Transparent, Lightweight, High Performance Polymer Films and Their Composites Yunyin Lin SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS OF THE DEGREE OF DOCTOR OF PHILOSOPHY School of Engineering and Materials Science, Queen Mary University of London, London, UK June 2020 1 Declaration Declaration I, Yunyin Lin, confirm that the research included within this thesis is my own work or that where it has been carried out in collaboration with, or supported by others, that this is duly acknowledged below and my contribution indicated. Previously published material is also acknowledged below. I attest that I have exercised reasonable care to ensure that the work is original, and does not to the best of my knowledge break any UK law, infringe any third party’s copyright or other Intellectual Property Right, or contain any confidential material. I accept that the College has the right to use plagiarism detection software to check the electronic version of the thesis. I confirm that this thesis has not been previously submitted for the award of a degree by this or any other university. The copyright of this thesis rests with the author and no quotation from it or information derived from it may be published without the prior written consent of the author. Signature: Date: 03/06/2020 2 Acknowledgements Acknowledgements First, I would like to give my greatest and sincere gratitude to my supervisors Prof. Ton Peijs, Dr. Emiliano Bilotti and Prof. Cees Bastiaansen for their generosity in guiding, supporting as well as sharing their knowledge and experience to me. Ton, always coming up with great ideas with his profound knowledge and comprehensive outlook, gave me plenty of constructive comments and supported me in all aspects over these four years. Emiliano’s helpful suggestions and encouraging support guided me through the whole process. The intensive discussions with Cees taught me how to think critically and to be rigorous when doing research. Without their patient guidance and precious input, I would not have been able to carry out my research. I would like to thank the China Scholarship Council (CSC) for their financial support and giving me an opportunity to study abroad. I gratefully acknowledge the facilities and support offered by School of Engineering and Materials Science and Nanoforce Technology Ltd. I also appreciate Dr. Russell Bailey, Dr. Alice Williams, Dr. Rory Wilson, Dr. Thomas Baumard, Dr. Giovanni Santagiuliana, Dr. Theo Saunders, Mr. Maurizio Leo, Ms. Sanam Ghaffari and Mr. Dennis Ife for their technical support. I greatly appreciate Dr. Wei Tu and Dr. Han Zhang for their useful suggestions and invaluable assistance. A huge thanks to my lovely colleagues and friends Dr. Yi Liu, Dr. Yaqiong Wang, Dr. Nan Meng, Dr. Min Yu, Dr. Hangfeng Zhang, Dr. Kan Chen, Dr. Jiyue Wu, Xintong Ren, Kening Wan, Hanchi Ruan, Xiangyan Yu, Rehan Bhatti, Arnaud Kernin, Leonardo Ventura, Claudia Bertei, Man Zhang and Yichen Wang for all the help in any shape or form. Also I would like to thank undergraduate student Ruhi Patel. 3 Acknowledgements I would also like to greatly acknowledge the support from the Stimuli-responsive Functional Materials & Devices (SFD) Group at Eindhoven University of Technology (Netherlands) and the SFD group members including Prof. Dick Broer, Prof. Albert Schenning, Dr. Lihua Shen, Tom Bus, Rob Verpaalen, Xinlong Pan, Simon Houben, Sarah Lafleur and Yuanyuan Zhan for their help. I spent a fabulous and unforgettable month in Eindhoven. My special thanks go to my parents and my boyfriend, Jun Cao, for their understanding, support and encouragement which helped me throughout my whole PhD period. 4 Abstract Abstract This thesis aims to develop novel polymeric material with an excellent balance in high optical transparency and mechanical properties. More particularly, the current thesis reports the processing and characterization of highly oriented transparent polyethylene films and their use in laminated composites. First, highly transparent high-density polyethylene (HDPE) films with high modulus and tensile strength were developed by regulating solid-state drawing conditions without the need of additives. The effects of drawing parameters like drawing temperature and draw ratio on optical and mechanical properties as well as morphology of these solid-state drawn HDPE films were methodically investigated. It was found that a fairly broad processing window can be utilized to tailor the required balance in optical and mechanical performance. Subsequently, the production of these ultra-drawn transparent HDPE films was carried out using a scalable and continuous cast-film extrusion and drawing process. High optical transparency of around 91 % was achieved even in the far field. A maximum modulus of ~ 33 GPa and tensile strength of ~ 900 MPa of these solid-state drawn HDPE films was attained without compromising optical transparency, which is an order of magnitude higher than mechanical properties of conventional transparent plastics such as polycarbonate (PC) and poly(methyl methacrylate) (PMMA). The influence of extrusion draw down and two-step drawing on optical and mechanical behaviours was also explored. Finally, these highly oriented transparent HDPE films were used as the reinforcing phase in high performance transparent composite laminates. The far field light transmittance of 5 Abstract 4-layer HDPE-reinforced laminates with either a unidirectional (UD) or bidirectional (BD) lay-up sandwiched between glass or PC skins, was maintained at around 85 %. The fabricated transparent composite laminates were shown to have not only a high tensile strength but also a high energy absorption capability, outperforming existing transparent glazing materials such as laminated glass or PC. 6 Table of Contents Table of Contents Declaration ....................................................................................................................... 2 Acknowledgements .......................................................................................................... 3 Abstract ............................................................................................................................ 5 Table of Contents ............................................................................................................. 7 List of Notations and Abbreviations ............................................................................ 11 List of Figures ................................................................................................................ 16 List of Tables .................................................................................................................. 27 Chapter 1 Introduction ................................................................................................. 29 1.1 Background ....................................................................................................... 29 1.2 Objective of the thesis ....................................................................................... 31 1.3 Scope of the thesis ............................................................................................. 32 Chapter 2 Literature Review ....................................................................................... 34 2.1 Conventional transparent inorganic glass .......................................................... 34 2.2 Transparent amorphous polymeric materials .................................................... 38 2.2.1 Polycarbonate (PC) ................................................................................... 39 2.2.2 Poly(methyl methacrylate) (PMMA) ........................................................ 40 2.2.3 Polystyrene (PS) ........................................................................................ 40 2.2.4 Polyvinyl chloride (PVC) .......................................................................... 41 2.2.5 Thermoplastic polyurethanes (TPU) ......................................................... 42 2.2.6 Other transparent amorphous polymers .................................................... 42 2.3 Transparent semi-crystalline polymeric materials ............................................. 43 2.3.1 Isotropic transparent semi-crystalline polymers ....................................... 44 2.3.1.1 Suppressed crystallization .................................................................... 44 7 Table of Contents 2.3.1.2 Reducing crystal size ............................................................................ 46 2.3.1.3 Matching refractive indices .................................................................. 50 2.3.2 Biaxially stretched transparent semi-crystalline polymers ....................... 51 2.3.3 Monoaxially stretched transparent semi-crystalline polymers .................. 56 2.3.3.1 Matching the refractive indices between polymer and voids ............... 57 2.3.3.2 Preventing the formation of voids ........................................................ 62 2.3.3.3 Whitening after overdrawing ................................................................ 65 2.4 Solid-state drawing ............................................................................................ 70 2.5 Transparent composite laminates ...................................................................... 76 2.6 High performance composites ........................................................................... 80 2.7 Summary ..........................................................................................................