Morphology and Microstructural Aspects

Unlike rubber-toughened epoxy polymers, the relationship between the microstructure and toughness of thermoplastic-toughened epoxy polymers is not fully understood. Several studies have been initiated the focusing on fracture properties and fractrography of thermoplastic-toughened epoxy systems to establish the definitive relationships between microstructure and fracture properties. However, as discussed in detail next, the complex nature of materials precludes straightforward interpretations. 

The microstructure of the modified epoxy networks largely depends on the cure condition. For a multistep cured system, the final microstructure depends on initial cure temperature, and post-curing condition has no role in microstructure development because phase separation gets arrested at gelation. An increase in initial cure temperature causes increase in particle size due to a decrease in system viscosity. 

These observations are similar to those observed in a rubber-toughened epoxy system. Yamanka and co-workers [114] observed a phase-inverted co-continuous structure by decreasing the cure temperature in their PES-modified epoxy system. They reported that a decrease in the cure temperature slowed down the rate of phase separation (based on spinodal decomposition process) without significantly reducing the rate of the chemical reaction. This arrested the phase separation at an early stage of phase separation and finally resulted in an interconnected globular epoxy particle in a co-continuous modifier-rich matrix. 

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