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Atomistic Simulations of Octacyclopentyl Polyhedral Oligomeric Silsesquioxane Polyethylene Nanocomposites

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Atomistic Simulations of Octacyclopentyl Polyhedral Oligomeric Silsesquioxane Polyethylene Nanocomposites Atomistic Simulations of Octacyclopentyl Polyhedral Oligomeric Silsesquioxane Polyethylene Nanocomposites by Franco M. Capaldi M.S. Mechanical Engineering Massachusetts Institute of Technology, 2001 B.S. Mechanical Engineering Brown University, 1999 SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTORATE OF PHILOSOPHY IN MECHANICAL ENGINEERING AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY JUNE 2005 MASSACHUSETTS INSTI UTE OF TECHNOLOGY 0 Franco M. Capaldi. All rights reserved. JU 62005 The author hereby grants to MIT permission to reproduce And to distribute publicly paper and electronic LIBRARIES copies of this thesis document in whole or in part. Signature of Author: Department of Mechanical Engineering 005 Certified by: . nOyce Professor of Mechanical Engineering ory C. Rutledge tical Engineering Accepted by: Lallit Anand Chairman, vepartmen Lommittee on Graduate Students BARKER Atomistic Simulations of Octacyclopentyl Polyhedral Oligomeric Silsesquioxane Polyethylene Nanocomposites Franco M. Capaldi Submitted to the Department of Mechanical Engineering On March 14, 2005 in Partial Fulfillment of the Requirements for the Degree of Doctorate of Philosophy in Mechanical Engineering As the scientific community develops the ability to create composites which incorporate nanoscopic filler particles, the detailed atomic arrangement and atomic interactions become significant in determining the composite properties. Nanoscopic fillers such as carbon nanotubes, polyhedral oligomeric silsesquioxane (POSS), and layered silicates have already been successfully used to improve the thermal and mechanical properties of polymers. On this length scale, details describing particle organization, interaction between particles, and interactions between particle and matrix are needed to understand the behavior of the composite. In this thesis, we use atomistic simulations to investigate the detailed behavior of a blended octacyclopentyl polyhedral oligomeric silsesquioxane (CpPOSS) / polyethylene (PE) nanocomposite. The model potential employed to describe the atomic interactions in these systems is capable of reproducing the experimental vibrational and crystal structures for the POSS well. The mechanical properties of an infinite crystal were calculated. They are anisotropic with a Reuss average isotropic elastic modulus of 11.78 GPa. Simulations of CpPOSS/PE composites revealed that the POSS had a stiffening effect on the polymer. Simulations revealed that both PE and POSS dynamics as measured by translational and rotational diffusivities decreased, the glass transition temperature increased, and both the density and modulus increased with increasing POSS content. Micromechanical models were fit to the composite modulus which allowed effective mechanical particle sizes to be determined. The POSS was found to aggregate into small necklace structures which have internal ordering similar to the crystal phase. The formation of crystallites was found to be energetically favorable in this system. A coarse grained potential which accounts for both the attraction and orientation between particles was developed to aid the further study of aggregation and crystallization in these composites. The interface between the POSS and the polymer is found to consist of polymer chains aligned tangentially to the POSS cage. This layer has increased mobility tangential to the surface of the particle and decreased mobility in the radial direction. Though it is very thin, consisting of only 1 or 2 monolayers of polymer, due to the small size of the POSS particle, the weight fraction of polymer in the interfacial region is as high as 43 % in the 25 weight percent CpPOSS/PE composite. These simulations reveal the formation of structure on both the angstrom length scale in the polymer near the interface and the mesoscopic length scale between the POSS particles. Thesis Supervisors: Mary C. Boyce Professor of Mechanical Engineering Gregory C. Rutledge Professor of Chemical Engineering ii Acknowledgements I have been extremely fortunate to work with many amazingly talented and helpful people. All have impacted my work and life in some way or another and for this I am thankful. I would like to thank my thesis advisors, Professor Mary Boyce and Professor Gregory Rutledge for their guidance, advice and for providing a first-rate research and learning environment. Thanks to my thesis committee members, Professor Robert Cohen and Professor Nicolas Hadjiconstantinou, for thoughtful discussions. I am grateful to the National Science Foundation and the Air Force Office of Scientific Research for the financial support of my graduate work. In the Mechanics and Materials group, I have known and worked with many great people who have made my time at MIT enjoyable. Though I cannot possibly list them all, special thanks to Mohit, Mats, Mike, Ethan, Rebecca, Dora, Sauri, Brian, Prakash, Tom, Hang, Jin, Matt, Rami, Steve, Antonio, Numan, Ahmed, Markus, Pieter, Fred, Sergei, Vikram, Amrit, Michelle, Ed, and Adam. Also, I would like to thank the staff of the Mechanical Engineering Department, and the mechanics group Una, Ray, Leslie and Joan for making my life much easier. Most of all, I express my gratitude to my entire family. Thanks to my mother, father and sister, Laura, for their constant encouragement and support, and the faith they have shown in me and especially to my wife, Irene, for providing inspiration, motivation, love, and just about everything else I have needed throughout my time here. iii Table of Contents Acknowledgem ents ............................................................................................................................... iii Table of Contents.................................................................................................................................. iV L ist o f F ig ure s ....................................................................................................................................... v i L ist o f T ab le s ....................................................................................................................................... v ii Chapter 1I Introduction ........................................................................................................................ 11 1.1 Polym er N anocomposites ....................................................................................................... 11 1.2 Polyhedral Oligom eric Silsesquioxane ................................................................................... 14 Chapter 2 M ethodology ...................................................................................................................... 25 2.1 M onte Carlo ............................................................................................................................... 25 2.2 M olecular Dynam ics.................................................................................................................. 27 Chapter 3 M odeling Cyclopentyl POSS ......................................................................................... 34 3.1 M odel Potential.......................................................................................................................... 35 3.2 Vibrational Frequencies ............................................................................................................. 39 3.3 Crystal Structures....................................................................................................................... 42 3.4 M echanical Properties................................................................................................................ 47 3.5 CpPOSS Cohesive Energy..................................................................................................... 51 3.6 Ring Dynam ics........................................................................................................................... 56 3.7 Discussion.................................................................................................................................. 62 Chapter 4 CpPOSS/Polyethylene Nano-composites........................................................................ 67 4.1 Description of Simulations...................................................................................................... 67 4.2 Length Scales............................................................................................................................. 69 4.2.1 A verage Particle Separation................................................................................. 69 4.3 Radius of Gyration..................................................................................................................... 72 4 .4 D iffu sion .................................................................................................................................... 7 3 4.5 Rotational Diffusion................................................................................................................... 76 4.6 Aggregation................................................................................................................................ 78 4.7 Cluster Analysis ......................................................................................................................... 83 4.8 Potential of M ean Force ............................................................................................................
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