Linear and non-linear rheological characterization of photo-oxidative degraded LDPE

Sheets of low-density polyethylene (LDPE) were photo-oxidatively treated in the presence of air. Characterization methods include solvent extraction method and rheology in the melt state by shear and uniaxial extensional experiments. For photo-oxidation, a xenon lamp was used to irradiate the samples for times between 1 day and 6 weeks. Linear viscoelasticity is strongly affected by increasing photo-oxidation due to crosslinking of LDPE, as corroborated by gel fraction. For samples which were degraded for up to three days, negligible gel content was found and the non-linear parameters fmax and β of the Molecular Stress Function (MSF) model indicate that the induced increase in strain hardening is due to the formation of long-chain branching, which is overcoming chain scission. Further degradation leads the parameter fmax to increase to values outside the experimental window, while the parameter β was found to correlate with the gel content and with the trend followed by the strain hardening index. The competition between chain scission and crosslinking could also be followed in linear viscoelasticity by the change in activation energy and in the loss angle δ. From the constitutive analysis, it is noticed that, within the experimental window, time-deformation separability is confirmed for all samples, independent of the degree of photo-oxidation. It is demonstrated that the sensitivity of rheological characterization with respect to structural modifications, when combined with constitutive analysis, can be used to elucidate the effects of photo-degradation on LDPE.