Influence of vinyl ester/styrene network structure on thermal and mechanical behavior

The influence of vinyl ester/styrene network structure on thermal and mecha nical properties was investigated. The crosslink density of the resins was altered by changing the molecular weight of the vinyl ester oligomer and by varying the amount of styrene used during the crosslinking reaction leadin g to variations in both the physical network structure and the chemical com position of the polymeric networks. The glass transition temperatures of th e network polymers were found to increase systematically with increasing cr osslink density without the additional influence of the chemical compositio n as determined from both differential scanning calorimetry (DSC) and dynam ic mechanical analysis (DMA). The breadth of the glass transition regions i ncreased with crosslink density for the DSC data, but the breadth assessed from the DMA data did not vary significantly for the network materials. A s econdary relaxation was observed for the materials using DMA, and this rela xation did not appear to be significantly affected by changes in either the crosslink density or the composition of the network. Cooperativity studies involving time-temperature scaling of dynamic mechanical data in the glass formation temperature region were also conducted. The degree of segmental cooperativity at T-g appeared to be primarily influenced by the chemical co mposition of the networks. These issues dealing with the structure of the n etworks provided insight into the associated fracture properties in the gla ssy state (ambient temperature). Specifically, an empirically based linear correlation was found between the fracture toughness of the networks and th e cooperative domain size at the glass transition temperature normalized by the crosslink density. (C) 2001 John Wiley & Sons, Inc.