Random copolymers were prepared by Candida antarctica lipase B (Novozyme-435) catalyzed copolymerization of omega-pentadecalactone (PDL) with epsilon-caprolactone (CL). Over the whole composition range PDL-CL copolymers are highly crystalline (melting enthalpy by differential scanning calorimetry, above 100 J/g; crystallinity degree by wide-angle X-ray scattering, WAXS, 60-70%). The copolymers melt at temperatures that linearly decrease with composition from that of poly(omega-pentadecalactone) (PPDL; 97 degrees C) to that of poly(epsilon-caprolactone) (PCL; 59 degrees C). The WAXS profiles of PCL and PPDL homopolymers are very similar, except for the presence in PPDL of the (001) reflection at 2theta = 4.58 degrees that corresponds to a 19.3 angstroms periodicity in the chain direction. In PDL-CL copolymers the intensity of this reflection decreases with increasing content of CL units and vanishes at 50 mol % CL, as a result of randomization of the ester group alignment and loss of chain periodicity. PDL-CL copolymers crystallize in a lattice that gradually changes from that of one homopolymer to that of the other, owing to comonomer isomorphous substitution. Cocrystallization of comonomer units is also shown by a random PDL-CL copolymer obtained in a polymerization/transesterification reaction catalyzed by C. antarctica lipase B (Novozyme-435) starting from preformed PCL and PDL monomer.

The crystal phase of random 3-hydroxybutyrate/3-hydroxyvalerate (3HB/3HV) copolyesters (3HV content Ck95 mol %) has been investigated by wide-angle X-ray scattering. Isodimorphism of the copolymer system is confirmed, P(3HB-co-3HV) crystallizing in either a P(3HB) or P(3HV) lattice for 3HV contents lower or higher than 40 mol % , respectively. P(3HB-co-41% 3HV) shows the coexistence of both

Blends of atactic poly[(R,S)-3-hydroxybutyrate], a-PHB, with poly(ε-caprolactone), PCL, and with poly(l-lactic acid), PLLA, were obtained in the form of compression-molded films. The phase behavior of the blends was different:  a-PHB and PCL were totally immiscible, whereas a-PHB/PLLA blends were miscible over the whole composition range. Biodegradation experiments were carried out on the blends and on the plain polymers in a buffered solution of PHB-depolymerase A from Pseudomonas lemoignei (Tris−HCl, pH = 8, T = 37 °C). None of the pure blend components (a-PHB, PCL, PLLA) showed any weight loss upon enzyme exposure. Conversely, both a-PHB/PCL and a-PHB/PLLA blends biodegraded. Analysis of the biodegradation products and of blend composition changes during biodegradation demonstrated that in both blends only the a-PHB component undergoes enzymatic hydrolysis. The results support the hypothesis that the crystalline polyester blended with a-PHB promotes a-PHB enzymatic hydrolysis by providing stable binding sites to the enzyme. The dependence of biodegradation rate on blend composition is different in (immiscible) a-PHB/PCL and (miscible) a-PHB/PLLA blends and can be explained in terms of different phase behavior and morphology.

Blends of bacterial poly(3-hydroxybutyrate) (P(3HB)) with cellulose acetate butyrate (CAB) and cellulose acetate propionate (CAP) were prepared by melt compounding. P(3HB)/CAB blends containing 5 5 0 % P(3HB) and P(3HB)/CAP blendawith 5-60% P(3HB) are transparent,stable homogeneous amorphous glasses, while blends with higher P(3HB) content are partially crystalline. When in the amorphous state, both P(BHB)/CAB and P(3HB)/CAP blends show a glass transition which regularly decreases with increasing P(3HB) content, in excellent agreement with the behavior predicted for totally miscible blends. Bothdynamic mechanical (DMTA) and calorimetric (DSC) measurements show that P(3HB) and CAB can crystallize from the blends only at temperatures higher than the composition-dependent Tg. When crystallization is induced by thermal treatments, the melting temperature of the crystalline phase obtained depends on composition, as expected for miscible blends of crystallizable polymers. Besides the strongly composition dependent glass transition, another relaxation is observed, located in proximity to the Tg of P(3HB) and slightly shifting to higher temperature with increasing CAB or CAP content. DSC measurements on melt-quenched blends containing more than 60% P(3HB) indicate contribution of both blend components to this glass transition process, on the basis of the very large specific heat increment observed. It is suggested that the two glass transitions are the manifestation of two mobilization processes coexisting in blends which appear in all respects to be single-phase, homogeneous mixtures.

Isothermal crystallization from the melt of blends of bacterial poly (3-hydroxybutyrate) (PHB) with two cellulose acetobutyrates (CE1 and CE2) has been investigated by hot-stage optical microscopy. Space-filling spherulites are observed at all compositions (50-100% PHB) and crystallization temperatures (50-130 °C) explored. The spherulite radius increases linearly with time, and the radial growth rate (G) strongly depends on the cellulose ester content of the blend. G decreases with increasing CE content: the crystallization rate of the blend containing 50% CE2 is 2.5 orders of magnitude lower than that of pure PHB. The spherulites show banding whose spacing increases with increasing Tc for a given blend, while at constant Tc banding decreases with increasing cellulose ester concentration. In PHB/CE1 blends, owing to the ability of CE1 to crystallize from the melt concomitantly to PHB at certain compositions and crystallization temperatures, a very unusual deviation from linearity of the radial growth of PHB spherulites is observed. The rate is seen to increase with time, reflecting the compositional changes which occur in the melt as a consequence of CE1 crystallization. Apart from the cases where crystallization of CE1 occurs, the morphology of melt-crystallized PHB/CE1 and PHB/CE2 blends is rather independent of the cellulose ester identity.

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