Crystallization Kinetics of PHB/PVAc Blends Using Time Resolved Dielectric Spectroscopy

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Polymers and High Performance Materials


Crystallization kinetics of poly(hydroxy butyrate), PHB, and its blends with poly(vinyl acetate), PVAc, have been thoroughly investigated using broadband dielectric technique over a wide range of frequencies (10(-2)-10(5) Hz) as functions of crystallization temperature and blend composition. The dielectric strength of the amorphous segments, Delta epsilon, which is directly proportional to the volume fraction of the mobile amorphous phase in the blend decreases exponentially with increasing the crystallization time. However, on the other hand, the dielectric strength of the rigid amorphous segments, Delta epsilon(alpha'), which is related to the percentage of crystallinity in the blend increases dramatically with increasing crystallization time. A great variation in the dynamical constraints of relaxation segments with increasing crystallization time has been observed as a result of different environments, which would lead to a variation in the consistency of the cooperative regions. The value of the dielectric constant, epsilon', decreases dramatically with increasing crystallization time, after that it reaches an equilibrium value at the end of the crystallization process. This dramatic decrease in the value of epsilon' as a result of crystallization at a given crystallization temperature, was taken as an accurate evaluation for the amount of the amorphous phase that has undergone crystallization considering the theoretical approach of Avrami. The Avrami exponent, n, was found to be crystallization temperature, T-c, independent (n similar to 3) indicating a three-dimensional crystal growth for pure PHB. The crystallization rate constant, k, increases greatly with increasing T-c due to the high crystallization rate. In the blend the value of n was found to be concentration dependent (n similar to 1.8-3.2). The different values of n indicate that the shapes of the growing crystals are affected by blend concentration. For n similar to 1.8, the crystals can either grow sporadically as rods or instantaneously as disks, while for n similar to 3 a three-dimensional crystal growth takes place. (C) 2007 Elsevier Ltd. All rights reserved.

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European Polymer Journal





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