Crystallization behavior and thermal properties of melt-crystallized poly[(R)-3-hydroxybutyric acid-co-6-hydroxyhexanoic acid] films

Melt-crystallized films of poly[(R)-3-hydroxybutyric acid-co-10mol% 6-hydro xyhexanoic acid] (P[(R)-3HB-co-6HH]) were prepared by isothermal crystalliz ation at various temperatures for 3 days, and subsequently stored at room t emperature after the films formed well-developed and volume-filled spheruli tes. The lamellar morphologies and properties of melt-crystallized films we re characterized by means of wide-angle X-ray diffraction, small-angle X-ra y scattering, differential scanning calorimetry, transmission electron micr oscopy, and atomic force microscopy. The melting endotherm of P[(R)-3HB-co- 6HH] films was composed of a broad peak starting around room temperature an d of a sharper peak starting above the isothermal crystallization temperatu re. The stacking of flat-on lamellae with lamellar periodicity of 8-10 nm w as detected on the surface of P[(R)-3HB-co-6HH] films after the primary cry stallization at 110 degrees C. On storage at room temperature above the T-g (-5 degrees C) of copolyester, thin crystals of 1-4 nm thickness appeared on the surface of P[(R)-3HB-co-6HH] films crystallized at 110 degrees C. Th ese results suggest that long sequences of (R)-3HB units in a random copoly ester form relatively thick P[(R)-3HB] crystalline lamellae during the prim ary crystallization process at a given crystallization temperature, while s horter sequences of (R)-3HB units, which are incapable of crystallizing at a given crystallization temperature, form relatively thin crystalline lamel lae during the subsequent crystallization process at room temperature. (C) 1999 Elsevier Science B.V. All rights reserved.