Determination of molecular weight distribution and composition dependence of monomeric friction factors from the stress relaxation of ultrahigh molecular weight polyethylene gels

The stress relaxation of ultrahigh molecular weight polyethylene (UHMWPE) gels in a broad composition range (0.025 ≤ φ_P ≤ 1.0) was studied within the framework of the reptation model. The molecular weight distribution (MWD) was determined at an optimum gel concentration (φ_P = 0.365) by regularization analysis of the stress relaxation data. The calculated relaxation moduli [G (t)] by using MWD evaluated at a certain composition (UHMWPE gels, φ_P = 0.365) were consistent with the experimental G (t) of other compositions in a broad range (0.218 ≤ φ_P ≤ 1.0). That is, the MWD (regularization method) was self-consistent within the framework of the reptation model. The composition dependence of the monomeric friction factor (ζ_eff) of the UHMWPE gels was examined by use of the double reptation (DR) model with the determined MWD. The Ferry's free volume model of the molecular weight dependence of ζ_eff can be extended to the composition dependence of ζ_eff. In the low-concentration gels (0.025 ≤ φ_P ≤ 0.17), the experimental G (t) relaxed faster than G (t) expected by the DR model.