Toughening mechanisms of block copolymer and graphene nanoplatelet modified epoxy polymers
File(s)
Author(s)
Chong, Huang Ming
Type
Thesis
Abstract
Epoxies are thermosetting polymers that have uses in a multitude of industrial and consumer applications. The high crosslink density of epoxies gives rise to exceptional mechanical, chemical and heat resistance properties. However, this also results in low toughness, i.e. poor resistance to crack initiation and propagation. The present work discusses the toughening mechanisms of four epoxy polymers modified with several novel modifiers, which create different morphologies.
Nanoscale core-shell rubber (CSR) particles are attractive as epoxy tougheners because they remain well dispersed even at high loadings, do not affect the Tg and most importantly, provide good toughness improvement (900% increase in GIC for a low Tg epoxy system). For a given weight percentage of modifiers, the tensile and compressive properties were better maintained for the much smaller CSR particles as these particles have a lower rubber content. However, the fracture performance of the CSR modified epoxies appear to be limited in low Tg epoxies. Examination of the fracture surfaces using high resolution scanning electron microscopy (SEM) show less plastic void growth due to the higher stresses required to initiate cavitation.
Amphiphilic triblock copolymers (BCP) represent the next generation of phase separating materials for toughening epoxies. The structure/property relationships of epoxies modified with symmetric and asymmetric triblock copolymers were determined. The complex 3D nanostructures that were created offer greatly increased fracture performance over conventional toughening agents for very tough epoxies, while minimising the decrease in mechanical properties. A measured increase in GIC of 1600% was noted. This increased to 2250% when a further addition of silica nanoparticles was considered. This complex nanostructure allows for more gradual and extensive plastic deformation of the epoxy matrix as shear yielding and plastic void growth initiated by debonding at the BCP/epoxy interface. The morphology was further studied numerically using a phase field model which identified parameters that control the evolution of the microstructure.
Graphene nanoplatelets (GNP) vary in size and quality depending on the method of preparation. Thus, a range of GNPs with different platelet sizes, thicknesses and aspect ratios were used to identify the properties that control the mechanical and fracture performance of GNP modified epoxies. The bulk GNP geometry and chemical makeup were first characterised. The optimum dispersion method was determined through systematic experiments using two solvents and an ultrasonic probe, and examined using SEM. Well dispersed GNPs improved the Young’s modulus, whereas the fracture energy increased for both well dispersed and poorly dispersed GNPs.
Nanoscale core-shell rubber (CSR) particles are attractive as epoxy tougheners because they remain well dispersed even at high loadings, do not affect the Tg and most importantly, provide good toughness improvement (900% increase in GIC for a low Tg epoxy system). For a given weight percentage of modifiers, the tensile and compressive properties were better maintained for the much smaller CSR particles as these particles have a lower rubber content. However, the fracture performance of the CSR modified epoxies appear to be limited in low Tg epoxies. Examination of the fracture surfaces using high resolution scanning electron microscopy (SEM) show less plastic void growth due to the higher stresses required to initiate cavitation.
Amphiphilic triblock copolymers (BCP) represent the next generation of phase separating materials for toughening epoxies. The structure/property relationships of epoxies modified with symmetric and asymmetric triblock copolymers were determined. The complex 3D nanostructures that were created offer greatly increased fracture performance over conventional toughening agents for very tough epoxies, while minimising the decrease in mechanical properties. A measured increase in GIC of 1600% was noted. This increased to 2250% when a further addition of silica nanoparticles was considered. This complex nanostructure allows for more gradual and extensive plastic deformation of the epoxy matrix as shear yielding and plastic void growth initiated by debonding at the BCP/epoxy interface. The morphology was further studied numerically using a phase field model which identified parameters that control the evolution of the microstructure.
Graphene nanoplatelets (GNP) vary in size and quality depending on the method of preparation. Thus, a range of GNPs with different platelet sizes, thicknesses and aspect ratios were used to identify the properties that control the mechanical and fracture performance of GNP modified epoxies. The bulk GNP geometry and chemical makeup were first characterised. The optimum dispersion method was determined through systematic experiments using two solvents and an ultrasonic probe, and examined using SEM. Well dispersed GNPs improved the Young’s modulus, whereas the fracture energy increased for both well dispersed and poorly dispersed GNPs.
Version
Open Access
Date Issued
2015-02
Date Awarded
2015-06
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Taylor, Ambrose
Kinloch, Anthony
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)