IMPROVEMENTS IN PROCESSING AND STRETCHABILITY OF SUPER GAS BARRIER MULTILAYER THIN FILMS A Dissertation by FANGMING XIANG Submitted to the Office of Graduate and Professional Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Chair of Committee, Jaime C. Grunlan Committee Members, Hong Liang Jodie Lutkenhaus Xinghang Zhang Head of Department, Andreas A. Polycarpou May 2015 Major Subject: Mechanical Engineering Copyright 2015 Fangming Xiang ABSTRACT Polymeric materials with gas barrier equivalent to metalized plastics have become increasingly important for food packaging, electronic device encapsulation, and vacuum insulation. Early attempts to develop alternative gas barrier materials have focused on thick (>10 μm) polymer/clay composites produced using physical mixing, but only limited improvement was achieved due to aggregation and random orientation of clay platelets. Layer-by-layer (LbL) assembly is a simple yet powerful bottom-up fabrication technique, which allows assembly of thin films with designed microstructure under ambient conditions. These multilayer thin films have many desirable properties: mechanical flexibility, transparency, and impermeability. This dissertation sought to further improve the processing and properties of high gas barrier thin films produced using LbL assembly. The influence of deposition time on gas barrier of polymer/clay multilayer thin films composed of polyethylenimine (PEI), poly(acrylic acid) (PAA), and clay was investigated. Multilple PEI/PAA bilayers (BL) were deposited between clay layers to construct multilayer assemblies with quadlayer (QL), hexalayer (HL), and octalayer (OL) sequences. Regardless of film structure, polymer/clay multilayer thin films prepared using shorter dipping time were thicker and exhibited better gas barrier than samples prepared using longer dipping time in the first few layers. This seemingly counterintuitive result was explained by considering desorption of previously deposited ii polymers between clay layers. Reduced deposition time helped to retain more polymer between clay sheets, leading to larger clay spacing and better gas barrier. In an effort to expand the applications and improve the throughput of layer-by- layer assembly, spray-assisted deposition of PEI/PAA bilayers was investigated. The influences of spraying time, spraying pressure, and flow rate on thickness, roughness, and gas barrier were evaluated. Spraying time was determined to be the most important parameter. A 7-bilayer PEI/PAA thin film assembled using optimized spraying parameters exhibited better gas barrier than a dip-coated multilayer prepared using the same deposition time for each layer. These findings pave the way for using the LbL technique commercially, where fast and continuous deposition of high performance thin films on large substrates is needed. Finally, the first example of super stretchy gas barrier was developed by combining polyacrylic acid and poly(ethylene oxide) (PEO). Oxygen transmission rate (OTR) of 1.58 mm thick natural rubber was reduced by an order of magnitude after deposition of a 20 BL PAA/PEO assembly. More importantly, no cracking was observed on the PAA/PEO coating after 100% strain. A 5X improvement in gas barrier was retained after this extreme stretching. Additionally, the effect of assembling condition on the permeability of PAA/PEO multilayer thin films was investigated. By setting the assembling pH at 2.75, the negative impacts of PAA ionization and COOH dimer formation could be minimized, leading to a 50% reduction in oxygen permeability. This unique combination of elasticity and gas barrier makes the PAA/PEO assembly an ideal candidate for improving the barrier of elastomeric materials. iii DEDICATION I dedicate this dissertation to my wife, dad, mom, grandpa, and grandma. Thank you for keeping me happy and motivated throughout my graduate study. iv ACKNOWLEDGEMENTS First and foremost, I would like to express my gratitude to my advisor, Dr. Jaime Grunlan, for his academic guidance, financial support, and enthusiastic encouragement throughout this research. I feel truly privileged to work in this great research group. I would like to thank my committee members, Dr. Hong Liang, Dr. Jodie Lutkenhaus, Dr. Xinghang Zhang for taking time out of their busy schedules to serve on my Ph.D. committee and for their valuable suggestions. I appreciate the assistance of Dr. Micah Green, and Dr. Oren Regev (of Ben-Gurion University of the Negev) for their investment in my research. I would like to thank the group members of the Polymer NanoComposites Lab for being great friends and coworkers. The help of my outstanding undergraduate assistants, Justin Sawyer, Sarah Ward, and Tara Givens, is highly appreciated. Thanks also go to former and current graduate students of the Polymer Nanocomposites Lab, Dr. You-Hao Yang, Dr. Galina Laufer, Dr. Gregory Moriarty, Bart Stevens, Ping Tzeng, David Hagen, Kevin Holder, Tyler Guin, Blake Teipel, Merid Haile, Yixuan Song, Ryan Smith, Morgan Plummer, Taylor Smith, Dr. Marcus Leistner, Dr. Chungyeon Cho, and Dr. Debabrata Patra. A special thanks to Dr. Morgan Priolo for being a great mentor. Thanks also go to my friends and colleagues and the department faculty and staff for making my time at Texas A&M University a great experience. Hanging out with Joseph Sargent and Pingjia Ming was the best pastime that I had in College Station. Additionally, I want to thank Dorsa Parviz for being a great collaborator, Dr. v Mohammad Naraghi for teaching the most interesting course, and Dr. Wilson Serem (of the Materials Characterization Facility at Texas A&M University) for AFM measurement. Finally, thanks to my dad, mom, grandpa, and grandma for their encouragement and to my wife for her patience and love. vi NOMENCLATURE AFM Atomic Force Microscopy ASTM American Society for Testing and Materials BL Bilayer EVOH Ethylene Vinyl Alcohol HL Hexalayer H-Bond Hydrogen Bonding LAP Laponite LbL Layer-by-Layer MMT Montmorillonite OTR Oxygen Transmission Rate OL Octalayer PAA Poly(acrylic acid) PAAM Polyacrylamide PAH Poly(allyl amine hydrochloride) PANI Polyaniline PDAD Poly(diallyldimethylammonium) PECVD Plasma-Enhanced Chemical Vapor Deposition PEI Branched Polyethylenimine PEO Poly(ethylene oxide) PET Poly(ethylene terephthalate) vii PHEA Poly(2-hydroxyethyl acrylate) PMAA Poly(methacrylic acid) PNIPAAM Poly(N-isopropyl acrylamide) PP Polypropylene PSS Poly(styrene sulfonate) PVCL Poly(N-vinylcaprolactam) PVD Physical Vapor Deposition PVME Poly(vinyl methyl ether) PVP Polyvinylpyrrolidone QCM Quartz Crystal Microbalance QL Quadlayer RH Relative Humidity SEM Scanning Electron Microscopy TEM Transmission Electron Microscopy VMT Vermiculite viii TABLE OF CONTENTS Page ABSTRACT .......................................................................................................................ii DEDICATION .................................................................................................................. iv ACKNOWLEDGEMENTS ............................................................................................... v NOMENCLATURE .........................................................................................................vii TABLE OF CONTENTS .................................................................................................. ix LIST OF FIGURES ..........................................................................................................xii LIST OF TABLES ....................................................................................................... xviii CHAPTER I INTRODUCTION ....................................................................................... 1 1.1 Background .............................................................................................................. 1 1.2 Objectives and Dissertation Outline ......................................................................... 4 CHAPTER II LITERATURE REVIEW ............................................................................ 8 2.1 Polymer Composites .............................................................................................. 10 2.1.1 Polymer Blends ............................................................................................... 10 2.1.2 Polymer/Platelet Composites .......................................................................... 14 2.2 Thin Film Gas Barrier ............................................................................................ 17 2.2.1 Thin Metal Films ............................................................................................. 17 2.2.2 Thin Metal-Oxide Films .................................................................................. 17 2.2.3 Super Gas Barrier Multilayers ......................................................................... 19 2.3 Layer-by-Layer Assembly...................................................................................... 20 2.3.1 Electrostatic Layer-by-Layer Assembly .......................................................... 22 2.3.2 Spray-Assisted Deposition of Electrostatic Bonded Layer-by-Layer Assembly .................................................................................................................. 26 2.3.3 Hydrogen-Bonded Layer-by-Layer Assembly ................................................ 29 CHAPTER III IMPROVING GAS BARRIER OF CLAY-POLYMER THIN FILMS USING SHORTER DEPOSITION