STRUCTURE-PROPERTY RELATIONSHIPS IN THIOL-ACRYLATE BASED MAIN-CHAIN LIQUID ...

132 Pages·2017·19.81 MB·English

STRUCTURE-PROPERTY RELATIONSHIPS IN THIOL-ACRYLATE BASED
MAIN-CHAIN LIQUID-CRYSTALLINE ELASTOMERS
by
MOHAND OSMAN SAED
B.S., University of Gezira, 2008
M.S., University of Colorado Denver, 2014

A thesis submitted to
Faculty of the Graduate School of the
University of Colorado in partial fulfillments
of the requirements of the degree of
Doctor of Philosophy
Mechanical Engineering Program
2017
©2017
MOHAND OSMAN SAED
ALL RIGHTS RESERVED

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The Thesis For Doctor Of Philosophy Degree by
Mohand Osman Saed
has been approved for the
Mechanical Engineering Program
by

Dana R. Carpenter, Chair
Christopher M. Yakacki, Advisor
Ronald Rorrer
Kai Yu
Carl P. Frick
Christopher N. Bowman
Date: May 13, 2017

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Saed, Mohand Osman (Ph.D., Mechanical Engineering Program)
Structure-Property Relationships in Thiol-Acrylate Based Main-Chain Liquid-Crystalline Elastomers
Thesis directed by Professor Christopher M. Yakacki
ABSTRACT
In this research, we used a profoundly new approach to synthesize liquid-crystalline elastomers
(LCEs) based on using a thiol-acrylate “click” reaction and two-stage thiol-acrylate Michael addition-
photopolymerization (TAMAP) reaction, both of which have not previously been investigated for
LCE synthesis. The thiol-acrylate reaction was used initially to synthesize polydomain LCEs and then
to examine the influence of crosslinking and spacer length. First, the influence of crosslinking on the
thermomechanical behavior of LCEs was investigated. The isotropic rubbery modulus, glass
transition temperature, and strain-to-failure showed strong dependence on crosslinker amount and
ranged from 0.9 MPa, 3°C, and 105% to 3.2 MPa, 25°C, and 853%, respectively. The isotropic
transition temperature (T) was shown to be influenced by the functionality of the crosslinker, while
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the crosslinker concentration had no effect. The magnitude of actuation can be tailored by controlling
the amount of crosslinker and applied stress. Actuation increased with increasing the applied stress
and decreased with greater amounts of crosslinking. Second, we hypothesized that tuning the LC
phases in main-chain LCE systems can be achieved by varying the spacer length while maintaining
the same mesogen (RM257). By increasing the length of spacers from two to eleven carbons along
the spacer backbone (C2 to C11), we can modulate the mesophase from nematic to smectic, tailor the
nematic to isotropic transition temperature between 90 and 140°C, and increase the average work
capacity from 128 to 262 kJ/m3. Phase segregation and the smectic C phase is achieved at room
temperature for the C6, C9, and C11 spacers. Upon heating, these samples transition into the nematic
and later, the isotropic phase. Furthermore, this segregation occurs along with polymer chain
crystallinity, which increasing the modulus of the networks by an order of magnitude; however, the
crystallization rate is highly time dependent on the spacer length and can vary between 5 minutes for
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the C11 spacer and 24 hours for shorter spacers. A novel TAMAP methodology was implemented to
synthesize monodomain LCEs using commercially available starting monomers. A wide range of
thermomechanical properties was tailored by adjusting the amount of crosslinker, while the actuation
performance was dependent on the amount of applied strain during programming.
The form and content of this abstract are approved. I recommend its publication.

Approved: Christopher M. Yakacki

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To my family.

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ACKNOWLEDGEMENTS
I would like to thank my advisor, Prof. Christopher M. Yakacki for his guidance, encouragement, and
continuous support over the past 5 years. Your passion for research, good work ethic, limitless ideas,
and creativity has inspired me greatly. Working in his laboratory has thought me so many skills that
will benefit me throughout my career.
I also would like to thank and acknowledge my thesis committee members, Prof. Dana Carpenter,
Prof. Ron Rorrer, Prof. Kai Yu, Prof. Carl. Frick, and Prof. Christopher Bowman, who agreed to
serve on my PhD committee despite their tense schedules and for their valuable feedback, which has
helped me to further understand my research. Special gratitude extents to Prof. Frick, and Prof.
Bowman for their willing to come from Laramie, WY, and Boulder, CO.
The interdisciplinary nature of my projects has taught me to collaborate extensively with many
groups around the country. Explicitly, I would like to thank our collaborators at University Wyoming
(Prof. Carl Frick and Dan Markel), University of Colorado Boulder (Rayshan Visvanathan, Prof.
Noel Clark, Matt McBride, Abeer Alzahrani, and Prof. Chris Bowman) and John Hopkins University
(Aurelie Azoug and Vicky Nguyen).
I would like to thank the Smart Materials and Biomechanics Lab (SMAB) members for their support
and encouragement. I would like to thank Dr. Amir Torbati, Ravi Patel, Ross Volpe, Nick Traugutt,
Sam Mills, Michael Bollinger, Lillian Chatham, and Ryan Anderson for useful discussions and the
wonderful time I spent working with them. I would also like to thank my undergraduate students
Brandon Mang, Ellana Taylor, Chelsea Starr, and Kristen Bonifield.
Finally, I would like to thank my family for their love, support, and encouragement throughout this
work. My wife, Omnia, has been a constant source of love and joy. I could not have accomplished my
PhD without her support

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TABLE OF CONTENTS
TABLE OF CONTENTS .................................................................................................................... xiii
LIST OF TABLE ................................................................................................................................... xi
LIST OF FIGURE ................................................................................................................................ xii
CHAPTER ........................................................................................................................... ….…..….....
I. INTRODUCTION AND BACKGROUND .................................................................................... 1
1.1 Liquid Crystals (LC) ................................................................................................................. 1
1.2 Liquid-crystalline Elastomers (LCEs) ...................................................................................... 2
1.2.1 Classification ..................................................................................................................... 3
1.2.2 Preparations ....................................................................................................................... 6
1.2.3 Crosslinking History .......................................................................................................... 7
1.2.4 Stress-Strain Behavior ....................................................................................................... 8
1.2.5 Actuation ............................................................................................................................ 9
II. RESEARCH MOTIVATION AND GOALS ............................................................................... 11
III. THIOL-ACRYLATE MAIN-CHAIN LIQUID-CRYSTALLINE ELASTOMERS WITH
TUNABLE THERMOMECHANICAL PROPERTIES AND ACTUATION STRAIN ..................... 15
3.1. Abstract ...................................................................................................................................... 15
3.2 Introduction ................................................................................................................................ 16
3.3. Experimental .............................................................................................................................. 19
3.3.1. Materials ............................................................................................................................. 19
3.3.2. Synthesis of Liquid-Crystalline Elastomers ....................................................................... 20
3.3.3. Gel Fraction Tests ............................................................................................................... 21
3.3.4. X-Ray Scattering ................................................................................................................ 21
3.3.5. Differential Scanning Calorimetry (DSC) .......................................................................... 22
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3.3.6. Dynamic Mechanical Analysis (DMA) .............................................................................. 23
3.3.7. Strain-to-Failure Tests ........................................................................................................ 23
3.3.8. Strain-Actuation Characterization ...................................................................................... 23
3.4. Results ....................................................................................................................................... 24
3.5. Discussion .................................................................................................................................. 32
3.6. Conclusions ............................................................................................................................... 38
3.6. Acknowledgements ................................................................................................................... 38
IV. MODULATED MESOPHASE LIQUID CRYSTAL ELASTOMERS ..................................... 39
4.1. Abstract ...................................................................................................................................... 39
4.2. Introduction ............................................................................................................................... 39
4.3. Results and Discussion .............................................................................................................. 42
4.4. Conclusions ............................................................................................................................... 54
4.5. Experimental Section ................................................................................................................. 55
4.6. Acknowledgements ................................................................................................................... 58
V. TAILORABLE AND PROGRAMMABLE LIQUID-CRYSTALLINE ELASTOMERS
USING A TWO-STAGE THIOL-ACRYLATE REACTION ............................................................. 59
5.1. Main ........................................................................................................................................... 59
5.2. Conclusions ............................................................................................................................... 66
5.3. Acknowledgments ..................................................................................................................... 67
VI. SYNTHESIS OF PROGRAMMABLE MAIN-CHAIN LIQUID-CRYSTALLINE
ELASTOMERS USING A TWO-STAGE THIOL-ACRYLATE REACTION .................................. 68
6.1. Abstract ...................................................................................................................................... 68
6.2. Introduction ............................................................................................................................... 69
6.3. Protocol ...................................................................................................................................... 71
6.3.1. Preparation of Liquid Crystalline Elastomers LCEs .......................................................... 71
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6.3.2. Kinetics Study of Two-stage Reaction with Real-time Fourier Transform Infrared .......... 73
6.3.3. Dynamic Mechanical Analysis (DMA) .............................................................................. 74
6.3.4. Strain-to-failure Tests ......................................................................................................... 75
6.3.5. Shape Fixity and Actuation Tests ....................................................................................... 76
6.5. Discussion .................................................................................................................................. 86
6.6. Acknowledgments ..................................................................................................................... 89
6.7. Materials ................................................................................................................................ 90
VII. CONCLUSIONS AND FUTURE WORK ................................................................................ 91
7.1 Conclusions ................................................................................................................................ 91
7.2. Recommendations for the Further Work ................................................................................... 93
BIBLIOGRAPHY ................................................................................................................................ 95
APPRNDIX ........................................................................................................................................ 103
A- Wide-Angle X-Ray Scattering Characterizations .................................................................. 103
B- Differential Scanning Calorimetry (DSC) .............................................................................. 108
C- Dynamic Mechanical Analysis (DMA) .................................................................................. 111

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