CRYSTALLIZATION OF POLY(HYDROXYBUTYRATE-CO-HYDROXYVALERATE) IN WOOD FIBER-REINFORCED COMPOSITES

Wood fiber-reinforced composites were prepared from poly(hydroxybuty r ate) (PHB) and poly(hydroxybutyrate-co-hydroxyvalerate) (PHB/HV) copol ymers containing 9 and 24% valerate. The effects of fibers on crystall ization-were investigated. Thermomechanical pulp, bleached Kraft fiber s, and microcrystalline cellulose filler were used as the reinforcing phase. The crystallization of PHB/HV in composite materials was examin ed using Modulated Differential Scanning Calorimetry (MDSC) and hot-st age microscopy. Hot-stage microscopy showed that polymer crystallites are nucleated on the fiber surface and that the density of nuclei was greater in fiber-reinforced composites than in unfilled material. Dyna mic crystallization experiments showed that bleached Kraft, thermomech anical pulp, and microcrystalline cellulose increased the crystallizat ion rate of PHB and PHB/HV both from the glass and melt. However, ulti mate crystallinity determined from the heat of crystallization was the same in unreinforced and reinforced materials. The kinetics of PHB/HV crystallization were examined using nonisothermal Avrami-type analysi s. Unreinforced and Kraft-reinforced PHB were characterized and compar ed with unreinforced PHB/9%HV. The Avrami exponent of crystallization, related to nucleation mechanism and growth morphology, is 2.0 for unr einforced PHB, 2.8 for kraft-reinforced PHB, and 3.0 for unreinforced PHB/9%HV. (C) 1997 John Wiley & Sons, Inc.