Development of amphiphilic cellulose nanocrystal/polymer composite particle and its application

Abstract

Cellulose nanocrystal (CNC), a rod-shaped nanoscale material isolated from the most abundant cellulose, has attracted a considerable research interest because of its distinct advantages including biocompatibility, biodegradability, lightweight, high tensile strength, stiffness and aspect ratio and low toxicity. This project aims to develop the synthetic methodology to prepare CNC-based composite particles and explore the potential application for enhancing mechanical properties of the epoxy resin. The CNC/polymer particles have been synthesized through a graft copolymerization of methyl methacrylate (MMA) from the chitosan/CNC complex through an emulsion polymerization. Material properties such as chemical composition, particle size and size distribution, as well as morphology have been carefully characterized using various advanced analytical techniques. Potential application of the PMMA/CTS-CNC composite particles for enhancing mechanical performances of the epoxy resin has been explored. Results show that the composite particles can improve the toughening performance of the epoxy as compared to those PMMA/CTS particles and pure CNCs. Thus, this type of CNC-based composite particle is a promising epoxy toughening agent. Chapter one introduces properties of cellulose nanocrystals and CNC-based nanomaterials. Fabrication methods of the CNC/polymer are reviewed. This chapter also describes the amphiphilic core-shell particles developed by Professor Pei Li's group including formation mechanism and properties. Finally, current toughening methods of epoxy resins using nanofillers and their toughening mechanisms are discussed. Chapter Two presents the rationale for the synthesis of the CNC-based composite core-shell particles and its potential use in epoxy toughening. Specific objectives of this project are listed in detail. Chapter three focuses on the synthesis and characterization of CNC-based composite particles. The particles were synthesized through a two-stage reaction: 1) Preparation of CNC/chitosan complexes through an electrostatic interaction and a hydrogen bonding; 2) Free-radical polymerization of MMA from the chitosan/CNC complexes, generating PMMA/CTS-CNC core-shell particles. Properties of CNC, CNC/chitosan complexes and composite particles were systematically investigated using Fourier transform infrared spectroscopy (FT-IR), dynamic light scattering (DLS), ξ-potential, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The CNCs have rod-like morphology with an average particle dimension of 202.9±34.5 nm in length and 18.1±5.4 nm in diameter, giving rise to an aspect ratio (L/d) of around 11.2±2.8. The measured ξ-potential of CNCs is -35.3 ±5.3 at pH 4.5, due to the presence of sulfuric acid groups on the surface of CNC. XRD results show that the CNC has high crystallinity (crystallinity index = 81.2 %). Complexation between the negatively charged CNCs and positively charged chitosan was carefully investigated through varying the CNC to chitosan weight ratios. Increasing the CNC to chitosan ratio resulted in increasing the complexed size and size distribution, as well as decreasing the positive surface charges of the particles. The CNC-based composite particles were then synthesized using MMA to CTS weight ratio of 4:1. Stable emulsions of the PMMA/CTS-CNC particles with high monomer conversations (> 90%) were obtained. Chemical structure of the PMMA/CTS-CNC particles was identified by the FTIR. The hydrodynamic diameter of composite particles vary from 230 to 290 nm with different CNC contents in particles, and have narrow particle size distributions (PDI <0.1). Scanning electron microscopy (SEM) images reveal that resulting composite particles have an oval-shaped morphology with an aspect ratio (length/width) of 1.34. The results indicate that the presence of CNC can influence the particles shape formation. AFM images further illustrate the unique morphology of PMMA/CTS-CNC composite particles. XRD profile detects two characteristic peaks of CNCs, confirming the presence of CNC in the resulting composite particles. Chapter Four explores the application of CNC-based composite particles in enhancing epoxy mechanical properties. The CNC-based hybrid particles show improved mechanical properties including the strain, strength, Young's modulus and toughness of epoxy composites. The last chapter provides conclusions of this thesis work and recommendations for further study. Significance and implications made in this project were also highlighted.