Comparison of miscibility and structure of poly((R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate) (P(HB-co-HHx)) (M-w = 638,000 g mol(-1)) and poly(L-lactic acid) (PLLA) (M,, = 150,000 g mol(-1)) with those of poly((R)-3-hydroxybutyrate) (PHB) (M-w = 600,000 g mol(-1)), and PLLA having the blending ratios was investigated by wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and infrared (IR) microspectroscopy techniques. WAXD reflection patterns show that the lattice parameters of a and b axes for PHB and P(HB-co-HHx) are unchanged in their blends with PLLA regardless of blending ratio. These WAXD results suggest that their crystalline structures are independent of the second component. The glass transition temperature (T-g) of blend components did not significantly change. The crystallization temperature (T-g) of PLLA reveal that both PHB/PLLA and P(HB-co-HHx)/PLLA blends form mixed semicrystalline systems. The T-g of PLLA in the PHB/PLLA blends and that in the P(HB-co-HHx)/PLLA blends shift to opposite directions indicating that both blends are immiscible but exhibit different levels of compatibilities. Micro IR spectra show that crystalline bands due to PHB appear even for the 20/80 blend but those due to PLLA are hardly observed for all the PHB/PLLA blends investigated. On the other hand, the crystalline bands of PLLA are observed in the micro IR spectra of some spots in the 20/80 P(HB-co-HHx)/PLLA blend. Micro IR spectra also show significant differences in the compatibility and crystalline structures between the P(HB-co-HHx)/PLLA and PHB/PLLA blends. (c) 2007 Elsevier Ltd. All rights reserved.