Figure 3.

Comparison of the structure of (a) a thermoplastic and (b) a thermoset polymer; for a given polymer chain, the process of cross-linking reduces the atomistic characteristic length l_a of the system from the entanglement length to the distance between two cross-links, and largely inhibits the sliding of chains past each other (by a reptation mechanism) therefore reducing relaxation time (based on the illustration by Henderson C., 2015, CC0). The constitutive behaviour of (c) short-chain polyethylene, a thermoplastic polymer, and (d) TGMDA epoxy, a thermoset polymer, for different strain rates ${\dot{ϵ}}_a$; decreasing the strain rate ${\dot{ϵ}}_a$ ceases to have an influence comparatively earlier for epoxy resin than for polyethylene, convergence of the behaviour appearing to be hardly reached for polyethylene in the range of test strain rates. (a) Polyethylene reptation length, (b) epoxy cross-link distance, (c) polyethylene uniaxial behaviour and (d) epoxy uniaxial behaviour.