Control of Corrosion on SS316L Using Surface Initiated Polymers

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Alexander Rupprecht

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CONTROL OF CORROSION ON SS316L USING SURFACE INITIATED

POLYMERS

A Dissertation

Submitted to the Bayer School of Natural and Environmental Sciences

Duquesne University

In partial fulfillment of the requirements for

the degree of Doctor of Philosophy

Alexander J. Rupprecht

December 2020

Alexander J. Rupprecht

2020

CONTROL OF CORROSION ON SS316L USING SURFACE INITIATED

POLYMERS

Alexander J. Rupprecht

Approved July 24, 2020

______Professor Ellen S. Gawalt, Ph.D. Professor Jeffrey Evanseck, Ph.D. Professor of Chemistry Professor of Chemistry (Committee Chair) (Committee Member)

______Professor Jennifer Aitken, Ph.D. Professor Wilson Meng, Ph.D. Professor of Chemistry Professor of Pharmaceutics (Committee Member) (Outside Reader)

______Dean Philip Reeder Professor Ellen S Gawalt, Ph.D Dean, Bayer School of the Natural and Chair, Department of Chemistry and Environmental Sciences Biochemistry Professor of Chemistry

iii ABSTRACT

CONTROL OF CORROSION ON SS316L USING SURFACE INITIATED

POLYMERS

Alexander Joseph Rupprecht

December 2020

Dissertation supervised by Professor Ellen S. Gawalt, Ph.D.

Pitting corrosion is arguably one of the most destructive and dangerous forms of corrosion, resulting in damage to structures, the environment, and public health.

The United States spent an estimated 575 billion USD repairing corrosion on architectural structures in 2016; moreover, estimated financial repercussions worldwide were 2.4 trillion dollars in 2014. In addition, damage caused to structures such as bridges, pipelines, and boats, corrosion also has a profound effect on the biomedical community. Implanted metallic devices (i.e., vascular stents and artificial joints) are prone to pitting corrosion caused by aggressive ions present in extracellular fluid.

To provide a corrosion-resistant surface on SS316L, films of poly(styrene), poly(methyl acrylate) and poly(methyl methacrylate) were formed using surface-initiated atom transfer radical polymerization (SI-ATRP). The resulting hydrophobic polymer films

iv had a fractional coverage of up to 99% and protection efficiencies of up to 99.6%, indicating that the modifications had excellent coverage of the surface and were capable of resisting corrosion, respectively. Furthermore, in additional cyclic voltammetry experiments, little change was observed in the voltammograms after repetitive cycling and immersion in 0.1M sodium chloride, which indicates that the films are stable in an electrochemical environment and to prolonged immersion.

Additionally, films of poly(oligo(ethylene glycol)acrylate) and poly(oligo(ethylene glycol)methacrylate) were formed on SS316L in an attempt to create a surface modification that was both cell and corrosion-resistant. However, cyclic voltammetry and electrochemical impedance spectroscopy revealed that the PEGylated SI-polymers were not capable of inhibiting corrosion to a high degree. To improve the corrosion resistance of the polymer films, an SI-copolymer approach was devised. A film a of poly(styrene) was formed using SI-ATRP, then chain extended with PEGylated acrylate and methacrylate. The resulting films were hydrophilic, indicating that the PEG chains were presenting at the interface, which is needed for cell resistant properties typically observed with in PEGylated films.

The SI polymerization of epoxides provides an attractive opportunity to impart both antifouling and corrosion-resistant properties to a surface. Several methodologies were employed to form SI-poly(ethers), including anionic ring-opening polymerization (ROP) and acid-catalyzed ROP. Films of poly(propylene oxide) were synthesized using anionic

ROP methodology; however, the result was not able to be replicated. To mitigate the side reactions that could be preventing repeatable polymerization, Lewis and Bronsted-Lowery acid-catalyzed approaches were examined. However, only small oligomers of poly(ethers)

v were able to be formed on the surface. Overall, several novel polymer films were formed on the native oxide surface of SS316L, which were able to provide improved corrosion resistance, while be further modified by chain extension.

vi DEDICATION

I would like to dedicate this to my parents, grandparents, and siblings for their love and encouragement in pursuit of my dreams.

vii ACKNOWLEDGEMENT

Frist and foremost, I would like to thank Dr. Ellen S. Gawalt, for her mentorship over the past three years. From changeling me to solve problems to listening to my crazy ideas, she has made me a better chemist, scientist, and researcher. In particular, I would like to thank her for accepting me into her research group when I had nowhere else to go.

Without her support, I would have never been able to accomplish my dissertation research.

Additionally, I would like to thank the members of my dissertation committee, Drs.

Jennifer Aitken, Jeffrey Evanseck, Wilson Meng, for their helpful conversations, careful guidance over the years, and support of my academic and career goals.

I would like to thank Dr. Gabby Pros, Dr. Ashly Blystone, Dr. Nina Reger, and

Mike Novak, who served as not only as mentors of mine but also as sounding boards for ideas, and most importantly friends. I would also like to thank Sean Fisher, Tell Lovelace, and Jen Armen, both friends and colleges that have always kept the lab an interesting environment. I have had the privilege to mentor some truly amazing undergraduate students, namely; Margret Marz, Megan Wasson, Emily Allego, Ethan Taylor, and Peter

Demjanenko.

I would like to thank family for the love and support over the past five years. Both my parents: Joe and Lynnette Rupprecht and my siblings: Rachael, Lauren, and Elliot

Rupprecht, for coming to visit me Pittsburgh. I would also like to thanks my grandparents, for their support over the years, whether it be helping me move and get settled in to my first apartment or helping me during middle school.

viii Finally, I would like to thank the Bayer School of the Natural and Environmental

Sciences and the Department of Chemistry and Biochemistry at Duquesne University for allowing me to complete my Ph.D.

ix TABLE OF CONTENTS

Page

Abstract ...... iv

Dedication ...... vii

Acknowledgement ...... viii

List of Tables ...... xviii

List of Figures ...... xx

List of Abbreviations ...... xxix

Chapter 1: Overview of Pitting Corrosion ...... 1

1.1 Introduction ...... 1

1.2 Pitting Corrosion of Stainless Steel ...... 3

1.2.1 Mechanism of Pitting Corrosion ...... 5

1.2.2 Stages of Pitting Corrosion ...... 5

1.2.1.1 Stage 1: Passive Film Breakdown ...... 6

1.2.1.2 Stage 2: Pit Initiation/Nucleation ...... 9

1.2.1.3 Stage 3: Metastable Pitting ...... 9

1.2.1.4 Stage 4: Stable Pitting ...... 10

1.3 Characteristics of Corrosion Resistant Coatings ...... 10

1.4 Conclusion ...... 12

1.5 Referances ...... 12

Chapter 2: Self-Assembled Monolayers and Surface Initiated Polymerizations ...... 22

2.1. Introduction ...... 22

2.1.1 Functionalization of Surfaces with SAMs ...... 22

x 2.1.2 Modification of Organic Thin Films ...... 23

2.2 Functionalization of Surfaces with Polymeric Films ...... 24

2.2.1 Grafting To ...... 24

2.2.2 Grafting From ...... 25

2.3 Techniques used to Form Surface Initiated (SI) Polymers ...... 26

2.3.1 Free Radical Polymerization (FRP) ...... 26

2.3.2 Controlled Radical Polymerization ...... 27

2.3.2.1 Atom Transfer Radical Polymerization (ATRP) ...... 28

2.3.2.2 Reversible Addition Fragmentation Chain-Transfer (RAFT) polymerization

...... 30

2.3.2.3 Nitroxide Mediated Polymerization (NMP) ...... 32

2.3.3 Ring-Opening Polymerization ...... 34

2.3.3.1 Ring-Opening Polymerization of Epoxides ...... 34

2.3.3.2 Ring-Opening Metathesis Polymerization ...... 36

2.4 Conclusion ...... 37

2.5 Referances ...... 38

Chapter 3: Functionalization of SS316L with Corrosion Resistant Polymer Films .52

3.1 Introduction ...... 52

3.2 Materials ...... 53

3.3 Methods ...... 54

3.3.1 Preparation of SS316L coupons ...... 54

3.3.2 Functionalization of SS316L with ATRP Initiators ...... 54

3.3.2.1 SAM Formation of 11-Bromoisbutryrylundecyl Phosphonic Acid ...... 54

xi 3.3.2.2 Formation of Mixed SAMs of 11-Bromoisbutryrylundecyl Phosphonic Acid

and Decyl phosphonic acid ...... 56

3.3.2.3 SAM Formation of 11-Hydroxyundecyl Phosphonic Acid ...... 57

3.3.2.4 Solution Immobilization of Bromoisobutyryl Bromide on Hydroxyl Terminated

SAMs ...... 58

3.3.2.5 Gas-phase Immobilization of Bromoisobutyryl Bromide on Hydroxyl

Terminated SAMs Under Passive Vacuum ...... 59

3.3.2.6 Gas-phase Immobilization of Bromoisobutyryl Bromide on Hydroxyl

Terminated SAMs Under Active Vacuum ...... 59

3.3.3 Functionalization of SS316L with Polymer Films ...... 61

3.3.3.1 SI-polymerization of Styrene ...... 61

3.3.3.2 SI-polymerization of Methyl Acrylate ...... 61

2.3.3.3 SI-polymerization of Methyl Methacrylate ...... 62

3.3.4 Surface Characterization ...... 62

3.3.4.1 Diffuse Reflectance Infrared Fourier Transform Spectroscopy ...... 62

3.3.4.2 Contact angle ...... 63

3.3.4.3 Atomic Force Microscopy ...... 63

3.3.4.4 Cyclic voltammetry ...... 64

3.3.4.5 Electrochemical impedance spectroscopy ...... 64

3.3.4.6 Cytotoxicity and Cell Adhesion Studies ...... 64

3.3.7 Statistics ...... 66

3.4 Results ...... 66

3.4.1 Formation of SAMs of BiBUPA ...... 66

xii 3.4.2 Formation of Mixed SAMs of DPA and BiBUPA ...... 67

2.4.3 Immobilization of Bromoisobutyryl Bromide on Hydroxyl Terminated SAMs ..69

3.4.3.1 Formation of HUPA SAMs ...... 69

3.4.3.2 Solution Immobilization of Bromoisobutyryl Bromide on Hydroxyl Terminated

SAMs ...... 70

3.4.3.3 Gas-phase Immobilization of Bromoisobutyryl Bromide on Hydroxyl

Terminated SAMs ...... 71

3.4.4. Surface Initiated Atom Transfer Radical Polymerization of Styrene, Methyl

Acrylate, and Methyl Methacrylate ...... 73

3.4.5 Contact Angle Analysis ...... 75

3.4.6 Atomic Force Microscopy ...... 77

3.4.7. Electrochemical Characterization ...... 79

3.4.7.1. Electrochemical Impedance Spectroscopy ...... 79

3.4.7.2 Cyclic Voltammetry ...... 82

3.4.8. Cytotoxicity and Normalized Cell Counts of NIH 3T3 Fibroblasts ...... 86

3.5 Discussion ...... 89

3.6 Conclusions ...... 96

3.7 References ...... 97

Chapter 4: Antifouling and Corrosion Resistant Surface Modification ...... 102

4.1 Introduction ...... 102

4.2 Equipment and Materials ...... 103

4.2.1 Equipment ...... 103

4.2.2 Materials ...... 104

xiii 4.3 Methods ...... 105

4.3.1 Preparation of SS316L coupons ...... 105

4.3.2 Functional of SS316L with ATRP Initiators ...... 105

4.3.2.1 SAM Formation of 11-Hydroxyundecyl Phosphonic Acid ...... 105

4.3.2.2 Gas-phase Immobilization of Bromoisobutyryl Bromide on Hydroxyl

Terminated SAMs ...... 106

4.3.3 Functionalization of SS316L with Polymer Films ...... 107

4.3.3.1 SI-polymerization of oligo(ethylene glycol)methyl ether acrylate ...... 107

4.3.3.2 SI-polymerization oligo(ethylene glycol)methyl ether methacrylate ...... 108

4.3.3.3 SI-polymerization styrene ...... 108

4.3.3.4 SI-block polymerization styrene and oligo(ethylene glycol) methyl ether

acrylate ...... 109

4.3.3.5 SI-block polymerization styrene and oligo(ethylene glycol) methyl ether

methacrylate ...... 110

4.3.4 Surface Characterization ...... 110

4.3.4.1 Diffuse Reflectance Infrared Fourier Transform Spectroscopy ...... 110

4.3.4.2 Contact angle ...... 111

4.3.4.3 Cyclic voltammetry ...... 111

4.3.4.4 Electrochemical impedance spectroscopy ...... 111

4.3.4.5 Cytotoxicity and Cell Adhesion Studies ...... 112

4.3.5 Statistics ...... 113

4.4 Results ...... 113

4.4.1 Formation of HUPA SAMs ...... 113

xiv 4.4.2 Immobilization of Bromoisobutryl bromide ...... 114

3.4.3 Formation and Characterization of Homopolymer films of OEGA and OEGMA

...... 115

3.4.3.1 Synthesis of Polymer Films ...... 115

Cyclic voltammetry ...... 117

4.4.3.3 Electrochemical Impedance Spectroscopy ...... 118

4.4.3.4 Cell Resistance Studies ...... 120

4.4.4 Formation of Poly(styrene) and Poly(styrene-co-OEG(M)A) Copolymer films 122

4.4.5 Contact Angle Analysis ...... 124

4.5 Discussion ...... 126

4.6 Conclusions ...... 130

4.7 References ...... 132

Chapter 5: A Novel Approach to a Poly(ether) Modified Surface ...... 138

5.1 Introduction ...... 138

5.2 Equipment and Materials ...... 141

5.2.1 Equipment ...... 141

5.2.2 Materials ...... 141

5.3 Methods ...... 142

5.3.1 Preparation of SS316L coupons ...... 142

5.3.2. SAM Formation of 11-Hydroxyundecyl Phosphonic Acid ...... 142

5.3.3 Functionalization of SS316L with Polymer Films ...... 143

5.3.3.1 SI-Anionic Ring-Opening Polymerization of Epoxides ...... 143

xv 5.3.3.2 SI- Toluenesulfonic Acid-Catalyzed Ring-Opening Polymerization of Epoxides

...... 144

5.3.3.3 SI- Lewis Acid-Catalyzed Ring-Opening Polymerization of Epoxides ...... 145

5.3.3.4 SI- Gas-Phase Lewis Acid-Catalyzed Ring-Opening Polymerization of

Epoxides ...... 146

5.3.4 Surface Characterization ...... 148

5.3.4.1 Diffuse Reflectance Infrared Fourier Transform Spectroscopy ...... 148

5.3.4.2 Contact angle ...... 148

5.3.4.3 Cyclic voltammetry ...... 149

5.4 Results ...... 149

5.4.1 Formation of HUPA SAMs ...... 149

5.4.2 Anionic Ring-opening polymerization ...... 150

5.4.3 Acid-Catalyzed Ring-Opening Polymerization ...... 153

5.4.4 SI-Polymerization of Gaseous Epoxide Monomers ...... 154

5.4.5 Contact Angle Analysis ...... 155

5.4.6 Cyclic Voltammetry ...... 157

5.5 Discussion ...... 158

5.6 Conclusions ...... 164

5.7 References ...... 165

Chapter 6: Conclusions ...... 172

2.1. Conclusions ...... 172

6.1.1 Functionalization of SS316L with Corrosion-Resistant Polymer Films...... 172

6.1.2 Antifouling and Corrosion Resistant Surface Modification ...... 173

xvi 6.1.3 A Novel Approach to a Poly(ether) Modified Surface ...... 174

6.2 Impact of Projects ...... 175

6.3 Future Work ...... 177

6.4 References ...... 179

xvii LIST OF TABLES

Page

Table 3.1: Solution and Aerosol deposition techniques used for the formation of BiBUPA

SAMs ...... 56

Table 3.2: Solution and Aerosol deposition conditions used for the formation of mixed

BiBUPA/DPA SAMs ...... 57

Table 3.3: Solution and Aerosol deposition techniques used for the formation of BiBUPA

SAMs ...... 59

Table 3.4: Summary of reaction conditions employed with the gas phase immobilization of BiBB ...... 60

Table 3.5 Average RMS roughness of modifications calculated from AFM micrographs

(n=9) ...... 79

Table 3.6. Charge transfer resistances and protection efficiencies for Bare and Modified

SS 316L ...... 81

Table 3.7. Fractional coverages for Bare and Modified SS 316L calculated from cyclic voltammetry ...... 84

Table 4.1 Charge transfer resistance values and protection efficiency from electrochemical impedance spectroscopy ...... 119

Table 5.1 Reaction conditions attempted for SI-Anionic Ring-Opening Polymerization of

Epoxides ...... 144

Table 5.2 Reaction conditions attempted SI Toluenesulfonic Acid-Catalyzed Ring-

Opening Polymerization of Epoxides ...... 145

xviii Table 5.3 Reaction conditions attempted for SI-Lewis Acid-catalyzed ring-opening polymerization of epoxides ...... 146

Table 5.4 Reaction conditions attempted for SI-Anionic Ring-Opening Polymerization of

Epoxides ...... 147

Table 5.5 Fractional coverages calculated from cyclic voltammetry ...... 158

xix LIST OF FIGURES

Page

Figure 1.1 Molecular structure of the passive oxide film on SS316L ...... 3

Figure 1.2. Proposed mechanism of pitting corrosion of SS316L in the presence of chloride ions. The cathodic and anodic half reactions are shown below the figure...... 4

Figure 1.3. Different stages of pitting corrosion a) intact passive oxide film, b) the breakdown of the passive oxide film caused by a flaw or damage, c) pit initiation d) metastable pitting, and e) stable petting with an iron hydroxide cap...... 6

Figure 1.4 Visualization of penetration mechanism for passive oxide breakdown; a) penetration of aggressive ions (chloride is used as an example) on pristine passive oxide layer b) deformation and corrosion under the passive oxide film, and c) rupture of the passive oxide film resulting in an initiated pit...... 7

Figure 1.5 Conceptualization of absorption and thinning mechanism for passive oxide breakdown; a) pristine passive oxide layer, b) absorption of aggressive ions (chloride is used as an example), and c) dissolution of metal cations...... 8

Figure 1.6 a) Dissolution of metal sulfides (MeS) and b) resulting in the formation of a pit.

...... 8

Figure 2.1 Structure of a) phosphonic acid SAMs with commercially available tail groups with tridentate coordination to the surface and b) carboxylic acid SAMs with commercially available tail groups with bidentate binding to the surface...... 23

Figure 2.2 Functionalization of carboxylic and phosphonic acid self-assembled monolayers peptides, small molecules, polymers, and proteins ...... 24

Figure 2.3 a) Grafting to vs. b) grafting from surface modification with polymers...... 25

xx Scheme 2.1 Proposed mechanism of FRP ...... 26

Scheme 2.2 Examples of SI-FRP using azo initiators a) from surfaces of nitinol, nickel, and stainless steel 316L and b) from the surface silica nanoparticles...... 27

Scheme 2.3 Proposed mechanism of ATRP in the presence of a copper catalyst and a chemical reducing agent. Pn-X denotes either an alkyl halide initiator or dormant, halide capped chain. Lm is a generic coordinating ligand with a denticity of m. RA and RAox represent reducing agent and oxidized reducing agent, respectively...... 29

Scheme 2.4 Examples of systems where ATRP has been using ...... 30

Scheme 2.5 Proposed mechanism of RAFT polymerization ...... 31

Scheme 2.6 Examples of SI-RAFT systems ...... 32

Scheme 2.7 Proposed mechanism of NMP ...... 33

Scheme 2.8 Examples of poly(styrene) formed using SI-NMP ...... 34

Scheme 2.9 a) SI ring-opening polymerization of glycidol and b) the ring-opening polymerization of various epoxides ...... 35

Scheme 2.10 The proposed mechanism of ROMP in the presence of a generic Ru ROPM catalyst and a generic olefin...... 36

Scheme 2.11 Surface initiated romp of cyclic alkenes in the presence of Grubbs Catalyst.

The first step involves the immobilization of a ruthenium carbene complex which forms the polymer chain when exposed to cyclic monomers ...... 37

Scheme 3.1 Functionalization of SS316L with polymer films of styrene, methyl acrylate, and methyl methacrylate ...... 52

Scheme 3.2 Functionalization of SS316L with SAMS of BiBUPA ...... 55

Scheme 3.3 Functionalization of SS316L with mixed SAMS of BiBUPA and DPA ...... 57

xxi Scheme 3.4 Functionalization of SS316L with SAMs of HUPA and subsequent immobilization of BiBB ...... 58

Figure 3.2. Diagram of the gas phase reaction setup for the immobilization of bromoisobutyryl bromide...... 60

Scheme 3.5 Functionalization of SS316L with polymer films of styrene, methyl acrylate, and methyl methacrylate ...... 61

Figure 3.3. Live/Dead stains used for mammalian cells...... 65

Figure 3.3 DRIFT Spectra of the a) methylene stretching region and b) ester stretching region of monolayers BiBUPA formed by aerosol deposition before rinsing (red trace), after rinsing with THF (green trace) ...... 67

Figure 3.4 DRIFT Spectra of the a) methylene stretching region and b) ester stretching region of mixed monolayers BiBUPA and DPA (9:1) formed by aerosol deposition after sonication with THF (green trace) ...... 68

Figure 3.5 DRIFT Spectra of the a) methylene stretching region and b) ester stretching region of mixed monolayers BiBUPA and DPA (9:1) formed by aerosol deposition after sonication with THF (green trace) ...... 69

Figure 3.6 DRIFT Spectra of the a) methylene stretching region and b) head group binding region of monolayers HUPA formed by aerosol deposition after sonication with THF. .70

Figure 3.7 DRIFT Spectra the a) methylene stretching region and b) carbonyl stretching region of monolayers HUPA (green) and solution immobilized bromoisobutyryl bromide on HUPA monolayers (red). Labels in green correspond to the spectrum of HUPA monolayers, where the labels in red correspond to the spectrum of immobilized bromoisobutyryl bromide ...... 70

xxii Figure 3.8 DRIFT Spectra of the a) methylene stretching region and b) carbonyl stretching region of monolayers HUPA (grey dashed trace) and gas-phase immobilized bromoisobutyryl bromide on HUPA monolayers (black trace). Labels in green correspond to the spectrum of HUPA monolayers, where the labels in red correspond to the spectrum of immobilized bromoisobutyryl bromide ...... 72

Figure 3.9 DRIFT Spectra of polymer films of a) styrene (black trace) b) methyl acrylate

(black trace) and c) methyl methacrylate (black trace). In each spectrum, the spectrum of bromoisobutyryl bromide immobilized on HUPA SAMs (grey trace) is provided as a comparison for before polymerization. All labels correspond to the spectra of the polymer films...... 74

Figure 3.10 Contact angle data for surface modifications. HUPA= HUPA modified surface, BiBB= Bromoisbutyl bromide immobilized on HUPA SAMs, PS = Polystyrene modified surface, PMMA = Polymethylmethacrylate modified surface, and

PMA=Polymethacrylate modified surface ...... 77

Figure 3.11 Atomic Force Micrographs of (a) native SS316L, (b) HUPA SAMs, (c) BiBB immobilized on HUPA SAMs, (d)SI-ATRP poly(styrene), (e) SI-ATRP poly(methyl methacrylate) and, (f) SI-ATRP poly(methyl acrylate)...... 78

Figure 3.12. Nyquist impedance spectra for poly(styrene) (grey dashed trace), poly(methyl methacrylate) (grey dash-dotted trace), poly(methyl methacrylate) (light grey dashed trace), BiBB immobilized on HUPA SAMs (black dash-dotted trace), HUPA SAMs (black dashed trace)and the native unmodified oxide surface (black trace) ...... 80

xxiii Figure 3.13. The equivalent circuit for electrochemical impedance spectra. Rsoln is solution resistance, and Rct and CPE are the charge transfer resistance and capacitance for the electrode, respectively...... 81

Figure 3.14 a) Voltammograms of the native oxide surface (black trace), HUPA SAM

(solid grey trace), and the BiBB immobilized (dashed grey trace). b) Voltammograms the native oxide surface (black trace), SI-poly(styrene) (solid grey trace), the SI-poly(methyl methacrylate) film (dashed grey trace), and the SI-poly(methyl acrylate) (dash-dotted grey trace) films...... 83

Figure 3.15 Multiple scan voltammograms of a) the native oxide surface, b) the SI- poly(styrene) film, c) the SI-poly(methyl methacrylate) film, and d) the SI-poly(methyl acrylate) film. In all sets of voltammograms, the red trace represents the first scan, and the blue trace represents the fifteenth scan. The gray traces represent the second to the fourteenth scans ...... 85

Figure 3.16 Time dependent voltammograms of a) the native oxide surface, b) the SI- poly(styrene) film, c) the SI-poly(methyl methacrylate) film, and d) the SI-poly(methyl acrylate) film. In all sets of voltammograms after immersion in 0.1M NaCl for 24 hours(orange trace), 48 hours(blue trace), 72 hours(yellow trace), 96 hours(green trace), and 120 hours(gray trace)...... 86

Figure 3.17 Live/Dead fluoresces microscopy images of modified and unmodified coupons. Red indicates a dead cell green indicates a lives cell ...... 87

xxiv each measurement with n=15. “*” indicates the statistical difference between control (Bare surface) and the modified surface with p< 0.05...... 88

Scheme 3.6 Functionalization of SS316L with polymer films of styrene, methyl acrylate, and methyl methacrylate ...... 89

Figure 3.19. a) A proposed mechanism of illustrating the formation of the metal-oxygen- phosphorus linkage to caused by the dehydration. b) The steric interactions caused by the ester terminus of the SAM...... 90

Figure 4.1 Formation of biofilms on bare metallic surfaces...... 102

Scheme 4.1 Formation of HUPA monolayers, immobilization of bromoisobutryl bromide, and the formation of the SI-ATRP homo-polymer films of poly(styrene) and poly(OEGA), and poly(OEGMA) and block copolymer films poly(styrene-co-OEGA) and poly(styrene- co-OEGMA)...... 106

Figure 4.2. Diagram of the gas phase reaction setup for the immobilization of bromoisobutyryl bromide in Section 4.3.2.2 ...... 107

Figure 4.3 DRIFT Spectra of the a) methylene stretching region and b) head group binding region of monolayers HUPA formed by aerosol deposition after sonication with THF. The spectrum of and labels for HUPA SAM is shown in black, and the spectrum of and labels for solid HUPA is shown in grey...... 114

Figure 4.4 DRIFT Spectra of the a) methylene stretching region and b) carbonyl stretching region of monolayers HUPA (dashed grey trace) and gas-phase immobilized bromoisobutyryl bromide on HUPA monolayers (black trace). Labels in green correspond to the spectrum of HUPA monolayers, where the labels in red correspond to the spectrum of immobilized bromoisobutyryl bromide ...... 115

xxv Figure 4.5 DRIFT Spectra of polymer films of a) OEGA and b) OEGMA. In each set of spectra, the spectrum of bromoisobutyryl bromide immobilized on HUPA SAMs (dashed grey trace) is provided as a comparison for before polymerization. All labels correspond to the spectra of the polymer films...... 116

Figure 4.6 Cyclic Voltammograms of a) native oxide (black trace), poly(OEGA) (dashed grey trace), and poly(OEGMA) (dash-dotted grey trace) ...... 117

Figure 4.7 Nyquist impedance spectra of a) native oxide, (black trace), poly(OEGA)

(dashed grey trace), and poly(OEGMA) (dash-dotted grey trace) ...... 119

Figure 4.8 Live/Dead fluoresces microscopy images of modified and unmodified coupons.

Red indicates a dead cell green indicates a lives cell ...... 120

Figure 4.9 a) Normalized cell counts and b) normalized cell viability for homopolymer and copolymer films from on SS316L. All counts are normalized to unmodified SS316.

“*” indicates a statistical difference from the native oxide surface...... 121

Figure 4.10 DRIFT Spectra of copolymer films of a) poly(styrene-co-OEGA) (red trace) and b) poly(styrene-co-OEGMA) (teal trace). In each spectrum, the spectrum styrene

(orange trace) is provided as a comparison for before chain extension. All labels correspond to the spectra of the copolymer films...... 123

Figure 4.11 Contact angle of water droplets defined as a) hydrophilic surface (θ < 90°) and b) hydrophobic surface (θ ≥ 90°)...... 124

Figure 4.12 Contact angle data (n=9) for surface modifications. BiBB= Bromoisobutryl bromide immobilized on HUPA SAMs, P(OEGA) = Poly(oligo(ethylene glycol)acrylate,

P(OEGMA) = Poly(oligo(ethylene glycol)methacrylate, P(S) = Poly(styrene), P(S—co-

xxvi OEGA) = Poly(styrene-co-oligo(ethylene glycol)acrylate and P(S-co-OEGMA) =

Poly(styrene-co-oligo(ethylene glycol)methacrylate modified surfaces...... 125

Figure 4.13 Collapse of solvent inflated polymer films (a) forming either b) an ordered collapse of the polymer films resulting in poly(styrene) block being “capped with” poly(OEGA) (or poly(OEGMA)) block or b) a disordered collapse of the polymer films resulting in poly(styrene) block and poly(OEGA)(or poly(OEGMA)) blocks presenting at the interface ...... 130

Figure 4.14 Polymer films that were synthesized as a part of this project. Poly(styrene) films (highlighted in purple) were forming in part of a previous project...... 131

Figure 5.1 Previous attempts used to form PEGylated surfaces; a) radical polymerization of PEGylated acrylates and methacrylates, b) grafting to of pre-synthesized polymer chains with reactive surface groups, and c) SI-polymerization of glycidol resulting in hyperbranched PEG...... 139

Scheme 5.3 Anionic SI-ROP of epoxides; Step 1: deprotonation hydroxyl-terminated monolayers with phosphazene base and Step 2: polymerization of propylene oxide ...... 140

Scheme 5.2 Formation of hydroxyl-terminated phosphonic acid SAMs on SS316L using

11-hydroxy undecyl phosphonic acid...... 142

Scheme 5.3 Anionic SI-ROP of epoxides; Step 1: deprotonation hydroxyl-terminated monolayers with phosphazene base and Step 2: polymerization of propylene oxide. ....143

Scheme 5.4 Acid-catalyzed SI-ROP of epoxides using; a Bronsted Lowery acid with hydroxyl-terminated monolayers (right) and a Lewis acid with hydroxyl-terminated monolayers (left) of (R=H), propylene oxide (R=Me), and 1,2 butylene oxide (R=Et). 145

xxvii Scheme 5.5 Gas-phase ring-opening polymerization of epoxides a) formation of the BF3 propylene oxide complex formed from BF3 diethyl etherate b) Ring-opening the BF3 propylene oxide complex resulting in SI-poly(propylene oxide) ...... 147

Figure 5.2 Vacuum deposition set up for gas-phase polymerization of propylene oxide using boron trifluoride diethyl etherate as a catalyst. The system was connected to vacuum the Schlenk manifold...... 148

Figure 5.3 DRIFT Spectra of the a) methylene stretching region and b) head group binding region of monolayers HUPA formed by aerosol deposition after sonication with tetrahydrofuran. Black trace is the spectrum of the HUPA SAM, and the dashed grey trace is the spectrum of solid HUPA. Labels in black correspond to the spectrum of the HUPA monolayers, whereas the grey corresponds to the spectrum of the solid HUPA...... 150

Figure 5.4 DRIFT Spectra of the a) methylene stretching region and b) ether stretching regions of HUPA monolayers (dashed grey trace) and SI- poly(propylene oxide) from by anionic ring-opening polymerization (black trace). Labels in grey correspond to the spectrum of the HUPA monolayers, whereas the black correspond to the spectrum of the

SI- poly(propylene oxide)...... 151

Figure 5.5 DRIFT Spectra of the a) methylene stretching region and b) ether stretching regions of bulk synthesized poly(propylene oxide) (dashed grey trace) and SI- poly(propylene oxide) (Black trace). Black labels correspond to the spectrum of the SI- poly(propylene oxide)...... 152

Figure 5.6 DRIFT Spectra of the a) methylene stretching region and b) ether stretching regions of HUPA monolayers (black trace) and SI- poly(propylene oxide) thought acid- catalyzed ring-opening (grey dashed trace). Labels in black correspond to the spectrum

xxviii of the HUPA monolayers, whereas the grey corresponds to the spectrum of the SI- poly(propylene oxide)...... 154

Figure 5.7 DRIFT Spectra of the a) methylene stretching region and b) ether stretching regions SI- poly(propylene oxide) thought SI-polymerization of gaseous propylene oxide and BF3-etherate before (green trace) and after (red trace) rinsing with THF. Labels correspond to the spectrum of the SI- poly(propylene oxide) before rinsing with THF.

...... 155

Figure 5.8 Contact angle of unmodified SS316L coupons, HUPA monolayer modified coupons, and SI- poly(propylene oxide) from by anionic ring-opening polymerization modified coupons...... 156

Figure 5.9 Cyclic voltammograms of unmodified SS316L coupons (black trace), HUPA monolayer modified coupons (dashed grey trace), and SI- poly(propylene oxide) from by anionic ring-opening polymerization modified coupons (grey trace)...... 157

Scheme 5.6 Possible routes to polymers formed in solution by a) deprotonation of propylene oxide resulting in ring-opening, and b) the deprotonation of THF resulting in ring-opening. Subsequent additions of propylene oxide result in poly(propylene oxide)

...... 160

Scheme 5.7 Possible Protonation of the phosphazene base by water ...... 161

Scheme 5.8 Ring-opening of epoxides promoted by BF3 in solution, :B is either another propylene oxide or THF molecule ...... 163

Figure 6.1 Functionalization of SS316L with polymer films of styrene, methyl acrylate, and methyl methacrylate ...... 172

xxix Figure 6.2 Polymer films that were synthesized as a part of this project. Poly(styrene) films were forming in part of a Chapter 4...... 174

Figure 6.3 Formation of hydroxyl modified surfaces (Step 1) and polymerization of ethylene oxide (R=H), propylene oxide (R=Me), and 1,2-butylene oxide (R=Et) (Step 2).

...... 175

Figure 6.4 Proposed formation and structure of crosslinked polymer films from by the introduction of bifunctional monomers ...... 177

Figure 6.5 Proposed formation and structure of antibiotic or nitric oxide releasing molecule modified PEGylated polymer films ...... 178

xxx LIST OF ABBREVIATIONS

AFM Atomic force microscopy

AIBN Azobisisobutyronitrile

AlCl3 Aluminum chloride

ANOVA One-way analysis of variance

ATRP Atom transfer radical polymerization

BF3 Boron trifluoride

BIBB Bromoisobutyryl bromide

BiBUPA 11-Bromoisobutyrateundecyl phosphonic acid

Br Bromine

CKRSR Peptide cystine-lysine-arginine-serine-arginine

Cl Chlorine cm-1 Wavenumber conc. Concentration

CPE Constant phase element

Cr Chromium

CRP Controlled radical polymerization

CuBr2 Copper bromide

DBEM Dulbecco’s modified Eagle medium

DI Deionized

DPA Decyl phosphonic acid

DRIFT Diffuse Reflectance Infrared Fourier Transform

Ecorr Corrosion potential

xxxi EDTA Ethylenediaminetetraacetic acid

EIS Electrochemical impedance spectroscopy

Epass Passivation potential

Epit Pit potential

Et Ethyl

Et3N Triethylamine

Fe Iron

Fe(OH)3 Iron (III) hydroxide

FRP Free radical polymerization

FT-IR Fourier Transform Infrared

G Centrifugal force

HUPA 11-hydroxyundecyl phosphonic acid iPrOH Isopropanol kHz Kilohertz kΩ Kiloohms

M Molar

Me Methyl

MeS Generic Metal Sulfide mL Milliliter mM Millimolar

MnS2 Manganese Sulfide.

Mo Molybdum

N/m Newton/meter

xxxii N2 Nitrogen

NaCl Sodium chloride

Ni Nickel

NMP Nitroxide mediated polymerization

NMR Nuclear magnetic resonance

NO Nitric oxide

º Degrees

OEGA Oligo(ethylene glycol) acrylate

OEGMA Oligo(ethylene glycol) methacrylate

PBS Phosphate buffered saline

PE Protection efficiency

PEEP Poly(ethylene ethylphosphate)

PEG Poly(ethylene glycol)

PEO Ethylene oxide photoATRP photo-induced atom transfer radical polymerization

PMA Poly(methyl acrylate)

PMMA Poly(methyl methacrylate)

Poly(S-co-OEGA) Poly(styrene-co- oligo(ethylene glycol) acrylate))

Poly(S-co-OEGMA) Poly(styrene-co- oligo(ethylene glycol) methacrylate))

PPO Propylene oxide

PS Poly(styrene)

RA Reducing Agent

RAFT Reversible Addition Fragmentation Chain-Transfer

xxxiii RAox Oxidized Reducing Agent

Rct Charge transfer resistance of unmodified coupon

Rct bare Charge transfer resistance of unmodified coupon

Rct mod Charge transfer resistance of modified coupon

Redox Reduction and oxidation

RMS Root mean square

ROP Ring-Opening Polymerization

ROPM Ring-opening metathesis polymerization

Rsoln Resistance of the solution

SAMs Self-Assembled Monolayers

SI Surface initiated

SI-ATRP Surface initiated atom transfer radical polymerization

SI-NMP Surface initiated nitroxide mediated polymerization

SI-RAFT Surface initiated Reversible Addition Fragmentation Chain-Transfer

SI-ROPM Surface initiated Ring-opening metathesis polymerization

SnIIEH2 Tin (II) 2-ethylhexanoate

Soln. Solution

SS316L Stainless Steel 316L

TBME Tertbutylmethlyether

Temp. Temprature

THF Tetrahydrofuran

TLC Thin Layer Chromtography

TNS Trypsin Neutralizing solution

xxxiv TPMA Tris(2-pyridylmethyl)amine

TsOH p-Toluenesulfonic acid

USD United States Dollar

V Volt v/v Volume to volume

ZnCl2 Zinc Chloride

µL Microliter

µm Micrometer

µ-oxos Bridging oxo groups

º C Degrees Celsius

øCV Fractional Coverage

Ω Ohm

xxxv Chapter 1: Overview of Pitting Corrosion

1.1 Introduction

Pitting corrosion is defined as localized corrosion forming deep pits on the surface of a metal,1-4 and is arguably one of the most destructive and dangerous forms of corrosion.

This is because the damage caused by the pits is severely underestimated.4, 5 In 2016, the financial repercussions of architectural corrosion were estimated at 575 billion USD in the

United States alone, excluding factors such as travel delays and environmental impacts.6

Furthermore, this value was estimated at 2.4 trillion USD worldwide in 2014.7 These values are expected to increase due to the growing gross domestic product in the United States, such that repairs could cost approximately 800 billion USD by the year 2025. In addition to costly repairs, just how dangerous corrosion can be is illustrated by recent history. In

1992, a catastrophic explosion occurred in Guadalajara, Mexico, caused by pressure build- up in sewer lines that were weakened by pitting corrosion, which led to the death of 184 people and injuring an additional 1500 people.8 More recently, in 2012, as a result of corrosion on a drilling rig in Norway, an estimated 125 barrels of crude oil was released, having a significant environmental impact.9 Also, in 2012, a corroded natural gas pipeline failed, which caused an explosion in West Virginia, destroying multiple homes and highways.10 These events indicate just how catastrophic the results of pitting corrosion can be, damaging life, property, and the environment.

In addition to structural failures of bridges, pipelines, boats, and drilling platforms, pitting corrosion also has a profound impact on the biomedical community. In biological systems, metallic implants can undergo pitting corrosion due to the presence of aggressive

1 ions such as chloride, phosphate, nitrate, and sulfate.11-21 This pitting corrosion ultimately leads to premature failure. In addition to aggressive ions present in bodily fluids, proteins such as bovine serum albumin and fibrinogen have been shown to expedite pitting corrosion as well.16, 22-24 In 2020, an estimated 512,00 hip25 and 1,340,000 knee26 replacements will be performed, which will require the insertion of a metallic implant. Of these, approximately 5% will need premature revision surgeries to correct either mechanical failure or infection within 10 years.26-28 Furthermore, corrosion can lead to heavy metal ions, such as chromium, nickel, and molybdenum leaching from the implant into the body which are taken up by proteins, with studies indicating that they persist in the body for years afterward, causing a myriad of adverse effects.11, 29, 30 Commonly, several different alloys are used in the medical field for implant devices, including stainless steels,22, 28, 31-37 titanium alloys,28, 34, 38-43 nitinol,28, 34, 41, 42 and cobalt-chromium.28, 34, 44-46

In particular, stainless steel 316L (SS316L) is highly susceptible to pitting corrosion and commonly used in both the biomedical and architectural fields. Thus, the development of corrosion-resistant coatings has become a critical area of research. This has generated several advances in both inorganic,47-53 organic,54-62 and hybrid inorganic/organic63-65 coatings for the prevention of corrosion. Many of these films formed on the surfaces of SS316L are either weakly absorbed to the surface or are exceedingly complex to prepare. The focus of this dissertation is to explore different corrosion-resistant polymeric coatings that are facile to prepare, strongly adhered to the surface, and can be used in marine and biological environments.

2 1.2 Pitting Corrosion of Stainless Steel

Pitting corrosion is one of the most damaging forms of corrosion because it’s difficult to manage and detect.1, 4, 5, 66, 67 While SS316L has been widely adopted because of its resistance to general surface corrosion, pitting corrosion is still a significant issue, particularly in marine and biological environments. The lack of resistance to pitting corrosion is a result of aggressive ions. The enrichment in the passive oxide film (relative to the bulk alloy) in chromium oxides is postulated to provide the corrosion resistance in

SS316L.68-70 The structure of the passive oxide film on SS316L is shown in Figure 1.1.

Figure 1.1 Molecular structure of the passive oxide film on SS316L

1.2.1 Mechanism of Pitting Corrosion

The mechanism by which pit formation is initiated has been an intense area of research. It has been postulated to occur where defects in the passive oxide layer are present, such as stress fractures, inclusions of metal sulfides, and grain boundaries.9, 36, 71-

77 These defects provide exposure of the bare metal surface, which in the presence of aggressive anions initiates pitting corrosion by preventing the passive oxide layer from reforming.77, 78 An aggressive ion is defined as an ion that is the conjugate base of a strong acid.77-79 Oxygen dissolved in an aqueous environment diffuses to the passive oxide surface, where it undergoes the cathodic half-reaction in corrosion, drawing electrons from a site where bare metal is exposed (Figure 1.2).9, 18, 71, 75, 80 In SS316L, this causes primarily iron(II) to dissolve from the interior surface of the pit; however, trace amounts of alloying

3 metals, such as nickel and chromium, are also released.11, 29, 30 The dissolution of metal ions from the bulk draws in chloride ions, which serve as a counter ion for the metal cations.9, 14, 71, 73, 74, 80 Further oxidation and reduction reactions cause iron (II) to be oxidized to iron (III), generating hydrogen ions, lowering the pH in the pit. This leads to a further acceleration of corrosion (Figure 1.2).9, 14, 71, 73, 74, 80 As the dissolved iron (III) ions diffuse to the exterior of the pit, it becomes insoluble in the high pH environment and precipitates. This, in turn, forms a cap over the pit, which limits the diffusion, concentrating the locally acidic environment inside,81, 82 which causes the corrosion in the pit to become autocatalytic and self-accelerating.81, 83

Figure 1.2. Proposed mechanism of pitting corrosion of SS316L in the presence of chloride ions. The cathodic and anodic half reactions are shown below the figure.

Several different potentials are relevant in pitting corrosion, including corrosion

9 potential (Ecorr), passivation potential (Epass), and pit potential (Epit). Ecorr corresponds to the potential required for electrons and cations to be released from the surface of a bulk metal. Conversely, Epass is the potential at which the bare metallic surface oxidizes and

4 results in a passive oxide film being formed. Both Epass and Ecorr are highly dependent on environmental conditions such as temperature, oxygen, and aggressive ion concentration.

High concentrations of dissolved oxygen favor the formation of the passive oxide film, whereas high concentrations of aggressive ions favor the formation of pits. Epit, defined as the potential that is present at the bottom of a pit, controls whether a pit will further corrode or re-passivate.9 For pitting corrosion on susceptible metals to occur, the microenvironment within the pit must generate an Epit that is between Ecorr, and Epass.

77, 82, 84 Furthermore, for a pit to re-passivate Epit must drop below Epass.

1.2.2 Stages of Pitting Corrosion

Pitting corrosion is generally subdivided into four different stages, which are based on the observed state of the passive oxide film, depth and size of pits, and stability of pitting; (1) passive film breakdown, (2) pit initiation, (3) metastable pitting, and (4) stable pitting (Figure 1.3).77, 83, 85 Due to the rapid rate of the breakdown of the film and microscopic nature of the events caused in the first and second stages of pitting corrosion, they are difficult to observe and study.86 However, the last two stages of the pitting

(metastable pitting and stable pitting) are easily observed, due to the size of pits and the time scale of the events.

5 x Flaw a) b) Passive oxide Passive oxide layer layer

Metal Metal

Stage 0 Stage 1

Intiated pit Metastable pit c) d) Passive oxide Passive oxide layer Electrolyte layer soluton Metal Metal

Stage 2 Stage 3

e) Stable pit with oxide cap

Fe(OH)3 Fe(OH)3 Passive oxide layer Electrolyte soluton Metal

Stage 4 Figure 1.3. Different stages of pitting corrosion a) intact passive oxide film, b) the breakdown of the passive oxide film caused by a flaw or damage, c) pit initiation d) metastable pitting, and e) stable petting with an iron hydroxide cap.

1.2.1.1 Stage 1: Passive Film Breakdown

The breakdown of the passive oxide film is the first event that must occur for pits to form on the surface. While this phenomenon has generated significant scientific curiosity, it remains poorly understood due to the difficulty of investigating the small changes in the surface and rapid time scale of the events (only lasting microseconds).86 The breakdown of the passive oxide film can occur either mechanically (abrasions and stress fractures) or chemically (discontinuities or degradation).77, 82, 86-88 While the damage to the oxide layer by mechanical means is easily explained, the chemical damage is slightly more nuanced. Discontinuities in the passive oxide layer, either grain boundaries of the oxide

6 layer or inclusions of metal sulfides, provide weak spots for pit initiation to occur. The chemical deterioration of the passive oxide layer can occur either by penetration or by absorption and thinning in the presence of aggressive ions.78, 84

The penetration mechanism mostly focuses on the ability of aggressive ions to permeate and intercalate into the oxide layer, resulting in a mixed oxide-aggressive ion layer on the surface of the bare metal (Figure 1.4).78 The additional penetration allows aggressive ions to fully permeate the passive oxide film, promoting corrosion underneath, and causing the oxide film to thicken and fail which results in a pit.78 This mechanism supports the induction time observed for pitting corrosion, the difference between introduction to a corrosive environment, and visible pit observation.78

Cl- Deformation of Passive oxide film Corrosion under a) b) Passive oxide film c) Initiated Pit Passive oxide layer Metal

A similar mechanism, the absorption and thinning of the passive oxide film, has also been proposed (Figure 1.5).84 Anions, in particular halide ions, can cause the oxide layer to become unstable, and as a result, cations are released from the oxide surface.78, 84

This process, for reasons that are not fully understood, becomes autocatalytically enhanced, causing pit formation on the site where aggressive ions were absorbed into the oxide layer.78, 84 In this mechanism, the induction time observed for pitting corrosion is caused by the time taken for the oxide layer to thicken to the point where it begins dissolving.78, 84

7 Cl-

+2/3 a) b) c) Fe (aq)

OH OH OH OH OH OH OH OH OH OH Cl OH OH OH OH OH OH OH OH OH O O O O O O O O O O O O O O O O Fe Fe Cr Fe Cr Fe Fe Fe Fe Cr Fe Cr Fe Fe Fe Fe Cr Cr Fe Fe O O Cl O O O O O O O O O O O O Cl O ClCl O O O O O O O O O O Cl O O O O O O Cl O O Fe Ni Fe Fe Ni Fe Fe Ni Fe Fe Ni Fe Fe Ni Fe Fe Ni Fe O O O O O O O O O O O O O O O O O O O O O

Additionally, chemical inconsistencies in the passive oxide film, such as grain boundaries where two semi-crystalline oxide phases meet, and inclusions of metal sulfides can be a point of initiation. Grain boundaries provide a location where a minimal passive oxide layer has been formed, and bare metal can be potentially accessed. Metal sulfide

77, 78, 89 inclusions, such as MnS2, are highly soluble in specific environments (Figure 1.6).

As these inclusions dissolve, they generate an acidic environment, adding to the already existing conditions inside of the pit, due to the production of sulfuric acid.77, 78, 89 This prevents the formation of the passive oxide film and provides a readily acidic environment for pitting corrosion to occur.

Metal Sulfide Intiated pit a) b)

MeS Me+2 + S-2

Figure 1.6 a) Dissolution of metal sulfides (MeS) and b) resulting in the formation of a pit.

8 1.2.1.2 Stage 2: Pit Initiation/Nucleation

Nucleation of a pit generally occurs at a defect site in the passive oxide layer of a material.82, 86 Pits rapidly form and depending on the environmental conditions such as dissolved oxygen and aggressive ion concentrations, pits can either re-passivate or further corrode.82 In the presence of low oxygen concentrations, a new passive oxide layer is unable to form on the exposed metal and instead undergoes the cathodic reaction, as shown in Figure 1.1. Defects in the passive oxide layer can come from damage (either chemical and/or mechanical) or native inconsistencies in the oxide film, as described in Section

1.3.1.2. A defect in the oxide layer can expose the bare metal underneath, and in the appropriate conditions (low oxygen and high aggressive ion concentration), an oxide film is unable to passivate the surface, which begins the corrosion process.9

1.2.1.3 Stage 3: Metastable Pitting

As initiated pits form, they can transition into metastable pits that are visible to the naked eye and contain a unique chemical microenvironment inside the pit. 78, 83, 88, 90

Generally, these pits are approximately a few hundred microns in size.88 However, these pits can rapidly switch between active pitting and re-passivation depending on the conditions inside the pit and in the environment surrounding the pit.78, 83, 88, 90 Similarly, to the initiation of pits, a low oxygen concentration inhibits a new passive oxide layer from being formed. Furthermore, even if pits can re-passivate, they are prone to reactivation if

91 Epit exceeds Epass, hence the name “metastable pitting”. For stable pitting to occur, metastable pits must undergo sufficient corrosion to develop an environment that prevents re-passivation without human intervention.82, 86, 92

9 1.2.1.4 Stage 4: Stable Pitting

In the final stage of pitting corrosion, stable pitting, the rate of corrosion accelerates due to conditions inside the pit. While the mechanism of pitting corrosion is not wholly understood, it is known that pitting at this stage becomes autocatalytic due to the low pH.86

This is due to iron (II) dissolution, which draws aggressive ions into the pit because of the

81, 82 ionic gradient. Hydrolysis reactions create an acidic condition and dissolved Fe(OH)3.

Coupled with a high pH environment outside the pit, a porous cap of Fe(OH)3 is formed in this stage, which further limits diffusion and concentrates the corrosive environment.92, 93

Ultimately, the rate of corrosion depends upon the materials composition, electrolyte concentration, and pitting potential. At this stage, significant amounts of material can be removed, causing severe damage to structures.82-84, 92, 94

1.3 Characteristics of Corrosion Resistant Coatings

To prevent corrosion or further pacify the surface of SS316L, various coatings have been applied. Generally, corrosion-resistant surface modifications operate under a combination of possible barrier, inhibitive, and/or galvanic effects.95 Barrier and inhibitive effects are generally observed when organic molecules are absorbed to the surface.95

Whereas the galvanic effect is observed when a surface is coated with a sacrificial metal.95

The barrier effect relies upon the ability of a surface modification to restrict the diffusion of ions, water, and oxygen to the metal surface.96-98 However, it has been postulated that the exact mechanism is more dependent on inhibiting the diffusion of ions to and from the bulk metal surface, and not the diffusion of water or oxygen.99-102 This increases the resistivity of the surface to ionic exchange between the solution and metal oxide surface,

10 resulting in a reduction of charge transfer, which is needed for corrosion to occur.103-105

Generally, these films are highly nonpolar, which creates a very hydrophobic surface that does not allow for the conduction of ions.104

In addition to barrier properties, corrosion-resistant coatings can also possess inhibitive properties that alter the potential at which electrochemical reactions occur.95

These effects are highly dependent upon the composition and structure of the bulk metal and the surface modification.95 Inhibitive effects tend to differ from the barrier effect by resisting the flow of ions and electrons between cathodic and anodic sites present in pitting corrosion instead of the surface and solution. Both the anodic and cathodic protection can be achieved using these systems. Anodic systems decrease the passivation potential (Epass), which results in a broader range in which the surface is pacified.95 Cathodic systems increase the potential at which corrosion will occur (Ecorr), generating a smaller accessible range of potentials.106-108 These effects are achieved by organic ligands coordination to the surface the modulate that redox potentials.

The last possible effect is a galvanic effect, which is not seen in systems where organic molecules are used to coat metal surfaces.95 In a galvanic system, a sacrificial anode (typically made of zinc) is attached to the stainless steel structure in such a way that electrical contact is provided.109-112 The anodic reaction, the reaction that is responsible for the dissolution of material, occurs preferentially at the sacrificial anode, while the cathodic reaction occurs on the passive oxide surfaces of the stainless steel.109-112 This protection will last if anode material remains present and connected to the steel.109-112

11 1.4 Conclusion

In summary, pitting corrosion is extremely detrimental to society and responsible for many costly repairs in the biomedical, structural, and marine industries. Several alloys are highly resistant to general corrosion, such as stainless steels, titanium alloys, nitinol, and cobalt-chromium due to their passive oxide film. However, they are plagued by localized (pitting) corrosion as a result of the passive oxide film being mechanically or chemically damaged. Because of the presence of aggressive ions, such as chloride, phosphate, nitrate, and sulfate, a locally acidic environment forms in the pit, further enhancing corrosion, and leading to large amounts of materials being removed, causing severe damage.

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21 Chapter 2: Self-Assembled Monolayers and Surface Initiated Polymerizations

2.1 Introduction

Functionalization of surfaces has been a long-standing method for modifying the properties of an interface. Modifications can lead to an improvement in the properties of surfaces for biomedical, industrial, and sensing applications. The absorption and covalent attachment of organic molecules to surfaces can be used to design interfaces that are resistant to corrosion1-3 and biofouling,4-7 or add features such as lubrication.8-12

2.1.1 Functionalization of Surfaces with Self-Assembled Monolayers

Self-assembled monolayers (SAMs) are a single molecule thick film that is covalently attached a surface with long-range order, forming a two-dimensional crystal- like structure (Figure 2.1).13-15 SAM modified surfaces can be prepared through either immersion,13, 16-20 vapor deposition,21, 22 or aerosol spraying23-25 of organic thiols or acids.

Traditionally, the model system of alkyl thiols on gold surfaces has been an intense area of research. While the real-world utility of these films are limited, due to the noble metal surface, valuable insights about the fundamental chemistry have been gained.20, 26-34 Over the past two decades a significant amount of research has been dedicated to the formation of organic acid self-assembled monolayers on industrially relevant oxide surfaces, such as stainless steels,17, 18, 23, 24, 31, 35 titanium,17, 18, 23-25, 35-39, and copper-based alloys17, 18, 27, 34, 35,

40, 41 to name a few.

22 O a)OH b) OH NH Br SH O OH Tail Group Br NH SH OH 2 O 2 O

Alkyl Chains

P P P P P P O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O Head Group Metal Oxide Metal Oxide Figure 2.1 Structure of a) phosphonic acid SAMs with commercially available tail groups with tridentate coordination to the surface and b) carboxylic acid SAMs with commercially available tail groups with bidentate binding to the surface.

SAMs consist of three parts: a head group, alkyl chains, and a tail group, all of which contribute to the stability of the monolayers (Figure 2.1).18, 35, 42-45 The head group provides covalent attachment to the surface by the formation of metal-oxygen-phosphorus

(Figure 2.1a) or metal-oxygen-carbon bonds (Figure 2.1b) for phosphonic and carboxylic acids, respectively18, 35, 42-45 Long alkyl chains, presenting in an all-trans orientation, provide reliable stabilizing interactions through van der Waals interactions. Hydrogen bonding interactions of tail groups on monolayer molecules have also been indicated to provide additional stability and control over the interfacial properties.35, 42, 44, 45

Furthermore, a large variety of tail groups are commercially available on long-chain phosphonic and carboxylic acids, which means that they can be modified by chemical transformations at the interface, further enhancing surface properties.

2.1.2 Modification of Organic Thin Films

Organic transformations at the interface have been used as a method for the immobilization of both small molecules and (bio)macromolecules with examples shown in

23 Figure 2.2.46-57 Covalent attachment of small molecules such as antibiotics,54-56 NO doner,46, 47 and antimicrobial peptides51, 58 have been used as methods to control biofilm formation. Conversely, immobilization of other bioactive molecules has been used to promote the attachment of specific cells, such as covalently attaching the peptide CKRSR to promote osseointegration, and to prevent aseptic loosening.55 Furthermore, SAMs have been used as molecular anchors for polymers, providing a method to finely tune surface properties of materials by adding corrosion1-3 or biofouling resistance,4-7 tribological effects,8-12 and control of surface energy.53, 57

CKRSR O R

NH N O 2 O O O OH OH S O HN NH NH OH NH O 2 O O O O 3 Reactant(s) 3 1 7 13 13 9 1 7 13 13 9 Conditions P P P P P P P O O O O O O O OH O O O O O O O O O O O O O O O O O O O Metal Oxide Metal Oxide

= KRWWKWIRW = Gentamicin (Antimicrobial Peptide)

CKRSR = Cell Addhesion Peptide = Horseradish Peroxidase

Figure 2.2 Functionalization of carboxylic and phosphonic acid self-assembled monolayers peptides, small molecules, polymers, and proteins

2.2 Functionalization of Surfaces with Polymeric Films

2.2.1 Grafting To

To functionalize surfaces with polymer films, either “grafting to” or “grafting from” methodologies are employed.59, 60 The “grafting to” technique requires the immobilization of presynthisized polymers onto a surface. This grafting strategy involves a functionalized surface reacting with polymer chains terminated with a compatible

24 functional group (Figure 2.3).59-61 An example of this is the immobilization of poly(ethylene oxide) on carboxylic acid terminated SAMs via esterification reactions.62-66

This type of grafting results in a low density of brushes attached to the surface due to increased steric hindrance as additional polymers chains try to access the reactive surface groups.59-61

Low Density a) Pre-sythesized Polymer Brush Polymers

Grafting To Substrate Substrate Reactive end group Reactive surface group

High Density b) Monomer solution Polymer Brush

Grafting From Substrate Substrate

Monomer Initiator Figure 2.3 a) Grafting to vs. b) grafting from surface modification with polymers.

2.2.2 Grafting From

In contrast to the “grafting to” process, the “grafting from” methods use a source of monomer and polymerization initiator, that is anchored to the surface, to form polymer films.59-61, 67 Polymerizations are surface-initiated utilizing this technique, meaning that the polymers are “grown” from the surface. This grafting strategy results in a high density of

25 polymer brushes, since the steric hindrance of large polymer chains blocking necessary functional groups is avoided.59-61, 67 While this is highly applicable to chain growth polymerizations, it has its limitations. Polymers grown in a step-growth type fashion are much less suitable.59-61, 67 Therefore, the remainder of this chapter will be focusing on methods used to form surface-initiated chain-growth polymers.

2.3 Techniques used to Form Surface Initiated (SI) Polymers

2.3.1 Free Radical Polymerization (FRP)

Free radical polymerization (FRP) is the simplest chain growth radical polymerization, where a thermal or photo radical initiators, such as peroxides or azo reagents, undergo hemolytic cleavage to generate radicals, initiating the polymerization

(Scheme 2.1).59, 68 These radicals then undergo propagation reactions where their added to a double bond, creating long polymer chains.59, 68 A propagating chain undergoes additions of alkenes until the radical undergoes irreversible termination reactions such as radical- radical coupling and disproportionation reactions.59, 68

Initiation

I I 2I

Propigation R R R I I

Pn Terminatinon Reactions

Pn + Pn Pn Pn

+ P P Pn Pn n -H n +H

Scheme 2.1 Proposed mechanism of FRP

26 FRP has previously been used to modify surfaces of gold,69 nitinol,53 nickel53, stainless steel 316L,57 and silica nanoparticles.70 For this to be a viable SI-polymerization method, a free radical initiator (such as an azo group) must be anchored to a surface, as shown in Scheme 2.2a and b.53, 57, 69, 70 After thermal treatment in presence of monomers, the decay of the azo group generates a radical, initiating the polymerization. The generated radical then adds to alkene monomers such as acrylates, methacrylates, and styrenics, producing surface-bound polymers.53, 69, 70 SI-polymers formed through FRP have been demonstrated to impart control of surface properties such as surface energy and corrosion resistance.53, 57 While this methodology has been exploited for the modification of surfaces in the past; it has been significantly overshadowed by controlled radical polymerizations due to the higher level of control over the surface structure.

O a) b) N O O O HN NH NC 2 R R R N O N N CN CN CN N O

90 C, 24 Hours O 90 C, 24 Hours O O HN NH HN NH Styrene or NH Strrene or NH NH O Pentofourostryene O Methyl Methacrylate

P P Si Si Si O O O O O O O O O O O O O O O Metal Oxide Metal Oxide SIO 2 SIO2 SIO2 Scheme 2.2 Examples of SI-FRP using azo initiators a) from surfaces of nitinol, nickel, and stainless steel 316L and b) from the surface silica nanoparticles.

2.3.2 Controlled Radical Polymerization

While free radical polymerization has been used to modify the surface of materials, the uncontrolled nature of the polymerization makes it challenging to finely tune surface structure, due to inevitable termination reactions. Over the past two decades, methods of

27 controlled radical polymerization (CRP) have been pioneered, which suppresses the radical termination reactions by reducing the concentrations of active radicals. CRP accomplishes this via reversibly deactivating propagating chains.59, 68 Additionally, the mild reaction conditions, high functional group tolerance, and uniformly growing polymer chains makes this attractive technique for modifying surfaces.59, 68

2.3.2.1 Atom Transfer Radical Polymerization (ATRP)

Ever since seminal reports of ATRP in 1995, it has been used as a method to modify the properties of existing materials and generate new materials.71, 72 The mechanism of copper-catalyzed ATRP is shown in Scheme 2.3. Copper-catalyzed ATRP relies on the equilibrium reaction between a copper (I) activator undergoing a one-electron oxidation with an alkyl halide (Cl or Br) forming copper (II) deactivator and a radical.73-78 Initiation occurs when an alkyl halide reacts with an activator generating a radical, which then undergoes the addition of alkene (monomer). After the addition of the monomer, the radical is deactivated by abstracting a halogen from the copper (II) deactivator, resulting in the dormant chain capped with a halogen (either Cl or Br). 73-78 After deactivation, the halogen capped chain can be re-activated, leading to the further growth of polymer chains.79 ATRP is initiated under inert conditions using air-sensitive copper (I) complexes. However, recent discoveries in the field have led to advances that permit polymerizations to be initiated under ambient conditions using air-stable copper (II) compounds and chemical reducing agents such as zero-valent copper, glucose, ascorbic acid, tin (II) 2-ethylhexanoate, excess ligand and light.80-84 85, 86 The reduction of copper (II) to copper (I) removes traces of dissolved oxygen in polymerization solutions.

28 I Cu Lm

P X Pn+1 X RA n

P RAox P n R n

II Cu LmX

Surface-initiated ATRP (SI-ATRP) has been extensively used in the past decade to impart tribological,9, 10, 12, 61 corrosion resistance,87-92 and antifouling61, 90, 91, 93-95 properties.

SI-ATRP has arguably become one of the most commonly used techniques for generating

SI polymer films in the past 10 years. The immobilization of an ATRP initiator on the surface is generally accomplished by the formation of a bromoisobutyrate ester on the terminus of a SAM(Scheme 2.4).88, 94, 96-100 The halogen terminated ester can then be used as an initiator for polymerization upon the introduction of copper catalysts and monomers into the system, with examples shown below in Scheme 2.4.

29 Br Br

Br R Br R O O O O

O O O O

9 R 9 9 R 9 Cu(L)X/[Cu(L)X][X] Cu(L)X/[Cu(L)X][X] P P Si Si O O O O O O O O O O O O Metal Oxide Metal Oxide SiO SiO2 2 Scheme 2.4 Examples of systems where ATRP has been using

2.3.2.2 Reversible Addition Fragmentation Chain-Transfer (RAFT) polymerization

Unlike ATRP, which relies on a transition metal catalyst to generate dormant polymer chains, RAFT polymerization relies on the equilibrium reaction between a dithioester and a propagating radical chain.101-105 RAFT is proposed to proceed by a mechanism that is similar to FRP, in which a thermal or photo-initiator generates radicals through initiation, which are then fed into initial propagation reactions.101, 103, 104 The

 resulting propagating chains (Pn ) then reversibly adds to dithioester in the RAFT pre- equilibrium phase, which generates a new radical (R) shown in Scheme 2.5 .101, 102, 104, 106

The new radical enters the re-initiation phase where it propagates new a polymer chain.101,

102, 104, 106 This polymer chain, as well as the dithioester, feeds into the main RAFT equilibrium in which the RAFT radical intermediate forms.101, 102, 104, 106 The intermediate then generates active propagating chains that can add to alkenes before being reversibly deactivated by the dithioester, reforming the RAFT radical intermediate. Polymerization is terminated by either radical coupling or disproportionation reactions.101, 103, 104

30 Initiation R R R R R I I 2I I I

P1 Pn RAFT Pre-equilibrium S S S S S S R P R P + Pn + n n R Z Z Z

Re-initiation R R R R R R R R

P1 Pn

Main RAFT Equilibrium S S S S S S R P P P + Pn + n n n Pn Z Z Z (RAFT Radical Intermediate) Terminatinon Reactions

Pn + Pn Pn Pn

+ P P Pn Pn n n

Scheme 2.5 Proposed mechanism of RAFT polymerization

For RAFT to be conducted in a surface-initiated manor (from here on out referred to as SI-RAFT) a dithioester must be anchored to the surface of a material, and a method of generating radicals must be present.107-114 Previously, the formation of organosilane

SAMs terminated with a RAFT agent (Scheme 2.6a) has been devised.107 After heating, the RAFT agent promoted the SI-polymerization of dimethyl acrylamide.107 Alternatively, an azo-reagent can be immobilized on the surface on a hydroxyl-terminated SAM, as shown in Scheme 2.6b.111 Upon immersion in a solution with monomer, AIBN, and chain transfer agent (dithioester), RAFT polymerization occurs, resulting in the formation of

31 poly(4-vinyl benzylchloride).111 These examples encompass the techniques used in the formation of SI-polymers using RAFT to modify surfaces of iron, silicon dioxide, gold, and high-density poly(ethylene).107-114 a) b) OH

O R R NC S S S R N S S S N CN CN N Cl S S 4-Vinyl benzyl chloride O Dimethyl acrylamide O O O AIBN, Chain Tranfer Agent O

NH NH O NH

8 8

Si Si Si Si O O O O O O O O O O O O

Metal Oxide Metal Oxide SIO2 SIO2 R = Bu, Ph R = Ph, Bn

Scheme 2.6 Examples of SI-RAFT systems

2.3.2.3 Nitroxide Mediated Polymerization (NMP)

Similarly to ATRP, nitroxide mediated polymerization (NMP) functions by forming dormant radical species.115-121 The first step in the proposed mechanism of NMP is the thermal or photo decomposition of an initiator, such as peroxides or azo compounds, generating radicals (Scheme 2.7).115, 117, 119, 120 The initiating radical then enters propagation reactions, where the addition of alkene occurs, producing a polymer chain.

After propagation, however, NMP takes advantage of the stability of nitroxide radicals, which can reversibly react with propagating chains forming a dormant radical intermediate.115, 117, 119, 120 The intermediate significantly reduces the concentration of the radicals, and as a result, a decrease in termination reactions is observed. Homolytic cleavage of the C-O bond (shown on the dormant radical intermediate in Scheme 2.7) on the dormant polymer chain results in a radical that undergoes the addition of alkenes,

32 extending the polymer chain.115, 117, 119, 120 Similarly to all radical polymerizations, some degree of termination reactions occur via disproportionation and radical coupling.116

Initiation

I I 2I

Propigation R R R R R I I I

P1 Pn

Reversable Deactivation R P O N Pn + O N n R (Dormant Radical Intermediate) Terminatinon Reactions

Pn + Pn Pn Pn

+ + Pn Pn Pn-H Pn+H

Scheme 2.7 Proposed mechanism of NMP

Surface initiated NMP (SI-NMP) has been used extensively to modify the surface properties of nanoparticles and bulk surfaces.115, 122-128 In the literature, three primary techniques have been identified for the formation of polymers using SI-NMP. The first involves the functionalization of acrylate terminated SAMs using AIBN and a stable nitroxide radical (Scheme 2.8a).125 The second uses initiation of an azo reagent modified surface in the presence of monomer and stable nitroxide radical (Scheme 2.8b) to generate the polymer.115, 128 An additional method that has been identified in the literature for SI-

NMP involves anchoring a phosphonic acid to the surface of a metal oxide that is

33 terminated with a deactivated nitroxide radical(Scheme 2.8c). Upon heating in the presence of monomer the C-O bond homolyzes, initiating polymerization. 126 Of the methods used to modify surfaces using SI-NMP, the procedures outline in Scheme 2.8a and b, are the most attractive due to the ability to functionalize commercially available tail groups on

SAM molecules.

a) b) Si(OEt)3

O O EtO P tBu O O EtO EtO P tBu NC N EtO OEt EtO tBu O P tBu N N O tBu O N Ph tBu O CN CN N P OEt N tBu O OEt O Ph O tBu O Styrene O O Styrene O NC NC tBu O O N CN O O O N P OEt NH NC N O OEt tBu Si Si Si Si Si O O O O O O O O O O O O O O O

Metal Oxide Metal Oxide Metal Oxide SIO2 SIO2

O O

N N O O Styrene Ph Ph P O P O O O O O Metal Oxide Metal Oxide Scheme 2.8 Examples of poly(styrene) formed using SI-NMP

2.3.3 Ring-Opening Polymerization

2.3.3.1 Ring-Opening Polymerization of Epoxides

Unlike the radical polymerization methods described above, ring-opening polymerization (ROP) of epoxides is generally conducted under anionic conditions.129-131

The initiating species is typically water or an alcohol which is deprotonated with a strong base such as organometallic reagents, 129-131 bicyclic guanidine, 129-131 or phosphazene

34 bases, 129-131 resulting in a strong nucleophile. After nucleophilic attack of the epoxide ring, it opens and generates a new nucleophile. Propagation reactions result in the formation of 1,2 poly(ethers) chains. This process has been extensively used for the preparation of polymers such as poly(ethylene oxide), poly(propylene oxide), and poly(butlylene oxide).129-131 Previously, only three reported attempts in the literature have formed SI-poly(ethers) films.132-134 The first two attempts both employed the polymerization of glycidol resulting in hyper-branched poly(ethers) on the surface of chemically modified glass for antifouling properties (Scheme 2.9a).132, 133 The other remaining attempt was formed on nanosheets of graphene oxide and were used in membranes for gas separation (Scheme 2.9b).134 All three reports have required the deprotonation of surface-bound hydroxyls to form a nucleophile which is used to open epoxide rings (Scheme 2.9).132-134 To date, no attempts have been reported in the literature describing the formation of SI linear poly(ethers) or the “grafting from” polymerization of epoxide monomers from metallic surfaces.

a) b) O H R O OH R OH OH OH O HO O O Graphene Oxide Graphene Oxide OH O HO O OH O O O HO O HO OH O O OH NaOMe O OH O Si Si Si Graphene oxide = OH OH O O O O O O O O O Glass Glass Glass O OH Scheme 2.9 a) SI ring-opening polymerization of glycidol and b) the ring-opening polymerization of various epoxides

35 2.3.3.2 Ring-Opening Metathesis Polymerization

Ring-opening metathesis polymerization (ROPM) was first reported in 1965, but wasn’t widely used until the 1990s, when new catalysts for olefin metathesis reactions were identified.135, 136 The initiation of the polymer chains starts with a catalyst, shown in

Scheme 2.10 as Ru, coordinating to a cyclic alkene (monomer). After coordination, the Ru- monomer complex undergoes a cycloaddition reaction, followed by a cycloreversion reaction.135-141 The adduct that forms feeds into the propagation stage where multiple additions of cyclic alkene will continue to occur until monomer is depleted, or termination reactions occur.135-141 The termination step generally occurs when the propagating end of the polymer chains undergo a cycloaddition with vinyl bond contained within the polymer backbone.135-141

Initiation

Ru Ru Ru Ru R Coordination R Cycloaddition R Cycloreversion R

Propagation

Ru Ru Ru R R m R

Terminatinon Reactions

X Y Ru X Y Ru m R R Scheme 2.10 The proposed mechanism of ROMP in the presence of a generic Ru ROPM catalyst and a generic olefin.

36 Surface-initiated ROMP (SI-ROMP) has been studied since the late 1990s and has been used to generate surfaces that have liquid crystals, adhesive biomimics, thin-film transistors, and peptides immobilized.142-146 To form the SI-polymers, a surface modified with norbornene terminated monolayers as shown in Scheme 2.11 is formed.142-154 After exposure to a ROMP catalyst, such as Grubbs Catalyst shown in Scheme 2.11, the norbornene moiety ring-opens resulting in the initiating species for the remainder of the polymerization reactions.142-154 Upon introduction of monomer (cyclic alkene) such as a norbornene , polymerization will commence generating poly(alkene)s.142-154

Cl Cl R (Cy) P 3 Ru P(Cy)3

P(Cy) O O 3 O Ph Ph NH Cl NH NH Ru R Cl Ph P(Cy) 3 R

O O O O O O Metal Oxide Metal Oxide Metal Oxide

Scheme 2.11 Surface initiated romp of cyclic alkenes in the presence of Grubbs Catalyst. The first step involves the immobilization of a ruthenium carbene complex which forms the polymer chain when exposed to cyclic monomers

2.3 Conclusion

In summary, tremendous advances have been made in the field of surface-initiated polymerizations, providing a route to the synthesis of advanced materials. The bulk of work is currently being done in the realm of controlled radical based polymerizations such as

ATRP, RAFT, and NMP, due to the high control over surface immobilized polymer architectures. This allows a large amount of functionality to be imparted to both metallic

37 and nonmetallic surfaces improving their qualities for applications where controlling the wettability, or imparting oleophobic, corrosion resistance, tribological, or antifouling properties are desired.

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51 Chapter 3: Functionalization of SS316L with Corrosion- Resistant Polymer Films

3.1 Introduction

Herein is presented a modification of SS316L with polymer films that are covalently grafted from the surface (Scheme 3.1) and their corrosion-resistant properties were examined. Bromoisobutyrate esters were anchored to hydroxyl-terminated self- assembled monolayers using vapor phase deposition of bromoisobutyryl bromide. The halogen modified surface was then used as an initiator for surface-initiated atom transfer radical polymerization (SI-ATRP). Films of poly(styrene), poly(methyl acrylate), and poly(methyl methacrylate) were formed using SI- ATRP. The resulting polymer films were characterized by both cyclic voltammetry and electrochemical impedance spectroscopy to measure the corrosion-resistant properties of each modification.

Br Monomers m R Br Styrene O O O O O O Monomer Methyl Acrylate 9 [Cu(TPMA)Br][Br] 9 O P Tin (II) Ethylhexanoate P O O O O O O O

SS 316L SS 316L Methyl Methacrylate

Scheme 3.1 Functionalization of SS316L with polymer films of styrene, methyl acrylate, and methyl methacrylate

52 3.2 Materials

11-Bromoisobutyrateundecyl phosphonic acid (>95%, BiBUPA), 11- hydroxyundecyl phosphonic acid (>95%, HUPA), decyl phosphonic acid (>95%, DPA),

II tin (II) 2-ethylhexanoate (95%. Sn EH2), dimethylformamide (99% anhydrous), phosphate-buffered saline and anisole (99.7%, anhydrous) were purchased from Sigma

Aldrich. Bromoisobutyryl bromide (97%, BIBB), styrene (99%), methyl acrylate (99%), and methyl methacrylate (99%) were purchased from Alfa Aesar. Triethylamie (98%) was purchased from Sigma Aldrich. Tetrahydrofuran (THF) was purchased from Marcon.

Stainless steel AISI 316L (Fe 69/Cr18 /Ni10/Mo3) foil with a 0.5 mm thickness was purchased from Goodfellow Inc. Invitrogen™ LIVE/DEAD™ Viability/Cytotoxicity Kit for mammalian cells (40 mM Calcein AM and 100 mM ethidium bromide homodimer) was obtained from Thermo-Fisher Scientific. Unless otherwise stated, all reagents were used as received without further purification. Prior to polymerization, styrene, methyl acrylate, and methyl methacrylate were passed through a silica column to remove inhibitors. Tris(2- pyridylmethyl)amine (TPMA) was prepared according to literature procedures.1 Following the synthesis of TPMA, it was recrystallized once from ethyl acetate then again from diethyl ether yielding pale crystals. Tetrahydrofuran was dried over sodium and benzophenone and distilled before use. Dichloromethane was dried over activated molecular sieves for at least 48 hours before use. The copper catalyst solution (0.1M

[Cu(TPMA)Br][Br]) was prepared by dissolving CuBr2 (223.4 mg, 1 mmol) and TPMA

(319.4mg, 1.1 mmol) in 10 mL of dimethylformamide amide.

53 3.3 Methods

3.3.1 Preparation of SS316L coupons

Stainless steel 316L coupons (1 cm x 1 cm) were prepared by sanding coupons with

160, 320, 400, and 600 grit sandpaper. Coupons were then placed in acetone and sonicated for 30 minutes to remove sanding debris and organic residues. Coupons were immediately transferred into methanol and boiled for 30 minutes to further remove organic residues from the surface. Coupons were then removed from the methanol and placed sanded side up and dried in a 120C oven overnight, then stored in sealed jars under ambient conditions until used. Coupons used for atomic force microscopy were further polished with 800 grit and 1200 grit sandpaper, followed by 1m diamond suspension until all visible scratches were removed. To remove polishing compounds, coupons were sonicated in DI water for

30 minutes and ethanol for 30 minutes. Polished coupons were then stored under vacuum until used.

3.3.2 Functional of SS316L with ATRP Initiators

3.3.2.1 SAM Formation of 11-Bromoisbutryrylundecyl Phosphonic Acid

Both solution and aerosol deposition techniques were employed to form stable and ordered SAMs of 11-bromoisbutryrylundecyl phosphonic acid (BiBUPA) on the surface of SS316L. Aerosol deposition was preform using a thin layer chromatography (TLC) sprayer. Coupons of SS316L were placed sanded side up on a glass Petri dish and then sprayed with the phosphonic acid solution. For solution deposition, coupons placed in a jar filled with a phosphonic acid solution, was capped for the desired time. Solutions of

BiBUPA were prepared at 1 mM and 2mM in 25 mL of dry THF. Solutions and coupons

54 were heated and cooled in order to optimize the self-assembly of the acid films on the surface. Also, triethylamine was added to help catalyze the formation of the covalent bonds to the surface.2 After deposition of BiBUPA, coupons were allowed to dry at ambient conditions or under vacuum overnight. In order to remove any physically absorbed acid molecules, coupons were rinsed and sonicated in dry THF. The formation of SAMs was confirmed via diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. All attempted deposition conditions are summarized in Table 2.1.

Br Br Br

O O O 11-Bromoisbutryrylundecyl O O O Phosphonic Acid 9 9 9 P P P OH OH OH OH OH OH O O O O O O O O O O O O O O SS 316L SS 316L Scheme 3.2 Functionalization of SS316L with SAMS of BiBUPA

55 Table 3.1: Solution and Aerosol deposition techniques used for the formation of BiBUPA SAMs

Deposition Deposition Substrate Stable and Trial Sprays Annealing soln. Conc. soln. Temp Temp Ordered SAMs 1 3 1mM Room Room Bench Top No 2 5 1mM Room Room Bench Top No 3 3 2mM Room Room Bench Top No 4 5 2mM Room Room Bench Top No 5 3 1mM 4°C 4°C Bench Top No 6 5 1mM 4°C 4°C Bench Top No 7 3 2mM 4°C 4°C Bench Top No 8 5 2mM 4°C 4°C Bench Top No 1mM + 0.75mM 9a 3 Room Room Bench Top No Et3N 2mM + 1.5 mM 10a 5 Room Room Bench Top No Et3N 11 3 1mM 60°C Room Vac line No 12 3 2mM 60°C Room Vac line No 13 3 1mM 60°C 4°C Bench Top No 14 3 2mM 60°C 4°C Bench Top No 15b 3 1mM 60 4 60 Oven No 16 5 1mM 60 4 60 Oven No 17 3 2mM 60 4 60 Oven No 2mM + 1.5 mM 18 3 60 4 60 Oven No Et3N 19b 3 1mM 40 4 Bench Top No 20b,c 18 hr 1mM 40 4 Bench Top No 21c 18 hr 1mM Room Room Bench Top No 22c 18 hr 2mM Room Room Bench Top No a Triethylamine was used as a catalyst to aid in the binding of the phosphonic acid to the oxide layer. b Dry dichloromethane was used as the solvent for the phosphonic acid solution. c Solution deposition. The number of sprays is the length of time.

3.3.2.2 Formation of Mixed SAMs of 11-Bromoisbutryrylundecyl Phosphonic Acid and Decyl phosphonic acid

Mixed monolayers of BiBUPA and decyl phosphonic acid (DPA) were formed through both aerosol and solution deposition. Aerosol deposition was preform using a TLC sprayer. Coupons of SS316L were placed sanded side up on a glass Petri dish and then sprayed with the phosphonic acid solution. For solution deposition, coupons were placed

56 in a jar filled with a phosphonic acid solution and capped for the desired time period. A solution of 0.9 mM BiBUPA and 0.1 mM DPA was prepared by adding 2.5 mL of 1 mM

DPA in dry THF to a 22.5 mL solution of a 1 mM BiBUPA in dry THF. Coupons were then dried overnight at ambient conditions. Deposition conditions are summarized in Table

2.2. Coupons were rinsed and sonicated in dry THF to remove any weakly absorbed acid molecules. The formation of SAMs was confirmed via DRIFT spectroscopy.

11-Bromoisbutryrylundecyl O Phosphonic Acid O Decyl Phosphonic acid 8 9 8 P P P OH OH OH OH OH OH O O O O O O O O O O O O O O SS 316L SS 316L Scheme 3.3 Functionalization of SS316L with mixed SAMS of BiBUPA and DPA

Table 3.2: Solution and Aerosol deposition conditions used for the formation of mixed BiBUPA/DPA SAMs Substrate Solution Stable and Trial Spray/Time Concentrations Annealing Temp. Temp. Ordered SAMs 0.9 mM BiBUPA 1 3 Sprays 60 °C 4 °C Vacuum No 0.1 mM DPA 0.9 mM BiBUPA 2 3 Sprays 60 °C 4 °C Bench Top 1/3 0.1 mM DPA 0.9 mM BiBUPA 3 18 hours Room Room Bench Top 1/3 0.1 mM DPA

3.3.2.3 SAM Formation of 11-Hydroxyundecyl Phosphonic Acid

SAMs of 11-hydroxyundecyl phosphonic acid were formed on SS316L using an aerosol deposition technique. Coupons of SS316L were placed sanded side up on a glass petri dish and cooled to 4C for 30 minutes. A 0.5mM solution of 11-hydroxyundecyl

57 phosphonic acid (3.1 mg, 0.0125 mmol) in dry THF (25mL) was prepared and heated to

60C in a sand bath. The warmed solution was transferred to a TLC sprayer and sprayed on the cooled coupons. Coupons were allowed to rest at 4C for 30 minutes, and the solution was returned to the 60C sand bath. This procedure was repeated twice more for a total of three sprays. Coupons were then transferred to a 60C oven and annealed overnight. Coupons were then rinsed and sonicated in THF to remove physically absorbed acid molecules and dried in a 60C oven.

O HO O

O 9 Br 9 11-Hydroxyundecyl Br Phosponic Aicd P P OH OH O O O O O 0.5 mM in THF O Conditions O SS 316L SS 316L SS 316L Step 1 Step 2 Scheme 3.4 Functionalization of SS316L with SAMs of HUPA and subsequent immobilization of BiBB

3.3.2.4 Solution Immobilization of Bromoisobutyryl Bromide on Hydroxyl Terminated SAMs

Coupons with SAMs of 11-hydroxyundecyl phosphonic acid were placed in a flask equipped with a stir bar and sealed with a septum. Either dry THF or DCM was used as the solvent for the reaction. In addition, various concentrations of bromoisobutyryl bromide were used to optimize the results of the immobilization. After immobilization, coupons were placed under vacuum to remove any residual solvent. DRIFT spectroscopy was used to evaluate immobilization.

58 Table 3.3: Solution and Aerosol deposition techniques used for the formation of BiBUPA SAMs Conditions Results THF, 1M BiBB, room temperature, substrates dipped Sub monolayers THF, 1mM BiBB, 1mM Et3N, room temperature, 10 minutes No reaction THF, 2mM BiBB, 2mM Et3N, room temperature, 10 minutes No reaction DCM, 1mM BiBB, 1mM Et3N, room temperature, 10 minutes Sub monolayers DCM, 2mM BiBB, 2mM Et3N, room temperature, 10 minutes Sub monolayers

3.3.2.5 Gas-phase Immobilization of Bromoisobutyryl Bromide on Hydroxyl Terminated SAMs Under Passive Vacuum

Coupons of SS316L modified with SAMs of 11-hydroxyundecyl phosphonic acid were placed into a vacuum deposition chamber shown in Figure 2.2, which was attached to a Schleck line. The deposition apparatus was then placed under vacuum, and the trap on the Schleck line was frozen with liquid nitrogen and an active vacuum was maintained for

30 minutes. After 30 minutes, the chamber was isolated from the vacuum and vapors of bromoisobutyryl bromide were introduced to the reaction chamber for 30 minutes.

Following the deposition of the bromoisobutyryl bromide, the chamber was returned to active vacuum for an additional 30 minutes. The procedure was optimized by repeating the number of depositions cycles for a total of one to three rounds of depositions. After the final round of deposition (conditions are summarized in Table 2.4) , the chamber was stored under vacuum with the coupons inside overnight. Coupons were rinsed with THF to remove any physically absorbed species. The initiator modified coupons were then stored under vacuum until used.

59 3.3.2.6 Gas-phase Immobilization of Bromoisobutyryl Bromide on Hydroxyl Terminated SAMs Under Active Vacuum

Coupons of SS316L modified with SAMs of 11-hydroxyundecyl phosphonic acid were placed into a vacuum deposition chamber, shown in Figure 2.2, which was attached to a Schleck line. The deposition apparatus was then placed under vacuum, and the trap on the Schleck line was frozen with liquid nitrogen and an active vacuum was maintained for

30 minutes. After 30 minutes, the chamber was exposed to vapors of bromoisobutyryl bromide while under active vacuum. After 30 minutes, exposure to bromoisobutyryl bromide was stopped and the coupons were stored under vacuum overnight. Samples were then rinsed with THF and then dried and stored under vacuum until used.

To Vacuum Reaction Chamber

BiBB

Figure 3.2. Diagram of the gas phase reaction setup for the immobilization of bromoisobutyryl bromide.

Table 3.4: Summary of reaction conditions employed with the gas phase immobilization of BiBB

Trial Vacuum Exposure time Exposures Results 1 Passive 15 min 1 Successful, stable though sonication 2 Passive 15 min 3 Successful, stable though sonication 3 Passive 30 min 1 Successful, stable though sonication 4 Passive 30 min 3 Successful, stable though sonication 5 Active 15 min 1 Successful, stable though sonication 6 Active 2 hours 1 Monolayer striped

60 3.3.3 Functionalization of SS316L with Polymer Films

Br Monomers m R Br Styrene O O O O O O Monomer Methyl Acrylate 9 [Cu(TPMA)Br][Br] 9 O P Tin (II) Ethylhexanoate P O O O O O O O

SS 316L SS 316L Methyl Methacrylate

Scheme 3.5 Functionalization of SS316L with polymer films of styrene, methyl acrylate, and methyl methacrylate

3.3.3.1 SI-polymerization of Styrene

Coupons of SS316L were functionalized with bromoisobutyryl bromide using the gas-phase passive vacuum method (described in Section 3.3.2.5) were placed in a 50 mL

Erlenmeyer flask with a stir bar and sealed with a septum. The flask was charged with 6.8 mL of styrene, 3.2 ml of anisole, 58 L of 0.1M [Cu(TPMA)Br][Br] in dimethylformamide. The resulting solution was purged with dry nitrogen for at least 20 minutes before 100 L of tin(II) 2-ethylhexanoate was added. The flask was then transferred to an oil bath thermostated at 80C for 18 hours. Coupons were then removed from the polymerization solution and rinsed and sonicated with ethyl acetate to remove weakly absorbed polymer chains.

3.3.3.2 SI-polymerization of Methyl Acrylate

Coupons of SS316L were functionalized with bromoisobutyryl bromide using the gas-phase passive vacuum method (described in Section 3.3.2.5) were placed in a 50 mL

Erlenmeyer flask with a stir bar and sealed with a septum. The flask was charged with 6.8

61 mL of methyl acrylate, 3.2 ml of anisole, 58 L of 0.1M [Cu(TPMA)Br][Br] in dimethylformamide. The resulting solution was purged with dry nitrogen for at least 20 minutes before 100 L of tin(II) 2-ethylhexanoate was added. The flask was then transferred to an oil bath thermostated oil bath at 80C for 18 hours. Coupons were then removed from the polymerization solution and rinsed and sonicated with ethyl acetate to remove weakly absorbed polymer chains.

2.3.3.3 SI-polymerization of Methyl Methacrylate

Coupons of SS316L were functionalized with bromoisobutyryl bromide using the gas-phase passive vacuum method (described in Section 3.3.2.5) were placed in a 50 mL

Erlenmeyer flask with a stir bar and sealed with a septum. The flask was charged with 6.8 mL of methyl methacrylate, 3.2 ml of anisole, 58 L of 0.1M [Cu(TPMA)Br][Br] in dimethylformamide. The resulting solution was purged with dry nitrogen for at least 20 minutes before 100 L of tin(II) 2-ethylhexanoate was added. The flask was then transferred to an oil bath thermostated oil bath at 80C for 18 hours. Coupons were then removed from the polymerization solution and rinsed and sonicated with ethyl acetate to remove weakly absorbed polymer chains.

3.3.4 Surface Characterization

3.3.4.1 Diffuse Reflectance Infrared Fourier Transform Spectroscopy

All infrared spectra were recorded using a Thermo Nicolet-NEXUS 470 FT-IR equipped with a Diffuse Reflectance Infrared Fourier Transform (DRIFT) attachment. All spectra were collected at 1024 scans with a resolution of 4 cm-1 using a SS316L coupon as

62 a background. Before background or any spectra was collected, the sample was placed in and the chamber was purged with N2 gas for 5 minutes.

3.3.4.2 Contact angle

Contact angles were recorded using a Rame-Hart goniometer and were measured using 2 μL drops of deionized water (Millipore 18Ω) to characterize the hydrophobicity of the surface modifications. Measurements were recorded on three spots on three different substrates (n=9), values were then averaged, and the standard deviation was calculated.

3.3.4.3 Atomic Force Microscopy

Atomic force microscopy (AFM) images were recorded using an Asylum

Instruments MFP-3D Bio AFM. AFM was used to characterize SS316L, before and after modifications with HUPA monolayers, immobilization BiBB, and SI poly(styrene), poly(methyl acrylate), and poly(methyl methacrylate). AFM micrographs (5μm x 5μm area) were collected under ambient conditions in tapping mode using silicon cantilevers with a resonant frequency of 190 kHz and a force constant of 58 N/m (AppNano ACL-20).

Three substrates were analyzed for each modification, and three micrographs were collected on each substrate for a total of 9 micrographs. Root mean square (RMS) roughnesses are reported as an average plus or minus the standard deviation (n=9). Average

RMS roughness was used to evaluate monolayer coverage of the bare substrates.

63 3.3.4.4 Cyclic voltammetry

All cyclic voltammograms were recorded on a NuVant Systems potentiostat using a standard three-electrode setup in 0.1M sodium chloride. The three-electrode setup was comprised of a SS316L coupon as the working electrode, a platinum wire as the counter electrode, and a silver-silver chloride electrode was used as the reference. In order to isolate the modified face of the coupon, the opposite face was masked with epoxy.

Voltammograms were scanned from 0.3 V to -1.2 V relative the Ag/AgCl couple. ImageJ was used to measure the surface area of each electrode, and the resulting voltammograms were normalized to the surface area.

3.3.4.5 Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) was performed using a NuVant

Systems potentiostat with the same electrochemical cell setup as described in Section

3.3.4.4. Measurements were recorded at, 0.4V vs. the Ag/AgCl couple with a frequency range of 0.1 Hz to 10 kHz and 5 data points per decade. Data was acquired using a 10 mV sinusoidal potential. Impedance spectra were fitted to a simplified Randles circuit using

EC-Lab Software and the surface area was measured using ImageJ software. The reported spectra are normalized relative to the surface area of the working electrode.

3.3.4.6 Cytotoxicity and Cell Adhesion Studies

Cell cultures for NIH 3T3 mouse fibroblasts were grown from frozen cultures stored at -78ºC. Cultures were grown to 80% confluency in T75 culture flasks using

Dulbecco's modified Eagle medium supplemented with fetal bovine serum and 10K/10K

64 penicillin/streptomycin. Cells were harvested from culture flasks by removal of growth media and addition of 5ml of trypsin/EDTA solution, followed by incubation for 5-7 minutes. The trypsin/EDTA solution was then removed and transferred to a sterile centrifuge tube. The culture flask was rinsed with trypsin neutralizing solution (TNS) and was then added to the centrifuge tube containing the trypsin/EDTA solution. The cell suspension was centrifuged at 2.1G for 5 minutes. The resulting cell pellet was resuspended in growth media, and cell concentration was determined using hemocytometry with trypan blue.

Figure 3.3. Live/Dead stains used for mammalian cells.

Coupons modified with polymer films were placed into 24 well plates with unmodified SS316L used as the control. The cell suspension was diluted to 10,000

65 cells/mL, and 1 mL was added to each well containing coupons. Coupons were incubated with cell cultures for one, four, and seven days, with media being changed every 24 hrs.

Before imaging, the media was aspirated from each well and rinsed with 1mL of phosphate- buffered saline (PBS) twice. Coupons were then incubated for 5 minutes with Calcein AM and ethidium bromide in PBS using the Live/Dead kit. Coupons were then imaged using a

Zeiss Axioskop 2 materials microscope under 10x magnification using fluoresces filters and AxioVision software.

3.3.7 Statistics

One-way analysis of variance (ANOVA) with a Tukey posthoc analysis was used to determine the statistical difference of averages at a p>0.05 unless otherwise stated,

Grubbs Tess for outliers was used to remove data points when applicable.

3.4 Results

3.4.1 Formation of SAMs of BiBUPA

Bromoisobutyrate ester functionalized surface are needed to provide an initiator for

SI-ATRP. Formation of BiBUPA SAMs was attempted on the surface of SS316L (Scheme

3.2, and Table 3.1). After initial experiments, stable and ordered monolayers that could

-1 survive sonication were not formed. After the deposition, the vCH2asym at 2917 cm and

-1 -1 vCH2asym at 2848 cm and carbonyl stretching at 1734 cm was observed (Figure 3.3).

However, after a THF rinse, the methylene and carbonyl stretching disappeared.

Triethylamine was added to the phosphonic acid solution to catalyze the formation of the covalent bonds between the head group and the oxide surface.2 Further attempts

66 (modification of solution temperature, coupon temperature, annealing), resulted in monolayers not forming. Since this failed to produce monolayers that were stable and ordered after sonication, alternate avenues for the functionalization of SS316L with bromoisobutyrate esters were explored.

Figure 3.3 DRIFT Spectra of the a) methylene stretching region and b) ester stretching region of monolayers BiBUPA formed by aerosol deposition before rinsing (black trace), after rinsing with THF (dashed grey trace)

3.4.2 Formation of Mixed SAMs of DPA and BiBUPA

Mixed self-assembled monolayers of BiBUPA and DPA, in a 9:1 ratio, were formed on the surface of SS316L using aerosol deposition (Scheme 3.3, Table 3.2 Entry

2). After rinsing and sonication in THF to remove any weakly absorbed acid molecules, only one coupon of three had a monolayer present on the surface. The resulting spectrum

-1 -1 with vCH2asym at 2916 cm and vCH2sym at 2847 cm indicated that the monolayer was ordered (Figure 3.4a).3-5 In addition, the peak at 1737 cm-1 in the spectrum of the ester stretching region (Figure 3.4 b) caused by carbonyl stretching indicates that BiBUPA is present in the monolayer since this functional group does exist on DPA molecules.

Figure 3.4 DRIFT Spectra of the a) methylene stretching region and b) ester stretching region of mixed monolayers BiBUPA and DPA (9:1) formed by aerosol deposition after sonication with THF

Additionally, solution deposition was also used in an attempt to form mixed self- assembled monolayers of BiBUPA and DPA using solution deposition (Scheme 3.3, Table

3.2 Entry 3). Similarly, to the results obtained from aerosol deposition, of the mixed

BiBUPA and DPA monolayers, only one coupon of three had monolayers after sonication

-1 -1 with vCH2asym at 2915 cm and vCH2sym at 2848 cm (Figure 3.5a). Additionally, carbonyl and geminal dimethyl stretching of the ester terminus was observed at 1736 cm-1 and 1465 cm-1, respectively (Figure 3.5b).

Figure 3.5 DRIFT Spectra of the a) methylene stretching region and b) ester stretching region of mixed monolayers BiBUPA and DPA (9:1) formed by aerosol deposition after sonication with THF

2.4.3 Immobilization of Bromoisobutyryl Bromide on Hydroxyl Terminated SAMs

Since consistent monolayer formation could not be accomplished via the formation of SAMs of BiBUPA or mixed SAMs of BiBUPA and DPA, immobilization of bromoisobutyryl bromide (BiBB) on monolayers of 11-hydroxyundecyl phosphonic acid

(HUPA) was employed instead. An esterification reaction was carried out between the hydroxyl terminus of the monolayer and bromoisobutyryl bromide to provide the bromoisobutyrate functionality needed for the ATRP initiator. Two different methodologies were employed to functionalize the monolayers with a bromoisobutyrate functionality

3.4.3.1 Formation of HUPA SAMs

To provide a stable and ordered surface for the immobilization of bromoisobutyryl bromide, SAMs of HUPA were formed on the surface of SS316L (Scheme 3.4, Step 1).

69 After sonication in THF the DRIFT spectrum of the monolayer had vCH2sym and vCH2assym at

2915 cm-1 and 2848 cm-1, respectively (Figure 3.6 a). In an ordered or crystalline-like film

-1 with methylene groups in an all-trans orientation, the vCH2sym ≤ 2918 cm and vCH2assym ≤

2848 cm-1.3-5 Additionally, the DRIFT spectrum of the phosphonic acid head group binding region (Figure 3.6 b) of the monolayer has a singular peak at 1016 cm-1 consistent with P-

O stretching.3-7 The absence of additional bands in the head group region corresponding to either P=O at 1231 cm-1 or P-O-H at 930 cm-1, as seen in the DRIFT spectrum of the bulk phosphonic acid (Figure 3.6b, dashed grey trace), indicates that the head group is coordinated to the surface in a tridentate fashion.3-7

Figure 3.6 DRIFT Spectra of the a) methylene stretching region and b) head group binding region of monolayers HUPA formed by aerosol deposition after sonication with THF.

3.4.3.2 Solution Immobilization of Bromoisobutyryl Bromide on Hydroxyl Terminated SAMs

The first method used to form the bromoisobutyrate esters on the surface was to expose coupons with HUPA monolayers to a solution of bromoisobutyryl bromide

(Scheme 3.4, Step 2). DRIFT spectrum of the HUPA modified coupons functionalized with

70 bromoisobutyryl bromide showed a decrease in the intensity of the methylene stretching to below 0.3% reflectance, indicating sub-monolayer coverage (Figure 3.7a).8 Furthermore, the region of the DRIFT spectrum where carbonyl or geminal dimethyl stretching is expected (approximately 1730 cm-1 and 1450 cm-1, respectively), shown in Figure 3.7b, no appreciable change is observed in comparison to the spectrum of the HUPA monolayer.

Further attempts led to the reaction either not occurring or the monolayer being removed from the native oxide layer.

Figure 3.7 DRIFT Spectra the a) methylene stretching region and b) carbonyl stretching region of monolayers HUPA (dashed grey trace) and solution immobilized bromoisobutyryl bromide on HUPA monolayers (black trace). Labels in grey correspond to the spectrum of HUPA monolayers, whereas the labels in black correspond to the spectrum of immobilized bromoisobutyryl bromide

3.4.3.3 Gas-phase Immobilization of Bromoisobutyryl Bromide on Hydroxyl Terminated SAMs

In an attempt to mitigate the consequences of the corrosive nature of bromoisobutyryl bromide observed in the solution-based immobilization strategies, coupons modified with HUPA monolayers were exposed to vapor phase bromoisobutyryl

71 bromide. After immobilization, the DRIFT spectrum of the methyl and methylene stretching region (Figure 3.8a) contained a band at 2968 cm-1, corresponding to the methyl group present on the bromoisobutyrate moiety, which was not present prior to

-1 -1 immobilization. In addition, the retention of CH2 stretching at 2925 cm and 2852 cm after immobilization indicates that the original monolayer remained present under the bromoisobutyrate moiety. The ester stretching region between 1800 cm-1 and 1150 cm-1 contained several new bands that were not present before immobilization. The peaks at

1736 cm-1 and 1465 cm-1 correspond to carbonyl and geminal dimethyl groups on the initiator. The broad band in the spectrum at 1584 cm-1 is caused by the hydrogen-bonding interactions between the carbonyl group and unreacted hydroxyl groups from the HUPA monolayer (Figure 3.8b). Formation of the ester linkage is evident by the appearance of both ester C-O stretching bands at 1276 cm-1 and 1164 cm-1, which are not present in the spectrum of the HUPA monolayer.

Figure 3.8 DRIFT Spectra of the a) methylene stretching region and b) carbonyl stretching region of monolayers HUPA (grey dashed trace) and gas-phase immobilized bromoisobutyryl bromide on HUPA monolayers (black trace). Labels in green correspond to the spectrum of HUPA monolayers, where the labels in red correspond to the spectrum of immobilized bromoisobutyryl bromide

72 To examine the robustness of this reaction methodology and optimize the immobilization of BiBB, several other vapor phase reaction conditions were attempted, including the additional exposures under passive vacuum, duration of exposure, and exposure under active vacuum which is summarized in Table 3.4. An increase in the number of exposures and increased exposure duration also resulted in the formation of the bromoisobutyrate ester, as indicated by DRIFT spectroscopy; however, after increased duration and number of exposures to the vaporous BiBB, the DRIFT spectra showed evidence of unreacted hydroxyl groups. The use of an active vacuum was used to “pull” vapors of BiBB across the surface of coupons. Since no marked improvements were observed by DRIFT spectroscopy, the original conditions from Table 3.4, Trial 1 was used as a starting point for further modifications with polymer films.

3.4.4. Surface Initiated Atom Transfer Radical Polymerization of Styrene, Methyl Acrylate, and Methyl Methacrylate

Coupons functionalized with bromoisobutyryl bromide were placed into a flask containing monomer, anisole, and [Cu(TPMA)Br][Br], and purged with nitrogen gas to remove dissolved oxygen. Tin(II) 2-ethlyhexanoate was then added to initiate SI-ATRP by reducing the copper(II) deactivator to the copper(I) activator and then heated to 80C

(Scheme 3.5). After being rinsed and sonicated in ethyl acetate to remove any weakly, physisorbed polymer chains and dried under vacuum to remove excess solvent molecules, the DRIFT spectrum of the polymer films was collected. The DRIFT spectrum of SI- poly(styrene) modified coupons presented with absorbances between 3080 cm-1 and 3024

-1 -1 -1, cm and between 802 cm and 700 cm which are indicative of C-Haromatic stretching and bending vibration, respectively, resulting from the phenyl groups on the polymer chains

73 (Figure 3.9a). In addition, absorbances at 1600 cm-1, 1492 cm-1, and 1452 cm-1 are all caused by the skeletal C-C stretching of the aromatic rings. The substantial increase at 2923 cm-1 and 2852 cm-1 in comparison to the BiBB immobilized spectrum is caused by the increase in methylene and methine groups on the polymer backbone chain. The spectral features located at 1735 cm-1 and 1261 cm-1 are a result of carbonyl and ester stretching of the immobilized BiBB under the polymer film, indicating that the monolayer is preserved, providing an anchor to the surface of SS316L.

Figure 3.9 DRIFT Spectra of polymer films of a) styrene (black trace) b) methyl acrylate (black trace) and c) methyl methacrylate (black trace). In each spectrum, the spectrum of bromoisobutyryl bromide immobilized on HUPA SAMs (grey trace) is provided as a comparison for before polymerization. All labels correspond to the spectra of the polymer films.

74 Because of the similarity of the polymer structures of SI-poly(methyl acrylate) and

SI-poly(methyl methacrylate), the spectra of both films are similar. In addition, both polymer films are structurally similar to the initiator modified surface. In both, the spectra of poly(methyl acrylate) (Figure 3.9b) and poly(methyl methacrylate) (Figure 3.9c) a significant increase is observed between 3000 cm-1 and 2850 cm-1 indicating a growth in the number of various C-Halkyl stretches, which corresponds to grafting of the polymer chains from the surface. In the spectrum of poly(methyl acrylate), an increase in the carbonyl stretching at 1740 cm-1 and both bands of the ester stretching at 1256 cm-1 and

1164 cm-1 were observed. Additionally, poly(methyl methacrylate) displayed a band at

1740 cm-1 of increased intensity corresponding to the carbonyl stretch of the ester. Both C-

O ester stretching modes are observed at 1245 cm-1 and 1150 cm-1. In both spectra, an infrared band at 1446 cm-1 corresponding to the geminal dimethyl stretching of the initiator terminated surface was observed. Similarly to the poly(styrene) modified surface, this indicates the monolayer is preserved under the polymer film.

3.4.5 Contact Angle Analysis

Contact angle goniometry was used to determine the wettability of the surface modifications. Changes in the wettability should correspond with changes in the functional groups that are presenting at the interface. Data for all surface modifications are presented in Figure 3.10 as the average of nine measurements with standard deviations. Contact angles of greater than or equal to 90º indicate a hydrophobic surface, whereas a contact angle less then 90º indicates a hydrophilic surface. Before functionalization, the native oxide layer of SS316L had a contact angle of 71 ± 3º, indicating a hydrophilic surface,

75 caused by hydrogen bonding interactions of water molecules with bridging -oxos and hydroxyls on the surface of SS316L. After modifying the surface of SS316L with HUPA monolayers, the contact angle rose to 75 ± 5º, which similarly to the bare oxide surface was hydrophilic. This was caused by favorable interactions with water molecules at the terminus of the SAM, indicating that the hydroxyl is presented at the interface in a consistent manner for further modification. The functionalization of the HUPA monolayer with BiBB caused the contact angle to increase from 75 ± 5º to 91 ± 4º indicating the formation of a hydrophobic surface. This switch indicates that the hydroxyl groups that were presented at the interface have been replaced with ester functionalities, confirming the attachment of BiBB. Additionally, this indicates that the bromine and methyl functionalities are presenting at the interfaces, which is necessary for further functionalization with polymer films.

After polymerization, the water contact angle increased for all polymer films when compared to the bromoisobutyryl bromide modified surface. In poly(styrene), the contact angle rose from 91 ± 4º to 109 ± 4º, indicating that the phenyl rings and the backbone of the SI-polymer are presenting at the surface. When the contact angle of the poly(methyl methacrylate) was measured, the contact angle resulted in a highly hydrophobic surface with a contact angle of 125 ± 6º. This indicates that the polymer backbone and the methyl groups are presented at the surface. However, in comparison, the polymer films form from methyl acrylate had a significantly lower contact angle of 103 ± 2º. This lower contact angle is likely due to a mixture of both the methyl ester side chains and the polymer backbone presenting at the interface, the ester allows for more favorable interactions with water than the poly(methyl methacrylate) films.

3.4.6 Atomic Force Microscopy

To further study the surface morphology of the SAM modified and polymer functionalized SS316L, atomic force microscopy was employed. Three locations on three different coupons were scanned in a 5um x 5um area, and the average RMS roughness is reported with the standard deviation (summarized in Table 3.5). Micrographs of the bare, native oxide surface were obtained and had an average roughness of 4.1 ± 1.5 nm (Figure

3.11a). The evidence of striations on the surface are a result of the final polishing with 1um diamond suspension and has been well noted in the literature.3, 9-11 After the coupons were modified with HUPA SAMs , there was a minimal change in the average RMS roughness to 4.1 ± 1.5 nm to 3.1 ± 0.6 nm; coupled with the evidence of the polishing lines still being present on the surface (Figure 3.11b) indicates that the SAMs follow the contours of the surface. Immobilizing BiBB on the surface resulted in a significantly rougher surface with

77 an average roughness of 15.1 ± 3.8 nm. In addition to the change in roughness that was observed, the polishing lines became obscured, indicating that small molecules have been immobilized on the surface (Figure 3.11c). After the formation of the poly(styrene) film, the morphology of the surface changed again, resulting in a film with many hills and valleys on the surface (Figure 3.11d), indicated by the high RMS roughness of 27.0 ± 6.0 nm. The film formed by SI-ATRP of methyl methacrylate resulted in a film that had approximately the same roughness as the unmodified SS316L coupons, 5.5 ± 1.3 nm, indicating that the polymer film is smooth (Figure 3.11e). In addition, polishing lines on the micrographs of the polymer films have been obscured, indicating complete coverage of the surface.

78 Table 3.5 Average RMS roughness of modifications calculated from AFM micrographs (n=9)

Modification RMS roughness (nm) Native SS 316 L 4.1 ± 1.5 HUPA SAM 3.1 ± 0.6 BiBB immobilized 15.1 ± 3.8 Poly(styrene) 27.0 ± 6.0 Poly(methyl methacrylate) 5.5 ± 1.3 Poly(methyl acrylate) 32.6 ± 11

3.4.7. Electrochemical Characterization

In order to determine the corrosion protection ability of the polymer films, electrochemical impedance spectroscopy and cyclic voltammetry were employed to monitor electrochemical reactions occurring at the surface. Electrochemical impedance spectroscopy was used to measure the charge transfer resistance of the surfaces; whereas cyclic voltammetry was used to observe redox reactions occurring. Functionalized coupons were placed in a three-electrode system as the working electrode. Coupons were immersed in a 0.1M sodium chloride electrolyte solution to mimic a saline environment. As a comparative analysis, electrochemical data for the native oxide is provided alongside the data collected for functionalized coupons.

3.4.7.1. Electrochemical Impedance Spectroscopy

The Nyquist impedance spectra, shown in Figure 3.12., are of the solution electrode interface, modeled using the equivalent Randles circuit represented in Figure 3.13. The circuit is used to model the solution electrode interface, where the Rsoln corresponds of the resistivity of supporting electrolyte solution, and the Rct (charge transfer resistance) and

CPE (constant phase element) corresponds to the resistance and capacitance, respectively, of the surface plus the modification. Previously the Rct has been shown to be an excellent

79 measure of the corrosion resistant nature of a surface since charge transfer is needed for corrosion to occur. 12-24 After the coupons were functionalized with HUPA monolayers there was an increase in the size of the capacitive loop resulting in an increase in the charge transfer resistance from 3.1 kΩ to 5.9 kΩ. Upon functionalization with BiBB, the Rct of the

SAM modified surface increased to 19 kΩ. Upon functionalization with polymer films, the charge transfer resistance increases again to 360 kΩ for poly(styrene), and 770 kΩ for the poly(methyl methacrylate), and 1900 kΩ for the poly(methyl acrylate) surfaces.

Figure 3.13. The equivalent circuit for electrochemical impedance spectra. Rsoln is solution resistance, and Rct and CPE are the charge transfer resistance and capacitance for the electrode, respectively.

Table 3.6. Charge transfer resistances and protection efficiencies (PE) for Bare and Modified SS 316L

Modification Rct(kΩ) PE Native SS 316 L 3.1 --- HUPA SAM 5.9 47.5 % BiBB immobilized 19 83.7 % Poly(styrene) 360 99.2 % Poly(methyl methacrylate) 770 99.5 % Poly(methyl acrylate) 1900 99.8 %

Furthermore, the protection efficiency (PE) can be calculated using the charge transfer resistances of both the modified surface (Rct mod) and the native oxide surface (Rct

20, 25-32 bare) using Equation 3.1. After HUPA monolayers were formed on the surface and

BiBB was immobilized, protection efficiencies of 47.5%, and 83.7% were observed, indicating that these thin films do have a modest impact on the corrosion resistivity of the surface, which has been shown in the literature. After grafting polymer films from the surface, protection efficiencies ranging from 99.2% to 99.8% were observed for films of poly(styrene), poly(methyl methacrylate), and poly(methyl acrylate) (Table 2.6)

81 푅 − 푅 푃퐸 = 푥100% 푅

Equation 3.1

3.4.7.2 Cyclic Voltammetry

Cyclic voltammograms were collected of the modified and unmodified coupons, from 0.3V to -1.2V (vs. Ag/AgCl couple). In the cyclic voltammogram of the bare oxide, a couple centered at 0.67 V corresponds to the oxidation/reduction of iron (II) oxides and hydroxides to the corresponding iron (III) species (Figure 3.14a, black trace).33, 34 A reduction in the current corresponds to a reduction in the number of redox reactions occurring at the interface, therefore indicates an increase in the corrosion resistance of the surface.24, 35-37 After functionalization with HUPA monolayers, the anodic current decreased; however, the cathodic wave shifted to a lower potential with slightly lower current (Figure 3.14a). Upon modification of HUPA monolayers with BiBB, both a decrease in both anodic and cathodic waves was observed in comparison to the native oxide surfaces. Upon the growth of polymer films from the surface, a dramatic decrease in the current was observed for all the polymer-modified surfaces.

Figure 3.14 a) Voltammograms of the native oxide surface (black trace), HUPA SAM (solid grey trace), and the BiBB immobilized (dashed grey trace). b) Voltammograms the native oxide surface (black trace), SI-poly(styrene) (solid grey trace), the SI-poly(methyl methacrylate) film (dashed grey trace), and the SI-poly(methyl acrylate) (dash-dotted grey trace) films.

Assuming the diffusion to the surface of the electrode is planner, the fractional coverage (θCV) can be calculated from the relative normalized currents in the

35-37 voltammograms using Equation 3.2 , where imod and ibare is the magnitude of the current observed in the modified and unmodified coupon, respectively. After the formation of the

HUPA SAMs, the fractional coverage was calculated at 0.29, and the immobilization of the BiBB on the surface of HUPA SAM further increased the fractional coverage to 0.70.

The formation of the polymer film of the surface increased to 0.99. 0.91, and 0.99 for poly(styrene), poly(methyl methacrylate), and poly(methyl acrylate) respectively.

83 Table 3.7. Fractional coverages for Bare and Modified SS 316L calculated from cyclic voltammetry

Modification θCV Native SS 316 L ----- HUPA SAM 0.29 BiBB immobilized 0.70 Poly(styrene) 0.99 Poly(methyl methacrylate) 0.91 Poly(methyl acrylate) 0.99

푖 휃 =1− 푖

Equation 2.2

In order to assess the electrochemical stability of these films, coupons were immersed in 0.1M NaCl, and the potential was cycled across the polymer film fifteen times.

In the voltammograms of the native oxide surface (Figure 3.15a), significant changes at both the anodic and cathodic waves are observed, indicating significant changes and an increasing amount of redox reactions occurring at the electrode electrolyte solution interface with each cycle. The repetitive voltammograms collected of the polymer films indicated (Figure 3.15b and c) minimal changes at the anodic wave, indicating a lack of the oxidation half-reaction occurring. A small increase at the cathodic wave indicates that the reduction Fe(III) mixed oxides and hydroxides to Fe(II) are still occurring.

Coupons modified with polymer films were placed in electrolyte solutions and voltammograms were recorded every 24 hours over the course of five days. The voltammograms collected of the native oxide (Figure 3.16a) display an increase at both the cathodic and anodic potential indicating an increase in the redox reactions. The voltammograms of the polymer films (Figure 3.16 b-d) show little change, indicating that the films remain intact on the SS316L surface.

3.4.8. Cytotoxicity and Normalized Cell Counts of NIH 3T3 Fibroblasts

As a first level cytotoxicity test, coupons were cultured with NIH 3T3 mouse fibroblasts for one, four, and seven days. After incubation with cell cultures, the coupons were rinsed with phosphate-buffered saline and stained with calcein AM and ethidium bromide homodimer, which cause live cells to fluoresce green and dead cells to fluoresce red. Three coupons were imaged in five different locations using a fluorescence materials microscope for a total of 15 images per modification per day (Figure 3.17).

86 Bare Poly(styrene) Poly(methyl acrylate) Poly(methyl methacrylate)

Day Day 1

Day Day 4

Day Day 7

Figure 3.17 Live/Dead fluoresces microscopy images of modified and unmodified coupons. Red indicates a dead cell green indicates a lives cell

After one day, there was no statistical difference observed for any of the normalized cell counts on day one with bare, poly(methyl acrylate), poly(methyl methacrylate), and poly(styrene) surfaces having 100 ± 16 cells, 99 ±15 cells, 82 ± 11 cells, and 112 ± 23 cells per image. On day four, 100 ± 7 cells, 113 ± 13 cells, 93 ± 12 cells, and 69 ±16 cells, were counted on the bare, poly(methyl acrylate), poly(methyl methacrylate), and poly(styrene) surfaces, with none of the data points being statistically different than the unmodified surface. On the final day, a statistical difference was observed between the unmodified surface with 100 ± 6 cells and poly(methyl methacrylate) with 130 ± 4 cells. The poly(methyl acrylate) surface had 84 ± 3 cells, whereas the poly(styrene) modified surfaces had 112 ± 4 cells, with both modifications not being statistically different from the unmodified samples.

Figure 3.18 a) Normalized live cell counts and b) normalized cell viability of NIH 3T3 mouse fibroblasts. Poly(MA) = poly(methyl acrylate), Poly(MMA) = poly(methyl methacrylate), and Poly(Sty) = poly(styrene). Error bars represent the standard error in each measurement with n=15. “*” indicates the statistical difference between control (Bare surface) and the modified surface with p< 0.05.

퐿푖푣푒 푐푒푙푙푠 Percent Viability = 퐿푖푣푒 푐푒푙푙푠 + 퐷푒푎푑 퐶푒푙푙푠

Equation 3.3

The percent viability was calculated using Equation 3.3. Over the course of seven days, little change in the normalized percent viability was observed. On day one, the normalized viability for the bare surface was measured at 100 ± 1.5% and poly(methyl acrylate), poly(methyl methacrylate), and poly(styrene) was measured to be 99 ± 0.7%, 97

± 1.2%, and 97 ± 1.5% (Figure 2.18b). After being incubated with fibroblast the normalized percent viability for the bare surface, poly(methyl acrylate), poly(methyl methacrylate), and poly(styrene) was 100 ± 1.4%, 106 ± 0.4%, 104 ± 1.0%, 103 ± 1.8% and , respectively,

88 with the value for poly(methyl acrylate) being statistically significant at day four (p< 0.05).

Furthermore, on day seven, a statistically significant increase in the viability of cells grown on the poly(styrene) and poly(methyl methacrylate) to 106 ± 0.9% and 104 ± 0.9%, respectively, from the bare surface at 100 ± 2.0%. However, the poly(methyl acrylate) was not statistically different from the bare surface, with an average viability of 100 ± 1.2%.

3.5 Discussion

The goal of this research was to form hydrophobic polymer films of poly(styrene) poly(methyl acrylate) and poly(methyl methacrylate) using SI-ATRP (Scheme 3.6). After the functionalization of SS316L with the bromoisobutyrate moieties, polymers of styrene, methyl acrylate, and methyl methacrylate were grown from the surface. The resulting films were then evaluated for their corrosion resistance.

Br Monomers m R Br Styrene O O O O O O Monomer Methyl Acrylate 9 [Cu(TPMA)Br][Br] 9 O P Tin (II) Ethylhexanoate P O O O O O O O

SS 316L SS 316L Methyl Methacrylate

Scheme 3.6 Functionalization of SS316L with polymer films of styrene, methyl acrylate, and methyl methacrylate

In order to have an anchor point for polymers to be grafted from the surface, self- assembled monolayers presenting a bromoisobutyrate moiety were prepared. Two different approaches were used in order to generate the surface; formation of monolayers with a bromoisobutyrate tail functionality, and functionalization of hydroxyl-terminated SAMs

89 with bromoisobutyryl bromide. The presence of this tail group is needed as an ATRP initiator to form polymer films on the surface of SS316L.

SAMs of BiBUPA were unable to be formed on the surface of SS316L. This is possibly due to not heating the metal coupon to a sufficient temperature to cause the formation of covalent metal-O-P linkage (Figure 3.19a), because of the heat sensitivity of the tail group. Heat has been proposed to drive off water molecules from the hydrogen bonding interactions formed between the phosphonic acid and the surface hydroxyls.38 A second explanation is the steric bulk that the geminal dimethyl and bromine moiety on the terminus creates, which inhibits the formation of van der Waals interactions (Figure

3.19.b). Since this interaction between alkyl chains is needed for stable SAMs to form, the ester terminus could prevent chains from forming interactions.3-5, 39 In order to investigate and mitigate the inability to form SAMs of BIBUPA, the immobilization of BiBB on hydroxyl-terminated SAMs was used.

a) Br b)

O Br O Br Br O O O O 9 O O O P OH Heat 9 O -H O 9 9 H 2 O P OH H P P OH O OH OH O OH O O O O O O O O O O SS 316L SS 316L SS 316L

90 BiBB was immobilized on the surface of hydroxyl-terminated monolayers via an acid bromine esterification reaction. SAMs of HUPA were formed on the surface of

SS316L. DRIFT spectroscopy indicated that the SAMs formed on the surface were ordered

-1 -1 3-5, 39 since vCH2asym ≤ 2918 cm and vCH2sym ≤ 2848 cm . Contact angle goniometry indicated a hydrophilic film on the surface of SS316L, and coupled with the DRIFT spectroscopy data; this indicates that the monolayer is ordered with the hydroxyl tail groups presenting at the interface in a consistent manor. This is necessary for further functionalization of the monolayer. In addition, a minimal change in the average roughness of the surface was observed from AFM, indicating that absorbed acid molecules follow the contours of the surface, further complementing the DRIFT and contact angle data in confirming monolayer coverage.

Vapor phase immobilization of BiBB was used to obtain the initiator modified film. DRIFT spectroscopy indicated the presence of IR signals corresponding to the carbonyl and methyl stretching that were not present in the monolayer spectrum, but are in the target molecule. Additionally, both modes of ester stretching are present in the spectrum of immobilized BiBB, indicating that the intended linkage to the surface was formed and the BiBB was not physisorbed. The retention of the methylene stretching indicates that the monolayer is preserved under the immobilized target molecule which is needed to provide a stable anchor for the polymer films. Contact angle data indicated a transition from a hydrophilic to a hydrophobic surface, which is consistent with a reduction in the groups that can participate in hydrogen bonding with water. Furthermore, the increase in the average roughness of the surface and the obscuration of the polishing lines indicates a small molecule is attached to the monolayer.

91 Following the immobilization of the BiBB, the bromoisobutyrate terminated surface was used as an initiator for SI-ATRP. Films of poly(methyl acrylate), poly(methyl methacrylate), and poly(styrene) were formed using activators (re)generated by electron transfer (ARGET) ATRP. Films of poly(styrene) present with stretching vibrations of C-

Haromatic and C-Caromatic bonds, as well as the corresponding bending vibrations of the aromatic rings. The increase in the intensity of the methylene stretching indicates an increase in the relative amount of methylene groups, which should accompany the growth of chains from the surface. A change in the water contact angle from 91 ± 4º to 109 ± 4º indicated a transition to a more hydrophobic surface, due to the backbone or phenyl rings of the polymer are presenting at the interface. AFM microscopy indicated that the polymer films completely covered surface, as the polishing lines observed in the bare SS316L were not present in the modified sample.

Films of poly(methyl acrylate) and poly(methyl methacrylate) grafted to the surface had similar IR absorptions when compared to the spectrum of immobilized initiator, due to the absence of unique functional groups. However, all of the absorptions with the exception of the band caused by the geminal dimethyl group, had a sharp increase in the percent reflectance indicating an increase in the number of corresponding functional groups, that would be associated with polymer chains growing from the surface. Contact angle measurements both polymer chains changed substantially from the BiBB modified surface, indicating a change in functional groups presenting at the interface. In the poly(methyl methacrylate) modified samples, the contact angle of 125 ± 6º indicates that methyl groups off the polymer chain, as well as the alkyl backbone, is presenting at the interface. However, the drop in the contact angle of the poly(methyl acrylate), when

92 compared to the poly(methyl methacrylate), is due to the lack of methyl groups on the polymer backbone, and indicating a mixture of ester and alkyl moieties presenting at the interface. In all DRIFT spectra of polymer modified surfaces, retained spectral features that are indicative of the initiator modified monolayer after being activated using ATRP. The presence of these vibrations indicates the monolayer is retained under the polymer film, providing covalent attachment to the surface of SS316L. In the AFM micrographs of the polymer film modified surface, polishing lines observed in the bare SS316L were not present in the modified sample, indicating the films completely covered surface.

The corrosion resistant properties of the polymer films were examined using cyclic voltammetry and electrochemical impedance spectroscopy. After the functionalization of

SS316L surfaces with monolayers, there was a modest increase in corrosion resistance of the surface, as noted by the cyclic voltammetry and electrochemical impedance spectroscopy by a decrease in the current and an increase in the charge transfer resistance, respectively. This increase in the corrosion resistance has been documented to occur upon the functionalization with SAMs in the literature and is within normal range.6, 11, 24 In the cyclic voltammograms of the HUPA SAM monolayer, the anodic wave shifted, possibly due to the oxidation and reduction of the terminal alcohol to either an aldehyde or a carboxylic acid. When functionalized with BiBB immobilized on HUPA, an increase in the fractional coverage was observed and was likely due to the increasing size of the terminus of the monolayer. Additionally, an increase in the charge transfer resistance from the HUPA monolayer was observed after functionalization with BiBB. The formation of the polymer films, all three modified surfaces presented with near-zero current passing though the coupon, indicating that the films were capable of resisting the flow of electrons

93 that would be needed for corrosion to occur. The high fractional coverage that was calculated from voltammograms is strongly supported by the AFM micrographs which show obscuration of polishing lines. Additionally, the electrochemical impedance revealed an increase in the charge transfer resistance by approximately two to three orders of magnitude, which was not observed when SS316L was modified with HUPA monolayers or immobilized BiBB. The increase in Rct has been linked to an increase in the corrosion resistant nature of the surface. The protection efficiency calculated from the comparison of the charge transfer resistance of the unmodified and modified surfaces, indicated that the polymer films are capable of resisting corrosion. Furthermore, the modifications are required to form an anchor for the polymer films show minimal corrosion resistance when compared to the polymer films. The protection efficiency tracked well with the fractional coverage that was obtained from cyclic voltammetry further confirming the pacified nature of the polymer film coated coupons.

Additional cyclic voltammetry experiments indicated that these films are stable to both repetitive current and prolonged immersion in an electrolyte solution. When the bare oxide surface was exposed to the repetitive potentials being cycled across a coupon, significant changes in the current at both the anodic and cathodic wave was observed.

When coupons modified with polymer films were subjected to repetitive CV, a small increase in the cathodic wave (in comparison to changes observed in the native oxide surface) indicated that the Fe(III) oxides at the interface are undergoing reduction to Fe(II).

However, the absence of the anodic peak, corresponding to the oxidation reactions, which are ultimately responsible for corrosion occurring, showed minimal charge. This indicates that even after having potential cycled across the films, they are still able to pacify the

94 surface. In a prolonged exposure to a saline solution, the films remained stable over a five- day period with no change in current at the anodic and cathodic waves of the voltammogram. This indicates that the films are stable and not delaminating from the surface.

In order to examine suitability of these polymer films in applications in biomedical situations, coupons were cultured with NIH-3T3 mouse fibroblasts. Studies on the polymer-modified surfaces indicated that they were not cytotoxic when compared to the bare metal oxide since there was no statistically significant decrease in the normalized cell viability after seven days. Additionally, after four days, there was an increase in the percent viability of the poly(methyl acrylate) that was statistically significant. Furthermore, after seven days, both poly(styrene) and poly(methyl methacrylate)also had a statistically significant increase in the viability. Since the SS316L has been widely used for the preparation of biomedical devices such as hip implants, vascular stents, and dental implants, 40-48 it can be assumed anything that has an average normalized cell viability that is not statistically lower, it would be safe in biomedical applications. This is expected since bulk polymers of poly(styrene) have been used for disposable cell culture supplies. In addition, poly((meth)acrylates) have been used in numerous biomedical devices such as optical implants.49, 50 In addition, the normalized cell counts showed little change over the course of the seven-day experiment, with the exception of a statistically significant increase observed for the poly(methyl methacrylate) film on day seven.

95 3.6 Conclusions

In conclusion, polymer films of styrene, methyl acrylate, and methyl methacrylate were formed on the surface of SS316L using SI-ARGET-ATRP. In order to polymerize the monomers from the surface, a bromoisobutyrate surface was needed. Multiple attempts were made to form SAMs of BiBUPA, but all attempts failed. In an alternate attempt to form monolayers terminated with the ATRP initiator, HUPA SAMs were exposed to vapors of BiBB. DRIFT spectroscopy indicated the formation and ester bond between the monolayer and the acid halide, confirming immobilization. The contact angle revealed that both the bare oxide surface and the HUPA monolayer formed favorable interactions with water as indicated by the hydrophilic contact angles. Polymer films, in addition to the initiator modified surfaces, displayed hydrophobic contact angles. Electrochemical analysis indicated that the polymer films were able to inhibit corrosion with up to a 99.9% protection efficiency. Additional studies aimed at evaluating the polymer films stability in electrochemical and aqueous saline environments revealed that a five-day exposure to 0.1

M sodium chloride and repeated cycling of potentials over the coupons yielded minimal changes. Cell cultures with NIH-3T3 fibroblast cells indicated that the polymer films were non-cytotoxic and has potential applications in biomedical applications.

This work contained within this chapter aims at furthering the study of corrosion resistant, polymer modified surfaces on SS316L through SI-ATRP. Previously films of poly(styrene) poly(methyl acrylate) and poly(methyl methacrylate) have not been formed on SS316L using SI-ATRP. The generated polymer films would be excellent corrosion resistance barriers in marine environments. Furthermore, the SI-polymer films of styrene,

96 methyl acrylate, and methyl methacrylate have never been evaluated for their cytotoxicity when formed though SI-ATRP or any other polymerization technique.

3.7 References

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100 46. Chen, S.; Frank Cheng, Y.; Voordouw, G., A comparative study of corrosion of 316L stainless steel in biotic and abiotic sulfide environments. International Biodeterioration & Biodegradation 2017, 120, 91-96.

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48. Zhang, B.; Yan, Q.; Yuan, S.; Zhuang, X.; Zhang, F., Enhanced Antifouling and Anticorrosion Properties of Stainless Steel by Biomimetic Anchoring PEGDMA- Cross-Linking Polycationic Brushes. Industrial & Engineering Chemistry Research 2019, 58 (17), 7107-7119.

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50. Hollick, E. J.; Spalton, D. J.; Ursell, P. G.; Pande, M. V., Biocompatibility of poly(methyl methacrylate), silicone, and AcrySof intraocular lenses: Randomized comparison of the cellular reaction on the anterior lens surface. Journal of Cataract & Refractive Surgery 1998, 24 (3), 361-366.

101 Chapter 4: Antifouling and Corrosion Resistant Surface Modification

4.1 Introduction

Biofouling is the process by which a surface becomes contaminated with proteins and cellular growth (Figure 4.1), and can ultimately lead to unwanted adhesions or bacterial biofilms.1-3 Corrosion and biofouling are two possible reasons for the failure of implant materials, causing the need for premature revision surgeries. Cells use absorbed proteins as a scaffold, to which they attach to the surface of materials.1-3 Previously, immobilization of antibacterial or biocidal molecules to the surface has been demonstrated to be effective in inhibiting surface fouling; however, over time, these modifications can lose their efficacy, and they do not treat the root cause of fouling, the absorption of proteins.4-9 Other techniques, such as micro-patterning and protein repulsive modifications, which aim to inhibit protein absorption, have been used to reduce or eliminate the growth of cells on the surface. 1-3 Micro-patterned surfaces do not allow for further functionalization, which limits their utility. In contrast, protein repulsive modifications rely on the immobilization of polymer films such as poly(ethylene glycol) (PEG)10-13 and poly(ethylene ethylphosphate)

(PEEP),14 which permit the for further functionalization of the surface.

Figure 4.1 Formation of biofilms on bare metallic surfaces.

102 Herein is presented the modification of SS316L surface with polymer films of oligo(ethylene glycol) acrylate (OEGA) and oligo(ethylene glycol) methacrylate

(OEGMA). The resulting films were examined for potential corrosion and cell resistant coatings. The PEGylated polymer films showed poor corrosion resistance properties, but good preliminary cell resistant properties. Since poly(OEGA) and poly(OEGMA) are well known in the literature to impart antifouling properties,15-24 the improvement of their corrosion resistance was attempted. To improve the corrosion resistance of the PEGylated films, poly(styrene) was chain extended with OEGA and OEGMA, as polymer films of styrene have proven to be effective in inhibiting corrosion. Hydroxyl-terminated monolayers were formed on the surface of SS316L and modified with bromoisobutyryl bromide, which serves as an atom transfer radical polymerization initiator (ATRP).

Polymer films of OEGA and OEGMA as well as block co-polymers films of poly(styrene- co-OEGA) and poly(styrene-co-OEGMA) were then formed on SS316L using ATRP.

4.2 Equipment and Materials

4.2.1 Equipment

All infrared spectra were recorded using a Thermo Nicolet-NEXUS 470 FT-IR equipped with a Diffuse Reflectance Infrared Fourier Transform (DRIFT) attachment. All electrochemical measurements were recorded using a NuVant System EZstat-pro potentiostat and EZware-pro and EZware-EIS software. The three-electrode system was comprised of a platinum wire as a counter electrode, an Ag/AgCl(aq) as the reference electrode, and SS316L coupon as the working electrode. Contact angles were recorded using a Rame-Hart goniometer. MelodySusie ultraviolet Lamp (36W) centered at 365nm

103 was used as a source of ultraviolet radiation for photo catalyzed polymerization. A incubator was maintained at 37°C and 5% carbon dioxide. All nuclear magnetic resonance

(NMR) spectra were performed in deuterated chloroform and were recorded on a Bruker

Advance 2 400 MHz spectrometer, with spectra being averaged over 16 scans.

4.2.2 Materials

11-Hydroxyundecyl phosphonic acid (>95%), tin (II) ethylhexanoate (95%), oligo(ethylene glycol)methyl ether acrylate (OEGA, %), oligo(ethylene glycol)methyl ether methacrylate (OEGMA, %) and anisole (99.7%, anhydrous) were purchased from

Sigma Aldrich. Bromoisobutyryl bromide (97%, BIBB) and styrene (99%) were purchased from Alfa Aesar. Tetrahydrofuran (THF) was purchased from Marcon. All reagents were used without further purification unless otherwise stated. Styrene was passed through a silica column prior to polymerization to remove inhibitors. OEGA and OEGMA were dissolved in an equal volume of dichloromethane and passed through a silica column.

Dichloromethane was evaporated, and the monomers were stored over molecular sieves prior to polymerization. THF was dried over sodium and benzophenone and distilled before use. Tris(2-pyridylmethyl)amine (TPMA) was prepared according to literature procedures.25 Following the synthesis of TPMA, it was recrystallized once from ethyl acetate then again from diethyl ether yielding pale crystals. Catalyst solution (0.01M

[Cu(TPMA)Br][Br]) was prepared by dissolving equal molar amounts of TPMA and copper(II) bromide in dimethyl formyl amide and mechanical stirring for 30 minutes prior to use. Stainless steel AISI 316L (Fe 69/Cr18 /Ni10/Mo3) foil with a 0.5 mm thickness was purchased from Goodfellow Inc. Dulbecco’s modified Eagle medium (DBEM),

104 10K/10K penicillin-streptomycin mixture, trypsin/EDTA, Fetal bovine serum (FBS), and trypsin neutralizing solution (TNS) were obtained from Lonza Walkersville Inc. D1-

Chloroform (99.8% D) was purchased from Sigma Aldrich and stored in a desiccator between uses. Phosphate buffered saline (Sterile filtered, PBS) was obtained from Sigma

Aldrich and stored at 4°C between uses. Invitrogen™ LIVE/DEAD™

Viability/Cytotoxicity Kit for mammalian cells was purchases from Fisher Scientific and stored at 0°C. Trypan blue was obtained from Sigma Aldrich

4.3 Methods

4.3.1 Preparation of SS316L coupons

Substrates were prepared by sanding stainless steel foils with 160, 320, 400, and

600 grit sandpaper. Substrates were then cleaned by sonication in acetone for 30 minutes and rinsed in boiling methanol for 30 minutes and placed in a 120 °C oven overnight.

Coupons were then stored in a sealed container under ambient conditions until used.

4.3.2 Functional of SS316L with ATRP Initiators

4.3.2.1 SAM Formation of 11-Hydroxyundecyl Phosphonic Acid

Monolayers of 11-hydroxyundecyl phosphonic acid (HUPA) were formed by aerosol deposition of 0.5 mM solution in dry tetrahydrofuran (THF) using a thin layer chromatography (TLC) sprayer (Scheme 4.1, Step 1). Coupons were cooled to 4°C for 30 minutes and were then sprayed with a 60°C solution of HUPA. This process was repeated two additional times, and coupons were allowed to rest at 4°C for 30 minutes between depositions. Coupons were then annealed in an oven overnight at 60°C. Samples were

105 rinsed for 15 minutes and sonicated for 15 minutes in dry THF to remove any physisorbed acid molecules.

Br O m R Br O O O O HO O O

PEGlyated (meth)acrylate 9 9 [Cu(TPMA)Br][Br] 9 11-HUPA P BiBB P Excess TPMA and hv P OH OH O O O O O O O 0.5 mM in THF O Vaccuum O O

SS 316L Step 1 SS 316L Step 2 SS 316L Step 3a SS 316L

Styrene [Cu(TPMA)Br][Br] Step 3b Tin (II) Ethylhexanoate 18 hours 80 OC O O Br Br

O n O n

O Br O m m m

O O O O O O

9 PEGlyated methacrylate 9 PEGlyated acrylate 9 [Cu(TPMA)Br][Br] [Cu(TPMA)Br][Br] P Excess TPMA and hv P Excess TPMA and hv P O O O O O O O O O

SS 316L SS 316L SS 316L Step 4a Step 4b Scheme 4.1 Formation of HUPA monolayers, immobilization of bromoisobutryl bromide, and the formation of the SI-ATRP homo-polymer films of poly(styrene) and poly(OEGA), and poly(OEGMA) and block copolymer films poly(styrene-co-OEGA) and poly(styrene- co-OEGMA).

4.3.2.2 Gas-phase Immobilization of Bromoisobutyryl Bromide on Hydroxyl Terminated SAMs

Coupons with SAMs of HUPA were modified with bromoisobutyryl bromide

(Scheme 4.1, Step 2). Coupons were placed in a vacuum deposition chamber and placed under vacuum on a Schlenk line for 30 minutes, as shown in Figure 4.2. After the trap was frozen, the coupons were exposed to vapors of bromoisobutyryl bromide for 30 minutes under a passive vacuum. Following the first exposure, the coupons were returned to

106 vacuum for 30 minutes. Exposure was repeated so that the coupons were exposed to bromoisobutyryl bromide three times. After three exposures, the system was placed under

N2, and the flask contains BiBB was removed, and the entire system was returned to vacuum for 12 hours. Coupons were rinsed with methanol to remove any physically absorbed bromoisobutyryl bromide.

To Vacuum Reaction Chamber

BiBB

Figure 4.2. Diagram of the gas phase reaction setup for the immobilization of bromoisobutyryl bromide in Section 4.3.2.2

4.3.3 Functionalization of SS316L with Polymer Films

4.3.3.1 SI-polymerization of oligo(ethylene glycol)methyl ether acrylate (OEGA)

Three coupons with bromoisobutyryl bromide immobilized on HUPA SAMs were placed in a 50ml Erlenmeyer flask with 0.1180g of tris(pyridylmethyl)amine (TPMA). The flask was sealed with a septum. The flask was then evacuated and refilled with nitrogen three times. To the flask, 6mL of OEGA, 0.4µL of methyl bromoisobutyrate, and 3 mL of

DMF were added. The resulting solution was purged for 30 min by bubbling with nitrogen gas through the solution. 0.568mL of 0.01M [Cu(TPMA)Br][Br] in DMF was added and bubbled through the solution for an additional 5 minutes (Scheme 4.1, Step 3a). The flask was placed on a stir plate and placed under a UV Lamp and stirred for 18 hours. Coupons were removed from the solution and rinsed with ethyl acetate to removing any unreacted

107 monomer. Coupons were dried by placing them under vacuum for six hours before further use.

4.3.3.2 SI-polymerization oligo(ethylene glycol)methyl ether methacrylate

Three coupons with bromoisobutyryl bromide immobilized on HUPA SAMs were placed in a 50 ml Erlenmeyer flask with 0.1180g of tris(pyridylmethyl)amine (TPMA) and a stir bar. The flask was sealed with a septum. The flask was evacuated and refilled with dry nitrogen gas three times. Into the flask was added 6mL of OEGMA, 0.4 µL of methyl bromoisobutyrate, and 3ml of anisole. The resulting solution was degassed by purging for

30 minutes by bubbling nitrogen gas through the solution. 0.568mL of 0.01M

[Cu(TPMA)Br][Br] in DMF was added and bubbled through the solution for an additional

5 minutes (Scheme 4.1, Step 3a). The flask was placed on a stir plate and placed under a

UV Lamp and stirred for 18 hours. Coupons were removed from the solution and rinsed with ethyl acetate in order to remove any unreacted monomer.

4.3.3.3 SI-polymerization styrene

Three coupons with bromoisobutyryl bromide immobilized on HUPA SAMs were placed in a 50 ml Erlenmeyer flask with a stir bar and was sealed with a septum. The flask was evacuated and refilled with dry nitrogen gas three times. Under inert conditions a 9mL solution containing 7.4M styrene, 0.5mM methyl bromoisobutyrate, and 21mM tin(II) 2- ethylhexanoate was prepared in DMF. Nitrogen was bubbled through the monomer solution for 20 minutes to degas the mixture. After degassing, 568µL of

[Cu(TPMA)Br][Br] 0.01M catalyst solution was added, and the solution was purged with

108 nitrogen for an additional 5 minutes (Scheme 4.1, Step 3b). The flask was then placed in a sand bath thermostated at 80°C and monitored with NMR. The reaction was removed when

70% conversion of the styrene monomer was reached via NMR. Coupons were rinsed and sonicated with ethyl acetate to remove any physically adhered polymer chains. Samples were dried under vacuum for at least six hours (~0.1 torr) before further use.

4.3.3.4 SI-block polymerization styrene and oligo(ethylene glycol) methyl ether acrylate

Three coupons, modified with SI-polystyrene, were chain extended by being placed in a 50 ml Erlenmeyer flask with 0.1180g of tris(pyridylmethyl)amine (TPMA) and a stir bar. The flask was sealed with a septum. The flask was evacuated and refilled with dry nitrogen gas three times. Into the flask was added 6mL of oligo(ethylene glycol)methacrylate, 0.4µL of methyl bromoisobutyrate, and 3ml of DMF. The resulting solution was purged for 30 min by bubbling nitrogen gas through the solution. 0.568mL of

0.01M [Cu(TPMA)Br][Br] in DMF was added and bubbled through the solution for an additional 5 minutes (Scheme 4.1, Step 4a). the flask was placed on a stir plate under a UV

Lamp, and the reaction solution was monitored with NMR. The reaction was removed when 70% conversion of the oligo(ethylene glycol)methacrylate monomer was reached via

NMR. Coupons were rinsed and sonicated with ethyl acetate to remove any physically adhered polymer chains. Samples were dried for at least six hours under vacuum (~0.1 torr) before further use.

109 4.3.3.5 SI-block polymerization styrene and oligo(ethylene glycol) methyl ether methacrylate

Three coupons, modified with SI-polystyrene, were chain extended by being placed in a 50 ml Erlenmeyer flask with 0.1180g of tris(pyridylmethyl)amine (TPMA) and a stir bar. The flask sealed was with a septum. The flask was evacuated and refilled with dry nitrogen gas three times. Into the flask was added 6mL of oligo(ethylene glycol)methacrylate, 0.4µL of methyl bromoisobutyrate, and 3ml of DMF. Nitrogen was bubbled through the monomer solution for 20 minutes to degas the mixture. The resulting solution was purged for 30 min by bubbling nitrogen gas through the solution. 0.568mL of

0.01M [Cu(TPMA)Br][Br] in DMF was added and bubbled through the solution for an additional 5 minutes (Scheme 4.1, Step 4b). The flask was placed on a stir plate under a

UV Lamp, and the reaction solution was monitored with NMR. The reaction was removed when 70% conversion of the oligo(ethylene glycol)methacrylate monomer was reached via

NMR. Coupons were rinsed and sonicated with ethyl acetate to remove any physically adhered polymer chains. Samples were dried for at least six hours under vacuum (~0.1 torr) before further use.

4.3.4 Surface Characterization

4.3.4.1 Diffuse Reflectance Infrared Fourier Transform Spectroscopy

110 coupon as a background. Before all spectra were collected, the sample chamber was purged

N2 gas for 5 minutes.

4.3.4.2 Contact Angle

Static contact angles were measured by placing a 2uL drop of deionized (18 MΩ) water on either a modified or unmodified coupon, and the contact angle was recorded.

Measurements were repeated twice on each coupon and thrice more measurements were taken on two additional coupons for a total of nine measurements. Contact angle averages and standard deviations were calculated for each modification.

4.3.4.3 Cyclic Voltammetry

Electrochemical measurements were recorded using 0.1M NaCl (aq) as a supporting electrolyte. Voltammograms and impedance spectra were collected using a standard three-electrode system with an SS316L coupon as the working electrode and a platinum wire as the counter electrode. An Ag/AgCl(aq) electrode was used as the reference electrode. Cyclic voltammograms were collected from +0.3 V to -1.2 V vs.

Ag/AgCl at a rate of 50 mV/sec. The surface area of each electrode was measured using

ImageJ software. Measurements were normalized to the surface area of each electrode.

4.3.4.4 Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) was carried out in the same electrochemical cell as described above. EIS was carried out at the corrosion potentials,

0.4 V vs. Ag/AgCl, with a frequency range of 0.1 Hz to 10 kHz with 5 data points per

111 decade. Data was acquired using a 10mV sinusoidal potential and collected with NuVant

EzWare EIS software. Impedance spectra were fitted to a modified Randel’s circuit, and charge transfer resistance was calculated using EC-Lab software.

4.3.4.5 Cytotoxicity and Cell Adhesion Studies

Cell cultures of NIH-3T3 mouse fibroblasts were grown from frozen cultures stored at -78ºC. Cultures were grown to 80% confluency in T75 culture flasks using Dulbecco’s modified Eagle medium supplemented with fetal bovine serum and penicillin/streptomycin. Cells were harvested from culture flasks by removal of growth media and addition of 5ml of trypsin/EDTA solution followed by incubation for 5-7 minutes. The trypsin/EDTA solution was removed and transferred to a sterile centrifuge tube. The culture flask was rinsed with trypsin neutralizing solution (TNS) and was then added to the centrifuge tube containing the trypsin/EDTA solution. The cell suspension was centrifuged at 2.1G for 5 minutes. The resulting cell pellet was resuspended in growth media, and cell concentration determined using hemocytometry with trypan blue.

Coupons modified with polymer films were placed into 24 well plates with unmodified SS316L coupons used as the control. The cell suspension was diluted to 10,000 cells/mL, and 1 mL was added to each well-containing coupons. Coupons were incubated with cell cultures for one, four, and seven days, with media being changed every 24 hrs.

Before imaging, the media was aspirated from each well and rinsed with 1mL of phosphate- buffered saline (PBS), twice. Coupons were then incubated for 5 minutes with calcein AM and ethidium bromide in PBS from Live/Dead kit. Coupons were then imaged using a Zeiss

Axioskop 2 fluorescence microscope under 10x magnification using AxioVision software.

112 4.3.5 Statistics

One-way analysis of variance (ANOVA) with a Tukey posthoc analysis was used to determine the statistical difference of averages at a p>0.05 unless otherwise stated,

Grubbs Test for outliers was used to remove data points when applicable.

4.4 Results

4.4.1 Formation of HUPA SAMs

Self-assembled monolayers of HUPA were formed on the surface of SS316L to provide a point of attachment for polymer films. The methylene stretching region of

-1 ordered and crystalline-like SAMs presents with vCH2asym ≥ 2918cm and vCH2asym ≥

2848cm-1 with 0.3% to 0.7% transmittance.26-31 After rinsing and sonication in THF to remove weakly adhered molecules, the DRIFT spectrum of monolayers presented with

-1 -1 vCH2asym 2915 cm and vCH2asym 2848 cm (Figure 4.3a), indicating that the monolayers are formed in an ordered, crystalline-like structure with alkyl chains in an all-trans orientation.

26, 28, 32

113

The monolayers were attached to the oxide surface of SS316L via the phosphonic acid head group. The DRIFT spectrum of the phosphonic acid-binding region contains a single absorption at 1016 cm-1 (Figure 4.3b), corresponding to the P-O stretching. The absence of additional peaks corresponding to P=O and P-O-H stretches at 1232 cm-1, and

-1 26, 28, 29, 930cm , indicates that the phosphonic acid is bound in a tridentate bonding pattern.

31, 33

4.4.2 Immobilization of Bromoisobutryl bromide

After monolayers of HUPA were formed on the surface of SS316L, the surface was further modified with bromoisobutyryl bromide. After immobilization, the spectra contained IR absorptions at 2925 cm-1 and 2852 cm-1 corresponding to the asymmetric and symmetric methylene stretching of the alkyl chains, respectively (Figure 4.4a).

114 -1 -1 Additionally, bands at 1736 cm and 1465 cm , corresponding to the carbonyl and the geminal dimethyl, respectively, verify the presence of the initiator, as they are not present in the spectrum of the monolayer. The presence of both C-O ester stretching bands at 1276 cm-1 and 1164 cm-1 confirm the formation of the targeted ester linkage (Figure 4.4b).

3.4.3 Synthesis and Characterization of poly(OEGA) and poly(OEGMA) of films

3.4.3.1 Synthesis of Polymer Films

Films of the poly(OEGA) and poly(OEGMA) were formed using a photoATRP approach. Coupons modified with BiBB were placed in a flask with monomer, anisole,

TPMA, and [Cu(TPMA)Br][Br]. Exposure to UV light was then used to generate activators and initiate polymerization. In the DRIFT spectra of the polymer film of PEGylated acrylate (Figure 4.5a), a large, broad peak at 2853 cm-1 corresponding to a variety of alkyl

C-H stretches, is caused by the PEG side chains and the polymer backbone. Additionally,

115 an increase in the intensity of the carbonyl stretching and the presence of C-O ester and C-

O ether stretching between 1270 cm-1 and 1117 cm-1 confirm the presence of these functionalities on the polymer. Additionally, the 1404cm-1 and 1348cm-1 peaks correspond to the methyl bending vibrations. Due to the similarity in the structures of the polymer films of OEGA and OEGMA, similar stretching and bending vibrations were observed in the DRIFT spectrum of the poly(OEGMA) film (Figure 4.5b). An increase in the percent

-1 transmittance at 2963cm indicates an increase in the amount of C-Halkyl groups present on the surface. Additional absorbances at 1740 cm-1 and between 1292cm-1 and 1109cm-1 correspond to the carbonyl and C-O ester and ether stretching, respectively. In both spectra, a peak at ~1450 cm-1 is indicative of the geminal dimethyl groups from the initiator chain end of the polymer film.

Figure 4.5 DRIFT Spectra of polymer films of a) OEGA and b) OEGMA. In each set of spectra, the spectrum of bromoisobutyryl bromide immobilized on HUPA SAMs (dashed grey trace) is provided as a comparison for before polymerization. All labels correspond to the spectra of the polymer films.

116 4.4.3.2 Cyclic Voltammetry

In order to characterize the corrosion-resistant nature of the polymer films, both cyclic voltammetry and electrochemical impedance spectroscopy were used. Coupons were placed in a standard three-electrode setup in an electrochemical cell with a 0.1M sodium chloride electrolyte solution. The coupon was used as the working electrode and platinum wire and Ag/AgCl as the counter and a reference electrode, respectively. Since cyclic voltammetry is used to monitor electrochemical reactions, and corrosion occurs as a result of such reactions, this makes it a suitable technique to monitor corrosion.34, 35 In the voltammogram of the bare oxide (Figure 4.6 black trace), the couple centered at 0.67 V relative to the Ag/AgCl couple corresponds to the reduction and subsequent oxidation of iron (III) oxides and hydroxides to the iron (II) analogs.33, 36-38 Upon functionalization with polymer films of OEGA, and OEGMA a decrease in the current passing through the coupon in comparison to the trace for the native oxide surface was observed. However, there is still a considerable amount of current passing through the electrode, indicating that redox reactions are still occurring and corrosion is still likely to occur.

Figure 4.6 Cyclic Voltammograms of a) native oxide (black trace), poly(OEGA) (dashed grey trace), and poly(OEGMA) (dash-dotted grey trace)

117

In addition to cyclic voltammetry’s application in monitoring electrochemical reactions at the interface of SS316L coupons, it can be used to measure the fractional coverage of modified coupons. Assuming that the charge transfer can only occur at unmodified sites on the electrode surface, and that diffusion is linear across the surface, the fractional coverage (θCV) can be calculated via Equation 4.1, where the imod and ibare is the magnitude the observed current at the modified and unmodified coupon, respectively. 36-38

After functionalization of the SS316L with films of poly(OEGA) and poly(OEGMA), fractional coverages of 0.80 and 0.50 were observed, respectively.

푖 휃 =1− 푖

Equation 4.1

4.4.3.3 Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy was used to gain additional insight about the surface modification by probing the charge transfer resistance of the surface.

Previously, the charge transfer resistance, the ability of an interface to retard the transfer of an electron from the electrode to the solution, has been used as a quantification of the corrosion-resistant nature of a modified surface when compared to the native oxide surface.

33, 39-50 After functionalization with homopolymer films of OEGA and OEGMA Rct of 78 k and 5.0 k, respectively, were observed (Figure 4.7, Table 4.1). In addition to the charge transfer resistance, the protection efficiency of the polymer films was calculated from Equation 4.2. 47, 51-58 The poly(OEGA) films had a protection efficiency of 96.2%, whereas the poly(OEGMA) films had a protection efficiency of 42.5%.

118

Figure 4.7 Nyquist impedance spectra of a) native oxide, (black trace), poly(OEGA) (dashed grey trace), and poly(OEGMA) (dash-dotted grey trace)

Table 4.1 Charge transfer resistance values and protection efficiency (PE) from electrochemical impedance spectroscopy

Modification Rct (k) PE (%) Native oxide 3.1 ---- Poly(OEGA) 78 96.2 Poly(OEGMA) 5.0 42.5

푅 −푅 푃퐸 = 푥100% 푅

Equation 4.2. Protection Efficiency

119 4.4.3.4 Cell Resistance Studies

To characterize the cell resistant nature of the polymer films, coupons were cultured with NIH 3T3 fibroblasts for one, four, and seven days. After incubation with cell cultures, coupons were stained using a Live/Dead assay, causing live cells to fluoresce green and dead cells to fluoresce red. Images were then recorded of three coupons with five images per coupon using a materials microscope with fluorescence filters.

A decrease in the number of adhered cells was observed for films of both

PEGylated polymer-modified coupons (Figures 4.8 and 4.9a). All values were normalized such that the bare SS316L surface was set to 100. After one day, the normalized cell counts for fibroblasts grown on coupons was 158 ± 6.6 and 6 ± 1.3 for poly(OEGA) and poly(OEGMA), respectively. After four days, the normalized cell counts for the poly(OEGA) and poly(OEGMA) modified surfaces were 1.1 ± 0.5 and 2.4 ± 0.9, respectively. After seven days, a decrease in the number of the adhered cells on the poly(OEGA) to and poly(OEGMA) to 2.7 ± 0.9 and 1.8 ± 0.5 form unmodified surface.

Bare poly(oligo(EG) poly(oligo(EG)

acrylate) methacrylate) Day Day 1

Day Day 4

Day Day 7

Figure 4.8 Live/Dead fluoresces microscopy images at 10x of modified and unmodified coupons. Red indicates a dead cell green indicates a lives cell

120 The average percentage of viable cells was determined using Equation 4.3 and was normalized such that the bare SS316L surface was set to 100. After functionalization with polymer films, the average viability on day one was 100 ± 0.85 and 63.7 ± 10% for poly(OEGA) and poly(OEGMA), respectively (Figure 4.9b). On day four, the average viability for all the samples decreased on the poly(OEGA) and poly(OEGMA) to 84.5 ±

24 % and 57.9 ± 13.3 %. The percent viability decreased on day seven for the poly(OEGA) and poly(OEGMA) to 42.2 ± 11.3 % and 40.1 ± 11.6 %, respectively, from the unmodified surface.

Figure 4.9 a) Normalized cell counts and b) normalized cell viability for PEGylated polymer films from on SS316L. All counts and viability data are normalized to unmodified SS316. “*” indicates a statistical difference from the native oxide surface (p<0.05).

퐿푖푣푒 푐푒푙푙푠 Percent Viability = 퐿푖푣푒 푐푒푙푙푠 + 퐷푒푎푑 퐶푒푙푙푠

Equation 4.3

121 4.4.4 Formation of Poly(styrene-co-OEGA) and Poly(styrene-co-OEGMA) Copolymer films

A copolymer film approach was devised to improve the corrosion-resistant nature of the poly(OEGA) and poly(OEGMA) films. A poly(styrene) “core” was formed first and then capped with polymer films of OEGA and OEGMA. Polymerization was reinitiated by the addition of TPMA and exposure to UV light. After exposure to UV light, the DRIFT spectrum of the styrene block OEGA copolymer film contained absorbances at 1452 cm-1,

1492 cm-1, and between 3076 cm-1 and 3024 cm-1 (Figure 4.10a), corresponding to the aromatic rings on poly(styrene) block, and are apparent before the modification with the second monomer (Figure 4.10). However, the appearance of bands at 1731 cm-1 and 1139 cm-1, which were not in the spectrum of poly(styrene), corresponds to the carbonyl and C-

O stretching.

122

Figure 4.10 DRIFT Spectra of copolymer films of a) poly(styrene-co-OEGA) (red trace) and b) poly(styrene-co-OEGMA) (teal trace). In each spectrum, the spectrum styrene (orange trace) is provided as a comparison for before chain extension. All labels correspond to the spectra of the copolymer films.

Similarly to the film of poly(stryrene-co-OEGA), the OEGMA analog was formed by chain extension of poly(styrene). The DRIFT spectrum of the copolymer film presented with two new absorptions at 1731 cm-1 and 1139 cm-1 corresponding to the carbonyl and mixture of C-O stretching caused by the ether and ester moieties, which were not present before chain extension (Figure 4.10b). After the chain extension, the IR absorptions corresponding to the poly(styrene) were still present between 3076 cm-1 and 3024 cm-1,

-1 -1 and between 1492 cm and 1452 cm , indicating poly(styrene) was retained under the film.

123 4.4.5 Contact Angle Analysis

Contact angle goniometry was used to examine the wettability of the surfaces and characterize what groups are presenting at the interface (Figures 4.11 and 4.12). Before functionalization with HUPA monolayers, the surface had a water contact angle of 71 ± 3° and after formation the SAMs, the surface had a contact angle of 75 ± 5°, indicating a hydrophilic surfaces. This is caused by favorable interactions, such as hydrogen bonding, which both interfaces can participate in, because of the presence of the hydroxyl groups on the bare oxide and HUPA monolayer surface. After the functionalization of the monolayer with BiBB, a hydrophobic contact angle of 91 ± 4° was observed, indicating that the hydroxyl groups were no longer presenting at the interface.

a) b)

θ θ

Hydrophilic Hydrophobic θ < 90° θ ≥ 90° Figure 4.11 Contact angle of water droplets defined as a) hydrophilic surface (θ < 90°) and b) hydrophobic surface (θ ≥ 90°).

124

Figure 4.12 Contact angle data (n=9) for surface modifications. BiBB= Bromoisobutryl bromide immobilized on HUPA SAMs, P(OEGA) = Poly(oligo(ethylene glycol)acrylate, P(OEGMA) = Poly(oligo(ethylene glycol)methacrylate, P(S) = Poly(styrene), P(S—co- OEGA) = Poly(styrene-co-oligo(ethylene glycol)acrylate and P(S-co-OEGMA) = Poly(styrene-co-oligo(ethylene glycol)methacrylate modified surfaces.

Following the functionalization with polymer films of poly(styrene), the surface had a hydrophobic contact angle of 109 ± 4°, indicating that either the aromatic rings or the polymer backbone presenting at the interface. Once the BiBB terminated surface was functionalized with polymer films of OEGA and OEGMA, a hydrophilic contact angles of

45 ± 5° and 72 ± 4° respectively, indicating that the PEG sidechains, which been well documented in the literature to have favorable interactions,1, 3, 59, 60 are presenting at the interface, which is necessary for antifouling properties.10-13, 61-64 After the chain end extension of the poly(styrene) modified surface with OEGA, the surface also had a hydrophilic surface with a contact angle of 51 ± 15°. After chain extension, of SI- poly(styrene) with OEGA, the contact angles were not significantly different (p<0.05).

Upon reinitiation of the poly(styrene) film with OEGMA resulting in the copolymer, the surface was hydrophilic, bearing a contact angle of 64 ± 4°. Similarly to the (styrene-co-

OEGA), statistical analysis indicated that the OEGMA and the poly(styrene-co-OEGMA),

125 contact angles were not significantly different (p<0.05), indicating there is no difference in the groups that are presenting at the interface.

4.5 Discussion

Polymer films of OEGA and OEGMA have been formed in previous reports on different metal oxide surfaces but not SS316L, and have been demonstrated to have excellent antifouling properites.15-24 Electrochemical studies contained herein, indicate that the PEGylated films formed on the surface do not provide a corrosion-resistant modification. To improve the corrosion-resistant properties of the PEGylated films, block- copolymer films of poly(styrene-co-OEGA) and poly(styrene-co-OEGMA) were used to provide a corrosion and cell resistant barrier on the surface of SS316L using SI-ATRP.

Hydroxyl-terminated monolayers of HUPA were formed on the native oxide surface of SS316L using aerosol deposition. After rinsing and sonicating, ordered films on the surface were observed with tridentate bonding to the oxide surface. An ordered monolayer is needed so that the hydroxyl groups on the terminus of the alkyl chain presents at the interface in a consistent manner allowing for functionalization with an ATRP initiator. Also, contact angle goniometry revealed a hydrophilic surface, which indicated favorable interactions forming with the surface of the monolayer. This was caused by hydrogen-bonding interactions between water molecules and the hydroxyl-terminated surface, indicating that the hydroxyls were indeed presenting at the interface.

To generate a bromoisobutyrate modified surface, which is needed for ATRP, bromoisobutyryl bromide was immobilized on the surface using a gas-phase technique.

After rinsing and sonicating to remove any weakly absorbed molecules, DRIFT

126 spectroscopy indicated the formation of carbon-oxygen ester bonds as well as the presence of carbonyl groups. Furthermore, the geminal dimethyl stretching present on the surface, coupled with the other bands that are not present in the monolayer spectrum indicate the successful immobilization. Contact angle goniometry of the initiator modified surface indicated a switch from a hydrophilic to a hydrophobic surface, which is diagnostic of the surface hydroxyls being consumed by the formation of ester bonds. The methyl and bromo groups on the ester terminus of the monolayer inhibit the formation of favorable interactions occurring between water droplets and the modified surface, indicating that the bromoisobutyrate ester is presenting at the interface for further modifications with polymer films.

Polymer films of OEGA and OEGMA were formed using a photo-initiated ATRP

(photoATRP) approach, where excess ligand and UV light serve to reduce the copper (II) deactivator species to the copper (I) activator species, initiating the polymerization. After excess, unreacted monomer was rinsed from the surface with THF, DRIFT spectroscopy of the samples presented with an increase in carbonyl and methylene stretching, indicating a growth of chains from the surface. Also, the presence of new peaks corresponding to various carbon-oxygen stretching indicates both the ester and the ether functionalities were present. These results, coupled together, indicate the polymer films of the PEGylated monomers were formed.

Cyclic voltammetry and electrochemical impedance spectroscopy was used to evaluate the surface modifications of SS316L for their corrosion resistance. After functionalization with polymer films of OEGA and OEGMA, a low fractional coverage was observed, indicating that the polymer films did not wholly restrict the access of the

127 solvent to the metal oxide surface. This is likely due to the highly hydrophilic nature of the polymers, allowing a large amount of solvent (water) to permeate the polymer film, which allows charges and ions to migrate from the solution to the oxide surface of the SS316L electrode. Additionally, electrochemical impedance spectroscopy indicated low charge transfer resistance, resulting in lower protection efficiencies for polymer films of OEGA and OEGMA. The electrochemical impedance spectroscopy coupled with data gained from cyclic voltammetry indicated that the PEGylated polymer films were not suitable for a corrosion resistant coating.

While the poly(OEGA) and poly(OEGMA) films did not have the desired corrosion resistance, they did have some cell resistant properties, with up to 98% reduction in adhered cells over seven days for both PEGylated polymers, except for the first day on poly(OEGA). To improve the corrosion resistance of PEGylated films, poly(styrene-co-

OEGA) and poly(styrene-co-OEGMA) were prepared. A poly(styrene) “core” was formed first and then capped with polymer films of OEGA and OEGMA. The choice of the poly(styrene) “core” was twofold: first, it has been demonstrated to be an excellent corrosion resistant polymer film on SS316L in Chapter 3; and it has unique IR absorbances from the PEGylated polymers.

In order to form the copolymer films, coupons modified with poly(styrene) were chain extended with PEGylated monomers. DRIFT spectroscopy of the resulting films showed an increase in the methylene stretching, indicating that additional methylene groups were present on the surface after polymerization, when compared to the spectra of poly(styrene). In addition to the increase in the methylene stretching, additional stretching caused by ester and ether groups is also observed in the copolymer spectra but not in the

128 spectrum of poly(styrene). The retention of absorbances corresponding to the aromatic stretching indicates that the films of polystyrene are preserved within the copolymer films.

Contact angle goniometry revealed hydrophilic contact angles for polymer films of

OEGA and OEGMA. This indicates that the PEG sidechains were presenting at the surface, allowing for the strong classical interactions between 1,2 poly(ethers) to occur, which has been documented in the literature.60 Additionally, the films of poly(styrene) that were formed using SI-ATRP displayed a hydrophobic contact angle, which is expected due to the polymer backbone and phenyl rings that should be presenting at the interface. After the chain extension of poly(styrene) with the PEGylated monomers, the evaporation of the solvent (either anisole from the reaction or THF from rinsing) trapped in the polymers causes the films to collapse (Figure 4.13). The resulting films can collapse into either an ordered or a disordered system, where the former results in a poly(styrene) “core” being

“capped” with PEGylated polymer chains (Figure 4.13b), and the latter results in a mixture of both poly(styrene) and PEGylated polymer chains presenting at the interface (Figure

4.13c). The contact angles decrease from poly(styrene), and are not statistically different from the PEGylated homopolymer surfaces, indicating that in the copolymer films, the

PEG sidechains were also presenting at the interface, and the films collapsed much like

Figure 4.13b. The presentation of these sidechains at the interface is necessary for the polymer films to have the desired antifouling properties associated with PEG-modified surfaces. Additionally, the contact angle indicates that the poly(styrene) “core” is maintained between the native oxide of the SS316L and the poly(ethylene glycol) modified layer. This “sandwiching” should lead to the poly(styrene) to serve as a corrosion resistant barrier, and the poly(OEGA) and poly(OEGMA) providing a cell resistant coating.

129

4.6 Conclusions

Films of poly(OEGA) and poly(OEGMA) were formed on the surface of SS316L using SI-ATRP. , which possess some cell resistant properties, however EIS and cyclic voltammetry studies, herein indicated poor corrosion resistance. Cell trials conducted using

NIH-3T3 fibroblasts indicate that the polymer films of OEGA and OEGMA grafted from

SS316L can provide a cell resistance surface, with premilitary results reducing the number of cells adhered to the surface by 98-95%, when compared to the control. Further studies, including cell proliferation studies and protein absorption assays with confirm the cell restraint and antifouling properties. Cyclic voltammetry revealed the PEGylated polymer

130 films allowed for charge transfer to occur, which was likely caused by the water permeability of the polymers, allowing for the conduction of ions between the surface and solution. Lack of a significant increase in the charge transfer resistance was further confirmed via EIS of poly(OEGA) and poly(OEGMA). In an attempt to improve the corrosion-resistant nature of the polymer films, copolymers between styrene and OEGA, as well as styrene and OEGMA, were prepared. Contact angle analysis revealed that the poly(OEGA), poly(OEGMA), poly(styrene-co-OEGA), and poly(styrene-co-OEGMA) were all hydrophilic, indicating that PEG chains are presenting at the interface.

O O

O Br O O Br O

O O Br Br O Br O

O O

O O O O O O O O O O

9 9 9 9 P P P P P O O O O O O O O O O O O O O O P(OEGA) P(S-co-OEGA) P(S-co-OEGMA) P(S) P(OEGMA) Figure 4.14 Polymer films that were synthesized as a part of this project. Poly(styrene) films were forming in part of a previous project.

Polymer films of OEGA and OEGMA (Figure 4.14 P(OEGA) and P(OEGMA)) have not been demonstrated previously in the literature on the surface of SS316L, nor have their corrosion-resistant modifications been electrochemically characterized on the surface, to date. Furthermore, the films of poly(styrene-co-OEGA) and poly(styrene-co-OEGMA)

131 (Figure 4.14 poly(S-co-OEGA) and poly(S-co-OEGMA)) have never been synthesized using an SI-polymerization. These new copolymer films could inhibit corrosion and provide an antifouling modification, finding excellent use in biomedical metallic devices.

Additionally, these films could be further utilized in marine environments where pitting corrosion and fouling (caused by algae and barnacles) occurs.

4.7 References

1. Higaki, Y.; Kobayashi, M.; Murakami, D.; Takahara, A., Anti-fouling behavior of polymer brush immobilized surfaces. Polymer Journal 2016, 48, 325.

2. Halperin, A., Polymer Brushes that Resist Adsorption of Model Proteins: Design Parameters. Langmuir 1999, 15 (7), 2525-2533.

3. Banerjee, I.; Pangule, R. C.; Kane, R. S., Antifouling Coatings: Recent Developments in the Design of Surfaces That Prevent Fouling by Proteins, Bacteria, and Marine Organisms. Advanced Materials 2011, 23 (6), 690-718.

4. Palchesko, R. N.; McGowan, K. A.; Gawalt, E. S., Surface immobilization of active vancomycin on calcium aluminum oxide. Materials Science and Engineering: C 2011, 31 (3), 637-642.

5. Palchesko, R. N.; Buckholtz, G. A.; Romeo, J. D.; Gawalt, E. S., Co-immobilization of active antibiotics and cell adhesion peptides on calcium based biomaterials. Materials Science and Engineering: C 2014, 40, 398-406.

6. Kruszewski, K. M.; Nistico, L.; Longwell, M. J.; Hynes, M. J.; Maurer, J. A.; Hall- Stoodley, L.; Gawalt, E. S., Reducing Staphylococcus aureus biofilm formation on stainless steel 316L using functionalized self-assembled monolayers. Materials Science and Engineering: C 2013, 33 (4), 2059-2069.

7. Buckholtz, G. A.; Reger, N. A.; Anderton, W. D.; Schimoler, P. J.; Roudebush, S. L.; Meng, W. S.; Miller, M. C.; Gawalt, E. S., Reducing Escherichia coli growth on a composite biomaterial by a surface immobilized antimicrobial peptide. Materials Science and Engineering: C 2016, 65, 126-134.

8. Reger, N. A.; Meng, W. S.; Gawalt, E. S., Antimicrobial Activity of Nitric Oxide- Releasing Ti-6Al-4V Metal Oxide. J Funct Biomater 2017, 8 (2).

9. Reger, N. A.; Meng, W. S.; Gawalt, E. S., Surface modification of PLGA nanoparticles to deliver nitric oxide to inhibit Escherichia coli growth. Applied Surface Science 2017, 401, 162-171.

132 10. Amiji, M.; Park, K., Prevention of protein adsorption and platelet adhesion on surfaces. Biomaterials 1992, 13, 682-692.

11. Xiao, Y.; Zhao, L.; Shi, Y.; Liu, N.; Liu, Y.; Liu, B.; Xu, Q.; He, C.; Chen, X., Surface modification of 316L stainless steel by grafting methoxy poly(ethylene glycol) to improve the biocompatibility. Chemical Research in Chinese Universities 2015, 31 (4), 651-657.

12. Chen, X.; Su, Y.; Shen, F.; Wan, Y., Antifouling ultrafiltration membranes made from PAN-b-PEG copolymers: Effect of copolymer composition and PEG chain length. Journal of Membrane Science 2011, 384 (1-2), 44-51.

13. Moore, E.; Delalat, B.; Vasani, R.; McPhee, G.; Thissen, H.; Voelcker, N. H., Surface-Initiated Hyperbranched Polyglycerol as an Ultralow-Fouling Coating on Glass, Silicon, and Porous Silicon Substrates. ACS Applied Materials & Interfaces 2014, 6 (17), 15243-15252.

14. Schottler, S.; Becker, G.; Winzen, S.; Steinbach, T.; Mohr, K.; Landfester, K.; Mailander, V.; Wurm, F. R., Protein adsorption is required for stealth effect of poly(ethylene glycol)- and poly(phosphoester)-coated nanocarriers. Nat Nanotechnol 2016, 11 (4), 372-7.

15. Zengin, A.; Tamer, U.; Caykara, T., Synthesis of superparamagnetic and thermoresponsive hybrid nanoparticles via surface-mediated RAFT polymerization of di(ethylene glycol) ethyl ether acrylate and (oligoethylene glycol) methyl ether acrylate. Journal of Polymer Science Part A: Polymer Chemistry 2013, 51 (16), 3420- 3428.

16. Zengin, A.; Caykara, T., RAFT-mediated synthesis of poly[(oligoethylene glycol) methyl ether acrylate] brushes for biological functions. Journal of Polymer Science Part A: Polymer Chemistry 2012, 50 (21), 4443-4450.

17. Yuan, S.; Wan, D.; Liang, B.; Pehkonen, S. O.; Ting, Y. P.; Neoh, K. G.; Kang, E. T., Lysozyme-coupled poly(poly(ethylene glycol) methacrylate)-stainless steel hybrids and their antifouling and antibacterial surfaces. Langmuir 2011, 27 (6), 2761-74.

18. Guo, W.; Zhu, J.; Cheng, Z.; Zhang, Z.; Zhu, X., Anticoagulant surface of 316 L stainless steel modified by surface-initiated atom transfer radical polymerization. ACS Appl Mater Interfaces 2011, 3 (5), 1675-80.

19. Rodda, A. E.; Ercole, F.; Nisbet, D. R.; Forsythe, J. S.; Meagher, L., Optimization of Aqueous SI-ATRP Grafting of Poly(Oligo(Ethylene Glycol) Methacrylate) Brushes from Benzyl Chloride Macroinitiator Surfaces. Macromol Biosci 2015, 15 (6), 799- 811.

20. Li, B.; Yu, B.; Ye, Q.; Zhou, F., Tapping the potential of polymer brushes through synthesis. Acc Chem Res 2015, 48 (2), 229-37.

133 21. Ma, H.; Hyun, J.; Stiller, P.; Chilkoti, A., “Non-Fouling” Oligo(ethylene glycol)- Functionalized Polymer Brushes Synthesized by Surface-Initiated Atom Transfer Radical Polymerization. Advanced Materials 2004, 16 (4), 338-341.

22. Kizhakkedathu, J. N.; Janzen, J.; Le, Y.; Kainthan, R. K.; Brooks, D. E., Poly(oligo(ethylene glycol)acrylamide) Brushes by Surface Initiated Polymerization: Effect of Macromonomer Chain Length on Brush Growth and Protein Adsorption from Blood Plasma. Langmuir 2009, 25 (6), 3794-3801.

23. Khabibullin, A.; Mastan, E.; Matyjaszewski, K.; Zhu, S., Surface-Initiated Atom Transfer Radical Polymerization. In Controlled Radical Polymerization at and from Solid Surfaces, Vana, P., Ed. Springer International Publishing: Cham, 2016; pp 29-76.

24. Yang, J.; Lv, J.; Behl, M.; Lendlein, A.; Yang, D.; Zhang, L.; Shi, C.; Guo, J.; Feng, Y., Functionalization of polycarbonate surfaces by grafting PEG and zwitterionic polymers with a multicomb structure. Macromol Biosci 2013, 13 (12), 1681-8.

25. Beni, A.; Dei, A.; Laschi, S.; Rizzitano, M.; Sorace, L., Tuning the Charge Distribution and Photoswitchable Properties of Cobalt–Dioxolene Complexes by Using Molecular Techniques. Chemistry – A European Journal 2008, 14 (6), 1804-1813.

26. Raman, A.; Quiñones, R.; Barriger, L.; Eastman, R.; Parsi, A.; Gawalt, E. S., Understanding Organic Film Behavior on Alloy and Metal Oxides. Langmuir 2010, 26 (3), 1747-1754.

27. Raman, A.; Gawalt, E. S., Self-Assembled Monolayers of Alkanoic Acids on the Native Oxide Surface of SS316L by Solution Deposition. Langmuir 2007, 23 (5), 2284- 2288.

28. Raman, A.; Dubey, M.; Gouzman, I.; Gawalt, E. S., Formation of Self-Assembled Monolayers of Alkylphosphonic Acid on the Native Oxide Surface of SS316L. Langmuir 2006, 22 (15), 6469-6472.

29. Quiñones, R.; Raman, A.; Gawalt, E. S., Functionalization of nickel oxide using alkylphosphonic acid self-assembled monolayers. Thin Solid Films 2008, 516 (23), 8774-8781.

30. Quiñones, R.; Raman, A.; Gawalt, E. S., An approach to differentiating between multi- and monolayers using MALDI-TOF MS. Surface and Interface Analysis 2007, 39 (7), 593-600.

31. Quiñones, R.; Gawalt, E. S., Study of the Formation of Self-Assembled Monolayers on Nitinol. Langmuir 2007, 23 (20), 10123-10130.

32. Buckholtz, G. A.; Gawalt, E. S., Effect of Alkyl Chain Length on Carboxylic Acid SAMs on Ti-6Al-4V. Materials 2012, 5 (7), 1206.

134 33. Quiñones, R.; Gawalt, E. S., Polystyrene Formation on Monolayer-Modified Nitinol Effectively Controls Corrosion. Langmuir 2008, 24 (19), 10858-10864.

34. Veleva, L.; Alpuche-Aviles, M. A.; Graves-Brook, M. K.; Wipf, D. O., Voltammetry and surface analysis of AISI 316 stainless steel in chloride-containing simulated concrete pore environment. Journal of Electroanalytical Chemistry 2005, 578 (1), 45- 53.

35. Veleva, L.; Alpuche-Aviles, M. A.; Graves-Brook, M. K.; Wipf, D. O., Comparative cyclic voltammetry and surface analysis of passive films grown on stainless steel 316 in concrete pore model solutions. Journal of Electroanalytical Chemistry 2002, 537 (1), 85-93.

36. Campuzano, S.; Pedrero, M.; Montemayor, C.; Fatás, E.; Pingarrón, J. M., Characterization of alkanethiol-self-assembled monolayers-modified gold electrodes by electrochemical impedance spectroscopy. Journal of Electroanalytical Chemistry 2006, 586 (1), 112-121.

37. Janek, R. P.; Fawcett, W. R., Impedance Spectroscopy of Self-Assembled Monolayers on Au(111): Sodium Ferrocyanide Charge Transfer at Modified Electrodes. Langmuir 1998, 14.

38. Shervedani, R. K.; Hatefi-Mehrjardi, A.; Babadi, M. K., Comparative electrochemical study of self-assembled monolayers of 2-mercaptobenzoxazole, 2- mercaptobenzothiazole, and 2-mercaptobenzimidazole formed on polycrystalline gold electrode. Electrochimica Acta 2007, 52 (24), 7051-7060.

39. Nagalakshmi, R.; Nagarajan, L.; Rathish, R. J.; Prabha, S. S.; Jeyasundari, N. V.; Rajendran, S., Corrosion Resistance Of SS316l In Artificial Urine In Presence Of D- Glucose. Int. J. Nano. Corr. Sci. Engg. 2014, 1, 39-49.

40. Mary, S. J.; Rajendran, S., Corrosion Behaviour of SS316L in Artificial Blood Plasma in Presence of Amoxicillin. Portugaliae Electrochimica Acta 2013, 31 (1), 33-40.

41. Duarte, R. G.; Castela, A. S.; Neves, R.; Freire, L.; Montemor, M. F., Corrosion Behavior of Stainless Steel Rebars Embedded in Concrete: an Electrochemical Impedance Spectroscopy Study. Electrochimica Acta 2014, 124, 218-224.

42. Yu, L.; Duan, J.; Du, X.; Huang, Y.; Hou, B., Accelerated anaerobic corrosion of electroactive sulfate-reducing bacteria by electrochemical impedance spectroscopy and chronoamperometry. Electrochemistry Communications 2013, 26, 101-104.

43. Siyah Murtdha, A.; Moradian, R.; Manouchehri, I., Electro chemical impedance spectroscopy (EIS) study of modified SS316L using radio frequency sputtering Ti6Al4V coating in Ringer solution. Anti-Corrosion Methods and Materials 2019, 66 (1), 27-33.

135 44. Feng, K.; Wu, G.; Li, Z.; Cai, X.; Chu, P. K., Corrosion behavior of SS316L in simulated and accelerated PEMFC environments. International Journal of Hydrogen Energy 2011, 36 (20), 13032-13042.

45. Al-Muhanna, K., Corrosion behavior of different stainless steel alloys exposed to flowing fresh seawater by electrochemical impedance spectroscopy (EIS). Desalination and Water Treatment 2011, 29 (1-3), 227-235.

46. Bonora, P. L.; Deflorian, F.; Fedrizzi, L., Electrochemical impedance spectroscopy as a tool for investigating underpaint corrosion. Electrochimica Acta 1996, 41 (7), 1073- 1082.

47. Ribeiro, D. V.; Abrantes, J. C. C., Application of electrochemical impedance spectroscopy (EIS) to monitor the corrosion of reinforced concrete: A new approach. Construction and Building Materials 2016, 111, 98-104.

48. Mansfeld, F., Electrochemical impedance spectroscopy (EIS) as a new tool for investigating methods of corrosion protection. Electrochimica Acta 1990, 35 (10), 1533-1544.

49. Applications of Impedance Spectroscopy. In Impedance Spectroscopy.

50. Mansfeld, F., Use of electrochemical impedance spectroscopy for the study of corrosion protection by polymer coatings. Journal of Applied Electrochemistry 1995, 25 (3), 187-202.

51. Appa Rao, B. V.; Narsihma Reddy, M., Formation, characterization and corrosion protection efficiency of self-assembled 1-octadecyl-1H-imidazole films on copper for corrosion protection. Arabian Journal of Chemistry 2017, 10, S3270-S3283.

52. Ghiamati Yazdi, E.; Ghahfarokhi, Z. S.; Bagherzadeh, M., Protection of carbon steel corrosion in 3.5% NaCl medium by aryldiazonium grafted graphene coatings. New J. Chem. 2017, 41 (21), 12470-12480.

53. Rajkumar, R.; Vedhi, C., A study of corrosion protection efficiency of silica nanoparticles acrylic coated on mild steel electrode. Vacuum 2019, 161, 1-4.

54. El-Rehin, S. S. A.; Ibrahim, M. A. M.; Khaled, K. F., 4-Aminoantipyrine as an inhibitor of mild steel corrosion in HCl solution. Journal of Applied Electrochemistry 1999, 29, 593-599.

55. Mahapatro, A.; Johnson, D. M.; Patel, D. N.; Feldman, M. D.; Ayon, A. A.; Agrawal, C. M., The use of alkanethiol self-assembled monolayers on 316L stainless steel for coronary artery stent nanomedicine applications: an oxidative and in vitro stability study. Nanomedicine 2006, 2 (3), 182-90.

56. Chang, K.-C.; Chen, S.-T.; Lin, H.-F.; Lin, C.-Y.; Huang, H.-H.; Yeh, J.-M.; Yu, Y.-H., Effect of clay on the corrosion protection efficiency of PMMA/Na+-MMT clay

136 nanocomposite coatings evaluated by electrochemical measurements. European Polymer Journal 2008, 44 (1), 13-23.

57. Le, H. N. T.; Garcia, B.; Deslous, C.; Xuan, Q. L., Corrosion protection of iron by polystyrenesulfonate-doped polypyrrole films. Journal of Applied Electrochemistry 2002, 32, 105–110.

58. Bhandari, H.; Srivastav, R.; Choudhary, V.; Dhawan, S. K., Enhancement of corrosion protection efficiency of iron by poly(aniline-co-amino-naphthol-sulphonic acid) nanowires coating in highly acidic medium. Thin Solid Films 2010, 519 (3), 1031-1039.

59. Khan, M.; Huck, W. T. S., Hyperbranched Polyglycidol on Si/SiO2 Surfaces via Surface-Initiated Polymerization. Macromolecules 2003, 36 (14), 5088-5093.

60. Aray, Y.; Marquez, M.; Rodríguez, J.; Vega, D.; Simón-Manso, Y.; Coll, S.; Gonzalez, C.; Weitz, D. A., Electrostatics for Exploring the Nature of the Hydrogen Bonding in Polyethylene Oxide Hydration. The Journal of Physical Chemistry B 2004, 108 (7), 2418-2424.

61. Amiji, M.; Park, K., Prevention of protein adsorption and platelet adhesion on surfaces by PEO/PPO/PEO triblock copolymers. Biomaterials 1992, 13 (10), 682-692.

62. Kishida, A.; Mishima, K.; Corretge, E.; Konishi, H.; Ikada, Y., Interactions of poly(ethylene glycol)-grafted cellulose membranes with proteins and platelets. Biomaterials 1992, 13 (2), 113-118.

63. Uchida, K.; Yamato, M.; Ito, E.; Kwon, O. H.; Kikuchi, A.; Sakai, K.; Okano, T., Two different types of nonthrombogenic surfaces: PEG suppresses platelet adhesion ATP-independently but HEMA-St block copolymer requires ATP consumption of platelets to prevent adhesion. Journal of Biomedical Materials Research A 2000, 50, 585-590.

64. Park, J. H.; Bae, Y. H., Hydrogels based on poly(ethylene oxide) and poly(tetramethylene oxide) or poly(dimethyl siloxane): synthesis, characterization, in vitro protein adsorption and platelet adhesion. Biomaterials 2002, 23, 1797–1808.

137 Chapter 5: A Novel Approach to a Poly(ether) Modified Surface

5.1 Introduction

Poly(ethers) have long been an interest in both academic and industrial research, because of the extensive properties that these polymers can possess.1-9 Poly(ethylene oxide)

(PEO) (commonly referred to as poly(ethylene glycol) (PEG)), has been known in the literature to be highly hydrophilic and provide excellent antifouling and tribological properties.7, 8 Similarly to PEO, poly(propylene oxide) possesses excellent lubricating and some antifouling properties, while being less hydrophilic.1-9 The potential for films of polymers to be grafted from the surface, resulting in covalently immobilized polymers has the potential to advance areas of antifouling and cell-resistant coatings.

Poly(ether) films are of interest by the marine industry, due to their ability to retard the adhesion of marine life such as barnacles10-12 and algae13-17 from adhering to underwater structures. As a result of marine life adhering to structures, such as ships, buoys, and other floating structures, a dramatic increase in the weight and roughness of surfaces occurs. The resulting increased drag causes an increase in fuel consumption resulting in a reduction in efficiency, between 40-80%, and reduced shipping speeds. Coupled together these can increase the cost of waterborne shipping by up to 77%.18-20 In addition to shipping, the impact is also felt in marine aquaculture, where biofouling causes and increased weight in nets, decreased water movement, and competition for food sources, causing an estimated

10% increase in the production costs of seafood.21

Over the past decade, attempts to form antifouling PEGylated surfaces have been mainly focused on surface-initiated (SI) polymerization of PEGylated acrylates and

138 methacrylates22-31 (Figure 4.1a) and grafting to of pre-synthesized poly(ethylene glycol)3,

4, 32-36 chains to the surface (Figure 4.1b). While poly(PEGylated (meth)acrylates), films do provide a high density of PEG chains and antifouling resistances, the resulting alkyl backbone can have some effect on the properties of the films. Furthermore, the grafting to approach to forming polymer films limits the grafting density of the polymer chains.

Additionally, the physisorption of polymers are prone to dissolution over time.

Substate Substate

Surface-bound initiator PEGylated (meth)acrylate

b) Pre-sythesized Polymers

Substate Substate

Surface-bound initiator PEG polymer chains c)

Substate Substate

Monomer Surface-bound initiator

139 To date, only two attempts at the SI-polymerization of epoxides initiated from a bulk surface (grafting from) have been attempted in the literature, and all have formed hyperbranched dendrimers on chemically modified glass surfaces resulting in excellent antifouling properties (Figure 4.1c).37, 38 In addition, poly(ether) films being formed on bulk surfaces, these films have been formed once on graphene oxide nanosheets, which were evaluated for the application in gas separation.39

Herein is presented an attempt to modify the surface of SS316L with polymer films of ethylene oxide, propylene oxide, and 1,2-butylene oxide. Self-assembled monolayers of

11-hydroxyundecyl phosphonic acid (HUPA) were formed on the surface of SS316L and used as initiators for surface-initiated ring-opening polymerization (SI-ROP). Several methods were employed to form SI-ROP films on the surface, including anionic ROP,

Lewis and Bronsted-Lowery acid-catalyzed ROP, and vapor phase Lewis acid-catalyzed

ROP. While films of the SI-poly(ethers) were not able to be formed in a repeatable, consistent fashion, the results contained herein present a promising avenue for the functionalization of metal oxide surfaces.

H O

R OH O

O 11-Hydroxylundecyl 9 9 Phosphonic Acid P R P OH OH OH O O O O O O O O

R= H, Me, Et Scheme 5.1 Formation of hydroxyl modified surfaces (Step 1) and polymerization of ethylene oxide (R=H), propylene oxide (R=Me), and 1,2-butylene oxide (R=Et) (Step 2).

140 5.2 Equipment and Materials

5.2.1 Equipment

All infrared spectra were recorded using a Thermo Nicolet-NEXUS 470 FT-IR equipped with a Diffuse Reflectance Infrared Fourier Transform (DRIFT) attachment.

Spectra were collected at 1024 scans with a resolution of 4 cm-1. All electrochemical measurements were recorded using a NuVant System EZstat-pro potentiostat and EZware- pro software. The three-electrode system was comprised of a platinum wire as a counter electrode, an Ag/AgCl(aq) as the reference electrode, and SS136L coupon as the working electrode. Contact angles were recorded using a Rame-Hart goniometer.

5.2.2 Materials

11- Hydroxyundecyl phosphonic acid (>95%), ethylene oxide (2M in THF), 1,2 butylene oxide (%), P4-tBu Phosphazene base as a 2 M solution in heptane were purchased from Sigma Aldrich. Propylene oxide (%) was purchased from TCI. Tetrahydrofuran,

(THF) was purchased from Marcon. Stainless steel AISI 316L (Fe 69/Cr18 /Ni10/Mo3) foil with a 0.5 mm thickness was purchased from Goodfellow Inc. All reagents were used without further purification unless otherwise stated. THF was dried over sodium and benzophenone and distilled before use. 1,2 Butylene oxide and propylene oxide were dried by stored over calcium hydride for at least 24 hours, then distilled. Propylene oxide was distilled under an N2 atmosphere at 35C. 1,2 Butylene oxide was distilled under reduced pressure at 45 C.

141 5.3 Methods

5.3.1 Preparation of SS316L coupons

Substrates were prepared by sanding stainless steel foils with 160, 320, 400, and

600 grit sandpaper. Substrates were then cleaned by sonication in acetone for 30 minutes and rinsed in boiling methanol for 30 minutes and placed in a 120 °C oven overnight.

Coupons were then stored in a sealed container under ambient conditions until used.

5.3.2 SAM Formation of 11-Hydroxyundecyl Phosphonic Acid

Monolayers of 11-hydroxyundecyl phosphonic acid (HUPA) were formed by aerosol deposition of 0.5 mM solution in dry THF using a thin layer chromatography (TLC) sprayer (Scheme 5.2, Step 1). Coupons were cooled to 4°C for 30 minutes and were then sprayed with a 60°C solution of HUPA. This process was repeated two additional times, and coupons were allowed to rest at 4°C for 30 minutes between depositions. Coupons were then annealed in an oven overnight at 60°C. Samples were rinsed for 15 minutes and sonicated for 15 minutes in dry THF to remove any physisorbed acid.

11- Hydroxyundecyl OH OH OH Phosphonic Acid 9 9 9 P P P OH OH OH OH OH OH O O O O O O O O O O O O O O SS 316L SS 316L

Scheme 5.2 Formation of hydroxyl-terminated phosphonic acid SAMs on SS316L using 11-hydroxy undecyl phosphonic acid.

142 5.3.3 Functionalization of SS316L with Polymer Films

5.3.3.1 SI-Anionic Ring-Opening Polymerization of Epoxides

A 50ml Erlenmeyer flask containing three SS316L coupons modified with HUPA monolayers and equipped with a stir bar was placed in a 120 °C oven for 12 hours prior to polymerization. The flask was removed from the oven, sealed with a septum and placed under vacuum until it reached room temperature. The flask was then back filled and evacuated three times with nitrogen. Freshly distilled THF (5 ml) was added to the reaction flask and was then cooled to -78°C in a dry ice/iPrOH bath for 15 minutes. In a typical experiment, 4 L of tBu-P4 was added to the flask, and the resulting solution was then stirred for 30 minutes at -78 °C. (Scheme 4.3, Step 1) Freshly distilled propylene oxide

(5mL) was then added to the reaction flask and allowed to come to slowly room temperature over 2 hours, and was then heated to 40 °C for 16 hours in a thermostated sand bath (Scheme 5.3, Step 2). To quench the reaction, 1ml of 1% acetic acid in DI water was added and stirred for 15 minutes. Coupons were removed from the reaction media, rinsed with THF, and stored under vacuum for at least 2 hours before DRIFT spectroscopy.

Optimizations of the procedure are summarized in Table 5.1.

H O N N P N OH O O O N N THF, tBu-P4(4mM) tBu-P4 = N P N P N 9 o 9 9 -78 C, 30 minutes -78oC to 40oC N N P P 18 hr P N P N O O O O O O O O O N

Scheme 5.3 Anionic SI-ROP of epoxides; Step 1: deprotonation hydroxyl-terminated monolayers with phosphazene base and Step 2: polymerization of propylene oxide.

143 Table 5.1 Reaction conditions attempted for SI-Anionic Ring-Opening Polymerization of Epoxides

Monomer (conc) Solvent Temp. Base (conc) Additive Result

Propylene oxide (50% (v/v)) THF 40 ºC tBu-P4 (2 mM) None No polymer

Propylene oxide (50% (v/v)) THF 40 ºC tBu-P4 (4 mM) None Polymer

Propylene oxide (50% (v/v)) THF 40 ºC DBU (4 mM) None No polymer

Propylene oxide (50% (v/v)) THF 40 ºC KOtBu (4 mM) None No polymer

Propylene oxide (50% (v/v)) THF 40 ºC tBu-P4 (4 mM) None No polymer

Propylene oxide (50% (v/v)) THF 40 ºC tBu-P4 (4 mM) AlCl3 No polymer (4mM)

Propylene oxide (50% (v/v)) THF 40 ºC tBu-P4 (4 mM) BF3 No polymer (4mM)

Ethylene oxide (2.1 M) THF 25 ºC tBu-P4 (4 mM) None No polymer

1,2- Butylene oxide (50% (v/v)) THF 65 ºC tBu-P4 (4 mM) None No polymer

Propylene oxide (50% (v/v)) Toluene 40 ºC tBu-P4 (4 mM) None No polymer

Propylene oxide (50% (v/v)) TBME 40 ºC tBu-P4 (4 mM) None No polymer

5.3.3.2 SI- Toluenesulfonic Acid-Catalyzed Ring-Opening Polymerization of Epoxides

A 50 ml flask was placed in a 120 °C oven with three SS316L coupons modified with HUPA monolayers and a stir bar. After drying in the oven for 12 hours, the flask and its contents were removed from the oven, and 5 mg of toluenesulfonic acid was added. The flask was then sealed with a septum then put under vacuum. After the flask cooled to room temperature, it was backfilled with nitrogen and evacuated two more times. Freshly distilled THF (5 mL) and either propylene oxide or 1,2-butylene oxide (5mL) was added to the flask and stirred. The flask was heated to reflux in a sand bath at 40 °C for propylene oxide and 65 °C for butylene oxide. (Scheme 5.4) After 18 hours the coupons were removed from the reaction solution and rinsed with THF. After rinsing, coupons were dried under vacuum and examined with DRIFT spectroscopy. Alternatively, neat monomer was

144 also used in place of the 5ml of freshly distilled THF and monomer, 10 mL of the epoxide was used, with all optimizations summarized below in Table 5.2

H H O O

R L.A. H R TsO O O OH O O O R R TsO- = O S 9 9 9 O P P P O O O O O O L.A. = Lewis Acid (BF3, ZnCl2) O O O R=H, Me or Et

Table 5.2 Reaction conditions attempted SI Toluenesulfonic Acid-Catalyzed Ring- Opening Polymerization of Epoxides

Acid (conc.) Conditions Polymer Formation TsOH (1mM) 50% (v/v) propylene oxide, THF, 40 ºC No, Possible oligomerization TsOH (1mM) Neat propylene oxide, 40 ºC No, Possible oligomerization TsOH (1mM) 50% (v/v) 1,2 butylene oxide, THF, 65 ºC No, Possible oligomerization TsOH (1mM) Neat 1,2 butylene oxide, 65 ºC No, Possible oligomerization

5.3.3.3 SI- Lewis Acid-Catalyzed Ring-Opening Polymerization of Epoxides

A 50 ml flask with three SS316L coupons modified with HUPA monolayers, and a stir bar were dried in a 120 °C oven for at least 12 hours. Coupons were placed in the flask and sealed with a septum. The flask and its contents were placed under vacuum until cooled, then backfilled with nitrogen gas. A 10mL solution of 50% (v/v) of propylene oxide in THF was prepared by charging the flask with 5ml of freshly distilled monomer and solvent. Lewis acid (BF3 diethyl etherate, AlCl3, or ZnCl2) was charged into the flask and

145 then heated to 40 °C in a thermostated sand bath. (Scheme 5.4) After 18 hours, the coupons were removed and rinsed with THF and stored under vacuum. To determine if the formation of the polymer films occurred, DRIFT spectroscopy was employed.

Table 5.3 Reaction conditions attempted for SI-Lewis Acid-catalyzed ring-opening polymerization of epoxides

Acid (conc.) Conditions Polymer Formation

BF3 (1mM) 50% (v/v) propylene oxide, THF, 40 ºC Polymer in solution, none attached

ZnCl2 (1mM) 50% (v/v) propylene oxide, THF, 40 ºC No

AlCl3 (1mM) 50% (v/v) propylene oxide, THF, 40 ºC No

5.3.3.4 SI- Gas-Phase Lewis Acid-Catalyzed Ring-Opening Polymerization of

Epoxides

SS316L coupons modified with HUPA monolayers were dried in a 120 °C oven for

12 hours before use. The glass vacuum deposition chamber, shown in Figure 5.2, was placed in a 120 °C oven overnight to remove moisture. After removing coupons and deposition glassware from the oven, coupons were placed under vacuum. After cooling to ambient temperature, the entire system was placed under nitrogen, and boron trifluoride diethyl etherate (BF3) and freshly distilled propylene oxide were placed into separate flasks. Each flask was isolated from the system by closing each respective flask’s stopcocks. The main reaction chamber was evacuated, and the flask containing BF3 was evacuated by opening the stopcock for 10 minutes and then was re-isolated from the system. The flask containing propylene oxide was evacuated by opening the stopcock for

10 minutes, and it was re-isolated from the system. The remainder of the system was left under vacuum for an additional 15 minutes. The entire system was isolated from the

146 vacuum, and the stopcocks on the flasks containing BF3 and propylene oxide were opened, and vapors of each reagent were exposed to the coupons for 1.5 hours. Alternately, with the system set up as described above, the entire system was kept under an active vacuum, and the stopcocks on the flasks containing BF3 and propylene oxide were opened. Vapors of each reagent were exposed to the coupons for 1.5 hours. Post-polymerization treatment of the polymer film treated is described in Table 5.4, below.

Scheme 5.5 Gas-phase ring-opening polymerization of epoxides a) formation of the BF3 propylene oxide complex formed from BF3 diethyl etherate b) Ring-opening the BF3 propylene oxide complex resulting in SI-poly(propylene oxide)

Table 5.4 reaction conditions attempted for SI-Anionic Ring-Opening Polymerization of Epoxides

Reaction conditions Post-polymerization Polymer Formation Passive Vacuum for 1.5 hours Rinsed Not stable to rinsing Passive Vacuum for 1.5 hours Annealed at 60ºC then rinsed Not stable to rinsing Active Vacuum for 1.5 hours Rinsed No

147 BF3

To Vacuum Reaction Chamber

Figure 5.2 Vacuum deposition set up for gas-phase polymerization of propylene oxide using boron trifluoride diethyl etherate as a catalyst. The system was connected to vacuum the Schlenk manifold.

5.3.4 Surface Characterization

5.3.4.1 Diffuse Reflectance Infrared Fourier Transform Spectroscopy

SS316L coupon as a background. Before all spectra were collected the sample chamber was purged with N2 gas for 5 minutes.

5.3.4.2 Contact angle

Static contact angles were measured by placing a 2L drop of deionized (18 Ω) water on either a modified or unmodified coupon, and the contact angle was measured.

This was repeated twice more and three more times on two additional coupons for a total of nine measurements. Averages and standard deviations were calculated for each modification

148 5.3.4.3 Cyclic voltammetry

Electrochemical measurements were recorded using 0.1M NaCl(aq) as a supporting electrolyte. Voltammograms were collected using a standard three-electrode system with

SS316L coupon as the working electrode and a platinum wire as the counter electrode.

Ag/AgCl(aq) electrode was used as the reference electrode. Cyclic voltammograms were collected from +0.3 V to -1.2 V vs. Ag/AgCl at a rate of 50 mV/sec. The surface area of each electrode was measured using ImageJ software. Measurements were normalized to the surface area of each electrode.

5.4 Results

5.4.1 Formation of HUPA SAMs

Self-assembled monolayers of HUPA were formed on the surface of SS316L to provide a point of attachment for polymer films. The methylene stretching region of and

-1 -1 ordered SAM presents with vCH2asym  2918 cm and vCH2asym  2848cm with 0.3% to

0.7% transmittance.40-46 Coupons were sprayed with 0.5 mM solution of the phosphonic acid in tetrahydrofuran using a thin layer chromatography sprayer. After rinsing and sonicating with THF to remove weakly adhered molecules, The DRIFT spectrum of the

-1 -1 monolayers presented with vCH2asym at 2915 cm and vCH2asym at 2848 cm (Figure 5.3a), indicating that the monolayers formed are ordered and crystalline with alkyl chains in an all-trans orientation.

149

The monolayers are attached to the oxide surface of SS316L via the phosphonic acid head group. The DRIFT spectrum of the phosphonic acid-binding region contains a single absorption at 1016 cm-1 (Figure 5.3b), corresponding to the P-O stretching. The absences of stretches at 1232 cm-1 and 930cm-1 indicate that P=O and P-O-H, respectively, are no longer present on the acid molecule, suggesting that the phosphonic acid is bound in a tridentate bonding pattern.41, 43-46

5.4.2 Anionic Ring-opening polymerization

The first technique used in an attempt to form films of surface-initiated poly(ethers) on the surface of SS316L was the anionic ring-opening polymerization outlined in Scheme

5.2. First, hydroxyl-terminated monolayers were deprotonated using a P4-tBu phosphazene base, and immediately following deprotonation, epoxide-based monomers were added to the system. Polymerization was then allowed to occur for 18 hours. After polymerization,

150 -l -l the advent of signals at 2970 cm and 2896 cm corresponding to vCH3 and an increase in

-l -l the percent transmittance of the vCH3 at 2935 cm and 2870 cm outside of the monolayer range40 in the DRIFT spectrum indicated polymer formation on coupons (Figure 5.4a) In

Figure 5.4b, additional peaks not present in the spectrum of the monolayer were observed at 1455 cm-l and 1373 cm-l caused by bending vibrations of C-H bonds of methyl groups on the poly(propylene oxide) chains. Absorbances at 1098 cm-l and 932 cm-l correspond to the symmetric and asymmetric C-O-C ether stretching of the polymer backbone, respectively. The presence of the ether stretching indicates that the desired bond of interest has formed under the reaction conditions.

To examine the films of poly(propylene oxide), the DRIFT spectra of the SI- polymer film was overlaid with the spectrum of the bulk synthesized polymer, formed by anionic ROP. Excellent agreement was observed in the methylene stretching region, as

151 indicated by the peaks at 2970 cm-l, 2935 cm-l, 2896 cm-l, and 2870 cm-l in the SI polymer films all match up well with the bulk synthesized polymers (Figure 5.5a). Additionally, the presence of the methyl bending at 1455 cm-l and 1373 cm-l and the ether stretching at 1098 cm-l and 932 cm-l in both spectra further confirm the formation of the same molecular architecture is present on the surface (Figure 5.5b).

While films of SI-poly(propylene oxide) were able to be formed once under the reaction conditions described in Scheme 5.3, the results were not able to be replicated. To this end, reaction optimization was attempted, which is summarized in Table 5.1 in section

5.3.3.1. of this chapter. All coupons presented with the absence of any IR bands attributed to the polymer. However, the successful formation of poly(ether) films was not able to be accomplished using this technique, and alternate routes were explored to form the poly(ether) modified surfaces.

152 5.4.3 Acid-Catalyzed Ring-Opening Polymerization

Two different acid-catalyzed approaches were attempted; one utilized a Lewis acid catalyst, and the other used TsOH as a Brønsted-Lowry acid. When BF3 ZnCl2 and AlCl3 were used as Lewis acid catalysts, no formation of SI-poly(propylene oxide) was observed on the surface of SS316L coupons, as indicated by DRIFT spectroscopy. In the experiment where BF3 was used as a Lewis acid catalyst, the solvent and unreacted monomer was evaporated from the reaction solution, and a considerable amount of solution grown polymer was isolated. However, upon the use of TsOH as a catalyst, the advent of stretching not observed in the monolayer spectra was present at 2959 cm-1, 1086 cm-1, and

902 cm-1 corresponding to methyl C-H stretching and the symmetric and asymmetric ether stretching (Figure 5.6a and b). However, the methylene stretching at 2922 cm-1 and 2852 cm-1 showed no appreciable increase in the percent transmittance in the DRIFT spectrum.

Further optimizations of this included the use of performing the reaction using neat conditions (no solvent) and the use of 1,2-butylene oxide, which allowed the polymerization to be conducted at higher temperatures (summarized in table in Table 5.2).

No further improvement, as indicated by DRIFT spectroscopy, was observed for the formation of polymer films.

153

5.4.4 SI-Polymerization of Gaseous Epoxide Monomers

In addition to the solution based SI-polymerization techniques utilized, a vapor phase SI-polymerization was also employed. The SS316L surface functionalized with

HUPA SAMs was exposed to a mixture of propylene oxide and boron trifluoride diethyl etherate vapors. After polymerization, similar peaks in the solution-based polymerization were observed. The methyl stretching was present at 2971 cm-1 and 2896 cm-1 and methylene stretching at 2931 cm-1 and 2871 cm-1 (Figure 5.7a black trace). Additionally, in the DRIFT spectra of the ether stretching region (Figure 5.7b black trace), the presence of methyl bending at 1451 cm-1 and 1371 cm-1, and the asymmetric and symmetric ether stretches at 1106 cm-1 and 1001 cm-1 occur, respectively. The presence of the ether stretches indicates that the desired bond was formed as a result of the polymerization, however, after rinsing with THF the modified surface presented with no absorbances related to the

154 polymer film, indicating that the polymer is no longer present on the surface (Figure 5.7a and b grey dashed trace). Additional attempts were made including annealing after polymerization (before rinsing), and the use of active vacuum in place of passive vacuum, which yielded no improvement (results are summarized in Table 5.4).

5.4.5 Contact Angle Analysis

As a result of the polymerizations discussed in the previous section of the results, further characterization of SI-poly(propylene oxide) formed on SS316L by anionic ROP using 4 mM P4-tBu phosphazene base (Section 4.4.2). To characterize the wettability of the surface, contact angle goniometry was employed. 2 L drops of deionized water (18

) were dropped on the surface, and the contact angle was recorded. Nine measurements were averaged, and the standard deviation was calculated. A contact angle of less than 90

155 is considered to be hydrophilic and a contact angle of greater than or equal to 90 is deemed to be hydrophobic.

Before functionalization with SAMs of HUPA, the surface had a contact angle of

71  3indicating a hydrophilic surface (Figure 5.8). The native SS316L oxide layer has a hydrophilic contact angle due to the hydroxyl and oxo groups on the surface allows for favorable interactions such as hydrogen bonding with water. Furthermore, after the functionalization of the native oxide layer with SAMs of HUPA, the contact angle was recorded to be 75  5, which is caused by the hydroxyl groups presenting at the interface, allowing for hydrogen-bonding interactions to occur. These groups are needed to present at the interface in order for the functionalization with poly(ether) films to be possible. After polymerization with propylene oxide, a contact angle of 49  1 was observed, which is expected due to the extreme hydrophilic nature of 1,2 poly(ether)s. Water can form a strong hydrogen bonding network with the resulting polymer films.

Figure 5.8 Contact angle of unmodified SS316L coupons, HUPA monolayer modified coupons, and SI- poly(propylene oxide) from by anionic ring-opening polymerization modified coupons.

156 5.4.6 Cyclic Voltammetry

Additionally, cyclic voltammetry studies were also able to be performed on the coupons modified with propylene oxide from Section 5.3.3.1. Studies indicated there wasn’t a significant change in the magnitude of the current flowing through the coupon modified with poly(propylene oxide) when compared to the HUPA modified electrode.

However, there was a modest decrease in the current passing through the SI-poly(propylene oxide) coupon in comparison to the native oxide surface. The small decrease indicates that poly(propylene oxide) does have a low degree of corrosion-resistant nature (Figure 5.9).

Furthermore, the fractional coverage can be calculated using Equation 5.1, where theta represents the fraction of the surface that is inaccessible by the solvent. After functionalization with SAMs, the surface had a fractional coverage of 0.29.

157 Table 5.5 Fractional coverages calculated from cyclic voltammetry

Modification θCV Bare 0.00 HUPA SAM 0.29 SI-Poly(propylene oxide) 0.25

푖 휃 =1− 푖

Equation 5.1

5.5 Discussion

Previous attempts at forming poly(ether) modified surfaces have been focused on either the chemisorption or physisorption of pre-synthesized polymer chains or the polymerization of PEGylated macromonomers. Few attempts to form SI-poly(ether) films have ever been reported.37-39 Here, the grafting from polymerization of ethylene oxide, propylene oxide, and 1,2- butylene oxide was attempted with the goal of surface-bound poly(ether) films. Polymer film formation was attempted to be grown using anionic and acid-catalyzed ROP polymerizations. These films have the potential to be useful in both the marine and biomedical environments, because of the ability to create cell resistant/antifouling surface modifications. While multiple attempts using several techniques were made to form SI-poly(ethers) of ethylene oxide, propylene oxide, and 1,2 butylene oxide, only one attempt at the formation of poly(propylene oxide) was successful.

As a result of the SI-polymerizations, poly(propylene oxide) was able to be formed under one set of reaction conditions. Deprotonation of the polymer films was used to improve the nucleophilicity of the surface hydroxyls, allowing them to open epoxide rings more easily. tBu-P4 phosphazene base, as well as DBU, were chosen because of their strong basic properties, ensuring almost complete deprotonation of the tail group, and that they

158 both form highly dissociated ion pairs in solution due to the size of the base.47 The resulting

DRIFT spectrum of the polymer-modified surface indicated an increase percent reflection of the methylene asymmetric stretch, indicating an increase in the number of CH2 groups.

The increase in methylene stretching is consistent with polymer chains being grown from the surface as a result of the increasing number of CH2 groups. Also, the presence of methyl stretching, which wasn’t present in the spectrum of the SAM modified surface, is coming from the methyl group on the polymer chain. The presence of ether stretching confirms the formation of the targeted bond/functional groups. Contact angle goniometry revealed an extremely hydrophilic surface, which is a result of the strong hydrogen bonding interactions that can be formed with poly(ethers) such as poly(propylene oxide) and poly(ethylene oxide).48-51 Electrochemical characterization of the SI-polymer films indicated that poly(propylene oxide) films had poor corrosion inhibition properties, because a considerable amount of current was able to pass through the electrode after functionalization. This is most likely caused by the hydrophilic nature of the polymers, which allow the films to be water permeable, and thus allows charge transfer between the surface of the coupon and the solution. However, after multiple, repeated attempts to repeat and optimization of the experiment, the results are not able to be replicated, and only small amounts of polymer were formed in solution. Thus, other techniques such as acid-catalyzed ring-opening polymerization and gas-phase Lewis acid-catalyzed polymerization were attempted, but unsuccessful.

One possible explanation why SI-ROP of epoxides was not able to be formed consistently under the anionic conditions is caused by the large pKa of the tBu-P4 phosphazene base’s conjugate acid. With a high pKa, of approximately 30 – 40. the base

159 can deprotonate monomer (propylene oxide) and solvent (THF) with the residual base from the deprotonation of the monolayer. The deprotonated monomer and THF species would then be able to undergo ring-opening reactions forming initiators for anionic ROP in solution (Scheme 5.6). Evidence of this exists from polymers being present after evaporation of unreacted monomer and solvent. In the case of propylene oxide, the proton on the methyl group can be abstracted, which then causes the ring to open, forming an sp2 bond, resulting in the vinyl alkoxide (Scheme 5.6a).52-54 This is favorable due to the high strain of the three-member epoxide ring.8 Furthermore, THF is also known to undergo degradation in the presence of strong bases, namely organolithium species,55, 56 which have

47 comparable pKa values to tBu-P4 phosphazene base. The deprotonation of THF can occur in two different positions, which allow for the formation of alkoxides (Scheme 4.6b).55, 56

Both resulting species can act as initiators for the ring-opening polymerizations of epoxides.

160 To investigate the potential downfalls caused by the deprotonation of monomer and solvent, ethylene oxide and tert-butlymethylether (TBME) were used. Ethylene oxide can’t undergo the ring-opening, as seen in propylene oxide in Scheme 5.6a. Additionally, TBME doesn’t have deprotonatable hydrogens due to the methyl and tert-butyl groups. However, after using these monomers and solvents for the polymerization, no further SI polymers were observed, indicating this was not likely the sole cause. Additionally, the 1,2-butylene oxide was chosen as a monomer, because of the higher boiling point in comparison to ethylene oxide or propylene oxide. The use of this monomer allows for the polymerization to be conducted at higher temperatures. The modification of reaction conditions led to no further improvement.

Additionally, this reaction is also highly sensitive to trace water due to the high pKa of the tBu-P4 phosphazene and the relative pKa values of alcohols and water. Since the pKa value of alcohol terminated monolayers (similar to the monolayers described here) have

57 been measured to be higher than 12, and water has a pKa of 7. Thus, the introduction of trace water would protonate the surface, inhibiting SI-polymerization. Furthermore, due to the concentration of SAM molecules absorbed to the surface, which is approximately 0.6 nmol./cm2,58, 59 even the introduction of trace water would cause irreversible termination of immobilized propagating polymer chains and chain transfer to solution under the reaction conditions. Even though rigorous inert reaction conditions were followed (i.e., drying of solvents and monomers, standard Schlenk procedures, and reactions being conducted under a positive pressure of dry N2 gas) trace amounts of water are still able to enter the system through transfer of reagents. To mitigate these effects, acid-catalyzed polymerizations were employed.

161

Scheme 5.7 Possible Protonation of the phosphazene base by water

Lewis and Brønsted Lowery acids were used as alternative methods of forming poly(ethers) via ROP from solutions of epoxides. Protonation of the epoxide oxygen with

TsOH causes a weakening of the C-O bond, making the epoxide more electrophilic, thus promoting ROP initiated by the HUPA monolayer. Lewis acids, such as BF3, AlCl3, and

ZnCl2 function analogously to TsOH (Brønsted-Lowery acid) by coordinating to the oxygen on the monomer with boron, aluminum or zinc, respectively, resulting in more electrophilic epoxide.

When TsOH was used as an acid catalyst for the polymerization of propylene oxide, small changes in the spectrum was observed. The appearance of ether stretching and methyl stretching bands occurring indicate the desired functional groups were present on the surface. However, an increase in the percent transmittance of the methylene stretching outside the range usually observed for monolayers was not observed. The presentation of

IR signals that are diagnostic of methyl and ether functional groups, indicates that possible oligomers (polymers with only a few repeat units) were formed. After switching monomers to 1,2 butylene oxide, which has a higher boiling point when compared to propylene oxide, it allowed for reactions to be conducted at higher temperatures. However, this also did not result in an increase in the percent reflectance outside the monolayer range for methylene stretching.

162 Lewis acids, such as BF3, AlCl3, and ZnCl2, were also used to catalyze the formation of poly(ethers). However, after 18 hours, no change in the spectrum was observed, indicating that the polymer films were not able to be formed using this methodology.

However, when BF3 was used as a catalyst, solution grown polymer was isolated. This is likely caused by BF3 being a strong enough Lewis acid to open epoxides, as shown in

Scheme 5.8. This has been exploited before as a methodology for the synthesis of small organic molecules.60, 61 Furthermore, to eliminate any effects of solvent, moisture, or other factors, a vapor phase polymerization technique was devised. Coupons modified with

HUPA monolayers were exposed to a mixture of propylene oxide and BF3 etherate vapors under vacuum. The coupons modified with propylene oxide and BF3 etherate vapors did present with an increase in the percent reflectance of IR absorptions, and peaks not present in the monolayer spectrum. However, the resulting films were only physisorbed and removed by a simple THF rinse. One possible route to the formation of polymers in both the solution and vapor phase polymerizations, that were not initiated from the surface, could be caused by BF3 opening the epoxide ring similar to the solution polymerization

(Scheme 5.8).

Scheme 5.8 Ring-opening of epoxides promoted by BF3 in solution, :B is either another propylene oxide or THF molecule

163 5.6 Conclusions

While multiple attempts using several techniques were made to form SI- poly(ethers) of ethylene oxide, propylene oxide, and 1,2 butylene oxide, only one attempt at the formation of poly(propylene oxide) was successful. Poly(propylene oxide) films that were formed through SI anionic ROP had a hydrophilic contact angle and had a fractional coverage of 0.25. Furthermore, electrochemical characterization via cyclic voltammetry in revealed the film would not be a good corrosion resistant barrier. As a result of anionic polymerization reaction conditions, deprotonation of both THF (solvent) and the epoxide monomers is possible, ultimately leading to chain transfer to solution, resulting in minimal

SI-polymerization. Additionally, the introduction of trace amounts of water would also lead to irreversible chain transfer. Similarly, under the reaction conditions for the acid- catalyzed polymerization with BF3, also can lead to epoxide ring-opening, causing initiation of non-surface bound polymer chains.

Previously, the only two reports of poly(ethers) grafted from the surface have been reported are films of hyperbranched poly(ethylene oxide) formed using anionic ring- opening polymerization on modified glass surfaces. This work aims to extend the current knowledge of the field as linear poly(ethers) have only ever been grafted from the surface of graphene oxide, and their electrochemical characterization and antifouling properties have never been examined. Furthermore, SI growth of poly(ethers) chains has never been demonstrated on metallic surfaces, which could be of use both the marine and biomedical environments to prevent biofouling of surfaces, leading to a potential increase in shipping efficiency and reduction in infections caused by biofilms on implant devices, respectively.

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35. Kingshott, P.; Wei, J.; Bagge-Ravn, D.; Gadegaard, N.; Gram, L., Covalent Attachment of Poly(ethylene glycol) to Surfaces, Critical for Reducing Bacterial Adhesion. Langmuir 2003, 19 (17), 6912-6921.

36. Amiji, M.; Park, K., Surface modification of polymeric biomaterials with poly(ethylene oxide), albumin, and heparin for reduced thrombogenicity. Journal of Biomaterials Science, Polymer Edition 1993, 4 (3), 217-234.

168 37. Khan, M.; Huck, W. T. S., Hyperbranched Polyglycidol on Si/SiO2 Surfaces via Surface-Initiated Polymerization. Macromolecules 2003, 36 (14), 5088-5093.

38. Moore, E.; Delalat, B.; Vasani, R.; McPhee, G.; Thissen, H.; Voelcker, N. H., Surface-Initiated Hyperbranched Polyglycerol as an Ultralow-Fouling Coating on Glass, Silicon, and Porous Silicon Substrates. ACS Applied Materials & Interfaces 2014, 6 (17), 15243-15252.

39. Wu, Y.; Jia, P.; Xu, L.; Chen, Z.; Xiao, L.; Sun, J.; Zhang, J.; Huang, Y.; Bielawski, C. W.; Geng, J., Tuning the Surface Properties of Graphene Oxide by Surface-Initiated Polymerization of Epoxides: An Efficient Method for Enhancing Gas Separation. ACS Appl Mater Interfaces 2017, 9 (5), 4998-5005.

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42. Raman, A.; Gawalt, E. S., Self-Assembled Monolayers of Alkanoic Acids on the Native Oxide Surface of SS316L by Solution Deposition. Langmuir 2007, 23 (5), 2284- 2288.

43. Raman, A.; Dubey, M.; Gouzman, I.; Gawalt, E. S., Formation of Self-Assembled Monolayers of Alkylphosphonic Acid on the Native Oxide Surface of SS316L. Langmuir 2006, 22 (15), 6469-6472.

44. Quiñones, R.; Raman, A.; Gawalt, E. S., Functionalization of nickel oxide using alkylphosphonic acid self-assembled monolayers. Thin Solid Films 2008, 516 (23), 8774-8781.

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51. Ngo, B. K. D.; Grunlan, M. A., Protein Resistant Polymeric Biomaterials. ACS Macro Letters 2017, 6 (9), 992-1000.

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170 58. Luschtinetz, R.; Oliveira, A. F.; Duarte, H. A.; Seifert, G., Self-assembled monolayers of alkylphosphonic acids on aluminum oxide surfaces – A theoretical study. Zeitschrift für Anorganische und Allgemeine Chemie 2010, 636, 1506-1513.

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171 Chapter 6: Summary

6.1 Conclusions

Several novel polymer films were formed on the native oxide surface of SS316L, which were able to provide improved corrosion resistance, while be further modified to impart additional properties by chain extension. The introduction of PEGylated monomers provided a surface that has the possibility of being antifouling, in addition to corrosion resistant, forming a dual pacified surface. Overall, polymer films on SS316L can provide highly advanced surface properties.

6.1.1 Functionalization of SS316L with Corrosion-Resistant Polymer Films

To inhibit the pitting corrosion of SS316L in marine environments, SI-ATRP was used to form polymer films of styrene, methyl acrylate, and methyl methacrylate on the oxide surface of SS316L (Figure 6.1). Several methods were examined to provide an initiator modified surface for SI-ATRP. Ultimately, a new strategy for the immobilization of vapor phase BiBB at the interface of HUPA SAMs was devised, as other methods failed to produce a consistent modification. Upon introduction of monomer, copper catalyst, and sacrificial reducing agent, hydrophobic polymer films were formed.

172 Br Monomers m R Br Styrene O O O O O O Monomer Methyl Acrylate 9 [Cu(TPMA)Br][Br] 9 O P Tin (II) Ethylhexanoate P O O O O O O O

SS 316L SS 316L Methyl Methacrylate

Figure 6.1 Functionalization of SS316L with polymer films of styrene, methyl acrylate, and methyl methacrylate

Electrochemical characterization of SS316L coupons modified with polymer films suggests that they can provided an excellent corrosion-resistant surface. EIS spectroscopy indicated charge transfer resistance values that were two to three orders of magnitude larger than those of the bare or initiator modified surfaces, resulting in excellent protection efficiencies of up to 99.9%. The significant increase in the charge transfer resistance indicates that the SAM and initiator modified surfaces do not significantly affect the corrosion-resistant nature of the surface. Cyclic voltammetry indicated a large decrease in the current passing through SS316L coupons when the polymer film modified coupons were compared to the native oxide. Polymer films of styrene, methyl acrylate, and methyl methacrylate had high fractional coverages of up to 0.99, indicating that the surfaces were completely covered. Upon immersion in 0.1M NaCl for five days and repetitive cyclic voltammetry, little change was observed in the voltammograms indicating films were not delaminating after 5 days and were stable to electrochemical reactions, respectively.

Coupled with the EIS data, the cyclic voltammetry data indicates the polymer films are outstanding corrosion resistance surface modifications.

173

6.1.2 Antifouling and Corrosion Resistant Surface Modification

Polymer films of oligo(ethylene glycol)acrylate (OEGA) and oligo(ethylene glycol)methacrylate (OEGMA) were formed on the surface of SS316L using SI-ATRP and evaluated for their corrosion resistance. Poly(OEGA) and poly(OEGMA) films have been known in the literature to be excellent antifouling modifications;1-10 however, no documented report examines the electrochemical characterization of these films.

Electrochemistry revealed a slight corrosion resistance by the increase in charge transfer resistance and a slight decrease in the current passing through the coupons in cyclic voltammograms, indicating poor corrosion resistance.

To increase the corrosion resistance of the PEGylated polymer films, SI- poly(styrene) was chain extended with OEGA and OEGMA (Figure 6.2). The resulting copolymer films, poly(styrene-co-OEGA) and poly(styrene-co-OEGMA), can provide both corrosion and cell resistance properties. DRIFT spectra confirmed that both styrene and PEGylated polymer films were present. Furthermore, contact angle goniometry revealed that the surfaces were hydrophilic, with contact angles that were not statistically significant from those of the PEGylated homopolymer films. This confirmed that PEG chains were presenting at the surface, which is needed for cell resistant properties.

174 Br O m O O Anitfouling 12

Br n n Corrosion Br Resistance

O O O O O O

Monomer Monomer 9 [Cu(TPMA)Br][Br] 9 [Cu(TPMA)Br][Br] 9 SnII(EH) SnII(EH) P 2 P 2 P O O O O O O O O O SS 316L SS 316L SS 316L Figure 6.2 Polymer films that were synthesized as a part of this project. Poly(styrene) films were forming in part of a previous project.

6.1.3 A Novel Approach to a Poly(ether) Modified Surface

The use of surface-initiated ring-opening polymerization (SI-ROP) of epoxides to generate a poly(ether) modified surface (Figure 6.3) has been relatively unexplored with few reports in the literature.11, 12 Here the surface of SS316L was functionalized with

HUPA monolayers, which were used as a polymerization initiator. Anionic SI-ROP was conducted by deprotonating the hydroxyl on a HUPA monolayer using P4-tBu phosphazene, DBU, or potassium t-butoxide then introducing monomer. However, only one attempt using P4-tBu phosphazene and propylene oxide formed SI-polymers. The resulting poly(propylene oxide) film was highly hydrophilic with a contact angle of 31, and cyclic voltammetry indicated a low fractional coverage of 0.25. The large amount of current passing through the polymer film in the voltammograms indicated poor corrosion resistance.

The most likely cause of the poor reproducibility was the introduction of trace amounts of water or the deprotonation of THF and monomer. To mitigate these side

175 reactions and generate higher quality polymer films, an acid-catalyzed approach was devised. After monomer was exposed to a HUPA modified surface, Lewis or Brønsted

Lowery acids were added to catalyze the formation of polymer films. However, absorbances in the DRIFT spectrum after polymerization never exceeded the intensity observed for a monolayer, indicating either polymer films had not formed, or small oligomers had formed.

H O

R OH O

O 11-Hydroxylundecyl 9 9 Phosphonic Acid P R P OH OH OH O O O O O O O O

R= H, Me, Et Figure 6.3 Formation of hydroxyl modified surfaces (Step 1) and polymerization of ethylene oxide (R=H), propylene oxide (R=Me), and 1,2-butylene oxide (R=Et) (Step 2).

6.2 Impact of Projects

The functionalization of SS316L with films of poly(styrene), poly(methyl acrylate), and poly(methyl methacrylate) formed thru SI-ATRP has not been reported in the literature to date. These films were corrosion resistant, which can find applications where corrosion is an issue, as an example when SS316L is used in marine applications.13-15 Furthermore, the toxicity of these films formed using SI-ATRP has never been examined using mammalian cell culture on any metal oxide surface. Since these films were found to be no more toxic than the native oxide of SS316L, they are also suitable for use in biological environments where corrosion may occur.

176 PEGylated films of poly(OEGA) and poly(OEGMA) have never been formed on the surface of SS316L, nor have the SI-poly(styrene-co-OEGA) or SI-poly(styrene-co-

OEGMA) been grafted from the surface of any metal oxide. Furthermore, the electrochemical characterization of these films has never been reported in the literature to date. The copolymer films have the potential to be excellent corrosion resistant and antifouling surface modifications.1-10 The copolymer films have applications where fouling and corrosion occur, such as marine environments. In marine conditions, pitting corrosion and growth of organisms such as algae or barnacles causes the need for extra maintenance and loss of shipping efficiency.16-20 Additionally, these films could find use in the biomedical industry to prevent implant corrosion and bacterial biofilm formation, reducing the number of revision surgeries.13, 21-27

The formation of SI poly(ether) films on the surface of bulk materials have only been studied twice, and have resulted in hyper-branched polymer film on the surface of glass.11, 12 The goal of this research was to explore and extend the current literature to include linear poly(ethers) and the formation of poly(ethers) on metal oxide surfaces. The synthesis of poly(ethers) films on the surface of metals has the potential to be used in marine and biomedical industries to inhibit fouling by organisms and bacteria.13, 16-23

Ultimately, several novel polymer films were formed on the native oxide surface of SS316L, which were able to provide improved corrosion resistance, while be further modified by chain extension. The

6.3 Future Work

A potential avenue to extend the work contained within this dissertation is to incorporate crosslinking monomers into the films of poly(methyl acrylate), poly(methyl

177 methacrylate), and poly(styrene) (Figure 6.4). This could be accomplished by substituting an amount of monofunctional monomers with bifunctional monomers. Theoretically, the incorporation of these monomers would generate an SI network polymer. Each resulting film would have multiple points of attachment due to the crosslinking between polymer chains, increasing longevity of these films. Additionally, the crosslinks could improve the corrosion-resistant properties by further restricting the motility of ions from the solution to the oxide surface.

Br Br Br

O O O O O O O O O O O O 11 11 P P 11 P 11 P P 11 P 11 O O O O O O Cu Catalyst O O O O O O O O O O O O Metal Oxide Metal Oxide

Figure 6.4 Proposed formation and structure of crosslinked polymer films from by the introduction of bifunctional monomers

To improve the known antifouling properties of PEGylated SI-polymers, post- synthetic modification of films can be used to impart additional fouling resistance. The selection of appropriate PEGylated monomers would provide a functional group handle, which could be used to immobilize antibiotic or nitric oxide releasing molecules such as vancomycin or S-nitroso-penicillamine. A potential example is the polymerization of azido terminated PEGylated macromonomer, then using orthogonal chemistry to immobilize target molecules (Figure 6.5). The attachment of antibiotic or nitric oxide releasing

178 molecules to a PEGylated film could provide additional resistance to biofilm formation in comparison to SI- polymers alone.

O NH

Br Br O N O m m N3 O R N N O R Br O O

O O O O O O O O N 9 O 9 O O 9 O N3 P P O P O O O O O O O O O

H2N

Figure 6.5 Proposed formation and structure of antibiotic or nitric oxide releasing molecule modified PEGylated polymer films

Additional studies using ellipsometry could be used to complement the data collected on polymer films. This has been used in the literature as a method to measure the thickness of the SI-polymer film.28-31 This particular analysis technique would provide meaningful results in Chapters 3, 4, and 5. In Chapter 3 the polymer film thickness could be correlated to the protection efficiency and corrosion resistance of the modified surface.

In Chapter 4, it would confirm the additional thickness of the polymer films gained from chain extension of poly(styrene) films. This would complement the collected DRIFT spectroscopy and contact angle data. Also, ellipsometry would complement DRIFT spectroscopy used in Chapter 5 for catalyzed ROP of epoxides. This would confirm that the small oligomers of propylene oxide and 1,2 -butylene oxide were formed during acid- catalyzed polymerizations.

179 6.4 References

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182