Crystallization and melting behavior in blends of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)s with a narrow composition distribution

Abstract

Crystallization and melting behavior of blends of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(HB"co-HV)) copolyesters, which were prepared by compositional fractionation of bacterially synthesized P(HB-co-HV) and have a narrow composition distribution, have been investigated and compared to those of P(HB-co-HV) before fractionation. Analysis of DSC melting thermograms and spherulite growth rate indicates four situations that might arise from the blend. These are: (i) An isomorphous crystalline phase being formed from the miscible melt. (ii) Two crystalline phases observed in situations (i) and (iii) being formed from the miscible melt. (iii) A crystalline phase of the component polyester having the largest crystallization rate being partitioned from the miscible melt. (iv) Two crystalline phase similar to the pure components being formed from the immiscible melt. Most bacterial P(HB-co-HV) copolyesters are in situation (i) and are indistinguishable from P(HB-co-HV) having a narrow composition distribution. Some bacterial P(HB-co-HV) copolyesters, which have a extremely broad composition distribution, show behavior different from P(HB-co-HV) with a narrow one. We need to take account of the details of composition distribution of these P(HB-co-HV) samples. Especially in situation (iii), we are apt to mistake the properties of the partitioned crystals for those of the isomorphous crystals. An observation of a single Tm is not sufficient to confirm the isomorphous crystals.