Study on the Thermal Degradation Kinetics of Biodegradable Poly(propylene carbonate) during Melt Processing by Population Balance Model and Rheology

The degradation behavior of poly­(propylene carbonate) (PPC) was investigated during melt processing to infer the mechanism and kinetics of thermal degradation. First, the degradation experiments were carried out in a miniature conical twin-screw extruder at different temperatures, rotating speeds, and processing times. Gel permeation chromatography (GPC) was applied to analyze the molecular weight and molecular weight distributions (MWDs) of melt processed PPC samples. The degradation process at various processing conditions was described by the population balance equations (PBEs) with random chain scission and chain end scission. By comparing the prediction of PBE model with the experimental evolution of molecular weight, it is proposed that random chain scission and chain end scission occur simultaneously. At temperature higher than 160 °C, random chain scission dominates with the activation energy about 120 kJ/mol. Second, a method combining the PBE model and rheology was suggested to determine the kinetics of degradation directly from the torque of mixer during melt processing without further measurements on molecular weight. Such method was applied to melt mixing of PPC in a batch mixer, from which a higher kinetic parameter of thermal degradation and similar activation energy were successfully determined as compared to those obtained from extrusion experiments

Mesophase Separation and Rheology of Olefin Multiblock Copolymers

Chain shuttling polymerization enables an efficient production of ethylene–octene block copolymers (OBCs) that combine different mechanical properties in a polymer chain. However, this method results in molecular weight polydispersity and multiblock chain structure. The melt-phase behavior and mesophase transition of the polydisperse OBCs with low octene content but different molecular weight and block composition were investigated by rheology, differential scanning calorimetry (DSC), atomic force microscopic (AFM), polarized optical microscopy (POM), and small-angle X-ray scattering (SAXS). Three rheological methods, namely the deviation of the scaling dependence of zero shear viscosity on molecular weight, the terminal behavior and the failure of time–temperature superposition (TTS), and two-dimensional rheological correlation spectrum, are used to reveal the mesophase separation with increasing sensitivity. The occurrence of mesophase separation transitions (MST) was observed in such low octene content and low molecular weight OBC systems, with much lower degree of segregation than the theoretical predictions in diblock copolymers. The extent of mesophase separation is further justified by its effect on subsequent crystallization behaviors