Phase behavior and thermal properties for binary blends of compositionallyfractionated poly(3-hydroxybutyrate-co-3-hydroxypropionate)s with different comonomer composition

Bacterially synthesized poly(3-hydroxybutyrate-co-3-hydroxypropionate) [P(3 HB-co-3HP)] samples were compositionally fractionated using solvent/nonsolv ent fractionation techniques. Binary blends between these compositionally f ractionated P(3HB-co-3HP) samples, with much narrower comonomer composition al distribution, were prepared, and their thermal and crystallization behav ior were studied using differential scanning calorimeter (DSC) and polarize d optical microscopy. It was found that, in the amorphous phase of these bl ends, the miscibility mainly depends on the difference of 3HP contents (or 3HB contents) between the two components of the blend. When the difference of 3HP content between two copolyesters is less than 30-40 mol %, they beco me miscible in the amorphous phase. The type of crystal lattice, miscibilit y, and difference of the spherulite growth rate between the two components of the blend appear to be the important factors controlling the crystalline phase behavior. The results are compared with those of unfractionated P(SH B-co-3HP)s and the effects of comonomer composition and its distribution on the properties of unfractionated copolymers are discussed.