Test of the energy landscape interpretation of fragility in polymers

Variation of the number of crosslinks in polyvinylethylene (PVE) networks h as a marked effect on the T-g-normalized temperature dependence ('"fragilit y") of the segmental relaxation times, even though the local chemical envir onment of virtually all repeat units is unchanged. Since the heat-capacity increment at T-g is a decreasing function of the number of crosslinks, ther e is an inverse correlation between fragility and the heat-capacity differe nce between PVE's glassy and liquid states; This result is opposite to expe ctations based on an energy landscape interpretation of fragility. [S0163-1 829(98)00245-8].