Phase morphology and fracture toughness of CNT/polytherimide modified epoxy polymer nanocomposites as matrices for advanced CFPR composites

Abstract

Thermosetting epoxy resins are extensively utilised as matrices for advanced carbon fibre reinforced polymer (CFRP) composites. Their relatively high stiffness, strength, glass transition temperature (Tg), and strong bonding to the reinforcing fibres, together with their ease of processing, are attractive attributes. The high crosslink density of cured epoxies, as a prerequisite for the high Tg and stiffness, is also the primary reason for the relatively low fracture toughness of these materials. This could negatively affect the interlaminar properties of the resulting FRP composites, as the toughness of the matrix is closely related to the delamination fracture toughness of the FRP composites, making them particularly susceptible to impact damage. In light of these issues, the motivation of this work was focused on enhancing the crack resistance of epoxy-based fibre reinforced composites through modifying the matrix via incorporation of micro-/nano-scale heterogeneities to create modified epoxy matrices and resulting FRP composites with significant improvements in fracture toughness for structural applications. These heterogeneities included carbon nanotubes (CNTs) and a phase separating thermoplastic (TP) material. Two types of epoxy resins were used and compared in this study, namely a high performance tetrafunctional epoxy and a standard bifunctional type. The resulting systems included various binary and ternary combinations of the epoxy matrices (bifunctional and tetrafunctional epoxies) and heterogeneities (TP and CNTs). Phase morphologies, mutual interactions and synergistic effects on fracture behaviour of the modified epoxy resins were investigated. The relationship between morphological characteristics and interlaminar fracture toughness of carbon fibre reinforced composites, based on the selected multiphase matrix materials, were also discussed. The first part of this work explores the underlying mechanism of the formation of phase-separated thermoplastic phases with distinct sizes, based on the two types of epoxy resins (bifunctional and tetrafunctional), and implications on fracture toughness. It is shown that the higher viscosity and cross-link density of the tetrafunctional epoxy are primary reasons for the thermoplastic phase being significantly smaller than the phase observed for the bifunctional epoxy at the same TP content. In both epoxy systems, a particulate morphology is readily observed when the TP content varies from 5 wt% to 15 wt%, and with the further increase of TP content to 20 wt%, the phase morphology turns to co-continuous, accompanied by a higher increase in fracture toughness. The highest fracture toughness, in terms of critical energy release rate (GIC), obtained in the bifunctional epoxy containing 20 wt% TP, is 5.5 times higher than the baseline resin (unmodified epoxy), and 2.7 times higher in the tetrafunctional epoxy system. The successfully developed analytical model for predicting the fracture toughness of TP/epoxy blends further confirms that both shear banding of the matrix and thermoplastic phase crack bridging are two major toughening mechanisms for systems with a particle/matrix morphology. In blends at higher TP contents (20 wt%) with a co-continuous morphology, crack bridging is identified as the dominant crack energy dissipation mechanism. The epoxy blends with 10 and 20 wt% TP are selected as matrices for CFRP composites. The presence of carbon fibres is shown to significantly affect TP phase separation in modified bifunctional systems. The resulting non-uniform phase morphologies of the matrix in the CFRP composites causes quite distinct fracture toughness improvements compared to the bulk epoxy system. For example, at high TP content (20 wt%), an epoxy-rich layer is formed around the carbon fibres, which leads to a 38% reduction in toughness enhancement value in FRP composite when compared to that in 20 wt% TP modified epoxy resin. In CFRPs, based on tetrafunctional epoxy blends, however, fibre reinforcement do not lead to the formation of a non-uniform phase morphology. Therefore, a steady fracture toughness enhancement in these composites can be obtained with increasing thermoplastic content. CNTs are added to the TP-modified epoxy blends to investigate the potential for further gains in fracture toughness by combining different crack energy dissipation mechanisms at both micro- and nano-length scales, and possible synergistic effects. CNTs are either initially added to the epoxy or to the TP, before mixing, to study the influence of the sequence of their introduction. It is shown that CNTs have a significant impact on different aspects of the phase separation process, and the resulting phase morphologies, depending on the introduction order and the types of epoxy (bi-functional and tetra-functional). Ternary blends, prepared by mixing nanotubes with epoxy first (1st approach), show a better CNT dispersion in comparison with the other approach where CNTs are pre-dispersed in the thermoplastic phase (2nd approach). The ternary nanocomposites in both bifunctional and tetrafunctional epoxy systems prepared by 1 st approach show the highest fracture toughness, which is 2.13 and 1.9 times higher than the fracture toughness of the unmodified counterparts respectively. Additionally, a synergistic increase in toughness of ternary blends made by the 1st approach for both epoxy systems is observed, arising from better CNT dispersion and thermoplastic deformability. Ternary epoxy blends used as matrix materials in CFRP composites show a higher degree of morphological homogeneity in comparison with CNT-free blends at similar TP content. CNTs seem to control the thermoplastic phase separation, to some extent, in a bifunctional epoxy system, through limiting molecular diffusion and accelerating the curing reaction. In a system based on a bifunctional epoxy resin at thermoplastic content of 20 wt%, CNTs are found to be mostly localised within the thermoplastic-rich domains, away from the brittle epoxy-rich domains surrounding carbon fibres. This has been identified as the main reason for the even lowered fracture toughness (0.90 kJ/m2 ) compared to the corresponding CNT-free systems (0.85 kJ/m2 ). This is not the case for the FRP composites based on the tetrafunctional epoxy resin where the resulting uniform phase morphology contributes to further improvements in fracture toughness (0.88 kJ/m2 for CFRP composite based on CNT/TP (20 wt%)/epoxy, and 0.82 kJ/m2 for CFRP composite without CNTs). In addition, the high aspect-ratio CNTs made in-house are added to the 20 wt% TP-modified bifunctional epoxy systems with the aim to control the phase morphology. The resulting CFRP specimens are shown to have even higher fracture toughness (0.99 kJ/m2 ), the highest achieved in the project. This is due to the formation of a more uniform phase separation caused by further limitations in molecular diffusion and polymer chain mobility with an increased matrix viscosity.