We have calculated by two different methods the shear modulus of a crosslinked polymer melt near the onset of rigidity, using molecular dynamics simulation. A macroscopic measurement called E from a volume preserving deformation yields a very slowly convergent average value. From an equilibrium fluctuation method, which probes the microscopic response of the system to deformation, significant variations in the average shear modulus mu are observed. And mu, after several million time steps, moves among values much smaller than E. A partitioning of the system into smaller cells reveals a constantly evolving system with regions of negative shear modulus corresponding to local structural changes. For a homogeneous system the two approaches will give equal results. For an inhomogeneous system, different values are expected, but if there is a well established ground state, mu should be larger than E, in the case of a system near percolation. We conclude from our study that the crosslinked polymer melt does not reach a well defined thermodynamic state, but oscillates continuously between several states in a manner reminiscent of a structural glass.
關聯:
APS-98 March Meeting,Los Angeles Convention Center,
