Polymorphic crystallization of fractionated microbial medium-chain-length polyhydroxyalkanoates

By focusing on a range of fractions of medium-chain-length polyhydroxyalkanoates (mcl-PHA), a more in-depth understanding of the thermal properties and the crystalline structure has been gained. Dual-mode melting was observed for all the fractionated samples. Further DSC analysis revealed that such phenomenon was due to the presence of two crystalline phases (phase I and phase II), forming at different temperatures. As noted, for the samples with higher 3-hydroxytetradecanoate (HTD) comonomer composition, the crystallization rate of phase II was much depressed, which made phase I dominant. The WAXD and FTIR results indicate that the two phases differ in the crystalline structure. The in situ IR and WAXD registrations indicate that phase I would transform into phase II through a melting-recrystallization process during heating. The high similarity of IR spectra in the range of 1500-900 cm−1 between phases I and II excludes the possibility of adopting different conformation. It is suggested that major differences between the two crystalline phases lie in the molecular packing.