Microscopic visualization on crystalline morphologies of thin films for poly[(R)-3-hydroxybutyric acid] and its copolymer

Thin films with average layer thickness 100 nm of poly[(R)-3-hydroxybutyric acid] (P[(R)-3HB]) and poly[(R)-3-hydroxybutyric acid-co-10 mol% 6-hydroxy hexanoic acid] were isothermally crystallized at a given crystallization te mperature after melting at 200 degrees C. The spherulitic and lamellar morp hologies of polyester thin films were investigated by means of optical micr oscopy, transmission electron microscopy and atomic force microscopy. After isothermal crystallization, the uniform two-dimensional spherulites which have the stacking texture of flat-on lamellae were developed throughout bot h polyester thin films. Each flat-on crystal gave a well-resolved electron diffractogram, and all electron diffractograms were identical. Accordingly, the 6-hydroxyhexanoic acid units were rejected from the tightly packed reg ion of lamellar crystals. Both the lamellar periodicity and lamellar width of polyester thin films increased with a rise in the crystallization temper ature. At the growth front of crystalline lamellae, microfibril crystals wi th 30-50 nm width were present. The enzymatic hydrolysis of thin films was carried out at 25 degrees C using an aqueous solution of PHB depolymerase f rom Alcaligenes faecalis. After enzymatic degradation, the jagged texture a long the crystal long-axis could be observed at the ends of crystalline lam ellae on the surface of the thin films. These observations suggest that the lamellar crystals of P[(R)-3HB] is composed of both tight molecular packin g regions of microfibril crystals and loose molecular packing boundary regi ons. (C) 1999 Elsevier Science Ltd. All rights reserved.