CRYSTALLIZATION KINETICS IN BLENDS OF ISOTACTIC AND ATACTIC POLY(BETA-HYDROXYBUTYRATE)

Isothermal crystallization kinetics in the miscible blend system isota ctic/atactic poly(P-hydroxybutyrate) (PHB) are examined by optical mic roscopy. The presence of the atactic component results in a reduction in spherulitic growth rate of the isotactic polymer. Analysis of the g rowth rates using the Lauritzen-Hoffman model shows that a regime II - -> III transition is present for all blend compositions investigated ( 0-50 wt% atactic), and that this transition occurs at a constant tempe rature of 140 degrees C. The ratio of the nucleation constants for the two regimes, K-g(III)/K-g(II), is found to deviate from the theoretic al value of 2.0 in the blend system. It is shown that a modified versi on of the Lauritzen-Hoffman equation incorporating entropic effects is unable to account for these deviations. The work of chain-folding, q, is calculated to be 5.1 kcal mol(-1) for isotactic PHB. The implicati ons of using assumed values of the transport parameters in the kinetic analysis are discussed.