, P. R. China, 1988 M.S. University of Cincinnati, Ohio, 2002

Committee Chair: Dr. Dale W. Schaefer

Abstract

This dissertation focuses on the relationship between substrate-surface chemistry

(cleaning), reaction mechanism, film structure and water-barrier properties of water- based bis-amino silane and vinyl triacetoxysilane films. The work was undertaked to fulfill a SERDP (Strategic Environment Research and Development Program) requirement to understand the mechanism of corrosion protection in silane-based coatings. The majority of the work focuses on mixtures of the above two silanes. Such coatings are referred to as AV coatings.

The chemistry of neat AV mixtures was studied by 13C NMR. The reaction mechanism was found to be the exchange of the hydrogen atom on the secondary amine group with the acetoxy group on the vinyl triacetoxysilane. The chemistry of the AV water solution was also investigated by 13C and 29Si NMR.

The influence of the cleaning solution pH on the CRS surface chemistry and AV absorption was examined. The corrosion performance examination on CRS panels showed that the CRS surface is very sensitive to cleaning protocol. Optimum anti- corrosion performance was obtained after cleaning at pH ∼ 9.5. The underlying mechanism for this observation is discussed.

The morphologies and water barrier properties of AV films were studied at different

A/V ratios. AV film was found susceptible to water penetration. About 30 vol% water is absorbed in the film with only slight thickness increase. Most water is physically

absorbed in the void space with the least amount being absorbed near the stoichiometric

A/V ratio of 3/1.

Kinetic investigation of water uptake enables us to monitor water ingress providing more details on water absorption. Time-resolved D2O ingress in bis-amino silane and bis-sulfur silane film was studied by situ neutron reflectivity and Fourier transform infrared reflection–absorption spectroscopy. The absorbed water exists in two populations: one is dissolved in the polymer matrix (Henry’s mode) and the other occupies unrelaxed free volume within the polymer (Langmuir mode). The Langmuir absorption mode dominates the D2O absorption in both films. The initial stage of water diffusion of both bis-amino silane and bis-sulfur silane was Fickian. However, the deviation from Fickian behavior was observed at the intermediate stage of water ingress.

iii

Acknowledgements

I extend my deep appreciation to my academic advisor, Prof. Dale W. Schaefer, for leading me to this exciting and challenging research field, for stimulating my research interest, and for invaluable advice and encouragement throughout the course of this work.

I am very grateful to my vice advisor Dr. Wim J. Van Ooij, for introducing me the research topic, for his insight, guidance and encouragements during the research.

I give my sincere appreciation to Dr. Relva C. Buchanan and Dr. Gregory Beaucage for their participation as my committee and their valuable insights and suggestions.

Specially, I would like to thank Dr. Paula Puomi for her help with FTIR analysis and valuable discussions. I will also give my thanks to Dr. Tammy L. Metroke from

Department of Chemistry, Oklahoma State University, Stillwater OK for her NMR experiments, analysis and suggestions.

This project was funded by the Strategic Environmental Research and Development

Program (www.serdp.org).

This work benefited the use of Surface Profile Analysis Reflectometer (SPEAR) at Lujan Neutron Scattering Center at Los Alamos National Laboratory, the x-ray reflectometer and grazing incidence angle small angle x-ray scatter (GISAXS) at x-ray

Operations and Research beamlines 1-BM and 12-ID at the Advanced Photon

Source, Argonne National Laboratory (ANL) as well as neutron reflectometer at High

Flux Isotope Reactor (HFIR) at Oak Ridge National Lab (ORNL). I also acknowledge the Statewide Shared NMR Facility at Oklahoma State University for supporting NMR experiments performed by Dr. Tammy Metroke.

I thank Jaraslaw Majewski, Erik Watkins, Byeongdu Lee, Jan Ilavsky and William A.

Hamilton for their effort in collecting the reflectivity and GISAXS data. I thank Mr.

Dima Barbash at the University of New Mexico for the XPS measurements, Mr. Kevin

Kubachka of the Department of Chemistry at the University of Cincinnati for the ICP-

MS measurements and Material Characterization Center for ESEM analysis.

I would like to thank my lab mates (past and present: Gurong Pan, Kevin Heitfeld,

Ryan Justice, Peng Wang, Doug Kohls), and members of SERDP group (Trilok, Chetan,

Akshay, John) for their advice and help. I also appreciate the support provided by the dedicated and enthusiastic staff of Chemical and Materials Engineering Department.

Special thanks to my husband Yun Gao, my daughter and son for their encouragement, love and understanding throughout my studies. I am grateful to my parents and brother in China for their love, support and inspiration in all my endeavors. This work is dedicated to each of them.

This dissertation is dedicated to

My dear husband, lovely daughter and son, my parents and brother, for their love, caring, and support

through the past years.

Table of Contents

Abstract··············································································································i

Acknowledgements ························································································iv

Table of Contents·····························································································1

List of Figures ··································································································7

Chapter 1. Introduction··················································································22

1.1 Research Significance and Objective·····················································22

1.1.1 Objective and relationship to Strategic Environment Research and

Development Program (SERDP) ···································································22

1.1.2 Research background···········································································24

1.2 The scope of research work····································································28

Chapter 2. Literature Review·········································································30

2.1 Chemical structure of organosilanes ·····················································30

2.2 Silane solution chemistry········································································32

2.2.1 Hydrolysis and condensation ······························································32

2.2.2 Other factors that affect hydrolysis and condensation ·····················35

2.3 Bonding mechanisms of silanes·····························································38

2.3.1 Bonding between silane and polymers ···············································38

2.3.2 Bonding and adsorption of silane on mineral substrate ···················39

2.4 Acid-base properties················································································44

2.5 Interaction of water with silane films······················································49

2.6 Sorption and diffusion of water in silane films······································54

2.7 X-ray and neutron reflectivity··································································55

Chapter 3. Experimental ················································································59

3.1 Sample Preparation··················································································59

3.1.1 Materials·································································································59

3.1.2 Procedures ····························································································60

3.1.2.1 Silane solution preparation ·······························································60

3.1.2.2 Substrate cleaning and coating deposition ·····································61

3.2 Characterization of silane films ······························································64

3.2.1 X-ray reflectivity ····················································································64

3.2.2 Neutron reflectivity················································································65

3.2.2.1 Test procedures ·················································································65

3.2.2.2 Data acquisition and analysis ···························································67

3.2.3 Grazing Incidence Small-Angle X-ray Scattering (GISAXS) ··············68

3.2.4 Nuclear Magnetic Resonance Spectroscopy (NMR) ··························70

3.2.5 Fourier-Transform Infrared Reflection-Absorption (FTIR-RA)

Spectroscopy ·································································································71

3.2.5.1 Cleaning study····················································································72

3.2.5.2 Water penetration kinetic study························································72

3.2.6 Inductively Coupled Plasma Mass Spectrometry (ICP-MS)···············74

3.2.7 Gravimetric measurement ····································································74

3.2.8 Scanning electron microscope (SEM) ·················································75

3.2.9 Surface energy ······················································································76

3.2.10 Electrochemical testing ······································································77

3.2.10.1 Electrochemical impedance spectroscopy (EIS)···························77

3.2.10.2 DC Potentiodynamic measurement ················································78

3.2.11 Salt water immersion test···································································78

Chapter 4. NMR Study of Reaction Mechanism of Bis-

[trimethoxysilypropyl]amine and Vinyl-triacetoxysilane mixture ··············79

4.1 Introduction ······························································································79

4.2 Results and discussion ···········································································80

4.2.1 NMR analysis of neat VTAS and bis-amino silane ·····························80

4.2.1.1 Neat bis-amino silane ········································································80

4.2.1.2 NMR analysis of neat VTAS·······························································83

4.2.1.3 NMR analysis of neat bis-amino silane and VTAS mixture ············86

4.2.2 NMR analysis of the AV silane in water solution································93

4.3 Conclusions····························································································103

Chapter 5. Effect of Substrate Cleaning Solution pH on Corrosion

Performance of Silane-coated Cold-rolled Steel ·······································105

5.1 Introduction ····························································································105

5.2 Results and discussion ·········································································108

5.2.1 Etch rate·······························································································108

5.2.2 Surface morphologies of as-cleaned CRS surfaces ························110

5.2.3 Surface energy of as-cleaned CRS panels········································111

5.2.4 Silane layer thickness on CRS···························································117

5.2.5 Fourier-transform infrared reflection-absorption (FTIR-RA)

Spectroscopy of silane-coated CRS···························································118

5.2.6 DC potentiodynamic results of CRS panels ·····································121

5.2.6.1 As-cleaned CRS panels ···································································121

5.2.6.2 Silane-coated CRS panels ·······························································123

5.2.7 Electrochemical Impedance Spectroscopy (EIS) of AV silane-The contained-primer-coated CRS panels ························································125

5.2.8 Salt water immersion of primer-coated CRS panels ························130

5.3 Conclusions····························································································131

Chapter 6. Water-Barrier Properties of Mixed Bis-

[trimethoxysilylpropyl]amine and Vinyl Triacetoxysilane Films ··············132

6.1 Introduction ····························································································132

6.2 Results and Discussion·········································································133

6.2.1 Calculation of X-ray and Neutron SLD of Possible Reaction Products

·······················································································································133

6.2.2 X-ray reflectivity study········································································135

6.2.3 GISAXS of dry AV films ······································································141

6.2.4 Neutron Reflectivity Study of the AV Mixture Coatings···················146

6.2.4.1 As-prepared Films············································································147

6.2.4.2 Films after D2O Vapor Conditioning ···············································149

6.2.4.3 Re-dried Films ··················································································150

6.3 Conclusions····························································································158

Chapter 7. Water absorption and transport in bis-silane films ················159

7.1 Introduction ····························································································159

7.2 Results and Discussion·········································································159

7.2.1 Neutron reflectivity study of bis-sulfur silane and bis-amino silane film·················································································································159

7.2.1.1 Water absorption in bis-amino silane film ·····································159

7.2.1.2 Water absorption in bis-sulfur silane film······································164

7.2.2 In situ FTIR-RA study the sorption of water in bis-sulfur silane and bis-amino silane film····················································································169

7.3 Conclusions····························································································179

Chapter 8. General Conclusions and Suggested Future Work ················181

8.1 General conclusions··············································································181

8.2 Impacts of present research on development of silane-enhanced superprimer corrosion protection coatings (SERDP)·······························184

8.3 Suggested future work ··········································································186

References····································································································189

List of Figures

Figure 2.1. Schematic of condensation and hydrolysis of silica at different pHs⋅⋅⋅⋅⋅⋅⋅35

Figure 2.2. Illustration of the bonding state of γ-APS in aqueous solution. The dashed lines indicate the hydrogen bond between amino group and silanol group. The silanol groups are hydrogen-bonded with amino group and hence retard the condensation between silanols⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅37

Figure 2.3. Mechanism for the formation of Si-O-M linkages during the adsorption of propyltrimethoxysilane onto a hydrated metal oxide surface. Condensation occurs between surface hydroxyl groups and a silanol species (Si-OH) ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅40

Figure 2.4. Models of absorption conformations. (a) surface bonding via protonated amine group; (b) surface bonding via protonated amine group and condensed silanol adsorption; (c) surface bonding via condensed silanols. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅41

Figure 2.5. The adsorption isotherm for PTMS in aqueous solution on polycrystalline iron oxide surface. The intensity ratios were calculated from the area under the Si 2S and Fe 2p3/2 XPS photoelectron peaks. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅41

Figure 2.6. The adsorption isotherm for PTMS in aqueous solution on polycrystalline aluminum oxide. The intensity ratios were calculated from the area under the Si 2s and

Al 2s XPS photoelectron peaks. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅42

Figure 2.7. Mechanism for formation of a strongly bound polysiloxane fragment. (a)

Adsorption of PTMS takes place from the solution. (b)Adsorbed PTMS molecule migrates to neighbor surface site. (c) Condensation reaction takes place resulting in a polysiloxane fragment with every monomer unit bound to the surface. The black solid line indicates bonds between silane and substrate or between silane molecules. ⋅⋅⋅⋅⋅⋅⋅⋅42

Figure 2.8. Mechanism for the formation of a weakly bound polysiloxane fragment. (a)

Condensation takes place directly from the solution. (b) The polysiloxane fragment formed is only attached to the surface via a single monomer unit. (c) The weakly bound polysiloxane fragment has a greater probability of desorbing from the surface. The black solid line indicates bonds between silane and substrate or between silane molecules. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅43

Figure 2.9. Schematic illustration of wetting angle at a solid-liquid-air interface at equilibrium. S stands for solid, L for liquid and V for air. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅46

Figure 2.10. The water absorption in bis-aminosilane and bis-sulfur silane films: the effects of bridging groups. Bis-amino silane absorbs more water than bis-sulfur silane.

A water-rich layer is found on the surface of bis-amino silane film but not on bis-sulfur silane film. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅53

Figure 2.11. The schematic illustration of reflection and refraction from parallel interfaces. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅55

Figure 2.12. Reflectivity calculated for a deuterated polystyrene film of thickness 500

Å deposited on a Si substrate, to illustrate the effect of diffuse interface. Thin solid

curve, σ1 = σ2 = 0; thin broken curve, σ1 = 20Å, σ2 = 0; thick broken curve, σ1 = 0, σ2 =

20Å. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅56

Figure 3.1. Chemical structure of bis-amino silane, VTAS and bis-sulfur silane. (a)

Bis-[trimethoxysilylpropyl]amine (bis-amino silane), and (b) Vinyl-triacetoxysilane

(VTAS) (c) Bis-[3-(triethoxysilyl)propyl]tetrasulfide (bis-sulfur silane). ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅59

Figure 3.2. Schematic illustration of the sample holder set up for neutron reflectivity

⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅66

Figure 3.3. Schematic illustration of SPEAR beamline setup. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅68

Figure 3.4. Illustration of the geometry and set-up of the GISAXS measurement. ⋅⋅⋅⋅⋅69

Figure 4.1. 13C NMR spectrum of bis-amino silane (a) 6-26 ppm, (b) 48-54 ppm. The presence of a MeOH peak and multiplets indicates that hydrolysis occurs readily. The occurrence of the multiplets around 23.7 and 50.2 ppm is due to the different hydrolysis species in the solution after hydrolysis. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅81

Figure 4.2. Chemical structure and calculated 13C NMR peak positions of the neat bis- amino silane and related hydrolysis products. (a) Neat bis-amino silane; (b) Low hydrolysis product; (c) Intermediate hydrolysis product; and (d) High hydrolysis product. The peak positions are calculated by Chemdraw software. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅82

Figure 4.3. 13C NMR spectra of neat VTAS. The occurrence of the doublets around

140.6/139.8 and 127/126.4 ppm of the vinyl groups and 168.9/169.3 ppm of the

acetoxy carbonyl carbon in Figure 4.3(a) and (b) is due to the hydrolysis and condensation of the VTAS. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅84

Figure 4.4. Chemical structures and 13C peak positions of neat VTAS. The peak position is calculated by Chemdraw software. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅85

Figure 4.5. 29Si NMR spectrum of the neat VTAS. The lower condensation products T1 of VTAS can be observed in the spectrum. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅85

Figure 4.6. 13C NMR spectra of (a) AV3.4 mixture after 5-hour reaction time, (b) neat bis-aminosilane, and (c) neat VTAS. The AV mixture silane prepared following the procedures described in section 3.2.4, the NMR spectra were collected after 5 hours of mixing of individual neat silanes. New peaks occur after mixing in 48-53 ppm and 18-

25 ppm range. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅87

Figure 4.7. Chemical structures and peak assignments for (a) amide complex, (b) neat bis-amino silane, and (c) neat VTAS. The peak positions are calculated by Chemdraw software. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅88

Figure 4.8. 13C NMR spectra of VTAS (green), bis-amino silane (red) and AV mixture

(blue). The AV mixture silane was prepared following the procedures described in section 3.2.4. The NMR spectra were collected after 5 hours of mixing of individual neat silanes. The carbon peak in methoxy group of bis-amino silane at 50.1-50.3 ppm and the propyl peak of neat bis-amino silane adjacent to N atom at 52.9 ppm decrease in intensity in the spectrum of AV mixture. Three new peaks at 51.3-51.7 ppm, 50.0-

50.6 ppm and the 48.5 and 48.7 ppm appear in the AV mixture spectrum due to the formation of amide complex. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅89

Figure 4.9 13C NMR spectra of the VTAS, bis-amino silane and AV mixture. The AV mixture silane prepared following the procedures described in section 3.2.4. The NMR spectra were collected 5 hours after the mixing of individual neat silanes. The C peak in propyl group of neat bis-amino silane shifts from the 23.5 to 21.5 ppm region and appears as multiplets with some residual intensity of propyl peak of neat bis-amino silane left at 24.1 ppm. The C peak on VTAS methoxy group at 22.3 ppm also decreases in intensity and shifts to the lower ppm region at around 21.5 ppm. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅90

Figure 4.10. 13C NMR spectra of VTAS, bis-amino silane and AV mixture. The AV mixture silane prepared following the procedures described in section 3.2.4. The NMR spectra were collected 5 hours after the mixing of individual neat silanes. The doublet peaks at 169.3 to 170.0 ppm due to the C=O in acetoxy group in neat VTAS almost disappear after reaction due to the exchange of the hydrogen atom on the secondary amine with the acetoxy group on VTAS. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅91

Figure 4.11. Primary reaction in neat bis-aminosilane and VTAS mixture from the

NMR results in Figure 4.6, 4.8, 4.9 and 4.10. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅91

Figure 4.12. 29Si NMR spectrum of neat AV mixture silane in -75 to -40 ppm range.

The NMR spectrum was collected 5 hours after the mixing of neat bis-amino silane and

VTAS. The condensation products of bis-amino silane of different stages (T1, T2 and T3)

and the first and second condensation products (T1 and T2) of VTAS are observed in the spectra. A VTAS hydrolysis product can also be observed in the spectrum at 55.6 ppm.

⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅93

Figure 4.13. 13C spectra of the neat AV3.4 mixture and 10 wt% AV3.4 water solution collected after 5-hour hydrolysis in water. The multiplet peaks at 52.9 ppm, 51.3-51.7 and 48.5-48.7 ppm of amide complex disappear. The peaks at 50.0-50.6 ppm decrease in intensity. The neat AV mixture is prepared following the procedure described in section 3.1.2.1 and the solution was made 5 hours after the mixing of neat AV silane mixture. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅94

Figure 4.14. 29Si NMR of 10 wt% AV mixture solution after 10 hours hydrolysis in water. The condensation product of bis-amino silane T2 and T3 are observed, but the condensation products (T1 and T2) of VTAS are absent. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅95

Figure 4.15. Possible reactions in the AV3.4 mixture water solution, (a) Decomposition of amide complex in water solution, (b) Hydrolysis of bis-amino silane, (c)

Condensation of bis-amino silane. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅96

The intensity of higher condensation product (T3) peak of bis-amino silane at 56.0-60.0

ppm increases with time. The lower condensation products peak (T1) of bis-amino silane and VTAS condensation product peak (T1 and T2) are absent from the spectra.

⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅98

Figure 4.17. Relative intensity of the neat bis-amino silane, VTAS and condensation products of bis-amino silane (T2 and T3) as a function of hydrolysis time of 10 wt% AV mixture silane in water solution. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅100

Figure 4.18. Schematic illustration of hydrogen bonding between secondary amine group on bis-amino silane and silanol group on VTAS in AV water solution. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅102

Figure 5.1. Schematic of the silane deposition and bonding on metal surface (a) before condensation: hydrogen-bonded interface; (b) after condensation: covalently bonded interface. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅107

Figure 5.2. Etch rates of CRS as determined by weight loss and Fe concentration as determined by ICP-MS after cleaning at different pH values. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅109

Figure 5.3. Surface morphology of the CRS surface cleaned at different pH values by

SEM (a) pH∼1.0, (b) pH∼9.5 and (c) pH∼12.4. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅110

Figure 5.4. Measured contact angle data of the as-cleaned CRS as function of the cleaning solution pH. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅111

Figure 5.5. Calculated polar and dispersion components of as-cleaned CRS surface cleaned in different pH conditions based on contact angle measurements. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅113

Figure 5.6. Schematic diagram of the mechanism of AV silane adsorption onto CRS.

Highly polar surface of CRS after cleaning in strongly acidic or alkaline conditions prevents the formation of bonding between CRS surface and silane. One the other hand, the neutral surface near the IEP point promotes the formation of hydrogen bonds between the silane and CRS surface. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅114

Figure 5.7. Calculated basic (a) and acidic (b) properties of the CRS surface cleaned in different pH conditions based on contact angle measurements following the procedure discussed in Chapter 3.2.9. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅116

Figure 5.8. AV silane layer thickness on CRS before curing as a function of cleaning solution pH. The AV silane coating procedure is decribed in detail in section 3.1.2.2 and cured at 100ºC for 60 minutes. The silane layer thickness was measured by the weight gain method following the procedure described in Chapter 3.2.7. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅117

Figure 5.9. FTIR spectra of the AV silane coated CRS cleaned in different pH solutions (a) before curing and (b) after curing at 100°C for 60 minutes. The AV silane coating procedure is described in detail in section 3.1.2.2. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅120

Figure 5.10. DC potentiodynamic curves of the as-cleaned CRS panels cleaned at 60ºC for 3 minutes in cleaning solution of different pHs. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅121

Figure 5.11. Ecorr of as-cleaned CRS panels after cleaning in different pH conditions at

60ºC for 3 minutes in cleaning solution of different pHs. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅123

Figure 5.12. DC potentiodynamic curves of the silane-coated CRS panels cleaned at 60

ºC for 3 minutes in cleaning solution of different pH. The AV silane coating procedure is described in detail in section 3.1.2.2. and cured under 100ºC for 60 minutes. ⋅⋅⋅⋅⋅⋅⋅124

Figure 5.13. EIS results on primer-coated CRS panels cleaned in different pH solutions on day 0, in 0.6 M NaCl solution (a) Modulus and (b) Phase angle plot. The formulation of the primer and the preparation of the primer coating follow the description in Chapter 3.1.2.2. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅126

Figure 5.14. EIS results on primer-coated CRS panels cleaned in different pH solutions after 14 days of immersion in 0.6 M NaCl solution, (a) Modulus and (b) Phase angle plot. The formulation of the primer and the preparation of the primer coating follow the description in Chapter 3.1.2.2. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅128

Figure 5.15. Comparison of the low-frequency impedance modulus of primer-coated

CRS panels cleaned in different pH solutions obtained from the EIS results on day 0 and after 14 days of salt water immersion The formulation of the primer and the preparation of the primer coating follow the description in Chapter 3.1.2.2. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅129

Figure 5.16. 7-day salt water immersion test results on the primer-coated CRS panels cleaned at different pH values. The formulation of the primer and the preparation of the primer coating follow the description in Chapter 3.1.2.2. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅130

in (b). Each reflectivity curve is offset by two decades for clarity. (b) The SLD profiles of the best-fit of the reflectivity data in (a). The bar in the graph indicates the calculated x-ray SLD range of AV monomer mixtures. The films were prepared following the procedure described in Chapter 3.1.2.1. The films were cured at 100ºC for 60 minutes.

⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅139

Figure 6.2. GISAXS horizontal line-cuts at different A/V ratios: (a) AV2; (b) AV3.4;

(c) AV5. θ is angle of incidence, θc, film is the critical angle of the AV films, θc, Si is the critical angle of Si substrate. When θ is less than the critical angle of the film, the beam does not penetrate the film and scattering arises from surface roughness. Above the critical angle scattering from the bulk of the sample is observed in addition to the surface scattering. The films were prepared following the procedure described in

Chapter 3.1.2.1. The films were cured at 100ºC for 60 minutes. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅142

Figure 6.3. The scattering from the bulk film (II) obtained by subtracting the surface scattering (Is) from the total scattering (IGISAXS). Although the data are not on absolute scale, they have not been shifted with respect to each other. The intensity has been normalized by the beam path length in the film. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅145

Figure 6.4. Neutron reflectivity and SLD of as-prepared AV films at different A/V ratios: (a) The reflectivity curve of the as-prepared film. The lines through the data points indicate the best-fit of the reflectivity data using the corresponding SLD in (b).

(b) The SLD profiles of the best fit to the reflectivity curves in (a). The bar indicates the calculated neutron SLD range of the AV monomer mixtures. The films were prepared

following the procedure described in Chapter 3.1.2.1. The films were cured at 100ºC for 60 minutes. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅148

Figure 6.5. Neutron reflectivity and SLD of AV2 film: (a) Reflectivity in the as- prepared, D2O-vapor-conditioned and re-dried states. The lines through the data points indicate the best-fit of the reflectivity data using the corresponding model SLD in (b).

(b) The corresponding SLD profiles of the best-fit to the reflectivity in (a). The film was prepared following the procedure described in Chapter 3.1.2.1 and cured at 100ºC for 60 minutes. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅151

Figure 6.6. Neutron reflectivity and SLD of AV3.4 film: (a) Reflectivity in as- prepared, D2O-vapor-conditioned and re-dried state. The lines through the data points indicate the best-fit of the reflectivity data using the corresponding model SLD in (b).

(b) The corresponding SLD profiles of the best-fit to the reflectivity in (a). The film was prepared following the procedure described in Chapter 3.1.2.1 and cured at 100ºC for 60 minutes.⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅152

Figure 6.8. Comparison of the void volume and D2O absorption during D2O conditioning in films of different A/V ratios. The D2O absorption tracks the void volume. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅156

Figure 6.9. Comparison of the chemical change after re-dry and physically absorbed

D2O during D2O conditioning. The chemical change is calculated from the SLD change between the dry and re-dried states using equation (6.5). D2O absorption is calculated by equation (6.6).⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅157

Figure 7.1. Change of reflectivity (a) and SLD (b) of the bis-amino silane film on exposure to D2O vapor. The lines through the data points of Figure 7.1(a) indicate the best-fit of the reflectivity data using the corresponding model SLD in Figure 7.1(b).

The data show a rapid uptake of D2O within first 30 minutes but equilibrium is not reached after 11.6 hours of exposure to D2O vapor. The films are prepared following the procedures described in 3.1.2.1 and cured at 180ºC for 60 minutes on Si substrate.

⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅161

Figure 7.2. Volume fraction of D2O in bis-amino silane film as function of conditioning time. Water rapidly occupies 30 vol% in 0.5 hour and then slowly increases to 33 vol% in the next 11.1 hours and equilibrium is reached after 11.6 hours of D2O conditioning. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅162

Figure 7.3. Thickness increase of the bis-amino silane film as the function of the water conditioning time. The thickness increase is slower than the SLD increase as shown in

Figure 7.2. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅162

Figure 7.4. Change of reflectivity (a) and SLD (b) of the bis-sulfur silane film on exposure to D2O vapor. The lines through the data points of (a) indicate the best-fit of the reflectivity data using the corresponding model SLD in (b). The data show a rapid uptake of D2O within first 30 minutes but equilibrium is not reached until 6 hours of exposure. The films are prepared following the procedures described in 3.1.2.1 and cured at 180ºC for 60 minutes on Si substrate. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅165

Figure 7.5. The D2O absorption in the bis-sulfur silane film as the function of the D2O conditioning time. The film is saturated with D2O water in 6 hours. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅166

Figure 7.6. The FTIR-RA spectra of the as-prepared bis-amino silane and bis-sulfur silane films. The films are prepared following the procedures described in 3.1.2.1 and cured at 180°C for 60 minutes on Si substrates. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅170

Figure 7.7. The IR spectra of the bis-amino silane film in as-prepared state and at equilibrium. The film is prepared following the procedure described in Chapter 3.1.2.1 and cured at 180°C for 60 minutes. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅171

Figure 7.8. The IR spectra of bis-sulfur silane film in as-prepared state and at equilibrium. The film is prepared following the procedure described in Chapter 3.1.2.1 and cured at 180°C for 60 minutes. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅172

Figure 7.9. The IR spectra of bis-amino silane in response to D2O conditioning. The films are prepared following the procedure described in Chapter 3.1.2.1 and cured at

180°C for 60 minutes on Si substrates. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅173

Figure 7.10. Normalized relative reflectance intensity increase of 1340 cm-1 IR peak of bis-amino silane film during D2O conditioning. The equilibrium of the D2O absorption is reached at around 1150 minute conditioning. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅174

Figure 7.11. The IR spectra of bis-sulfur silane in response to D2O conditioning. The films are prepared following the procedure described in Chapter 3.1.2.1 and cured at

180°C for 60 minutes on Si substrates. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅174

Figure 7.12. Normalized relative intensity increase of 1340 cm-1 IR peak of bis-sulfur silane film during D2O conditioning. Equilibrium D2O absorption is reached at around

250 minutes, which is much faster than for bis-amino silane as shown in Figure 7.10.

⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅175

Figure 7.13. Best-fit of the FTIR intensity increase data for bis-amino silane calculated from Eq. (7.4) with D = 3.95 × 10-15 cm2/s and m = 8. The calculated value fits the data.

The slight derivation of the data from calculated value indicates the deviation from

Fickian behavior in the intermediate stage of diffusion. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅177

Figure 7.14. Best-fit of the FTIR intensity increase for bis-sulfur silane calculated from

Eq. (7.4) with D = 2.30 × 10-15 cm2/s and m = 8. The slight derivation of the data from calculated value indicates the deviation from Fickian behavior in the intermediate stage of diffusion. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅178

List of Tables

Table 2.1. List of common mono-silanes and bis-silanes for corrosion protection⋅⋅⋅⋅⋅⋅⋅31

Table 3.1. Origin, purity, surface energy of probing liquids⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅76

Table 5.1. IR peaks and their assignments for the silane-coated CRS, (s stands for strong, m for medium and w for weak) ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅118

Table 5.2. Icorr from the DC potentiodynamic measurements on as-cleaned in different pH solutions and silane-coated CRS panels⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅124

Table 6.1. Calculated SLD values of initial materials and by-products in AV system

⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅133

Table 6.2. Calculated SLD values of AV monomer physical mixtures. ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅134

Table 6.3. Calculated SLD values of possible reaction products in AV system. ⋅⋅⋅⋅⋅⋅⋅⋅136

Chapter 1. Introduction 1.1 Research Significance and Objective

1.1.1 Objective and relationship to Strategic Environment Research and Development Program (SERDP)

Silane coatings have emerged in recent years as promising candidates for replacement of toxic chromate treatment of metals to improve adhesion and corrosion performance.1-11 The work performed here supports a SERDP-funded project to develop pretreatment and primer with no sacrifice of corrosion performance compared to chromate-containing coatings.

The proposed system is based on a University of Cincinnati (UC) invention termed

“superprimer.” The superprimer, consisting of silane, organic resin and nano-particle filler, is totally water-borne and is amenable to dipping, spraying, or brushing onto any clean metal surface. No conversion coating, either phosphate or chromate, is required.

The key innovation underlying this technology is bis-silane based primers. The water- based bis-amino silane and vinyl triacetoxysilane (VTAS) mixture (AV) system developed by Van Ooij et al.9, for example, shows excellent stability in water, rapid hydrolysis and good compatibility with various polymer systems. The AV system provides good bonding with metal surface as well as with the polymer top-coat. More importantly, the AV primer and AV-polymer superprimer offer good corrosion protection on metal substrate as well as low volatile organic compounds (VOCs).12

Despite of the above advantages of AV system, the molecular basis of stability, bonding and anti-corrosion performance is unclear both for primers and super primers.

This research was undertaked to clarify substrate surface chemistry, silane film chemistry, film morphology and water absorption behavior. The work provides new insights to guide the development of optimum films and identifies potential vulnerabilities of the new technology. The key finding is the rapid absorption of water in all films studied including those that provide excellent anticorrosion protection.

Water is absorbed with minimal change in film thickness. Water is absorbed in the free volume present in the film. Water absorption in this manor generates minimal stress in the film and seems not to be detrimental to corrosion performance. In fact water can serve to leach pigments to provide scratch protection. The hydrophilicity of the films, however, does raise concern regarding the long-term susceptibility of the silanes to hydrolytic degradation.

Of all the silane system for anti-corrosion applications, bis-silanes with the structure

(RO)3Si(CH2)3–R′–(CH2)3Si(OR)3, are of particular interest because they are highly crosslinked, leading to very robust films. Previous study suggests that the bridging group is critical for favorable properties.1,9,13-15

Bis-amino silane and bis-sulfur silane are common silanes used in anti-corrosion primer coatings.12,15-17 Bis-amino silane is hydrophilic due to the secondary-amine bridging group. Bis-sulfur silane, on the other hand, is hydrophobic due to the Sx bridging group.

Bis-amino silane and bis-sulfur silane behave differently during water absorption.14,18

Bis-amino silane absorbs more water (41.0 vol%) than bis-sulfur silane (7.8 vol%).

The difference in water absorption was attributed to the difference in the bridging groups. Following the previous study on equilibrium water absorption, this work targets the kinetics of water penetration. Time-resolved water diffusion and water distribution in the film was monitored during water penetration.

1.1.2 Research background

Organofunctional silanes are known as adhesion promoters between organic polymers and mineral substrates. The silane may function as (1) a finish or surface modifier, (2) a primer, or (3) an adhesive, depending on the thickness of the silane layer at the interface.

Organofunctional silanes have the general structure X3Si(CH2)nY, where X is a hydrolyzable group and Y is an organofunctional group such as amine or epoxy. When a silane coupling agent is applied to the polymer-inorganic interface, the hydrolyzable group, X, and the organofunctional group, Y, can interact with the polymer and the inorganic interface, respectively, providing a strong chemical and/or physical linkage.

Organosilanes, especially bis-type silanes, with the general of X3Si(CH2)3-R’-

1- (CH2)3SiX3, offer effective corrosion protection on metals such as aluminum and steel.

5,9,11

The silane bonding mechanism between metal substrate and the polymer top-coating is reasonably well established.1,19,20 When the silane solution is applied onto the metal

surface, silanol (-SiOH) groups absorb rapidly through hydrogen bonds. During the subsequent curing and drying, two condensation reactions occur. Silanols (-SiOH) condense with the hydroxyl groups (MeOH) on the metal surface and form covalent metallo-siloxane bonds (SiOH (solution) + MeOH (metal substrate) → SiOMe + H2O), and the excess silanol groups self-condense to form a siloxane network (SiOH(solution) +

SiOH(solution) →SiOSi + H2O). The SiOMe linkages are responsible for the strong bonding to the metal substrate. The siloxane network in bulk silane film is assumed to be responsible for the water-barrier properties. The organofunctional groups on silane molecule, on the other hand, interact with the polymer top-coating in two ways: they can either forms chemical bonds with reactive groups of polymer or they can incorporate into the polymer and form an interpenetrating network (IPN). The two forms can co-exist.20

Recently the effect of the metal surface cleaning on silane adsorption has been recognized.20-28 As the metal surface hydroxyl groups are directly related to the formation of the metallo-siloxane bonds, the adhesion of the silane coating is sensitive to the surface cleaning procedures of the metal substrate. Evidence shows that silane adsorption on a mineral substrate surface bears a close relationship to the pH of the silane solution.3,24,29 In attempt to understand the effects of the substrate pretreatment on the adhesion and corrosion performance the surface chemistry, surface energy, surface morphology and corrosion performance on cold-rolled steel (CRS) was evaluated .

Generally, silanes can be classified as solvent-based or water-based depending on their hydrophobicity. The former, represented by bis-[triethoxysilyl]ethane (BTSE) and bis-

[triethoxysilylpropyl]tetrasulfide (bis-sulfur silane), show effective corrosion protection for many metals. Unfortunately, the use of organic solvents such as methanol and ethanol cannot be avoided during the preparation for these hydrophobic silanes. Such volatile organic compounds (VOCs), however, raise environmental concerns.

Regarding water-based silane, bis-[trimethoxysilypropyl]amine (bis-aminosilane) and vinyl-triacetoxysilane (VTAS) mixtures containing-coatings have emerged as outstanding candidates with corrosion protection performance comparable to the solvent-based silanes on many metal substrates.9,15 According to previous work, the following advantages have been observed for bis-aminosilane and VTAS mixture coatings: 1) the mixture is water-soluble; 2) the system hydrolyzes rapidly in water yet does not gel; 3) the system exhibits broader compatibility with paints than most silanes; and 4) the ratio of the between bis-amino silane and VTAS can be adjusted to optimize the anti-corrosion performance.

Bis-amino silane is hydrophilic, traceable to the secondary amine bridging group, which promotes good bonding to metal substrates. However, bis-amino silane is not stable by itself at high concentration in water. Bis-amino silane solutions must be used within time-window since gelation occurs after prolonged hydrolysis. VTAS, on the other hand, is hydrophobic and not readily hydrolyzable in water. Interestingly, when the two silanes are mixed at certain ratios, rapid hydrolysis, higher stability, good

bonding and good corrosion-protection performance are achieved. In order to elucidate the basis for such enhanced properties, the reaction mechanism of pure AV silane mixture as well as the silane water solutions were studied by 13C and 29Si NMR.

Despite the well-understood adhesion promoting mechanism of silane coupling agents mentioned above, the mechanism of silane corrosion inhibition is not as well established. According to previous electrochemical research,2,10 silane acts as a physical barrier rather than an electrochemical barrier. Some important factors may contribute to the physical barrier properties of the silane layer. One assumption is that the dense hydrophobic siloxane network in silane layer acts as the water barrier to prevent water from penetrating to the metal-silane interface. Another possible mechanism is that the siloxane bond formed between silanol and metal surface oxide saturates metal surface bonding sites and thus retards the reaction with water and other corrosive species. Both of the above mechanisms can contribute to the anti-corrosion properties of silane coatings.15 In order to clarify the anti-corrosion mechanism, we will investigate the interaction of water with silane films using x-ray and neutron reflectivity (NR).

Equilibrium water absorption of silanes reveals that bridging group controls water absorption behavior.14,18 Bis-amino silane absorbs more water (41.0 vol%) than bis- sulfur silane (7.8 vol%) at equlibrium. In addition to the water uptake at equilibrium, water transport kinetics and the absorption mechanism of water in polymer film are of interest. The generally accepted model of sorption of water in polymers is the so-called dual-mode-sorption model.30,31 Unlike the study of equilibrium water uptake,13,14,18,32-36

, the in situ kinetic study enables us to monitor the water penetration into the film and provides more details on water absorption. In this study, bis-silane films were exposed to the saturated D2O vapor and NR and FTIR were monitored during water penetration.

The results are compared with that of the as-prepared and re-dried films. A sorption model of water in bis-silanes is discussed.

1.2 The scope of research work

This dissertation summarizes the study on the substrate surface chemistry, and pretreatment protocol on the corrosion performance, reaction mechanism, morphology, water sorption and water transport kinetics in water-based bis-amino and VTAS mixed silane system (denoted as AV).

This dissertation consists of 8 chapters:

Chapter 1: Introduction. The research objective, background and scope of the research work are summarized.

Chapter 2: Previous work on silane chemistry, surface pretreatment, bonding, water transport and water absorption are reviewed. The theory of x-ray and neutron reflectivity is also given.

Chapter 3: The sample preparation, characterization methods, and experimental procedures are described.

Chapter 4: The reactions of water-based AV system are reported. The reaction mechanism between neat bis-amino silane and VTAS was characterized by 13C, and

29Si NMR. The 29Si NMR spectral evolution was monitored as a function of time. A stabilization mechanism of AV water solution is proposed.

Chapters 5: The influence of the cleaning solution pH on surface chemistry and corrosion performance is reported for cold-rolled-steel (CRS) panels. The underlying mechanism of cleaning is discussed.

Chapter 6: The morphologies of bis-amino silane (A) and vinyl triacetoxy silane (V) mixed films of different A/V ratios are reported. The response of these films to saturated water vapor is investigated with x-ray and neutron reflectivity. The interaction of water with the film during the conditioning is discussed. A dual mode mechanism of water absorption is proposed.

Chapter 7: Water transport mechanism in bis-sulfur silane and bis-amino silane films is investigated with in situ neutron reflectivity (NR) and FTIR in the presence of saturated

D2O vapor. Time-resolved water ingress is calculated from the scattering length density

(SLD) profile. The absorption and transport behavior is compared for bis-amino silane and bis-sulfur silane. FTIR is used to track the water ingress. A water absorption model and kinetic model of water transport are proposed.

Chapter 8: General conclusions are summarized and future work is proposed.

Chapter 2. Literature Review 2.1 Chemical structure of organosilanes

Organosilanes with the general structure X3Si(CH2)nY have been used as coupling agents between the polymer and mineral surfaces since 1940s.20 The coupling is attributed to a stable link between the polymer top-coat and the organofunctional group,

Y, of silane as well as between the hydrolyzable alkoxy groups, X, of silane and surface hydroxyl group (-OH) on mineral surface. The organofunctional groups are chosen for reactivity or compatibility with polymers. The hydrolyzable alkoxy groups form intermediate silanols that react with surface hydroxyls on the mineral surface.

In recent years, silane-based surface treatment of metals has emerged as a promising alternative for chromate treatment in the metal-finishing industry.1,3,4 The use of silanes for corrosion protection is motivated by the need for an alternative to conventional chromating in metal-finishing industries. So far, chromates are still the most efficient within the current repertoire of inhibitors. Nevertheless, the well-recognized toxicity and carcinogenicity of hexavalent chromium ions (Cr6+) has led to restriction of the chromates. As a result, alternative candidates have been explored. Among the candidates, organosilanes appear to be the most promising.1

Protective silanes can be divided into two categories in terms of their chemical structure: mono-silylfunctional (mono-) and bis-silylfunctional (bis-) silane. Mono-silanes have the general structure of X3Si(CH2)nY, where X is a group that can be hydrolyzed, and Y is an organofunctional group. Compared to mono-silanes, bis-silanes with the general

structure of (RO)3Si-R’-Si(OR)3, provide more crosslinks, stronger bonding with mineral surface, and better corrosion protection.1-3,9-11,37-39

Table 2.1. List of common mono-silanes and bis-silanes for corrosion protection

Name Chemical structures

Mono-silane Vinyl triacetoxysilane (VTAS) CH2=CHSi(COCOCH3)3

Vinyl triethoxysilane (VS) CH2=CHSi(OC2H5)3

γ-aminopropyltriethoxysilane (γ-APS) H2N(CH2)3Si(OCH3)3

Bis-silane Bis-[3-(triethoxysilyl)propyl]tetrasulfide (H5C2O)Si-(CH2)3-S4-

(bis-sulfur silane) (CH2)3-Si(OC2H5)

Bis-[trimethoxysilyl]amine (CH3O)3Si-NH-Si(CH3O)3

(bis-amino silane)

Bis-[triethoxysilyl]ethane (BTSE) (C2H5O)3Si-(CH2)2- Si(C2H5O)3

Organosilane-based coatings offer effective corrosion protection on metals such as aluminum and steel. 2,10,20,21,40-42 Hydrophobic silanes such as bis-[triethoxysilyl]ethane and bis-[triethoxysilylpropyl]-tetrasulfide are particularly effective in corrosion protection. Unfortunately volatile organic compounds (VOCs) such as methanol and ethanol cannot be avoided during preparation of the coatings from hydrophobic precursors. Development of environmentally benign corrosion-protection coatings, therefore, is the driver for understanding the fundamental properties of water-based silane films.

2.2 Silane solution chemistry

2.2.1 Hydrolysis and condensation

The hydrolysis and condensation reactions in water-containing silane solutions can be expressed by the following two reactions:43

Hydrolysis:

R'Si(OR)3 +H2O → R'Si(OH)3 + 3ROH (2.1)

Condensation:

R'Si(OH)3 + 3ROH → R'Si(OR)3 +H2O (2.2)

Hydrolysis and condensation of silanes in water solutions are influenced by factors such as the nature of organofunctional groups, solution pH, concentration, hydrolysis time, and temperature. The quality of water and even the containers used also affect the hydrolysis of alkoxysilanes. However, pH is often the most important factor that determines the behavior of a particular silane.

The mechanism of acid- and base-catalyzed hydrolysis of alkyltrialkoxy-silanes in aqueous solution was reported by Pohl43 and Plueddemann.20 The hydrolysis of alkoxy esters group of organotrialkoxysilanes occurs in a stepwise manner:

k1 R’Si(OR)3 + H2O R’Si(OR)2OH + ROH (2.3) -k1 k2 R’Si(OR)2OH + H2O R’Si(OR)(OH)2+ ROH (2.4) -k2 k3

-k3

R’Si(OR)(OH)2 + H2O R’Si(OH)3 + ROH (2.5)

According to the NMR study by Pohl et al.19 on the hydrolysis of vinyl trialkoxysilane, the ratios k1/k2 and k1/k3 are small indicating that the first step of the hydrolysis is slow, and thus becomes the rate-determining step in the process of silane hydrolysis.

The kinetics of hydrolysis is generically expressed by19

n m o p q -d[S]/dt = kspon[H2O] [S] + kH[H+][H2O] [S] + kHO[HO] [H2O] [S] + k[B][H2O] [S]

(2.6) where [S] is the concentration of the alkoxysilane, -d[S]/dt is hydrolysis rate, and [B] is

- the concentration of any basic species other than hydroxide anion (OH –). The first term on the right of Eq. (2.6) is the spontaneous reaction rate without catalysis. This term is of negligible interest due to the slow spontaneous hydrolysis.

The third and fourth terms on the right of Equation (2.6) describe the contributions of acids and bases to silane hydrolysis. Both acids and bases accelerate silane hydrolysis but in different ways. Acid-catalyzed hydrolysis involves the attack on an alkoxy

+ oxygen by a hydronium ion (H ) followed by bimolecular SN2-type displacement of the leaving alkoxy group by water. Alkali-catalyzed hydrolysis involves attack on silicon by a hydroxyl ion (OH–) to form a pentacoordinate intermediate followed by a bimolecular displacement of alkoxy group by OH. Rates of hydrolysis by both mechanisms are influenced by the nature of the alkyl group on silicon as well as the leaving alkoxy group.19

The effect of solution pH on the hydrolysis of γ-glycidoxypropyltrimethoxysilane (GPS) was reported by Osterholtz and Pohl etc.19 NMR spectra of dilute aqueous solutions show no significant concentration of intermediate methoxysilanols, but only the silanetriol and its condensed siloxane oligomer, which indicates that the second and third alkoxy groups hydrolyze faster than the first.

The final hydrolysis product, monomeric silane triol in the form of R’Si(OH)3, is not stable in concentrated aqueous solutions. Condensation occurs as soon as –Si-

1 (OH)3 forms. These silaneltriols condense to form oligomeric siloxanols,

R’(SiOSi)n(OH). Only very dilute silane solutions, e.g., 0.15% for γ- aminopropyltriethoxysilane (γ-APS) and 1% for vinyltriethoxysilane (VS), remain as monomeric.44 Thus, in most practical cases, silane solutions contain oligomeric siloxanols rather than monomeric silanetriols as the silane concentration is normally above 1%.

The condensation and rehydrolysis are in equilibrium in aqueous solution. A kinetic expression describing the rate of silanol condensation to disiloxane is given as19

+ 2 – 2 2 –d[S’]/dt = kH [H ] [S’] + kHO [HO ] [S’] + k[B] [S’] (2.7) where S’ = SiOH, and B = any basic species other than OH–. Pohl and Osterholz19 studied the rate of condensation of silanols in water and observed that the hydrolysis of most alkoxy groups are catalyzed by both acids and bases. The slowest hydrolysis is at

approximately neutral pH (pH∼7) as shown schematically in Figure 2.1.45 A change of pH by one unit in either the acid or the base direction produces a ten-fold acceleration in hydrolysis. Thus, at pH∼4, the hydrolysis of silane is about 1000 times faster than at pH∼7. The anion of any acid may also accelerate the hydrolysis. Unlike the hydrolysis, the condensation rate is generally minimized at pH∼4, although substitution at the silicon atom can shift this minimum. To attain the maximum solution stability and more silanols, the pH is usually adjusted to around 4. Higher pH favors the condensation of

SiOH groups, leading to a premature gelation.

Figure 2.1. Schematic of condensation and hydrolysis of silica at different pHs45

2.2.2 Other factors that affect hydrolysis and condensation

Steric effects also influence both hydrolysis and condensation in silane solutions.15 The rate of hydrolysis is generally associated with steric bulk of the alkoxy group. That is,

the smaller the size of the alkoxy groups, the faster the hydrolysis rate, CH3O > C2H5O

46 > t-C4H9O. Arkles et al. concluded that isobutyltrimethoxysilane hydrolyzes 7.7 times faster than its ethoxy analog in acid solution. The hydrolysis of mixed ethoxymethoxy silane is 87% of that of the trimethoxysilane.

The hydrolysis rate also increases with the number of organic substitution: Me3SiOMe

47 > Me2Si(OMe)2 > MeSi(OMe)3. It was also reported that increasing size of alkyl group increases the stability of intermediate silanols and favors cyclic siloxane formation upon condensation.20

Due to the competition between silane hydrolysis and alcoholysis, the condensation of silanols in protic solvents, such as water, methanol, ethanol, etc, approaches equilibrium rather than completion. This fact enables us to tailor the silanol concentration by adjusting the solution parameters (pH, concentration etc.). On the contrary, the condensation in non-protic solvents such as dioxane, tends to completion.19

Silanols are stable in a very dilute solution. It is seen in Equation (2.7) that the rate is second order with respect to the silanol concentration. Thus, low silane concentration

(silanol concentration) minimizes condensation and favors solution stability.

Interestingly, it was found that some water-based silanes are stable at relative high concentration. A relatively concentrated γ-APS aqueous solution, for example, has a good stability. The amine group and silanol group on γ-APS forms hydrogen bonds,

hence retards the condensation and stabilizes the aqueous γ-APS solution. Figure 2.2 explains the stabilization mechanism of aqueous γ-APS solution. The silanol groups form hydrogen bond with amino groups of γ-APS and prevent the condensation between silanols.20

N OH H HO H Si O Si OH O H O H OC H H N 2 5 H OH H N Si

H OC2H5

Adhesion properties and anti-corrosion performance depend largely on the hydrolysis of silane. However, among all the silane molecules, only a few organosilanes with hydrophilic groups such as amine salts are immediately miscible with water. The majority are not soluble until the alkoxy group hydrolyzes. Hydrolysis of the alkoxy groups, however, requires molecular contact with water. For this reason, it is difficult to hydrolyze some alkoxysilane directly, and it is indeed a very slow process to hydrolyze alkoxysilanes in hydrocarbon solvents.20 In water, organosilanes are commonly

dissolved by shaking or stirring vigorously with acidified water until a clear solution results.

2.3 Bonding mechanisms of silanes

2.3.1 Bonding between silane and polymers

The possibility of combining the properties of organic and inorganic compounds is a challenge that dates to the beginning of the industrial era. Pigments, minerals, clays, etc. were added to polymers to improve properties. An important feature of hybrid materials is the interaction between the organic and inorganic phases, as the final properties of these materials strongly depend on the strength and durability of the interfacial bonds.21,48

Covalent bonding between the organic phase and the inorganic phase can be achieved with the introduction of organosilane with reactive organofunctional groups like acrylic, epoxy, amino etc. The organofunctional group can either form chemical bonds with reactive groups on the polymer or incorporate into the polymer as an interpenetrating organosilane network (IPN).20 A good example of bonding is glass-fiber-reinforced polyester. The glass fibers are covalently bonded to the organic matrix by pretreating these fibers with silane coupling agents. The strength of these composites does not degrade with time in water, which suggests that the Si-C bonds do not hydrolyze easily.

2.3.2 Bonding and adsorption of silane on mineral substrate

The mechanism by which silanes bond to metals has attracted a great deal of attention in recent years. The most widely accepted theory postulates that the condensation between the silanols (-SiOH) and the hydroxyl groups (MeOH) on the metal surface through hydrogen bonding during deposition. The formation of the covalent metallo- siloxane bonds (SiOH (solution) + MeOH (metal substrate) → SiOMe + H2O) occurs during the subsequent drying and curing. The effects of glass/metal surface conditions and organofunctional groups (e.g., amino groups) on the bonding between silanes and mineral surface have been extensively investigated.20,21,24-26,28,49

However, recent studies26,50,51 present evidence that silanes with organofunctional groups can also be absorbed onto the metal surface through the functional groups. The bonding depends on the availability and nature of the functional groups as well as the pretreatment of the metal surface.

The absorption kinetics of both functional and non-functional silane on metal (Fe and

Al) was studied by Quinton et al.26,51-55 with XPS and SIMS. These authors observed that non-functional silanes, such as propyltrimethoxysilane (PTMS) absorb on metal surfaces by complex adsorption involving hydrogen bonding as shown in Figure 2.3.54

For silanes with multiple moieties (γ-APS), the absorption occurs through both the functional groups (e.g. the amine group in γ-APS) and the OH groups as shown in

Figure 2.4.26

The study of adsorption of simple silanes on Al and Fe indicates that the silane adsorption mechanism depends on the substrate. Quinton etc.51 compared the absorption of silane on Fe and Al with XPS and found that the adsorption of the propyl trimethoxysilane (PTMS) on Fe proceeds much more rapidly than on Al.51 The absorption on iron is relatively straightforward as shown in Figure 2.5. The silane concentration on Fe surface increases monotonically until saturation is reached.

Adsorption isotherms indicate a Langmuir mechanism whereby the adsorbing PTMS occupy empty sites in a non-interacting manner. As the number of free sites decreases, the PTMS molecules are forced to combine. Polymerization takes place and a siloxane polymer is formed.

Figure 2.6, however, indicates that the formation of PTMS film on aluminum substrate is more complicated.26,51,52,54 A decrease in coverage occurs in the intermediate

exposures indicating that adsorption mechanism depends fundamentally on the substrate.51

Figure 2.6. The adsorption isotherm for PTMS in aqueous solution on polycrystalline aluminum oxide. The intensity ratios were calculated from the area under the Si 2s and Al 2s XPS photoelectron peaks.51

Figure 2.7. Mechanism for formation of a strongly bound polysiloxane fragment. (a) Adsorption of PTMS takes place from the solution. (b)Adsorbed PTMS molecule migrates to neighbour surface site. (c) Condensation reaction takes place resulting in a polysiloxane fragment with every monomer unit bound to the surface.51 The black solid line indicates bonds between silane and substrate or between silane molecules.

Figure 2.8. Mechanism for the formation of a weakly bound polysiloxane fragment. (a) Condensation takes place directly from the solution. (b) The polysiloxane fragment formed is only attached to the surface via a single monomer unit. (c) The weakly bound polysiloxane fragment has a greater probability of desorbing from the surface.51 The black solid line indicates bonds between silane and substrate or between silane molecules.

The model proposed by Quinton et al.51 states that condensation of the polysilane is an equilibrium process. An isolated adsorbed PTMS molecule may condense with another

PTMS molecule in one of the two ways. If a second PTMS molecule occupies a neighbor surface site, then a siloxane bond will be formed with both molecules anchored to the metal surface (for example, Fe) as shown in Figure 2.7. Alternatively, if the second molecule condenses from the solution then a siloxane bond will be formed with only one of the molecules anchored to the surface as shown in Figure 2.8. The polysiloxane fragment thus formed, which is only bound to the metal surface (for example, Al) by a single silicon-oxygen-substrate link, therefore has a greater possibility of desorbing. If the desorption rate is sufficiently high, then a mechanism exists by which PTMS molecules in solutions may effectively remove adsorbed groups from the surface and hence produce the observed decrease in the adsorption isotherm at intermediate concentrations. In addition, both desorption and adsorption are functions

of the strength of the silane-metal interaction accounting for the differences between

PTMS adsorption on aluminum and iron oxide substrates.

Harding et al.50 observed that the bond strength between silane and metal increases with the number of amino groups for amino-functional silane series. It has also been observed that γ-APS absorption on the metal substrate varies with isoelectric point (IEP) of the substrate and the solution pH.47 However, the study of Van Schaftinghen et al.56 shows no improvement in the anti-corrosion performance using bis-amino silane or γ-

APS as compared with the non-functional silane coatings. The observation implies other factors, such as surface hydroxyl-group properties (acid-base, polar-nonpolar) and surface-oxide species, might dominate the amino-functional silane adsorption.

2.4 Acid-base properties

It is well known that the surface oxide film on metal terminates in a layer of hydroxyl groups. These groups can be acidic or basic, depending on metal cation present in the oxide. There have been numerous reports on the relationship between acid-base properties and the isoelectric point (IEP) of bulk oxides, but only a limited number of studies have reported on that of oxide film on metals.24,50,57,58

In aqueous solution, surface hydroxyl groups may remain undissociated, in which case the pH of the solution is the same as the isoelectric point of the oxide. If the pH is less than the isoelectric point, the surface will acquire a positive charge:24

+ + − MOH surf + H (aq) ⇔ −MOH 2surf (2.8)

On the other hand, if the pH is greater than the isoelectric point, the surface will acquire a negative charge:

− − − MOH surf + OH ⇔ −MOsurf + H 2O or

− + − MOH surf ⇔ −MOsurf + H (2.9)

There have been several studies on the effect of pH on the wetting of solid surfaces.

Chau and Porter59 derived an expression showing that the variation of the water contact angle with pH goes through a maximum at the IEP of the surface. A more general treatment, applicable to any oxide film, has been published by McCafferty et al.24

McCafferty24 studied the interaction of liquids of different pH with different metal surfaces and verified that contact angles go through the maximum at the isoelectric point of the metal surface. The contact angle varies after surface modification due to the change of the isoelectric point.

For a solid surface (S) in contact with liquid (L) exposed to air (V) at equilibrium as illustrated in Figure 2.9, the relation between the solid-vapor interfacial energy (γS), the solid-liquid surface free energy (γSL) and the liquid-vapor surface free energy (γL) per unit area can be expressed by Young’s equation in the air:

0 = γ S − γ SL − γ L cosθ

where θ is the contact angle.

γL

Air

γS θ Liquid γ Solid SL

Figure 2.9. Schematic illustration of contact angle at a solid-liquid-air interface at equilibrium. S stands for solid, L for liquid and V for air.

Dupré expressed the relationship between the work of adhesion between the liquid and solid as:

ΔGSL = −ΔWSL = γ SL − γ S − γ L

where ΔGSL is the surface free energy change of solid and liquid interface per unit area,

ΔWSL is the adhesion energy of the solid and liquid surfaces or energy needed to separate solid and liquid from solid-liquid interface.

Thus the contact angle can be used to determine the surface free energy change at solid- liquid interface (if other interfacial energies are known) using the Young-Dupré equation:

γ L (1+ cosθ ) = −ΔGSL = ΔWSL (2.10)

The acid-base properties of polymer surfaces were determined by the method of van

Oss and Good.60,61 This approach combines the Young-Dupré equation (2.10) with the

Fowkes assumption that the surface free energy, γ i , of a phase i consists of the sum of

LW AB Lifshitz-van der Waals (γ i ) and acid-base (γ i ) contributions:

LW AB γ i =γ i +γ i ,

which leads to

LW AB ΔGSL = ΔGSL + ΔGSL (2.11)

where ΔGSL is the total surface free energy change of solid and liquid interface per unit

LW area, ΔGSL is the surface free energy change of solid and liquid interface per unit area

AB due to the contribution from Van der Waals force. ΔGSL is the surface free energy change of solid and liquid interface per unit area due to the contribution from acid-base interaction.

The Good-Girifalco rule for the Lifshitz-Van Waals energy across an interface is

LW LW LW 1/ 2 ΔGSL = −2(γ S γ L ) (2.12)

LW LW where γ s and γ L are the surface free energies of solid and liquid due the

contributuion of van der Waals component.

For the acid-base interaction across an interface, van Oss and Good introduced the

+ − concept of the acid and base components of the surface free energy, γ i and γ i ,

respectively, and the combining rule:

AB + − 1/ 2 − + 1/ 2 ΔGSL = −2[(γ s γ L ) + (γ S γ L ) ] (2.13)

Equations (2.10)-(2.13) give the result

1 (γ LW γ LW )1/ 2 + (γ +γ − )1/ 2 + (γ −γ + )1/ 2 = (1+ cosθ )γ (2.14) S L S L S L 2 L

LW + − Equation (2.14) is an equation in three unknowns: γ S , γ S and γ S . The first of these is

determined by the measuring the contact angle of an apolar liquid, such as methylene

+ − iodide, for which both γ L and γ L are zero. Equation (2.14) reduces to

(1+ cosθ ) 2 γ LW = γ , (2.15) S L 4

LW which allows evaluation of the parameter γ S from the surface energy, γ L , of the apolar liquid.

+ − After this step, Eq. (2.14) contain two unknown γ S and γ S , which are determined by

+ − measuring the contact angles of two different polar liquids with known γ L and γ L and solving the resulting two equations simultaneously. The polar liquids used include water, formamide, ethylene glycol, and glycerol.

It is recommended62 that water be used as one of the two polar liquid in solving the two

equations. However, van Oss’s model (Eq. 2.14) is known to overestimate the basic

− + component (γ S ) on the cost of the acid component (γ S ) of the surface energy due to

the lack of appropriate probing liquids62, which will cause an elevated basic component

as compared with acidic component, which will addressed later in the text.

2.5 Interaction of water with silane films

Water absorption is the concern for corrosion and adhesion applications especially

when high energy surfaces such as metal or metal oxides are involved. Water is the

common media of the corrosion electrolytes and oxygen. Seemingly, a hydrophobic

coating on a metal surface would be sufficient to prevent corrosion as the water

solubility in the polymer is small. However, in many cases, a simple coating is

insufficient and moisture accumulation at the interface can lead to blistering, adhesion

loss and corrosion. It has been shown that water concentration near the interface can be

significantly higher than in bulk polymer film near a high energy surface.33,34 On the

other hand, Doshi et al.63 observed a reduced water density at a hydrophobic substrate

surface with neutron reflectivity. Thus the equilibrium water absorption of polymer

film is a function of many parameters, such as processing conditions (curing

temperature, time) and the nature of substrates and surfaces.

Vogt etc.64 investigated the moisture absorption in poly(4-tert-butoxycarbonyl- oxystyrene) and polyelectrolyte films on an Al2O3 surface by neutron and x-ray

reflectivity. A water-rich layer was detected at the film-substrate interface. The water

absorption was found to be independent of the film thickness. It was found that the

swelling of the film on Al2O3 is less than the swelling of a film of the same thickness on SiOx due to the lower moisture accumulation at the Al2O3/polymer interface.

The water absorption behavior of silane films is important for both adhesion promotion

and corrosion inhibition. However, water absorption and barrier properties of silane are

still not well understood. Traditional methods for determining the rate of water

diffusion in polymers generally employ gravimetric means. In contrast of the

gravimetric methods, FTIR (reflection-absorption infrared (RAIR) and attenuated total

reflectance infrared spectroscopy (ATR) enable in situ monitoring of the chemical

structure during water penetration. McKnight etc.65 examined the water diffusion in the

polypropylene-vinylbenzyl-(trimethoxysilyl)-propylethanediamine silane film with

FTIR-ATR and observed changes in the spectra during the water diffusion. The

changes were indicative of hydrolysis of the siloxane backbone in a silane layer at the

silane-metal interface. The results provide evidence of a degradation mechanism in

silane adhesion.

Compared to FTIR and gravimetric methods, neutron reflectivity (NR) and X-ray

reflectivity (XR) are non-destructive methods that enable us to examine water

distribution within a thin film, to monitor thickness change and to determine the water

absorption quantitatively, simultaneously.

Kent etc.33 examined equilibrium water absorption in silane and epoxy films by NR and found that the distribution of water within the silane layer is not uniform. A thin, water- rich layer is present adjacent to the silicon oxide surface, while a much lower level of water is present in the bulk silane film. The silane used in the study was not a water barrier. More water is present within the silane layer relative to the bulk epoxy film during both elevated temperature and room temperature conditioning. Without the silane film, no excess water was detected in the interface region after humidity conditioning of epoxy samples. The uptake of water at the interface region is nearly reversible upon re-dry. With the presence of silane layer, however, water redistribution and accumulation at the interface were detected after re-dry in vacuum.

The equilibrium water intake of bis-amino silane, bis-sulfur silane and their mixtures was investigated by Pan et al.13,14,18,35,66 Bis-silane films were exposed in the saturated

D2O vapor for several hours until equilibrium was reached. Neutron reflectivity was

performed in a closed sample holder with the presence of D2O-saturated-vapor. The equilibrium water absorption in bis-amino silane and bis-sulfur silane and mixed films as well as the effects of bringing groups, substrates and curing temperature on the properties of silane films were investigated. Pan et al.14,18 found that the bis-amino

silane and bis-sulfur silane itself is not an effective water barrier. 5-40 vol% water is

absorbed during the water conditioning depending on the silane species.

The bridging group in bis-silanes was found to be the key factor that controls water

resistance of silane films as illustrated in Figure 2.10.14 Water absorption in the silane

depends largely on the silane species and the hydrothermal conditioning temperature.

BTSE is a perfect water barrier1, and bis-sulfur swells less in water than bis-amino

silane because of the hydrophobic nature of the bridging group. The reflectivity of both

bis-amino silane and bis-sulfur silane film is reversible after room-temperature water

conditioning but not after 80°C conditioning, indicating chemical alteration of the film

at 80°C. The water resistance of mixed silanes is roughly that of both components

weighted by their volume fraction. Based on the enhanced shrinkage that occurs

following water-vapor conditioning of the mixed film, condensation is accelerated in

the mixed silane. The bis-amino silane may act as a catalyst in the hydrolysis of silane.

Processing variables during the preparation of silane films such as curing temperature

also play an important role regarding to the water barrier properties.13 For bis-sulfur

silane, substantially less water is absorbed when cured at 180◦C. The further

condensation of residual silanol groups and breakdown of polysulfide linkages to bis-

sulfur and mono-sulfur linkages may responsible for the lower water absorption. By

contrast, the scattering length density (SLD) of bis-amino silane films cured at 180◦C is

close to that of films cured at 80◦C. There is also no significant decrease in water volume fraction absorbed, as observed in the case of bis-sulfur silane. These results

confirm that bis-amino silane is fully hydrolyzed and condensed at the curing

temperature of 80◦C. Further increase in temperature does not affect the bulk structure

of the film.

Figure 2.10. The water absorption in bis-aminosilane and bis-sulfur silane films: the effects of bridging groups.13 Bis-amino silane absorbs more water than bis-sulfur silane. A water-rich layer is found on the surface of bis-amino silane film but not on bis-sulfur silane film.

2.6 Sorption and diffusion of water in silane films

In addition to the equilibrium water absorption, it is also important to understand the

kinetics of the moisture absorption into silane films. Moisture absorption in polymer

films can be complex. Many models have been formulated to describe the process. The

so-called dual-mode sorption model31,67 postulates that penetrants absorbed in a

polymer exist in two populations: one is dissolved in polymer matrix according to

Henry’s law, while the other occupies unrelaxed free volume within the polymer in the

so-called Langmuir mode.31

The quantitative description dual-mode sorption model in a glassy polymer was

achieved by the Vieth and Michaels etc.68,69 Glassy polymers have an abnormally high solubility for inert gases. The isotherms for these systems are non-linear and can be decomposed into a Henry’s law component and a Langmuir component. Vieth67 postulated that only gas molecules held by the Langmuir sites are immobilized, allowing mathematical formulation of diffusion process as well. Gupta et al.30 suggested that the Langmurian mode involves the entry of water molecules into preexisting gaps, while in the Henry’s law’s mode, the water molecule occupies gaps created by segmental motion. It has been pointed out by Michaels et al.68 that since water molecules in the Langmurian mode absorb in voids already present in the sample, little energy is needed. More energy is needed for a molecule to enter the Henry’s mode since separation of polymer chains is required for sorption by this mechanism. So below Tg the Langmurian absorption dominates, whereas above Tg the Henry’s mode

becomes more important.

2.7 X-ray and neutron reflectivity

Reflectivity, R(qz), defined as the intensity ratio between reflected and incident beam, is

measured as a function of the normal component of the scattering vector, qz =

(4π/λ)sinθ, where θ is the angle-of-incidence on the wafer and λ is the wavelength. x- ray and neutron reflectivity have emerged as powerful tools for the investigation of the surface behavior of polymers. While neither technique is new, their employment for study of the polymers has only recently been realized in last 20 years. Both techniques provide excellent spatial resolution down to 1.0 nm with penetration depths of hundreds of nanometers.

Medium 0 θ θ σ2 θ1 Medium 1 t σ1 Medium 2 θ2

Figure 2.11. The schematic illustration of reflection and refraction from parallel interfaces

For a uniform film as shown in Figure 2.11, the reflectivity curve R(qz) oscillates as a

71 function of qz. The so-called Kiessig fringes are caused by the interference of waves

reflected from both interfaces of the film. When qz is significantly far from the critical

edge, the layer thickness, d, can be estimated from the ∆qz spacing of the minima of

two neighboring fringes, by d = 2π / Δqz . Diffuse interfaces or low contrast between film and substrate can dampen the fringes as shown in Figure 2.12. This effect is

2 2 approximated by a factor exp(−qzσ ) , where σ is the width of the diffuse interface. If

the interface contains some degree of roughness (or waviness), the reflectivity will also

be diminished. If the size-scale of the roughness is less than the coherence length (the

distance between two points on the sample from which scattered rays will interfere

coherently at the detector), the effect on the reflectivity would then be the same as that

produced by the diffuse interface. For large-scale roughness, the overall effect is then

similar to that produced by a divergent incident beam on a planar surface. Therefore,

samples need to be flat and smooth in order to get rich information from neutron

reflectivity.

Figure 2.12. Reflectivity calculated for a deuterated polystyrene film of thickness 500 Å deposited on a Si substrate, to illustrate the effect of diffuse interface. Thin solid curve, σ1 = σ2 = 0; thin broken curve, σ1 = 20Å, σ2 = 0; thick broken curve, σ1 = 0, σ2 = 20Å.70

The scattering length density (SLD) of a material is a function of density and atomic

composition, expressed as70

ρ × N A 2 SLD = ∑bα × n = ∑bα × , ( .16) α α ∑ M α α where n is the number density of the molecular species and ∑bα is sum of the α scattering lengths of all the atoms in the molecule. The number density is obtained from the mass density, ρ, divided by the sum of atomic masses, Mα, times Avogadro’s

number, NA.

For x-ray, the SLD is proportional to the atomic number. In general, the range in mass density for polymers is small and the contrast (SLD difference between different materials), strictly from mass density variations is not large. With neutrons on the other hand, the scattering length, b, does not vary in a systematic manner with atomic number as in the case of x-rays. The neutron scattering lengths have to be obtained experimentally. One of the largest differences in scattering length is that between the proton and the deuteron. The scattering length for hydrogen and deuterium are -0.374×10-6 Å-2 and 0.667×10-6 Å-2, respectively. This large difference in the scattering lengths provides a unique means of labeling polymer molecules with minimal perturbation to the thermodynamics.71 Thus x-rays are more sensitive to density changes and neutrons are sensitive to both the chemical information and density.

Detailed interfacial profiles and species distributions within layers are gained from the

SLD profile obtained by fitting the reflectivity data with a model. Because of the usual

"phase problem" endemic to diffraction experiments, the measured R(qz) cannot be

uniquely inverted to give the SLD distribution. Inversion of R(qz) involves simulating

the reflectivity from a candidate model structure and optimizing the model parameters

by means of nonlinear regression to obtain agreement between the simulated and measured reflectivity. The recursive Parratt formalism72 is used here. The method is

exact in that the reflectivity is calculated exactly based on the real-space model. More

than one real-space model, however, may fit the data. In the Parratt procedure, one

postulates a layered structure with the thickness, SLD and interface width of each layer

18 as variables. The Parratt recursion scheme for stratified media calculates R(qz).

Chapter 3. Experimental

3.1 Sample Preparation

3.1.1 Materials

H CO OCH3 3 H N Si H3CO Si OCH3 OCH H3CO 3

(a) Bis-[trimethoxysilylpropyl]amine (bis-amino silane)

C CH3 O O

H2C C Si O C CH H 3 O C CH3 O

(b) Vinyl triacetoxysilane (VTAS)

C H O 2 5 OC2H5 C H O Si S S 2 5 Si OC2H5 C H O S S 2 5 OC2H5

Figure 3.1. Chemical structure of bis-amino silane, VTAS and bis-sulfur silane. (a) Bis- [trimethoxysilylpropyl]amine (bis-amino silane), and (b) Vinyl-triacetoxysilane (VTAS) (c) Bis-[3-(triethoxysilyl)propyl]tetrasulfide (bis-sulfur silane)

Bis-[trimethoxysilylpropyl]amine (H3CO)3Si-(CH2)3-NH-(CH2)3-Si(OCH3)3 and Bis-

[3-(triethoxysiyl)propyl]tetrasulfide were provided by GE Silicones (Friendly, WV).

Vinyl triacetoxysilane, (H2C)=(CH)-Si-(OCOCH3)3, was purchased from Gelest Inc.

(Morrisville, PA). The molecular structures of these silanes are shown in Figure 3.1.

The silanes were used without further purification. One-side-polished 2 and 4-inch (111)

Si wafers were purchased from Wafer World Inc. (West Palm Beach, FL. USA). The

sulfuric acid and hydrogen peroxide used in substrate cleaning were mixed at the volume ratio of 7:3. The D2O (99.9 atom%), glacial acetic acid, sodium hydroxide

(99.5 atom%) and hydrochloride acid (99.9 atom%) were obtained from Aldrich (St.

Louis, MO. USA) and used as received. The water-borne epoxy resin EPI-REZ 3540-

WY-55 was obtained from Hexion Specialty Chemicals, Houston, TX.

CRS 1018 panels purchased from Stillwater Steel and Welding Supplies, LLC,

(Stillwater, OK) were used as substrate for cleaning study.

3.1.2 Procedures

3.1.2.1 Silane solution preparation

A) Mixed bis-amino silane and VTAS solution

Neat bis-amino silane and VTAS were mixed at mol ratios (A/V) of 2, 3.4 and 5. After

4 hours from mixing of neat AV silane, 5-wt% water solutions of the AV mixtures were prepared by adding the neat AV into DI water while stirring. To retard condensation and to facilitate hydrolysis, an additional 0.12 wt% (of the total concentration) of glacial acetic acid was added to the water prior to the addition of neat silane mixtures.

The diluted AV solutions were aged at ambient temperature for another 4 hours prior to

spin coating.

B) Bis-amino silane and bis-sulfur silane films

A 5 wt% bis-amino silane solution was prepared by adding silane into a mixture of

deionized (DI) water and methanol. The volume ratio of bis-amino silane/DI

water/ethanol was 5/5/90 (wt/wt/wt). Acetic acid was added to lower the pH to 4 in

order to facilitate the hydrolysis of silane and retard condensation. The solution was

aged continuously in the ambient environment for 9 hours to ensure maximum silanol

concentration. The silanol concentration decreases at both shorter and longer hydrolysis time due to the incomplete hydrolysis and and excessive condensation, respectively.

A 5 wt% bis-sulfur silane solution was made in a similar way except that the organic solvent was ethanol rather than methanol. Acetic acid was added to lower the pH to 6.

The solution was then aged in the ambient atmosphere for 17 hours before spin coating on a Si wafer.

3.1.2.2 Substrate cleaning and coating deposition

A) Si substrate for x-ray and neutron reflectivity

The wafers were cleaned by immersion in sulfuric acid/hydrogen peroxide

(concentrated H2SO4/H2O2 = 7:3 v/v) at room temperature for at least 30 min. After immersion, the substrates were rinsed repeatedly with deionized (DI) water and blow- dried.

The films were deposited using a Larrel single-wafer spin processor (North Wales, PA,

USA). The silane solution was pipetted onto the wafer followed by one-minute

stabilization to allow wetting and reaction with the substrate. The wafer was then

accelerated to 2000 rpm or 3000 rpm and held for 30 s to spin off the excess solution

and dry the film. To remove all traces of solvent and cure the film, the samples were

then dried in an oven at 100 °C or 180 °C for 1 hour. The samples were kept in a

desiccator until further measurements.

B) The CRS substrate for cleaning study

The cleaning solutions of different pH were prepared by adjusting the amount of HCl

and NaOH in the solution. The pH-values investigated were: 1.0, 3.6, 6.8, 8.5, 9.5, 10.5

and 12.4.

The CRS substrate panels were first mechanically cleaned with Scotch Brite™, rinsed

with deionized (DI) water and blow-dried. The panels were then cleaned ultrasonically

in acetone for 10 minutes to further degrease the panels. Thereafter, the CRS substrates

were immersed in the cleaning solutions at the different pH-values mentioned above at

60°C for 3 minutes. After cleaning, the panels were immediately rinsed with DI water, blow-dried and stored in a desiccator until further characterization. The blank control panel was also degreased as described above without using cleaning in cleaning solution.

For corrosion studies, some of the as-cleaned panels were dip-coated with silane and

some were coated with a silane-containing primer. A 10 wt% aqueous solution of bis-

amino silane and VTAS in a weight ratio of 5:1 was used for the silane coatings of the

CRS panels. To facilitate the hydrolysis process, acetic acid was added to the DI water to adjust the pH to 5.5 prior to the addition of bis-amino silane/VTAS mixture. This AV mixture solution was aged for 4 hours at ambient temperature before it was applied to the as-cleaned CRS substrate. The AV silane coatings were applied by dipping the

panels in the solution at room temperature for 30 seconds. The silane-coated panels

were cured at 100°C for 1 hour in air.

The primer formulation contained 80-wt% epoxy resin, 10 wt% bis-sulfur silane, 9 wt% of AV silane and 1 wt% TEOS crosslinker. The primer formulation was applied by a draw-down bar. The primer-coated panels were cured at 100°C for 1 hour in air.

The panels coated with the AV mixture solution are hereafter referred to as silane-

coated panels. The panels coated with the silane-containing primer are referred to as primer-coated panels. The thickness of the primer coating was found to be around 8 μm

as measured by a DCF-2000 coating thickness gage (Electromatic Equipment Co., Inc.,

Cedarhurst, NY).

3.2 Characterization of silane films

3.2.1 X-ray reflectivity

X-ray reflectivity (XR) measurement was performed at 1-BM beamline at the

Advanced Photon Source (APS), Argonne National Laboratory, Argonne, IL, USA, using a standard 4-circle Huber diffractometer. An Oxford Cyberstar scintillation detector with YAP (YAlO3:Ce)-head was mounted on the arm of the diffractometer

with two sets of collection of slits separated by an evacuated beam path. The distances

from the sample to slits were 350 mm (guard slits) and 825 mm (detector slits). The

beam was focused both horizontally and vertically onto the focal plane behind the

scintillation detector using the beamline optics. The slits in front of the detector were

set to about 0.8 mm in width and 0.2 mm in height. The width of the beam at the

sample position was about 1.5 mm.

The wavelength of x-ray was 1.016 Å. Another two sets of incident beam slits were

placed at the entrance to the hutch (about 2040 mm before the sample) and right before

the sample (270 mm). These slits were separated by a vacuum path. The instrument was

operated using SPEC (Certified Scientific Software) and data were reduced using set of custom Igor macros provided by Jan Ilavsky.

Reflectivity, R(qz), defined as the intensity ratio between reflected and incident beam, is measured as a function of the normal component of the scattering vector, which in specular reflectivity is related to the angle of incidence, α, as qz = (4π/λ)sinα, where λ

is the wavelength. During the measurement, qz was varied by changing the incident

angle at a fixed wavelength of 1.016 Å. In all cases, the exit angle equals the incident

angle (specular reflectivity).

Before the x-ray reflectivity measurement, the as-prepared films were kept in a sealed

sample holder with desiccant ambient temperature. A sealed aluminum can with

Kapton™ windows was used as the sample holder for the in situ water conditioning study. The x-ray scattering of the Kapton widow was negligible.

3.2.2 Neutron reflectivity

3.2.2.1 Test procedures

A) Equilibrium water absorption study

Reflectivity of the as-prepared films was first measured with desiccant in a sealed

sample holder at ambient temperature. For the water-vapor conditioning measurement

in neutron reflectivity, the sample was exposed in D2O vapor in a closed container for

12 hours at ambient temperature to ensure equilibrium between film and water vapor.

During water conditioning, the desiccant was removed from the sample holder as

shown in Figure 3.2 and D2O was added for the neutron study. The samples were then transferred to a sealed vapor-saturated sample holder at ambient temperature. After finishing the measurement in the “wet” condition, the samples were again desiccated at ambient temperature for 10 hours before the measurement in the re-dried state. Finally the reflectivity of the “re-dried” film was again measured with desiccant in a sealed sample holder at ambient temperature.

Incident neutron beam Reflected neutron beam

θ θ

Open D2O reservoir Silane thin film

Si substrate

Figure 3.2, Schematic illustration of the sample holder set up for neutron reflectivity

For the neutron study, a sealed aluminum can was used as the sample holder for the in

situ D2O conditioning study. The neutron beam travels through a thin Al wall and is

incident on the film from the air-side surface. There is almost no flux loss through the

Al wall.

B) In situ water sorption study

During the in situ water sorption study, reflectivity of the as-prepared and re-dried films

was measured with desiccant in a sealed sample holder at ambient temperature as

mentioned above. During the D2O conditioning study, the desiccant was removed and

D2O water reservoir was put into the sealed sample holder as shown in Figure 3.2.

Instead of waiting for the system to reach equilibrium state, the neutron reflectivity was

measured as soon as D2O was transferred into the sample holder. The reflectivity was

repeatedly recorded until the equilibrium state was reached. To facilitate the rapid

measurement and decrease the run time for each cycle, only the reflectivity in the low-q

range was recorded for each run cycle during the D2O conditioning measurement.

3.2.2.2 Data acquisition and analysis

NR was performed using the Surface Profile Analysis Reflectometer (SPEAR) at

LANSCE at Los Alamos National Laboratory. The schematic illustration of beamline set up of SPEAR is shown in Figure 3.3.

SPEAR is a time-of-flight (TOF) neutron reflectometer, which means that qz was varied by collecting intensity for a range of wavelengths at a fixed angle of incidence. The wavelength range of LANSCE is 1.4 to 16 Å. In a typical spallation neutron source, protons are accelerated onto a target. Neutrons with a broad energy distribution are generated, which pass through moderators and filters before reaching the sample. The wavelength of neutron is determined by the speed, which is determined by time of flight. The reflectivity is obtained by measuring the reflectivity for each wavelength of the available spectrum with fixed incident angle, with each wavelength corresponding to different scattering wave-vector magnitude qz. To get a large qz range, the reflectivity curves were usually obtained by merging data from two angles of incidence. The merge enables us to measure scattering length density (SLD) profiles of silane film up to 3000

Å.

During the data fitting, we accept the simplest layered model that fits the data. All the data are consistent with a four-layer model consisting of substrate, native oxide, silane film and air. The fitting allows for interlayer roughness at all interfaces.

Figure 3.3. Schematic illustration of SPEAR beamline setup

3.2.3 Grazing Incidence Small-Angle X-ray Scattering (GISAXS)

Grazing-incidence small-angle x-ray scattering (GISAXS) is a versatile tool for

characterizing nanoscale density correlations and/or the shape of nanoscopic objects at

surfaces, at buried interfaces, or in thin films. GISAXS combines features from small-

angle x-ray scattering and diffuse x-ray reflectivity. The technique was originally

introduced in 1980’s,73 but has only recently comes to flourish for the study of

nanoscopic systems.

Figure 3.4 shows the schematic illustration of the geometry of GISAXS. X-ray comes

at an incident angle α. A 2-D detector records the scattering intensity of over a range of

exit angles, β, and scattering angles, ψ, in the surface plane. A beam stop blocks direct

beam spill-over. The reflected beam and the diffuse scattering in the scattering plane

were both recorded during the GISAXS measurement.74 Information of both lateral

structure parallel to the sample surface and normal density profile can be obtained from

GISAXS.

Figure 3.4. Illustration of the geometry and set-up of the GISAXS measurement74

The GISAXS was performed at the Advanced Phonon Source at Argonne National

Laboratory 12-ID beamline. The GISAXS samples were prepared following the same procedures as the XR samples described above on one-side polished Si wafers. During the experiment, the sample was mounted on a motor-controlled tilting stage. The incident beam path was evacuated to minimize air scattering. The incident wavelength,

λ, was fixed at 1.033 Å. The scattering was monitored with a CCD detector located

2013 mm from the sample. The pixel size of the CCD detector was 0.07884 mm. The

2D GISAXS images of AV2, AV3.4 and AV5 were collected in dry state at room

temperature. For each sample x-ray reflectivity was performed prior to the GISAXS to

determine the critical angles of the film and Si substrate.

To facilitate quantitative comparison, GISAXS horizontal line-cuts below the critical

angle of the film (θ < θc,film) and between the critical angles of the film and the Si substrate (θc, film < θ < θc, substrate) were compared. The blank images (GISAXS without any sample) were subtracted from the GISAXS data image before the cut. To ensure the maximum scattering intensity, all horizontal line-cuts were made at the critical angle of the film (θc, film). The 1D line-cut is plotted as the scattered intensity, I, versus the modulus, qxy, of in-plane component of scattering vector q (q is the scattering vector

from the scattering point on the substrate surface to the measuring point on the detector

plane). All line-cuts of different A/V ratios were normalized by the exposure time and

x-ray path length in the film.

3.2.4 Nuclear Magnetic Resonance Spectroscopy (NMR)

NMR spectroscopy is one of the principal techniques used to obtain physical, chemical,

electronic and structural information about a molecule. NMR is a selective technique,

distinguishing among many atoms within a molecule or collection of molecules of the

same type but which differ only in terms of their local chemical environment.

The neat AV silane mixture in the NMR examination was prepared by mixing the bis-

amino silane and VTAS at 3.4: 1 mol ratio since previous work observed optimum anti-

corrosion performance at this ratio.9 The mixture was stirred continuously for 5 hours

before the NMR measurement. During this time, the color of the mixture changed from

clear to a deep amber.

In order to monitor the hydrolysis and condensation, a 10 wt% solution was prepared

by adding 1 g neat AV3.4 mixture to 9 g DI water. Prior to mixing, 0.5 g glacier acetic

acid was added in the DI to retard condensation. The solutions were stirred

continuously for 5 hours before the NMR measurement.

The neat silanes, the AV mixture and a water solution of the mixture were analyzed by

13C, and 29Si NMR using acetone d-6 as a deuterated lock solvent. Liquid-state 13C

NMR spectra were collected using a Varian Inova 400-MHz spectrometer with a

Varian broadband probe and a single pulse sequence using a 10.2-µs pulse width with a

3-second pulse delay and 256 scans. Liquid-state 29Si NMR spectra were collected using a Varian Inova 600 MHz spectrometer with a Varian broadband probe and a single pulse sequence using an 8-µs pulse width with a 3-second pulse delay and 3000 scans. Cr(acac)3 was added as a relaxation agent. All chemical shifts are referenced to internal tetramethylsilane (TMS). The Chemdraw software was used in this study to calculate 13C and 29Si peak positions.

3.2.5 Fourier-Transform Infrared Reflection-Absorption (FTIR-RA)

Spectroscopy

IR spectroscopy is mostly performed in the mid-IR region. When qualitative analysis is

performed, two important sub-regions are considered, the group frequency region

(1200-3600 cm-1) and the fingerprint region (600-1200 cm-1). In the group frequency

region, it is possible to determine what functional groups are present in the molecule.

On the other hand, the finger print region is used for the determination of the whole

structure of the molecule, since the overall structure gives rise to very specific spectral

features in this region.75 The IR spectra were recorded following the procedures

described below.

3.2.5.1 Cleaning study

CRS samples were polished and cleaned as described above. Then a 0.3 wt% AV silane

solution was prepared and hydrolyzed for 4 hours. The as-cleaned panels were dipped

into the silane solution for 30 seconds and then dried in air. After drying the silane-

coated CRS was cured at 100°C for 60 min. FTIR-RA measurements were conducted

on a Perkin-Elmer Spectrum One spectrometer in mid-IR range (500 - 4000 cm-1). All

IR spectra were obtained with an incidence angle of 45° to the surface with a spectral resolution of 4 cm–1. In each measurement 64 scans were collected.

3.2.5.2 Water penetration kinetic study

A 5 wt% bis-amino silane solution was prepared by adding silane into a mixture of

deionized (DI) water and methanol. The volume ratio of bis-amino silane/DI

water/ethanol was 5/5/90 (wt/wt/wt). Acetic acid was added to lower the pH to 4 in

order to facilitate the hydrolysis of silane and retard condensation. The solution was

aged in the ambient environment for 9 hours to ensure maximum silanol concentration.

The silanol concentration decreases at both shorter and longer hydrolysis time due to

the incomplete hydrolysis and excessive condensation, respectively.

A 5 wt% bis-sulfur silane solution was made in a similar way except that the organic

solvent was ethanol rather than methanol. Acetic acid was added to lower the pH to 6.

The solution was then aged in the ambient atmosphere for 17 hours before spin coating

on a Si wafer.

The films were deposited using a Larrel single-wafer spin processor (North Wales, PA,

USA). The silane solution was pipetted onto the wafer followed by one-minute

stabilization to allow wetting and reaction with the substrate. The wafer was then

accelerated to 2000 rpm and held for 30 s to spin off the excess solution and dry the

film. To remove all traces of solvent and cure the film, the samples were then dried in

an oven at 180 °C for 1 hour. The samples were kept in a desiccator until further measurements.

FTIR-RA measurements were conducted on a Perkin-Elmer Spectrum One spectrometer in mid-IR range (800 - 4000 cm-1). All IR spectra were obtained with an incidence angle of 45° normal to the surface with a spectral resolution of 4 cm–1. For the dried and re-dried samples 64 scans were collected for each measurement. For the

in situ sorption study, 16 scans were collected to ensure the rapid measurement. In this case, each spectral collection takes approximately 1.5 minutes.

The background spectra on bare Si wafer were collected before each run. The

background spectra were subtracted from each scan to obtain the IR spectra. The

background spectrum was checked several times during the experiment to make sure no

change in background spectra occurred during the measurement. No change in

background was observed, which confirms no condensation on optics during the

experiment. The as-prepared dry-state of sample was measured with the desiccant

present in the sealed sample chamber. During the D2O conditioning study, the desiccant was removed and D2O water reservoir was put into the sealed sample chamber. Instead

of waiting for the system to reach equilibrium, the FTIR-RA was measured as soon as

D2O was transferred into the sample holder. The FTIR-RA spectrum was repeatedly

recorded until the equilibrium state was confirmed by no further change with time.

3.2.6 Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

The ICP-MS characterization was carried out on an instrument equipped with an

Agilent 7500CE detector (Agilent Technologies, Tokyo, Japan) with a 1500 W RF

power and an argon carrier gas flow rate of 1.23 L/min. During the experiment, 1 ml of

the cleaning solution was collected both before and after cleaning and diluted 105 times

by volume. The background concentrations of 56Fe in the solutions before cleaning

were deducted from the results during 56Fe concentration calculation.

3.2.7 Gravimetric measurement

In order to evaluate the effect of cleaning solution pH on the silane adsorption, the as-

cleaned CRS panels were dipped into the 10 wt% aqueous solution of the AV mixture

as described above and blow-dried in air. During the test, the weight and area of both as-cleaned CRS panels and silane-coated CRS panels were measured. Three replicate samples were analyzed for each pH. The silane layer thickness was calculated from the weight gain between silane-coated panels and as-cleaned only panels by equation (3.1) assuming a coating density of 1.06 g/cm3, which is the density of bis-amino silane

(1.040 g/cm3) and VTAS (1.167 g/cm3) averaged by the volume fraction in the AV

36 silane mixture:

W −W t = a b , (3.1) 2dA

where t is the calculated thickness of the silane layer, Wa is the weight of the panel after

coating, Wb is the weight of the panel before coating but after cleaning, d is the density of the silane and A is the surface area of panel. The average weight gain after silane absorption was around 0.01g (10 mg). The standard deviations of the measurements were in the range between 0.2 and 0.6 mg, which is in the repeatability range of the balance.

3.2.8 Scanning electron microscope (SEM)

A Philips XL-30 FEG (field emission gun) Environmental Scanning Electron

Microscope (ESEM) was employed to observe the surface morphology of the as-

cleaned CRS surfaces. The instrument has an ultimate resolution of 12-15 Å. The

working distance was 8–10 mm. The chamber pressure was: 120-173 Pa. The

acceleration energy of the electrons was 20 kV.

3.2.9 Surface energy

The CRS substrates were first cleaned in solutions of different pH as described above.

The substrate was rinsed with DI water immediately after cleaning, followed by blow-

dry with compressed Ar gas. The contact angle measurements were performed within

20 minutes after cleaning.

The contact angle measurements were carried out by using a VCA 2000 Video

goniometer system (Advanced Surface Technology Inc., Billerica, MA). During the

contact-angle measurement, three liquids were employed as probing liquids. Two

liquids (water and ethyl glycol) are polar and one (methylene iodide) is apolar. Table

3.1 gives the origin, purity, total surface tension and the acid, base and dispersive

components for each probing liquid. Each contact angle was an average of four

measurements. The geometric mean model76 was used to calculate the dispersive and the polar surface energy of the as-cleaned CRS substrate as mentioned above. The Van

Oss-Good model62 was used to determine the acid-base properties.

Table 3.1 Origin, purity, surface energy of probing liquids24,60,61,77 TOT LW - + AB Liquid Origin and purity γL γL γL γL γL

Water Tedia Company, Inc. >99.9 % 72.8 21.8 25.5 25.5 51.0 Ethyl glycol Fisher Scientific, >99.0 % 48.0 29.0 47.0 1.92 19.0 Methylene iodide Sigma-Aldrich, >99.0 % 50.8 50.8 − − −

3.2.10 Electrochemical testing

3.2.10.1 Electrochemical impedance spectroscopy (EIS)

Electrochemical impedance is usually measured by applying an AC potential to an

electrochemical cell and measuring the current. The response to this potential is an AC current signal. This current can be analyzed as a sum of sinusoidal functions (a Fourier series). Electrochemical impedance is normally measured using a small excitation signal so that the cell's response is pseudo-linear. Linearity is described in more detail in the following section. In a linear (or pseudolinear) system, the current response to a

sinusoidal potential will be a sinusoid at the same frequency but shifted in phase.

The popular presentation method of EIS result is the Bode Plot. The impedance is plotted with log frequency on the X-axis and both the absolute values of the impedance

(|Z| = Z0) and the phase-angle on the Y-axis.

EIS measurements were employed to evaluate the anti-corrosion performance of the

primer-coated CRS panels. During the EIS experiments, the panels were immersed in a

0.6 M NaCl electrolyte solution of pH∼6.5. The EIS measurements were carried out

using an SR 810 frequency response analyzer connected to a Gamry PC-3 potentiostat.

The measured frequency range was varied from 10-2 to 105 Hz, with an amplitude of 10

mV. A commercial saturated calomel electrode (SCE) was used as the reference

electrode and coupled with a graphite counter electrode. The surface area exposed to

the electrolyte was 5.16 cm2. The panels were immersed in the electrolyte for 5 hours in order to reach a steady state before the first measurement.

3.2.10.2 DC Potentiodynamic measurement

Anodic and cathodic DC potentiodynamic tests were carried out on both as-cleaned and

silane-coated CRS panels after immersion in aerated 0.6 M NaCl solution at pH∼6.5 for

10 minutes with a CMS 100 Corrosion Measurement System (Gamry Instrument, Inc.,

Warminster, PA.). A commercial saturated calomel electrode (SCE) and a platinum

mesh were used as the reference and counter electrodes, respectively. The exposed area

was 0.78 cm2. On the average, 5 replicate samples were used for each condition. The

data were recorded from Ecorr -0.50 V/SCE to Ecorr (where, Ecorr is the corrosion

potential of the sample) in the cathodic polarization tests, and from Ecorr to Ecorr +0.50

V/SCE in the anodic polarization tests. The scan rate was 1 mV/s.

3.2.11 Salt water immersion test

A salt water immersion test (ASTM D714-56) was employed to evaluate the corrosion

resistance of the primer-coated CRS panels. During salt water immersion, three forths

of a primer-coated CRS panel was immersed in aerated 0.6 M NaCl solution at 25°C

for one week.

Chapter 4. NMR Study of Reaction Mechanism of

Bis-[trimethoxysilypropyl]amine and Vinyl- triacetoxysilane mixture

4.1 Introduction

Previous studies 9 have also show that the A/V ratio in the AV mixture is critical for stability and corrosion performance of AV coatings. Zhu and Van Ooij et al.9

investigated stability of the AV silane mixture at different A/V ratios and found that the

solution is stable only with more than 50 vol% bis-amino silane present in the mixture.

Without excess bis-amino silane, VTAS will self-condense and gel. The best anti-

corrosion performance is achieved at an A/V weight ratio of 5, which is 3.4A: 1.0V mol

ratio. This formula is designated as AV3.4 in the following discussion.

Despite the good anti-corrosion performance and solution stability, the underlying

chemistry of AV reaction and stabilization is still unknown. NMR is a versatile tool for

investigation of the reaction chemistry. Liquid-state NMR, for example, has been used

extensively to monitor silane hydrolysis and condensation.78-81 In the present study, 13C and 29Si NMR were used to explore the reaction between bis-amino silane and VTAS.

13C NMR was used to determine the reaction between the amino and acetoxy

functionalities. 29Si NMR was employed to monitor the hydrolysis and condensation

reactions in water solution.

4.2 Results and discussion

4.2.1 NMR analysis of neat VTAS and bis-amino silane

4.2.1.1 Neat bis-amino silane

Figure 4.1 (a) (b) shows the 13C NMR spectrum of bis-amino silane. The corresponding amino silane structure and NMR 13C peak assignments are shown in Figure 4.2.The 13C

NMR of the neat bis-amino silane (Figure 4.1) exhibits peaks at approximately 52.8,

50.1 (triplet), 23.7 (triplet, not shown here), 10.1 (not shown here), 8.8 (triplet) and 7.2 ppm. These peaks are in good agreement with the calculated peak positions shown in

Figure 4.2. The appearance of MeOH peak at 49.2 ppm indicates some hydrolysis occurs in neat bis-amino silane due to presence of trace amount of water absorbed from the air. The calculated peak position for the C adjacent to the Si atom shifts from 7.2 ppm to 9.7, 12.2 and 14.7 ppm corresponding to the non-hydrolyzed bis-amino silane and silane hydrolysis products after 1, 2 and 3 hydrolysis steps as shown in Figure 4.2.

The actual resonances observed are centered at 7.3, 8.7 and 10.2 ppm, due to the neat, non-hydrolyzed silane and the lower and intermediate hydrolysis peaks. A peak due to higher hydrolysis product is not observed. The hydrolysis of the bis-amino silane is also responsible for the occurrence of the multiplet appearance of the peaks at approximately 50.2 and 23.7 ppm for C of methoxy groups and the C in the middle of the propyl chain in bis-amino silane. Multiplets have been observed in other silanes due to the different hydrolysis species and differences in local chemical environment after hydrolysis.78-80

7.3 23.7

8.7

10.2

26 24 22 20 18 16 14 12 10 8 6 ppm

(a)

52.8

50.3 50.2

MeOH 50.1

54 53 52 51 50 49 48 ppm

(b)

50.2 H N 50.2 50.2 25.8 O 25.8 O 53.1 53.1 OSi Si O 7.2 7.2 O O 50.2

50.2 50.2

(a)

50.2 H N 50.6 50.2 25.5 O 25.8 O 53.1 53.1 OSi Si OH 7.2 9.7 O O

50.6 50.2

(b)

50.2 H N 50.2 25.2 O 25.8 OH 53.1 53.1 OSi Si OH 7.2 12.2 O O

50.9 50.2

(c)

50.2 H N 50.2 24.9 O 25.8 OH 53.1 53.1 OSi Si OH 7.2 14.7 O OH

50.2

(d)

In summary, the neat bis-amino silane is partially hydrolyzed due the presence of absorbed water from the air. The lower, intermediate hydrolysis product of bis-amino silane are observed from the 13C spectra, the higher hydrolysis product is absent in neat bis-amino silane. The hydrolysis in neat bis-amino silane is due to the presence of trace amount of water absorbed from the air.

4.2.1.2 NMR analysis of neat VTAS

As shown in Figure 4.3, the 13C NMR spectrum of VTAS exhibits peaks at

approximately 168.9, 140.6/139.8, 127/126.4, and 22.3 ppm. Figure 4.4 shows the

calculated peak positions. The peak at 168.9 ppm is due to the acetoxy carbonyl carbon

(C=O), the peaks at 140.6 and 126.2 ppm are due to the two carbon atoms in the vinyl

group (C=C). The peak at 22.3 ppm is assigned to carbon in the acetoxy CH3 group.

Doublets of the vinyl (C=C) (126.2/127.2 and 139.8/140.6 ppm) and acetoxy carbonyl carbon (C=O) (168.9/169.3 ppm) are observed indicating the existence of hydrolysis products at different hydrolysis stages in neat VTAS. VTAS is hydrophobic and barely dissolves in water. It has been reported, however, that VTAS hydrolysis occurs after prolonged contact with water and vigorous stirring.9 The presence of water absorbed

from air during storage may responsible for hydrolysis in neat VTAS.

C=O 168.9

169.3

172 171 170 169 168 167 166 ppm

(a)

CH2=CH2 CH2=CH2 140.6 126.2

139.8 127.2

125 127 129 131 133 135 137 139 141 143 ppm

(b)

22.3

CH3

25 24 23 22 21 20 ppm

(c)

Figure 4.3. 13C NMR spectra of neat VTAS. The occurrence of the doublets around 140.6/139.8 and 127/126.4 ppm of the vinyl groups and 168.9/169.3 ppm of the acetoxy carbonyl carbon in Figure 4.3(a) and (b) is due to the hydrolysis and condensation of the VTAS.

23.4

176 C O

23.4 C O Si C CH2 139.4 176 H O 139.3

176 C O

23.4

Figure 4.4. Chemical structures and 13C peak positions of neat VTAS. The peak position is calculated by Chemdraw software.

Figure 4.5 shows the 29Si NMR spectrum. The lower condensation products T1 are observed in the spectrum, which confirms the occurrence of the condensation in the neat VTAS.

-61.6 T0 VTAS

-63.9 T1 VTAS

-57 -59 -61 -63 -65 ppm

Figure 4.5. 29Si NMR spectrum of the neat VTAS. The lower condensation products T1 of VTAS can be observed in the spectrum.

In summary, neat VTAS is partially hydrolyzed due to the presence of the water

absorbed from the air. The doublet peaks as well as condensation products are observed from the 13C NMR and 29Si NMR spectra, respectively.

4.2.1.3 NMR analysis of neat bis-amino silane and VTAS mixture

As discussed above, AV mixtures have favorable properties that are not possessed by the individual component silanes. Zhu and Van Ooij9 suggested that the color change after mixing in AV system is due to the chemical reaction occurring between bis-amino silane and VTAS. Presumably these reactions play a role in the corrosion performance of the mixed film.

The AV mixture silane was prepared following the procedures described in section

3.2.4. Figure 4.6 compares the 13C liquid NMR spectra in 0-60 ppm region of AV3.4 silane mixture after 5 hours from mixing. Figure 4.7 shows the related structures, calculated peak positions, and assignments for the major reaction products calculated using Chemdraw software.

Some changes are observed after 5 hours reaction time (Figure 4.6a) spectrum as compared to the spectra of the neat silanes. In this time period, the colorless solution became amber indicating the formation of a complex. Two product peaks due to the propyl chain in the amide complex are observed as multiplets in Figure 4.6a in 48-53 ppm and 18-25 ppm range. A new peak due to the acetoxy methyl group of the complex

(Figure 4.6a) is observed at approximately 24 ppm.

MeOH

Acetone –d6

AV mixture

(a) Bis-amino silane

(b) VTAS

60 50 40 30 20 10 0 (c) ppm

Figure 4.6 13C NMR spectra of (a) AV3.4 mixture after 5-hour reaction time, (b) neat bis-amino silane, and (c) neat VTAS. The AV mixture is prepared following the procedures described in section 3.2.4. The NMR spectra were collected after 5 hours of mixing of neat silanes. New peaks occur after mixing in 48-53 ppm and 18-25 ppm range.

Examination of the 13C NMR spectra in the 53.5 to 48.0 ppm range shown in Figure 4.8 indicates that the peak at approximately 52.9 ppm due to the propyl group adjacent to the N atom in neat bis-amino silane molecule decreases in intensity during the reaction, while peaks in the 50.0 to 50.6 ppm region due to the same propyl group on the amide complex shown in Figure 4.7(a) increase. These peaks are observed as multiplets due to the different hydrolysis stages. The two other sets of multiplet peaks (51.3 -51.7 and

48.5-48.7 ppm) are possibly due to the same complex but with replacement of the original methoxy group by SiOH and Si-O-Si termination. The 13C doublet peaks at

50.1-50.3 ppm due to the methoxy group on bis-amino silane disappear after reaction

due to hydrolysis of bis-amino silane by water generated from the condensation of

reacted VTAS. The methanol peak at 49.1 ppm remains after mixing.

(a) Amide complex 21.1

168.9 C O

N 22.2 22.2 OH OH 50.0 50.0 HO Si Si OH 7.2 7.2 OH OH

(b) Bis-amino silane H N 50.2 50.2 25.8 25.8 O O 53.1 53.1 50.2 O Si Si O 50.2 7.2 7.2 O O

50.2 50.2

23.4

176 C O

O 176 139.3 139.4

23.4 C O Si C CH2 H O O

176 C O

23.4

Figure 4.7. Chemical structures and peak assignments for (a) amide complex, (b) neat bis-amino silane, and (c) neat VTAS. The peak positions are calculated by Chemdraw software.

52.9 50.1-50.3 50.0-50.6

VTAS

49.1 MeOH

Bis-amino silane 51.3-51.7 48.5-48.7

AV3.4

53.5 53.0 52.5 52.0 51.5 51.0 50.5 50.0 49.5 49.0 48.5 48.0 ppm

Figure 4.8 13C NMR spectra of VTAS (green), bis-amino silane (red) and AV mixture (blue). The AV mixture was prepared following the procedures described in section 3.2.4. The NMR spectra were collected 5 hours after mixing of individual neat silanes. The carbon peak in methoxy group of bis-amino silane at 50.1-50.3 ppm and the propyl peak of neat bis-amino silane adjacent to N atom at 52.9 ppm decrease in intensity in the AV mixture. Three new multiplets peaks at 51.3-51.7 ppm, 50.0-50.6 ppm and 48.5 and 48.7 ppm appear in the mixture due to the formation of amide complex.

Another look at the 13C NMR spectra in 18 to 25 ppm region in Figure 4.9 indicates that the C peak in propyl group of neat bis-amino silane shifts from the 23.5 to 21.5 ppm region and appears as multiplets with some residual intensity in propyl peak of neat bis-amino silane at 24.1 ppm. The appearance of the multiplet peaks in 13C spectrum of the mixture may be due to the different local chemical environment. The

13C NMR peak on VTAS methoxy group at 22.3 ppm also decreases in intensity and

shifts to the lower ppm region at around 21.5 ppm, which agrees with the calculated C

peak position on Figure 4.7 (a).

22.3 (CH3 VTAS)

CH2 Bis-amino 23.5-23.9

VTAS

Residual Bis-amino silane Bis-amino Residual 21.4-21.7 20.0-21.4 CH3 VTAS AV mixture

25 24 23 22 21 20 19 18 ppm

Figure 4.9. 13C NMR spectra of the VTAS, bis-amino silane and AV mixture. The AV mixture is prepared following the procedures described in section 3.2.4. The NMR spectra were collected 5 hours after the mixing of individual neat silanes. The C peak in propyl group of neat bis-amino silane shifts from the 23.5 to 21.5 ppm region and appears as multiplets with some residual intensity of propyl peak of neat bis-amino silane left at 24.1 ppm. The C peak on VTAS methoxy group at 22.3 ppm also decreases in intensity and shifts to the lower ppm region at around 21.5 ppm.

Evidence for the complex reaction can also be observed in the 13C NMR spectra in the

168-172 ppm range as shown in Figure 4.10. The doublet peaks at 169.3 to 170.0 ppm due to the C=O in acetoxy group in neat VTAS almost disappears after reaction due to the exchange of the hydrogen atom on the secondary amine with the acetoxy group on

VTAS.

170.0 C=O VTAS

169.3 VTAS

Bis-amino silane

169.5 AV3.4

172 171.5 171 170.5 170 169.5 169 168.5 168 ppm

Figure 4.10. 13C NMR spectra of VTAS, bis-amino silane and AV mixture. The AV mixture silane is prepared following the procedures described in section 3.2.4. The NMR spectra were collected 5 hours after the mixing of individual neat silanes. The doublet peaks at 169.3 to 170.0 ppm due to the C=O in acetoxy group in neat VTAS almost disappear after reaction due to the exchange of the hydrogen atom on the secondary amine with the acetoxy group on VTAS.

C CH3 O H3CO O H OCH3 3 H3CO Si N + Si OCH3 H2C C Si O C CH H 3 H3CO OCH 3 O C CH3 O O CH3 OH C OCH H3CO 3 3 H CO Si N Si OCH3 + H2C C Si OH 3 H H CO OCH3 3 OH Figure 4.11. Primary reaction in neat bis-aminosilane and VTAS mixture from the NMR results in Figure 4.6, 4.8, 4.9 and 4.10.

These above results indicate the primary reaction is exchange of the acetoxy group on

VTAS with the secondary amine hydrogen, forming an amide complex as shown in

Figure 4.11. The stoichiometric ratio of primary reaction is therefore 3. When bis-

amino silane and VTAS are mixed near the stoichiometric ratio (AV3.4), the reaction in

the mixture is near completion. The primary reaction is followed by a series of

hydrolysis and condensation reactions of both bis-amino silane and the reacted VTAS

due to the presence of the water from the condensation of VTAS. The reaction is non-

reversible with time due to the further condensation of the reaction product e.g. reacted

VTAS and bis-amino silane in the mixture as shown below.

Figure 4.12 shows the 29Si NMR spectrum of neat AV mixture in -75 to -40 ppm range.

The NMR spectrum was collected 5 hours after the mixing. The condensation products of bis-amino silane of different stages (T1, T2 and T3) and the first and second condensation products (T1 and T2) of VTAS are observed in the spectrum. VTAS hydrolysis product is also observed at 55.6 ppm.

The above results show that the primary reaction is followed by the condensation between silanol groups on the reacted VTAS from the primary reaction. The water generated from the VTAS condensation then further hydrolyzes bis-amino silane and produces methanol. The condensation between hydrolyzed bis-amino silane generates additional water. The hydrolysis and condensation products of bis-amino silane and

VTAS are observed in neat AV3.4 mixture as shown in Figure 4.12.

2 1 T bis-amino silane T bis-amino silane 49.8-51.0 41.8-42.8 T3 bis-amino silane 58.0-60.0 VTAS hydrolysis product T1 VTAS 55.6 63.9 T2 VTAS 72.6

-40 -45 -50 -55 -60 -65 -70 -75 ppm

Figure 4.12 29Si NMR spectrum of neat AV mixture in -75 to -40 ppm range. The NMR spectrum was collected 5 hours after the mixing. The condensation products of bis- amino silane of different stages (T1, T2 and T3) and the first and second condensation products (T1 and T2) of VTAS are observed in the spectrum. VTAS hydrolysis product is also observed at 55.6 ppm.

4.2.2 NMR analysis of the AV silane in water solution

Figure 4.13 compares 13C spectra of the neat AV3.4 and 10 wt% AV3.4 water solution.

The water solution was prepared by adding 1g neat AV3.4 mixture to 9 g DI water

which had been acidified with 0.5 g glacial acetic acid. The neat AV mixture was

prepared by following the procedure described in section 3.1.2.1 and the solution was

made 5 hours after the mixing of neat AV silane mixture. The AV solution was

hydrolyzed continuously in water for 5 hours before the NMR measurement. The peaks

at 52.9 ppm (propyl C next to nitrogen atom in neat bis-amino silane), 51.3-51.7 and

48.5-48.7 ppm (propyl C next to nitrogen atom in amide complex in primary reaction

product) disappear. The peaks at 50.0-50.6 ppm (propyl C next to nitrogen atom in

amide complex in primary reaction product) decrease in intensity. The methanol peak

shifts in position due to the change to solution environment (neat oil vs water). The

increase in intensity of methanol peak in water solution indicates further hydrolysis of

bis-amino silane.

50.0-50.6

Neat AV3.4 mixture 48.5-48.7 51.3-51.7 Methanol 52.9

AV3.4 water solution

54 53 52 51 50 49 48 ppm

Figure 4.13 13C spectra of the neat AV3.4 mixture and 10 wt% AV3.4 water solution collected after 5-hour hydrolysis in water. The multiplet peaks at 52.9 ppm, 51.3-51.7 and 48.5-48.7 ppm of amide complex disappear. The peaks at 50.0-50.6 ppm decrease in intensity. The neat AV mixture is prepared following the procedure described in section 3.1.2.1 and the solution was made 5 hours after the mixing of neat AV silane mixture.

29Si NMR was used for analysis and identify of the hydrolysis and condensation reactions and products. The 29Si NMR spectrum of AV silane solution after 10 hours

hydrolysis in water is shown in Figure 4.14. The T0 (unhydrolyzed bis-amino silane),

T2 (siloxane oligomer with two silanol group condensed), and T3 (siloxane oligomer

with three silanol group condensed) peaks of bis-amino silane can be clearly identified.

However, the lower condensation product peak (T1) of bis-amino silane is absent.

T0 Bis-amino silane 40.5

T2 Bis-amino silane 49.6 Bis- amino silane hydrolysis products T3 Bis-amino silane 56.0-60.0 0 VTAS hydrolysis T VTAS products 61.8 39.2-39.5

-75 -70 -65 -60 -55 -50 -45 -40 -35 ppm

Based on the above analysis, the reaction of neat AV silane in water solution can be summarized as: a) decomposition of the amide complex formed in neat AV mixture, b) hydrolysis of the bis-amino silane, VTAS and related reaction products, and c)

condensation of hydrolyzed silane products in water. The reactions are illustrated in

Figure 4.15. a) Decomposition of amide complex in water solution

O CH3 OH H3CO C

HO Si N Si OH + H2O

HO OCH3 OH H3CO H O N HO Si Si OH + H3C C OH HO OCH3

b) Hydrolysis of bis-amino silane

H CO OCH3 3 H N Si H3CO Si OCH3 + 4 H2O

H3CO OCH3

H CO OH 3 H N HO Si Si OH + 4 CH3OH

HO OCH3

c) Condensation of bis-amino silane

H CO OH HO OH 3 H H HO Si N Si OH + HO Si N Si OH

OCH OCH3 HO 3 H3CO

OH H CO O H 3 H Si N Si OH HO Si N Si O OCH HO 3 OCH3 OCH3

Figure 4.15. Possible reactions in the AV3.4 water solution, (a) Decomposition of amide complex in water solution, (b) Hydrolysis of bis-amino silane, (c) Condensation of bis-amino silane.

In order to monitor the hydrolysis and condensation of AV silane in water solution, 29Si

NMR spectra were recorded as the function of hydrolysis time from 80 minutes to 1740 minutes. The spectra are shown in Figure 4.16. At the initial stage, both the T0 peaks of

neat bis-amino silane (40.5 ppm) and VTAS (61.8 ppm) are present. The secondary

condensation product of bis-amino silane (T2) (49.6 ppm) and the hydrolysis products of bis-amino silane (39.5 ppm) are also observed in the spectra at the beginning. The higher condensation product peak of bis-amino silane is absent at the beginning of hydrolysis. However, with increasing hydrolysis time, the neat bis-amino silane peak

(T0) at 40.5 ppm decreases in intensity. The third condensation product of bis-amino

silane (T3) at 56.0-60.0 ppm appears and increases in intensity at the cost of secondary

condensation product of bis-amino silane (T2). In the mean time, the hydrolysis products of bis-amino silane at 39.5 ppm decrease and disappear after 920 minutes. The lower condensation product (T1) of bis-amino silane is absent from all the spectra.

The 29Si NMR spectra at different hydrolysis stages indicate that the hydrolysis of the

bis-amino silane dominates in the initial stage. The AV solution is populated with the

bis-amino silane hydrolysis products and the intermediate condensation product (T2) of bis-amino silane. The higher condensation product (T3) of bis-amino silane is absent.

As time increases, however, the higher condensation product (T3) of bis-amino silane

appears and increases at the cost of secondary condensation product (T2) and the hydrolysis product of bis-amino silane, which implies the second condensation product

(T2) reacts with hydrolysis product of bis-amino silane and forms T3. The intermediate

(T2) and higher condensation products (T3) of bis-amino silane co-exist. Similar results

were observed by Nishiyama et al.79 on the NMR study of γ-methacryloxypropyl-

trimethoxysilane (γ-MPTS). The weak neat VTAS and bis-amino silane (T0) peaks are

also observed in the spectrum, indicating incomplete reaction and hydrolysis.78,79

However, both neat VTAS and bis-amino silane peaks decrease with time and the neat

VTAS peak disappears after 1740 minutes hydrolysis. The absence of the lower condensation product (T1) of bis-amino silane may due to the absence of lower hydrolysis products in the solution, which was also observed by Osterholtz and Pohl etc.19 in their study of the hydrolysis of γ-glycidoxypropyltrimethoxysilane (GPS).

T2 Bis-amino T0 Bis-amino T3 Bis-amino 0 silane silane T VTAS Silane VTAS hydro 49.6 40.5 61.8 products 55.6 1740 minutes Amino silane hydro products

39.5 920 minutes

640 minutes

360 minutes

80 minutes

-75 -70 -65 -60 -55 -50 -45 -40 -35 ppm

Figure 4.16. 29Si NMR spectra of 10 wt% AV silane water solution after different aging times. The intensity of neat bis-amino silane peak (T0) at 40.5 ppm, intermediate condensation products peak (T2) at 49.6 ppm, bis-amino silane hydrolysis products peak at 39.5 ppm and neat VTAS peak (T0) decrease in intensity as hydrolysis proceeds. The intensity of higher condensation product (T3) peak of bis-amino silane at 56.0-60.0 ppm increases with time. The lower condensation products peak (T1) of bis- amino silane and VTAS condensation product peak (T1 and T2) are absent from the spectra.

Surprisingly, with the continuous decrease and final disappearance of the VTAS T0

peak (61.8 ppm), the condensation products of VTAS T1and T2, which should be appear at 64.0 ppm and 72.5 ppm respectively, are not observed in Figure 4.16. Also the VTAS hydrolysis product (-SiOH) is barely visible at 55.6 ppm.

Recall that the VTAS T1 (64.0 ppm) and T2 (72.5 ppm) peak are clearly observed in

29Si spectra of neat AV mixture in Figure 4.11. The absence of VTAS condensation in

the AV solution with continuously decrease in T0 peak of VTAS (61.8 ppm) suggests

that hydrolyzed VTAS is stabilized in water solution preventing further condensation.

The minor peak of VTAS hydrolysis product can be barely observed at 55.6 ppm. The lack of VTAS hydrolysis products in the solution may due to the bonding of the -OH group of VTAS with the –NH group on bis-amino silane as discussed below.

Figure 4.17 shows the normalized the intensity of bis-amino silane T0, T2, T3 and

VTAS T0 peak intensities of 10 wt% AV solution as a function of hydrolysis time (from

Figure 4.16). The intensity of bis-amino silane T0 and T2 peaks decreases with hydrolysis time. On the other hand, the higher condensation product of bis-amino silane

(T3) increases, reaching a maximum after 780 minutes and then decreases and levels off after prolonged hydrolysis. The decrease and level-off are probably due to the elevated acetic acid in solution after the hydrolysis of VTAS. The higher bis-amino silane condensation products may re-hydrolyze in acidic conditions. The VTAS T0 peak

decreases with hydrolysis time indicating the hydrolysis of the VTAS. However no

addition peaks of VTAS condensation are observed.

1.0

0.8

0 0.6 T Bis-amino silane 2 T Bis-amino silane 3 0.4 T Bis-amino silane 0 T VTAS 0.2 Relative intensity (a.u.)

500 1000 1500 2000 2500

Hydrolysis time (minutes)

Generally speaking, silanols are not stable in solution, as condensation occurs

simultaneously with hydrolysis. Silanols are only stable in a very dilute solution.

However, it has been shown that some water-based silanes are stable at relative high

concentration in water solution compared with the solvent-based solutions.20 In the

previous study, a relatively concentrated γ-APS aqueous solution was found to have

good stability. The amine group and silanol group on γ-APS form hydrogen bonds,

hence retarding the condensation and stabilizing the aqueous γ-APS solution.20

Bis-amino silane is not stable by itself in water solution. The condensation of bis-amino silane is catalyzed by the basic secondary amine group. However, the water solution of

AV mixture silane was found to be stable for at least 2 years with no gelation. The fact

100

that no VTAS condensation product is observed in the 29Si NMR spectra in Figure 4.16 indicates that the silanol group of VTAS is stabilized in water solution of AV mixture.

VTAS is hydrophobic. The hydrolysis of VTAS proceeds slowly in water. Bis-amino silane, on the other hand, hydrolyzes and gels rapidly due to the catalytic effect of the basic secondary amine group in water solution. However, when mixed with bis-amino silane, VTAS reacts with bis-amino silane by exchanging the acetoxy group with the hydrogen atom on the bis-amino silane and forms the amide complex as shown in

Figure 4.11. In this way, VTAS is hydrolyzed through the primary reaction in neat AV mixture.

In water solution, the acetoxy group on amide complex decomposes into bis-amino silane and acetic acid. The silanol group (-OH) on the hydrolyzed VTAS forms a hydrogen bond with the secondary amine group (-NH) on bis-amino silane. Since the silanol group is weakly acidic and the secondary amine group is strongly basic, the

NH···OH hydrogen bond is more stable than hydrogen bond between silanols Si-

OH···OH-Si bond.9 The OH group is thus stabilized by secondary amine group

retarding condensation between the silanol groups of bis-amino silane and VTAS. A

schematic illustration of the solution stabilization mechanism is shown in Figure 4.18.

101

OC2H5

Si OH

CH2 OC2H5 H OC2H5 O N HO Si Si H OH O H OH OH Si H N OH OH

Si CH3 HO

Figure 4.18 Schematic illustration of hydrogen bonding between the secondary amine group on bis-amino silane and the silanol group on VTAS.

Zhu et al.9 observed that the solution stability of AV depends on the A/V ratio. When

bis-amino silane is more that 50 mol%, the solution is stable. When bis-amino silane is

less than the 50 mol%, silane solution becomes hazy, indicating the onset of

condensation reaction. The bis-amino silane plays a critical role in stabilizing the AV

solution. This result further confirms the above silane stabilizing mechanism. Similar

ideas on solution stability of γ-aminopropyltrimethoxysilane (γ-APS) were advanced

by Chiang et al.82

When bis-amino silane is more than the 50 mol%, both homo-condensation and hetero- condensation of VTAS are retarded. However, the homo-condensation between bis- amino silane molecules can proceed since most of silanols on bis-amino silane are not

102

bonded. The evidence for homo-condensation is observed in Figure 4.14 and 4.16 as

the T1 and T2 and T3 peaks of bis-amino silane. In the absence of VTAS condensation,

however, the total condensation in AV3.4 water solution is reduced, leading to long-

term solution stability.

As the A/V ratio increases or decreases, the availability of the “unstablized silanols” on

bis-amino silane or VTAS also increases, which will, in turn, increase the possibility of

condensation between bis-amino silane or VTAS molecules, thus impacting the

stability of the solution. Hence, the optimum A/V ratio for solution stability is around 3.

At this ratio, the solution has almost equal populations of -OH on VTAS and -NH

groups on bis-amino silane, which yields the highest stability, less condensation and

higher silanol concentration. This result agrees with the previous observation by Zhu et

al.9 on optimum corrosion resistance, stability at A/V ratio around 3. Previous work36

on water barrier properties of AV mixture silane films also shows less water absorption

and higher density of the AV mixture film at near stoichiometric A/V ratio.

4.3 Conclusions

1. 13C NMR spectra show that the primary reaction in neat AV3.4 silane mixture is the exchange of the hydrogen atom on secondary amine group of bis-amino silane with the

acetoxy group of VTAS forming an amide complex and hydrolyzed VTAS. The

primary reaction is followed by a series of condensation and hydrolysis reaction of bis-

amino silane and VTAS and their reaction by-products.

103

2. In water solution, the amide complex formed in the neat AV mixture decomposes

into bis-amino silane and acetic acid. Bis-amino silane condenses into higher

2 3 condensation products (T and T ). The lowest condensation product of bis-amino silane (T1) is absent.

3. Time-resolved 29Si NMR reveals that the intermediate condensation product of bis- amino silane (T2) dominates due the high hydrolysis rate of bis-amino silane in the initial stage of hydrolysis. As hydrolysis time increases, the hydrolysis rate of neat bis- amino silane decreases, and the higher condensation products (T3) increases at the cost of the intermediate condensation products (T2) as well as the uncondensed hydrolysis product of bis-amino silane. The condensation products of VTAS are absent in the AV solution even when all the VTAS is hydrolyzed at 1740 minutes, which indicates the hydrolyzed VTAS is stabilized in the water solution.

4. Based on the NMR results and previous observations on the stability of the AV

mixture, a model of the AV solution stabilization is proposed. The weakly acidic silanol

group (-OH) on hydrolyzed VTAS forms hydrogen bonds (NH···OH) with the strongly

basic secondary amine group (-NH) on bis-amino silane. The OH group of VTAS is

thus stabilized by secondary amine group and the condensation between the silanol

groups of bis-amino silane and VTAS is retarded. The decrease of the solution pH by

release of the acetic acid through the decomposition of the amide complex in the water

solution also contributes to the stability of the AV solution.

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Chapter 5. Effect of Substrate Cleaning Solution pH on Corrosion Performance of Silane-coated

Cold-rolled Steel1

5.1 Introduction

The adhesion and structure of the silane coating is sensitive to the surface oxide as well as to the surface cleaning procedures. Based on recent studies, a silane bonding mechanism has been established.20-28 When a silane solution is applied on a metal surface, silanol (-SiOH) groups adsorb rapidly through hydrogen bonds. During the subsequent curing and drying, two condensation reactions occur. Silanols from the silane solution condense with the hydroxyl groups (MeOH) on the metal (Me) surface to form the covalent metallo-siloxane bonds (MeOSi); SiOH (solution) + MeOH (metal

substrate) → SiOMe + H2O. The excess of silanol groups condense to form a siloxane network, SiOH(solution) + SiOH (solution) →SiOSi + H2O, as shown in Figure 5.1. The

SiOMe bond contributes to the strong bonding of the silane coating to the metal surface and the SiOSi moiety is strongly hydrophobic. Both groups are responsible for the corrosion resistance of the silane coating.

The amount of hydroxyl on the surface is directly related to the cleaning conditions of the metal substrate. During the coating process, the AV silane can absorb onto CRS via hydrogen bonding. Previous work on γ-APS have proven that the silane is absorbed on

1 Published in J. Adhesion Sci. Technol., Vol. 21, No. 10, (2007) 935–960

105

the iron substrate within a few seconds.26 Zhu et al.15 also observed that the silane layer thickness is controlled by the silane solution concentration independent of the dipping time. Silane is absorbed instantly after dipping. The same observation was also reported by Zhang.83 The surface is barely altered by the solution environment of during the short time. The inherited surface chemical state from the prior cleaning history therefore plays important role in silane absorption and hence in the anti- corrosion performance of silane coatings.

Adsorption bears a close relationship to metal surface chemistry.3,24,29 However, the the relationship between cleaning solution pH and corrosion performance has not been widely addressed. The present work aims at understanding the relationship between the cleaning solution pH and the corrosion performance of silane and primer-coated CRS.

The properties of as-cleaned, AV-silane-coated and AV-silane-containing primer- coated CRS panels cleaned at different pH values were characterized by contact angle,

FTIR spectroscopy, SEM, XPS, DC potentiodynamic measurement and EIS.

106

O O O O OH OX Si OX Si S Si XO OH HO R HO R R R OH HO OH OX Si Si S Si HO HO OH O O O O H H H H H H H H

O OH O O O

Me Me Me Me Me

(a)

Si Si O Si O Si R R R R R

Si R Si O Si O Si

O OH O O O

Me Me Me Me Me

(b)

Figure 5.1. Schematic of the silane deposition and bonding on metal surface (a) before condensation: hydrogen-bonded interface; (b) after condensation: covalently bonded interface

107

5.2 Results and discussion

5.2.1 Etch rate

The etching effect of the cleaning solutions on CRS was examined by weight loss

56 measurements of the CRS panels after cleaning and by measuring the Fe

concentration in the cleaning solution both before and after cleaning using inductively

coupled plasma mass spectrometry (ICP-MS).

Figure 5.2 shows the etch rate of the as-cleaned CRS panels and the 56Fe concentration in the solution after cleaning at different pH. The etch rate and the 56Fe concentrations

in the solution after cleaning follow a similar trend. The etch rate in acidic conditions is

much higher than in alkaline conditions with the maximum etch rate of CRS observed

at pH∼1.0. The minimum etch rate is around zero in neutral cleaning conditions

(pH∼6.8).

The above etching behavior is not unusual. Below pH = 4, no oxide layer can be

formed. Etching is accelerated due to the dissolution of Fe according to Equation (5.1):

0 + 2+ Fe + 2H → Fe + H 2 (g) (5.1)

When the pH of the solution is between 4 and 9, a porous film of FeO formed.2,84 The

diffusion of oxygen through the porous coating controls the etch rate. The etch rate

decreases sharply with the increase of pH.85 In neutral conditions, not many H+ or OH- ions are available for the etching reaction, leading to an etch rate near zero. In mildly

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- alkaline conditions (pH∼ 9 to 11) the OH ions react with Fe to form non-porous Fe2O3,

which passivates the surface, resulting in a low etch rate. The slight increase in etch rate

and Fe concentration in the solution with pH in alkaline conditions may be related to

the availability of oxygen in different pH environments.84 As the pH of the solution

- increases further, the soluble ferrite ion, HFeO2 is formed according to equation (5.2):

0 − + − Fe + 2H 2O → HFeO2 + 3H + 2e (5.2)

and the etch rate increases again.

0.25 3.00E-03

Calculated by weight loss 2.50E-03

) 0.2 Fe concentration

2.00E-03 0.15 m/minute m/minute μ 1.50E-03 0.1 1.00E-03 Fe concentration (g/ml) Etching rate ( rate Etching 0.05 5.00E-04

0 0.00E+00 02468101214 pH value

Figure 5.2 Etch rates of CRS as determined by weight loss and Fe concentration as determined by ICP-MS after cleaning at different pH values.

109

5.2.2 Surface morphologies of as-cleaned CRS surfaces

Figure 5.3 shows the surface morphology of CRS after cleaning at different pHs

(pH∼1.0, pH∼9.5 and pH∼12.4). A coarse platelet-like surface oxide (Figure 5.3a) was

observed on the CRS surface cleaned at pH∼1.0, which may be related to rapid

dissolution of the Fe surface. A fine, dense and spheroidal surface oxide was detected

on CRS surface cleaned in alkaline environments (pH∼9.5 and pH∼12.4) as shown in

Figures 5.3b and 5.3c.

(a) (b)

(c)

Figure 5.3 Surface morphology of the CRS surface cleaned at different pH values by SEM (a) pH∼1.0, (b) pH∼9.5 and (c) pH∼12.4.

110

The coarse, rough surface oxide layer formed in acidic conditions is detrimental to the

formation of a dense bonding layer between silane and CRS. The fine, dense surface

oxide formed in alkaline condition, on the other hand, is desirable for the formation of a

dense bonding layer.

5.2.3 Surface energy of as-cleaned CRS panels

As mentioned in Chapter 3, the surface energy components of a mineral surface can be

determined by the contact angle measurements using three different probing liquids (2

polar and 1 apolar). In this research methylene iodide, water and ethylene glycol were

used. The contact angle measurement follows the procedure described in Chapter 3.2.9.

The measured contact angles of the three probing liquids of different pH are shown in

Figure 5.4.

70 Ethyl glycol 60 Methylene iodiode Water 50 40 30 20 Wetting angle (Degree) Wetting 10 0 2 4 6 8 10 12

Figure 5.4 Measured contact angle of the as-cleaned CRS as function of the cleaning solution pH.

111

The maximum contact angles are observed at cleaning pH near the IEP point (pH 8.5-

10.5) for polar probing liquids. The non-polar liquid, methylene iodiode, shows the

lowest contact angle on CRS surface, whereas water, the most polar probing liquid,

shows the highest contact angle over the whole pH range with the contact angle value

of ethylene glycol falling in the middle. The higher contact angle value of polar liquid

and lower contact angle of non-polar probing liquid suggests that the surface energy is

dominated by the dispersive component.

Figure 5.5 shows the dispersive and polar components of the CRS panels after cleaning

at different pH, respectively. The dispersive component dominates the surface energy

of CRS. The dispersive part is relativly constant over the entire pH range whereas the

polar component varies with the cleaning pH. A minimum in polar surface energy is

detected under mild alkaline condition. Considering the error bar, however, it is hard to

fix the actual value of the polar component.

In aqueous solutions, the surface hydroxyl groups may remain undissociated, in which

case the pH of the aqueous solution is the same as the IEP of the surface oxide. If the

pH is less than the IEP, the surface will acquire a positive charge:24

+ + − MOH + H ⇔ −MOH 2

If the pH of the solution is greater than the IEP, the surface will acquire a negative charge:

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− − − MOH + OH ⇔ −MO + H 2O

Both the negative and the positive charges should increase polar component at higher

and lower pH than the IEP. The lower polar component at mild alkaline condition as

shown in Figure 5.5 suggests that the surface cleaned the near IEP (pH ~ 9.5) is closer

to a neutral surface than the surfaces cleaned in other conditions. It must be recognized, however, that the apparent minimum at pH ~ 9.5 is within the error of the measurement. ) 2

10 (a) 0 Dispersive surface energy(mJ/m 0 2 4 6 8 10 12 pH

) 10 2

(b) 0 Polar surface energy (mJ/m energy surface Polar 2 4 6 8 10 12 pH

Figure 5.5 Calculated polar and dispersion components of as-cleaned CRS surface cleaned in different pH conditions based on contact angle measurements

113

NH NH NH Si Si Si Si Si

Si Si Si Si NH OH OH NH OH NH + + + + + - - - OH2 OH2 HO OH2 OH2 HO OH2 HO HO HO HO O O O

Low pH CRS High pH

pH of solution IEP pH 9.5

Figure 5.6 Schematic diagram of the mechanism of AV silane adsorption on CRS. The polar surface of CRS after cleaning in strongly acidic or alkaline conditions prevents the formation of bonding between CRS surface and silane. One the other hand, the neutral surface near the IEP point promotes the formation of hydrogen bonds between the silane and the surface.

The surface charge and the isoelectric point of CRS are schematically depicted in

Figure 5.6. The hydrolyzed AV silane solution contains weakly acidic silanol groups

and strongly basic secondary amine groups. The solution is stabilized, because the

weakly acidic silanol groups form hydrogen bonds with the strongly basic secondary

amine groups rather than between themselves as mentioned in Chapter 4. The latter will

otherwise result in siloxane formation and condensation of the silane in solution.

114

In present study, silane is absorbed through both silanol groups and secondary amine

groups. Strongly alkaline or acidic cleaning of CRS results in a charged surface as

shown in Figure 5.6, which is less favorable as a basis for the adsorption of silane via

hydrogen bonding for silanols and/or secondary amine groups. However, at near neutral

conditions, close to the IEP of CRS, both of these species are able to adsorb through

hydrogen bonding, which leads to increased bonding of AV silane onto CRS at mildly

alkaline pH around 9.5.

Since the secondary amine group of bis-amino silane is highly basic; it is thus

preferably adsorbed on the acidic surface sites. The silanol group, on the other hand, is

weakly acidic, and will be preferably absorbed on the basic surfaces sites.

In order to further characterize the acid-base properties of CRS surfaces, the Lewis

acid-base properties were evaluated using the van Oss–Chaudhury-Good model.76

Figure 5.7 shows the Lewis acid-base properties of the as-cleaned CRS surface. The basic component of the surface energy shows a minimum at mild alkaline conditions

(pH~8.5 to 9.5), which agrees with the polar component results of Figure 5.5. However, due to the relative large error bar of acidic component as shown in Figure 5.7 (b) no useful information can be drawn from the acidic component data.

115

) 2 50

10 (a)

Basic surface energy (mJ/m 0 0 2 4 6 8 10 12 pH

)

2 0.6

(b) 0.4

0.2

0.0

Acidic surface energy (mJ/m Acidic -0.2 0 2 4 6 8 10 12 pH

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5.2.4 Silane layer thickness on CRS

1.8

1.6 m) μ 1.4

1.2

Thickness ( 1

0.8

0.6 02468101214 Cleaning solution pH

Figure 5.8. AV silane layer thickness on CRS before curing as a function of cleaning solution pH. The AV silane coating procedure is decribed in detail in section 3.1.2.2 and cured at 100°C for 60 minutes. The silane layer thickness was measured by weight gain following the procedure described in Chapter 3.2.7.

Figure 5.8 shows the calculated AV silane thickness before curing as a function of cleaning pH. A thicker silane layer is observed on CRS cleaned at both pH∼1.0 and

pH∼9.5. The thicker layer at pH∼9.5 is possibly due to the abundant bonds formed

between the functional groups of AV silane and the CRS surface close to the IEP as

discussed above. This conclusion agrees with Franquet and co-workers’ 29 observation that a thicker and more uniform silane layer is formed on aluminum alloys after alkaline cleaning than on alloys without alkaline cleaning. The increase in thickness at pH∼1.0 is believed to be related to the fact that a large amount of silane is trapped in

117

the pits and cavities of the coarse surface produced by cleaning in highly acidic

conditions.

5.2.5 Fourier-transform infrared reflection-absorption (FTIR-RA)

Spectroscopy of silane-coated CRS

Table 5.1 IR peaks and their assignments for the silane-coated CRS20,86-89 (s stands for strong, m for medium and w for weak) Wavenumber (cm-1) Assignment References

~1610, s -NH 13, 90, 91 ~1560, m to s Hydrogen-bonded –NH- 13, 90, 91

1480-1300, m to s Bending of CH2 and CH3 13, 90 ~1264, w -COO- 13, 88, 89

~1190, s -OCH3 13, 90-92, 94 ~1100, s Branched, long-chain 13, 90-92, 94 siloxane ~1010, s Cyclic, short-chain siloxane 13, 90-92, 94 ~920, m Si-OH 90-95 ~750, m to w -NH 13, 90, 91 + ~690, m to w Protonated –NH2 13

The FTIR-RA spectra of the silane-coated panels before curing are shown in Figure

5.9(a) and after curing at 100°C for 60 minutes in Figure 5.8(b). The AV silane coating

procedure is described in detail in section 3.1.2.2. The peaks and their assignments are

listed in Table 5.1. The band at 1010 cm-1 is assigned to the short-chain or cyclic

siloxane (Si-O-Si), whereas the band at 1100 cm-1 is assigned to long and branched- chain siloxane.20,86-88,90 The peaks at 1560 cm-1 and 1610 cm-1 are assigned to the

hydrogen-bonded and free –NH, respectively.20,86,87 The peak at 920 cm-1 is most likely due to silanol (Si-OH) groups.86-91

118

Before curing, the differences between the IR spectra of the fresh silane films on CRS

cleaned in the different pH solutions are not significant, as shown in Figure 5.9(a). All

spectra show unreacted silanol groups (∼920 cm-1) and a similar shape of the siloxane

double peak (∼1010 cm-1 and ∼1100 cm-1). The peak corresponding to the short-chain siloxane (~1010 cm-1) is more pronounced than the longer-chain siloxane peak at 1100

cm-1. Both hydrogen-bonded NH (∼1560 cm-1) and free NH (∼1610 cm-1) can be found before curing. The hydrogen bonded -NH may come from the bonds between

secondary amine groups (-NH) and the silanol groups (Si-OH), or from the hydrogen bonds between the NH groups with surface hydroxyl groups (Fe-OH).

After curing at 100°C for 60 minutes some changes are observed for the AV silane films cleaned at the different pH values, as shown in Figure 5.9(b). The difference between the long-chain siloxane peaks at about 1100 cm-1 and the short chain siloxane peak at 1010 cm-1 increases for the panels cleaned at pH∼1.0 and pH∼9.5, which

indicates further crosslinking of silanols during curing. For the panel cleaned at

pH∼12.4, however, the above difference is not observed as compared with the coating

before curing. In the meantime, the intensity of the hydrogen bonded -NH peaks at

1560 cm-1 decreases after curing regardless of cleaning pH. During curing at 100°C, the silanol groups hydrogen bonded with –NH groups condense among themselves. As a result, hydrogen-bonded –NH groups are liberated and the –NH peak at 1560 cm-1

decreases.

119

pH=1.0

pH=9.5

pH=12.4 Reflectance 691 757 1267 1190 920

1405 1100 1557 1605 1022

500 750 1000 1250 1500 1750 2000 -1 Wavenumber (cm ) (a)

pH=1.0

pH=9.5

pH=12.4 Reflectance

1267 686 750 1190 920 1405 1557 1605 1100 1020

500 750 1000 1250 1500 1750 2000 -1 Wavenumber (cm ) (b)

Figure 5.9. FTIR spectra of the AV-silane-coated CRS cleaned in different pH (a) before curing and (b) after curing at 100°C for 60 minutes. The coating procedure is described in detail in section 3.1.2.2.

120

5.2.6 DC potentiodynamic results of CRS panels

5.2.6.1 As-cleaned CRS panels

In order to understand and compare the corrosion resistance of the surface oxide layers formed in different pH, DC potentiodynamic measurements were performed on the as- cleaned CRS panels. The cleaning procedure is described in section 3.1.2.2 at 60°C for

3 minutes in cleaning solution of different pHs. Differences in the DC polarization indicate differences in the stability of the surface oxide formed after cleaning. The stability of the surface oxide layer is further related to the composition, density and thickness of the layer.

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

Potential (V/SCE) pH=1.0 -0.8 pH=9.5 -0.9 pH=12.4 Control -1 -6.0 -5.0 -4.0 -3.0 -2.0 Log current density (A/cm2)

Figure 5.10. DC potentiodynamic curves of the as-cleaned CRS panels cleaned at 60°C for 3 minutes in cleaning solution of different pHs.

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Figure 5.10 shows the DC potentiodynamic results for the CRS cleaned at different pH.

Compared with the control panel, the cathodic reaction is suppressed whereas the

anodic reaction is promoted after cleaning in acidic conditions. On the other hand, the

anodic reaction was suppressed and the cathodic reaction was promoted for CRS

cleaned in both mildly alkaline conditions (pH~9.5) and strongly alkaline conditions

(pH~12.4) but to different extent. The suppression of the anodic reaction indicates the

suppression of the dissolution of Fe, which may result from the dense, fine oxide layer

mentioned above. The abundant-grain-boundary structure is most likely the cause for

the higher current density of panels cleaned in strongly alkaline conditions (pH~12.4).

The porous surface accounts for the high current density in the strongly acidic condition

(pH~1.0). It is also interesting to note from the anodic reaction in Figure 5.10 that the

surface is passivated at pH∼9.5. The passive layer protects the iron surface from further

dissolution and may account for the slow anodic reaction of CRS.

Figure 5.11 shows the relationship between the pH-value and the Ecorr for as-cleaned

panels. The Ecorr value of the panels cleaned in mildly acidic and all alkaline conditions

(pH from 3.6 to 12.4) is stable at around 0.5 V, indicating that a dense and inert surface

oxide is formed after cleaning in these conditions and re-exposure to air. However, for

the panel cleaned in strongly acidic conditions, the Ecorr shows a dramatic decrease, which may be associated with the coarse and porous surface oxide layer formed after cleaning in strongly acidic conditions mentioned above. This result agrees well with the

SEM results.

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-0.4

-0.45

-0.5

-0.55 (V)

corr -0.6 E Control -0.65

-0.7

-0.75 02468101214 pH

Figure 5.11 Ecorr of as-cleaned CRS panels after cleaning in different pH conditions at 60°C for 3 minutes in cleaning solution of different pHs.

5.2.6.2 Silane-coated CRS panels

Figure 5.12 shows the DC potentiodynamic curves of the silane-coated CRS panels

cleaned at different pH values. Compared with the shape of the curves shown in Figure

5.10, the DC polarization curves remains the same. This observation is consistent with

that reported by Zhu15 on aluminum. This result implies that the AV mixture coating

serves as a physical barrier rather than an electrochemical barrier during corrosion of

CRS. Being a physical barrier, the bonding between the silane coating and the CRS

substrate is critical, because only a dense and strong bonding layer can prevent

electrolyte from penetrating into the silane-metal interface.

123

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7 pH=1.0

Potential (V/SCE) -0.8 pH=9.5 pH=12.4 -0.9 Control panel coated with silane -1 -6.0 -5.0 -4.0 -3.0 -2.0 Log current density (A/cm2)

Figure 5.12. DC potentiodynamic curves of the silane-coated CRS panels cleaned at 60°C for 3 minutes in cleaning solution of different pH. The AV silane coating is cured at 100°C for 60 minutes.

Table 5.2 Icorr from the DC potentiodynamic measurements on as-cleaned and silane-coated CRS panels Sample pH=1.0 pH=9.5 pH=12.4 Control

Icorr(as-cleaned) 1 3.55E-5 2.82E-5 5.01E-5 5.01E-5 2 (A/cm ) 2 3.89E-5 3.21E-5 5.24E-5 4.87E-5 3 4.07E-5 2.97E-5 5.07E-5 4.76E-5

Icorr (silane-coated) 1 1.41E-5 7.08E-6 2.51E-5 5.01E-5 (A/cm2) 2 1.91E-5 1.03E-5 2.56E-5 4.89E-5 3 2.06E-5 7.53E-6 2.44E-5 4.72E-5

The Icorr values of the bare CRS panels and silane-coated panels cleaned in different pH conditions are shown in Table 5.2. As shown in Figure 5.12 and Table 5.2, the Icorr of the silane-coated CRS panels cleaned in mildly alkaline condition (pH~9.5) shifts to a lower current density by half a decade after silane deposition. The decrease in Icorr

124

indicates that the coating serves as a barrier against electrolyte. However, compared

with the corresponding as-cleaned panels, the reduction in current density is not so

significant after coating for the silane-coated control panels and those cleaned at

strongly acidic (pH~1.0) and strongly alkaline (pH~12.4) condition. Interestingly, very

little reduction in current densities is observed on control CRS panels after silane

deposition. The different Icorr values before and after silane deposition indicate that cleaning is essential for good anti-corrosion performance. The silane coating deposited onto the CRS cannot serve as an effective barrier layer on CRS surfaces cleaned in strong acidic or alkaline conditions due to the porous and loose surface oxide layer formed after cleaning in strongly acidic condition (pH~1.0) and less available bonds after cleaning in strongly alkaline condition (pH~12.4) as mentioned above.

5.2.7 Electrochemical Impedance Spectroscopy (EIS) of AV silane-

The contained-primer-coated CRS panels

The EIS measurements were performed on primer-coated CRS maintained in 0.6 M

NaCl solution for the following periods of times: 0, 1, 2, 7 and 14 days. The primer

formulation contained 80-wt% epoxy resin, 10 wt% bis-sulfur silane, 9 wt% of AV

silane and 1 wt% TEOS crosslinker. The primer formulation was applied by a draw-

down bar. The primer-coated panels were cured at 100°C for 60 minutes in air.

Figures 13(a) and 13(b) show Bode plots of impedance and phase angle for primer- coated panels cleaned in different conditions at the beginning of the EIS experiment on day 0. The impedance moduli of panels cleaned in mildly alkaline conditions (pH∼8.5

125

to 10.5) are around 104 to 106 Ω, which is larger by 2 decades than for panels cleaned in the other conditions. This result agrees well with the DC polarization results.

6 10 Day 0 pH=1.0 5 10 pH=9.5 pH=12.4 4 Control 10 (ohm) 3

mod 10 Z

2 10

1 10 -2 -1 0 1 2 3 4 5 10 10 10 10 10 10 10 10 Frequency (Hz) (a)

-80 Day 0 pH = 1.0 pH = 9.5 -60 pH = 12.4 Control -40

-20 Phase angle (degree)Phase

0 -2 -1 0 1 2 3 4 5 10 10 10 10 10 10 10 10 Frequency (Hz)

(b)

Figure 5.13. EIS results on primer-coated CRS panels cleaned in different pH solutions on day 0, in 0.6 M NaCl solution (a) Modulus and (b) Phase-angle plot. The formulation of the primer and the preparation of the primer coating follow the description in Chapter 3.1.2.2.

126

More importantly, the primer-coated panel cleaned at pH∼9.5 reveals two time constants in the phase-angle plot, while the panels cleaned in the other conditions possess only one time constant at intermediate frequencies (Figure 13(b)). The presence of an extra time constant at low frequency is usually assigned to the formation of a layer between the coating and the metal oxide.2,3,9,22,23 The difference in time constants supports the conclusion that bonds between the primer coating and the CRS surface are most abundant after cleaning in mildly alkaline (pH∼9.5) conditions. For CRS cleaned in strongly acidic conditions, the phase angle at low frequencies is approximately 45°, suggesting the existence of a Warburg diffusion mechanism and the formation of a porous primer coating (Figure 13(a)), which agrees with the SEM surface morphology observation results.

Figures 5.14(a) and 5.14(b) show the Bode plots of primer-coated panels cleaned in different conditions after 14 days immersion in 0.6 M NaCl solution. As compared with the above EIS results on day 0, the low-frequency modulus drops by one or two orders after 14 days of salt water immersion. However, the panels cleaned in mildly alkaline conditions (pH∼9.5) still have higher impedance than those cleaned in all other conditions. For the panels cleaned at pH∼9.5, the second time constant still exists, indicating survival of the bonding layer between the primer coating and the substrate after 14 days of immersion. For the control panels and those cleaned in acidic (pH~1.0) as well as in strongly alkaline (pH~12.4) conditions, only one time constant was observed after 14 days as on day 0.

127

6 10 Day 14 pH=1.0 5 10 pH=9.5 pH=12.4 4 10 Control (ohm) 3

mod mod 10 Z

2 10

1 10 -2 -1 0 1 2 3 4 5 10 10 10 10 10 10 10 10 Frequency (Hz)

(a)

-80 Day 14 pH = 1.0 pH = 9.5 -60 pH = 12.4 Control

-40

-20 Phase angle Phase (degree)

0 -2 -1 0 1 2 3 4 5 10 10 10 10 10 10 10 10 Frequency (Hz) (b)

Figure 5.14 EIS results on primer-coated CRS panels cleaned in different pH solutions after 14 days of immersion in 0.6 M NaCl solution, (a) Modulus and (b) Phase angle plot. The formulation of the primer and the preparation of the primer coating follow the description in Chapter 3.1.2.2.

128

Figure 5.15 shows the relationship between the cleaning pH and the low frequency impedance (0.01 Hz) for the primer-coated CRS panels obtained on day 0 and after 14 days of salt-water immersion. As can be expected, the largest low frequency impedance values are observed for the mildly alkaline condition at both times after exposure to 0.6

M NaCl solution.

1.0E+08

Day 0 1.0E+06 Day 14 Ω)

1.0E+04 Impedance ( Impedance 1.0E+02

1.0E+00 02468101214 pH

Figure 5.15 Comparison of the low-frequency impedance modulus of primer-coated CRS panels cleaned in different pH solutions obtained from the EIS results on day 0 and after 14 days of salt water immersion The formulation of the primer and the preparation of the primer coating follow the description in Chapter 3.1.2.2.

129

5.2.8 Salt water immersion of primer-coated CRS panels

Water line

Control pH=1.0 pH=3.6

Water line

pH=6.8 pH=8.5 pH=9.5

Water line

pH=10.5 pH=12.4

Figure 5.16 7-day salt water immersion test results on the primer-coated CRS panels cleaned at different pH values.

Figure 5.16 shows the images of the primer-coated CRS panels after immersion in 0.6

M NaCl solution for 7 days. The formulation of the primer and the preparation of the

130

primer coating follow the description in Chapter 3.1.2.2. Delamination and blistering

were observed on all panels. The panels cleaned in mildly alkaline conditions (pH∼9.5

to 10.5), however, show less rust and delamination than the other panels. On the other

hand, the control panels and those cleaned in strongly acidic and alkaline conditions

show delamination and rust, which agrees with the EIS and DC polarization results.

5.3 Conclusions

CRS was cleaned in different pH solutions in order to investigate optimum cleaning

condition for the AV silane and the AV-silane-containing primer. Both the as-cleaned

surfaces and the as-cleaned silane-coated surfaces were characterized with several

analytical methods and the corrosion performance of the as-cleaned, silane-coated and

primer-coated panels was evaluated by electrochemical methods and/or salt water

immersion.

The best performance was obtained for panels cleaned in mildly alkaline conditions

(pH~9.5) near the isoelectric point of CRS. SEM, XPS and surface energy examination

of CRS cleaned in mildly alkaline cleaning condition detected a fine, dense, spheroidal

surface oxide and a nearly neutral surface. The neutral surface promotes hydrogen

bonding of both silanols and secondary amine groups to surface, which could result in

the most abundant number of bonds and a thicker silane film. Moreover, FTIR indicates that acidic (pH∼1.0) and mildly alkaline cleaning conditions (pH∼9.5) result in the

formation of long-chain siloxane, whereas the strongly alkaline cleaning conditions

(pH∼12.4) inhibits the long-chain siloxane formation.

131

Chapter 6. Water-Barrier Properties of Mixed

Bis-[trimethoxysilylpropyl]amine and Vinyl

Triacetoxysilane Films2

6.1 Introduction

Recent electrochemical research2,10 including that reported above reveals that silane coatings act as a physical barrier rather than as an electrochemical corrosion inhibitor.

One generally accepted assumption is that the crosslinked siloxane network acts as a water barrier preventing water penetration to the silane-metal interface. However, the mechanism of silane corrosion inhibition is still unclear. Therefore, investigation of water-barrier properties of the coatings is important in elucidation of the origin of corrosion-protection.

In an attempt to sort out the issues regarding the water-barrier properties, we studied the properties of mixed AV films under exposure to water vapor. The properties of the films, including the scattering length density (SLD) profile and thickness change on exposure to water, were investigated as a function of A/V ratio using both x-ray and neutron reflectivity. Nuclear magnetic resonance (NMR) was employed to clarify the reaction between bis-amino silane and VTAS in the neat oil. Grazing angle small-angle x-ray scattering (GISAXS) was used to characterize the void volume.

2 Published in Journal of Physical Chemistry, B, 111 (2007) 7041-7051.

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6.2 Results and Discussion

6.2.1 Calculation of X-ray and Neutron SLD of Possible Reaction

Products

As mentioned the Chapter 4, the primary reaction in AV mixture is exchange of the acetoxy group on VTAS with the secondary amine hydrogen, forming an amide complex as shown in Figure 4.11. The stoichiometric ratio of primary reaction is therefore 3. The primary reaction is followed by a series of hydrolysis and condensation reactions of both bis-aminosilane and the reacted VTAS in water solution.

The calculated x-ray and neutron SLD values of initial materials and by-products in AV system are shown in Table. 6.1.

Table 6.1. Calculated SLD values of initial materials and by-products in AV system

materials density 106 x SLD (Å-2) 3 (g/cm ) x-ray neutron Bis-amino silane (C12H31O6Si2N) 1.04 9.660 0.299 Vinyl triacetoaxysilane (H12C8O6Si) 1.167 10.340 1.430 Water (H2O) 1.00 9.460 -0.560 Heavy water (D2O) 1.10 9.190 6.286 Methanol (CH4O) 0.791 7.493 -0.379 Acetic acid (C2H4O2) 1.049 9.425 1.046

Assuming no volume change or further chemical reaction upon mixing, the SLD of the mixture is that of the two components weighted by their volume fractions:

133

SLDT = φ A SLDA + φV SLDV , (6.1)

where SLDT, SLDA and SLDv are the total calculated SLD of the AV mixture, the SLD of neat bis-aminosilane, the SLD of neat VTAS, respectively. φA and φV are the corresponding volume fractions. The SLDs of AV monomer physical mixtures at different A/V ratios are shown in Table 6.2.

The calculated SLD of all the possible reaction products of the primary reaction from the NMR analysis and the possible subsequent hydrolysis and condensation reactions in

AV system are shown in Table 6.3. The SLDs are calculated by assuming that the reaction products have the same density as the corresponding monomer reactant.

Table 6.2. Calculated SLD values of AV monomer physical mixtures 6 -2 AV mixture 10 x SLD (Å ) x-ray neutron AV2 9.82 0.562

AV3.4 9.76 0.470

AV5 9.73 0.421

It can be seen from Tables 6.1, 6.2 and 6.3 that the x-ray SLD values of the initial

materials and all possible reaction products are nearly identical whereas the neutron

SLD values vary substantially for different chemical compositions in AV system.

134

For x-rays, the SLD of a material is proportional to the electron density, which varies

little for substances composed of light atoms. XR therefore is sensitive to film density

but insensitive to chemical composition. NR, on the other hand, is sensitive to both

composition and density. A combination of the two methods allows us to determine

density and the chemical change in AV films.

6.2.2 X-ray reflectivity study

Figure 6.1(a) shows the XR data of as-prepared films as a function of A/V ratio. The

films were prepared following the procedure described in Chapter 3.1.2.1 on Si

substrates. The films were cured at 100ºC for 60 minutes. Figure 6.1(b) shows the SLD

profile corresponding to the best-fit to the reflectivity curve in Figure 6.1(a). A typical

SLD profile can be divided into three regions: the wafer substrate, the native surface

oxide and the silane film. The bottom surface of the native SiO2 layer is designated as

zero distance.

From the best-fit SLD profiles in Figure 6.1(b) it can be seen that all the films have a

nominal thickness of around 1200 Å. The experimentally determined SLDs range from

-6 -6 -2 6 ×10 – 8 ×10 Å for different AV ratios, which is substantially less than the

calculated SLDs of both the monomer mixtures and all-possible reaction products

(Tables 6.1 and Table 6.3 in supporting materials). Since the monomer densities were

employed in the SLD calculation, these results imply that the film densities are below

that of the monomer precursors. The reaction between bis-amino silane and the VTAS

135

must produce considerable void space either in the form of pores or molecular-level

free volume.

Table 6.3. Calculated SLD values of possible reaction products in AV system formula density 106 x SLD (Å- (g/cm3) 2) products neutro x- n ray H CO 1 3 OCH3 NH H CO Si H31C12O6Si2N 3 Si OCH3 1.040 0.300 9.66

H CO 3 OCH3 2 O

C CH3 O O H 10.3 Si O C H C O Si 1.167 1.43 H2C C CH3 12 8 6 4 O C CH3

O 3 O CH3 C OCH H3CO 3 H33C14O7Si2N N 1.040 0.31 9.54 H3CO Si Si OCH3 OCH H3CO 3 4 OH O

H2C C Si O C 10.2 H CH H C O Si 1.167 1.28 O 3 10 6 5 2 8 O C CH3 5 OH 10.3 H2C C Si OH H6C2O3Si 1.167 0.82 H 8 OH H CO OCH3 6 3 H N H29C11O6Si2N H3CO Si Si OH 1.040 0.33 9.53

OCH H3CO 3 HO OH 7 H H19C6O6Si2N HO Si N Si OH 1.040 0.52 9.40

HO OH 8 O CH3 C OCH3 OH H3CO H C O Si N H 23 8 8 3 1.100 0.68 9.88 N Si C H3CO Si O Si CH

H3CO OCH3 OH

136

9 O CH3 H CO C 3 O H H19C16O6Si2N N Si O Si C CH 1.100 1.15 9.73 H3CO Si 2 O H3CO 10 O O H9C5O4.5Si 10.3 H2C C Si O C 1.167 1.24 H O CH3 5 O C CH3 11 O H3C2O1.5Si 10.2 H2C C Si O 1.167 1.33 H 0 O

12 H CO OCH3 3 H N H28C11O5.5Si2N H3CO Si Si O 1.040 0.35 9.53

OCH H3CO 3 13 O O H H13C6O3Si2N O Si N Si O 1.040 0.81 9.31

O O 14 H CO OCH3 O 3 H N H31C13O7Si2N H3CO Si Si O C CH3 1.040 0.49 9.49

OCH H3CO 3

15 O O H C C CH3 3 C O H C O Si N O O H O 31 18 12 2 1.040 1.12 9.29 C O Si N Si O C CH3 H3C O O H C C CH3 3 C O O 16 O

H3C C O OCH3 H H C O Si N 25 11 6 2 1.040 0.62 9.43 O Si N Si O H CO 3 OCH3

137

17 O O

H3C C C CH3 O O H H25C14O9Si2N O Si N Si O 1.040 1.068 9.30

O O H3C C C CH3 O O

18 DO OCH3 H H22DC9O5Si2N O Si N Si O 1.040 0.685 9.44

OCH H3CO 3

19 DO OD H H13D4C6O5Si2N O Si N Si O 1.040 1.664 9.22

DO OD 20 DO OD H H D C O Si N O Si N Si O 13 3 6 4.5 2 1.040 1.48 9.24 OD O

21 H3CO OCH3 D H DC O Si N O Si N Si O 24 10 5 2 1.040 0.635 9.46

OCH H3CO 3 22 DO OD D H12D5C6O5Si2N O Si N Si O 1.040 1.924 9.19

DO OD

23 O O D H DC O Si N O Si N Si O 12 6 3 2 1.040 1.130 9.27

O O

138

4 10 As-prepared 3 AV2 10 AV3.4

2 AV5 10

1 10

0 10

-1

Reflectivity 10

-2 10

-3 10 -3 30 40 50 60 70 80x10 -1 q (Å ) z (a)

30 As-prepared 25 AV2

) AV3.4

-2 20 AV5 Calculated monomer mixture 15 SLD(Å X

6 10 10 5

0 0 500 1000 1500 Distance from Si (Å)

(b)

Figure 6.1. X-ray reflectivity and SLD profiles of the as-prepared AV films at different A/V ratios: (a) The reflectivity curve of the as-prepared films. The lines through the data points indicate the best-fit of the reflectivity data using the SLD profile in (b). Each reflectivity curve is offset by two decades for clarity. (b) The SLD profiles of the best-fit of the reflectivity data in (a). The bar in the graph indicates the calculated x-ray SLD range of AV monomer mixtures. The films were prepared following the procedure described in Chapter 3.1.2.1. The films were cured at 100ºC for 60 minutes.

139

The density of the films can be estimated by Eq. (2.16) as 0.69 g/cm3, 0.80 g/cm3 and

0.65 g/cm3 for AV2, AV3.4 and AV5, respectively. The volume fraction of void space in the film, ϕs, can be estimated as:

SLDmono − SLD f −dry ϕs = (6.2) SLDmono

where SLDmono is the calculated SLD of the monomer mixture shown in Table 6.2, and

SLDdry is the SLD of the as-prepared film obtained from fitting the XR data (Figure

6.1(b)). According to this calculation, the volume fraction of void space in the as- prepared film is: 32.6 vol%, 30.3 vol% and 38.2 vol% for AV2, AV3.4 and AV5, respectively.

Figure 6.1(b) also shows that the largest x-ray SLD is observed near the stoichiometric ratio. As shown in Figure 4.11, the stoichiometric ratio for the primary reaction in A/V system is 3. As discussed in the introduction, VTAS is hydrophobic and is not readily hydrolyzable in water. During the reaction shown in Figure 4.11, however, the exchange of secondary amine on bis-amino silane and acetoxy group on VTAS generates a silanol group on VTAS. At the stoichiometric ratio, the completion of the reaction results in more silanol groups on VTAS and hence higher crosslink density and

SLD in AV3.4 film after the subsequent drying and curing, The AV5 film has the

lowest SLD due to both the lower crosslink density resulting from incomplete reaction

between the bis-amino silane and VTAS and the slightly lower x-ray SLD value of the

bis-amino silane compared with that of VTAS (Table 6.1).

140

6.2.3 GISAXS of dry AV films

In order to understand the nature of the ~30-vol% void space detected in the as-

prepared films, grazing incidence small angle scattering (GISAXS) was performed.

Two-dimensional (2d) GISAXS images of the scattered intensity were obtained as described above.

GISAXS is sensitive to fluctuations in electron density in the length-scale range < 100

Å. Films with distinct pores with radii in this range show scattering that decreases with

increasing q. Figure 6.2 (a - c) compares the I-qxy horizontal line-cut plots of the 2d

GISAXS data of AV2, AV3.4 and AV5 for incident angle above and below the critical angle. In all cases the line-cut was taken at the critical exit angle of the film (θc, film), which is determined by x-ray reflectivity measurement before the GISAXS measurement.

Each figure compares GISAXS line-cuts at two different angles of incidence: below the critical angle of the film and between the critical angle of the film and substrate. When the incident angle is less than θc, film, the entire scattering is parasitic since the beam is totally reflected from the film surfaces and does not penetrate the film. When the incident x-ray beam falls between θc,film and Si substrate (θc, Si), the beam penetrates

the film and is totally reflected from the substrate. In this case small-angle scattering

will be observed if there are SLD fluctuations in the film. By comparing the GISAXS

for these two cases, one can determine if there is significant scattering above the

background from the bulk of the film.

141

1000 1000

100 100

10 -2 10 -3

1 AV2 1 AV3.4 θ < θ θ < θ

c,film Intensity (a.u.)

Intensity (a.u.) c,film 0.1 0.1 θc,film < θ < θc,Si θc,film < θ < θc,Si 0.01 0.01 2 3 4 5 6 7 8 9 2 2 3 4 5 6 7 8 9 2 0.1 -1 0.1 -1 qxy (Å ) qxy (Å ) (a) (b)

1000

100 -3 10 AV5 1 θ < θc,film Intensity (a.u) 0.1 θc,film < θ < θc,Si

0.01 2 3 4 5 6 7 8 9 2 -1 0.1 qxy (Å ) (c) Figure 6.2. GISAXS horizontal line-cuts at different A/V ratios: (a) AV2; (b) AV3.4; (c)

AV5. θ is angle of incidence, θc, film is the critical angle of the AV films, θc, Si is the critical angle of Si substrate. When θ is less than the critical angle of the film, the beam does not penetrate the film and scattering arises from surface roughness. Above the critical angle scattering from the bulk of the sample is observed in addition to the surface scattering. The films were prepared following the procedure described in Chapter 3.1.2.1. The films were cured at 100ºC for 60 minutes.

142

Following Lee et al.92, the GISAXS intensity from an amorphous porous film sample can be written as follows under the kinematic approximation.

IGISAXS = I S + I I (6.3)

where I S and I I stand for the surface roughness scattering and scattering from film

interior, respectively. The surface roughness scattering is written as92,93

2 I S (q) = (ns f s ) Lx Ly S(q) (6.4)

∞ 2 2π −qz g (R) / 2 where S(q) = dR ⋅ R ⋅ e J 0 (qxy R) . 2 ∫0 qz

S(q) is the structure function per unit area surface for an isotropic rough surface and is

2 2 expressed in terms of qz and qxy = qx + qy , with qz and qxy being the moduli of the out-of-plane and in-plane components of scattering vector, q , respectively. The surface

2 roughness is modeled by the distribution function g(X ,Y) ≡ 〈[z(x' , y ' ) − z(x, y)] 〉 , where z(x, y) is the height of the surface at the coordinates (x,y),

' ' 2 2 1/ 2 (X ,Y) ≡ (x − x, y − y) , and R ≡ (X + Y ) . ns and f s stand for the average atomic

number density and average atomic scattering factor of a substrate. Depending on the

shape of the roughness distribution of g(R) in the structure factor S(q) , I S (q) will

show different profiles, including power law behavior.12

In Eq. (6.4), I S is a function of Lx , the beam foot print length. Thus, as incident angle

increases I S will decrease. Since I S is unrelated to the bulk film, I S should be the

143

same shape whether measured above or below the critical angle in the ranges applied in

this work.

Compared to I S , the contribution from I I depends on the angle of incidence. Since the

x-ray beam cannot penetrate the film if the incident angle is below the critical angle, I I

measured below the critical angle is very weak. Once the incident angle is larger than

the critical angle, however, I I increases due to the scattering from the bulk film. The

total beam path in the film will then contribute to I I .

The data in Figure 6.2, show a clear power-law decrease with q due to the diffuse

scattering from surface roughness fluctuations.93 AV2 shows slope of -2 while AV3.4 and AV5 show -3. The difference in power-law exponent is due to the different surface roughness features, the origin of which is unknown.

The curves measured above critical angle also show power-law behavior with nearly the same slope in low-q region as that measured below the critical angle. The magnitude of the intensity, however, is decreased due to the reduction of beam

footprint, Lx , at the higher angle of incidence. More importantly, in the high-q region,

excess scattering ( I I ) is observed due to the scattering from SLD fluctuations in the film. The fact that excess intensity is observed only at the larger q indicates the size scale of the fluctuations is small (comparable to molecular length scales).

144

The I s part from the GISAXS line-cut of above and below the critical angle should be

the same except for a shift due to the x-ray footprint length. Thus, by matching the low-

q region above and below the critical angle and subtracting the surface roughness

scattering, I s from the total scattering, IGISAXS , the scattering from bulk film, I I , can be

extracted (Figure 6.3). These data are normalized for the path length in the film due to

the different incident angles and film thicknesses.

AV2 2 I = 25 + 430 qxy 50 AV3.4 AV5 40

2 I = 28 + 90 qxy 20

2 10 I = 17 + 16 qxy Relative Intensity (a.u.) Intensity Relative 0 -3 0 10 20 30 40 50x10 -2 q 2 (Å ) xy

Figure 6.3. The scattering from the bulk film (II) obtained by subtracting the surface

scattering (Is) from the total scattering (IGISAXS). Although the data are not on absolute scale, they have not been shifted with respect to each other. The intensity has been normalized by the beam path length in the film.

Roe et al.94 and Rathje et al.95 studied thermal and frozen density fluctuations in

amorphous polymers. Their data are similar to Figure 6.3. The excess scattering at high

q is presumably due to the frozen density fluctuations locked in during polymerization

of the highly functional silane. The intensity of scattering increases with q as

145

bq2 2 I = I0e = I0 (1+ bq + L) , b being a constant. This behavior is to be contrasted with scattering from the smallest detectible distinct pore (~ 5 Å), which would give Guinier-

1 like scattering of the form as I = I (1− q 2 R + ) for a pore of radius-of-gyration, 0 3 g L

Rg. This analysis indicates that the scattering does not arise from distinct pores, but

from short-scale density fluctuations.

The bulk scattering varies among the films of different A/V ratios. The lowest scattered

intensity is observed for AV3.4, implying that the near-stoichiometric film is the most

uniform. AV3.4 also shows the weakest q-dependence. The origin of the q-dependence

is not known, but may arise from the tail of the first diffraction peak, which occurs

outside the GISAXS q range. With this interpretation, the degree of disorder is gauged

2 by the slope, b, of I vs qxy . The uniformity of the near-stoichiometric film results from

the completion of the initial AV reaction as discussed with regard to the NMR analysis.

This result also agrees with the XR data. AV2 film shows highest intensity and

strongest q-dependence, both of which imply that it is least homogeneous. Since this

film has excess hydrophobic VTAS, the inhomogeneity may arise from incomplete

hydrolysis of VTAS.

6.2.4 Neutron Reflectivity Study of the AV Mixture Coatings

Neutron reflectivity (NR) yields structural information normal to the film surface on

length scales from ~10 Å to ~2000 Å. The sensitivity of NR to different isotopes allows

146

us to track specific components by isotope substitution. In this study, NR was measured

for the films as-prepared, after exposure to saturated D2O vapor and after re-dry.

6.2.4.1 As-prepared Films

The AV films were prepared following the procedure described in Chapter 3.1.2.1 and

cured at 100ºC for 4 hours. Figure 6.4(a) shows the NR data of as-prepared films at

different A/V ratios. Figure 6.4(b) shows the SLD profile corresponding to the best fit to reflectivity curve in (a). The films have a nominal thickness of 580 Å - 780 Å. The

smaller film thickness compared to that observed in XR experiment is due to the higher

spinning rate (3000 rpm) during spin coating, which is required to produce optimum

films for NR. The highest SLD is observed near stoichiometric ratio (AV3.4). The SLD

of the film decreases both above and below the stoichiometric ratio with the lowest

SLD observed for the AV5 film, consistent with the trend observed in XR study

described above.

The experimentally determined neutron SLDs for AV films range from 0.76 ×10-6 –

1.00 ×10-6 Å-2, which is much higher than the calculated neutron SLDs of AV monomer

mixtures shown in Table 6.2 (0.40 × 10-6 Å-2 – 0.56 × 10-6 Å-2), and is significantly

different from the XR results discussed above. The difference between XR and NR is

understandable. During curing, H2O is liberated. H2O has a negative neutron SLD (-

-6 -2 0.560 × 10 Å ), so the liberation of H2O increases the overall film SLD. On the other hand, the x-ray SLD of H2O is almost the same as the bis-amino silane and VTAS

monomer mixture. Hence XR is insensitive to water liberation so the change in SLD

147

between the cured films and the initial monomers indicates only that the film density

decreases after curing.

1 As-prepared AV2 0.1 AV3.4 AV5

0.01

Reflectivity 0.001

0.0001 -3 10 15 20 25 30 35 40x10 -1 q (Å ) z (a)

4 As-prepared AV2

) AV3.4 -2 3 AV5

Calculated monomer mixture

SLD(Å 2 X 6

10 1

0 0 200 400 600 800 1000 Distance from Si (Å)

(b) Figure 6.4. Neutron reflectivity and SLD of as-prepared AV films at different A/V ratios: (a) The reflectivity curve of the as-prepared film. The lines through the data points indicate the best-fit of the reflectivity data using the corresponding SLD in (b). (b) The SLD profiles of the best fit to the reflectivity curves in (a). The bar indicates the calculated neutron SLD range of the AV monomer mixtures. The films were prepared following the procedure described in Chapter 3.1.2.1. The films were cured at 100ºC for 60 minutes.

148

6.2.4.2 Films after D2O Vapor Conditioning

Heavy water (D2O) has similar chemical properties but much higher neutron SLD

(6.286×10-6 Å-2) than light water (-0.560 ×10-6 Å-2). The higher neutron SLD allows us

to track the water penetration by measuring the SLD change after D2O conditioning. In

order to investigate the water absorption, NR was performed on films of different A/V

ratio after exposure to saturated D2O vapor and on re-dry. Figure 6.5, 6.6 and 6.7 show

the NR data and the corresponding SLD profiles.

Compared to the as-prepared state, the SLDs of the D2O-conditioned films increase

substantially with very small thickness increase, regardless of A/V ratio. This

observation is consistent with the results observed by Pan et al.18 on bis-amino silane

films. Most water resides in the void space created by frozen density fluctuations with

very little swelling of the film.

All D2O-conditioned films show enhanced SLD on both the air-side surface and silane

and Si substrate interface implying a D-rich region attributed to D2O absorption and the protonated amino groups near the substrate, repectively.18 The AV system contains the

reaction products of VTAS and bis-amino silane mixture as mentioned above. The

hydrophilic properties of the film are thus inherited from the secondary amine group on

bis-amino silane. The amino group is hydrophilic, protonated and able to form

hydrogen bonds with surface hydroxyl groups and absorb on metal surface.26,42,50,96

Previous work on the tri-functional analog of bis-amino silane: γ-aminopropyl-

triethoxysilane (γ-APS) by Fowkes et al.96 also observed free amino groups located at

149

the air-side of the γ-APS film surface with angle-resolved XPS. The hydrophilic free

amino group at the air-side surface can account for the D2O-rich layer near the film

surface.

6.2.4.3 Re-dried Films

32,33 Kent et al. proposed that the chemical change during the D2O-vapor-conditioning can be measured by comparing the reflectivity in the re-dried state with that of the as- prepared dry state. As mentioned above, most of the water is physically absorbed in the void space. As shown in Figure 6.5, 6.6, 6.7, the SLDs of the re-dried films almost return to their original as-prepared (dry) values after re-dry, which suggests the films are quite stable in water vapor. The small SLD increase between the dry and re-dried states may result from a chemical reaction resulting in the incorporation of deuterium into the film during conditioning.

The degree of chemical alteration of polymer skeleton after re-dry can be quantified by comparing the relative SLD change between the as-prepared state and the re-dried state:

ΔSLD SLD −SLD = re-dry dry (6.5) SLD SLDdry

The calculated relative SLD change is 9.1%, 20.3% and 4.8% for AV2, AV3.4 and

AV5 films, respectively.

150

1 AV2 As-prepared

0.1 D2O vapor 25ºC 16h Re-dried

0.01

Reflectivity 0.001

0.0001 -3 10 15 20 25 30 35 40x10 -1 q (Å ) z (a)

4 ) -2 3 AV2 As-prepared

SLD(Å 2

X D2O vapor 25ºC 16h 6 Re-dried 10 1

0 0 200 400 600 800 1000

Distance from Si (Å) (b)

Figure 6.5. Neutron reflectivity and SLD of AV2 film: (a) Reflectivity in the as-

prepared, D2O-vapor-conditioned and re-dried states. The lines through the data points indicate the best-fit of the reflectivity data using the corresponding model SLD in (b). (b) The corresponding SLD profiles of the best-fit to the reflectivity in (a). The film was prepared following the procedure described in Chapter 3.1.2.1 and cured at 100ºC for 60 minutes.

151

1 AV3.4 As-prepared

0.1 D2O vapor 25º 16h Re-dried

0.01

Reflectivity 0.001

0.0001 -3 10 15 20 25 30 35 40x10 -1 qz (Å )

(a)

4 ) -2 3 AV3.4 As-prepared SLD(Å 2 D O vapor 25ºC 16h

X 2 6 Re-dried 10 1

0 0 200 400 600 800 1000

Distance from Si (Å) (b)

Figure 6.6. Neutron reflectivity and SLD of AV3.4 film: (a) Reflectivity in as-prepared,

D2O-vapor-conditioned and re-dried state. The lines through the data points indicate the best-fit of the reflectivity data using the corresponding model SLD in (b). (b) The corresponding SLD profiles of the best-fit to the reflectivity in (a). The film was prepared following the procedure described in Chapter 3.1.2.1 and cured at 100ºC for 60 minutes.

152

1 AV5 As-prepared

0.1 D2O vapor 25ºC 16h Re-dried

0.01

Reflectivity 0.001

0.0001 -3 10 15 20 25 30 35 40x10 -1 q (Å ) z (a)

) 3 -2

AV5 2 As-prepared

SLD(Å D2O vapor 25ºC 16h X

6 Re-dried

10 1

0 0 200 400 600 Distance from Si (Å)

(b)

Figure 6.7. Neutron reflectivity and SLD of AV5 film: (a) Reflectivity in the as- prepared, D2O-vapor-conditioned and re-dried states. The lines through the data points indicate the best-fit of the reflectivity data using the corresponding model SLD in (b). (b) The corresponding SLD profiles of the best-fit to the reflectivity in (a). The film was prepared following the procedure described in Chapter 3.1.2.1 and cured at 100ºC for 60 minutes.

153

Despite the near return of the SLD to the as-prepared state after re-dry, the film

thickness remains at the swollen value after re-dry following D2O conditioning. The

change of film thickness after swelling and re-dry confirms relaxation of the siloxane

network during D2O conditioning and the persistence of the siloxane network in the

relaxed state after re-dry. The calculated relative thickness changes between as-

prepared and re-dried state are 5.41%, 7.07% and 3.98% for AV2, AV3.4 and AV5,

respectively.

From the above calculations, both the largest relative SLD and thickness changes are

observed for the near-stoichiometric film (AV3.4), which implies some connection

between the siloxane network relaxation and the incorporation of D in the film during

the D2O conditioning. NMR results, which were reported in Chapter 4, show evidence

of the reaction of amide complex with water during water conditioning as following:

(CH3O)3-Si-(CH2)3-N(COCH3)-(CH2)3-Si-(H3CO)3 + D2O → (CH3O)3-Si-(CH2)3-ND-

(CH2)3-Si-(H3CO)3 + CH3COOD. Since the amide complex is maximized at

stoichiometry as shown in Figure 4.11, the incorporation of D would also be maximized at stoichiometry.

The above analysis shows that the water is absorbed in the films in two forms. Most of the water is physically resident in the void space. A small amount of water, however, reacts with the polymer skeleton and results in relaxation of the siloxane network, which is responsible for the SLD and thickness increase after re-dry. This pattern is consistent with the so-called dual-mode sorption model,97-99 which postulates that

154

penetrants exist in two populations: one of which is the dissolved in polymer matrix according to Henry’s law, and the other of which occupies unrelaxed void space within

the silane films.

The SLD change after D2O conditioning includes both the contribution from physical

water absorption in void volume and the smaller contribution from the chemical

incorporation of D. The total D2O-equivalent volume fraction, ϕD2O, can be estimated with the following equation:

SLD f −D2O − SLD f −redry ϕ D2O = , (6.6) SLDD2O

where SLDf-D2O is the SLD of the film after conditioning, SLDf-redry is the SLD of the film after re-dry and SLDD2O is the SLD of the D2O (Table 6.1). The calculated volume

fractions are: 29.7 vol%, 28.2 vol% and 35.1 vol% for AV2, AV3.4 and AV5,

respectively.

As mentioned previously, the curing process involves condensation reactions and hence

the liberation of small, volatile by-products (e.g., H2O, CH3OH, etc.). The composition of the film depends on the progress of the condensation reaction. Hence the composition of the films is difficult to predicate. Fortunately, the combination of XR and NR techniques enables us to investigate interaction of the film with water without knowing the exact composition of the film. By comparing the SLDs of the dry, water- conditioned and re-died state for each composition, useful information such as void

155

volume, water uptake and chemical alternation of the films can be obtained as shown in

Eq. (6.2), (6.5) and (6.6).

Percentage (%) Percentage 25 Void volume Physically absorbed D2O 20 2.0 3.0 4.0 5.0 A/V ratio

Figure 6.8. Comparison of the void volume and D2O absorption during D2O conditioning in films of different A/V ratios. The D2O absorption tracks the void volume.

Figure 6.8 compares the void volume (Eq. (6.2)) with the water absorbed (Eq. (6.6)).

The amount of void space in the as-prepared “dry” coating and the volume occupied by

water during conditioning follow the same trend, with minimum water absorption near

the stoichiometric ratio. This trend again implies that the water absorption is

determined by the amount of void space in the film. Water occupies the void space

instead of swelling the film. This result also confirms that the near-stoichiometric film

is the densest. More water is absorbed in the film of above the stoichiometric A/V ratio

(AV5) than below (AV2) due to the hydrophilic nature of excess bis-amino silane.

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Percentage (%) Percentage 10 Physically absorbed D2O Chemical change after redry 0 2.0 3.0 4.0 5.0 A/V ratio

Figure 6.9. Comparison of the chemical change after re-dry and physically absorbed

D2O during D2O conditioning. The chemical change is calculated from the SLD change

between the dry and re-dried states using equation (6.5). D2O absorption is calculated by Eq.(6.6).

Figure 6.9 shows the comparison of the D2O absorption applying Eq. (6.6) and the

calculated chemical alternation (relative SLD change) on re-dry following D2O

conditioning by Eq. (6.5). Despite the lowest water absorption, the highest relative

chemical alternation is observed near the stoichiometric ratio (AV3.4) possibly due to

the more abundant amide complex at stoichiometry after reaction. More D incorporated

into the film during the reaction of D2O with amide complex during D2O conditioning as mentioned above.

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6.3 Conclusions

The structure of AV films was studied with XR and GISAXS, and the interaction of

water in the films was also explored with NR as function of A/V mol ratio. The XR

experiments show that considerable void space exists in as-prepared films regardless of

AV ratio. The void space is minimized near the stoichiometric ratio due to higher

crosslink density at stoichiometry. GISAXS indicates that the void space exists as

molecular-level density fluctuations.

NR experiments on as-prepared, water-conditioned and re-dried films indicate that

water is absorbed in two populations in AV film. One population is dissolved in

polymer matrix and the other physically occupies unrelaxed void space. Most of the

water is physically absorbed and accommodated in void space of film during water-

vapor conditioning with the least amount being absorbed near stoichiometry.

A small SLD change and irreversible thickness change in redried film implies chemical interaction of the film with water. The largest degree of chemical alteration between the as-prepared and the re-dried states is observed near the stoichiometric A/V ratio. The chemical alteration is attributed to reaction of water with the amide complex.

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Chapter 7. Water absorption and transport in bis-silane films

7.1 Introduction

In this study, in situ neutron reflectivity (NR) was performed on bis-sulfur silane and

bis-amino silane films in the presence of saturated D2O vapor. The time-resolved water

ingress was calculated from the scattering length density (SLD) profile. Fourier

transform infrared reflection–absorption (FTIR-RA) was also used to monitor the water

penetration in the silane film. Kinetic models of water transport are proposed for

different bis-silanes.

7.2 Results and Discussion

7.2.1 Neutron reflectivity study of bis-sulfur silane and bis-amino

silane film

7.2.1.1 Water absorption in bis-amino silane film

Figure 7.1(a) shows the neutron reflectivity (NR) of the bis-amino silane in the as-

prepared state, after 0.5, 2.0, 8.4, 11.6, 24 hours of water conditioning and in the re- dried state. Figure 7.1(b) shows the SLD profiles corresponding to the best-fit of the neutron reflectivity curves in Figure 7.1(a). The films are prepared following the procedures described in 3.1.2.1 and cured at 180°C for 60 minutes on Si substrate.

159

As shown in Figure 7.1, during the water conditioning, the reflectivity and the critical

edge of the bis-amino silane film increase rapidly in the first 0.5 hour indicating a rapid

increase of the SLD of the film. After 0.5 hour, the reflectivity curve slowly increases

until equilibrium is reached at 11.6 hours.

The SLD profile shown in Figure 7.1(b) shows that the SLD of the bis-amino silane

also increases abruptly within 0.5 hour of water conditioning with only a small

thickness increase. After 0.5 hour, the SLD increases slowly untill 11.6 hours. The film

reaches saturation after 11.6 hours water conditioning.

By employing Eq.(7.1), the D2O-equivalent volume fraction absorbed by the film, ϕD2O,

can be estimated:

SLD f −D2O − SLD f −dry ϕ D2O = , (7.1) SLDD2O

where SLDf-D2O is the SLD of the film after conditioning, SLDf-dry is the SLD of the as-

-6 -2 prepared dry film and SLDD2O is the neutron SLD of the D2O (6.286 × 10 Å ). The calculated volume fractions of D2O absorbed in bis-amino silane film at different D2O conditioning time are shown in Figure 7.2.

Figure 7.2 shows that 33 vol% of film is filled with water at equilibrium with 30 vol%

D2O absorption observed in the first 0.5 hour of water conditioning. The water

absorption reaches an equilibrium state after 11.6 hours of water conditioning. The

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equilibrium water sorption agrees with an earlier study on equilibrium water sorption of

bis-amino silane and related silane mixtures.18,36

(a)

(b) Figure 7.1. Change of reflectivity (a) and SLD (b) of the bis-amino silane film on exposure to D2O vapor. The lines through the data points of Figure 7.1(a) indicate the best-fit of the reflectivity data using the corresponding model SLD in Figure 7.1(b).

161

Figure 7.2 Volume fraction of D2O in bis-amino silane film as function of conditioning time. Water rapidly occupies 30 vol% in 0.5 hour and then slowly increases to 33 vol% in the next 11.1 hours and equilibrium is reached after 11.6 hours of D2O conditioning.

Figure 7.3 Thickness increase of the bis-amino silane film as the function of the water conditioning time. The thickness increase is slower than the SLD increase as shown in Figure 7.2.

162

Interestingly, in spite of 33 vol% of D2O absorption, the increase of the film thickness

during water conditioning is only 2.3 vol%. Figure 7.3 shows the thickness increase

profile of the bis-amino silane as function of the water conditioning time. The thickness

increase also lags behind the increase of water absorption (Figure 7.2). The thickness

reaches its equilibrium after 11.6 hour D2O conditioning, which is consistent with the

SLD increase. By assuming the film swells uniaxially in thickness direction, which

should be valid given the large surface area to volume ratio and the impermeable

substrate,64 the total increase in the film volume is only 2.3 vol%. The large difference

between the volume of water absorption and swelling of the film suggests that about 30

vol% of D2O is accommodated in the free space in the as-prepared film. The existence

of about 30 vol% free volume was observed in our previous study on AV films.36

The D2O-conditioned bis-amino silane film shows enhanced SLD on the air-side

surface regardless of the conditioning time implying a D-rich region attributed to D2O

18 absorption on the hydrophilic amino groups near the air-side surface. This D2O-rich layer has been repeatedly observed for hydrophilic silane and epoxy films.13,14,18,100

Previous work on the tri-functional analog of bis-aminosilane: γ-aminopropyltriethoxysilane (γ-APS) by Fowkes et al.96 also observed free amino groups located at

the air-side of the γ-APS film surface with angle-resolved XPS.

The thickness of the film decreases after re-dry as compared with the as-prepared state.

The decrease in thickness after re-dry indicates further hydrolysis and condensation of

the bis-amino silane film during and after D2O conditioning.

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7.2.1.2 Water absorption in bis-sulfur silane film

Figure 7.4(a) shows the neutron reflectivity of the bis-sulfur silane in the as-prepared

state, in the D2O-conditioned state at different times and in the re-dried state. Figure

7.4(b) shows the SLD profiles corresponding to the best-fit of the neutron reflectivity

curves in Figure 7.4(a). The films are prepared following the procedures described in

3.1.2.1 and cured at 180°C for 60 minutes on Si substrate.

Compared to bis-amino silane film, little change is observed in the reflectivity after the

24 hours D2O conditioning. An increase in R(q), however, is noticeable from the fact that the first minimum is lower and the tail of the curve is slightly higher relative to the as-prepared state. By fitting the reflectivity curve, the SLD profile is obtained and shown in Figure 7.4(b). The thickness of as-prepared bis-sulfur silane film is 906 Å, which is much less than that of the bis-amino silane film even though the same spin rate and same solution concentration are employed during preparation. The smaller thickness is due to the hydrophobic nature of bis-sulfur silane and thus less bonding between silane and the SiO2 surface.

As with bis-amino silane, the SLD of the bis-sulfur silane increases rapidly in 0.5 hours but the degree of absorption is much smaller. Also, after 0.5 hour, minimal SLD increase is observed during prolonged D2O conditioning. Equilibrium in water

absorption is reached after 6 hours. The more rapid approach to equilibrium of bis-

sulfur film correlates to the lower absorption and more hydrophobic nature of bis-sulfur

silane film. No D2O-enhanced layer is observed on the film surface also due to the

hydrophobic nature of bis-sulfur silane.14

164

(a)

(b)

Figure 7.4. Change of reflectivity (a) and SLD (b) of the bis-sulfur silane film on

exposure to D2O vapor. The lines through the data points of (a) indicate the best-fit of the reflectivity data using the corresponding model SLD in (b). The data show a rapid

uptake of D2O within first 30 minutes but equilibrium is not reached until 6 hours of exposure. The films are prepared following the procedures described in 3.1.2.1 and cured at 180°C for 60 minutes on Si substrate.

165

By employing Eq. (7.1), the D2O absorption after different conditioning time can be calculated as shown in Figure 7.5. The total water absorption at equilibrium is 4.6 vol%, which is substantially less than the 33 vol% observed for bis-amino silane.

Figure 7.5 The D2O absorption in the bis-sulfur silane film as the function of the D2O

conditioning time. The film is saturated with D2O water in 6 hours.

For the bis-sulfur silane, however, the increase in film thickness is much smaller than that of bis-amino silane. The time-resolved trend of thickness increase is hard to predict due to the comparable error bar. However, the equilibrium film total thickness increase can still be calculated. The calculated thickness increase after saturation is only 0.45 ±

0.18 %. By assuming uniaxial swelling in thickness direction, the volume of the film also increases by 0.45 ± 0.18 vol%. Compared with the 4.6 vol% D2O absorption in the

film we can conclude that most D2O is physically accommodated in the free space of

the film instead of swelling the film.

166

Although the interaction of D2O with silane and the swelling of silane film are observed

for both silane films, the water absorption and swelling in different silanes are very

different due to different bridging groups. Bis-sulfur silane is hydrophobic due to its Sx group, on the other hand, bis-amino silane is hydrophilic due to the hydrophilic nature of the secondary amine group (-NH). The difference in hydrophobicity results in the different water absorption behaviors of bis-silane films during water conditioning, which are reflected by the lower water absorption and the presence of the surface D2O-

rich layer.

The sorption isotherms of small molecule penetrants in many glassy polymers are

generally described by the dual-mode sorption model.31,67 This model postulates that

the penetrant exists in two populations: one is dissolved in the polymer matrix

according to Henry’s law, which usually involves the relaxation of the molecular chain

and swelling of the film, and the other of which occupies pre-existed un-relaxed free

volume within the polymer according to the Langmuir isotherm.

The dual-mode sorption isotherm is given as follows:31,67

C ' bp C = C + C = k p + H (7.2) D H D 1+ bp

where C is the solubility of the penetrant in the polymer, CD represents the solubility of normally diffusible species, CH represents the sorption of species in microvoids or free

volume in the polymer matrix, KD is the temperature-dependent Henry’s law constant,

167

b is the temperature-dependent affinity constant of the water for the Langmuir sites (or

-1 so-called hole affinity constant in Pa ), CH’ is the maximum capacity of the polymer for the penetrant in the Langmuir sorption sites, and p is the gas or vapor pressure (Pa).

Since water molecules in the Langmurian mode are absorbed in void space already present in the sample, little activation energy is needed. On the other hand, more activation energy is needed for a molecule to absorb in the Henry’s law mode since the separation of the polymer chains is required. Below Tg, the Langmuir mode dominates the sorption process. At higher temperature, the Henry’s law mode will become more important since this mode involves more energy.

Based on the above NR results, D2O is absorbed in bis-silane films in both Langmuir

and Henry’s modes. However, since the present experiment is conducted at room

temperature, below the Tg of both bis-amino silane and bis-sulfur silane, the sorption is

66 dominated by the Langmuirian mode especially in the initial stage. D2O rapidly

occupies and saturates the free space of the film, which results in the rapid SLD

increase during the initial 0.5 hour of the D2O conditioning with negligible alternation of film network or swelling (thickness increase) of the film. As time going on, Henry’s law mode becomes noticeable. D2O begin to interact with the silane leading to

relaxation and swelling the film. Since the interaction requires more time, this process

is slower than the Langmuir mode, so the swelling of the film lags behind the water

absorption in the film.

168

Unlike bis-amino silane, the Langmuir affinity constant, b, of water in eq. (7.2) is much

smaller for bis-sulfur silane due to the hydrophobicity of bis-sulfur silane. D2O cannot

be easily accommodated in the free space in the bis-sulfur silane film, so the water

absorption in bis-sulfur silane film is much lower.

7.2.2 In situ FTIR-RA study the sorption of water in bis-sulfur silane

and bis-amino silane film

Neutron reflectivity allows us to monitor water absorption and distribution across the

interface as well as the thickness increase of the film. However, the measuring time for

each individual NR run in the low-q range is about 30 minutes, which is relatively long.

The above NR study shows most of D2O is absorbed during the initial 30 minutes of

water conditioning for both silane films. So following the NR study, FTIR-RA was

employed to trace the rapid water ingress in the initial stage and understand chemical

state of D2O in the silane films.

FTIR-RA was first performed on both silane films in the as-prepared state in the range

of the 800 cm-1 to 4000 cm-1. The films are prepared following the procedures described in 3.1.2.1 and cured at 180°C for 60 minutes on Si substrates. The spectra are shown in Figure 7.6. The spectra of the bis-amino and sulfur silanes at high wavenumber region is similar. The 2880 cm-1 peak is assigned to stretching of the

-1 unhydrolyzed ester group (Si-OCH2CH3). The broad peak at about 3200 cm

- 87,91 represents secondary amine or protonated -NH2 . In the low frequency region, the

-1 peaks in the 1530 to 1490 cm range are due to the bending of CH2 and CH3. The peaks

169

in 1070 to 1110 cm-1 range are assigned to asymmetric Si-O stretching in siloxane (Si-

O-Si).87,91 It is noticeable, however, that the siloxane peak of bis-sulfur silane shifts to

higher frequency (1110 cm-1) and is much sharper and stronger compared with that of

bis-amino silane (1070 cm-1), which may associated with the formation of much longer or branched siloxane chains in bis-sulfur.11,15,86,87,90 The peak at 970 cm-1 is assigned to

asymmetric Si-O stretching of unhydrolyzed –Si-O-CH2CH3 bonds. The peak at 890-

870 cm-1 is assigned to the hydrogen-bonded silanol (–SiOH).

2880 970 1490 890 As-prepared Bis-sulfur silane 1110 Bis-amino silane

1070 2880 3200 Reflectance (a.u.) Reflectance 870 1530 970

1000 1500 2000 2500 3000 3500 -1 Wavenumber (cm )

Figure 7.6 The FTIR-RA spectra of the as-prepared bis-amino silane and bis-sulfur silane films. The films are prepared following the procedures described in 3.1.2.1 and cured at 180°C for 60 minutes on Si substrates.

The isolated heavy water (D2O) molecule has three vibration modes that correspond to

the symmetrical (ν1) and asymmetrical (ν3) stretching vibration of O-D bond, and the

bending vibration involving the D-O-D angle (ν2). Because of the intermolecular

reactions, however, the IR spectrum of liquid D2O is more complex than that of the

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101 isolated molecules. The IR spectra of D2O-conditioned bis-amino and bis-sulfur silane in the as-prepared state and at equilibrium after 1210 minutes of D2O

conditioning are shown in Figure 7.7 and Figure 7.8, respectively.

In Figure 7.7 and 7.8, both bis-amino silane and bis-sulfur silane spectra at equilibrium

-1 show a bending vibration involving the D-O-D angle (ν2) at 1340 cm , and the

-1 combination band (ν2 + νL) at 1470 cm with νL being the liberation band of D2O, a

sharp band of Si-OD at 2710 cm-1 with two shoulders of symmetric and asymmetric

-1 -1 102 103,104 stretching of –OD at 2640 cm (ν1) and 2768 cm (ν3) respectively. Finally

-1 -1 the combination band of ν1 + ν2 is in the range of 3620 cm to 3780 cm . The spectra

are qualitatively identical for both bis-amino silane and bis-sulfur silane in the

wavenumber between 800 cm-1 and 4000 cm-1 except for a much sharper siloxane peak

171

observed for bis-sulfur silane, which indicates bis-sulfur silane film has more long or

branched chain siloxane than bis-amino silane film.

Figure 7.8. IR spectra of bis-sulfur silane film in as-prepared state and at equilibrium. The film is prepared following the procedure described in Chapter 3.1.2.1 and cured at 180°C for 60 minutes.

Figure 7.9 and 7.11 show the IR spectra of D-O-D bending vibration peak at (1340 cm-1)

of the bis-amino silane and bis-sulfur silane as a function of time. The reflectance

intensity of the all the D-related peaks increases with the conditioning time for both bis-

sulfur silane and bis-amino silane. By potting normalized intensity increase of the D-O-

-1 D bending vibration peak at 1340 cm against the D2O conditioning time, water

absorption profiles are obtained (Figure 7.10 and Figure 7.12).

The normalized intensity increase of the D-O-D bending peak (1340 cm-1) increases

with D2O conditioning time for both silanes, indicating D2O absorption is increase with

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D2O conditioning time. The equilibrium water absorption is observed after 1120 and

250 minutes for bis-amino silane and bis-sulfur silane film, respectively. Recall that the

time to reach equilibrium was 700 and 360 minutes for bis-amino silane and bis-sulfur

silane, respectively, in the NR study. Although some differences in time to reach

equilibrium exist between the two test methods, the fact that a shorter saturation time is

required for bis-sulfur silane is still valid. The difference in the time to equilibrium

between NR and FTIR experiment may originate from the lower sensitivity of NR to

the small film structure changes as mentioned above.

Figure 7.9. IR spectra of bis-amino silane film in response to D2O conditioning. The films are prepared following the procedure described in Chapter 3.1.2.1 and cured at 180°C for 60 minutes on Si substrates.

173

100

0 Normalized intensity increase (%) increase intensity Normalized 0 400 800 1200

D2O conditioning time (minutes)

Figure 7.10. Normalized relative reflectance intensity increase of 1340 cm-1 IR peak of

bis-amino silane film during D2O conditioning. The equilibrium is reached at around 1150 minute conditioning.

Figure 7.11. IR spectra of bis-sulfur silane film in response to D2O conditioning. The films are prepared following the procedure described in Chapter 3.1.2.1 and cured at 180°C for 60 minutes on Si substrates.

174

It is also interesting to note that the long chain siloxane peak (1110 cm-1) for bis-sulfur

film remains constant during the D2O condition, which suggests the long-chain siloxane

in bis-sulfur silane film is more robust than in bis-amino silane. On the other hand, as

shown in Figure 7.9, the short-chain siloxane peak (1070 cm-1) of bis-amino silane film

first increases before 110 minutes and then decreases with continued D2O exposure,

which indicates further hydrolysis of the short-chain siloxane in bis-amino silane film

during D2O conditioning. This result agrees with the NR result mentioned above on the

decrease of the film thickness of bis-amino silane film after re-dry from D2O conditioning.

100

Normalized intensity increase (%) 0 100 200 300 400

D2O conditioning time (minutes)

Figure 7.12. Normalized relative intensity increase of 1340 cm-1 IR peak of bis-sulfur

silane film during D2O conditioning. Equilibrium D2O absorption is reached at 250 minutes, which is much faster than for bis-amino silane as shown in Figure 7.10.

For thin films with the length and width much greater than thickness, the adsorption

and transport of gas and vapors can be treated as one-dimensional diffusion. The

175

simplest model to describe such a system is Fick’s law which assumes weak interaction

between the penetrant molecules and film matrix:105

∂C ∂ ∂C = − (D ) (7.3) ∂t ∂x ∂x

where C is the concentration of the penetrant at time t and at a distance of x from the film-vapor surface and D is the diffusion coefficient.

For an infinite slab with constant diffusion coefficient and zero initial penetrant concentration in the film, the solution of Eq. (7.3) is given by:97

M 8 ∞ 1 ⎛ D(2m +1) 2 π 2t ⎞ t = 1− exp⎜− ⎟ (7.4) 2 ∑ 2 ⎜ 2 ⎟ M ∞ π m=0 (2m +1) ⎝ L ⎠ where Mt is the amount of gas or vapor absorbed at time t, M∞ is the equilibrium

sorption, m is an integer, and L is the film thickness.

Taking the average thickness of the bis-amino silane film measured from the NR

experiment (1890 Å) and varing the diffusion coefficient, D, the best agreement

between the data and Eq. (7.4) is achieved with D = 3.85 × 10-15 ± 0.10 cm2/s.

Adequate convergence is achieved with m = 8. Figure 7.13 compares the calculated

curve and the experimental data. The calculated curve fits the data, which indicates that

the diffusion process is basically Fickian.

176

A slight deviation from the calculated value from Eq. (7.4) is observed after longer

diffusion time. Since the curve is normalized at t = ∞ (Mt/M∞ =1), the deviation

detected at intermediate diffusion time indicates the deviation from Fick’s law after

prolonged diffusion. The implies some interaction of bis-amino silane with D2O such as relaxation of siloxane chains in Henry’s mode, which agrees with the result of the earlier NR experiment.

1.0

0.8

0.6

0.4 Bis-amino silane Relative intensity increase 0.2 Calculated value using Equation (7.4) -15 2 with D=3.85 x10 cm /s

Normalized intensity increase (a.u.) increase intensity Normalized 0.0 0 400 800 1200 D O conditioning time (minutes) 2

Figure 7.13. Best-fit of the FTIR intensity data for bis-amino silane calculated from Eq. (7.4) with D = 3.85 × 10-15 cm2/s and m = 8.

For bis-sulfur silane, Eq. (7.4) is also used to fit the intensity data by varying diffusion

coefficient D. Taking the average thickness of the bis-sulfur silane film as 920 Å from

the neutron reflectivity measurement, the best-fit is shown in Figure 7.14. The best-fit

diffusion coefficient is 2.30 × 10-15 ± 0.10 cm2/s.

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1.0

0.8

0.6

0.4 Bis-sulfur silane Relative intensity increase Calculated value using Equation (7.4) -15 2 0.2 with D=2.30 x10 cm /s

Normalized intensity increase (a.u.) 0.0 0 100 200 300 400 D O conditioning time (minutes) 2

Figure 7.14. Best-fit of the FTIR intensity increase for bis-sulfur silane calculated from Eq. (7.4) with D = 2.30 × 10-15 cm2/s and m = 8. The derivation of the data from calculated value from Eq. (7.4) indicates the deviation from Fickian behavior in the intermediate stage of diffusion.

The deviation from calculated value of Eq. (7.4) again implies some interaction of bis- sulfur silane film with D2O such as relaxation of siloxane chains in Henry’s mode.

Recall that the largest thickness change was observed during the intermediate stage of diffusion of bis-amino silane from the NR experimental results (Figure 7.3). Both NR and FTIR indicate that most relaxation and swelling occurs in the intermediate stage of diffusion. As we mentioned before, during the initial stage of the diffusion, water physically occupies the free space in the film (Langmuir mode) without swelling of the film. As the conditioning time increases, D2O dissolves in the polymer, relaxes the chains and swells the film.

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7.3 Conclusions

1) Bis-amino silane film absorbs substantially more D2O (33 vol%) than the bis-sulfur silane film (4.6 vol%) after equilibrium. Bis-sulfur silane film reaches equilibrium more rapidly than bis-amino silane film. Two absorption modes are observed in both films,

Henry’s mode, which related to the relaxation of the siloxane network and Langmuir’s mode, which is related to the physical occupation of existing free space. Langmuir absorption mode dominates the total absorption of both films.

2) The thickness increase of both bis-amino silane and bis-sulfur silane film is much smaller than expected based on the volume of D2O absorbed, which implies that the

most of water resides in the free space instead of swelling the film. The Langmuir

absorption mode dominates the D2O absorption process. The increase of the film

thickness lags behind the water absorption indicating the Langmuir absorption is more

rapid than the Henry absorption mode. A D2O-enriched layer is observed at the air-side of hydrophilic bis-amino silane film surface. No D2O-rich layer is detected on hydrophobic bis-sulfur silane surface.

3) FTIR analysis agrees with the NR experiment. Bis-sulfur silane film reaches equilibrium more rapidly than bis-amino silane film. The diffusion of D2O both films

follows Fickian’s behavior with slight deviation at the intermediate stage of diffusion.

The deviation from Fickian kinetics may result from of the relaxation of the molecular

chain and swelling of the films.

179

The above study of the water penetration and absorption in bis-amino silane and bis- sulfur silane films indicates both films are susceptible to water. Water penetrates rapidly into the films. This result indicates the interfacial bonding between silane and substrate must be critical in anti-corrosion performance and also raises concern regarding the long-term susceptibility of the silanes to hydrolytic degradation.

Development of silane and polymer mixture (primer) coating may increase the long- term stability of the films.

180

Chapter 8. General Conclusions and Suggested

Future Work

8.1 General conclusions

The anti-corrosion performance of the silane films depends on chemical structure of

silane and also on the adsorption characteristics of the substrate. The present study

focuses on the chemistry, substrate pretreatment and water absorption properties of the

water-based bis-amino-VTAS mixture system. The following conclusions can be drawn

from the above discussion:

1) 13C NMR spectra study shows that the primary reaction in neat AV3.4 silane mixture

is the exchange of the hydrogen atom on secondary amine group on bis-amino silane

with the acetoxy group on VTAS forming an amide complex and hydrolyzed VTAS.

The primary reaction is followed by a series of condensation and hydrolysis reactions

of bis-amino silane and VTAS and their reaction by-products.

2) In water solution, the amide complex formed in the neat AV mixture decomposes

into bis-amino silane and acetic acid. Bis-amino silane condenses into higher

2 3 condensation products (T and T ). The lowest condensation product of bis-amino

silane (T1) is absent, probably due to the absence of mono-silanol.

3) The time-resolved 29Si NMR reveals that the intermediate condensation products of

bis-amino silane (T2) dominate in the initial stage of bis-amino condensation in water

181

solution due the high hydrolysis rate of bis-amino silane in the initial stage of

hydrolysis. As hydrolysis time increases, the hydrolysis rate decreases, and the higher

condensation products (T3) increase at the cost of the intermediate condensation

products (T2) as well as the “free” hydrolysis product of bis-amino silane. The condensation products of VTAS are absent in the AV water solution even when all the

VTAS is consumed after 1740 minutes hydrolysis, which indicates the VTAS is stabilized in the hydrolyzed state in the water solution.

5) Based on the NMR results and previous observations, a model of the AV solution stabilization is proposed. The weakly acidic silanol group (-OH) on hydrolyzed VTAS forms hydrogen bonds (NH-OH) with the strongly basic secondary amine group (-NH) on bis-amino silane. The OH group of VTAS is thus stabilized by the secondary amine group on bis-amino silane. The condensation between the silanol groups of bis-amino silane and VTAS is retarded and hence the AV solution is stabilized.

6) CRS was cleaned in different pH solutions in order to determine the optimum cleaning pH for AV and the AV-silane-containing primers. The best performance is obtained at mildly alkaline conditions (pH~9.5) near the isoelectric point of CRS. The neutral surface at the IEP point (pH~9.5) promotes absorption and hydrogen bonding of both silanols and secondary amine groups to the CRS surface. This chemistry resulted in the most abundant bonds and a thicker silane film on CRS cleaned at pH ~ 9.5. SEM examination of CRS cleaned after mildly alkaline cleaning shows a fine, dense, spheroidal surface oxide morphology. EIS and DC potentiodynamic measurements

182

confirm the optimum anti-corrosion performance and adhesion for silane-coated CRS at

mild alkaline condition (pH~9.5).

7) The structure of AV films was studied with XR and GISAXS as function of A/V mol

ratio. The XR experiments show that considerable void space exists in as-prepared

films regardless of AV ratio. The void space is minimized near the stoichiometric A/V

ratio due to higher crosslink density at stoichiometry. GISAXS indicates that the void

space exists as molecular-level density fluctuations.

8) NR experiments on as-prepared, water-conditioned and re-dried AV films prove that

water is absorbed in two populations. One population is dissolved in polymer matrix

and the other physically occupies unrelaxed void space. Most of the water is physically

absorbed and accommodated in void space with the least amount being absorbed near

stoichiometry.

9) NR and FTIR-RA were performed on bis-sulfur silane and bis-amino silane films in

the presence of saturated D2O vapor to monitor the water penetration in the silane film.

Bis-amino silane film absorbs substantially more D2O (33 vol%) than the bis-sulfur

silane film (4.6 vol%) after equilibrium. Bis-sulfur silane film reaches equilibrium more

rapidly than bis-amino silane film. Two absorption modes are observed in both films,

Henry’s mode, which related to the relaxation of the siloxane network and Langmuir’s

mode, which is related to the physically occupation of existing free space in the

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siloxane network. The Langmuir absorption mode dominates the total absorption of both films.

10) The thickness increase of both bis-amino silane and bis-sulfur silane film is much smaller than that expected based on the volume of D2O absorbed, which implies that the most of water resides in the free space instead of swelling the film (Langmuir mode). The increase of the film thickness lags behind the water absorption indicating the Langmuir absorption is more rapid than the Henry absorption mode. A D2O- enriched layer is observed at the air-side of hydrophilic bis-amino silane film surface.

No D2O-rich layer is detected on hydrophobic bis-sulfur silane surface.

11) FTIR analysis agrees with the NR experiment. Bis-sulfur silane film reaches its equilibrium more rapidly than bis-amino silane film. The diffusion of D2O in bis-amino silane and bis-sulfur silane films follows Fickian behavior with slight deviation from

Fickian’s behavior in the intermediate stage of diffusion. The deviation from Fickian behavior may result from of the relaxation of the molecular chains (Henry mode) and swelling of the films.

8.2 Impacts of present research on development of silane-enhanced superprimer corrosion protection coatings (SERDP)

The goal of SERDP project is to develop a water-borne surperprimer consisting of silane, organic resin and nano-particle filler with no sacrifice of corrosion performance compared to chromate-containing coatings. Previous research of Van Ooij et

184

al.1,9,12,16,17,106 suggests that silane is the critical component in the anti-corrosion primer coatings. The water-based AV system shows excellent stability in water, rapid hydrolysis as well as good compatibility with various polymer systems. Previous work of the SERDP group12 reveals that the AV-containing primer offers good corrosion

protection on metal substrates as well as low VOCs, which is promising for SERDP

applications.

The NMR study confirms that the primary reaction in the neat AV system is the

exchange of the hydrogen atom on the secondary amine group of the bis-amino silane

and the acetoxy group on VTAS. The stoichiometric ration of the primary reaction

between A and V is 3. The results explain the optimum anti-corrosion performance and

solution stability at near stoichiometric ratio (A/V=3.4) observed before and provide

guidance on optimization of the AV silane formulation (AV ratios). The 29Si NMR study also clarified that the excellent stability of AV mixture water solution origins from the preferred-bonding between the secondary amine group of the bis-amino silane with the silanol groups in the solution and consequently the retarding of the condensation between silanol groups in the solution. The stablization mechanism can be applied to other amine-related water-based silane systems to obtain stable water solution.

Surface chemistry of metal substrate serves an important role on the bonding and hence

anti-corrosion resistance of the silane and primer coatings. The cleaning study of CRS

clarifies the relationship between the surface chemistry, pretreatment and the anti-

185

corrosion performance of the water-based AV silane and AV silane-containing primer

coatings. This work also provides guidance on the CRS surface pretreatment process

before AV silane and AV silane-containing primer coating. The mechanism of

preferred silane absorption at IEP of the substrate can be applied to other mineral or

metal substrates and provide guidance on the metal surface pretreatment before silane

coating.

Both the study of water barrier properties of AV silane and the investigation of

penetration kinetic in bis-amino silane and bis-sulfur silane show rapid absorption of

water in the films including those that provide excellent anticorrosion protection. Water

is absorbed with minimal change in film thickness. The absorbed water is

accommodated in the free volume present in the film. These findings provide new

insights to guide the development of optimum films and identify potential

vulnerabilities of the new technology. Water absorption in this manor generates

minimal stress in the film and seems not to be detrimental to corrosion performance. In

fact water can serve to leach pigments to provide scratch protection. The hydrophilicity

of the films, however, does raise concern regarding the long-term susceptibility of the

silanes to hydrolytic degradation.

8.3 Suggested future work

Previous research and applications1,9,12,15-17 of silanes and silane-containing primer suggests that the primer of AV silane, bis-sulfur silane and bis-amino silane silane offer excellent anti-corrosion performance. However, the study on AV silane, bis-amino

186

silane and bis-sulfur silane systems shows that these silanes are susceptible to water

penetration. So the enhanced anti-corrosion resistance of primer must be associated

with one of the following mechanisms: 1) the reaction between the polymer and silane

in the silane-containing primer coating; 2) the adhesion and bonding layer between

silane and metal substrate; and 3) inhibition of ion or oxygen transport of the silane or

silane containing primer coating.

Based on the study, further investigation should be done in the following area:

1) Investigation water penetration mechanism in superprimer (silane and silane mixture)

including study the reaction between silane and polymer in primer system .The same

NMR and NR methods can be applied.

2) The present research shows that the reaches the silane-substrate interface rapidly, so

the enhanced anti-corrosion properties of the silane film must be related adhesion and

bonding between silane and metal substrate. Earlier research,20,107 however, shows that

the covalent bonding between silane and mineral surface is susceptible for the

rehydrolysis after prolonged exposure to water, a potential vulnerability of the silane

system. Furth investigation of long-term stability is therefore required.

- + 3) Water is a common carrier for corrosive ion or oxygen species (Cl , O2, Na , etc.).

- Corrosion, however, does not progress without mineral ion or oxygen (e.g. Cl and O2).

Ionic attack (Cl-, Na+ etc.) is the most common cause for metal corrosion. Al and its alloys suffer severe pitting corrosion, as the passive Al oxide film is broken down by

187

the local attack of chloride ions. The enhanced anti-corrosion performance may relate to the ion-exclusion properties of the silane film. So it is important to investigate the barrier properties of silane and silane-containing primer films to ion penetration. NR

- experiments can be designed to monitor the penetration of mineral ions (e.g. Cl ) into silane films.

4) The study of the bis-amino silane and bis-sulfur silane films indicates that water is absorbed following dual -mode sorption. Water penetrates rapidly films following

Fick’s law. A previous study, however, revealed that the bis-amino and bis-sulfur silane mixture (AS) system provides better anti-corrosion performance than individual neat silanes.11,15,38,39 The enhanced anti-corrosion properties of the AS mixture may result from a chemical reaction between the bis-amino silane and bis-sulfur silane. Further investigation on reaction and water penetration kinetics of the AS mixture system should be conducted. The same characterization methods of FTIR, NR and NMR can be applied.

A previous study on nano-particle-loaded AV silane coating indicates improvement on the anti-corrosion performance with nano-particle filler.108 Further work needs to be carried on to understand the role of nanoparticle fillers on the anti-corrosion performance.

188

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