Melting and Crystallization Behaviors of Biodegradable Polymers Enzymatically Coalesced from Their Cyclodextrin Inclusion Complexes

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Abstract
Inclusion complexed (IC) and coalesced biodegradable poly(epsilon-caprolactone) (PCL), poly(L-lactic acid) (PLLA), and their diblock copolymer (PCL-b-PLLA) were achieved by forming ICs between host alpha-cyclodextrin(alpha-CD) and guest PCL, PLLA, and PCL-b-PLLA, followed by removing the alpha-CD host with an amylase enzyme. FTIR spectra of the coalesced polymers reveal that the host alpha-CD can be completely removed, without polymer degradation, by treatment with an amylase enzyme. The melting and crystallization behavior of these CD-IC treated polymers, which are crystallizable, biodegradable, and bioabsorbable, are investigated by differential scanning calorimetry (DSC) and polarized optical microscopy. Results show that coalescence increased the crystallinities of the homopolymers but decreased that of the diblock copolymer. The Avrami exponent (n), derived from both isothermal and nonisothermal crystallization models for homo-PCL and -PLLA and the PCL and PLLA blocks in the diblock copolymer samples coalesced from their ICs, is close to 4, indicating homogeneous crystallization, whereas crystallization of the blocks in the as-synthesized diblock copolymer yields an Avrami exponent around 3, indicating heterogeneous crystallization. All of these results demonstrate that the PCL and PLLA homopolymers and blocks in the IC-coalesced samples are more readily and homogeneously crystallized than those in the as-synthesized samples or their physical blend, even though the level of crystallinity in the IC-coalesced diblock copolymer is significantly lower. Moreover, unlike the as-synthesized diblock copolymer, the crystallization of PCL and PLLA blocks in the IC-coalesced diblock copolymer are not influenced by their covalent connection.

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