Formation of Inclusion Complexes of Poly(3-hydroxybutyrate)s with Cyclodextrins. 1. Immobilization of Atactic Poly(R,S-3-hydroxybutyrate) and Miscibility Enhancement between Poly(R,S-3-hydroxybutyrate) and Poly(ε-caprolactone)

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Abstract

Atactic poly(R,S-3-hydroxybutyrate) (a-PHB) was synthesized by anionic polymerization of β-butyrolactone with potassium methoxide as an initiator. This completely amorphous polyester is capable of forming a crystalline inclusion complex (IC) with γ-cyclodextrin (γ-CD) adopting a channel structure. There is no evidence showing that a-PHB may form IC with either α-CD or β-CD. On the basis of these discoveries, a common IC was formed with two polymer chains, a-PHB and poly(ε-caprolactone) (PCL), randomly distributed into the channels of γ-CD-PCL/a-PHB IC crystals. Nevertheless, in the formation of the common IC, PCL inclusion appears superior to a-PHB inclusion. Therefore, the molar ratio of a-PHB and PCL in the coalesced sample has been detected to be lower than that used in the formation of the common IC. Washing the common IC with hot water removed the γ-CD, and the molecular chains of the two polymers were coalesced. Very interestingly, only a single glass transition temperature (Tg), dependent on the composition, was observed between the Tg's of a-PHB (∼5 °C) and PCL (∼−60 °C) in the differential scanning calorimetry (DSC) measurements of the coalesced samples. To the contrary, Tg of a-PHB (∼5 °C) was found to remain unchanged in the physical blend with PCL. Tm's of coalesced blend samples are lower than that of pure PCL, while the Tm of the physical blend is almost the same as that of pure PCL. These results strongly demonstrate that the miscibility of the inherently immiscible a-PHB/PCL pair has been improved in the coalesced blends. Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), 13C solid-state NMR, DSC, and wide-angle X-ray diffraction (WAXD) measurements were employed to demonstrate IC formation.

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