Liquid-nitrogen quenching was applied to study the enthalpy effect on the stress-induced crystallization of microbial polyesters. Crystallization bands of poly(3-hydroxybutyrate) exhibited the potential to reveal the stress distribution in the melt; while crystallization of poly[(3-hydroxybutyrate)-co-(3-hydroxyhexanoate)] gave shishkebab structures. Polarized-light micrographs confirmed that the enhanced nucleation was attributed to the tensile stress. Furthermore, control of the quenching direction provides a method to direct the lamellar growth. Polarized-light micrographs ofPHBfilm crystallized at 90 8C after quenching in liquid nitrogen from the melt. The normal of the bands, namely the lamellar growth direction, runs predominantly parallel to the stress direction.

Real-time atomic force microscopy observation was carried out during crystallization in thin films of chiral poly(R-3-hydroxybutyrate-co-R-3-hydroxyhexanoate) copolymer, and the development details of single lamellae in the banded spherulites are revealed for the first time. The lamellae exhibit complicated growth behaviors: twisting, bending, backward growth, and branching. The lamellae continuously twist to show alternating edge-on and flat-on views along the radii of the spherulites. Giant screw dislocations bring forth to the birth of new lamellae. Interaction between the leading and trailing lamellae contributes to cooperative stacking of the twisting crystals. The lamellae twist before screw dislocations appear, demonstrating that screw dislocations are not causal of twisting. All the observed twisting occurs in the right-handed sense, likely resulting from the chirality of the crystal structure. Increased crystallization temperature results in decreased magnitude of lamellar twisting and bending. Models for morphological development are discussed in the context of these observations.

Banded spherulites of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3HB-3HV)) random copolymer were prepared by isothermal crystallization at 90℃ for 10h. Using tapping-mode atomic force microscopy (AFM), the concentric periodic ridges and valleys on the surface of the banded spherulites of this polymer were found to consist of edge-on and flat-on lamellae, respectively. The periodic concentric ridges and valleys observed by AFM corresponded to the periodic extinction rings observed by polarized optical microscopy. AFM measurements showed that the interaction between the AFM probe and the sample surface can be significantly influenced by lamellar orientation.

Strain Pseudomonas stutzeri 1317 was able to synthesize a variety of polyhydroxyalkanoates (PHAs) when grown in glucose and/or fatty acids. The monomer structures of the PHAs were found to be dependent on the structures of related fatty acids. Up to ten PHA monomers have been found in PHAs, including 3-hydroxybutyrate (HB), 3-hydroxyvalerate (HV), 3-hydroxyhexanoate (HHx), 3-hydroxyheptanoate (HHp), 3-hydroxyoctanoate (HO), 3-hydroxynonanoate (HN), 3-hydroxydecanoate (HD), 3-hydroxyundecanoate (HU), 3-hydroxydodecanoate (HDD) and 3-hydroxytetradecanoate (HTD). In the presence of butyric acid, valeric acid to nonanoic acid, HB, HV to HN were all synthesized. Mixed substrates of glucose and fatty acids led to the formation of fatty acid structure related PHA monomers. Monomer percentage contents in PHA could be adjusted by regulating ratios of fatty acids in growth substrates. During cell growth in glucose and octanoic acid, the addition of heptanoic acid stimulated the rapid synthesis of HHp in PHA. It can be concluded that PHA structures can be tailor made by using structure-related fatty acids and varying ratios of fatty acids.

On banded spherulites of poly (3-hydroxybutyrate) and its copolymers, fine circles were observed between either cross-polarizers or without polarizers. Atomic force microscopy was applied to confirm that these circles are terraces with heights up to several hundred nanometers rather than cracks. Real-time observation demonstrated that the terrace forms at the front of the growing spherulites just before or exactly when two spherulites impinge on each other. Terraces were observed on the spherulites crystallized from melt confined between glass or polyimide slides rather than poly(ethylene terephthalate) slides. The formation of the terraces may have resulted from instability of the moving boundary of the melt film confined between the spherulite surface and cover slide. Wettability of the substrate played an important role in the formation of the terraces.