Crystallization kinetics in blends of isotactic and atactic poly(β-hydroxybutyrate)

Abstract

Isothermal crystallization kinetics in the miscible blend system isotactic/atactic poly(β-hydroxybutyrate) (PHB) are examined by optical microscopy. The presence of the atactic component results in a reduction in spherulitic growth rate of the isotactic polymer. Analysis of the growth 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 temperature of 140°C. The ratio of the nucleation constants for the two regimes, K III g K II g , is found to deviate from the theoretical value of 2.0 in the blend system. It is shown that a modified version 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 implications of using assumed values of the transport parameters in the kinetic analysis are discussed.