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Effect of Copolymer Sequence on Mechanical Properties of Polymer

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Effect of Copolymer Sequence on Mechanical Properties of Polymer Nanocomposites from Molecular Dynamics Simulations Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Alex Trazkovich, B.S., M.S. Graduate Program in Chemical Engineering The Ohio State University 2019 Dissertation Committee: Dr. Lisa M. Hall, Advisor Dr. Kurt Koelling Dr. Isamu Kusaka Dr. Yiying Wu c Copyright by Alex Trazkovich 2019 Abstract When incorporated into polymers, nanoparticles are known to modify the struc- ture and dynamics of nearby polymer chains. Because nanoparticles have a high surface area to volume ratio, the properties of the polymer–nanoparticle interphase region can have a significant effect on the overall composite properties even at rel- atively low nanoparticle loading. In this work, we study the polymer–nanoparticle interphase region using molecular dynamics simulations, and we analyze the impact of a nanoparticle on local structure, dynamics, and viscoelastic properties. Of particular interest here is a class of systems which consists of nanoparticles incorporated into two-component copolymers where one component of the copolymer interacts more favorably with the nanoparticle than the other. In these systems, modifying the particular copolymer sequence may modify the interphase properties, and composite properties may therefore be adjusted even while maintaining the same overall monomer ratio. These systems have been the subject of several simulation studies focused on nanoparticle dispersion and assembly; however, relatively little simulation work has focused specifically on the impact of copolymer sequence on properties of the copolymer–nanoparticle interphase. ii We simulate a simple nanocomposite consisting of a single spherical nanoparticle surrounded by coarse-grained polymer chains. The polymers are composed of two dif- ferent monomer types that differ only in their interaction strengths with the nanopar- ticle. By studying a series of regular multiblock copolymers with adjustable block length as well as a random copolymer, we examine the effect of copolymer sequence blockiness on the structure as well as the end-to-end vector autocorrelation, bond vector autocorrelation, and self-intermediate scattering function relaxation times as a function of distance from the nanoparticle surface. We find that, depending on block length, blocky copolymers can have faster or slower interphase dynamics than a random copolymer. Certain blocky copolymer sequences also lead to relaxation times near the nanoparticle surface that are slower than those of homopolymer systems composed of either component monomer. To analyze viscoelastic mechanical properties in the interphase, we measure local atomic stress fluctuations and use them to estimate the local stress autocorrelation as a function of distance from the nanoparticle. This local stress autocorrelation is then used to estimate the local dynamic modulus. This allows us to examine the effect of adjusting copolymer sequence on the dynamic modulus as a function of both frequency of excitation and distance from the nanoparticle. Notably, we find that certain copolymer sequences can lead to a higher viscoelastic hysteresis in the interphase than either homopolymer system, suggesting that tuning copolymer sequence could allow for significant control over nanocomposite dynamics. iii To demonstrate a possible application of adjusting material properties using copoly- mer sequence, we briefly consider a design challenge motivated by tire tread com- pounds, in which improving traction without sacrificing fuel economy requires in- creasing high-frequency hysteresis while maintaining low-frequency hysteresis. By considering an additional set of sequences motivated by our results from studying regular multiblock copolymers, we show how further adjusting copolymer sequence can be used to make progress toward this goal. iv To Pat, the reason I am here. & To Remi, for keeping my smile bright. v Acknowledgments My road to this degree has been a bit unusual, and as a result, there are several individuals to whom I owe more than the usual debt of gratitude owed by a typical Doctoral student. Thank you most to Dr. Lisa Hall, who took a chance on this odd robotics engineer and who tolerated the distraction and sporadic attendance as I split time between campus and my job. She has been a role model, an inspiration, and an excellent board game partner. Of course, I never even would have met Dr. Hall without Dr. Pat Majors, without whom, quite literally, none of this would have happened. As my supervisor at Cooper Tire, Dr. Majors repeatedly advocated for my degree, introduced me to Dr. Hall, and then supported me for years, giving me the flexibility I needed to pursue my studies. He has also been a valuable resource, a wealth of knowledge, and, I’m proud to say, a dear friend. I also owe a more general thanks to Cooper Tire, which funded my first two semesters at Ohio State, and I would like to specifically thank Curt Selhorst, Jeff Endicott, and Chuck Yurkovich, who, along with Dr. Majors, advocated to the administration on my behalf. Now that I have moved on to SEA Ltd., I should also thank my supervisors, Dr. Gary Heydinger and Jared Henthorn, for their kind support and flexibility while I vi wrote my dissertation, and I also thank Dr. Anmol Sidhu and An Nguyen, teammates who covered for me during occasional jaunts to campus. Thank you to the Hall Research group, especially Kevin Shen, Jeff Ethier, Dr. Youngmi Seo, Dr. Janani Sampath, and Dr. Jon Brown, who always made me feel like a full member of the team despite my often ephemeral presence. I also thank my Qualifier, Candidacy, and Dissertation Defense committee mem- bers, Dr. Kurt Koelling, Dr. Isamu Kusaka, and Dr. Stuart Cooper, who challenged me in a way that significantly improved this work. I was fortunate enough to be aided during this study by several talented interns, most notably Tarik Akyuz and Mitchell Wendt, who each made major contributions to the analysis code and coauthored publications with me. Finally, from the bottom of my soul, I thank my wife, Remi, for always taking care of me as the stress of pursuing a degree while working a full-time job took its toll. Remi, you are the light of my life, and my heart ached every time I had to choose research instead of spending time with you. But guess what? I’m done. vii Vita 2010 . .B.S. Robotics Engineering, Olin College of Engineering 2013 . .M.S. Mechanical Engineering, Northwestern University 2014-present . .Graduate Student, The Ohio State University Publications 1. A.J. Trazkovich, T.H. Akyuz, L.M. Hall, “Effects of Copolymer Sequence on Ad- sorption and Dynamics Near Nanoparticle Surfaces in Simulated Polymer Nanocom- posites”, Tire Science and Technology, 2019, In Press. 2. A.J. Trazkovich, M.F. Wendt, L.M. Hall, “Effect of Copolymer Sequence on Structure and Relaxation Times Near a Nanoparticle Surface”, Soft Matter, 2018, 14, 5913–5921. 3. A.J. Trazkovich, M.F. Wendt, L.M. Hall, “Effect of Copolymer Sequence on Local Viscoelastic Properties Near a Nanoparticle”, Macromolecules, 2019. doi: 10.1021/acs.macromol.8b02136. Fields of Study Major Field: Chemical Engineering Areas of Interest: Molecular Simulations, Polymer Physics, Tire Mechanics viii Table of Contents Page Abstract . ii Dedication . .v Acknowledgments . vi Vita......................................... viii List of Abbreviations . xi List of Symbols . xii List of Figures . xv 1. Introduction . .1 1.1 Motivation . .1 1.1.1 Design Challenge in Tire Treads . .5 1.1.2 Coarse-Grained Modeling . .8 1.2 Structure of Dissertation . 10 2. Model and Simulation Methods . 12 2.1 Kremer-Grest Bead-Spring Model . 12 2.2 Polymer Nanocomposite System Details . 14 3. Structure and Relaxation Times of Homopolymer Systems . 18 3.1 Background . 18 3.2 Simulation Details . 19 ix 3.3 Effect of Polymer–Nanoparticle Interaction Strength on Interphase Structure . 21 3.4 Effect of Polymer–Nanoparticle Interaction Strength on Interphase Relaxation Times . 26 4. Structure and Relaxation Times of Copolymer Systems . 36 4.1 Background . 36 4.2 Simulation Details . 38 4.3 Effect of Copolymer Sequence on Interphase Structure . 41 4.4 Effect of Copolymer Sequence on Interphase Relaxation Times . 50 4.5 Discussion . 57 5. Effect of Copolymer Sequence on Interphase Dynamic Modulus . 58 5.1 Background . 58 5.2 Definition of Dynamic Moduli and Relationship to Hysteresis . 62 5.3 Measurement Methods . 67 5.3.1 Influence of Bond Vibrations on Modulus Results . 72 5.3.2 Effect of Pre-Processing Data . 74 5.4 Results . 76 5.5 Discussion . 92 6. Adjusting Copolymer Sequence to Modify Hysteresis . 95 6.1 Motivation . 95 6.2 Comparison of Hysteresis in Original Systems . 97 6.3 Investigating Additional Sequences . 99 6.3.1 Triblock and New Regular Multiblock Copolymers . 99 6.3.2 Modifications to Random Copolymers . 106 6.4 Discussion . 108 7. Concluding Remarks . 111 7.1 Summary of Results . 111 7.2 Future Work . 114 Bibliography . 119 x List of Abbreviations BVACF Bond Vector Autocorrelation Function EEACF End-to-End Vector Autocorrelation Function COM Centers of Mass FENE Finitely Extensible Nonlinear Elastic LAMMPS Large-scale Atomic/Molecular Massively Parallel Simulator LJ Lennard-Jones LSACF Approximation of the Local Stress Autocorrelation Function MD Molecular Dynamics MSD Mean-Squared Displacement NPT Isobaric, isothermal, and fixed particle number ensemble NRI Nanoparticle Radial Vector
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