A Dissertation

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A Dissertation

entitled

Effect of Negative Thermal Expansion Material Cubic ZrW2O8 on Polycarbonate Composites

Xiaodong Gao

Submitted to the Graduate Faculty as partial fulfillment of the requirements for the

Doctor of Philosophy Degree in

Chemistry

______Cora Lind-Kovacs, Ph.D., Committee Chair

______Maria R. Coleman, Ph.D., Committee Member

______Terry P. Bigioni, Ph.D., Committee Member

______Jon R. Kirchhoff, Ph.D., Committee Member

______Patricia R. Komuniecki, Ph.D., Dean College of Graduate Studies

The University of Toledo

May 2015

This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author. An Abstract of

Effect of Negative Thermal Expansion Material Cubic ZrW2O8 on Polycarbonate Composites

Xiaodong Gao

Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Chemistry

The University of Toledo

May 2015

Research on control of thermal expansion of polymers has attracted significant attention, since polymers exhibit excellent mechanical and electronic properties, but suffer from high thermal expansion due to the thermal motion of their long molecular chains. Such problems can be addressed through formation of composites that contain an inorganic filler material. Filler materials reduce the thermal expansion of polymers through restriction of polymer chain motion. One particular area of interest is the introduction of negative thermal expansion (NTE) materials into polymer composites. The NTE property is expected to have an additional effect on the reduction of the coefficient of thermal expansion (CTE) of the composites. Several papers have demonstrated successful reduction of the CTE of polymer composites using cubic

ZrW2O8, however, it is still unclear how much of this effect is caused by the NTE behavior, and how much is due to chain stiffening. To address whether the use of expensive NTE materials is justified, this project is designed to investigate the exact effects of NTE and chain stiffening on the reduction of thermal expansion of polymer

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composites. This objective was achieved through the preparation and testing of two sets of composites containing isomorphic particles with opposite thermal expansion (ZrW2O8 and

ZrW2O7(OH)22H2O), which possess identical chain stiffening effects.

The first goal of the project was to synthesize two different particles that have identical morphology but opposite thermal expansion, with cubic ZrW2O8 as the NTE material of choice. The initial idea was to use α-Al2O3 (corundum), which has a known positive CTE value, as the second material. This phase can be obtained through heat treatment of AlOOH at about 1100 °C. The synthesis of AlOOH with controlled morphologies based on choice of synthetic conditions has been reported. Attempts on the synthesis of AlOOH were made through two different routes. Neither of them delivered particles with similar size as cubic ZrW2O8. Additionally, it was found that the heat treatment at high temperature caused sintering of the particles, resulting in the formation of large particles. To circumvent this problem, the precursor of ZrW2O8,

ZrW2O7(OH)22H2O, was used as the counterpart for the comparison, since the topotactic transformation between the two phases results in unchanged morphology, giving rod-like shape for both materials.

The synthesis of ZrW2O7(OH)22H2O was optimized to prepare particles with small size, high crystallinity, and good resistance to hydration after converting to the cubic

NTE phase. The effects of acid concentration and reaction time were explored. The products were examined by powder X-ray diffraction (PXRD) and scanning electron microscopy, and the hydration rates were also estimated based on the PXRD patterns.

Final reaction conditions were chosen as 6 M HCl at 230 °C for 7 d. The coefficient of thermal expansion was determined for ZrW2O7(OH)22H2O using Pawley refinements of

-6 -1 variable temperature PXRD data, and values of a = 11 × 10-6 ± 1 × 10 K and c = 2.6

× 10-6 ± 0.3 × 10-6 K-1, respectively, were found. Rietveld refinements were carried out on

PXRD patterns of both types of particles mixed with silicon to estimate their amorphous content. Results indicated that both particles were close to fully crystalline.

To improve the interaction between the particles and polymer, surface modification was carried out via in-situ polymerization in the presence of the particles using triphosgene and bisphenol A as monomers. Soxhlet extraction was used to purify the recovered particles. Thermogravimetric analysis was used to determine the surface coverage of the products and the presence of unbound polymer, and the required time for extraction was revealed to be 96 h based on the TGA results. Infrared spectroscopy was also used to examine the modified particles, which confirmed the presence of surface bound oligomers.

Optimization of synthetic conditions, including monomers ratio, reaction time and amount of particles, was carried out to obtain the highest possible coverage. It was found that the optimum ratio for the monomers is between 2.2 : 1 and 1.3 : 1. Leveling off was observed for the surface coverage after 21 h of reaction time. Smaller amounts of particles gave higher surface coverages, but resulted in very low quantities of recovered particles due to losses during recovery steps. To recover more particles from a single batch reaction, the particles were subjected to two consecutive modification steps, resulting in both high coverage and high recovered amounts. The precursor particles could be modified under the same optimum conditions found for NTE particles. The interaction between the particles and polymer was found to be improved after the modification.

Solution casting was used to prepare the composite films. A custom made glass vessel was created to provide an inert atmosphere with reduced pressure. This can lower the moisture level and increase the evaporation rate of the casting solvent, which can prevent moisture deposition and crystallization of the polymer. The interaction between the two phases was further enhanced through reprecipitation blending. Under optimized conditions, composite films loaded with bth types of particles were prepared with weight loadings ranging from 2 wt% to 25 wt%. Films with loadings above 12 wt% showed agglomeration on optical images. The homogeneity of the particle dispersion within the films was still acceptable based on combustion analysis.

Several properties of the composites were measured, including tensile properties, thermal stability, glass transition temperature and coefficient of thermal expansion. All films without agglomeration showed enhance thermal stability. On the other hand, most films with agglomeration exhibited slightly lower thermal stability. Similar trends were seen for the stress and strain at yield for both types of composites. The composites with lower thermal stability showed lower stress and strain at yield than pristine PC films, whereas the rest showed similar values for these two properties. The Young’s modulus of both types of composite films was found to slightly increase with the addition of the filler particles. All composites exhibited similar values as pristine PC. However, the local structure of the two types of the composites was revealed to be different by dynamic mechanical analysis. The films loaded with the precursor particles exhibited earlier softening than pristine PC, while a delay in softening was found for the ones loaded with

NTE particles.

The coefficient of thermal expansion (CTE) was measured for the film samples at the University of Mulhouse. This instrument produced faulty numbers that required corrections for instrument contributions. The correction for instrument contributions was checked by comparing the corrected values of three selected film samples to values obtained through analysis at West Kentucky University. The composites blended with

NTE particles showed consistently lower CTE values than pristine PC and decreased with increased particle loading, whereas the values of the other set of composites showed no clear trends. Overall, considering the errors associated with the CTE values, the difference caused by the NTE behavior of the particles may not be very significant. Additional samples with higher loadings need to be tested to obtain a clearer picture, and data should be collected on well calibrated instruments to reduce errors.

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I dedicate my dissertation work to my lovely parents, Yuehua Zhang and Yong Gao, who have supported me substantially and spiritually by all means ever since I was born.

Without them, I cannot achieve any of this. I also dedicate this dissertation to my wife,

Zhen Sun. She has accompanied me in every bit of my life since the beginning of the work, encouraging and looking after me. I cannot imagine life without her through all these years.

Acknowledgements

I would like to thank my advisor Dr. Cora Lind-Kovacs. She offered a flexible timetable for research, and held strict standard in the meantime. I learned how discretion and precision should be kept with science. She also provided many valuable suggestions to the project I have been doing, and taught me a lot of chemistry knowledge. The tutorial from her on presentations and writing through these years was even more helpful. I would also like to thank all the previous and current group members, whom I spent a lot of time with. It is impossible to focus on the work if there is not a nice and friendly environment around you. The next person I need to thank is Pannee M. Bruckel. She has offered me as much help as possible in terms of instrument reservation, training, maintenance of the instruments, troubleshooting, etc.. Other staff, including Steven D.

Moder, Anthony J, Kaminski, Youming Cao and Dr. Yong Wah Kim, also provided a lot of help through the course. It also needs to be acknowledged that National Science

Foundation grant DMR-0545517 supported the project, and CRIF-0840474 supported the purchase and maintenance of scanning electron microscope used in this project. Finally, I need to thank my family members, my parents, Yuehua Zhang and Yong Gao, and my wife, Zhen Sun. They stand by me all the time, so that I can make these years through when I am thousands of miles away from home. I appreciate what they did to me.

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Table of Contents

Abstract.……………………………………………………………...….…...………..…iii

Acknowledgements.………………………………………………………………...…..viii

Table of Contents.……………………………………………………...…….…..………ix

List of Tables…………………………………………………………………….…...... xiv

List of Figures………………………………………………………………………..…xvi

1 Introduction ...... 1

1.1 Negative Thermal expansion ...... 1

1.2 Zirconium tungstate (ZrW2O8) ...... 3

1.3 Polymers and thermal expansion control ...... 6

1.4 Polymer composites ...... 7

1.5 Polymer/filler interface in composites ...... 11

1.6 Mechanisms of CTE reduction in composites...... 15

1.7 Objectives of thesis project ...... 18

2 Characterization ...... 21

2.1 Powder X-ray diffraction ...... 21

2.2 Scanning electron microscopy ...... 25

2.3 Infrared spectroscopy ...... 26

2.4 Thermogravimetric/differential temperature analysis ...... 27

2.5 Differential scanning calorimetry...... 30

2.6 Rheometry ...... 31

2.6.1 Storage, loss modulus and dissipation factor ...... 31

2.6.2 Coefficient of thermal expansion ...... 33

2.7 Tensile testing ...... 33

2.8 Thermomechanical analysis ...... 35

3 Synthesis of filler particles...... 36

3.1 Introduction ...... 36

3.1.1 Selection of filler particles ...... 37

3.1.2 Desired properties of NTE particles ...... 38

3.1.3 Hydrothermal synthesis ...... 39

3.2 Experimental ...... 40

3.2.1 Materials and characterization methods ...... 40

3.2.2 Synthesis of ZrW2O7(OH)2·2H2O and ZrW2O8 ...... 40

3.2.3 Synthesis of boehmite ...... 41

3.3 Results and discussions ...... 42

3.3.1 Synthesis of ZrW2O7(OH)2·2H2O and ZrW2O8 ...... 42

3.3.2 Synthesis of boehmite (AlOOH) ...... 56

3.3.3 Determination of thermal expansion of ZrW2O7(OH)2·2H2O ...... 61

3.3.4 Determination of amorphous contents of ZrW2O7(OH)2·2H2O and ZrW2O8

...... 66

3.4 Conclusions ...... 68

4 Surface modification of filler particles ...... 71

4.1 Introduction ...... 71

4.2 Experimental ...... 74

4.2.1 Characterization ...... 74

4.2.2 Surface modification of ZrW2O8 and ZrW2O7(OH)2·2H2O ...... 74

4.3 Results and discussion ...... 75

4.3.1 Surface modification of filler particles ...... 75

4.3.2 Soxhlet extraction...... 76

4.3.3 Surface modification of ZrW2O8 ...... 77

4.3.4 Surface modification of ZrW2O7(OH)2·2H2O particles ...... 88

4.3.5 Challenges throughout the course of the project ...... 90

4.4 Conclusion ...... 93

5 Preparation of the composite films ...... 94

5.1 Introduction ...... 94

5.1.1 Crystallization of polycarbonate...... 94

5.1.2 Direct blending and particle flow during solution casting ...... 95

5.1.3 Reprecipitation blending ...... 96

5.2 Experimental methods and characterization ...... 97

5.2.1 Materials ...... 97

5.2.2 Direct blending ...... 99

5.2.3 Reprecipitation blending ...... 99

5.3 Results and discussion ...... 100

5.3.1 Direct blending ...... 100

5.3.2 Reprecipitation blending ...... 105

5.3.3 Composite films with two types of filler particles ...... 107

5.4 Conclusions ...... 111

6 Properties of NTE/PC and PTE/PC composite films ...... 113

6.1 Introduction ...... 113

6.1.1 Interaction at the interface between fillers and polymer ...... 113

6.1.2 Thermal properties ...... 114

6.1.3 Tensile properties ...... 115

6.1.4 Coefficient of thermal expansion ...... 115

6.1.5 Re-casting of composite films ...... 116

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6.2 Characterization ...... 116

6.2.1 Determination of crystallinity ...... 116

6.2.2 Homogeneity of composite films ...... 116

6.2.3 Tensile properties ...... 117

6.2.4 Interface morphology ...... 117

6.2.5 Thermal degradation ...... 118

6.2.6 Glass transition temperature ...... 118

6.2.7 Coefficient of thermal expansion ...... 118

6.3 Results and discussion ...... 118

6.3.1 Effect of surface modification on interface and dispersion ...... 118

6.3.2 Properties of composite films ...... 120

6.3.3 Thermal expansion coefficient of the composites ...... 135

6.4 Conclusions ...... 143

7 Summary and future work ...... 146

References ...... 149

Appendix A……………………………………………………………………………..159

Appendix B……………………………………………………………………………..162

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List of Tables

1.1 CTE values of polymer composites investigated in the previous research...... 9

2.1 Linear trend lines for TGA calibration measurements on empty balance arms...... 29

3.1 Samples of ZrW2O7(OH)22H2O prepared with different acid concentrations...... 45

3.2 Samples of ZrW2O7(OH)22H2O prepared for different times...... 47

3.3 Lattice constants of several ZrW2O8 samples over a period of 10 d...... 53

3.4 AlOOH samples prepared under different conditions using Route 1...... 59

3.5 AlOOH samples prepared under different conditions using Route 2...... 59

3.6 Morphology of AlOOH samples from SEM images...... 60

4.1 Synthetic conditions for surface modification of ZrW2O8 with variable ratios of monomers...... 82

4.2 Synthetic conditions for surface modification of ZrW2O8 particles with variable reaction time...... 84

4.3 Synthetic conditions for surface modification of ZrW2O8 particles with variable amount of filler particles...... 85

4.4 Synthetic conditions for surface modification of ZrW2O8 particles with two consecutive modification steps...... 87

4.5 Synthetic conditions for surface modification of ZrW2O7(OH)22H2O particles...... 89

5.1 Thickness of composite film samples (at 8 weight% loading)...... 105

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6.1 Amount of filler particles in composite films...... 120

6.2 Surface coverage of modified particles used for composite preparation...... 121

6.3 True weight loading of the composites obtained from homogeneity tests...... 124

6.4 Initial and final weight of composites...... 127

6.5 Tensile properties of composite films...... 129

6.6 Glass transition temperatures of PC and composite films...... 133

6.7 Raw CTE values for PC and composite films obtained in France...... 137

6.8 CTE values measured for an Al metal strip at different grip distances...... 138

6.9 CTE values of selected film samples tested at WKU...... 139

6.10 Linear fit results for the CTE values of the composites...... 141

List of Figures

1-1 Mechanism of NTE in corner-shared polyhedral networks...... 3

1-2 A unit cell of cubic ZrW2O8...... 4

1-4 Schematic illustration of polymer composites with pristine and surface modified filler particles...... 13

1-5 CTE of cyanate ester composites loaded with fumed silica of different sizes along with results from theoretical models...... 16

1-6 Schematic illustration of chain stiffening and CTE effects on overall composite expansion...... 19

2-1 TGA curve for empty balance arms...... 28

3-1 PXRD patterns of (a) ZrW2O7(OH)22H2O, and (b) ZrW2O8...... 43

3-2 SEM images of (a) ZrW2O7(OH)22H2O, and (b) ZrW2O8...... 43

3-3 SEM images of ZrW2O7(OH)22H2O prepared in (a, b) 7.2 M HCl (XG.31.raw), (c, d)

6 M HCl (XG.43.raw) and (e, f) 4.8 M HCl (XG.42.raw)...... 46

3-4 SEM images of ZrW2O7(OH)22H2O prepared in (a)-(d) 6 M HCl for (a) 1 d

(XG.50.raw), (b) 3 d (XG.43.raw), (c) 7 d (XG.41.raw) and (d) 14 d (XG.45.raw), and

(e)-(g) 4.8 M HCl for (e) 1 d (XG.37.raw), (f) 3 d (XG.78.raw), (g) 7 d (XG.72.raw) and

(h) 14 d (XG.75.raw)...... 49

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3-5 SEM images of heat treated and raw sample prepared in 4.8 M acid (a) XG.37.650a

(1 d), (b) XG.37.raw, (c) XG.78.645a (3 d), (d) XG.78.raw, (e) XG.72.645a (7 d), (f)

XG.72.raw, (g) XG.75.645a (14 d) and (h) XG.75.raw...... 51

3-6 verlaid patterns scanned after d - - d d and ) 10 d of exposing ZrW2O8 to air: (a) XG.43.650a, (b) XG.40.650a, (c) XG.41.650a, (d)

XG.45.650a, (e) XG.37.650a, (f) XG.72.645a and (g) XG.75.645a...... 55

3-7 SEM images of AlOOH samples synthesized using Route 1: (a) XGA.4.raw (pH 8), and (b) XGA.8.raw (pH 3.7)...... 56

3-8 SEM images of AlOOH samples synthesized using Route 2: (a, b) PMA.7.raw

(0.0011 M H2SO4), (c, d) PMA.13.raw (0.0011 M H2SO4), (e, f) PMA.5.raw (0.0023 M

H2SO4), (g) PMA.1.raw (0.0046 M H2SO4), and (h) -Al2O3 sample PMA.1.1200. .... 58

3-9 Overlaid PXRD patterns collected for ZrW2O7(OH)2H2O on a variable temperature stage at temperatures between 25 °C (trace 1) and 205 °C (trace 19)...... 62

3-10 Pawley refinement results for a mixture of Si and ZrW2O7(OH)22H2O at 25 °C.

Observed ( and calculated patterns a difference curve and tic mar s indicating the calculated peak positions of Si (bottom) and ZrW2O7(OH)22H2O (top) are displayed. .. 64

3-11 Unit cell length of (a) a-axis and (b) c-axis of ZrW2O7(OH)2H2O as a function of temperature under ambient pressure (×) and vacuum ()...... 64

3-12 Unit cell volume of ZrW2O7(OH)22H2O as a function of temperature under ambient pressure (×) and vacuum ()...... 65

3-13 bserved and calculated patterns and difference curve for a mixture of i and ZrW2O7(OH)22H2O at RT. Tick marks indicate the calculated peak positions of Si

(bottom) and ZrW2O7(OH)22H2O (top)...... 67

xvii

4-1 In-situ polymerization of polycarbonate in the presence of filler particles...... 72

4-2 Enhanced interaction between filler particles and polymer matrix through surface modification...... 73

4-3 TGA curves of typical modified ZrW2O8 sample A after 6 h ---) 48 h and

() 56 h of Soxhlet extraction...... 78

4-4 IR spectra of (a) pure polycarbonate, (b) plain ZrW2O8, and (c) plain

ZrW2O7(OH)22H2O...... 79

4-5 IR spectra of (a) modified ZrW2O8, and (b) modified ZrW2O7(OH)22H2O. Insets are zoomed-in spectra portions from 1800 cm-1 to 1450 cm-1...... 80

5-1 Conformation of polymer chains in cold and hot DMAc...... 97

5-2 Custom-made glass vessel with gas inlet (clear tubing) and water vacuum (black tubing)...... 98

5-3 Film samples prepared by direct blending under ambient atmosphere with (a) pristine polycarbonate, (b) polycarbonate and ZrW2O8 in an incompletely dried dish, and (c) polycarbonate and ZrW2O8 in a dried dish...... 102

5-4 Films prepared using custom made glassware: (a) pristine polycarbonate with crystalline part in the center caused by slow evaporation, and (b) pristine polycarbonate under high argon flow...... 104

5-5 Scanner images of composite film samples (a) to (e) NTE1 to NTE5, respectively, and (f) to (i) PTE1 to PTE5, respectively...... 109

5-6 Camera images of composite film samples (a) to (e) NTE1 to NTE5, respectively, and (f) to (i) PTE1 to PTE5, respectively...... 110

5-7 PXRD patterns of film samples, (a) clear pristine PC, and (b) NTE1...... 108

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6-1 SEM images of composites prepared by direct blending with (a) raw NTE particles,

(b) raw precursor particles, (c) modified NTE particles, and (d) modified precursor particles...... 119

6-2 PXRD patterns of (a) ZrW2O8, (b) vacuum grease, (c) clear PC, (d) discolored PC, and (e) to (i) NTE1 to NTE5...... 122

6-3 PXRD patterns of (a) ZrW2O7(OH)22H2O, (b) clear PC, and (c) to (g) PTE1 to

PTE5...... 122

6-4 TGA curves of (a) PTE composites, and (b) NTE composites...... 126

6-5 SEM images of the fracture surface of composite films (a) NTE3, (b) PTE3, (c) PTE5 and (d) NTE5...... 132

6-6 Overlaid tan  curves of (a) PTE composites, and (b) NTE composites...... 135

6-7 SEM images for filler particles used for PTE1 and NTE1, (a) precursor particles, and

(b) NTE particles...... 140

6-8 CTE values of pristine PC (), PTE () and NTE () composites...... 141

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Chapter 1

1 Introduction

1.1 Negative Thermal expansion

Thermal expansion is a property that describes the volume change of a material in response to a temperature change. The thermal expansion coefficient α is defined by equation-1.1, which allows quantitative determination of thermal expansion. Depending on the nature of materials α can adopt negative zero or positive values.

dV 1  V dT V (Equation-1.1)

Negative thermal expansion (NTE) is a special property that only a few materials possess. NTE materials show a reduction of some of their crystallographic axes or their unit cell volume with increasing temperature.1-6 Five mechanisms have been identified that can result in NTE behavior: (1) In some AMO3 oxides with perovskite structure, such as BaTiO3, close to a phase transition, some of the metal-oxygen bonds in MO6 and

AO12 polyhedra become shorter with increasing temperature as the polyhedra become more symmetrical.7 This behavior shows up close to the tetragonal to cubic phase transition. In this narrow temperature range, the a and b axes have a positive value of α and the c axis contracts resulting in a negative value of α for volume expansion. The

shortening of some bond distances upon heating is due to the fact that the phase transition gives rise to more regular polyhedra. (2) In hexagonal cordierite (Mg2Al2Si5O18 β-

8 9 eucryptite (LiAlSO4) and NZP (NaZr2P3O12), positive expansion occurs along the a and b axes, whereas the c axis is shortened due to the expansion along the other two axes and the 3D connectivity of the framework.10 This mechanism alone does not give rise to an overall unit cell volume contraction. The volume contraction of β-eucryptite and NZP is caused by cation movement in addition to the framework connectivity. They move from tetragonal sites or octahedral sites to interstitial sites with increasing temperature,

5, 10 resulting in unit cell contraction. (3) In the ZrW2O8 family and other corner-sharing

11 2, 12 13 14-16 frameworks (e.g., AM2O7 family, A2M3O12 family, zeolites, AlPOs, the transverse motion of oxygens in linear metal-oxygen-metal linkages pulls the two metal atoms closer together, causing volume contraction (Figure 1-1).2 (4) In some magnetic metal alloys, such as Fe-Pt and Fe-Ni, a negative contribution to thermal expansion due to changes in magnetic moments exists below their magnetic ordering temperature, resulting in close to zero overall thermal expansion. (5) Some cyanide compounds were

17 discovered to show NTE behavior as well, which was first observed in Zn(CN)2 and

18 Fe[Co(CN)6] . These compounds adopt a cubic structure similar to ZrW2O8 that consists of corner-connected polyhedra. The difference is that diatomic cyanide bridges replace the corner-sharing oxygen atoms in the compounds described in (3). The NTE is caused by transverse motions of the M-CN-M bonds, where the cyanide bridges bring additional flexibility to the framework.19 Therefore, this type of compounds usually exhibits

-6 -1 20 significantly negative α values. For example the αl value of Cd(CN)2 is -33  10 K ,

-6 -1 21 and that of Mn3[Co(CN)6]2 is -48  10 K . Another similar type of compound is

Ag3[Co(CN)6], which crystallizes in a trigonal cell, and is composed of alternating layers

+ 22 of Ag and [Co(CN)6]. The expansion of this compound is highly anisotropic, with αa

-6 -1 -6 -1 between 130  10 K and 150  10 K whereas αc is negative with values between -

130  10-6 K-1 and -120  10-6 K-1.23 The favorable Ag+ - Ag+ interaction (argentophilic interaction) upon heating is responsible for this thermal expansion behavior.22, 24

Figure 1-1. Mechanism of NTE in corner-shared polyhedral networks.

1.2 Zirconium tungstate (ZrW2O8)

3-6, 10, 25-31 Cubic ZrW2O8 is one of the most investigated NTE materials. This compound exhibits isotropic NTE over a wide temperature range with an αl value of -

9.1×10-6 K-1 from 0.3 K to 430 K and -4.9×10-6 K-1 from 430 K to 1050 K.6, 30 This is the widest temperature range for NTE behavior known up to date, and the isotropic αl value of -9.1 ×10-6 K-1 is one of the largest value among NTE materials with the exception of the less thermally stable cyanide compounds. Two phases α- and β-ZrW2O8, exist in this temperature range. A phase transition between them occurs at 430 K.6 The main structural features are retained, and the phases only differ by an orientational disordering of the WO4 tetrahedra. A phase transition from cubic to orthorhombic symmetry occurs upon compression to 0.2 GPa.25, 32 The orthorhombic phase is metastable after

decompression. Heating this phase to 390 K results in conversion to the cubic phase again. At higher pressures between 1.5 to 3.5 GPa, the cubic phase becomes amorphous under non-hydrostatic conditions, and heating to 923 K is necessary to recrystallize the cubic phase.33 This transition occurs at even lower pressure when the material is pressurized non-hydrostatically.33

Figure 1-2. A unit cell of cubic ZrW2O8.

ZrW2O8 has a cubic structure with ZrO6 octahedra and WO4 tetrahedra linked in a corner-sharing network (Figure 1-2). The metal atoms (Zr, W) are located at the centers of the polyhedra, and oxygen atoms on the corners of each polyhedron. Each WO4 tetrahedron is connected to three ZrO6 octahedra, leaving one oxygen as a terminal atom.

Two features that are responsible for NTE are (1) stiff metal-oxygen-metal bonds that form angles close to 180°, and (2) a framework with a large amount of open space.

An increase in temperature results in increased thermal motion of atoms in all materials. However, in NTE materials, the nearly linear M-O-M linkage and strong bonding make longitudinal vibrations of oxygen atoms unfavorable. Instead, the oxygens undergo a transverse vibration.2 This transverse motion of the oxygen pulls the two metal atoms together. Due to such motions, the polyhedra connected through M-O-M linkages undergo a rotation, which results in long range shrinkage of the entire framework.2

Previous research in the group observed the incorporation of atmospheric

28 moisture in ZrW2O8. The hydration causes increased coordination of the tungsten atoms, and a reorientation of the polyhedra, resulting in a smaller unit cell than unhydrated ZrW2O8. In addition, the M-O-M linkages are no longer linear so that the transverse motion does not shorten the metal-metal distance. The hydrated ZrW2O8 thus loses the NTE behavior, and shows weak positive thermal expansion (PTE) instead.

Hydrated ZrW2O8·xH2 was first prepared by leight’s group via a hydrothermal method at temperatures above 170 °C.27 Complete hydration gives a composition of

ZrW2O8·H2O, which has a lattice constant of 8.84 Å compared to 9.14 Å for ZrW2O8.

Previous work in our group showed that nano-sized ZrW2O8 hydrates much more readily than micron-sized samples, and readily takes up water under ambient conditions.28 For the smallest particles, the rate is fast enough to impede any proposed application that tries to take advantage of the NTE property.28 It was also found by our group that the hydration rate is influenced by synthetic conditions, and that it can be slowed down significantly through synthesis at higher temperatures.28 It appears that only

a thin surface layer may hydrate in these particles, resulting in materials that still show

NTE.

1.3 Polymers and thermal expansion control

A polymer is a macromolecule that consists of a large number of repeating units that are connected via covalent bonding. Some polymers exhibit excellent mechanical and electronic properties.34 They also have some advantages over metals and ceramics, such as ease of processing, resistance to corrosion and low density.35 However, polymers display high thermal expansion coefficients. Temperature fluctuations can result in distortion or failure of materials with large α values. In devices made from multiple materials, failure can be caused by stress at interfaces generated from different expansion of the materials.36, 37 Therefore, control of thermal expansion is important in these cases.

The high thermal expansion of polymers is caused by extensive thermal motion of polymer chains upon heating. To reduce the chain motion and thus the expansion of polymers, two approaches are commonly used: (1) Enhancing the rigidity of the polymer structure by incorporating monomers that stiffen the chain,38 and (2) incorporation of fillers with low α to ma e polymer composites. The reduction of the CTE of the polymer by the second method originates from the restriction of polymer chain motion through the incorporation of rigid filler particles.

While thermal expansion can be reduced with either approach, the first method changes the original polymer structure, which often results in changes of bulk properties as well. For instance, flexibility and processibility could be reduced. In addition, the synthesis of custom monomers is usually time consuming and expensive. Therefore,

incorporation of fillers is a better way to control thermal expansion while maintaining the bulk properties.

1.4 Polymer composites

Control of thermal expansion of polymers by formation of polymer composites has attracted a lot of attention. Various polymers have been chosen as target systems, such as phenolic resins,39 epoxies,40-42 polyesters,43 cyanate esters,44-47 and polyimides.48-

50 Fillers that have been researched include silica,40, 51 alumina,40 alumina coated silicon nitride,40 nanoclay,52, 53 and zirconium tungstate.39, 41, 42, 44-46 The results show that addition of fillers can reduce the thermal expansion of the polymers.

Kessler’s group carried out research on fumed silica/bisphenol E cyanate ester

(BECy) composites.47 Commercial fumed silicas (Aerosil 200 and Aerosil OX 50) were used, which had average primary particle sizes of 12 nm and 40 nm, respectively. The authors reported that these fumed silica particles formed aggregates of 200 nm to 300 nm in diameter, but no SEM images were presented. According to technical specifications of the silicas, the surface areas of the two types of particles were 200 m2/g and 50 m2/g, respectively. The cyanate ester was blended with 12 nm silica particles at loadings ranging from about 0.3 vol% to 3.4 vol%, and with 40 nm silica particles at loadings of about 0.3 vol% to 20.7 vol%. A reduction in CTE of about 27% (by 17  10-6 K-1) and

6% (by 4  10-6 K-1) was observed for composites with 20.7 vol% of 40 nm silica and 3.4 vol% of 12 nm silica, respectively. The same research group later presented work on

BECy/ZrW2O8 composites. The cyanate ester was mixed with nano-sized ZrW2O8 at volume loadings ranging from 1% to 10%. A CTE reduction of about 18% (by 10  10-6

K-1) was observed at 10 vol% loading. A reduction of 14% (by 9  10-6 K-1) was achieved in a 40 nm fumed silica composite at the same volume loading.

Wong et al. carried out investigations on epoxy composites using three different types of particles, silica-coated aluminum nitride, alumina and silica.40 All particles were blended with the same epoxy matrix at loadings of 10 vol% to 50 vol%. At the highest particle loading, the silica composite showed a 67% reduction in the CTE of the epoxy

(by 60  10-6 K-1); the reduction for the silica coated aluminum nitride and silica composites was about 59% (by 52  10-6 K-1) and 57% (by 50  10-6 K-1), respectively.

Huang et al. presented work on polyimide composites containing polyhedral oligomer silsesquioxane (POSS).50 The POSS cage was modified to form octa(aminophenyl)silsesquioxane, followed by blending with a solution of polyamic acid.

The polyimide composites were then formed via solution casting and heat treatments. A

38% reduction (by 25  10-6 K-1) in the CTE of the polyimide was observed for a composite with 10 wt% loading of POSS. Leveling off on the CTE was shown at 18 wt%

(~30 vol% if particles are represented by the density of SiO2) loading, which showed a reduction of 42% (by 28  10-6 K-1).

Table 1.1 CTE values of polymer composites investigated in the previous research.

CTE of CTE of CTE of the Particle Authors Polymer polymer Particle particle composites loading (10-6 K-1) (10-6 K-1) (10-6 K-1) 12 nm fumed 20.7 vol% 47 silica BECy 63.5 0.5 Goertzen 40 nm fumed 3.4 vol% 60 et al.47 silica nano-sized BECy 56.5 10 vol% 47 ZrW2O8 alumina 6.6 50 vol% 38 silica 0.5 50 vol% 28 Wong epoxy 88 et al.40 silicon coated 4.4 50 vol% 36 aluminum nitride Huang 10 wt% 41 50 polyimide 65.8 POSS et al. 18 wt% 38

polyimide 6 wt% 62 73 (soft) Magaraphan 11 wt% 60 nano-clay 0.05 et al.52 polyimide 2.5 wt% 34 40 (hard) 9 wt% 34 Sullivan micron-sized 49 polyimide 33 22 vol% 23 et al. ZrW2O8 Tani micron-sized 39 phenolic 46 52 vol% 30 et al. ZrW2O8 Miller micron-sized 41 epoxy 200 40 vol% 90 et al. ZrW2O8 Shi epoxy 94 micron-sized 56 43 30 vol% et al. polyester 54 ZrW2O8 16 Chu micron-sized 42 epoxy 45.5 40 vol% 23 et al. ZrW2O8 Sharma nano-sized 48 polyimide 78 15 vol% 62 et al. ZrW2O8

Magaraphan et al. has researched polyimide/nanoclay (montmorillonite) composites.52 Two types of polyimides, one with flexible and one with a rigid backbone, were used. The clay was modified by reacting the sodium montmorillonite with dodecylamine, followed by dispersing in a solution of polyamic acid. The polyimide composite films were then obtained through solution casting and heat treatment. The flexible polyimide loaded with 6 wt% (~5 vol%, calculated based on a density of 2 g/cm3) showed a 15% reduction in CTE (by 11  10-6 K-1) compared to the pristine polyimide. At 11 wt% (8.5 vol%) loading, only a slightly larger reduction (by 13  10-6

K-1) was observed. On the other hand, the rigid polyimide composites with 3 wt%, 6 wt% and 9 wt% clay all showed a reduction of about 15% in their CTEs (by 6  10-6 K-1).

Sullivan et al. prepared ZrW2O8/polyimide films containing 0.8 vol% to 22 vol%

49 of filler particles. Micron- and nano-sized ZrW2O8 was synthesized via hydrothermal and sol-gel routes, respectively. The surfaces of the particles were modified with 3- aminopropyltriethoxylsilane (APTES), followed by dispersing in BTDA ’ 4 4’ - benzophenometetracarboxylic dianhydride) - ODA 4 4’ - oxydianiline) polyamic acid resins. The mixture was then thermally cured to form polyimide composite films. The composite with 22 vol% of micron-sized ZrW2O8 showed a 30% reduction in CTE (by 10

-6 -1  10 K ); the composite with 1.7 vol% loading of nano-sized ZrW2O8 exhibited a 10% reduction in CTE (by 3  10-6 K-1).

Tani et al. prepared phenolic resin/ZrW2O8 composites with unmodified filler particles that had 7 vol% to 52 vol% particle loading using a hot press, which exhibited a reduction of 7% to 70% (by 3  10-6 to 30  10-6 K-1) in CTE compared to a pristine resin, respectively.39

Shi et al. presented the work on polyester and epoxy composites containing

-6 -1 unmodified ZrW2O8, which exhibited a reduction of about 40% (by 38  10 K ) and

70% by (38  10-6 K-1) in composite CTE with 30 vol% particles loading, respectively.43

Miller et al. prepared ZrW2O8/epoxy composites through thermal curing in a

41 module. Unmodified, silane modified and carboxylic acid modified ZrW2O8 particles were used. The largest CTE reduction of the epoxy (55%, reduced by 110  10-6 K-1) was achieved for mixing with 40 vol% of ZrW2O8 that was modified with benzeotriazole-5- carboxylic acid.

Chu et al. also obtained acrylic acid coated ZrW2O8/epoxy composites through thermal curing.42 Composites with particle loadings of 5 vol% to 40 vol% were prepared.

A 55% reduction in CTE (by 23  10-6 K-1) was observed for a sample with 40 vol% of particles.

A previous group member (Gayathri Sharma) investigated polyimide composites

48 containing zirconium tungstate. Both micron- and nano-sized ZrW2O8 particles were incorporated into polyimide, and the thermal expansion behavior and other mechanical properties were determined. The moisture uptake of the composite was also monitored. A

20% reduction (by 16  10-6 K-1) in the CTE of the polyimide was achieved with 15 vol% of APTES-polyimide modified ZrW2O8.

1.5 Polymer/filler interface in composites

Given that polymer composites consist of two phases, polymer and filler particles, the interface between the two phases becomes extremely important, since this is the only area where interaction between the particles and polymer matrix occurs. The extent of

interaction at the interface is crucial to the dispersion of filler particles throughout the polymer matrix and the resulting property transfer between the two phases. The factors that affect the interaction at the interface include inherent surface properties of polymer and particles, and surface to volume ratio of filler particles.

Polymers are organic macromolecules, while filler particles are usually inorganic materials. These materials contain functional groups that may or may not favorably interact with each other. For instance, metal oxides that have hydroxyl group terminated surfaces are expected to have more favorable interactions with epoxy resins than with aromatic polyimides. The surface to volume ratio of the filler particles is another very important factor, which determines the surface area of the particles that is in contact with the polymer matrix. Particles with larger surface-to-volume ratios possess more surface area to interact with the matrix. This means that the same extent of interaction can be achieved with a smaller amount of particles, and that a higher extent of interaction can be achieved with the same amount of materials.

One way to improve the interaction at the interface between these two types of materials is to reduce the size of the filler particles, which increases the corresponding surface-to-volume ratio. Another way is to alter the particle surface by treating the particles with organic compounds to form a chemically or physically bound organic layer, which is called surface functionalization or surface modification (Figure 1-3).

olymer matrix urface oligomer article

Figure 1-3. Schematic illustration of polymer composites with pristine and surface

modified filler particles.

The bonding of a layer of organic molecules on the particle surface enables more favorable interaction between the particles and polymer, since the structure of organic molecules is more similar to that of the polymer. This approach has been successfully used for NTE oxides incorporated into polymer matrices. Sullivan et al. modified

ZrW2O8 by reacting 3-aminopropyltriethoxysilane (APTES) with the hydroxyl groups on the surface of the particles.49 The modified particles were shown to interact better with the polyimide matrix in the SEM images of the composite sample. Similarly, previous group member Gayathri Sharma modified ZrW2O8 with APTES as well. The interface interaction was enhanced as seen in SEM images of the composite films. The CTE of the composites prepared with the APTES modified particles was lower than the CTE of

48 composites with unmodified particles. Miller et al. presented a study of ZrW2O8/epoxy resins, in which ZrW2O8 was modified with both APTES and benzotriazole-5-carboxylic

acid.41 Both types of the composites showed enhanced interactions at the interface of the two phases.

In addition to grafting small organic moieties on particle surfaces, oligomers that have a similar or identical structure to the polymer matrix can also be grown on the particle surfaces. Longer chains can extend from the particle surfaces, and thus provide better contact with the matrix. Chu et al. modified the particle surfaces of ZrW2O8 used in epoxy composites with oligomers of acrylic acid using plasma enhanced chemical vapor deposition.42 No comparison was made between composites with modified and unmodified particles.

On the other hand, Gayathri Sharma successfully grafted covalently bonded

48 polyimide oligomers onto the surface of APTES-modified ZrW2O8. The APTES served as a linker, since neither of the monomers can directly react with the hydroxyl groups on the particle surfaces, while APTES can react with the hydroxyl groups as well as the monomers. The surface modification by chemical bonding is expected to result in better property transfer between the two phases due to the much stronger binding force. At the highest volume loading (15%), the composites with APTES-polyimide modified particles showed a more significant reduction in CTE than composites with unmodified and

APTES-modified particles.

Similar approaches were used by the Coleman group for filler particles and polymers that have functional groups in common. Oligomers of polycarbonate were directly grown on the surface of alumina through in-situ polymerization of polycarbonate in the presence of the hydroxyl terminated particles.54 Bisphenol A and triphosgene were the monomers used in the polymerization, which involves the condensation of the

hydroxy groups of bisphenol A with triphosgene. As the particles are hydroxyl terminated, they can also react with triphosgene, which then continues to reacts with bisphenol A to grow oligomers on the particle surface. The interface between the two phases showed good adhesion in SEM images of composite films.

1.6 Mechanisms of CTE reduction in composites

While a number of papers report the reduction of CTE values for composites using a variety of fillers, a thorough understanding of the mechanisms of CTE reduction is lacking. It is also impossible to compare the results obtained from different projects in detail, since they were all based on different polymers. Although some of them used the same type of polymers, such as epoxy resins, the monomers were different. However, some insights can still be gained from these investigations.

Figure 1-4. CTE of cyanate ester composites loaded with fumed silica of different sizes along with results from theoretical models.

As discussed in section 1.4, the surface-to-volume ratio of filler particles affects the property transfer at the interface between particles and polymers. This factor can be quantified by surface area (m2/g), as shown in Goertzen’s wor on two fumed silicas that

possessed different surface area and resulted in different expansion of the cyanate ester composites. The CTE of composites containing 12 nm particles was only slightly lower

(less than 0.1  10-6 K-1) than for those prepared with 40 nm particles at low volume loadings of 0.2% and 0.5% (total reduction of about 0.5  10-6 K-1 in both cases).

However, at volume loadings of 1% and 2.5%, the differences became more significant, which are 0.1  10-6 K-1 and 1  10-6 K-1, respectively. Experimental data for 12 nm filler particles were only available up to 3.5% loading due to processing difficulties at higher loadings. However, these results suggest that particle size and surface area of the fumed silica affected the CTE of the composites.

Another important factor that could affect the reduction of CTE of polymers is particle shape. The morphology of particles changes how the particles disperse in polymers and how they affect the movement of polymer chains. This is evident through comparison of composites with filler particles that possess identical or similar CTE values, but result in different reduction of composite expansion. Similarly, work on nanoclay/polyimide composites with soft an rigid polyamide matrices showed a high reduction in CTE (15% in both cases) at low particle loadings of 6 wt% (4.6 vol%) and

2.5 wt% (1.9 vol%), respectively, with lower amounts of filler particles exhibiting the same magnitude of reduction for the softer polyimide.52 Another example is found in the comparison of several polyimide composites. POSS/polyimide composites showed a 38% reduction in CTE at 10 wt% loading (16 vol%).50 Each unit of POSS only has a radius of

1.3 nm after modification and is approximately spherical. Although leveling off of the

CTE was observed at higher loadings, the initial reduction in CTE was dramatic

48 compared to what Sharma achieved in ZrW2O8/polyimide composites. Due to its NTE,

ZrW2O8 would be expected to exhibit a more pronounced effect, however, at low loadings, POSS showed superior performance. This could be related to the smaller particle size and better dispersion of the POSS, or the distinct particle shapes. The

ZrW2O8 particles were rod-shaped, while the POSS cages were spherical. This shows that the morphology of particles has to be considered to achieve meaningful comparisons between different composites.

No further comparative insights can be gained from the remaining papers discussed in section 1.4, as multiple variables, including polymer structure, surface area of filler particles, particle morphologies and processing routes, were changed between papers.

1.7 Objectives of thesis project

NTE materials have been promoted as superior fillers, as they can exhibit a contractive force on the polymer matrix due to their NTE behavior, in addition to the stiffening effect from the interaction between the particles and polymers. Therefore, it was expected that the use of ZrW2O8 in polymer composites can achieve a more significant reduction in the CTE of polymers compared to filler particles that exhibit positive expansion. However, no research has been carried out to show how much of the

CTE reduction of polymers arises from the NTE behavior.

different effect with respect to chain stiffening

same effect with respect to chain stiffening

different effect of particle CTE

Figure 1-5. Schematic illustration of chain stiffening and CTE effects on overall composite expansion.

The goal of this project is to prepare composites that contain NTE and positive thermal expansion (PTE) particles of identical size and shape. As shown in Figure 1-5, composites will be subject to identical surface modification and polymer matrix formation approaches, resulting in composites for which the thermal expansion of the filler particles is the only variable. This will allow determination of the relative contributions of NTE and chain stiffening exhibited by the presence of any filler particles to the reduction in the CTE of the polymer composite. The only preliminary comparison is given by research carried out by Kessler’s group on BECy composites using fumed silica and ZrW2O8. At a volume loading of 10%, NTE composites showed an additional

4% CTE reduction compared to silica composites. However, the two types of particles possess very different morphologies, which could also result in differences.

Therefore, this project is designed to distinguish the effect of NTE from chain stiffening, and to determine whether the use of NTE materials for the reduction of the

CTE of polymers is justified. Particles with exactly the same size, shape and surface area are required in this comparison to eliminate any differences in chain stiffening effects.

Polycarbonate was chosen as the matrix material, which has not been studied for the purpose of control of CTE. It has high impact resistance, flame resistance, and transparency, which makes it a good matrix for this research.55 A comparison between two different types of composites is necessary to address the two effects, where one contains ZrW2O8, and the other is prepared with a positive thermal expansion compound.

Chapter 2

2 Characterization

2.1 Powder X-ray diffraction

X-ray diffraction is the most powerful technique to study crystalline materials, including single crystalline and polycrystalline materials. Powder X-ray diffraction

(PXRD) is used to study polycrystalline materials. The X-ray beam is scattered by electrons in the atoms that constitute matter. When the beam is scattered by an array of atoms that are arranged in a periodic repeat pattern in three dimensional space, diffraction occurs, and diffraction patterns can be collected by appropriate detectors.

X-ray diffraction from crystalline materials follows Bragg’s law where λ is the wavelength of the incident X-ray beam, d is the spacing between two lattice planes that can be described by Miller indices, and  is the incident angle between the X-ray beam and the lattice plane. Miller indices can be used to describe arrays of imaginary planes within the unit cell of materials, a concept developed to visualize X-ray diffraction.

Miller indeces use three integers, h, k, and l, to represent an array of parallel planes. The indices correspond to the reciprocal of the intercepts of the plane closest to the unit cell origin with the edges of the unit cell, a, b and c, respectively.

  (Equation 2-1)

The instrument used to conduct this experiment is called a diffractometer, which consists of an X-ray tube, sample stage, detector and some optics in between. The X-ray beam is generated in a tube that contains a target composed of a specific metal, which is bombarded with high energy electrons (usually 45 kV) from a tungsten filament. The identity of the metal determines the characteristic wavelengths of the X-rays produced through removal of core electrons. The most common target used for PXRD experiments is copper. The X-ray beam contains several different wavelengths, which are designated as Cu Kα1 Cu Kα2, Cu Kβ, etc. The X-ray tube and the detector are mounted on the diffractometer circle, and the sample is mounted in the center of the circle. Common detectors used in diffractometers are gas ionization point detectors, scintillation counters, and semi-conductor point or line detectors. The most common instrument setup uses

Brag-Brentano parafocusing geometry and flat plate sample holders. Samples of interest are packed on a sample holder with a leveled, smooth surface. The sample holders are usually made from aluminum, glass or miscut silicon single crystal wafers used as zero background sample holders. Optics such as slits, monochromator and filters can be attached on either the tube or detector side. During data collection, the tube and detector move at the same speed along the perimeter of the diffractometer circle.

In this project, the major purpose of PXRD experiments is to identify the crystalline phases present in samples. The collected patterns are compared with patterns in the Powder Diffraction File (PDF) database to confirm the identity of the phase. The crystallite sizes of the particles can also be estimated using the Scherer equation, which

correlates the full width at half max (FWHM) of diffraction peaks to the crystallite sizes according to Equation 2-2.

(Equation 2-2)

where  is the crystallite size, K a constant related to the experimental setup,  the

X-ray wavelength,  the FWHM in radians, and  the diffraction angle of the peak.

PXRD patterns can also be analyzed by Rietveld refinement, which is a whole pattern fitting method implemented in software programs like GSAS56, 57, GSAS-II or

TOPAS. A theoretical pattern is calculated based on input variables like crystal structure and instrument parameters. The difference between experimental and calculated patterns is minimized by refining variables like peak shape, background, zero offset, sample height, phase fractions, lattice parameters and atomic positions. Detailed structural information can be extracted based on the best fit.

When an internal standard that is fully crystalline is mixed with a sample in equal amounts, the amorphous content of the sample can be estimated through Rietveld refinement. The refinement returns phase fractions of all crystalline materials. For a fully crystalline sample, the extracted phase fractions for sample and standard should be equal.

Any deviation from equal phase fractions is attributed to the existence of an amorphous content in the sample. This technique was used to determine the amorphous contents of the two types of filler particles, ZrW2O8 and ZrW2O7(OH)22H2O.

Rietveld refinement was also used in this project to determine the coefficient of thermal expansion of ZrW2O7(OH)22H2O, which was previously unknown. This can be achieved because the lattice constants of the materials are related to the diffraction angles of the peaks. A mathematical expression for the relationship between lattice constants

and d-spacing for a cubic system is given in Equation 2-3, where h, k and l are Miller indices, a is the length of the edge of the unit cell, and d is the d-spacing of the peak.

Variable temperature PXRD data can be collected on specialized stages that are equipped with a heater and a thermocouple.

(Equation 2-3) √

The final use of PXRD in this project was to compare the crystallinity of composite films to clear PC films. This was achieved by extracting the FWHM of the strongest scattering feature in the corresponding patterns, which is discussed in more detail in Chapter 6.

In this research, a PANalytical ’pert ro diffractometer was used for all analysis. The powder samples were ground thoroughly, and uniformly packed on aluminum sample holders. Patterns were collected in Bragg-Brentano geometry using Cu

Kα radiation with an accelerating voltage of 45 kV and a current of 40 mA. Film samples were scanned in the same way after being cut and flatly stuck onto the back of an aluminum holder with vacuum grease and/or clamps.

To determine the amorphous content of ZrW2O8 and ZrW2O7(OH)2·2H2O, the samples were mixed with an equal weight of silicon powder, followed by uniformly packing on an aluminum sample holder. Phase fractions were extracted by Rietveld analysis using Topas Academic.

To determine the thermal expansion coefficient of ZrW2O7(OH)2·2H2O, it was mixed with an equal weight of silicon powder in methanol. The mixture was uniformly packed on top of a flat nickel-alloy sample holder of an Anton Paar variable temperature stage, with a little vacuum grease applied for adhesion of the sample. Scans were

collected in 10 °C intervals at temperatures ranging from 25 °C to 205 °C. Lattice constants were extracted by Rietveld refinement.

2.2 Scanning electron microscopy

Scanning electron microscopy (SEM) is the most common technique to exam the morphology of small particles. The sample is mounted on an appropriate holder, placed into a chamber under vacuum, and bombarded by an electron beam with an energy of 1 kV to 30 kV. This bombardment can remove secondary electrons from atoms in the sample, which are then collected by a designated detector to generate an image. The morphology of materials can be examined at magnifications of up to 100,000. Analysis of insulating samples at high magnifications can be difficult, as the electron bombardment can result in charge accumulation on the particle surfaces. This charging effect is reduced for more conductive materials. To obtain images with good quality, the energy of the beam needs to be low (usually 1 kV to 4 kV). The samples can also be coated with conductive materials to enhance the quality of images. Coating materials commonly used are gold and carbon. Additionally, charge compensation mode can also be used to reduce the charge. A bias voltage (up to 2 kV) can be applied on the specimen holder, which can help remove the electrons accumulated on the sample.

The materials can also be imaged using transmitted electrons. Unlike images generated by secondary electron, transmission mode delivers two dimensional images with higher resolution. Nanometer sized features can be imaged in scanning transmission mode.

In this project, the morphologies of filler particles were examined under charge compensation mode without coating. This allows observation of the original morphology.

The SEM was also used to examine the composite films. Films were coated with gold in a sputtering coater, and examined using charge compensation mode. Imaging of uncoated samples was attempted, however, the charge build-up on the insulating polymer could not be eliminated in charge-compensation mode.

A JEOL 7500F field emission scanning electron microscope was used for all imaging purpose for particles and composite films. The particles were examined with no coating using a secondary electron detector with charge compensation mode, whereas the films were coated with gold, and examined using the same detector.

2.3 Infrared spectroscopy

Infrared (IR) spectroscopy is a technique for structural analysis based on vibrations of chemical bonds within materials. Any vibration of covalent bonds that causes changes in dipole moments can be observed by IR. The IR light is absorbed at specific frequencies depending on the functional groups the materials possess. These frequencies are characteristic for different functional groups, especially for organic groups. Based on absorption bands present in the spectra, the presence of the respective functional groups can thus be determined.

IR was used in this project to examine filler particles, polycarbonate and surface modified particles. The results can be used to prove the success of the surface modification of the filler particles.

A Perkin Elmer Instruments Spectrum GX FT-IR spectroscopy was utilized in this project. The modified particles were mixed with potassium bromide in a weight ratio of 1 : 100. The mixture was then ground and pressed into a pellet. A background spectrum was collected on an empty sample holder. The spectrum of the sample was then scanned, and the background was automatically subtracted.

2.4 Thermogravimetric/differential temperature analysis

Thermogravimetric analysis (TGA) is a technique that reveals weight changes of materials upon heating. It enables the quantification of the change in chemical composition of materials at elevated temperature due to dehydration, decomposition and oxidation. Samples are placed in a small pan made from materials that do not react with the samples, such as platinum, alumina or aluminum. This pan and an identical empty pan are then placed onto a dual-beam balance located in a sealable furnace. Experiments can be carried out under different gas atmospheres, and the weight difference between the sample and empty pans is recorded continuously during heating.

TGA was utilized in this project to determine the actual amount of oligomer bound on the particle surface during surface modification. The weight loss from the samples was very small, which made data correction due to buoyancy effects necessary.

Buoyancy effects refer to the change in a sample’s apparent weight due to the change in density of the medium surrounding it, as the sample pan displaces the same volume of gas. However, the density of the gas changes as the temperature is increased.

When the instrument was run with no pan on the balance, a small weight loss was still observed upon heating. This weight loss is subtle when the weight loss from the sample is

large, but it became significant enough to interfere with quantification when the weight loss from the sample was only a few percent. Fortunately, this buoyancy-related weight loss depends linearly on temperature below 700 C, which is the highest temperature used in this project. Therefore, the measured weight losses were corrected by subtracting the weight loss observed without sample. The TGA curve of the empty balance arms is shown in Figure 2-1.

-0.02

-0.04

Weight (mg) -0.06

-0.08

-0.1 0 100 200 300 400 500 600 700 Temperature (°C) Figure 2-1. TGA curve for empty balance arms.

The baseline was measured three times, and each curve was fitted with a linear trend line. The average of the trend lines was used to correct all experimental data.

Table 2.1 Linear trend lines for TGA calibration measurements on empty balance arms.

Baseline slope intercept 1 -1.27  10-4 9.94  10-3 2 -1.28  10-4 -4.91  10-3 3 -1.25  10-4 -3.94  10-3

As shown in Table 2.1, the slope of the trend line was very consistent from run to run, whereas the intercept changed slightly each time. This intercept was supposed to be zero, as the balance was tared before each run. However, the weight signal from the balance was not completely stable after taring, which eventually caused slight shifts of the baseline. As the same subtle shifts could also occur during experimental runs, the intercept of the trend line was neglected during data correction.

It should be noted that the baseline of this instrument might exhibit drift over time, thus it should be checked regularly. Any maintenance work on the instrument might also cause a change of the baseline, such as adjusting or replacing the beams, and calibration of the balance and thermocouples.

TGA was also used to study the composite films prepared in this project to analyze thermal degradation and homogeneity of particle dispersion. Since all polymer portions of the composites decomposed during the test, the weight loss of the samples was very large, which only left a few weight percent residue. This revealed some serious problems with the TGA instrument. The weight measured by the balance of the instrument was not stable, and changed whenever the beams were tapped, as well as during heating-cooling cycle of the furnace. The offset was observed to be as large as 0.2

mg. Therefore, a microbalance was used to precisely measure sample weight before and after TGA runs.

In this project, a TA Instruments SDT 2960 Simultaneous TGA-DTA was used for analysis of modified particles and composites. About 5 to 8 mg of modified particles were added to a platinum pan, which was placed on the balance. The sample was then heated to 600 C at 10 C/min in air.

For the study of composite film homogeneity, five small pieces were cut from each examined film sample, one from the center and four from the edges in four directions that are about 90° to each other. The pieces were weighed on a PerkinElmer

AD-4 autobalance, and placed in platinum pans designed for TGA experiments. These pans were placed in ceramic crucibles and heated to 650 C in a furnace. The residual pan contents were weighed on the autobalance again to determine the filler weight loading of the sample. The combustion of the films can also be carried out on the TGA, however, use of a larger furnace allows analysis of multiple samples simultaneously.

2.5 Differential scanning calorimetry

Differential scanning calorimetry (DSC) is a technique that is similar to DTA.

However, instead of detecting a temperature difference, it measures the difference in heat flow between a sample and an empty pan while holding both pans at the same temperature. The pans used for this test are usually made from aluminum, which are crimped shut to give better thermal contact. The instrument is capable of detecting thermal events that result in a change in heat content, such as glass transition, crystallization, curing, melting and decomposition. The instrument can be attached to a

gyro system, which allows supercooling of samples. The events that occur during cooling can also be recorded and studied. This allows detection of structural differences caused by supercooling.

DSC was utilized in this project to determine the glass transition temperature (Tg) of the film samples. The glass transition of polymers causes a change in heat capacity of the materials, which is represented by a step change in the heat flow curve. The inflection point corresponding to Tg is the midpoint of the step.

A PerkinElmer Pyris 1 DSC was used to determine the glass transition temperature of the film samples. A small piece of about 2 mg was cut off the film, and placed in an aluminum pan, followed by sealing with a crimper. The samples were then heated to 180 C at 25 C/min, cooled to 50 C, and heated again to 180 C at 10 C/min under a N2 flow of 50 mL /min.

2.6 Rheometry

2.6.1 Storage, loss modulus and dissipation factor

Storage, loss modulus and dissipation factor describe the viscoelastic behavior of polymers. The strain of the material is measured while a sinusoidal stress is applied, and the storage and loss modulus of the material can be obtained from the data. Temperature sweep tests and frequency sweep tests can be carried out on this type of instrument.

When the temperature is swept, the complex modulus can be obtained as a function of temperature, which allows calculation of the dissipation factor (tan ). This factor is the ratio of loss modulus to storage modulus as a function of temperature, where Tg can be determined from the maximum of the tan  curve. Another advantage of this

measurement is that the local structure of the composite samples can be revealed from the peak shape of the tan  curve, which is discussed in detail in Chapter 6.

It must be noted that Tg values determined by DMA often differ from those determined using DSC, since the two characterization techniques measure the same property by different approaches. DSC measures the heat capacity of the materials upon heating, while DMA measures the storage modulus of the materials in response to an applied force during heating. DMA results give insights into localized behavior, while

DSC represents the properties of bulk materials. The values of Tg obtained from these two methods should not be directly compared to each other.

The instrument used in this project for measurements of tan  was an Anton Paar

MRC 302 rheometer, which does not have a heating function. To perform temperature sweeps, the instrument was placed into an oven. This instrument was also lacking an extension mode, which is the typical setup for thin film tests. Measurements were thus carried out by applying a torsional force, which twists the films during the test.

Unfortunately, samples are more easily deformed in this mode, especially when the samples become more viscous around the glass transition temperature. Fortunately, the obtained data are consistent in terms of peak width and onset point. However, the tan  peaks are not quite symmetric, which may be attributed to the twisting mode used for the tests.

The storage and loss modulus of the samples were measured from 30 C to 170

C at 5 C/min with a twisting force at a frequency of 1 Hz and a maximum strain amplitude of 0.05%. This test was carried out on the same strips used previously for CTE measurements. This test has to be carried out after all other characterization has been

completed, since the temperature range is higher than the Tg of the film samples, which causes deformation of the materials due to the external force.

2.6.2 Coefficient of thermal expansion

An Anton Paar MRC 302 rheometer was used to determine the CTE of the composites. Four rectangular strips of about 5 mm in width and 20 mm in length were cut off the film. The strips were measured with a caliper, followed by mounting vertically in the grips. A constant force of 0.1 N was applied, and the length of the film piece between the grips was measured while the temperature was held at 40 C for 5 min, followed by heating to 130 C at a heating rate of 10 C/min in air. The heating rate used for the composites containing NTE particles was 5 C/min. Due to time constraints, these samples were not tested again at 10 C/min. However, three pristine PC samples were tested at 5 C/min again, showing no difference from the results obtained at 10 C/min.

However, it is unclear whether this instrument is still well calibrated due to the lack of a standard material. Therefore, the CTE values for three samples were also determined on a thermomechanical analyzer (see 2.8). A correction factor was obtained based on measurements of an aluminum strip, and confirmed by comparing the average results of several samples with those obtained on the calibrated instrument.

2.7 Tensile testing

Tensile testing is a technique that measures the tensile properties of materials. In this project, the measured properties were stress, the corresponding strain and Young’s modulus. Stress is defined as the external force that materials can endure per unit area at

yield (softening) or break of the materials, which is reported as a pressure (amount of force per unit area). Strain is the relative change in dimension of the materials as a function of external force. Stress and strain at yield were used to compare samples in this project. Data at higher forces were unreliable, as some samples showed random behavior.

This is attributed to random entanglement of the polymer chains, resulting in random breaking points of the materials. Thus comparison of stress or strain at breaking became meaningless. Yield of materials can be defined in multiple ways. The definition used in this work is the point where stress does not increase any more with increased strain, corresponding to a zero slope in the stress-strain curve. Young’s modulus indicates the stiffness of materials, and has the same units as pressure. This modulus can be determined by creating a tangent to the stress-strain curve from the origin. The slope of the corresponds to Young’s modulus.

The instrument used for the tensile test was an Instron tensile tester. Films were cut into a certain shape using a designated metal die, followed by mounting in two grips aligned horizontally. A tensile force is applied on the sample while the strain of the film is measured. Data analysis was carried out using the instrument’s software which automatically returns stress strain and Young’s modulus based on the stress-strain curves after sample testing.

The American Standard Testing Method (ASTM) is a standard method that has been widely used for all kinds of property measurements. ASTM-638 describes the standardized testing method for tensile properties.57 Five types of specimens for different size samples are described, which all resemble dog-bone shapes. This shape assures that the specimens can be broken in the narrower middle part by the tensile force instead of

breaking close to the grips. However, the only available die (ASTM-638 type V) was too large for the film samples prepared in this project, resulting in incomplete wider portions of the dog bone portion of the cut. Thus rectangular samples (about 2 inches long, 0.25 inch wide) were obtained by using the straight middle section of the die cut. Several tests attempted on pristine polymer films cut to varying lengths gave consistent results when the grip distance of the instrument was reduced from the standard setting of 2.5 inch to

0.5 inch. Therefore, the rectangular specimens cut with the die were used for testing the composites. These pieces were then mounted in the grips of an Instron tensile tester, where a pre-load force ranging from 0.3 N to 1.0 N was applied. The stress exerted on the samples was measured while the grips were moved apart at a rate of 1 in/min until either the materials broke, or 20% strain in the material was reached.

2.8 Thermomechanical analysis

Thermomechanical analysis (TMA) is a technique that determines the coefficient of thermal expansion of materials by recording dimensional changes of specimens during temperature changes.

Samples of pristine PC, NTE2 and PTE1 were also tested using a TA Instruments

2940 thermomechanical analyzer with film fiber probe at West Kentucky University.

Samples with the same dimensions were cut and mounted in the grips. With a constant extensional force of 0.1 N applied, the grip distance was measured from room temperature to 140 C at 10 C/min in air. During data analysis, only the data between 40

C and 130 C were used, which matched the temperature range for the test carried out in

France.

Chapter 3

3 Synthesis of filler particles

3.1 Introduction

Polymer composites have been explored to add valuable properties to those of the pristine polymeric material. Depending on the desired effects, specific filler particles must be deliberately chosen. One purpose of composite preparation has been to provide better mechanical strength and to reduce thermal expansion. It is well known that thermal expansion of polymers can be controlled through incorporation of filler particles, which constrain thermal motion of polymer chains and thus lower the inherent thermal expansion of polymers. If filler particles exhibit NTE behavior, an additional contribution resulting from the filler particle contraction becomes effective. To distinguish the effects of chain motion constraints and NTE on the thermal expansion of polymer composites, the contributions of both factors must be separated. The morphology of filler particles is the only factor that affects how particles restrict the motion of polymer chains. Thus, use of two types of particles with identical size and shape but opposite α values should give composites that differ only by the expansion coefficient of the filler particles. The difference in composite thermal expansion will therefore be directly related to the difference in  values of the negative and positive thermal expansion filler particles.

3.1.1 Selection of filler particles

ZrW2O8 was chosen as the NTE filler. When this compound is synthesized at low temperatures via topotactic transformation from a hydrated precursor, it exhibits rod-like particle shapes, and particle size can be tuned through synthetic conditions.28, 58 Thus, a contrasting filler with the same particle shape was needed, and the sizes also needed to be tunable to match those of ZrW2O8.

riginally corundum or α-Al2O3, was chosen as a PTE filler, as it had been extensively studied in the literature. This compound was especially attractive as it has a

-6 -1 59 nown positive αv value of 30 × 10 K , which is of comparable magnitude to the

-6 -1 negative v value of ZrW2O8 (-27 × 10 K ). Corundum is accessible by dehydration of boehmite (AlOOH), which can be synthesized with a variety of particle shapes under different synthetic conditions.60, 61 Therefore, attempts were made to synthesize AlOOH with similar particle morphology as ZrW2O8, and then to convert AlOOH to α-Al2O3 while retaining the pre-existing morphology. As it was not clear whether the particle size and morphology would be preserved during this conversion, use of boehmite as the contrasting filler was considered as an alternative plan. Boehmite’s  value has not been reported to date, thus determination of the expansion coefficient would be necessary in this case before the particles can be used.

Unfortunately, precise morphology control of boehmite particles proved challenging, and an exact match to ZrW2O8 was not achieved. Since the morphology of the PTE particles is crucial for successful comparison between composites, a back-up plan was needed. An alternative was found by realizing that ZrW2O8 was obtained from

ZrW2O7(OH)2·2H2O via a topotactic transformation, which only requires small atomic

rearrangements within the existing structure. This means that the morphology of the hydrated precursor particles should be retained during the conversion, resulting in identical size and shape of both types of particles. Thus, it was decided to use

ZrW2O7(OH)2·2H2O as a contrasting particle in the comparison. The only disadvantage of this approach lies in the fact that the temperature range over which

ZrW2O7(OH)2·2H2O composite expansion can be evaluated is limited due to dehydration upon heating.

3.1.2 Desired properties of NTE particles

Good dispersibility is an important factor for filler particles, which usually can be achieved by reducing the particle size. For the same amount of particles, the ones with smaller size possess a larger surface-to-volume ratio, resulting in better interaction with the matrix. This would suggest that the smallest possible particles are most desirable.

However, another factor had to be taken into account for ZrW2O8 as well. It was found previously in our group that nanosized ZrW2O8 takes up moisture from air, and forms

28 ZrW2O8·xH2O, where the water molecules coordinate to metal atoms in the framework.

This autohydration causes the compound to lose its NTE behavior, and thus the comparison based on opposite α values of the two types of filler particles would become impractical. In addition, it would be impossible to predict the  value of the filler particles, as it would change with hydration state. Autohydration rates depend on the size of the particles, and are known to be faster for particles with smaller sizes and more defects. It has been proposed that this is a result of water molecules first adsorbing on the surface of particles, and then diffusing into the framework over time. Smaller particles

adsorb more water molecules on the surface with respect to the same volume, leading to a higher tendency for water uptake. More defective particles also display a higher surface

28 area that water can adsorb on. Autohydration of nanosized ZrW2O8 cannot be stopped completely, but can be slowed down by increasing the particle size and crystallinity.

While dispersibility of the particles can be enhanced by reducing the particle size, autohydration of the particles also needs to be slowed down. Thus, a compromise between the two properties had to be achieved in this work.

3.1.3 Hydrothermal synthesis

Hydrothermal synthesis is a “soft” chemistry route. Compared to conventional solid-state synthesis, samples with small and controllable particle sizes can be obtained through this method. Such a synthesis is usually carried out in an autoclave, or a Parr bomb, with a Teflon liner serving as reaction vessel. The autoclave can be sealed to prevent vapor from escaping the vessel. “Hydro” means that water is present in the synthesis and “thermal” means that the synthesis is carried out at an elevated temperature. Small amounts of organic solvents may sometimes be mixed with water in a hydrothermal synthesis. When the organic solvent becomes the major phase instead of water the synthesis method is called “solvothermal”. Hydro- and solvothermal methods have the advantage that reactions can be heated at temperatures above the boiling point of the solvent, which often improves crystallinity of the products.

3.2 Experimental

3.2.1 Materials and characterization methods

Zirconium oxychloride (ZrOCl2·xH2O, Alfa Aesar), 1-propanol (Alfa Aesar), sodium tungstate (Na2WO4·2H2O, Strem Chemicals), hydrochloric acid (Fisher

Scientific) and deionized (DI) water were used as starting materials for the synthesis of

ZrW2O7(OH)2·2H2O and ZrW2O8. Aluminum chloride (AlCl3·6H2O, Fisher Scientific), ammonia (NH3·H2O, Fisher Scientific), sulfuric acid (H2SO4, Fisher Scientific) and DI water were used for the synthesis of AlOOH.

Powder X-ray diffraction was used for phase identification, crystallite size estimation, amorphous content analysis, and determination of thermal expansion (see section 2.1).

SEM was used to examine the morphology of filler particles. SEM images were collected using a secondary electron detector at 2 kV accelerating voltage in charge compensation mode with a bias voltage of 2 kV.

3.2.2 Synthesis of ZrW2O7(OH)2·2H2O and ZrW2O8

ZrW2O7(OH)2·2H2O was synthesized via a hydrothermal method, which can achieve good particle size control in contrast to traditional solid-state methods. In a typical synthesis, ZrOCl2·xH2O (2.3 g, 8.2 mmol) and Na2WO4·2H2O (3.0 g, 9 mmol) were each dissolved in 10 mL of DI water. These solutions were added simultaneously to a 125 mL Parr bomb containing 2.5 mL of 1-propanol. Concentrated hydrochloric acid

(30 mL) was added to the white precipitate formed in the previous step, the Parr bomb was sealed, and heated at 210 C for 24 h. The amounts of all chemicals were scaled

down by a factor of five to synthesize small batches in 23 mL Parr bombs during exploration of synthetic conditions. The product was collected by centrifugation, washed with DI water until the pH of the supernatant was around 6, and dried at 65 C overnight.

To obtain cubic ZrW2O8, ZrW2O7(OH)2·2H2O was heated to a temperature ranging from

600 C to 650 C over a period of 2 h, and held at this temperature for 30 min. The crucibles were quenched in air after heating to avoid decomposition of ZrW2O8. Heating at 650 C resulted in better crystallinity of the cubic phase, however, it was found that for some samples, ZrW2O8 decomposed at this temperature. Heating at 645 C can avoid this problem, and still result in excellent crystallinity.

3.2.3 Synthesis of boehmite

The synthesis of boehmite was carried out using two different approaches, one of which required ion exchange in an attempt to better control particle morphology. These routes are described in the next paragraphs, and will be referred to as routes 1 and 2.

Route 1: An aqueous solution of Al(NO3)3 (1.1 M) was prepared by dissolving

0.75 g of Al(NO3)3·9H2O in 20 mL of DI water. Ammonium hydroxide (0.3 M) was dripped into the Al(NO3)3 solution until the pH reached 8.4. A white precipitate formed when the pH was increased, which was collected by filtration. The recovered solid was washed with DI water and recovered by centrifugation until the pH of the supernatant was around 6. This precipitate was then sealed in a 25 mL Parr bomb with 10 mL of DI water, and heated at 250 °C for 12 h.

Route 2: To synthesize AlOOH, 2.4 g AlCl3·6H2O were dissolved in 15 mL of DI water. A solution of 3 M NH4OH was dripped into the AlCl3 solution, leading to

formation of a white transparent sol. The sol was washed with DI water and centrifuged

- until all chloride had been removed. AgNO3 was used to test for the presence of Cl . The sol was placed in a 125 mL Parr bomb with 50 mL of diluted H2SO4 of various concentrations ranging from 0.3 to 7.5 mM, and heated to 250 °C for different periods of time ranging from 18 h to 88 h.

3.3 Results and discussions

3.3.1 Synthesis of ZrW2O7(OH)2·2H2O and ZrW2O8

All samples were characterized by X-ray diffraction to verify that the desired phase was obtained. Typical XRD patterns are shown in Figure 3-1, which shows traces for a precursor and the corresponding cubic material. The morphology of

ZrW2O7(OH)2·2H2O and ZrW2O8 was analyzed by SEM. ZrW2O8 exhibited identical particle size and shape as the hydroxide hydrate precursor (Figure 3-2). Therefore, the precursor is a perfect contrasting filler for use in polymer composites.

(b)

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10 20 30 40 50 60 70 2 (degrees)

Figure 3-1. PXRD patterns of (a) ZrW2O7(OH)22H2O, and (b) ZrW2O8.

Figure 3-2. SEM images of (a) ZrW2O7(OH)22H2O, and (b) ZrW2O8.

As discussed in the previous section, an optimized particle size needs to be found that displays a slow autohydration rate while maintaining good dispersibility. Synthetic conditions including temperature, heating duration, concentration of acid and volume of

alcohol were found to affect the size and crystallinity of the particles, which are the crucial factors that determine the autohydration rate and dispersibility. Based on previous results by Nathan Banek (summarized in 3.3.1.1), initial conditions were chosen, and further optimization work was carried out on the synthetic conditions of the precursor particles (3.3.1.2 and 3.3.1.3).

3.3.1.1 Choice of reaction temperature, alcohol type and volume

Reactions at different temperatures ranging from 130 °C to 230 °C were carried out in the past by a previous group member.62 The temperature of 230 °C was found to be too high for the precursor phase to crystallize. It was also found that high reaction temperatures up to 210 °C resulted in faster kinetics of crystallization for the particles and overall better crystallinity. Therefore, a reaction temperature of 210 °C was used for all precursor particle syntheses.

Another variable investigated in the past was volume of alcohol.62 Addition of alcohol was found to significantly break down the size of agglomerates formed by nanosized individual particles, and to also influence the exact particle size. Alcohols with different chain length from methanol to 1-heptanol in hydrochloric acid were used in the synthesis. Methanol was found to result in poor crystallinity, while alcohols with chain lengths of five carbons or longer were not miscible enough with water, resulting in broad size distributions of the particles. Ethanol, 1-propanol and 1-butanol had similar effects on the particle morphology.

For syntheses in 23 mL Parr bombs, small volumes of alcohol around 0.5 mL were sufficient to decrease the agglomerate size to the nanoscale. Higher volumes of

alcohol decreased the particle length and adversely affected crystallinity. Synthesis with

0.5 mL of 1-propanol was thus a good starting point for the optimization of particle size.

The volume of alcohol can be adjusted depending on characterization results.

3.3.1.2 Concentration of acid

Hydrochloric acid was the only acid used in the synthesis, since the precursor phase crystallizes most rapidly in this acid. To explore the effect of acid concentration on the particle morphology, several samples were prepared with acid concentrations ranging from 4.8 M to 7.2 M at 210 °C for 3 days. The crystallite size of the samples was initially estimated from the PXRD patterns. It was known from previous work that the estimated crystallite size is close to the average width of the rod-shaped particle, so that such estimates offered first insights into the particle size.

Table 3.1 Samples of ZrW2O7(OH)22H2O prepared with different acid concentrations.

Sample Acid Reaction time Crystallite size concentration XG.31.raw 7.2 M 3 d 61 ± 4 nm

XG.43.raw 6 M 3 d 56 ± 5 nm

XG.42.raw 4.8 M 3 d 75 ± 9 nm

As shown in Table 3-1, particles prepared with an acid concentration of 4.8 M gave a crystallite size of 75 ± 9 nm. Smaller crystallite sizes of 56 ± 5 nm and 61 ± 4 nm were observed for samples prepared with higher acid concentrations, 6 M and 7.2 M, respectively. These concentrations resulted in similar crystallite sizes, while the use of

4.8 M acid caused a large increase in crystallite size.

Figure 3-3.SEM images of ZrW2O7(OH)22H2O prepared in (a, b) 7.2 M HCl (XG.31.raw), (c, d) 6 M HCl (XG.43.raw) and (e, f) 4.8 M HCl (XG.42.raw).

The particle sizes of these samples showed the same trend as the estimated crystallite sizes (Table 3.1). The majority of the particles prepared with an acid concentration of 4.8 M (XG.42.raw) showed a size of about 100 nm to 200 nm in width, and 1000 nm to 1800 nm in length. A small portion of the particles were about 80 nm wide and 500 nm to 800 nm long. The samples prepared with acid concentrations of 6 M

and 7.2 M were composed of much smaller particles. Sample XG.31.raw (7.2 M) and

XG.43.raw (6 M) showed particle sizes of about 50 nm to 100 nm in width, and 300 nm to 800 nm in length. No significant difference between the particle sizes of these two samples was observed in the SEM images, which agreed with their similar estimated crystallite sizes. Thus, distinct sizes can be prepared by adjusting the acid concentration.

3.3.1.3 Reaction time

Reactions were also carried out for different lengths of time to explore the effect of heating time on the particle morphology (Table 3-2).

Table 3.2 Samples of ZrW2O7(OH)22H2O prepared for different times.

Sample [acid] t Crystallite Crystallite size after size heating XG.50.raw 6 M 1 d 46 ± 3 nm 44 ± 2 nm (600 °C) XG.37.raw 4.8 M 1 d 90 ± 14 nm 46 ± 2 nm (650 °C) XG.43.raw 6 M 3 d 56 ± 5 nm 55 ± 5 nm (650 °C) XG.40.raw 6 M 3 d 62 ± 8 nm 57 ± 6 nm (650 °C) XG.78.raw 4.8 M 3 d 109 ± 19 nm 54 ± 3 nm (645 C) XG.41.raw 6 M 7 d 54 ± 4 nm 54 ± 3 nm (650 °C) XG.46.raw 6 M 7 d 67 ± 8 nm 51 ± 3 nm (600 °C) XG.72.raw 4.8 M 7 d 94 ± 14 nm 52 ± 5 nm (645 °C) XG.45.raw 6 M 14 d 63 ± 5 nm 58 ± 6 nm (650 °C) XG.75.raw 4.8 M 14 d 103 ±22 nm 50 ± 3 nm (645 °C)

Reactions were carried out with hydrochloric acid concentrations of 4.8 M and 6

M for 1 d, 3 d, 7 d and 14 d. Sample XG.50.raw, prepared with 6 M acid for 1 d, showed the smallest crystallite size of 46 ± 3 nm. The crystallite size increased to 56 ± 5 nm and

62 ± 8 nm when the reaction time was elongated to 3 d (XG.43.raw and XG.40.raw, respectively). When the samples were prepared for 7 d and 14 d, the crystallite sizes stayed similar, although some variance was observed with samples XG.41.raw (7 d),

XG.46.raw (7 d) and XG.45.raw (14 d), which showed sizes of 54 ± 4 nm, 67 ± 8 nm and

63 ± 5 nm, respectively. This indicated that the crystallite growth in 6 M acid was complete after 3 d. On the other hand, samples prepared with 4.8 M acid showed faster crystallization rates. Reaction times of 1 d, 3 d, 7 d and 14 d resulted in samples with similar crystallite sizes of 90 ± 14 nm (XG.37.raw), 109 ± 19 nm (XG.78.raw), 94 ±14 nm

(XG.72.raw) and 103 ± 22 nm (XG.75.raw), respectively. This indicated that crystallite growth was already complete after 1 d.

The precursor particle sizes of the samples correlated well with the corresponding crystallite sizes. Particularly, the widths of most particles were about the same as the estimated crystallite sizes. With longer reaction time, small particles under a certain threshold are likely to re-dissolve and grow onto the large ones, which is called Ostwald ripening.

When ZrW2O8 was investigated, the effect of heating on crystallite size was found to be different for samples prepared with the two different acid concentrations. Those prepared in 6 M acid retained the crystallite sizes of the precursors, whereas the ones prepared in 4.8 M showed a significant reduction in crystallite sizes. This is unexpected for a topotactic transformation that only involves a slight re-arrangement of atoms.

Figure 3-4. SEM images of ZrW2O7(OH)22H2O prepared in (a)-(d) 6 M HCl for (a) 1 d (XG.50.raw), (b) 3 d (XG.43.raw), (c) 7 d (XG.41.raw) and (d) 14 d (XG.45.raw), and (e)-(g) 4.8 M HCl for (e) 1 d (XG.37.raw), (f) 3 d (XG.78.raw), (g) 7 d (XG.72.raw) and (h) 14 d (XG.75.raw).

The decrease in crystallite size upon heating suggested that the morphology might have changed during the heat treatment.

The morphologies of samples XG.37, XG.78, XG.72 and XG.75 (prepared from

4.8 M of acid for 1 d, 3 d, 7 d and 14 d) before and after heat treatment are shown in

Figure 3-4. After heating, all samples showed the emergence of many small rods along with the large rods that were present in the precursor sample, indicating that the original morphology was at least partially lost. This could be due to the large crystallite size of the precursor, which could result in formation of multiple ZrW2O8 nuclei in a single precursor particle, and thus crystallites break apart into smaller ones. This indicates that samples prepared in 4.8 M acid should not be used to synthesize the filler particles for the further comparison of NTE and PTE composites.

Additionally, it was unknown whether samples prepared for different times were resistant enough to autohydration to be filler particles. To determine the optimum reaction time, the hydration rate of the particles prepared under different conditions was screened.

Figure 3-5. SEM images of heat treated and raw sample prepared in 4.8 M acid (a) XG.37.650a (1 d), (b) XG.37.raw, (c) XG.78.645a (3 d), (d) XG.78.raw, (e) XG.72.645a (7 d), (f) XG.72.raw, (g) XG.75.645a (14 d) and (h) XG.75.raw.

3.3.1.4 Estimation of hydration rate

Hydration of ZrW2O8 causes compression of the framework, which leaves no space for the atoms to undergo transverse motions, resulting in loss of NTE behavior. As a consequence of the framework compression, the lattice constant of the particles decreases with hydration. This causes a shift of the diffraction peaks to higher angles in

PXRD patterns, making PXRD an excellent method to detect hydration as a function of time in air. Depending on what volume fraction of the particles undergoes hydration, the shift of the diffraction peaks will appear different. A shift of the entire peak is observed when most particles undergo hydration and hydrate throughout the particles. In contrast, only peak tails at higher angles are observed when only a small fraction of the particles or only an outer layer of each particle are hydrated, resulting from the shift of a small fraction of the peak to higher angles.

The lattice constants of each sample can be correlated to the positions of its diffraction peaks through Equation 2-3, which is valid for a cubic unit cell. The shift of the peaks to higher angles results in smaller d-spacings, and thus smaller values of a.

PXRD patterns of the samples were collected on different days after they were exposed to ambient atmosphere. Pawley refinement in TOPAS was used to extract the lattice constants from the corresponding patterns.

Table 3.3 Lattice constants of several ZrW2O8 samples over a period of 10 d.

Sample a (original) a (after 3 d) a (after 7 d) a (after 10 d)

XG.43.650a 9.156 Å 9.153 Å 9.151 Å 9.149 Å

XG.40.650a 9.156 Å 9.153 Å 9.153 Å 9.152 Å

XG.41.650a 9.156 Å 9.153 Å 9.153 Å 9.147 Å

XG.45.650a 9.157 Å 9.156 Å 9.153 Å 9.153 Å

As shown in Table 3-3, lattice constants were calculated for all samples from peak positions extracted from the PXRD patterns. No significant change was observed for the lattice constant of any of the samples prepared for different reaction times. This indicated that the majority of the particles did not undergo hydration. To examine the hydration behavior more carefully, the PXRD patterns obtained over time for the same sample were overlapped after scaling and offsetting the patterns to account for small variations in sample height as indicated by Rietveld refinements.

It was found that formation of peak tails was obvious in the overlapped patterns

(Figure 3-6), suggesting that a small portion of the particles underwent hydration. The peak tail became more intense and more spread out as more particles became hydrated over time. Thus, the hydration rate can be estimated by examining changes of the peak tail. Samples XG.40.650a, XG.41.650a and XG.43.650a exhibited similar rates of autohydration (Figure 3-6 (a), (b) and (c)), which was expected due to the similar crystallite and particle sizes. Sample XG.45.650a showed a smaller peak tail in the patterns over the same period of time. This result can be explained by the fact that less small sized particles were present in this batch due to the longer time reaction (14 d).

Smaller particles tended to re-dissolve in the solvent and combine with large ones over

time. The smaller NTE particles tended to exhibit higher hydration rates, thus the elimination of these particles was expected to slow down the autohydration.

These observations indicated that particle size was important in determining hydration rate. The higher surface-to-volume ratio of smaller particles allows more water to be adsorbed on the surface with respect to the same volume. This means that more water molecules diffuse into the framework over the same amount of time, leading to a higher hydration rate. All four samples discussed above gave acceptable hydration rates, as evident from the unchanged lattice constant and moderate change in peak shape. As the time efficiency of the synthesis should also be considered, 3 d or 7 d reactions would be preferred, although the sample prepared for 14 d showed a slightly slower hydration rate.

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2000 1000 1000

0 0 0 20 21 22 23 24 25 20 21 22 23 24 25 20 21 22 23 24 25 2 (degrees) 2 (degrees) 2 (degrees)

Figure 3-6. verlaid patterns scanned after d - - d d and ) 10 d of exposing ZrW2O8 to air: (a) XG.43.650a, (b) XG.40.650a, (c) XG.41.650a, (d) XG.45.650a, (e) XG.37.650a, (f) XG.72.645a and (g) XG.75.645a.

Interestingly, sample XG.37.650a, prepared with 4.8 M acid for only 1 d, showed a slower hydration rate than XG.45.650a (Figure 3-6 (e)). The latter sample exhibited the slowest rate among all samples prepared with 6 M acid. The crystallite size of

XG.45.650a was 58 ±6 nm, while that of XG.37.650a was 46 ± 2 nm. Samples

XG.72.645a (7 d) and XG.75.645a (14 d) showed a similar hydration rate as XG.37.650a

due to their similar particle size (Figure 3-6 (a), (c) and (e)). Thus, no Ostwald ripening effect on the hydration rate of the particles was observed. This might be attributed to the fact that the breaking apart of the precursor particles during heat treatment eliminates the effect caused by ripening during synthesis.

3.3.2 Synthesis of boehmite (AlOOH)

Several synthetic attempts using two different routes were made to control the particle shape of aluminum oxide hydroxide to match ZrW2O8. Phases present in all samples were verified to be AlOOH using PXRD, and the crystallite sizes were calculated as well. In Table 3.4 and Table 3.5, the synthetic conditions for both routes and the resulting crystallite sizes for AlOOH samples are summarized. Morphology and particle size of the samples were analyzed by SEM, and the results are summarized in

Table 3.6.

Figure 3-7. SEM images of AlOOH samples synthesized using Route 1: (a) XGA.4.raw (pH 8), and (b) XGA.8.raw (pH 3.7).

For reactions using the first route, platelets were obtained when the pH of the solution was around 8 (Figure 3-7 (a)). The particles became elongated in one direction

as the pH decreased. However, even the most anisotropic platelets were still not close to the morphology of the NTE particles. As shown in Figure 3-7 (b), the particles of AlOOH are elongated rhombic platelet, whereas the NTE particles are rods with square cross sections. Therefore, a second route to synthesize AlOOH was introduced to match the morphology of ZrW2O8.

Using the second route, platelets of AlOOH can be obtained when no sulfate ions are present during the crystallization. Addition of sulfate has a strong effect on particle shape. AlOOH adopts an orthorhombic structure, and sulfate anions are preferentially adsorbed on the 100 and 010 faces. This allows the c axis to grow faster, leading to a rod- like shape. The concentration of H2SO4 significantly affects the crystal growth of

AlOOH, with a lower concentration of H2SO4 giving larger particle sizes along the a and b axes. As shown in Figure 3-8 (a), rod shaped particles were successfully obtained.

Reducing the concentration of sulfuric acid resulted in increased crystallite sizes (Table

3.4 and Table 3.5), which agreed with the proposed mechanism. However, the rods showed different diameters in different directions, indicating that the particles were elongated rectangular prisms. The samples that showed the most similar size to ZrW2O8

(PMA.7.raw and PMA.13.raw) were prepared in 0.0011 M sulfuric acid. The larger dimension of the width was 40 to 80 nm, while the smaller dimension was only about 25 nm (Figure 3-8 (a) to (d)). Crystallite size analysis by PXRD showed that different reflections gave significantly different size estimates. The (020) reflection gave a much smaller size than other reflections that contained components from other axes, indicating that the b axis most likely corresponds to the thinner width of the particles.

Figure 3-8. SEM images of AlOOH samples synthesized using Route 2: (a, b) PMA.7.raw (0.0011 M H2SO4), (c, d) PMA.13.raw (0.0011 M H2SO4), (e, f) PMA.5.raw (0.0023 M H2SO4), (g) PMA.1.raw (0.0046 M H2SO4), and (h) -Al2O3 sample PMA.1.1200.

Table 3.4 AlOOH samples prepared under different conditions using Route 1.

Samples [Al(NO3)3] Final pH t Crystallite size Crystallite size (020) XGA.1.raw 1.1 M 8.4 15 h Not recovered XGA.2.raw 1.1 M 8.4 12 h 20 ± 10 nm 10 nm XGA.3.raw 1.1 M 8.4 25 h Not recovered XGA.4.raw 4.3 M 8 15 h 13 ± 5 nm 8 nm XGA.5.raw 1.1 M 8 15 h 18 ± 7 nm 11 nm XGA.6.raw 1.1 M 5.5 15 h 36 ± 11 nm 22 nm XGA.7.raw 2.9 M 4.1 44 ± 10 nm 32 nm XGA.8.raw 2.8 M 3.7 18 h 43 ± 14 nm 28 nm XGA.9.raw Not recorded 3.3 14 h 38 ± 10 nm 26 nm

Table 3.5 AlOOH samples prepared under different conditions using Route 2.

Sample Vtotal [H2SO4] t Crystallite size Crystallite size (020) XGA.10.raw 50 mL 0.0195 M 12 h 20 ± 5 nm 14 nm XGA.11.raw 50 mL 0.0105 M 12 h 9 ± 2 nm 6 nm XGA.12.raw 50 mL 0.0075 M 12 h 9 ± 2 nm 7 nm PMA.1.raw 50 mL 0.0046 M 18 h 10 ± 2 nm 7 nm PMA.2.raw 50 mL 0.0023 M 69 h 15 ± 4 nm 9 nm PMA.3.raw 50 mL 0.0023 M 22 h 9 ±4 nm 6 nm PMA.4.raw 50 mL 0.0023 M 87 h 17 ± 5 nm 11 nm PMA.5.raw 50 mL 0.0023 M 22 h 40 ± 11 nm 28 nm PMA.6.raw 50 mL 0.0010 M 25 h 9 ± 2 nm 6 nm PMA.7.raw 6 mL 0.0011 M 22 h 28 ± 10 nm 17 nm PMA.8.raw 6 mL 0.0016 M 22 h Not recovered PMA.9.raw 6 mL 0.0005 M 22 h 18 ± 4 nm 13 nm PMA.10.raw 6 mL 0.0003 M 21 h 44 ± 5 nm 45 nm PMA.11.raw 6 mL 0.0003 M 91 h 49 ± 13 nm 33 nm PMA.12.raw 6 mL 0.0003 M 22 h 24 ± 8 nm 18 nm PMA.13.raw 6 mL 0.0011 M 22 h 28 ± 10 nm 17 nm PMA.14.raw 6 mL 0.0005 M 22 h 23 ± 5 nm 17 nm

Table 3.6 Morphology of AlOOH samples from SEM images.

Sample Particle shape Particle size

XGA.2.raw Platelets W: 30 - 80 nm; T: ~10 nm

XGA.4.raw Platelets W: 30 - 80 nm, T: ~10 nm

XGA.5.raw Platelets (some stretched) W: 30 - 80 nm; T: ~10 nm (~100 nm for stretched dimension) XGA.6.raw Rectangular platelets L: 100 - 200 nm; W: 40 - 80 nm; T: ~20 nm

XGA.7.raw Rectangular platelets L: 100 to 500 nm; W: 40 to 80 nm; T: ~25 nm

XGA.8.raw Rectangular platelets L: 100 to 500 nm; W: 40 to 80 nm; T: ~25 nm

XGA.9.raw Rectangular platelets L: 100 to 500 nm; W: 40 to 80 nm T: ~30 nm

XGA.10.raw Platelets W: 20 to 50 nm; T: ~10nm

XGA.11.raw Rods with slight difference in L: 100 to 300 nm; W: 15 to 25 nm width in two dimensions XGA.12.raw Rods with slight difference in L: 100 to 300 nm; W: 20 to 25 nm width in two dimensions PMA.1.raw Entangled fibers with slight L: 200 to 500 nm; W: 20 to 40 nm difference in width in two dimensions PMA.2.raw Rods with different width in two L: 100 to 500 nm; W: 40 to 60 nm; T: ~20 nm dimensions PMA.3.raw Rectangular platelets (some L: ~30 nm; W: 50 to 100 nm; T: ~10 nm small rods) PMA.4.raw Rods with different width in two L: 100 to 500 nm; W: 40 to 60 nm; T: ~20 nm dimensions PMA.5.raw Square platelets W: 50 to 200 nm; T: 30 to 50 nm

PMA.6.raw Rods with slight difference in L: 200 to 500 nm; W: 20 to 40 nm width in two dimensions PMA.7.raw Rods with different width in two L: 100 to 500 nm; W: 40 to 60 nm; T: ~25 nm dimensions PMA.9.raw Rods with different width in two L: 100 to 500 nm; W: 40 to 60 nm; T: ~25 nm dimensions PMA.10.raw Square platelets W: 50 to 150 nm; T: ~50 nm

PMA.11.raw Square platelets W: 20 to 80 nm; T: ~25 nm

PMA.12.raw Square platelets W: 30 to 150 nm; T: 30 to 50 nm

PMA.13.raw Rectangular platelets L: 100 to 300 nm; W: 40 to 80 nm; T: ~25 nm

PMA.14.raw Square platelets W: 30 to 100 nm; T: ~25 nm

L = length, W = width, T = thickness

Lower acid concentrations were investigated to increase the size of the elongated rods. However, samples prepared from lower concentrations of acid showed unexpected and inconsistent behavior. Sample PMA.9.raw, prepared with 0.0005 M acid, showed similar sizes as PMA.7.raw and PMA.13.raw. In contrast, the other sample prepared under the same conditions showed platelet shaped particles. None of the samples prepared with the lowest concentration used in this research (0.0003 M) produced rod- like particles. This concentration might be too low to observe the effect of preferential sulfate coordination (Figure 3-8 (e) and (f)).

The NTE filler particles that had the best balance between autohydration rate and dispersibility had sizes around 100 nm in width. This cannot be matched by the width of

AlOOH particles for the synthesis with the lowest concentration of sulfuric acid.

Attempts to synthesize particles that had identical sizes to ZrW2O8 failed. Attempts on the conversion from AlOOH to corundum also became meaningless, since crystallization of corundum only occurs above 1000 °C. Compared to the original raw particles (Figure 3-8

(g)), massive sintering of the particles was observed at this temperature, leading to the formation of severely agglomerated chunks (Figure 3-8 (h)). To save time for the rest of the work on this project, the back-up plan was used instead of searching for other routes to synthesize AlOOH particles with identical shapes to ZrW2O8.

3.3.3 Determination of thermal expansion of ZrW2O7(OH)2·2H2O

Originally, corundum was selected as the PTE filler particle, partially for its well- known thermal expansion coefficient since the α values for both inds of filler particles in the comparison of composites had to be well nown. The α value of

ZrW2O7(OH)·2H2O had not been reported in the past, so that it had to be determined in this project.

Figure 3-9 Overlaid PXRD patterns collected for ZrW2O7(OH)2H2O on a variable temperature stage at temperatures between 25 °C (trace 1) and 205 °C (trace 19).

Scans were collected at temperatures ranging from 25 °C to 205 °C, however, the sample started to undergo a phase transition to orthorhombic ZrW2O8 at 205 °C (Figure

3-9), making data collected at higher temperatures unusable. To determine the lattice constants of the sample at each temperature, Pawley refinements were carried out on

PXRD patterns of a mixture of silicon and ZrW2O7(OH)·2H2O at temperatures from 25

°C to 195 °C (Figure 3-10). Silicon was used as an internal standard to account for changes in sample height at elevated temperatures. The correct value of the lattice constant for

silicon as a function of temperature is known, so that peak shifts due to changes in sample height can be accounted for. During the refinement, the sample height for

ZrW2O7(OH)·2H2O and Si were constrained to be identical. After correcting for sample height offsets, accurate lattice constants for the hydroxide hydrate could be determined.

The a-axis showed smooth increases up to 195 °C, while a sudden decrease in the c-axis was observed at 185 °C (Figure 3-11). This decrease caused a leveling off of the volume change of the unit cell (Figure 3-12). This suggested that the dehydration already started at 185 °C, and that data above 175 °C should be excluded. Therefore, the axial thermal

-6 -6 expansion coefficients of ZrW2O7(OH)·2H2O were determined to be 11 × 10 ± 1 × 10

K-1 for the a axis and 2.6 × 10-6 ± 0.3 × 10-6 K-1 for the c axis between 25 °C and 175 °C.

The relative volume coefficient of thermal expansion was found to be 24 × 10-6 ±

1 × 10-6 K-1 in the same temperature range (Figure 3-12). The volume expansion

-6 -1 coefficient for ZrW2O8 in this temperature range is -27 × 10 K . These two values were different enough to investigate the effect of thermal expansion on the polymer composite.

7000

6000

5000

4000

3000

Intensity (counts) 2000

1000

-1000 20 40 60 80 100 2 (degrees)

Figure 3-10. Pawley refinement results for a mixture of Si and ZrW2O7(OH)22H2 at 25 C. bserved and calculated patterns a difference curve and tic mar s indicating the calculated peak positions of Si (bottom) and ZrW2O7(OH)22H2O (top) are displayed.

11.46 12.5

11.45 12.5

a axis (Angstrom) a axis c axis (Angstrom)c axis

11.44 12.49

11.43 12.49 0 50 100 150 200 0 50 100 150 200 Temperature (°C) Temperature (°C)

Figure 3-11. Unit cell length of (a) a-axis and (b) c-axis of ZrW2O7(OH)2H2O as a function of temperature under ambient pressure (×) and vacuum ().

1641

1640

)

3 1639

1638

Volume(Angstrom 1637

1636

1635 0 50 100 150 200 Temperature (°C)

Figure 3-12. Unit cell volume of ZrW2O7(OH)22H2O as a function of temperature under ambient pressure (×) and vacuum ().

3.3.3.1 Dehydration of ZrW2O7(OH)2·2H2O

The phase transition observed at 205 °C in the PXRD patterns collected under ambient pressure was caused by dehydration of the hydroxide hydrate phase. This transition occurred at 115 °C when the variable temperature PXRD data were collected under vacuum. However, as stated in the above section, the length of the c axis already started to decrease at 185 C under ambient pressure. Under vacuum, this change started at 105 C. The deviation of the c-axis length from the linear relationship shows that the particles cannot be exposed to temperatures above 105 C under vacuum, or above 185

°C under ambient pressure. It is important to realize this, as the composite made with the precursor particles has to be dried under vacuum for complete solvent removal.

During the transition, an orthorhombic ZrW2O8 phase was formed upon dehydration, which is expected to exhibit a different α value. This phase is also characterized by poor crystallinity. Both of these factors would cause problems for the comparison of composites, especially if the transition was not detected and occurred during sample drying in later experiments. The dehydration of this material must be avoided by all means in all later experiments. Sample drying for composites was usually carried out overnight, which allowed thorough heat transfer throughout the sample, thus resembling the case of heating on the platinum strip. Thus, temperatures of 185 °C under ambient pressure and 105 °C under vacuum were chosen as upper limits of temperature that these particles were exposed to.

3.3.4 Determination of amorphous contents of ZrW2O7(OH)2·2H2O and ZrW2O8

To calculate the effect of thermal expansion of a filler in a composite, it is convenient to assume that the volume thermal expansion of the crystalline phase is representative of the volume expansion of the particles. However, this assumption will only hold true if all particles are fully crystalline. The presence of an amorphous phase would ma e this estimate inaccurate since the α value of the amorphous component would be unaccounted for. Therefore, to validate whether the filler particles can be described by the  values of the crystalline phases, the amorphous content of both types of filler particles had to be determined.

3 104

2.5 104

2 104

1.5 104

1 104

Intensity (counts)

5000

-5000 20 40 60 80 100 120 2 (degrees)

Figure 3-13. bserved and calculated patterns and difference curve for a mixture of Si and ZrW2O7(OH)22H2O at RT. Tick marks indicate the calculated peak positions of Si (bottom) and ZrW2O7(OH)22H2O (top).

The phase ratio of silicon standard to filler particles was determined by Rietveld refinement of PXRD data collected on a mixture of equal weights of both materials

(Figure 3-13). Since the silicon used was known to be fully crystalline, a 1:1 phase ratio of the two phases was expected if the filler particles were fully crystalline as well. A phase fraction of 43% was obtained for nano-sized ZrW2O7(OH)·2H2O, whereas a micron-sized sample (prepared previously in the group, CLA117RT) that should possess higher crystallinity only gave 38%. It is unclear what exactly caused this difference, however, amorphous content estimates by PXRD are subject to errors of a few percent.

This suggests that the hydrothermally synthesized precursor particles possess high crystallinity. On the other hand, a 33% phase fraction was obtained for ZrW2O8, corresponding to 66% crystallinity. This was surprising, as the presence of 33% amorphous material can usually be optically detected in PXRD patterns, while the patterns of ZrW2O8 looked well crystallized. Such discrepancies can sometimes arise due to microabsorption when several phases in a mixture have very different absorption coefficients. To correct data for quantification errors resulting from absorption mismatches between the particles and the standard, a mixture of silicon with fully crystalline ZrW2O8 prepared by solid-state methods at high temperature was also refined.

A percentage of 33% was also obtained for this sample. However, 4% of ZrO2 and a trace amount of WO3 were also found to be present within the sample, resulting in very similar phase percentage of the sample as the micron-sized precursor after correction for these impurities. This suggested that the difference in absorption between silicon and the two types of particles was responsible for the lower calculated phase fraction. The hydrothermally synthesized precursor and NTE samples thus were both estimated to be highly crystalline.

Both types of filler particles were proven to possess excellent crystallinity, which suggested that it would be reasonable to approximate the expansion of the particles by the volume expansion coefficients of the crystalline phases.

3.4 Conclusions

ZrW2O7(OH)·2H2O was successfully synthesized via a hydrothermal method, followed by conversion to ZrW2O8 at 650 °C. Aluminum oxide hydroxide synthesis with attempts to match the morphology of ZrW2O8 was unsuccessful. Given the identical

morphology of the hydroxide hydrate and ZrW2O8 particles, the ZrW2O7(OH)·2H2O particles were thus chosen as PTE filler particles for preparation of polymer composites.

Acid concentration and reaction time were varied to investigate the effect on the morphology of the particles. Distinct changes were observed for different acid concentrations. An acid concentration of 4.8 M resulted in larger crystallite sizes, while 6

M and 7.2 M gave smaller crystallites. The width of the particles observed by electron microscopy was comparable to the corresponding crystallite sizes. For samples prepared in 6 M acid, reaction times of 3 d, 7 d and 14 d were found to result in similar crystallite sizes of the particles, which were slightly larger than for the sample prepared for 1 d. The sample heated for 14 d showed an increased number of larger particles, resulting from

Ostwald ripening. Use of 4.8 M acid resulted in similar crystallite and particle sizes for all reaction times.

The results of hydration rate studies on these particles showed trends that could be related to crystallite and particle sizes. The samples with larger crystallite and particle sizes tended to hydrate more slowly. The sample prepared in 6 M acid for 14 d showed the slowest hydration rate due to the absence of extremely small particles. However, this rate was still faster than those observed for the samples prepared in 4.8 M acid, which showed similar rates of hydration regardless of reaction time. No effect of Ostwald ripening on autohydration was observed for these samples. This observation might be attributed to the fact that the precursor particles prepared at this concentration break apart during heat treatment. As a result, the effect of the ripening on the particle size was lost to the more significant changes cause by particle breakage.

The volume thermal expansion coefficient of ZrW2O7(OH)·2H2O was determined to be 24 × 10-6 ± 1 × 10-6 C-1 from 25 C to 175 C. This value can be used to describe the expansion of the particles, since the crystallinity of the precursor and NTE particles prepared with the proposed route was excellent.

Chapter 4

4 Surface modification of filler particles

4.1 Introduction

Surface modification of filler particles is an inevitable step before the incorporation of the particles into the polymer. For the purpose of this project, the modification becomes even more critical. Good adhesion at the interface between particles and polycarbonate is extremely important to allow the effect of NTE and chain stiffening from the particles to fully transfer to the polymer matrix. If voids exist between the two phases, the interaction between the two phases will be weakened or eliminated, resulting in less or no reduction in the CTE of the composites. In addition, the route used for the modification has to be carefully considered. The approaches used in similar investigations were already discussed in section 1.5. Most of these approaches introduce an alien species into the composite system to achieve bonding on the surface. In the cases of polyimide/ZrW2O8 composites, APTES was used to react with the hydroxyl groups on

48, 49 43 43 the particle surfaces; in the cases of epoxy/ZrW2O8, carboxylic acid, APTES and polyacrylic acid42 were used. For the purpose of reducing the CTE of polymers, these modifiers are useful, and do not introduce any disadvantages. However, this project is

designed to distinguish the effect of expansion coefficient and chain stiffening on the polymer. Thus, the introduction of any species other than filler particles and monomers might result in additional factors that could influence the CTE of the polymer composite.

To avoid this, only monomer species can be used for the modification of ZrW2O8.

A different route called in-situ polymerization was mentioned in section 1.5,

54 which was utilized in the preparation of PC/Al2O3 composites. The metal oxide particle surfaces are terminated by oxygen atoms, as metal terminated sites are usually oxidized in air. Reactions with atmospheric moisture result in formation of hydroxyl groups on the particle surfaces. This allows the oxide particles to react with monomer species used in polycarbonate formation.

Figure 4-1. In-situ polymerization of polycarbonate in the presence of filler particles.

As shown in Figure 4-1, one of the monomers that forms PC, triphosgene, can directly react with the hydroxyl groups on the particle surface, followed by reaction with the other monomer, bisphenol A, thus resulting in formation of PC oligomers on the filler particle surfaces. This route introduces no alien species into the composite system, and can be utilized when the monomers and particles have functional groups in common.

Both types of filler particles used in this project, ZrW2O8 and ZrW2O7(OH)22H2O, have multiple oxygen atoms per formula, resulting in the existence of hydroxyl groups on the surface. This suggests that the same approach as for Al2O3 should allow growth of PC

oligomers on the surface of both types of particles. While the feasibility of surface modification by in situ polymerization was proven by previous work, optimized conditions were not established. To achieve high oligomer coverage, reaction conditions were optimized in this project.

Figure 4-2. Enhanced interaction between filler particles and polymer matrix through surface modification.

A detailed depiction of the effect of surface modification is shown in Figure 4-2.

The oligomer of PC grown on the surface of the particles has the same structure as the PC matrix, enabling it to interact more strongly with the matrix than the surface hydroxyl groups. In addition, the oligomer is longer than the surface hydroxyl groups, which extends the effective interaction area of the particles, and is likely to result in entanglement of oligomer and polymer chains. This entanglement also enhances the interaction between the two phases. The use of modified particles ensures optimal

property transfer between the two different phases, and the difference in the CTE of the two types of composites can fully represent the effect of the filler CTE on the expansion of the composites.

4.2 Experimental

4.2.1 Characterization

IR spectroscopy was used to examine the bonding of PC oligomers on the particle surface. The samples were mixed with potassium bromide in a weight ratio of 1:100. The successful bonding of PC oligomer was also verified by TGA. The samples were heated to 700 C at a rate of 10 C min-1.

4.2.2 Surface modification of ZrW2O8 and ZrW2O7(OH)2·2H2O

The procedure reported for solution based in-situ polymerization of polycarbonate in the presence of alumina nanowhiskers by Coleman et al. was modified for this work.

This approach resulted in synthesis of bulk polycarbonate along with formation of surface bound polymer chains through reactions with hydroxyl groups. In a typical reaction, bisphenol A (1.5 g, 6.5 mmol) was dissolved in 30 mL of pyridine in a round bottom flask, followed by suspending about 0.2 g of filler particles in the same flask. The particles were dried at 140 C to remove surface moisture prior to this step. This suspension was sonicated for 30 min to disperse the nanoparticles well. Triphosgene (1.1 g, 3.7 mmol) was dissolved in 10 mL of dichloromethane, and this solution was slowly dripped into the bisphenol A/nanoparticle/pyridine mixture, followed by dripping in triethylamine (1.4 mL) as catalyst. The final mixture was stirred for various lengths of

time at room temperature to allow the polymerization to progress. Centrifugation was used to collect the modified nanoparticles, which were then washed with methanol and collected by centrifugation until the supernatant became clear. After the sample was dried at room temperature, Soxhlet extraction was used to remove residual PC oligomers that remained entangled with the modified nanoparticles during centrifugation. About 200 mL of dichloromethane were used to extract the PC oligomers. The extraction process took up to about 96 h until all unbound polymer was washed out. Modified particles were dried at 90 C overnight.

4.3 Results and discussion

4.3.1 Surface modification of filler particles

To obtain the best coverage of oligomers on the particle surface, the synthetic conditions were optimized. Results were judged using IR spectra and TGA data collected on modified samples. Small variations could result from the fact that particles from different batches were used in the optimization. This was necessary because a single batch of particles did not provide enough material for the whole investigation. To obtain consistent results, only particles from the same batch were compared, since those from different batches might have different surface area or number of defects.

The original sample names of modified particles are used in Tables 1 and 2 (e.g.,

XGF.69.645a.1). Each sample starts with the initials of the experimenter (XG), and the addition of the letter “F” indicates functionalization. The next number corresponds to the sample number in chronological order of raw particle synthesis followed by either “raw” or a three digit number. “ aw” samples correspond to precursor particles and the heat

treatment temperature is indicated for NTE particles. A lower case letter following the temperature indicates the batch of heat treatment, while the last number entry indicates the batch of surface modification.

For convenience of sample referencing in the text, batches of the particles are identified by letters A through F in parentheses. Numbers following these letters refer to batches of surface modification for the same particle batch (e.g., A1 and A2).

4.3.2 Soxhlet extraction

During the in-situ polymerization, the formation of PC occurred simultaneously on the particle surface and in the solvent. The modified particles were collected by centrifugation, which removed both particles and some unbound oligomers from the solution. The true surface coverage of the modified particles can only be evaluated after removal of unbound PC. Soxhlet extraction was carried out on the modified particles. In

Soxhlet extraction, a selected solvent is placed in a round bottom flask and heated until it evaporates into a Soxhlet extractor connected to the flask. The vapor reaches a water condenser that is connected to the extractor, and the solvent is condensed. In the extractor, a cellulose thimble is placed in the bottom. A piece of filter or weighing paper usually is placed on top of the thimble to contain the modified particles, which is to prevent the particles from getting stuck in the cellulose. When the level of the solvent becomes higher than the siphon arm, the solvent flows back to the flask, taking any dissolved compounds with it. These dissolved compounds remain in the round bottom flask if their boiling points are higher than that of the solvent. The solvent undergoes many cycles of evaporation and condensation throughout an extraction. In this project,

THF was initially used as the solvent, however, it was found that THF sometimes cannot

completely remove unbound polymer. To circumvent this problem, dichloromethane was used instead.

4.3.3 Surface modification of ZrW2O8

4.3.3.1 TGA curves of modified ZrW2O8 particles

The surface coverage of modified particles was quantified by TGA. To obtain meaningful results, it is important to ensure that all unbound polymer has been removed by Soxhlet extraction prior to quantification. To monitor the progress of extraction over time, a sample was tested by TGA three times over the progress of a Soxhlet extraction.

As shown in Figure 4-3, the weight loss between 200 °C and 300 °C was around 2 wt%,

0.5 wt% and 0.2 wt% after 36 h, 48 h and 54 h of extraction, respectively. However, the weight loss above 300 °C remained at the same level. This strongly indicated that unbound polycarbonate was lost below 300 °C, while bonded oligomer was only lost above this temperature. This showed that TGA is an effective way to examine the completeness of the extraction. It was also found that the necessary extraction time for complete removal of all unbound polymer varied slightly from sample to sample. Some samples showed no indication of leftover unbound polymer after 3 d, however, many samples with higher surface coverage tended to require longer extraction times. An extraction time of 96 h was long enough to ensure the complete removal in all cases.

100

Weight% 97

95 0 100 200 300 400 500 600 700 Temperature (°C)

Figure 4-3. TGA curves of typical modified ZrW2O8 sample A after 6 h

(---) 48 h and () 56 h of Soxhlet extraction.

A small portion of the bonded oligomer decomposed at 300 °C to 350 °C, whereas the majority was lost between 400 °C and 550 °C. The surface coverage of all samples was determined from the weight loss between 300 °C and 550 °C, which was not affected by any residue of unbound polycarbonate due to incomplete separation. In addition, slight variances were observed for the TGA results of modified particles when analyses were duplicated. To account for this fact and allow for better comparison between different conditions, especially for samples giving very similar results, some samples were subjected to triplicate TGA analyses, so that averages and standard deviations could be considered in the final comparison.

4.3.3.2 IR spectra of ZrW2O8

The IR spectra of plain particles (ZrW2O8 and ZrW2O7(OH)22H2O) and polycarbonate are shown in Figure 4-4.

(c)

(b)

(a)

Transmittance (arbitrary units) Transmittance (arbitrary aliphatic C-H C=O aromatic C-C 4000 3500 3000 2500 2000 1500 1000 Wavenumber (cm-1)

Figure 4-4. IR spectra of (a) pure polycarbonate, (b) plain ZrW2O8, and (c) plain

ZrW2O7(OH)22H2O.

Successfully modified particles showed the features of both plain particles and polycarbonate. The most distinct bands of polycarbonate that do not overlap with bands of filler particles are at 1780 cm-1 and 1500 cm-1, corresponding to carbonyl group stretching and aromatic C=C stretching, respectively. An additional band, corresponding to aliphatic C-H stretching, was also observed at 2900 cm-1 in the IR spectra of modified samples. A representative IR spectrum of modified NTE particles is shown in Figure 4-5

(a). Bands corresponding to PC were clearly observed at 1500 cm-1, 1780 cm-1 and 2900 cm-1. The relatively low intensity of the bands for this sample could be attributed to the

very low amount of functional groups present in the IR beam, which is close to the limit of detection of the instrument. The total amount of bonded oligomer was around 1 wt% of the particles, and only 5 to 8 mg of modified particles were used in IR sample preparation. In the IR spectrum of the modified precursor particles (Figure 4-5 (b)), the additional peaks corresponding to PC showed even lower intensity than those in the IR spectrum of the modified NTE particles. This was attributed to the stronger absorption of

IR light from the precursor particles, resulting in lower relative intensities of the PC peaks. The signal corresponding to aliphatic C-H stretching at about 3000 cm-1 was not visible at all, since it was completely covered by a signal from the precursor particles, which contain hydroxy groups in their structure.

Figure 4-5. IR spectra of (a) modified ZrW2O8, and (b) modified ZrW2O7(OH)22H2O.

Insets are zoomed-in spectra portions from 1800 cm-1 to 1450 cm-1.

4.3.3.3 Influence of amount of monomers

During in-situ polymerization for surface modification, a complex set of reaction kinetics must be considered. Triphosgene monomers can directly react with hydroxy groups on the particle surfaces, producing surface end groups that can in turn react with bisphenol A. In addition to the stepwise formation of polymer chains on the particle surfaces, monomers will react with each other in solution to produce polymer chains of varying lengths. Lastly, shorter oligomer chains that are carbonate terminated may also graft onto the particle surfaces through a condensation reaction of the surface hydroxyl and oligomer end groups. Direct grafting of oligomers will only occur for relatively short chains, as longer chains tend to entangle in solution, resulting in unfavorable sterics for surface reactions. In addition, growing surface coverage will also reduce the kinetics of additional surface grafts due to steric effects. The effect of reaction time, monomer ratio and nanofiller concentration on surface coverage by the PC was investigated to optimize the functionalization procedure. Reactions were carried out with 30 mL bisphenol

A/pyridine and 10 mL triphosgene/dichloromethane solutions unless specified otherwise.

The amount of monomers used in the reaction was found to affect the oligomer coverage on the particles. Surface modifications carried out with monomers in stoichiometric ratio (sample A1, around 10.0 mmol of bisphenol A and 3.3 mmol of triphosgene) resulted in 1 wt% oligomer coverage on the particles. However, the surface coverage increased when the modification was carried out with a reduced amount of bisphenol A for the same period of time (samples A2 to A4).

Table 4.1 Synthetic conditions for surface modification of ZrW2O8 with variable ratios of monomers.

Sample Bisphenol Triphosgene ZrW2O8 t Bonded ESD Yield A (mmol) (mmol) (g) (h) polymer (g) (wt%) XGF.68.645b. 10.3 3.8 0.19 16 1.0 0 0.07 4 (A1) XGF.68.645b. 8.1 3.7 0.17 17 1.6 0.1 0.04 2 (A2) XGF.68.630. 6.9 3.8 0.19 17 1.5 0.1 0.06 645a.2 (A3) XGF.68.630. 4.8 3.7 0.18 17 1.5 0.2 0.06 645a.3 (A4) XGF.30.600a. 1.9 3.7 0.28 24 1.1 0.1 0.24a 4 (B1) XGF.30.600a. 4.8 3.6 0.31 18 1.8 0.1 0.20a 3 (B2) a Samples were extracted differently than other samples, many particles were lost on the glass fiber extraction thimble. All other samples used a cellulose thimble.

Samples prepared with reduced bisphenol A amounts as the only variable (about

8.0 mmol, 6.7 mmol and 4.5 mmol) resulted in surface coverages of 1.6 ± 0.1 wt%, 1.5 ±

0.1 wt% and 1.5 ± 0.2 wt% of bonded oligomer, respectively. No significant difference in coverage was observed for these three samples, suggesting similar reaction kinetics for monomer ratios in this range. The increased coverage for reduced amounts of bisphenol

A was attributed to the fact that excess triphosgene shifted the ratio of reactive end groups. This favored reaction with the hydroxy groups on the particle surfaces due to the shortage of bisphenol A in solution, resulting in faster kinetics of surface modification.

When stoichiometric amounts of monomers were used, triphosgene favored reaction with dissolved bisphenol A over reaction on solid particle surfaces, which are less accessible.

In addition, the monomer ratio affects the polymerization kinetics of unbound polymer.

The relative tendencies for stepwise polymer formation on the particle surfaces and formation of growing chains in solution shift with changes in monomer ratio, which can explain the plateau in surface coverage for a range of monomer stoichiometries. The coverage was found to decrease when the amount of bisphenol A was reduced to about

4.5 mmol to 2.8 mmol (samples B1 and B2, respectively). The former showed 1.8 wt% coverage after reacting for 18 h, while the latter had only 1.1 wt% oligomers bound with a longer reaction time of 24 h. This indicated that the extent of the reaction is reduced when the amount of bisphenol A is too low. The optimum amount was found to be between 4.5 mmol and 8.0 mmol. A ratio of about 6.7 mmol of bisphenol A to 3.7 mmol of triphosgene was used for the remaining work.

4.3.3.4 Influence of reaction time

Polymerizations with varied reaction times were carried out to investigate the effect of time on surface coverage. Particles from the same batch were modified under identical conditions except for different reaction times.

Table 4.2 Synthetic conditions for surface modification of ZrW2O8 particles with variable reaction time.

Sample Bisphenol Triphosgene ZrW2O8 t Bonded ESD Yield A (mmol) (mmol) (g) (h) polymer (g) (wt%) XGF.71.645a. 6.9 3.7 0.18 8 1.3 0.1 0.09 1 (C1) XGF.71.645a. 6.9 3.7 0.18 12 1.5 0.1 0.09 3 (C2) XGF.71.645a. 6.8 3.7 0.18 21 2.5 0.3 0.06 6 (C3) XGF.71.645a. 6.9 3.9 0.16 30 2.5 0.3 0.05 7 (C4) XGF.71.645b. 6.6 3.7 0.16 48 2.8 0.2 0.04 1 (C5)

For the same batch, coverage increased slightly from 1.3 ± 0.1 wt% (sample C1) to 1.5 ± 0.1 wt% (sample C2) when the reaction time was increased from 8 h to 12 h, respectively. Longer reaction times of 21 h, 30 h and 48 h produced higher coverages of

2.5 ± 0.3 wt% (sample C3), 2.5 ± 0.3 wt% (sample C4) and 2.8 ± 0.2 wt% (sample C5), respectively, indicating that coverage was leveling off after 21 h. This was attributed to the fact that all monomers had either reacted with surface groups, or formed polymer chains in solution that were too long to graft onto the particle surfaces. Thus, reaction times of around 21 h were used for modification of the particles used in composite preparation.

4.3.3.5 Influence of the amount of particles suspended in solution

The amount of particles present in the modification reaction was another factor that influenced the resulting coverage. Unsurprisingly, using fewer particles in the

reaction resulted in a higher weight percentage of oligomer bonded on the surface within the same reaction time, which was attributed to less available surface sites that reacted with the same concentration of monomers.

Table 4.3 Synthetic conditions for surface modification of ZrW2O8 particles with variable amount of filler particles.

Sample Bisphenol Triphosgene ZrW2O8 t Bonded ESD Yield A (mmol) (mmol) (g) (h) polymer (g) (wt%)

XGF.69.645a 7.1 3.8 0.08 16 3.4 0.03 .4 (D1) XGF.69.645a 6.8 3.9 0.37 17 0.9 0.1 0.18 .1 (D2) XGF.72.645a 13.5 7.5 0.17 24 2.7 0.06 .2 (E1) XGF.72.645a 7.1 3.5 0.39 29 1.0 0.30 .1 (E2)

A sample prepared with 0.08 g of particles, D1, resulted in bonding of 3.4 wt% of oligomer. This was a significantly higher coverage than for the sample prepared with about 0.37 g of particles (D2) modified for the same period of time, which gave only 0.9 wt% of oligomer on the surface. The same trend was observed for particles from another batch, and seems to be related to the particle to monomer ratio. Sample E1, prepared with

0.17 g of particles with twice the amount of solvent and other chemicals, had 2.7 wt% of oligomer bonded, whereas sample E2 prepared with the normal solvent volume gave a much lower coverage of 1.0 wt% (0.39 g of particles). While these results suggest that small batches of particles are ideal to achieve good surface coverage, for practical reasons, the amount of particles that can be recovered for use in composite preparation

has to be considered as well. The recovery yield after Soxhlet extraction was increased when more particles were used in the reaction, since a small amount of particles was always lost during the extraction, due to the particles that stick to the surface of the cellulose thimble. About 0.03 g (around 35%) of particles were recovered for sample D1.

The recovery yield remained similar when the amounts of all chemicals were scaled up by a factor of two (sample E1, 0.06 g). Modification reactions of larger batches (> 0.35 g) of particles generally resulted in lower surface coverage but higher recovery yields (50-

70%), while reactions with around 0.18 g of particles in the smaller solvent volume showed surface coverage and recovery yields in between these values. It is clear that a suitable amount of sample with excellent surface coverage can be recovered by scaling up the reaction conditions with the smallest amount of particles explored. However, this also results in use of significant quantities of solvents and monomers. As the leveling off of surface coverage with time is likely related to consumption of monomers by oligomer formation in solution, it was hypothesized that the addition of new monomers to already modified particles will result in additional surface coverage. This alternative route allows reduction in the amount of solvent and chemicals used, without sacrificing the desired surface coverage. About 0.3 g of particles were subjected to two consecutive modification steps, which can increase the coverage significantly, while preserving the same high recovery yield obtained for single step modifications.

4.3.3.6 Increasing surface modification through double modification

A second modification reaction was carried out on several batches of pre- modified particles to achieve better oligomer surface coverage (designated by addition of

’ in sample name . The pre-modified particles were collected by centrifugation without

further washing. The collected solid was immediately subjected to a second modification step. The optimized synthetic conditions for the first modification step were used again.

Table 4.4 Synthetic conditions for surface modification of ZrW2O8 particles with two consecutive modification steps.

Sample Bisphenol Triphosgene ZrW2O8 t Bonded ESD Yield A (mmol) (mmol) (g) (h) polymer (g) (wt%)

XGF.72.645a.1 7.1 3.5 0.39 29 1.0 0.30 (E2) XGF.72.645a.1 6.8 3.9 0.29 23 1.8 0.1 0.15 .2nd E2’ XGF.72.645a.3 6.8 4.1 0.38 26 (E3b) XGF.72.645a.3 6.7 3.9 24 1.6 0.3 0.24 .2nd E ’ XGF.73.645a.1 13.3 7.3 0.60 24 (F1b) XGF.73.645a.1 13.2 7.4 24 1.9 0.2 0.36 .2nd F1’ XGF.73.645a.2 3.3 2.0 0.16 23 (F2b) XGF.73.645a.2 3.3 1.8 24 3.0 0.5 0.06 .2nd F2’ b Sample was not recovered between modification steps.

The resulting coverage of the particles was increased, while the recovery yield remained high, similar to the yields observed for single step reactions. The first modification step on sample E3 was carried out with 0.38 g of particles for 26 h. The coverage of sample E ’ obtained by subjecting all of the precipitate from sample E to a second surface modification reaction, was found to have 1.6 ± 0.3 wt% oligomers bound after 24 h. The two modification step procedure was also performed with the use of

Soxhlet extraction in between steps as a control. Sample E2 showed a coverage of 1.0 ±

0.1 wt% after 29 h of modification which increased to 1.8 wt% for sample E2’ after an additional modification reaction for 23 h. This indicated that the purification in the middle has little or no effect on the resulting coverage. The same process was carried out on sample F2 with all the chemicals including solvents scaled down by a factor of two.

ample F2’ showed a coverage of . ± .5% after two modification steps on .159 g of particles. However, only about 0.06 g of particles were recovered, which was lower than the yield for sample E ’ but still comparable to most of the samples processed with a single modification step. Compared to the route where all chemicals are scaled up

(sample E1), this route showed slightly lower surface coverage, but a higher amount of recovered sample. To obtain the same amount of product (0.24 g vs. 4 × 0.06 g), the route of double modification only consumes a quarter of the monomers and solvents that the other route would require. Even when comparing to the least favorable sample F2’ only half of the chemicals were consumed to deliver the same amount of product. Thus, to obtain sufficient quantities of filler particles with high surface coverage for composite films in an efficient manner, the two-step modification was chosen to obtain particles for composite film preparation.

4.3.4 Surface modification of ZrW2O7(OH)2·2H2O particles

ZrW2O7(OH)2·2H2O particles were also modified with two consecutive reactions using the optimum conditions established for ZrW2O8. The ZrW2O7(OH)2·2H2O particles behaved similarly to the ZrW2O8 particles in modification reactions that lasted 16 to 22 h.

Table 4.5 Synthetic conditions for surface modification of ZrW2O7(OH)22H2O particles.

Sample Bisphenol Triphosgene Particles t Bonded ESD Yield A (mmol) (mmol) (g) (h) polymer (g) (wt%) XGF.68.raw.1 6.7 3.7 0.32 18 0.9 0.16 (A5) XGF.68.raw.1 6.8 4.0 0.19 16 2.2 0.06 .2nd A5’ XGF.73.raw.1 12.0 7.1 0.61 23 (F2)b XGF.73.raw.1 13.1 7.1 22 2.7 0.4 0.36 .2nd F2’ b Sample was not recovered between modification steps.

ample A5’ retained .9 wt% and 2.2 wt% of oligomer bonded on the surface for the first and second modification step respectively. ample F2’ prepared from .61 g of particles in a reaction scaled up by a factor of two without intermediate purification, showed a coverage of 2.7 ± 0.4 wt% after the second modification step. About 0.36 g of modified particles were recovered, indicating that the recovery yield was as high as for

NTE particles. The additional IR bands corresponding to C=O and aromatic C=C stretches were even harder to observe than in the case of ZrW2O8, as the intensity of the peak at 1650 cm-1, corresponding to a feature of the precursor particles, was much stronger than that of ZrW2O8. For modified precursor particles, the peaks caused by aliphatic C-H stretching were undetectable due to a band of the precursor particles in the same region (Fig. 6(b)).

4.3.5 Challenges throughout the course of the project

4.3.5.1 Inconsistencies during Soxhlet extraction with THF

In the beginning, only IR spectroscopy was utilized for the analysis of surface coverage of the modified particles. The intensities of the two additional peaks from PC at

1780 cm-1 and 1500 cm-1 were compared to that of the peak from the particles at 1600 cm-1. Increased intensities of the PC peaks indicated more surface bound oligomer. The first dozen modified ZrW2O8 samples characterized in this way showed clear trends with respect to how the amount of monomers and reaction time affected the surface coverage, and those results agree with the observations described in the sections above. However, the next batch of samples showed less coverage in IR spectra. It was realized that the extraction time was elongated for these samples (48 h instead of 24 h, suggesting that the unbound polymer was not fully removed for the former samples and contributed to the IR signals. Further studies revealed that THF did not dissolve all unbound oligomers even after extractions lasting several days. The reason for this was not thoroughly studied. It was most likely caused by the uptake of moisture by the solvent, leading to lower solubility of polycarbonate. It is also possible that some decomposition of PC occurred, and that the decomposition products interfered with continued extraction.

Dichloromethane was then introduced as a substitute for the extraction, which did not show the same behavior. The distinct temperature ranges for the decomposition of unbound and bound polymer were finally discovered using TGA. The lack of systemic study on how much extraction time was required for complete removal of unbound oligomer was mainly why the inadequacy of THF was not found earlier. This study should have been carried out first, since it reveals the true surface coverage of the

modified particles, based on which the conditions can be optimized. Once the unbound polymer was fully removed, the IR study became less meaningful, since the difference in the surface coverage of the particles under different synthetic conditions caused no noticeable change in the intensities of the additional IR peaks from PC.

4.3.5.2 Evaluation of success of surface modification of ZrW2O7(OH)2·2H2O particles

Initially, the modification of ZrW2O7(OH)22H2O were carried out by a different route, which was designed based on observations that later turned out to be incorrect.

This mistake was also caused by the problem of incomplete extraction using THF, and the consequent misjudgment of the surface coverage of the filler particles.

When the precursor and NTE particles from the same batch were modified under the same conditions for the first two times (XGF.48.raw.1 & 2 and XGF.48.600a.1 & 2), very weak intensities of the two additional peaks at 1780 cm-1 and 1500 cm-1 were observed for the precursor particles. Similarly, only a weak signal from PC was observed for XGF.48.600a.1, whereas strong intensity was observed for XGF.48.600a.2. The observations for XGF.48.600a.1 were thought to be an outlier, and the low intensity shown in the IR spectra of the precursors was thought to be due to poor reactivity of the precursor surface. Looking back at these results with the current knowledge, the IR spectra actually implied that the unbound polymer was almost fully removed in three of the samples, and that entangled unbound oligomers caused the strong signals for

XGF.48.600a.2. This sample was extracted with the same batch of THF as several previous samples, which could possibly result in the reduced extraction efficiency of the solvent. In addition, these four samples were prepared with a different amount of filler

particles (0.45 g), which would be predicted to result in low surface coverage. This explains the weak signal observed in the IR spectra of the first three samples compared to other samples modified at that time.

Due to the above described observations, it was concluded that the surface of the precursor could not be modified using an identical route as for ZrW2O8. A new route was designed to circumvent the perceived problem. The precursor was dehydrated to obtain orthorhombic ZrW2O8, which should have similar surface properties as cubic ZrW2O8.

This phase was then modified under the same conditions, and the surface coverage was confirmed to be similar to that of the cubic phase based on IR spectroscopy. The modified particles were then converted to the precursor phase again by re-hydrating under hydrothermal conditions. However, this treatment can also hydrolyze some of the polycarbonate bonds. Consequently, the re-hydrated particles showed only a very small weight loss by TGA, which corresponded to the leftover oligomers bound on the surface.

The detected coverage was considered inadequate for use in composites. However, it seemed likely that the bound oligomers could undergo additional reactions with fresh monomers, resulting in enhanced coverage after a second modification step. The results obtained from IR indeed showed improved coverage after the second step, and this exploration eventually resulted in the design of the two-step modification procedure used for both types of filler particles. Unfortunately, a large number of samples was prepared over the course of half a year before the problems leading to the inconsistent observations were discovered.

4.4 Conclusion

Surface modification of PTE and NTE filler particles was successfully achieved via in-situ polymerization. The unbound polymer was removed with dichloromethane through Soxhlet extraction for three to four days. It was found that THF is not an adequate Soxhlet solvent for this system.

Optimized synthetic conditions use monomer molar ratios of bisphenol A to triphosgene between 1.3:1 and 2.2:1, and reaction times of approximately 20 h. It was also found that smaller quantities of particles resulted in higher coverage, however, final yields were poor for small batches. A two-step modification process on larger batches of particles (0.3 to 0.4 g) was used to obtain reasonable amounts of recovery and good surface coverage. Both types of particles showed similar surface modification results under the same conditions.

Chapter 5

5 Preparation of the composite films

5.1 Introduction

Solution-cast films were chosen as the form of composites to be studied, as no special equipment such as an extruder is required, and only a small amount of materials is needed to prepare each sample. In solution casting, filler particles and polycarbonate were mixed in a solvent that dissolves the polymer, followed by casting in a Petri-dish dish. Composite films were obtained after the solvent completely evaporated. Both types of filler particles were mixed with commercial polycarbonate at the same weight percentage for comparison, ranging from 2% to 12%, where inhomogeneous dispersion of particles was observed in the films. It is important to avoid certain problems during film casting, which are discussed in detail in the following sections.

5.1.1 Crystallization of polycarbonate

Polycarbonate is known to easily crystallize during solution casting.

Crystallization results in partially ordered packing of polymer chains, which is called glassy state. However, the rubbery state, where polymer chains are randomly oriented, is

preferred. The structural change between glassy and rubbery state causes changes in the properties of the polymer, including mechanical and thermal properties. It is especially important to note that upon the formation of the ordered structure, the motion of the packed polymer chains is constrained by lattice energy from intermolecular forces, which is likely to change the thermal expansion coefficient.45, 47 Since the site and degree of crystallization cannot be controlled during solution casting, it has to be prevented completely to ensure consistency of inherent properties of the polymer between different samples. Therefore, a custom-made glass vessel (Figure 5-2) was used to prevent crystallization.

5.1.2 Direct blending and particle flow during solution casting

Direct blending is a simple and fast way to prepare both pure polymer and composite film samples. The filler particles and polycarbonate are mixed in a volatile solvent, directly followed by film casting. This is a convenient approach to explore the conditions used for casting.

A problem that is likely to occur during casting is the flow of solvent during evaporation. This flow originates from different rates of evaporation of solvent throughout the container. Due to the surface tension of the solvent, the casting solution forms a drop-like shape, resulting in curvature and larger surface area at the edges, and thus higher rates of evaporation.63 Additionally, the area covered by the casting solution will remain unchanged as the solvent evaporates. If uneven evaporation occurs, a flow of the casting solution is generated from areas that exhibit slower evaporation to those with higher rates, which compensates for the evaporation loss from the latter areas.63 In the

case of composite films, movement of suspended particles with this flow results in inhomogeneous dispersion if the particles can travel at a different rate than the polymer solution. Therefore, uneven dispersion of filler particles is most likely to occur when the interaction between particles and polymer is not sufficient. In addition, this flow results in changes in film thickness throughout the film. Solvent takes longer to evaporate from thicker areas of the film, which gives particles more time to settle, especially if the interactions with the polymer matrix are insufficient.

The last chapter discussed that surface modification of filler particles can improve their interaction with the polymer. Better interaction between particles and polymer reduces the mobility of the filler particles. This interaction can be enhanced even more by using a different process for solution casting, which is called reprecipitation blending.

5.1.3 Reprecipitation blending

In the reprecipitation blending approach, filler particles are mixed with a polymer solution at an elevated temperature. The mixture is then precipitated in a non-solvent, followed by drying, redissolution and film casting. The advantage of this method lies in the use of elevated temperatures, which allows polymer chains to extend during mixing, and enhances the interaction between polymer and filler particles (Figure 5-1).64, 65

heat

olymer ligomer article matrix oligomer Figure 5-1. Conformation of polymer chains in cold and hot DMAc.

During the precipitation step, the extended polymer chains wrap around the filler

particles. This entanglement is preserved even during subsequent dissolution and film

casting steps, which enhances the homogeneity of the dispersion.

5.2 Experimental methods and characterization

5.2.1 Materials

5.2.1.1 Chemicals

Polycarbonate (MW ~64000 g/mol, Sigma Aldrich) and dichloromethane (Fisher

Scientific, ACS grade) were used for direct blending. N,N-dimethylacetamide (Alfa

Aesar, anhydrous, 99.8%) and methanol (Fisher Scientific, ACS grade) were the

additional solvents used for reprecipitation blending.

5.2.1.2 Specialized glassware

A custom-made glass chamber was created in the glass shop for the preparation of

film samples (Figure 5-2). The bottom edge of the chamber can be greased, allowing

creation of an airtight seal with a piece of glass (e.g. bottom part of crystallization dish).

The chamber can be purged with argon through a rubber tube connection, which allows

control of the flow rate and lowers the moisture level. A combination of argon flow and

water vacuum can be used to control the pressure inside the vessel, as well as lower the

moisture level inside the chamber.

Figure 5-2. Custom-made glass vessel with gas inlet (clear tubing) and water vacuum (black tubing).

5.2.2 Direct blending

To prepare pure polycarbonate films, about 0.8 g of PC was dissolved in about 10 mL of dichloromethane. The solution was then stirred at room temperature until all PC was dissolved, followed by pouring into a Petri-dish. Three different sized dishes were used, which had diameters of 5.5 cm, 8.5 cm and 9 cm. The dichloromethane was allowed to evaporate, which left a film in the Petri-dish. For the preparation of polymer composites, filler particles with various weight loadings (2 wt%, 5 wt%, 8 wt% and 12 wt%) were mixed with about 0.8 g of PC in about 10 mL of dichloromethane. The mixture was stirred at room temperature until all the particles were suspended in the solution instead of settling on the bottom. This usually took no more than 4 h. Sonication or shaking aided in the suspension of the particles. The composite film was cast in the same way as the pure PC film.

Film preparation was also carried out in a special glass chamber. The bottom of the chamber was greased and set on a flat glass surface. One outlet of the chamber was connected to an argon tank using rubber tubing, and the other outlet was connected to water vacuum (Figure 5-2). The film was cast in the Petri-dish inside the chamber under about 150 mL/min of argon flow and slight vacuum.

5.2.3 Reprecipitation blending

To prepare each of the composites with NTE particles, about 0.8 g of PC was mixed with cubic ZrW2O8 at various weight loadings (2 wt%, 5 wt%, 8 wt% and 12 wt%) in 1 mL of N N’-dimethylacetamide (DMAc). The mixture was then heated to 95 °C for

1 h, followed by precipitation in about 70 mL of methanol. The precipitate was ground

into a very fine powder in an industrial blender, collected by filtration, and dried at 140

°C under vacuum. The dried powder was subjected to the film casting procedure described for direct blending in 5.1.2.

The preparation of composite films with precursor particles only differed in the drying step. The recovered precipitate was suspended overnight in about 100 mL of methanol, followed by drying at 95 °C under vacuum for 2 d. This procedure was necessary to avoid dehydration of the precursor particles, which occurs above 95 C under vacuum. It is difficult to completely remove dimethylacetamide under these conditions. The overnight stirring in methanol helps to exchange dimethylacetamide adsorbed on the surface of the powders for methanol, which can be removed at lower temperatures under vacuum.

5.3 Results and discussion

5.3.1 Direct blending

Direct blending was used to explore conditions for film casting, since it is a fast and simple process. It was discovered that three factors were crucial to obtain a high quality film, which included the moisture level around the casting solution, the evaporation rate of the solvent and the protection of the forming film from any disturbance.

5.3.1.1 Moisture around casting solution and protection from disturbance

As the solvent evaporates during film casting, energy is absorbed from the surroundings to supply the heat of evaporation. Especially for dichloromethane, a volatile

100

solvent with a boiling point of 39.6 °C, the energy absorbed during evaporation is significant. The evaporation lowers the temperature of the dish and forming film, causing moisture in the air to condense on the surface of the casting solution. The presence of moisture can be detected by the white discoloration of the film surface, which is retained after solvent evaporation (Figure 5-3 (a)). Another potential source of moisture is the glassware that the casting solution comes in contact with. Mixing of particles and PC in an incompletely dried flask results in aggregation of the particles in the composite film

(Figure 5-3 (b)). The detailed reasons for this are unclear, but this could be related to the evaporation behavior of the solvent in the presence of moisture. Clearly, extra precautions are necessary to ensure moisture free glassware, and all glassware should be dried at 115 °C for multiple hours before use in film casting.

The effect of moisture on the properties of the film is unclear, as the extent of penetration is unknown. While moisture may just affect the surface of the film, it is better to avoid exposure altogether. In addition to thorough drying protocols for glassware, ambient humidity is critical, which cannot be controlled if the film is cast under ambient atmosphere. The extent of moisture deposition also depends on the evaporation rate of the solvent. The faster the solvent evaporates, the lower the temperature of the dish, making condensation more likely.

Another issue is that the air flow in the hood is strong enough to stir the casting solution. In the case of composite films, this can result in flow of the filler particles, leading to decreased homogeneity due to poor particle dispersion (Figure 5-3 (c)). It is impossible to control the air flow consistently without additional equipment.

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Figure 5-3. Film samples prepared by direct blending under ambient atmosphere with (a) pristine polycarbonate, (b) polycarbonate and ZrW2O8 in an incompletely dried dish, and (c) polycarbonate and ZrW2O8 in a dried dish.

To circumvent these two problems, film casting was carried out in a custom made glass chamber (Figure 5-2).

5.3.1.2 Evaporation rate of the solvent

When a confined environment was used for film casting, slow evaporation of solvent resulted, which was found to favor crystallization of PC. The center of the film

102

started to crystallize after about one hour. Since the temperature of the solution is reduced along with the temperature of the dish during film casting, thermal motion of molecules becomes slower, resulting in lower kinetic energy of PC chains. Formation of hydrogen bonds and aromatic stacking interactions between the chains of PC reduces the potential energy, thus lowering the internal energy. This might compensate for the decrease of entropy associated with crystallization, making formation of crystalline PC favorable.

Crystallization of polycarbonate is easily recognized, as the packed polymer chains block visible light, resulting in opaqueness of the film (Figure 5-4 (a)).

The evaporation rate of the solvent can be increased by increasing the flow rate of the argon. However, if the argon flow is too high, the resulting film severely wrinkles, arising from the strong disturbance of the casting solution as a consequence of high gas flow (Figure 5-4 (b)). Therefore, water vacuum was applied to the second outlet to reduce the argon pressure in the chamber, which is a more effective way to enhance the rate of evaporation without disturbing the surface of the film. This setup does not eliminate all traces of moisture, but the reduction achieved under these conditions is sufficient to result in films without discoloration. To prepare high quality films, the vacuum and argon flow must be controlled carefully.

The flow rate of argon was set to about 150 mL/min, which does not agitate the solution, but is high enough to keep the moisture level low to avoid condensation. Under this condition, most of the solvent evaporates within about 45 min of casting, and a free standing film can be obtained within an hour. However, the film is left to dry until it starts peeling off from the dish, which usually takes about 1.5 h. The peeling indicates that even the solvent under the film has dried out. Residual solvent trapped inside the film

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then needs to be removed at elevated temperatures in an oven.

Figure 5-4. Films prepared using custom made glassware: (a) pristine polycarbonate with crystalline part in the center caused by slow evaporation, and (b) pristine polycarbonate under high argon flow.

5.3.1.3 Composition of casting solution

The concentration of the casting solution can affect the evaporation rate as well.

This parameter is determined by the amount of polymer and solvent. Unsurprisingly, evaporation takes less time as the amount of solvent decreases. However, the solvent volume needs to be large enough to cover the evaporation dish, so that the size of the films is consistent between samples. Thus, the volume of solvent must be at least 12 mL for a dish with a diameter of about 8.5 cm. This amount of solvent was used for all experiments. For the same volume of solvent, the evaporation becomes slower with increasing amounts of polymer. This can be rationalized by the fact that the motion of solvent molecules is slowed down due to interactions with the long polymer chains, which consequently decelerates the evaporation of solvent.

In a Petri-dish with a diameter of 8.5 cm, different amounts of polycarbonate

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ranging from 0.1 g to 1.0 g were used in attempts to prepare films. While lower polymer concentrations accelerate evaporation, the reduced amount of material also makes it difficult to recover free standing films. On the other hand, high polymer concentrations result in slower evaporation of the solvent during casting, and thus increase the risk of crystallization. It was found that amounts ranging from 0.5 g to 0.8 g were optimal to balance the structural stability of the film with the desired fast evaporation. The thickness of the resulting films was measured using a caliper (Table 5-1).

Table 5.1 Thickness of composite film samples (at 8 weight% loading).

Weight of polycarbonate (g) Thickness (µm) STD (µm) 0.1 14 7 0.3 38 14 0.5 60 22 0.8 101 21 1.0 126 31

The amount of filler particles is another variable, but was not found to be critical to the evaporation of the solvent. This is likely due to the low volume of filler particles used, which makes their effect negligible.

5.3.2 Reprecipitation blending

As mentioned above, polymer chains tend to form coils at room temperature, resulting in less than optimal interaction between filler particles and polymer. Therefore, reprecipitation blending for film preparation was explored to optimize this interaction.

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5.3.2.1 Temperature of heating

To extend the polycarbonate chains, the polymer solution containing the filler

particles was heated in dimethylacetamide. Polycarbonate decomposed within 1 h at 110

°C, which may be caused by the attack of DMAc on the acidic carbon in the carbonate

group. The polymer formed a powder when it was precipitated in methanol after heating

at 110 °C, indicating that the molecular weight of the polymer had dramatically

decreased. Polymers with high MW should form entangled fiber-like solids due to their

long molecular chains. The decomposition can be avoided by heating below 100 °C. No

decomposition was observed at temperatures between 95 °C and 100 °C even after 6 h.

5.3.2.2 Film casting after precipitation

The same film casting procedure as for direct blending was used for the mixture

of polymer and filler particles after precipitation and thorough drying. The complete

removal of DMAc required a temperature close to its boiling point under vacuum.

ZrW2O8 can withstand this temperature, however, the dehydration of the precursor occurs

above 95 °C under vacuum. This temperature is too low to remove all DMAc adsorbed

on the composite sample. If DMAc molecules remain until the film casting step, their

limited tendency to evaporate can cause crystallization of PC. In addition, retention of

DMAc in the film could affect composite film properties and interfere with comparison

of PTE and NTE filler composite films. Thus, the mixture was stirred in excess methanol

overnight, which allows enough time to completely remove the DMAc molecules from

the composite.

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5.3.3 Composite films with two types of filler particles

With the optimized conditions for film casting, composite films with both types of filler particles were prepared at various weight loadings, including 2%, 5%, 8%, 12% and

25%.

As shown in Figure 5-6, the transparency of the films decreased gradually with increasing amounts of filler particles. Use of optimized conditions ensured low flow of the casting solution and good homogeneity of the particle dispersion. Uneven thickness of the films could not be completely avoided due to the shape of the Petri-dish. The homogeneity of the films was examined using TGA, results of which will be discussed in the next chapter.

At a weight loading of 12%, aggregation of the filler particles was observed for both types of composites (Figure 5-6 (d), (e), (i) and (j)). The particles in the films formed islands instead of evenly dispersing. This indicated that the interaction between the filler particles began to exceed that between polymer and particles. The effect of NTE might be compromised in this case due to the lack of interaction at the interface.

No crystallization was observed in the resulting films as evidenced by PXRD patterns. The broad peak at 15 to 20° 2 sharpens significantly when PC crystallizes. As shown in Figure 5-5, the amorphous features of clear, pristine PC films and composite films were very similar. More details about crystallinity of films will be presented in chapter 6.

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Table 5-2 Composite film samples with precursor and NTE particles.

No. Sample PC (g) Particles Wt (%) (g) NTE1 XGF.69.645a.1.2nd.2% 0.803 0.019 2.3 NTE2 XGF.69.645a.1.2nd.5% 0.828 0.043 4.9 NTE3 XGF.73.645a.1.2nd.8%a 0.327 0.028 7.9 NTE4 XGF.73.645a.1.2nd.12% 0.297 0.041 12.1 NTE5 XGF.73.645a.1.2nd.26% 0.314 0.103 24.7 PTE1 XGF.69.raw.1.2nd.2% 0.803 0.019 2.3 PTE2 XGF.69.raw.1.2nd.5% 0.828 0.043 4.9 PTE3 XGF.71.raw.1.3rd.8% 0.806 0.066 7.6 PTE4 XGF.73.raw.1.2nd.12%a 0.307 0.043 12.3 PTE5 XGF.73.raw.1.2nd.26% 0.295 0.107 26.6 aThe film was cast in a dish with diameter of 5.5 cm, while the others used a 9 cm dish.

(b)

(a)

Intensity (arbitary Intensity units)

10 15 20 25 30 2 (degrees)

Figure 5-5. PXRD patterns of film samples, (a) clear pristine PC, and (b) NTE1.

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Figure 5-6. Scanner images of composite film samples (a) to (e) NTE1 to NTE5, respectively, and (f) to (i) PTE1 to PTE5, respectively.

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Figure 5-7. Camera images of composite film samples (a) to (e) NTE1 to NTE5, respectively, and (f) to (i) PTE1 to PTE5, respectively.

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5.4 Conclusions

Direct blending was found to be a convenient approach to explore film casting conditions. Two problems encountered during film casting were crystallization of films and flow of casting solution. The flow of the casting solution is caused by uneven evaporation rates in different areas of the casting solution. Large differences in evaporation rates result in highly uneven thickness of the films. In the case of composite films, the flow of filler particles occurs with the flow of solution, leading to inhomogeneous dispersion of the filler particles. Large film thickness causes crystallization, since the evaporation of the solvent is slowed down by the presence of more polymer. A thickness of about 60 µm to 100 µm was found to be optimal. Thinner films did not produce stable free standing films, while thicker films reduced the evaporation rate of the solvent. Evaporation rates were controlled using a custom- designed glass vessel, where argon gas flow and water vacuum can be applied simultaneously. Around 100 mL/min of argon flow and slight water vacuum were used to avoid agitation of the casting solution while accelerating solvent evaporation and providing a moisture free environment.

To enhance the interaction between filler particles and polycarbonate, reprecipitation blending was used. The mixture was heated in DMAc at 100 C for 1 h, followed by precipitation in excess methanol. Higher temperatures caused decomposition of polycarbonate in DMAc. The residual DMAc was removed by vacuum drying at about

140 C for 2 d for ZrW2O8/PC mixtures, while ZrW2O7(OH)22H2O/PC mixtures were stirred in excess methanol overnight, followed by drying at 95 C under vacuum for 2 d, to avoid dehydration of the inorganic phase. Composite films with both types of filler

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particles were successfully prepared by solution casting in a Petri-dish. Attempted weight loadings of the films were 2%, 5%, 8%, 12% and 25%. Inhomogeneous dispersion of both filler particles was already observed at 12% weight loading, since islands of filler particles were formed.

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Chapter 6

6 Properties of NTE/PC and PTE/PC composite films

6.1 Introduction

Polycarbonate (PC) composite films with ZrW2O8 and ZrW2O7(OH)22H2O as filler particles were successfully prepared at weight loadings of about 2%, 5%, 8%, 12% and 25% using reprecipitation blending (Figure 5-6 and Figure 5-7). In this chapter, the interactions between PC and particles with and without surface modification were examined, and the effect of filler particles on various properties like glass transition temperature, thermal degradation, tensile strength and linear thermal expansion coefficient were determined.

6.1.1 Interaction at the interface between fillers and polymer

As discussed in chapter 4, surface modification of the filler particles was carried out to improve the interaction between the particles and PC. To reveal the true effect of the surface modification on the interaction between the two phases, composites with raw

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and modified particles were examined using SEM and combustion analysis.

SEM images of the composites allow visualization of the filler particles embedded in the PC matrix. Tight adhesion between the particles and PC is expected when the interaction between them is good, while gaps between particles and polymer are expected for poor interactions. To visualize the filler particles in the matrix, this test was carried out on the cross-section of the film samples. The cross-section of the films can be exposed by cutting with a knife, or after breaking a film with a tensile tester. The examination after tensile tests is most suitable to demonstrate how strong the adhesion between the particles and polymer matrix is, as the tensile force will separate particles that do not strongly interact with the polymer matrix.

Unfortunately, SEM images can only focus on a very small area containing a few particles, and can only detect particles at the surface of the examined specimen. To address the large scale, three-dimensional homogeneity of the films, combustion analysis is utilized to exam the dispersion of the filler particles. The polymer component is less thermally stable than the filler particles, so the matrix can be burned off at elevated temperature while the filler particles are left behind. Homogeneous dispersion of filler particles will result in similar leftover weight percentages for all areas of the film. To calculate the weight percentage of precursor particles in the film, the remaining weight percentage was collected for weight loss due to dehydration of the precursor particles during combustion.

6.1.2 Thermal properties

The effect of filler particles on glass transition temperature and thermal

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degradation was studied. Polymer chains exhibit a sudden change in mobility at the glass transition temperature, and transition from glassy state to rubbery state. In the glassy state, the polymer is a solid, and the molecular chains do not possess much mobility. In contrast, in the rubbery state, the material becomes soft, and the chains are mobilized to a high degree. It is expected that PC blended with filler particles should possess less chain mobility than the pristine PC, resulting in increased Tg values. Interactions between polymer chains and filler particles might also affect how the molecular chains degrade at elevated temperatures. Therefore, it is worth examining these properties as a function of weight loading and type of filler particles to understand how these filler particles interact with PC.

6.1.3 Tensile properties

The mechanical properties measured in this project include Youngʹs modulus tensile strength and strain. Youngʹs modulus measures the stiffness of the material.

Tensile strength and strain measures the stress and strain the material experiences at the breaking point. With the presence of additional filler particles in the PC matrix, the bulk mechanical properties of the composite films are expected to change compared to pristine

PC. The magnitude of the change depends on the inherent properties of the filler and polymer, as well as the interaction at the interface between them.

6.1.4 Coefficient of thermal expansion

The ultimate goal of this project requires measurement of the linear CTE of composites with two different types of filler particles. The effect of NTE can be extracted

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from the difference in the CTE of the two composites, since the only difference between them is the opposite CTE that the two filler particles exhibit.

6.1.5 Re-casting of composite films

Considering the amount of the material needed for all kinds of tests, the size of the film samples was too small to carry out all tests. Therefore, a re-casting process was inevitable during the property tests. The films were dissolved in dichloromethane again, followed by direct film casting. The films were initially cut for the expansion measurements, and then re-cast for other tests.

To prove that the weight loading of the films remains unchanged after re-casting, combustion analysis on several re-cast films was carried out by recording the initial and final weight during thermal degradation. The loadings can thus be determined and compared to the original values.

6.2 Characterization

6.2.1 Determination of crystallinity

Portions of the films were cut out, and mounted on a glass slide with a very small amount of grease on the surface. The grease was necessary to keep most films flat on the glass slide. If the sample stays flat on the slide without the grease, the grease should be avoided, since it gives a hump around 10 2, which interferes with peak fitting during analysis.

6.2.2 Homogeneity of composite films

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Five small pieces were cut from each examined film sample, one from the center and four from the edges in four directions that are about 90° to each other. The pieces were weighed on a PerkinElmer AD-4 autobalance and placed in platinum pans designed for TGA experiments. These pans were placed in ceramic crucibles and heated to 650 C in a furnace. The residual pan contents were weighed on the autobalance again to determine the filler weight loading of the sample. For the PTE composites, weights were corrected for the loss of three water molecules per formula unit during combustion.

According to the chemical formula, the hydrate water has a weight fraction of 8.4%.

6.2.3 Tensile properties

The tensile properties of the film samples were tested using the method outlined in section 2.7.

6.2.4 Interface morphology

SEM imaging was used to evaluate the interface between filler particles and polymer matrix. Film samples were immersed in liquid nitrogen for about half a minute, followed by cutting off small pieces with a knife. These pieces were then mounted vertically onto carbon tape with the cross-section facing up. A gold coating was applied to the samples using a gold sputter coater.

After the samples were fractured during tensile tests, the fractured cross-section of the films was examined by SEM after coating with gold.

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6.2.5 Thermal degradation

The thermal stability of the composite films was tested using a TA Instruments

SDT-2960 thermogravimetric analyzer. Films were cut into small pieces that weighed about 2 mg, and placed into a platinum pan. The samples were heated to 650 C at 10

C/min under an air flow of 50 ml/min.

6.2.6 Glass transition temperature

The glass transition temperature of the composites was measured using both an

Anton Paar MRC 302 rheometer and a Pyris 1 differential scanning calorimeter. Details of the procedures were described in section 2.6.1 and 2.5.

6.2.7 Coefficient of thermal expansion

The CTE of the composite films was measured using an Anton Paar MRC 302 rheometer placed in a heating oven. Samples of pristine PC, NTE2 and PTE1 were also tested using a TA Instruments 2940 thermomechanical analyzer at West Kentucky University. Sample preparation and parameters used for the tests were described in section 2.8.

6.3 Results and discussion

6.3.1 Effect of surface modification on interface and dispersion

Composites prepared by direct blending with modified and unmodified particles were examined, so that the improvement in the interaction between the two phases due to surface modification could be demonstrated.

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Figure 6-1. SEM images of composites prepared by direct blending with (a) raw NTE particles, (b) raw precursor particles, (c) modified NTE particles, and (d) modified precursor particles.

As shown in Figure 6-1, gaps were observed between both types of unmodified particles and the PC matrix, indicating insufficient interaction between them. After treating with in-situ surface modification, the interface between them became continuous, indicating that the interaction at the interface was improved.

Due to the limitation in the scale of SEM images, this examination cannot cover a large area. Therefore, combustion tests were carried out on these samples to determine the average weight loading. By comparing values for several film pieces, the homogeneity of particle dispersion can be evaluated.

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Table 6.1 Amount of filler particles in composite films.

Weight% 1 2 3 4 5 6 Average ESD Targeted

Filler

NTE 3.2 7.7 5.9 4.8 3.1 4.5 4.9 1.7 5.6

Modified NTE 4.8 5.9 5.4 4.3 4.8 4.6 5.0 0.6 5.2

PTE 2.2 3.4 4.7 4.8 4.0 4.6 3.9 1.0 4.9

Modified PTE 4.9 4.8 4.8 4.9 4.8 4.9 4.9 0.1 5.3

As shown in Table 6.1, the weight loading of the composites with unmodified and modified particles was determined. The standard deviation was calculated based on values obtained for several film pieces. Lower ESD values indicated more homogenous particle dispersion. It is clearly evident that the surface modification of the particles improved their dispersion in the PC matrix. Both types of composites showed a reduction in ESD by about 1% from 1.7% to 0.6% and from 1.0% to 0.1%, for PTE and NTE particles, respectively. The results obtained by combustion analysis and the interface images are strong evidence that surface modification improves the interaction between the particles and polymer.

6.3.2 Properties of composite films

The surface coverages of the modified particles used for preparation of composites are shown in Table 6.2. The amounts of bonded oligomers ranged from 1.9 wt% to 2.4 wt% for the NTE particles, while coverages of 2.2 wt% to 2.9 wt% were observed for the precursor particles. The composites were named with short codes for

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convenience, as described in section 5.3.3.

Table 6.2 Surface coverage of modified particles used for composite preparation.

Filler particles Film samples Surface coverage (wt%)

XGF.69.raw.1.2nd PTE1, PTE2 2.2 ± 0.3

XGF.71.raw.1.3rd PTE3 2.9*

XGF.73.raw.1.2nd PTE4, PTE5 2.5 ± 0.4

XGF.69.645a.1.2nd NTE1 2.4 ± 0.3

XGF.71.645a.4.2nd NTE2 2.0 ± 0.4

XGF.73.645a.1.2nd NTE3, NTE4, NTE5 1.9 ± 0.2

*No error could be determined as sample was completely used up

6.3.2.1 Crystallization of polycarbonate

Crystalline PC films usually show white or grey discoloration, often show a rough surface, or feel more rigid than amorphous, clear PC films. The crystallinity of any part of the films of interest can be examined using PXRD. In contrast to well crystallized inorganic materials, polymers usually only show broad humps in PXRD patterns, corresponding to the short-range order within their structures. Even peaks corresponding to crystalline polymers are often broad.

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(i)

(h)

(g)

(f)

(e)

(d)

Intensity (arbitary units) (arbitary Intensity (c)

(b)

(a)

10 15 20 25 30 2 (degrees)

Figure 6-2. PXRD patterns of (a) ZrW2O8, (b) vacuum grease, (c) clear PC, (d) discolored PC, and (e) to (i) NTE1 to NTE5.

(g)

(f)

(e)

(d)

(c)

Intensity (arbitary units) Intensity (arbitary (b)

(a)

10 15 20 25 30 2 (degrees)

Figure 6-3. PXRD patterns of (a) ZrW2O7(OH)22H2O, (b) clear PC, and (c) to (g) PTE1 to PTE5.

Clear PC films show a hump from about 15 to 20 2 in PXRD patterns (Figure

6-2 (c)). When the films become more crystalline, this hump becomes narrower,

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suggesting that the long range order of the chains increases (Figure 6-2 (d)). The first broad feature in Figure 6-2 (d) that occurs at a slightly lower angle than the PC feature is due to scattering from the grease beneath the films (Figure 6-2 (b)). The humps from amorphous composite films should be identical after proper scaling of the diffraction intensity and any minor offsets in diffraction angle due to sample height variations.

Different film thickness and cross sections within the x-ray beam can result in different intensity, and non-planar film samples give slight shifts in diffraction angles. The stacked

PXRD patterns for all PTE composites and filler particles are shown in Figure 6-3. The diffraction intensity from the filler particles increases with increased weight loading. All composite patterns showed very similar humps to the clear PC film in terms of peak width and intensity, indicating that the PC in all composite films is amorphous. However, some NTE composite films show slight peak tails at higher angles, suggesting that a small amount of the filler particles can still hydrate inside the films. Thus, all property measurements were carried out on freshly dried films.

6.3.2.2 Homogeneity of composite films

The properties of the composite films depend on the inherent properties of the particles and polymer, as well as the interaction between them. Changes in weight loading of the filler particles cause changes in the interface region volume, which is expected to affect the properties of the composite. A composite film with homogenous dispersion of filler particles delivers uniform properties throughout the sample. Good homogeneity of the dispersion also suggests good interaction between the two phases, so that efficient property transfer at the interface is expected as well.

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Combustion tests were carried out on different film pieces that were cut off from different areas of the composite samples. PC completely degrades in air at 600 C, whereas the inorganic particles are still present, allowing determination of the true weight loading from the initial and final masses of the sample.

Table 6.3 True weight loading of the composites obtained from homogeneity tests.

Samples Target Spot Spot Spot Spot Spot Ave ESD Vol%

wt% 1 2 3 4 5

PTE1 2.4 1.5 1.1 0.9 1.3 0.8 1.1 0.3 0.2

PTE2 5.2 4.2 4.4 4.1 5.5 5.4 4.7 0.3 1.1

PTE3 8.2 6.8 6.4 6.8 6.0 6.5 0.4 1.5

PTE4 12.1 11.6 10.6 9.8 10.8 11.3 10.8 0.7 2.6

PTE5 26.6 26.3 25.7 27.0 26.4 27.0 26.5 0.5 7.3

NTE1 2.2 1.5 2 1.8 1.9 1.4 1.7 0.3 0.4

NTE2 5.5 4.7 5.1 5.1 5.3 5.9 5.2 0.4 1.2

NTE3 8.4 7.3 7.5 7.9 8.3 7.0 7.6 0.5 1.7

NTE4 12.4 9.9 10.7 12.3 11.7 11.9 11.3 1.0 2.7

NTE5 24.7 23.9 24.0 23.7 24.8 22.2 23.7 0.9 6.4

Table 6.3 summarizes the results of TGA tests on the composite films. A standard deviation for each sample was calculated based on the five tests on each film. This number offers insights into how different the weight loadings of the filler particles throughout the film are. All composite samples with loadings up to 8 wt% showed similar

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variances in filler particle content, with standard deviations of 0.5% or less. This indicated that excellent dispersion of the filler particles can be achieved using surface modified particles and reprecipitation blending. The films with loadings above 10 wt% showed slightly worse homogeneity than the other films. This result is not surprising, as agglomeration of filler particles had already been detected by optical inspection. The weight loading could still be estimated reasonably well, as the pieces cut from the film are large enough to contain considerable numbers of aggregates. Lower values for real weight loading compared to the targeted loading were observed for most film samples, indicating that some loss of particles occurred during processing. It is likely that some losses occurred during the filtration step that was used to collect the precipitated composites after DMAc treatment. Any nanoparticles that were not well wrapped by the polymer could be easily lost during filtration and solution transfer.

6.3.2.3 Thermal degradation

The thermal degradation of PC and composite films was studied as a function of weight loading of filler particles and type of filler particles using TGA. The resulting curves for the composites with both types of particles are shown in Figure 6-4.

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PTE

(a) 100

Weight% 40

pristine PC 20 PTE1 PTE2 PTE3 PTE4 0 PTE5

200 300 400 500 600 Temperature (°C)

NTE

(b) 100

Weight% 40

20 prinstine PC NTE1 NTE2 NTE3 0 NTE4 NTE5

200 300 400 500 600 Temperature (°C)

Figure 6-4. TGA curves of (a) PTE composites, and (b) NTE composites.

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Table 6.4 Initial and final weight of composites.

Onset T of Initial weight Final weight Weight degradation (mg) (mg) loading% (C) Pristine 430 2.511 0 0 PTE1 450 2.435 0.042 1.7 PTE2 465 2.665 0.113 4.2 PTE3 450 2.460 0.172 7.0 PTE4 445 2.361 0.271 11.5 PTE5 395 2.188 0.586 26.7 NTE1 450 2.576 0.045 1.7 NTE2 450 1.995 0.119 6.0 NTE3 450 2.527 0.216 8.5 NTE4 425 2.596 0.302 11.6 NTE5 400 2.869 0.690 24.1

As shown in Table 6.4, the initial and final weights of each sample were determined on a microbalance to calculate the real weight loadings, as the traces reproducibly returned physically meaningless numbers (e.g., negative residual weights) due to instrument errors.

Comparison of the determined weight loadings to those in section 6.3.2.2 showed that the weight loading of the re-cast films was very similar to that of the original samples. Therefore, the effect of re-casting on weight loading is negligible.

Pristine PC showed a two-step thermal degradation in air atmosphere. According to literature, the first step is associated with degradation caused by decarboxylation of the carbonate, and chain scission of the isopropylidene group. The resulting compounds from this decomposition then form large or branched aromatic compounds through radical reactions with each other, which are decomposed at higher temperatures, corresponding

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to the second step in the curve.66-68 The curve obtained for pristine PC showed the onset of degradation at about 420 C. An increase in the onset temperature to 440 °C was seen for PTE1, indicating slightly enhanced thermal stability of this composite. This effect

69 70 70 was also observed in silica/PC, -Fe2O3 and CuO composites in other research projects. Similar temperatures were observed for PTE3 and PTE4, while PTE2 showed an additional shift to higher temperature (about 450 C). The enhanced thermal stability of this composite could arise from the interaction between the particles and PC, which delays the breakdown of the molecular linkages. In contrast, PTE5 showed a decreased onset temperature (~395 C). Both PTE4 and PTE5 composites showed particle agglomeration, however, only PTE5 showed lower thermal stability than pristine PC.

This might suggest that the local environment of the particles is different due to different dispersion for these two samples. The tensile properties shown in section 6.3.2.4 showed the same trend, where only PTE5 exhibited lower tensile strength than pristine PC, whereas PTE4 did not.

The NTE composites behaved in a similar fashion as the ZrW2O7(OH)22H2O composites. NTE1, NTE2 and NTE3 exhibited similar thermal stability, with a decomposition onset temperature of about 450 C. In contrast, NTE4 and NTE5 showed

TGA curves shifted to lower onset temperatures than even pristine PC. Both of these samples contained agglomerated particles, and showed lower tensile strength than pristine PC (section 6.3.2.4).

Comparing the results from both types of composites, films with up to 8 wt% particle loading showed similar enhanced thermal stability due to the surface interaction between the particles and matrix, indicating that the presence of filler particles affects the

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polymer matrix. In contrast, for composites with agglomerated filler particles (PTE5,

NTE4 and NTE5), a decrease in thermal stability was observed. This suggested that the interaction at the interface may be affected by particle agglomeration, which could lead to different stiffening effects on the PC matrix. Therefore, these samples may also show deviations from expected behavior during other property measurements.

6.3.2.4 Tensile properties

The tensile properties of the composite films including stress at yield, strain percentage at yield and Young’s modulus are summarized in

Table 6.5. It was noticed that the composites became very brittle at the highest filler loading, resulting in breakage of several films before yield. In this case, the stress and strain percentage at break was used.

Table 6.5 Tensile properties of composite films.

Samples Stress at yield (MPa) Strain% at yield Young’s modulus (MPa)

Pristine PC 57 ± 3 8.5 ± 0.6 940 ± 50 PTE1 62 ± 4 8.6 ± 0.9 960 ± 90 PTE2 59 ± 2 8.2 ± 0.3 970 ± 50 PTE3 59 ± 5 8.6 ± 0.2 970 ± 100 PTE4 62 ± 7 8.1 ± 0.5 1000 ± 260 PTE5 53 ± 1 7.4 ± 0.6 1136 ± 168

NTE1 66 ± 5 9.3 ± 0.1 920 ± 80 NTE2 63 ± 4 9.0 ± 0.8 960 ± 140 NTE3 56 ± 2 8.1 ± 0.2 940 ± 40 NTE4 52 ± 6 7.0 ± 0.9 990 ± 80 NTE5 53 ± 2 7.1 ± 0.1 1071 ± 85

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The pristine PC gave values of 57 ± 3 MPa, 8.5 ± 0.6 and 940 ± 50 MPa for stress at yield strain percentage at yield and Young’s modulus respectively. Composites TE1 through TE4 showed very similar Young’s modulus and strain at yield values as pristine

PC, while the stress at yield was slightly higher (5% - 10%) than for pristine PC. No differences were observed between these four composites within error, and no systematic changes in properties with filler loading were observed. The difference started to be more distinguishable for PTE5, which exhibited a slightly larger Young’s modulus than all other composites, while the stress and strain at yield were lower than for pristine PC.

Young’s modulus for composites NTE1 through NTE4 was comparable to TE composites and pristine PC. However, the NTE composites showed clear trends in stress and strain with filler loading. NTE1 and NTE2 showed increases in stress at yield by about 15% and 10% compared to pristine PC. With increased weight loading, the stress at yield decreased, and NTE3 showed the same stress at yield as pristine PC, while NTE4 and NTE5 gave a lower stress at yield than pristine PC. Similarly, the strain values for

NTE1 and NTE2 were higher than for pure PC, but decreased to values lower than PC for

NTE3, NTE4 and NTE5. The reduction in the stress and strain for NTE4 and NTE5 were very similar, with values close to those of PTE5. It should be noted that the addition of filler particles was not expected to significantly improve the tensile properties of PC due to the small size of the particles, which cannot stop any crack propagation within the material.

6.3.2.5 Adhesion at the fractured interface

The fracture surface created during the tensile tests was studied by SEM, which

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allows close examination of the interface between the particles and PC. SEM images of the fracture surface of PTE3 and NTE3 are shown in Figure 6-5. Both types of filler particles showed close interaction with the polymer matrix around them. Even after the films were fractured mechanically by a stretching force, no gaps were observed at the interface between the two phases. This is strong evidence that excellent interaction existed between the particles and the PC matrix. Along with the results obtained from

TGA and DMA experiments, this clearly demonstrates the excellent properties of composites prepared with surface modified particles and reprecipitation blending.

At the highest particle loadings (around 25 wt%), the distribution of the particles became less optimal. Especially in NTE5, some agglomeration can be easily observed in

SEM images (Figure 6-5 (c) and (d)). Most of the particles were still embedded in the matrix, while some were separated from the matrix and located on top of the fracture surface after film fracture. The images for PTE5 and NTE5 agree well with results from combustion analysis, which suggested that PTE5 has more homogeneous particle dispersion than NTE5 based on the lower ESD.

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Figure 6-5. SEM images of the fracture surface of composite films (a) NTE3, (b)

PTE3, (c) PTE5 and (d) NTE5.

6.3.2.6 Glass transition temperature

The glass transition temperatures of the composite films were determined by two different techniques, namely from the dissipation factor (tan ) as a function of temperature obtained from DMA measurements, and the heat flow curve obtained from

DSC experiments. The values for all samples are listed in Table 6.6.

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Table 6.6 Glass transition temperatures of PC and composite films.

Peak of Tan  (C) DSC (C) Pristine 152 142 PTE1 151 142 PTE2 150 143 PTE3 150 142 NTE1 154 143 NTE2 153 143 NTE3 152 142

Pristine PC showed a peak at 152 C in the tan  plots, and slight decreases of the peak position were observed for PTE composites. The peaks were recorded at 151 C,

150 C and 150 C for PTE1 to PTE3, respectively. On the other hand, for NTE composites, the peaks in the tan  curves occurred at 154 C, 153 C and 152 C for

NTE1 to NTE3, respectively. The samples that showed agglomeration of filler particles were excluded from this discussion due to the particle agglomeration that was optically observed. The difference between the peak positions of all samples was not significant, indicating that Tg is not affected by small quantities of filler particles. However, a more distinctive difference was found in the onset of the peaks, which represents the local environment within the composites (Figure 6-6). The tan  curves obtained for PTE composites exhibited earlier onsets than pristine PC, whereas the curves for NTE composites showed a delayed onset. The different effect of the two types of fillers must be related to the opposite CTE of the particles. The PTE precursor particles behave as a plasticizer in the polymer matrix, resulting in separation of polymer chains and thus

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increases in free volume between chains. This additional free volume allows for easier movement of polymer chains under an applied force at elevated temperatures. Because of this, the polymer chains start to show viscous behavior at slightly lower temperatures than pristine PC. This effect was also observed in PC/ZnO and PC/ZnO/Ionic liquid composites by others.71 In contrast, when the filler particles exhibit negative thermal expansion, the particle shrinkage upon heating causes an additional stress on the matrix and constrains the motion of the chains against the external force, resulting in delayed emergence of viscous behavior. This observation indicates excellent interaction between

ZrW2O8 and the PC matrix, since the negative expansion from the particles affects the behavior of the matrix.

The glass transition temperature was also determined using DSC. Pristine PC showed a Tg of 142 C. All composite samples gave very similar Tg values of 142 C to

143 C, suggesting that the presence of the two types of filler particles hardly affects the chain mobility of the polymer upon heating. These results agree with the trends observed for the peak positions of the tan  curves, which suggest that the addition of the filler particles does not cause changes in the glass transition temperature of the bulk composites.

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1.5 (a) prisitne PC PTE-1 PTE-2 PTE-3

tan deltatan

0.5

0 130 135 140 145 150 155 160 Temperature (°C)

1.5 (b) prisitne PC NTE-1 NTE-2 NTE-3

tan delta tan

0.5

0 130 135 140 145 150 155 160 Temperature (°C)

Figure 6-6. Overlaid tan  curves of (a) PTE composites, and (b) NTE composites.

6.3.3 Thermal expansion coefficient of the composites

6.3.3.1 Effect of ZrW2O8

The CTEs of the two types of composites were determined using a rheometer

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located at University of Mulhouse in France. A summary of the obtained CTE values for all film samples is shown in Table 6.8. Due to the constrained timeline and limited access to the instrument, PTE5 and NTE5 have not been tested. Additionally, high CTE values above 200 ppm/K were obtained for all samples, which were much larger than the accepted value for pristine PC (about 70 ppm/K). To check the validity of the data, a metal strip of aluminum (cut from a DSC pan) was also tested on the same machine. CTE values in excess of 100 ppm/K were observed for these samples, which were also dramatically higher than the accepted value for aluminum (about 24.7 ppm/K).72

Unfortunately, no certified standard materials were available for this instrument, making direct correction of CTE values impossible. Additionally, it was also found that the resulting CTE is affected by the grip distance. To account for this variable, the metal strip was measured at different grip distances. As show in Table 6.8, the CTE values were inversely proportional to the grip distance, suggesting an unaccounted contribution by the instrument setup itself. The contribution of the instrument to the CTE values was estimated by subtracting the change caused by the metal sample from the total change. A linear trend line was then fit through these data, which showed the instrument contribution as a function as grip distance. The equation of this line was used to correct the corresponding values for all composites for the instrument contribution. After adjusting all data for each film, averages and standard deviations were determined (Table

6.7).

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Table 6.7 Raw CTE values for PC and composite films obtained in France.

Sample Grip CTE Corrected Average STD distance (ppm/K) CTE (mm) (ppm/K) 16* 202 82.4** 16* 207 87.4 15 219 89.7 PC 88.8 1.1 15 229 99.7** 15 219 89.7 16 208 88.4 15 225 95.4 15 221 91.4 PTE1 93.3 1.9 15 224 94.4 16 212 92.0 16 209 89.0 16 200 80.0 PTE2 85.8 4.0 16 208 88.0 16 206 86.0 15 224 94.4 PTE3 15 219 89.4 90.0 4.2 16 206 86.0 15 220 90.4 15 212 82.4 PTE4 85.6 3.7 15 216 86.4 16 203 83.0 13 249 95.8 14 224 83.4 NTE1 87.4 7.4 15 209 79.4 16 211 91.0 15 216 86.4 15 216 86.4 NTE2 87.2 3.0 15 214 84.4 15 221 91.4 12 263 95.0** NTE3 14 226 85.4 85.9 0.7 15 216 86.4 14 226 85.4 14 224 83.4 NTE4 85.2 3.1 15 219 89.4 15 212 82.4 *Samples were measured at a rate of 5 C/min. **Values were ruled out based on Iglewicz’s outlier test.

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Table 6.8 CTE values measured for an Al metal strip at different grip distances.

Grip Measured CTE Change of Change of Change of GD distance (ppm/K) total GD GD from Al from grip (mm) (10-6 mm) (10-6 mm) (10-6 mm)

12.1 191.0 2311 298.9 2012 N/A 13.0 177.5 2308 321.1 1986 14.0 166.1 166.5 2328 345.8 1982 15.0 154.5 154.6 2318 370.5 1948 16.0 144.3 143.6 2304 395.2 1909

Before devising the correction method described above, standardization of CTEs was attempted by mailing two composite films (PTE1 and NTE2) and a pristine PC film for testing on an instrument located in the Thermal Analysis Lab at Western Kentucky

University. These samples were tested on a well-calibrated instrument with an original goal of obtaining a correction factor that would allow use of the comprehensive data collected in France. After devising the method for correcting the data collected in France, the values from WKU were used to confirm whether the correction method was acceptable. The CTEs of these samples measured in France were 88.8 ± 1.1 ppm/K, 93.3

± 1.9 ppm/K and 87.2 ± 3.0 ppm/K for pristine PC, PTE1 and NTE2, respectively. The

CTEs of the same samples tested at WKU were 83.0 ± 4.1 ppm/K, 87.0 ± 4.7 ppm/K and

98.5 ± 12.4 ppm/K, respectively (Table 6.9). Surprisingly, NTE2 showed much higher expansion than the pristine PC. However, the individual results of the triplicate measurements varied significantly (Table 6.9), suggesting that the data are not reliable. A possibly reason for this could be that the NTE particles within the composites had hydrated during mailing or handling of samples at WKU, which could explain the disparate results for different parts of the same sample. This is corroborated by the

138

observation of the hydration tails in the PXRD patterns of the NTE composites in Figure

6-2. On the other hand, pristine PC and PTE1 gave similar values to the corrected

Mulhouse data, indicating that the correction applied to the original data was usable.

Table 6.9 CTE values of selected film samples tested at WKU.

Sample Grip CTE Average ESD distance (ppm/K) (mm) 16 78.3 PC 16 85.2 83.0 4.1 16 85.6 16 88.2 PTE1 16 81.9 87.0 4.7 16 91.0 16 84.5 NTE2 16 103 98.5 12.4 8 108

The final CTE data of all films is plotted in Figure 6-8. At volume loadings of 0.2 vol%, 1.1 vol%, 1.5 vol% and 2.6 vol%, the PTE composite films showed CTE values of

93.3 ± 1.9 ppm/K-1, 85.8 ± 4.0 ppm/K-1, 90.0 ± 4.2 ppm/K-1 and 85.6 ± 3.7 ppm/K-1, respectively. On the other hand, the NTE films showed values of 87.4 ± 7.4 ppm/K-1,

87.2 ± 3.0 ppm/K-1, 85.9 ± 0.7 ppm/K-1 and 85.2 ± 3.1 ppm/K-1 NTE1 to NTE4 at volume loadings of 0.4, 1.2, 1.7 and 2.7%, respectively. The large error for the CTE value of

NTE1 was surprising, and prompted additional investigation of this sample. SEM imaging revealed that this sample exhibited an altered morphology of the NTE particles

(Figure 6-7).

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Figure 6-7. SEM images for filler particles used for PTE1 and NTE1, (a) precursor particles, and (b) NTE particles.

For unexplained reasons, it was found that large aggregates were formed during the phase transition. Unfortunately, the particles were not imaged by SEM until after composite preparation and measurements had been carried out, as it was prepared under identical conditions to many other samples used in composite preparation, all of which showed identical particle morphologies. The large difference in size and shape between the small particles and aggregates could result in less homogeneous films and thus larger errors than for more homogeneous samples.

Composite PTE1 reproducibly showed much higher CTE values than pristine PC and other PTE samples, indicating that effects other than restriction of chain motion or additive expansion behavior must dominate. It has been shown for ZnO/PC and

ZnO/PC/ionic liquid composites that incorporation of filler particles can result in further separation of the polymer chains, resulting in additional space for chain movement upon heating, and thus higher glass transition temperatures.71 DMA measurements carried out in this work suggest that similar behavior may be occurring for the PTE composites, which can cause higher CTE values of the composites at low loadings, since thermal

140

expansion of polymers is strongly correlated to the chain motion.

Figure 6-8. CTE values of pristine PC (), PTE () and NTE () composites.

Table 6.10 Linear fit results for the CTE values of the composites.

Slope Intercept R2 Fit for NTE series with -1.45 ± 0.14 88.8 0.88 constraint Fit for NTE series excluding -1.38 ± 0.20 88.7 ± 0.33 0.96 NTE1 without constraint Fit for PTE series with -0.87 ± 0.90 88.8 0.18 constraint Fit for PTE series excluding -0.96 ± 1.28 88.8 ± 2.04 0.22 PTE1 without constraint

The CTE data of both sets of composites were described by linear trend lines that

141

were constrained to include the data point for pristine PC (Figure 6-8). Only PTE1 shows strong plasticizer behavior of the filler, and the intercept obtained for a trendline of PC and PTE2 through PTE4 has a very similar intercept to this value even without restrictions, suggesting that the intercept constraint is a reasonable choice (Table 6.10).

The NTE samples showed a relatively clean, linear trend for the reduction of CTE as a function of particle loading despite the large error bars on several data points. In contrast, the values for the PTE samples scattered significantly around the line, and statistical analysis of the data clearly illustrated that the data are not sufficient to draw clear conclusions, as the error on the slope of the trendline is larger than the slope itself. In addition, the CTEs of PTE and NTE composites with similar weight loadings are identical within error. Additional data points at higher weight loadings, and repeated measurements on well calibrated instruments that result in lower errors, would be useful to more clearly establish trends for both sets of particles. As such data were not available, only qualitative conclusions can be drawn. If the trendlines are used as guides, both types of composites show slightly lower CTE values than predicted by the rule of mixtures

(RoM), although the error bars for most composites include the values predicted by the

RoM model. This rule assumes that no interaction exists between filler and polymer phases. While it was clear before the start of this thesis project that the rule of mixtures does not apply to the overall behavior of polymer composites due to chain stiffening effects of filler particles, the contribution of the filler particle CTE itself was not understood. The NTE behavior may cause a slight additional effect on the polymer matrix, but better data will be necessary to address this. Effect of agglomeration of filler particles on the CTE of composites

142

In addition to the reason stated above, measurements of the CTE of the samples at higher loadings would be very helpful to obtain a more thorough understanding on how the NTE behavior affects the CTE of the composites when agglomeration of the particles increases. Agglomeration of the particles in the matrix was observed for samples with loadings above 2 vol%. The resulting particle-particle interactions in these composites most likely reduce the interface area between filler and polymer. Therefore, the magnitude of interactions between the two phases, especially the effect of NTE on the polymer, might be affected as well. It is unclear whether the CTEs of composites at higher loadings would follow the trend lines created in Figure 6-8. Agglomeration was observed to affect the thermal stability and tensile properties of PTE5 and NTE5, and lower stress and strain at yield were found for PTE5, NTE4 and NTE5.

6.4 Conclusions

Composites with two types of filler particles were successfully prepared. Surface modification enhanced the interaction between the particles and the PC matrix as shown by SEM and TGA. No gaps were observed between the modified particles and polymer, and better dispersion of the modified particles was achieved as shown by the combustion test. Most composites showed higher onset temperatures for decomposition than pristine

PC.

Various properties of PTE and NTE composites were tested, including homogeneity of particle dispersion, thermal degradation, tensile properties, morphology of fractured surface, glass transition temperature and coefficient of thermal expansion.

All composites showed only small variations in the weight loading at different areas

143

across the samples in the TGA tests, indicating good homogeneity of particle dispersion.

All composites except NTE4, NTE5 and PTE5 showed enhanced thermal stability due to protective effects from interface interactions. NTE4 showed similar stability as pristine

C while NTE5 and TE5 exhibited lower thermal stability. The Young’s modulus values of all composites were comparable to that of pristine PC within error. Tensile strength and strain at yield for PTE1 through PTE4 showed similar values as pristine PC, while a decreasing trend was observed for samples NTE1 through NTE4 with loading.

The curves leveled off at higher loadings (PTE5 and NTE5). The stress and strain values for PTE5 and NTE5 were found to be similar to NTE4, giving distinctly lower values than pristine PC. The effect of particle agglomeration on the tensile properties of PTE composites emerged at slightly higher loadings than for NTE samples. SEM images of the cross-section of the film samples fractured during tensile tests confirmed that the particles and polymer showed excellent adhesion. Small differences in the averaged CTE were observed for the two types of composites, and data may indicate that NTE particles exhibit an additional effect on the polymer due to the larger CTE mismatch. However, considering the large errors associated with each value, this difference is not quantifiable for the currently available data, and it is unclear whether a significant additional reduction can be caused by the NTE behavior of ZrW2O8. In addition, the agglomeration of filler particles at the highest loadings might diminish the reduction of the composite

CTE due to NTE behavior of the filler particles. Therefore, composites with higher loadings of both fillers need to be tested to obtain clearer trends, and CTE values with lower error bars are needed for the samples measured to date.

If the averaged trend was considered, the difference in the CTE of the two

144

composites was very close to that between fumed silica/BECy composites loaded with 12 nm and 40 nm sized particles, respectively, indicating that optimization of particle morphology may result in similar improvements.47 In theory, the effects of particle size/morphology optimization and NTE can be combined, however, for ZrW2O8, the autohydration observed for small nanoparticles imposes restrictions to particle size optimization. Therefore, the use of ZrW2O8 should be carefully considered in terms of cost and effect, as other particles with smaller size and low positive CTE values may be able to achieve similar results. If extreme reductions in CTE are necessary, use of a different NTE filler that does not suffer from size/morphology limitations may be promising.

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Chapter 7

7 Summary and future work

This project was designed to distinguish two effects from filler particles that reduce the CTE of polymer composites, namely chain stiffening due to particle morphology and stress caused by NTE of filler particles. The preparation of PC composites was attempted using two isomorphic filler particles with opposite α

ZrW2O7(OH)22H2O and cubic ZrW2O8. As a result, no difference existed in the effect from chain stiffening for these two particles. Thus any difference in the CTE of the composites was attributed to the difference in the α values of the particles.

The two filler particles were synthesized using a hydrothermal synthesis method.

The synthetic conditions were mostly explored by previous group members. Some fine tuning of conditions was carried out, including acid concentration and reaction time.

Kinetic studies of the hydration rate of NTE samples were also conducted in an attempt to find a balance between particle size and hydration rate. The NTE particles prepared from 4.8 M the acid showed very limited hydration, however, the particle size changed dramatically between precursor and NTE particles due to breaking apart of larger particles. ZrW2O8 synthesized under the chosen conditions (6 M acid/7 d/210 C) showed fairly small particle size and slow hydration rate.

146

To compare the CTE of the two composites the α value of ZrW2O7(OH)22H2O was determined by Rietveld refinement of PXRD data collected at various temperatures.

Additionally, the amorphous content of both types of samples was determined to be close negligible, allowing representation of the α values of the filler particles by the values determined for the crystalline phases.

Surface modification was successfully achieved via in-situ polymerization of PC in the presence of the particles. To achieve the highest coverage of PC oligomers on the surface, the synthetic conditions, including monomer ratio, particle amount and reaction time were optimized with the aid of IR spectroscopy and TGA. A two-step modification procedure was developed to improve the amount of modified particles recovered without sacrificing surface coverage.

PC composite films containing both types of particles were prepared using solution casting. Difficulties during the processing, including strong flow of the solution and slow rate of evaporation, were overcome by using a custom made glass vessel allowing for application of controlled argon flow and water vacuum. A reprecipitation blending method was used to enhance the interaction between the particles and PC by precipitating after heating in hot dimethylactamide. Five composites with different filler loadings were prepared for each type of particle. Agglomeration of particles was optically observed for both types of composites at loadings of 2.6 vol% and above.

Several properties of the composite films were tested, including homogeneity of particle dispersion, crystallinity, thermal degradation, glass transition temperature, tensile properties and coefficient of thermal expansion. The proposed route was found to result in composites with good homogeneity and completely amorphous PC. The tensile

147

properties of the composites were found to be close to those of pristine PC, except composites NTE4, NTE5 and PTE5 that showed slightly lower stress and strain at yield.

The fracture surface of the samples after tensile testing was studied by SEM, showing that the adhesion between the particles and polymer was excellent.

The CTE values of the composites was measured to determine the difference between the two types of composites. Linear trend lines were fitted for the averaged CTE values of both sets of composites. Slight differences were observed between the slopes of the lines. However, due to the large errors, the differences could not be quantified, and it is unclear how significant this additional reduction caused by the NTE behavior of

ZrW2O8 is. The expansion of the PTE5 and NTE5 composites needs to be measured to gain a better understand of how the NTE behavior affects the reduction of the CTE of the composites.

It would be also interesting to choose a different NTE filler that does not suffer from autohydration and therefore does not pose any limitations with respect to size and morphology optimization. This would allow comparison to the morphology effect observed in fumed silica/BECy composites loaded with 12 nm and 40 nm sized particles, respectively, which suggest that optimization of particle morphology may result in similar improvements as changes from PTE to NTE fillers. This may allow preparation of composites in cases where extreme reductions in CTE are necessary.

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157

Appendix A

Samples of ZrW2O7(OH)22H2O

Samples Vacid t T Acid type Comments XG.1.raw 4.4 mL 3 d 130 °C NaCl 2.98 g XG.2.raw 6.1 mL 23 h 130 °C NaCl 2.98 g XG.3.raw 6.1 mL 3 d 130 °C NaCl 2.98 g XG.4.raw 4.4 mL 1 d 130 °C NaCl 2.98 g XG.5.raw 6.1 mL 2 d 130 °C NaCl 2.98 g XG.6.raw 4.4 mL 2 d 130 °C NaCl 2.98 g XG.7.raw 6.1 mL 3 d 130 °C NaCl 2.98 g XG.8.raw 22 mL 1 d 130 °C NaCl 14.9 g XG.9.raw 4.4 mL 3 d 130 °C NaCl 2.99 g HClO / XG.10.raw 6.1 mL 3 d 130 °C 4 NaCl 2.92 g NaCl XG.11.raw 3.5 mL 3 d 130 °C NaCl 2.98 g XG.12.raw 5.3 mL 3 d 130 °C NaCl 2.98 g XG.13.raw 4.4 mL 3 d 170 °C NaCl 2.98 g XG.14.raw 6.1 mL 3d 170 °C NaCl 2.98 g XG.15.raw 4.4 mL 3 d 150 °C NaCl 2.98 g XG.16.raw 5.3 mL 3 d 150 °C NaCl 2.98 g XG.17.raw 3.5 mL 3 d 130 °C NaCl 2.98 g XG.18.raw 4.4 mL 3 d 130 °C NaCl 2.98 g XG.19.raw 4.4 mL 3 d 170 °C NaCl 2.98 g XG.20.raw 6 mL 1.5 h 220 °C XG.21.raw 6 mL 2 h 220 °C 0.7 ml 1-propanol XG.22.raw 6 mL 1.5 h 220 °C HCl/ XG.23.raw 30 mL 1.5 h 220 °C 1-propanol XG.24.raw 37.5 mL 1.5 h 220 °C (0.5 mL) Scaled up by a factor of 6 XG.25.raw 30 mL 4 h 220 °C XG.26.raw 30 mL 4 h 220 °C

158

Samples Vacid t T Acid type Comments XG.27.raw 30 mL 1 d 200 °C XG.28.raw 30 mL 14 h 200 °C XG.29.raw 30 mL 12 h 200 °C XG.30.raw 30 mL 1 d 200 °C XG.31.raw 6 mL 3 d 210 °C XG.32.raw 6 mL 7 h XG.33.raw 6 mL 1 d XG.34.raw 6 mL 3 h XG.35.raw 4 mL 3 h XG.36.raw 4 mL 12 h XG.37.raw 4 mL 1 d XG.38.raw 2 mL 20 h XG.39.raw 6 mL 3 d XG.40.raw 5 mL 3 d XG.41.raw 5 mL 7 d XG.42.raw 4 mL 3 d XG.43.raw 5 mL 3 d XG.44.raw 5 mL 7 d XG.45.raw 5 mL 14 d XG.46.raw 5 mL 7 d HCl/ XG.47.raw 6 mL 14 d 1-propanol XG.48.raw 6 mL 1 d (0.5 mL) XG.50.raw 25 mL 1 d 210 °C XG.51.raw 30 mL 4 d XG.52.raw 30 mL 9 d XG.53.raw 25 mL 3 d XG.54.raw 30 mL 5 d XG.55.raw 25 mL 5 d XG.56.raw 23 mL 5 d XG.57.raw 23 mL 3 d XG.58.raw 25 mL 4 d XG.59.raw 5 mL 1 h XG.60.raw 5 mL 16 min XG.61.raw 5 mL 1 h XG.62.raw 5 mL 45 min XG.63.raw 5 mL 45 min XG.64.raw 5 mL 30 min XG.65.raw 20 mL 14 d XG.66.raw 27.5 mL 14 d XG.67.raw 5.5 mL 2 d XG.68.raw 25 mL 7 d

159

Samples Vacid t T Acid type Comments XG.69.raw 25 mL 7 d XG.70.raw 25 mL 7 d XG.71.raw 25 mL 7 d XG.72.raw 20 mL 7 d XG.73.raw 25 mL 9 d HCl/ XG.74.raw 4 mL 5 d 210 °C 1-propanol XG.75.raw 4 mL 5 d (0.5 mL) XG.76.raw 5 mL 4 d XG.77.raw 5 mL 4 d XG.78.raw 4 mL 3 d XG.79.raw 5 mL 7 d

160

Appendix B

Samples of modified ZrW2O8 and ZrW2O7(OH)22H2O “raw” in the sample name indicates ZrW2O7(OH)22H2O, while all other samples correpond to NTE samples Bis- Tri- Particles t Samples phenol A phosgene Comments (g) (h) (g) (g) PMF.2.600.1 2.277 1.089 0.306 4 PMF.2.600.2 2.283 0.981 0.304 24 PMF.2.600.3 2.238 1.094 0.306 24 PMF.2.600.4 2.279 1.104 0.318 22 PMF.2.600.5 2.243 1.106 0.321 22 PMF.2.600.6 2.235 1.094 0.3 8 XGF.21a 2.282 1.287 0.318 XGF.20a 2.283 1.29 0.298 4 XGF.22a 2.289 1.285 0.316 4 No XGF.23a 2.279 1.289 4 record XGF.23b 2.284 1.082 0.244 4

XGF.23c 2.283 1.086 0.298 4 All particles were XGF.24a 2.284 1.091 0.389 4 heat-treated at XGF.24b 2.285 1.091 0.439 4 600 °C. XGF.25a 2.283 1.089 0.298 4 XGF.21a was not XGF.25b 2.286 1.086 0.306 4 recovered. XGF.25c 2.288 1.092 0.301 5 aThe monomers XGF.25d 2.283 1.083 0.641 7 were added after 24 XGF.25e 2.282 1.082 0.606 4 h. The triphosgene XGF.26.600a.5 1.528 1.09 0.307 24 was dissolved in 2 0.334a 0.233a 20 mL of XGF.26a.1 2.282 1.084 0.329 4 dichloromethane XGF.26a.2 2.237 1.091 0.308 4 XGF.26a.3 0.288 1.09 0.299 4 XGF.26a.4 2.286 1.093 0.301 4 XGF.29a.1 2.283 1.088 0.3 4 XGF.29a.2 2.281 1.098 0.3 4

161

Bis- Triphosg Particles t Samples phenol A Comments ene (g) (g) (h) (g) XGF.29a.3 2.288 1.091 0.242 4 XGF.29a.4 2.282 1.082 0.303 3 All particles were heat-treated at Oligomer for 2.281 1.093 4 600 °C. XGF.29a.5 b 0.537 g of XGF.29a.5b 0.598 0.224 3 oligomer was used XGF.29a.6 1.161 0.523 0.22 24 for XGF.29a.5. XGF.29a.7 2.281 1.085 0.29 4 XGF.30.600a.1 2.242 1.096 0.301 23 XGF.30.600a.2 2.29 1.091 0.311 11 XGF.30.600a.3 1.077 1.081 0.308 18 XGF.30.600a.4 0.433 1.098 0.28 24 XGF.30.600b.1 1.112 1.093 0.307 19 XGF.30.600b.2 1.502 1.085 0.315 8 XGF.30.600b.3 1.098 1.098 0.308 26 XGF.30.600b.4 1.508 1.089 0.314 24 XGF.41.650a.1 1.508 1.114 0.313 31 XGF.43.raw.1 1.517 1.094 0.163 39 XGF.44.600a.1 1.719 1.096 0.256 ~12 XGF.44.raw.1 1.727 1.088 0.234 24 XGF.46.raw.1 1.56 1.09 0.2 7 XGF.48.600a.1 1.77 1.093 0.426 16 XGF.48.600a.2 1.509 1.089 0.453 16 XGF.48.600a.3 1.506 1.095 0.404 17 XGF.48.raw.1 1.731 1.093 0.423 16 XGF.48.raw.1.2nd 1.51 1.104 0.196 60 XGF.48.raw.2 1.502 1.091 0.451 16 XGF.48.raw.3 2.000 1.093 0.372 17 XGF.50.600a.1 1.499 1.105 0.416 18 XGF.50.600a.2 1.51 1.091 0.31 17 XGF.50.600a.3 1.507 1.097 0.3 24 XGF.50.600a.4 1.509 1.087 0.273 41 LC: low crystallinity phase, XGF.50.LCa 1.508 1.094 0.396 16 patericles were heated at 210 °C XGF.50.LCb 1.500 1.091 0.407 18 Heated at 400 °C XGF.50.LCb.hydrate. modify 1.507 1.091 0.126 17

162

Bis- Tri- Particles t Samples phenol A phosgene Comments (g) (h) (g) (g) XGF.50.LCc 1.513 1.084 0.313 17 Heated at 500 °C XGF.51.600a.1 1.519 1.086 0.406 24 XGF.51.600a.2 1.505 1.098 0.406 21 XGF.51.LCa 1.519 1.067 0.293 16 Heated at 500 °C XGF.51.LCa.hydrate. modify 1.519 1.092 0.04 17 XGF.51.LCb 1.519 1.091 0.291 16 Heated at 200 °C XGF.51.LCc 1.511 1.081 0.302 17 Heated at 200 °C XGF.51.LCc.hydrate. modify 1.505 1.103 0.102 17 XGF.52.650b.1 3.008 2.187 0.849 23 O: orthorhombic phase, same as “LC” phase the XGF.52.Oa 1.505 1.089 0.606 20 two labels might be interchanged sometimes. Heated at 200 °C XGF.52.Oa.hydrate. modify 1.512 1.083 0.469 22 XGF.53.650a.1 3.016 2.193 0.715 24 XGF.53.650b.1 1.837 1.324 0.304 ~24 XGF.54.300a 1.519 1.105 0.433 30 XGF.54.300a.hydrate 170.modify 1.473 1.094 0.553 24 XGF.54.300a.hydrate 170.modify.2nd 1.587 1.113 0.241 24 XGF.54.300b 1.528 1.107 0.552 25 XGF.54.300b.hydrate Solvent dried out 1.511 1.099 0.156 n/a 170.modify by high Ar flow. XGF.54.650a 1.515 1.11 0.300 49 XGF.55.650b.1 3.046 2.182 0.811 19 XGF.55.650b.2 1.506 1.072 0.314 25 XGF.56.400a 1.503 1.099 0.464 24 XGF.56.400a.hydrate 150.modify 1.515 1.097 0.108 46 XGF.56.400a.hydrate 170.modify 1.525 1.13 0.122 47 XGF.56.400b.hydrate 1.528 1.093 0.132 170

163

Samples Bis- Tri- Particles t Comments phenol A phosgene (g) (h) (g) (g) XGF.56.400b.hydrate 1.548 1.121 0.246 n/a Not recorded 170.2 XGF.56.650b 1.533 1.154 0.400 48 XGF.57.300a.hydrate 150 1.519 1.098 0.165 51 XGF.57.300a.hydrate 150.2 1.526 1.119 0.171 46 XGF.57.650a.2 3.025 2.173 0.608 22 XGF.57.300b.hydrate 170 1.517 1.109 0.163 48 XGF.57.400a.hydrate 150 1.54 1.102 0.16 50 XGF.58.400a.hydrate 150 1.552 1.134 0.187 48 XGF.58.400a.hydrate 170 1.53 1.163 0.164 48 XGF.58.400b 1.523 1.092 0.321 26 Only made 1 batch XGF.58.400c.1 1.516 1.115 0.3 12 XGF.62.raw 1.511 1.088 0.129 48 XGF.63.raw 1.53 1.104 0.178 24 XGF.66.630a.1 1.327 1.111 0.173 17 XGF.66.630a.2 1.513 1.115 0.176 17 XGF.66.630a.3 1.084 1.093 0.182 17 XGF.66.630a.4 1.731 1.092 0.174 ~17 XGF.66.630a.5 2.31 ~1.1 0.178 22 Aliquot was also taken at 5 h, 9 h, 21 XGF.66.630a.6 3.034 2.161 0.344 49 h, 28 h, 33h and 39 h. XGF.66.630a.7 1.511 ~1.1 0.174 17 XGF.66.630a.8 1.516 1.104 0.18 5 XGF.66.630a.9 1.51 1.086 0.17 2 XGF.66.630a.10 1.5 1.11 0.178 16 XGF.66.630a.11 1.506 1.119 0.167 4.5 XGF.66.630a.12 1.555 1.117 0.155 12 XGF.68.400a.hydrate. 1.505 1.163 0.199 17 170 XGF.68.630.645a.1 1.507 1.094 0.183 17 XGF.68.630.645a.2 1.544 1.133 0.189 17 XGF.68.630.645a.3 1.064 1.088 0.177 17

164

Bis- Tri- Particles t Samples phenol A phosgene Comments (g) (h) (g) (g) XGF.68.645b.1 1.509 1.16 0.185 8 XGF.68.645b.2 1.816 1.102 0.172 17 XGF.68.645b.3 1.516 1.117 0.18 12 XGF.68.645b.4 2.312 1.114 0.186 16 XGF.68.raw.1 1.498 1.104 0.317 18 XGF.68.raw.1.2nd 1.520 1.110 0.138 16 XGF.69.645a.1 1.523 1.148 0.368 17 XGF.69.645a.1.2nd 1.525 1.142 0.152 18 XGF.69.645a.2 1.525 1.116 0.178 23 XGF.69.645a.3 2.318 1.106 0.178 16 XGF.69.645b.1 1.539 1.116 0.175 17 XGF.69.645b.2 1.54 1.162 0.077 16 XGF.69.645b.3 1.525 1.111 0.185 12 Not recorded, it XGF.69.645d.1 1.551 N/A 0.176 12 was around 1.1 g. XGF.69.raw.1 1.527 1.126 0.312 16 XGF.69.raw.1.2nd 1.512 1.176 0.191 22 XGF.71.645a.3 1.549 1.095 0.175 12 XGF.71.645a.4 1.58 1.16 0.36 12 XGF.71.645a.4.2nd 1.555 1.112 0.259 11 XGF.71.645a.5 1.546 1.089 0.177 24 XGF.71.645a.6 1.516 1.106 0.179 21 XGF.71.645a.7 1.546 1.166 0.156 30 XGF.71.645b.1 1.473 1.095 0.16 48 XGF.71.650a.1 1.54 1.105 0.18 8 XGF.71.650a.2 1.516 1.124 0.184 10 XGF.71.raw.1 1.557 1.162 0.358 12 XGF.71.raw.1.2nd 0.240 11 XGF.71.raw.1.3rd 1.521 1.164 0.168 11 XGF.72.645a.1 1.596 1.04 0.393 29 XGF.72.645a.1.2nd 1.525 1.147 0.287 23 XGF.72.645a.2 3.021 2.23 0.167 24 XGF.72.645a.3 1.53 1.201 0.383 26 XGF.72.645a.3.2nd 1.491 1.144 24 XGF.72.raw.1 1.542 1.087 0.394 35 XGF.72.raw.1.2nd 1.494 1.094 0.238 23 XGF.72.raw.2 1.536 1.092 0.382 24 XGF.72.raw.2.2nd 1.494 1.16 24

165

Bis- Tri- Particles t Samples phenol A phosgene Comments (g) (h) (g) (g) XGF.72.raw.3 1.408 1.149 0.281 36 XGF.72.raw.3.2nd 1.539 1.24 28 XGF.73.645a.1 2.977 2.151 0.6 24 XGF.73.645a.1.2nd 2.957 2.158 24 XGF.73.645a.2 0.746 0.587 0.159 22 XGF.73.645a.2.2nd 0.758 0.53 24 XGF.73.raw.1 2.734 2.093 0.607 23 XGF.73.raw.1.2nd 2.99 2.11 22

166