PALS free volume and mechanical properties in dimethacrylate-based thermosets

Dynamic mechanical thermal analysis, impact strength, bulk density and double-bond conversion are reported for a series of dimethacrylate-based networks in which the addition of styrene or/and divinylbenzene co-monomer leads to variations in the morphology and crosslink density. The glass transition region was very broad for DVB-rich networks and this was interpreted in terms of network heterogeneity. The mean size and the size distribution of free-volume holes were estimated via analyses of positron annihilation lifetime spectroscopy (PALS) measurements at 22 degreesC and were found to be inversely correlated with the bulk density and dependent on the network heterogeneity. The PALS free volume was raised for DVB-rich systems, due to the poor molecular packing in these highly crosslinked systems and because some regions in this heterogeneous network were in a rubbery state. The impact strength was found to be lower for the more highly crosslinked systems but was inversely correlated with the level of molecular mobility as measured by DMTA. (C) 2004 Society of Chemical Industry.