Influence of cure via network structure on mechanical properties of a free-radical polymerizing thermoset

An improved understanding has been achieved regarding the relationships among cure chemistry, network structure, and final physical properties of vinyl ester (VE) resins, a thermoset polymer often used as the matrix of fiber reinforced polymers. Mechanical properties of the polymer are found to depend on both cure schedule and cure formulation. The possibilities of phase separation and micro-gel formation being the cause for these differences in mechanical properties are examined. The VE/styrene (S) system does not phase separate under the conditions studied. Though bulk properties of the resin are unaffected by the details of the cure, the microscopic morphology, in particular the type of cross-link formed (intermolecular bond or intramolecular bond), is sensitive to both cure temperature and initiation mechanism as determined by cure formulation. An analysis of cure kinetics shows that both temperature and initiation mechanism affect the apparent 'reaction order' of the VE/S system as determined by the autocatalytic equation. This apparent reaction order is interpreted as being an indication of the degree of heterogeneity in the resin. By controlling cure temperature and cure formulation, it is possible to minimize the apparent reaction order and thereby optimize physical properties. Finally, a theory is adapted from other non-network polymer systems to qualitatively describe how cure temperature and initiation mechanism may alter the heterogeneity in network structure via micro-gel formation and how these changes in structure affect changes in the mechanical properties. (C) 2002 Elsevier Science Ltd. All rights reserved.