The crystallization behavior of three double crystalline diblock copolymers containing poly-( -caprolactone) and poly(p-dioxanone) has been studied via differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide-angle X-ray scattering (WAXS). Crystallization and melting temperatures and enthalpies are compared among copolymers and to those of the corresponding homopolymers. Only one crystallization exotherm was observed for the diblocks. DSC and WAXS indicated that during isothermal crystallization PPDX crystallized first, followed by PCL. POM revealed a transformation of crystalline morphologies at around 50 °C, from granular aggregates at high temperature (where only PPDX is crystalline) to banded spherulites at lower temperature, where both blocks were crystalline. The kinetics of crystallization were studied in detail via spherulite growth rates obtained from POM, and it was found that PPDX crystallization in the diblocks occurred much more slowly than in the homopolymers, this being responsible for the observed coincident crystallization of the two blocks during DSC cooling scans. On the other hand, the crystallized PPDX acts to nucleate the PCL block, so that heterogeneous crystallization was always observed even in copolymers containing a minority of this component (23%), for which "confined" crystallization might be expected. The crystallization kinetics of the PCL in the copolymers is accelerated by the presence of the crystalline PPDX block.

The crystallization kinetics of each constituent of poly(p-dioxanone)-b-poly(epsilon-caprolactone) diblock copolymers (PPDX-b-PCL) has been determined in a wide composition range by differential scanning calorimetry and compared to that of the equivalent homopolymers. Spherulitic growth rates were also measured by polarized optical microscopy while atomic force microscopy was employed to reveal the morphology of one selected diblock copolymer. It was found that crystallization drives structure formation and both components form lamellae within mixed spherulitic superstructures. The overall isothermal crystallization kinetics of the PPDX block at high temperatures, where the PCL is molten, was determined by accelerating the kinetics through a previous self-nucleation procedure. The application of the Lauritzen and Hoffman theory to overall growth rate data yielded successful results for PPDX and the diblock copolymers. The theory was applied to isothermal overall crystallization of previously self-nucleated PPDX (where growth should be the dominant factor if self-nucleation was effective) and the energetic parameters obtained were perfectly matched with those obtained from spherulitic growth rate data of neat PPDX. A quantitative estimate of the increase in the energy barrier for crystallization of the PPDX block, caused by the covalently bonded molten PCL as compared to homo-PPDX, was thus determined. This energy increase can dramatically reduce the crystallization rate of the PPDX block as compared to homo-PPDX. In the case of the PCL block, both the crystallization kinetics and the self-nucleation results indicate that the PPDX is able to nucleate the PCL within the copolymers and heterogeneous nucleation is always present regardless of composition. Finally, preliminary results on hydrolytic degradation showed that the presence of relatively small amounts of PCL within PPDX-b-PCL copolymers substantially retards hydrolytic degradation of the material in comparison to homo-PPDX. This increased resistance to hydrolysis is a complex function of composition and its knowledge may allow future prediction of the lifetime of the material for biomedical applications.

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Two types of mixtures were prepared by solution blending: high molecular weight polyhydroxybutyrate (PHB)/poly(ε‐caprolactone) (PCL) and PHB/low molecular weight chemically modified PCLs (mPCL). The morphology, crystallization, and enzymatic degradation of the blends were studied by differential scanning calorimetry, polarized light optical microscopy, scanning electron microscopy, 1H NMR, and weight loss measurements. In addition, enzymatic degradation studies were performed by an exposure to Aspergillus flavus. High molecular weight PHB/PCL blends were found to be immiscible in the entire composition range. Phenomena such as PCL fractionated crystallization and a decrease in PHB nucleation density were detected. When PHB was blended with mPCLs, the blends were partially miscible; two phases were formed, but the PHB‐rich phase exhibited clear signs of miscibility through a depression of both the Tm and the Tg of the PHB component (which was stronger with lower molecular weight mPCL), and an increase in the growth rate of PHB spherulites in the blends as compared to neat PHB or to the PHB component in the PHB/PCL blends. The biodegradation by a exposure to A. flavus showed that the blends are synergistically attacked in comparison to the homopolymers. Two factors may influence the improved degradation rate of the blends: the dispersion of the components and their crystallinity that was reduced in view of the fractionated crystallization and impurities transfer. In the case of the PHB/mPCL blends, the increased miscibility between the components caused a reduction in the degradation rate.magnified image

The morphology, crystallization and self nucleation behavior of double crystalline diblock copolymers of poly(p‐dioxanone) (PPDX) and poly(ϵ‐caprolactone) (PCL) with different compositions have been studied by different techniques, including optical microscopy (OM), atomic force microscopy (AFM) and differential scanning calorimetry (DSC). The two blocks crystallize in a single coincident exotherm when cooled from the melt. The self‐nucleation technique is able to separate into two exotherms the crystallization of each block. We have gathered evidences indicating that the PPDX block can nucleate the PCL block within the copolymers regardless of the composition. This effect is responsible for the lack of homogeneous nucleation or fractionated crystallization of the PCL block even when it constitutes a minor phase within the copolymer (25% or less). Nevertheless, we were able to show that decreasing amounts of PCL within the diblock copolymer still produces confinement effects that retard the crystallization kinetics of the PCL component and decrease the Avrami index. On the other hand evidence for confinement was also obtained for the PPDX block, since as its content is reduced within the copolymer, a depression in its self‐nucleation and annealing temperatures were observed.