Melting and crystallization in poly(b-hydroxyalkanoate) blends

Crystallization and melting are investigated in blends involving bacterial poly($ beta$-hydroxyalkanoates), PHA's. Two different blend systems are investigated. The first comprises bacterial, isotactic poly($ beta$-hydroxybutyrate), PHB, blended with synthetic, atactic PHB. The second is composed of bacterial poly($ beta$-hydroxyvalerate), PHV, blended with poly($ beta$-hydroxybutyrate-co-hydroxyvalerate), P(HB-co-HV), and bacterial PHB.Crystallization kinetics are measured for isotactic/atactic PHB blends. The atactic component is shown to depress the spherulitic growth rate of the isotactic PHB. The Lauritzen-Hoffman model is used to analyze the crystallization data. A regime II $ to$ III transition is observed at a constant temperature of 140$ sp circ$C in the blends. The ratio of nucleation constants for the two regimes, K$ rm sb g sp{III}$/K$ rm sb g sp{II}$, is found to deviate from the theoretical value of 2.0 in the blend system. Equilibrium melting points are measured for the isotactic component and are found to be depressed by the atactic component. The Nishi-Wang equation is used to obtain an interaction parameter, $ chi sb{12}.$ Multiple melting of isotactic PHB is reported and analyzed. It is shown that a melt/recrystallization process is operative and a model is proposed to explain the melting behaviour in terms of recrystallization and annealing effects.Crystallization kinetics of bacterial PHV are analyzed using the Lauritzen-Hoffman model. Crystallization is shown to occur in regime II. The equilibrium melting point of PHV is measured to be 130$ sp circ$C. A blend of PHV with P(HB-co-HV) of 77 mol% HV is shown to exhibit cocrystallization. The homopolymers PHB and PHV are shown to be phase-separated in the melt, thus precluding cocrystallization