Isothermal Crystallization of Poly[3-Hydroxybutyrate- Co -4-hydroxybutyrate] Mixtures

The effect of melt processing temperature on the isothermal crystallization behavior of poly(3-hydroxybutyrate-co-4-hydroxybutyrate)[P(3HB-co-4HB)] is studied using differential scanning calorimetry and polarizing microscopy. P(3HB-co-4HB) undergoes thermal degradation above its melting temperature. The degree of degradation increases with increase in the 4HB content in copolymer. The formation of low molecular weight P(3HB-co-4HB) from thermal degradation reduces the crystallization temperature. Thus, the isothermal crystallization rate and crystallinity decrease at a processing of 190 degree celsius compared with 180 degree celsius where less thermal degradation occurs. This is more evident in the case of P(3HB-co-4HB) of high 4HB content. The P(3HB-co-4HB) crystals formed in the isothermal crystallization process have different perfectness in the 4HB rich phase and the 3HB rich phase depending on the melt processing temperature. This results in the appearance of two distinct melting peaks with different melting temperatures and melting enthalpies. Our results indicated that thermal history in the isothermal crystallization process will affect these crystals differently.