Phase separation and mechanical properties of epoxy / thermoplastic blends

Abstract

Thermosetting epoxy resins blended with aromatic thermoplastics are of significant commercial interest in several areas including aerospace, transportation and electronics. These materials offer an excellent combination of mechanical properties ease of processing and overall component cost. A more complete and fundamental understanding of these materials could potentially lead to a new generation of thermoplastic-thermoset blends with enhanced properties that would extend their use into new applications. This thesis studies the reaction induced phase separation behaviour of a thermosetting epoxy resin blended with an aromatic polyethersulphone thermoplastic and explores the link between morphology and physical properties. The phase separation behaviour of the thermoplastic-thermoset blend is studied using a combination of Small Angle Light Scattering (SALS), Differential Scanning Calorimetry (DSC) and Polarised Light Microscopy. A large part of this thesis involved the design and build of specialised SALS equipment that enabled the phase separation of the thermoplastic-thermoset blends to be studied dynamically. The studies show that in this thermoplastic-thermoset blend the mechanism of phase separation appears to be, without exception, spinodal decomposition. There was no evidence of phase separation occurring by nucleation and growth in these blends. The particulate morphologies seen in the thermoplastic-thermoset blends appear to form by the break up of a percolating network that initially forms by spinodal decomposition. This behaviour is termed Percolation to Cluster Transformation (PCT).Cahn-Hilliard theory was used to analyse the SALS data from the reacting thermoplastic-thermoset blends to give kinetic information about the phase separation process. These studies show that this particular blend exhibits Lower Critical Solution Temperature (LCST) behaviour. The work also shows that Cahn-Hilliard theory does not fully describe the spinodal decomposition processes occurring in these thermoplastic-thermoset blends. The effect of blend composition, cure temperature and epoxy functionality on phase separation behaviour are studied. This shows that in certain formulations especially around the critical formulation of the blend both primary and secondary spinodal decompositions occur during cure. This can lead to thermoplastic-thermoset blend formulations that have very unique, and previously unreported, morphologies for these systems. These novel morphologies resemble sub-included or salami morphologies similar to that observed in rubber modified polystyrene (HIPS) but have in fact formed by multiple PCT occurring during cure. This study shows that moisture can significantly influence the properties of epoxy resin based systems. It also appears that a phase inverted morphology consisting of a continuous thermoplastic rich phase with a dispersed thermosetting rich phase appears to offer great advantage in terms of fracture and mechanical properties after both dry and moisture conditioning. The phase-inverted morphology also appears to significantly reduce the moisture ingress of the epoxy blend.