A Rheological Study On The Molecular Structure Of Biodegradable Polycarbonate

Biodegradable polycarbonate has attracted widespread attentionbecause of its readily accessible raw materials and relatively smoothbiological degradation. The thermal degradation is of practicalimportance. The knowledge of biodegradable polycarbonate thermalstability and pyrolysis may help to optimize the processing conditionsand modify its performance. In this paper, the degradation behavior ofpoly(propylene carbonate)(PPC) was investigated during melt processingby combination of gel permeation chromatograph (GPC), rheology andkinetic model to infer the mechanism and kinetic of thermal degradationfor the first time. The degradation behavior of PPC was also predicted viasimply online monitoring of torque curve. However, biodegradablepolycarbonate’s abroad applications are limited by its low melt strength,low tensile strength. In order to modify above defects, Long chainbranching (LCB) poly(butylene carbonate)(PBC) was prepared viafunctional group reactions by processing, and its topological structuresand properties were evaluated and characterized.The degradation experiments were carried out in a miniatureconical twin-screw extruder at different temperatures, rotating speedsand processing time. The degradation process at various processingconditions was described by the population balance equations (PBEs) with random chain scission and chain end scission. Gel permeationchromatography (GPC) was applied to analyze the molecular weight andmolecular weight distributions (MWDs) of melt processed PPC samples.By comparing the prediction of PBE model with the experimentalevolution of molecular weight, it is proposed that random chain scissionand chain end scission occur simultaneously. At temperature higher than160°C, random chain scission dominates with the activation energyabout113kJ/mol. The mechanism of chain end scission has an increasinginfluence on the degradation at short mixing time as the temperaturedecreases.The dependence of the shear viscosity on the molecular weight andtemperature was successfully established via combining rheometer withGPC. Moreover, a method combining the PBE model and rheology wassuggested to determine the kinetics of degradation directly from thetorque of mixer during melt processing without further measurementson molecular weight. Such method was applied to melt mixing of PPC ina batch mixer, from which a higher kinetic parameter of thermaldegradation and similar activation energy were successfully determinedas compared to those obtained from extrusion experiments.LCB PBC was successfully prepared by the successive reactions ofthe end hydroxyl groups of PBC with pyromellitic dianhydride (PMDA)and triglycidyl isocyanurate (TGIC). Gel permeation chromatography (GPC) and rheology were adopted to prove the existence of LCB PBC andpredict functional group reaction mechanism. In order to obtainquantitative information on the chain topological structure, their linearand nonlinear viscoelasticity were simulated by branch-on-branch (BOB)model and Pom-Pom model respectively.