Rheological characterization of thermal, thermo-oxidative and photo-oxidative degradation of LDPE

Abstract

Rheology has been used to study thermal degradation (V. H. Rolon-Garrido et al., Rheol. Acta 50, 519-535, 2011), thermo-oxidative degradation (V. H. Rolon-Garrido et al., Rheol. Acta 50, 519-535, 2011; V. H. Rolon-Garrido et al., J. Rheol. 57, 105-129, 2013) and photo-oxidative degradation (V. H. Rolon-Garrido and M. H. Wagner, Polym. Degrad. Stab. 99, 136-145, 2014; V. H. Rolon-Garrido and M. H. Wagner, J. Rheol. 58, 199-22 2, 2014; V. H. Rolon-Garrido et al., Polym. Degrad. Stab. 111, 46-54, 2015) of low-density polyethylene (LDPE). This contribution presents the analogies and differences between these types of degradations of LDPE on the linear (by use of van-Gurp Palmen plots) and non-linear viscoelastic properties (by use of the parameters of the MSF model, fmax2 and β), as well as on the failure mode of the samples (through the maximum strain and stress achieved experimentally). In contrast to thermal and thermo-oxidative degradation, the linear viscoelastic properties of photo-oxidated samples were more affected by degradation. In the non-linear regime, for thermal and thermo-oxidative treated samples, the elongational measurements elucidated the role of chain scission and long-chain branching (LCB) formation, while for photo-oxidated LDPE even the competition between chain scission, LCB formation, and gel formation was demonstrated. The failure behavior was found to be determined by a constant maximum strain in thermo-oxidative degradation, if the LDPE has high content in branching points, or in photo-oxidative degraded LDPE, if a considerable portion of gel structure is present. Otherwise, either the maximum strain or stress measured was found to be strain-rate dependent.