Rheological and Thermal Properties of Peroxide-Modified Poly(L-lactide)s for Blending Purposes

Poly-L-lactide (PLLA) is a widely used sustainable and biodegradable alternative to replace synthetic plastic materials in the packaging industry. Unfortunately, its processing properties are not always optimal, e.g. insufficient melt strength at higher temperatures needed in extrusion coating processes is recognized. In the present work, one have addressed the problem by peroxide modification of commercial PLLA in order to obtain chain branching. Reactive extrusion of PLLA has been carried out in the presence of 0.1, 0.3 and 0.5 wt% of various peroxides [tert-butyl-peroxybenzoate, 2,5-dimethyl-2,5-(tert-butylperoxy)-hexane (Lupersol 101) and benzoyl peroxide]. The peroxides were chosen due to their different decomposition rates at a reactive extrusion temperature of 190 °C. Changes in thermal properties (differential scanning calorimeter) and dynamic rheology, where studied. The rheological analyses were conducted at 240 °C as to mimic typical PLLA extrusion coating conditions. The peroxide-treated PLLAs showed increased complex viscosity and storage modulus at lower frequencies indicating the formation of branched/cross linked architectures. The branching is also supported by the size exclusion chromatography-chromatogram signals revealing the development of higher molecular weight species. The material property changes were dependent on the peroxide and the used peroxide concentration. Gel fraction analysis showed that the peroxides afforded different gel contents and especially 0.5 wt% peroxide produces both an extremely high molar mass and a cross linked structure, not perhaps well suited e.g., for further use in a blending step. The thermal behavior was somewhat unexpected as the materials prepared with 0.5 wt% peroxide showed the highest ability for crystallization and cold crystallization despite substantial cross linking.