Zhihua Gan

Solid-state structures and thermal properties of aliphatic–aromatic poly(butylene adipate-co-butylene terephthalate) copolyesters

Abstract Aliphatic–aromatic copolyesters of poly(butylene adipate-co-butylene terephthalate) [P(BA-co-BT)] with wide copolymer compositions were synthesized by melt polycondensation. Solution 13 C NMR analyses show that the copolyesters are statistically random copolymers of BA and BT units. Their solid-state microstructures and thermal properties were investigated by wide-angle X-ray diffraction (WAXD), solid-state 13 C NMR, differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The WAXD shows that the copolyesters with 10–25 mol% BT contain PBA crystals, while those with 27.5–80 mol% BT contain PBT crystals. In contrast, solid-state 13 C NMR analyses demonstrate that the copolyesters with 20–30 mol% BT units contain both PBA and PBT crystals, but the minor one cannot be detected by X-ray diffraction due to the small sizes and amounts. Both the melting temperature and crystallinity of copolyesters show minimum values at around 25 mol% BT content which is the transition point from PBA crystal structure to PBT crystal structure. The composition dependences of melting temperature, crystallinity, crystallization rate and spherulite morphology are discussed and correlated to the biodegradability of the P(BA-co-BT) copolyesters.

Surface structure, morphology and stability of polyhydroxyalkanoate inclusions characterised by atomic force microscopy

Atomic force microscopy was used to analyse the surface structure, morphology and stability of polyhydroxyalkanoate (PHA) inclusions isolated from various microorganisms. The isolated PHA inclusions adsorbed firmly onto positively charged polyelectrolyte film on glass cover slip. Freshly isolated inclusions appeared spherical, smooth surfaced and without any structural features. With time, the adsorbed inclusions showed drastic morphological changes. PHA inclusions containing short-chain-length monomers (C4, C5) isolated from Comamonas acidovorans showed the presence of membrane-like layers on the surface, to which globular particles of about 30 nm in apparent size were attached. Repeated centrifugation during inclusion isolation resulted in the disappearance of the membrane-like layers. Prolonged storage of the adsorbed inclusions at ambient conditions resulted in the formation of lamella stacks, with a uniform thickness of about 5 nm. PHA inclusions isolated from Pseudomonas oleovorans containing predominantly C8 monomers showed a complex surface structure that appeared as a network of microfibril-like structures.

Direct observation of polyhydroxyalkanoate granule-associated-proteins on native granules and on poly(3-hydroxybutyrate) single crystals by atomic force microscopy

Polyhydroxyalkanoate (PHA) granules isolated from Comamonas acidovorans revealed the presence of globular particles on the granule surface when imaged using atomic force microscopy (AFM). The globular particles appeared to form a monolayer on the granule surface. Repeated washing with pure water resulted in granules with smooth surface without any globular particles. Purified granule-associated-proteins and PHA single crystals were used to construct a model representing native granule surface for AFM imaging. For this purpose, PhaR and PhaP proteins, purified from Paracoccus denitrificans were adsorbed onto poly(3-hydroxybutyrate) [P(3HB)] single crystals. PhaR is a 22 kDa protein involved in the regulation of PhaP (phasin) protein (16 kDa). Both purified proteins were morphologically different but adsorbed uniformly on the single crystal surface forming a monolayer. Immunogold-labeling was used to further confirm the adsorption and identity of PhaR protein on P(3HB) single crystals. The study shows for the first time, that proteins associated with PHA granules can be imaged directly and characterised using AFM. In addition, AFM images obtained in this study provide direct evidence for the binding of PhaR and PhaP proteins to the hydrophobic PHA surface.

Direct observation of polyhydroxyalkanoate chains by atomic force microscopy

Atomic force microscopy in the tapping mode was used to investigate aqueous acetone-treated polyhydroxyalkanoate (PHA) inclusions freshly isolated from a recombinant bacterium. The PHA is a copolymer containing about 95 mol% 3-hydroxybutyrate units while the rests are units of 3-hydroxyhexanoate, 3-hydroxyoctanoate, 3-hydroxydecanoate, and 3-hydroxydodecanoate. Polymer chains extending to several micrometers in length were observed on glass cover slips upon the evaporation of the aqueous acetone. The polymer chains seem to exist in the form of fibrillar aggregates. The height of the microfibrils was about 1 nm. Upon prolonged standing at ambient conditions, the microfibrils dissociated into finer strands of about 0.5 nm in height. The results suggest that biosynthesized PHA are stored in the inclusions in an amorphous state but with minimal chain entanglement. This is possible because the PHA chains exist in the form of fibrillar aggregates that may be the product of a special biosynthesis mechanism.

The Solid-State Structure, Thermal and Crystalline Properties of Bacterial Copolyesters of (R)-3-Hydroxybutyric Acid with (R)-3-Hydroxyhexanoic Acid

Bacterial poly(3-hydroxyalkanoic acid)s (PHAs) are interesting biodegradable and biocompatible thermoplastics produced by a wide variety of microorganisms from various carbon sources.1,2 The PHAs are known to accumulate inside bacterial body as intracellular storage materials for biological carbon and energy sources1–4. Until now PHAs with more than 100 different monomeric units as constituents have been found5. These biodegradable PHA thermoplastics have attracted much attention in the recent two decades as they are environmentally friendly materials which can be degraded to carbon dioxide, water and biomass by a wide of microorganisms. Therefore the bacterial PHAs are a prospective candidate to replace the normally used plastics, which result in serious environmental pollution by waste polymers.