Memory effects in polyethylenes: influence of processing and crystallization history

The experimental timescale of memory effects related to restoring equilibrium viscoelastic properties in solution-cast linear and long-chain branched polyethylenes has been investigated. It was found that the crystallization history of low-density polyethylene and ultra-high-molecular-weight polyethylene, with respect to solid-state and melt properties, is lost after heating into the melt after a short period of time. The experimental timescale of memory effects is at least one order of magnitude shorter than the longest relaxation times in these materials, which correspond to reptative motion of macromolecules. The experimental results indicate that long-term memory effects related to the physical entanglement network in polyethylenes are absent and that equilibrium viscoelastic properties of polyethylene melts and solids and the concept of entanglement coupling are related to segmental rather than to macromolecular mobility.