BIOPOLYESTER MELT BEHAVIOR BY TORQUE RHEOMETRY

A torque rheometer method has been developed to evaluate the melt behavior of biopolyesters. The main features of this technique include: (i) a modified equip ment configuration, so as to maintain the melt temperature within +/- 1 degrees C of the set temperature after the first few minutes of polymer melting and thermal equilibration, and (ii) a kinetic equation that can be used to determine the melt behavior from the torque-time data. Applying this methodology to a poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer shows that the degradation kinetics follow Arrhenius behavior with temperature, that the degradation rate increases with increasing shear, and that the activation energy for random chain scission is independent of shear rate. This effect of shear on the degradation process may be a result of viscous heating. The degradation rate is increased because of the increased internal energy of the polymer chains as a result of the mechanical deformation. There is no evidence of direct mechanical degradation. Last, the viscosity values obtained with the torque rheometer are consistent with those obtained by capillary rheometry.