Temperature dependence of the free volume holes in polyhydroxybutyrate biopolymer: A positron lifetime study

The size of the free volume holes and their distributuion in polyhydroxybutyrate (PHB) biopolymer have been studied using positron annihilation lifetime spectroscopy (PALS). The measurements were performed as a function of the temperature from 243 to 363 K. The temperature dependence of the free volume hole shows a glass transition temperature, T-g at 278 K. The free volume behaviour of PHB shows a small linear increase with temperature below T-g and a steeper increase above T-g. The thermal expansion coefficients of the free volume hole were determined to be 4.78 x 10(-3) and 12.5 x 10(-3) / K below and above T-g, respectively. The free volume hole distribution shows a narrow distribution below T-g. With increasing temperature, the maximum of the distributions of the free volume holes shifts to higher values. Meanwhile, widening of the distribution is smaller in lower temperature ranges and more pronounced at higher temperatures, especially above T-g. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.