List of papers This thesis is based on the following papers: I Improvement of toughness and electrical properties of epoxy composites with carbon nanotubes prepared by industrially relevant processes R Hollertz, S Chatterjee, H Gutmann, T Geiger, F A Nüesch and B T T Chu Nanotechnology, 22,125702 (2011) [1] II Comparing carbon nanotubes and graphene nanoplatelets as reinforcements in polyamide-12 composites S Chatterjee, F A Nüesch and B T T Chu Nanotechnology, 22,275714 (2011) [2] III Investigation of crystalline and tensile properties of carbon nanotube- filled polyamide-12 fibers melt-spun by industrially-related processes S Chatterjee, F A Reifler, B T T Chu and R Hufenus Journal for Engineered Fibers and Fabrics (accepted for publication) [3] IV Mecahnical reinforcement and thermal conductivity in expanded graphene nanoplates reinforced epoxy composites S Chatterjee, J W Wang, W S Kuo, N H Tai, C Salzmann, W L Li, R Hollertz, F A Nüesch and B T T Chu Chemical Physics Letters 531, 6 (2012) [4] V The size and synergy effects of graphene nanoplatelets in mechanical properties of epoxy composites S Chatterjee, F Nafezarefi, N H Tai, L Schlagenhauf, F A Nüesch and B T T Chu under preparation [5] Reprints were made with permission from the publishers. Preface Preface This doctoral thesis presents a study carried out to understand the rein- forcing effects of carbon nanofillers on the structure and physical prop- erties of thermoplastic and thermoset polymer matrices. All the exper- iments have been performed during my tenure as a Ph.D. student be- tween 2008 - 2012 at the Swiss Federal Laboratories for Materials Science and Technology (Empa), Switzerland and the Division of Molecular and Condensed Matter Physics at the Department of Physics and Astron- omy, Uppsala University, Sweden. Most of the activities were carried out by me with the help and support of my colleagues at three departments of Empa namely, Functional Polymers, Advanced Fibers and Protection and Physiology. In addition some experiments were done at the Materi- als Science Department of Swiss Federal Institute of Technology (ETH), Zürich. Some of the important measurements were carried out at the synchrotrons MAX-lab, Sweden and Swiss Light Source, Switzerland. The last four years have been a overwhelming journey for me as I have worked with many talented scientists and have enriched my knowledge with their support. In the following pages I put together the interesting findings which have made my doctoral studies so rewarding. I take this opportunity to thank you for your interest in my research work. v Preface Comments on my own participation The project has been carried out based on the framework of the pro- posal accepted by the Swiss National Science Foundation. I have per- formed most of the experimental techniques described in the thesis with adequate training and support from technical personnel. Among the five publications mentioned in this thesis I have had the main responsibility for planning and executing the experimental part and subsequent data analysis as well as writing the manuscript for four of them. For the pub- lication in which I am the second author I have carried out part of the study and have assisted the first author in manuscript preparation. I would be happy to provide further information regarding any aspect of the research mentioned in the thesis. vi Abbreviations List of Abbreviations ASTM American Society for Testing and Materials CNT Carbon nanotube CVD Chemical vapour diposition DR Draw ratio DSC Differential scanning calorimetry ECI Equatorial percentage crystallinity EGNP Expanded graphene nanoplatelet Ext Extrusion factor FTIR Fourier transform infra red spectroscopy GnP Graphene nanoplatelet HPH High pressure homoginizer FDTD Finite difference time domain ISO International Organization for Standardization MAS Magic angle spinning MWCNT/MWNT Multi-walled carbon nanotube NG Natural graphite NMR Nuclear magnetic resonance PA Polyamide PA12 Polyamide-12 SENT Single edge-notched tension SEM Scanning electron microscope Str Strain factor SWCNT/SWNT Single-walled carbon nanotube TEM Transmission electron microscope TGA Thermogravimetric analysis WAXD Wide angle x-ray diffraction vii Contents Preface . v 1 Introduction . 1 1.1 Carbon nanofillers . 2 1.2 Polymer nanocomposites . 4 1.2.1 Properties of polymer nanocomposites . 5 1.3 Motivation . 7 2 Materials . 11 2.1 Synthesis . 11 2.1.1 Carbon nanotube . 11 2.1.2 Graphene . 13 2.2 Properties . 13 2.3 Polymer composites of CNT and graphene . 14 2.3.1 Dispersion . 14 2.3.2 Methods for manufacturing polymer nanofiller com- posites . 16 2.4 Thermoplastic polymer: Polyamide . 17 2.4.1 Polyamide nanocomposites . 18 2.5 Thermoset polymer: Epoxy . 20 2.5.1 Epoxy nanocomposites . 21 2.6 Specifications of materials used . 22 3 Experimental details . 25 3.1 Processing of polyamide-12 . 25 3.2 Processing of epoxy . 28 3.3 Characterization . 28 4 Polyamide Composites . 35 4.1 Dispersion of nanofillers . 35 4.2 Nuclear magnetic resonance (NMR) . 37 CONTENTS 4.3 Wide angle x-ray diffraction (WAXD) . 37 4.4 Thermal analysis . 43 4.4.1 Differential scanning calorimetry (DSC) . 43 4.4.2 Thermogravimetric analysis (TGA) . 45 4.5 Mechanical properties . 45 4.6 Electrical conductivity . 50 5 Epoxy Composites . 57 5.1 Dispersion . 57 5.2 Mechanical properties . 61 5.3 Electrical conductivity . 65 5.4 Thermal conductivity . 66 6 Conclusions and Outlook . 69 Summary . 75 Populärvetenskaplig sammanfattning . 79 7 Bibliography . 83 Acknowledgement . 95 x LIST OF FIGURES List of Figures 1.1 Graphene as building block . 4 1.2 Composite application in automobile . 6 2.1 TEM image of SWNT . 12 2.2 TEM image of graphene. 14 2.3 Surface modifications of carbon nanotubes. 16 2.4 Structure of PA12. 19 2.5 SEM images of PA12 composites. 20 2.6 Structure of Epoxy resin. 21 2.7 SEM images of nanofillers used . 23 3.1 Micro-extruder . 26 3.2 In-house spinning set-up . 27 3.3 Processing steps for epoxy composites . 29 4.1 SEM images showing agglomeration sites within the com- posites. 36 4.2 SEM images showing fractured surfaces of the composites. 36 4.3 WAXD images of films . 38 4.4 Deconvolution of WAXD peak for film sample . 39 4.5 WAXD images of fibers . 41 4.6 Deconvolution of WAXD peak for fiber . 41 4.7 Herman’s orinetation factor of fibers . 43 4.8 DSC curves . 44 4.9 TGA curves . 45 4.10 Modulus of toughness . 47 4.11 specific tensile strength in relation to the strain factor . 50 4.12 Electrical conductivity of films . 51 xi LIST OF FIGURES 4.13 Electrical percolation in films . 54 5.1 TEM images of CNT dispersion . 58 5.2 Matlab simulation of CNT dispersion . 59 5.3 STEM images of CNT network in matrix . 59 5.4 TEM images of amine-EGNP dispersion . 60 5.5 TEM images of CNT and GnP mixture dispersion . 60 5.6 Mechanical properties of CNT composites . 61 5.7 Mechanical properties of EGNP composites . 63 5.8 Meachnical properties of GnP composites . 64 5.9 Mechanical properties of CNT/GnP mixture composites . 65 5.10 Electrical conductivity of CNT composites . 66 5.11 Thermal conductivity of EGNP and GnP composites . 67 xii LIST OF TABLES List of Tables 2.1 A summary of the most common methods of graphene synthesis. 14 2.2 A summary of properties of CNT and graphene. 15 4.1 Chemical shifts (in ppm) of 13C in pure PA12 samples. 37 4.2 Equatorial crystallinity index (% crystallinity) for a set of PA12 films with 0.5 wt% CNT and GnP loading. 40 4.3 Crystallinity of pure and composite films from DSC analysis 44 4.4 Summary of extrusion factors, strain factors and tensile strengths. 49 4.5 Percolation threshold in PA12 composite samples for the different nanofillers. 53 xiii CHAPTER 1. INTRODUCTION 1. Introduction There is no higher or lower knowledge, but one only, flowing out of experimentation - Leonardo da Vinci. Eagerness for experimentation has always gifted mankind with novel materials and the ever curious mind has never rested to improvise on all that already exist. Reinforcement is "to make a structure or material stronger, especially by adding another material to it". It is the effects of "adding another material" that fascinate a large section of the sci- entific community today. The idea of using fillers as reinforcing agents is not new, it possibly started with the use of straw to reinforce mud bricks in about 4000 BCE. Reinforcement grids, plates or fibers have been incorporated to strengthen concrete. This method was invented in 1849 and even patented in 1867. In more recent times, fibres made from materials such as alumina, glass, boron, silicon carbide and most signifi- cantly carbon have been used as fillers in composites. These conventional fibres have dimensions on the meso-scale with diameters of tens of mi- crons and lengths of the order of millimetres limiting their capabilities of mechanical reinforcement. Polymer composites with additives mixed in thermoplastic, thermoset and elastomer matrices are considered as an important group of materials for their wide variety of applications. In contrast to the incorporation of micro-scale fillers, nanofillers, due to their size ensure a very small inter-particle distance which influences the polymer matrix properties even at very low filler concentrations. Polymer nanocomposites came into limelight with the discovery, at Toyota research center in the 1980s, that on addition of a small fraction of nanoclay to PA6 dramatic improvement could be recorded in strength, modulus, heat distortion temperature and gas barrier properties [6]. 1 CHAPTER 1. INTRODUCTION Carbon, a remarkable element is known for its extraordinary ability of catenation. A property to combine with itself and other chemical el- ements in various ways forms the basis of organic chemistry and thus life.