Four commercially available samples of bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] containing from 6.5 to 21.8 mol % 3HV have been fractionated by chloroform/reheptane mixed solvent. They have been separated into several fractions with wide composition ranges. These results show that the composition distribution of as received bacterial P(3HB-co-3HV)s is extremely broad and/or has many peaks over a wide composition range. The effects of the complex composition distribution on physical properties have been analyzed through the comparison of melting and crystallization behavior between samples before and after fractionation. Three as received P(3HB-co-3HV)s show the behavior corresponding to the average composition in spite of their complex composition distribution. The melting temperature and spherulite growth rate correspond to the values expected from extrapolation of the data from the fractionated samples. In these copolyesters, cocrystallization of the chains within the wide composition range occurs. However, one as received P(3HB-co-3HV) has a higher melting temperature and faster growth rate than might be expected. The apparent crystallization behavior corresponds to that of P(3HB-co-3HV) with lower 3HV content. These data suggest that only the component chains of relatively low 3HV content are crystallized in this as received P(3HB-co-3HV). The crystallization of components of high 3HV content is significantly restricted. The extremely broad composition distribution of this copolyester affects the crystallization (melting) behavior.
ABSTRACT:Crystallization and melting behavior of blends of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(HBco-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.KEY WORDS Composition Distribution / Polymer Blend / Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) / Recently, productions of various hydroxyalkanoate copolymers (PHAs) by bacterial fermentation have been reported. The importance of this family owes to biodegradability combined with thermoplasticity. Poly(3-hydroxybutyrate) (PHB) and its copolyesters with 3-hydroxyvalerate (PHB-HV) are the best-known members of this family. 1 -3 A variety of properties 4 · 5 such as melting temperature, 6 -8 solid-state structure, 6 • 9 -10 -12 crystallization behavior, 11 • 13 mechanical properties, 14 · 1 5 and enzymatic degradation processes, 16 have been studied for this copolyester. Most of the studies focused on the dependence of these properties on the comonomer composition. These properties certainly vary with the HV content. In these studies, however, a narrow composition distribution was expected without any careful consideration.Every copolymer is considered to be a kind of polymer blends by its nature, that is, a copolymer material is composed of molecules over a range of comonomer compositions. In general, the range in a synthesized copolymer is believed to be very narrow. We can treat a copolymer material as a mass of identical macromolecules and the average composition as a composition of every molecule. In this study, we refer to only the copolymer with a narrow composition distribution as a pure copolymer. However, we found recently that bacterial PHB-HV copolyesters have a complex feature of composition distribution, i.e., their compo...
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