No abstract

Aeromonas caviae produced a random copolymer of 3-hydroxybutyric acid (3HB) and 3-hydroxyhexanoic acid (3") under aerobic conditions when sodium salts of alkanoic acids of even carbon numbers ranging from Clz to CIS and olive oil were fed as the sole carbon source. On the other hand, a copolymer of 3HB and 3-hydroxyvaleric acid (3HV) was produced by A. caviae from alkanoic acids of odd carbon numbers from Cll to (217. The weight-average molecular weights of P(3HB-co-3") were in the range (2-11) x lo5. The structure and physical properties of P(3HB-co-3HH) with compositions of 5-25 mol % 3HH were characterized by IH and 13C NMR spectroscopy, X-ray diffraction, differential scanning calorimetry, mechanical tensile measurement, and optical microscopy. The degree of X-ray crystallinity of solvent-cast P(3HB-eo-3") films decreased from 60 to 18% as the 3HH fraction was increased from 0 to 25 mol %, suggesting that 3HH units are excluded from the P(3HB) crystalline phase. The isothermal radial growth rates of spherulites of P(3HB-co-3HH) were markedly reduced with an increase in the 3HH fraction. Enzymatic degradations of P(3HB-co-3") films were carried out at 37 "C in an aqueous solution of P(3HB) depolymerase from Alcaligenes faecalis. The rates of enzymatic erosion increased markedly with an increase in the 3HH fraction to reach a maximum value at 15 mol % 3HH, followed by a decrease in the erosion rate. The above results were compared with the solid-state properties of two other microbial copolymers, P(3HB-co-3HV) and P(3HB-co-3HP) (3HP: 3-hydroxypropionic acid).

Like most of the materials used by humans, polymeric materials are proposed in the literature and occasionally exploited clinically, as such, as devices or as part of devices, by surgeons, dentists, and pharmacists to treat traumata and diseases. Applications have in common the fact that polymers function in contact with animal and human cells, tissues, and/or organs. More recently, people have realized that polymers that are used as plastics in packaging, as colloidal suspension in paints, and under many other forms in the environment, are also in contact with living systems and raise problems related to sustainability, delivery of chemicals or pollutants, and elimination of wastes. These problems are basically comparable to those found in therapy. Last but not least, biotechnology and renewable resources are regarded as attractive sources of polymers. In all cases, water, ions, biopolymers, cells, and tissues are involved. Polymer scientists, therapists, biologists, and ecologists should thus use the same terminology to reflect similar properties, phenomena, and mechanisms. Of particular interest is the domain of the so-called "degradable or biodegradable polymers" that are aimed at providing materials with specific time-limited applications in medicine and in the environment where the respect of living systems, the elimination, and/or the bio-recycling are mandatory, at least ideally.

Polymorphism phenomenon of melt-crystallized poly(butylene adipate) (PBA) has been studied by wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC). It has been found that the isothermal crystallization leads to the formation of PBA polymorphic crystals, simply by changing the crystallization temperature. The PBA alpha crystal, beta crystal, and the mixture of two crystal forms grow at the crystallization temperatures above 32 degrees C, below 27 degrees C, and between these two temperatures, respectively. The relationship between PBA polymorphism and melting behaviors has been analyzed by the assignments of multiple melting peaks. Accordingly, the equilibrium melting temperatures Tm degrees of both alpha and beta crystals were determined by Hoffman-Weeks and Gibbs-Thomson equations for the purpose of understanding the structural metastability. The Tm degrees of the PBA alpha crystal was found to be higher than that of the beta crystal, indicating that the PBA alpha crystal form is a structurally stable phase and that the beta crystal form is a metastable phase. The analysis of growth kinetics of PBA polymorphic crystals indicates that the metastable PBA beta crystal is indeed the kinetically preferential result. Based on the thermal and kinetic results, the phenomenon of stability inversion with crystal size in melt-crystallized PBA was recognized, in terms of the growth mechanisms of PBA alpha and beta crystals and the transformation of beta to alpha crystals. The PBA beta --> alpha crystal transformation takes place at a sufficiently high annealing temperature, and the transformation has been evident to be a solid-solid-phase transition process accompanied by the thickening of lamellar crystals. The molecular motion of polymer chains in both crystalline and amorphous phases has been discussed to understand the thickening and phase transformation behaviors.

A 5.0-kbp EcoRV-EcoRI restriction fragment was cloned and analyzed from genomic DNA of Aeromonas caviae, a bacterium producing a copolyester of (R)-3-hydroxybutyrate (3HB) and (R)-3-hydroxyhexanoate (3HHx) [P(3HB-co-3HHx)] from alkanoic acids or oils. The nucleotide sequence of this region showed a 1,782-bp poly (3-hydroxyalkanoate) (PHA) synthase gene (phaC Ac [i.e., the phaC gene from A. caviae]) together with four open reading frames (ORF1, -3, -4, and -5) and one putative promoter region. The cloned fragments could not only complement PHA-negative mutants of Alcaligenes eutrophus and Pseudomonas putida, but also confer the ability to synthesize P(3HB-co-3HHx) from octanoate or hexanoate on the mutants' hosts. Furthermore, coexpression of ORF1 and ORF3 genes with phaC Ac in the A. eutrophus mutant resulted in a decrease in the polyester content of the cells. Escherichia coli expressing ORF3 showed (R)-enoyl-coenzyme A (CoA) hydratase activity, suggesting that (R)-3-hydroxyacyl-CoA monomer units are supplied via the (R)-specific hydration of enoyl-CoA in A. caviae. The transconjugant of the A. eutrophus mutant expressing only phaC Ac effectively accumulated P(3HB-co-3HHx) up to 96 wt% of the cellular dry weight from octanoate in one-step cultivation.The utilization of biological systems for production of biodegradable materials is becoming important as a solution of the problems concerning plastic waste and the global environment. Poly(3-hydroxyalkanoates) (PHA) are produced by a wide variety of bacteria as intracellular carbon-and energystorage materials from renewable carbon resources, such as sugars or plant oils (2,6,18,25). Since these bacterial PHA are biodegradable thermoplastics, they have attracted industrial attention as possible candidates for large-scale biotechnological products. At present, more than 90 different monomeric units have been found as constituents of PHA (37).Bacterial PHA can be divided into two groups, depending on the number of carbon atoms in the monomeric units (35). One group of bacteria, including Alcaligenes eutrophus, produces short-chain-length PHA with C 3 -to-C 5 monomer units, while the other group, including Pseudomonas oleovorans, synthesizes medium-chain-length PHA with C 6 -to-C 14 monomer units. Only a few reports are available for bacteria which can synthesize PHA consisting of both short-and medium-chainlength monomer units. For example, Rhodospirillum rubrum (3), Rhodocyclus gelatinosus (19), and Rhodococcus ruber (12) produce terpolymers consisting of C 4 , C 5 , and C 6 3-hydroxyalkanoate (3HA) units from hexanoate, and some pseudomonad strains accumulate PHA consisting of C 4 -to-C 12 3HA units (16,38). Our laboratory has found that a random copolymer of 3-hydroxybutyrate (3HB) and 3-hydroxyhexanoate (3HHx), P(3HB-co-3HHx), is produced by Aeromonas caviae FA440 isolated from soil (7,32). This bacterium synthesizes the copolyester from alkanoic acids of even carbon numbers or from plant oils up to approximately 30 wt% of the cellular dry weight, with a 3HHx fra...

No abstract

Regime transitions of poly[(S)-lactide] (PLA) crystal growth from the melt were investigated by studying the morphological changes and carrying out kinetic analysis using microscopic techniques. PLA thin films with an average layer thickness of 100 nm were isothermally crystallized at a given crystallization temperature after melting at 220 degrees C. Following isothermal crystallization at a temperature below 145 degrees C, uniform two-dimensional spherulites having stacked flat-on lamellar texture were developed throughout the PLA thin films. On the basis of electron diffraction analysis for two-dimensional spherulites of PLA, it was found that the average growth direction of an individual lamellar crystal was parallel to the crystallographic b axis. At temperatures above 150 degrees C, hexagonal lamellar crystals were formed from the melt. Electron diffractograms of these lamellae showed that the crystal had orthogonal packing of PLA molecules and a truncated-lozenge-shaped growth behavior. The growth surfaces of the hexagonal crystal were parallel to either the crystallographic (110) or the (100) plane. The PLA crystal growth rate along the b axis direction was evaluated at various crystallization temperatures of the thin films. Kinetic analysis of crystal growth in the PLA thin film demonstrated that the regime transitions of PLA crystal growth, from regime III to regime II and from regime II to regime I, occur at around 120 and 147 degrees C, respectively. The transition from regime II to regime I induced morphological changes in the crystalline aggregates whereby spherulitic aggregates transformed into hexagonal lamellar stacking. As for the transition between regimes II and III, no obvious morphological change in the spherulitic crystal aggregates was observed.

No abstract