BIOPOLYESTER MELT BEHAVIOR BY TORQUE RHEOMETRY

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