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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).

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.

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.

Studies of solid-state structures under different crystallization conditions, melting behavior, and crystal growth kinetics are reported for biodegradable aliphatic poly(butylene succinate) (PBS) and its random copolyesters of poly(butylene succinate-co-14 mol %ethylene succinate) [P(BS-co-14 mol %ES)] and poly(butylene succinate-co-15 mol %hexamethylene succinate) [P(BS-co-15 mol %HS)]. The crystal structures of two copolyesters determined by wide-angle X-ray diffraction (WAXD) are the same as that of the PBS homopolymer, suggesting that the second comonomers ES or HS units are excluded from the crystalline core and are in an amorphous state. The further investigations on the crystalline and amorphous phase structures and sizes by small-angle X-ray scattering (SAXS) method have provided quantitative evidence that the existence of second comonomers increases the amorphous thickness of copolyesters but that the influences of comonomers on the lamellar crystal structure and size of copolyesters are complicated. The origin of the complexity and its influences on the equilibrium melting temperature T(m)(0) of copolyester are discussed in the present paper. Gibbs-Thomson and Hoffman-Weeks equations were applied for the determination of T(m)(0) of PBS, P(BS-co-14 mol %ES), and P(BS-co-15 mol %HS)-three samples. Two equations gave different T(m)(0) values for each sample. On the basis of the Gibbs-Thomson equation, the P(BS-co-15 mol %HS) copolyester has the same T(m)(0) value as that of the PBS homopolymer, while the P(BS-co-14 mol %ES) copolyester has a lower one. This is due to the different influences of the second comonomers on the crystalline phase structure indicated by SAXS results, and the Gibbs-Thomson equation has been suggested to be more reliable. The crystal growth kinetics of the three samples was analyzed by using the secondary nucleation theory, and the influences of various parameters on the regime transition and nucleation constant were studied. Meanwhile some interrelations between regime transition temperature and melting behavior were found for the three samples.

The films of random copolymers of (R)-3-hydroxybutyric acid with different hydroxyalkanoic acids such as (R)-3-hydroxypentanoic acid, (R)-3-hydroxyhexanoic acid, 4-hydroxybutyric acid, 6-hydroxyhexanoic acid, and (S,S)-lactide were prepared by the melt-crystallized method at various crystallization temperatures. The solid-state structures and thermal properties of melt-crystallized films were characterized by means of wide-angle X-ray diffraction, small-angle X-ray scattering, differential scanning calorimetry, optical microscopy, and scanning electron microscopy. Both the long period distance and the lamellar thickness of melt-crystallized polyester films were increased with an increase in the crystallization temperature. The melting temperature of polyester films also increased with an increase in the crystallization temperature. From the relationship between lamellar thickness and melting temperature, it has been concluded that randomly distributed second monomer units except for the (R)-3-hydroxypentanoic acid unit in copolyesters act as defects of poly[(R)-3-hydroxybutyric acid] crystal and are excluded from the crystalline lamellae. The enzymatic degradation of melt-crystallized films was carried out at 37 °C in an aqueous solution (pH 7.4) of PHB depolymerase from Alcaligenes faecalis. After enzymatic degradation, the smooth and rough planes were observed mutually along the radial direction of spherulites on the surface of melt-crystallized films, suggesting that PHB depolymerase predominantly hydrolyzes polymer chains on the edges of crystalline lamellar stacks. The enzymatic erosion rate of crystalline region in polyester films decreased with an increase in the lamellar thickness.

The melting behavior, crystal structure, and spherulitic morphology of melt‐crystallized poly(butylene adipate) (PBA) at a temperature range from 25 to 35 °C have been investigated by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD) and optical microscopy. Two distinct melting behaviors with double peaks have been observed after melt‐crystallizing at different temperatures. X‐ray analysis proves that these two behaviors arise from two forms of PBA crystal structures. The β‐form crystals are formed at temperatures below 31 °C, while the α‐form crystals are formed above 29 °C. When the crystallization temperature is 30 ± 1 °C, a mixture of α‐form and β‐form crystals has been formed. The influences of annealing treatment and sample preparation method on the two forms of PBA crystal structures have been discussed. The results indicate that the low temperature‐formed β‐form crystals are very sensitive to the preparation conditions.

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