Structure, dispersibility, and crystallinity of poly(hydroxybutyrate)/poly(L-lactic acid) blends studied by FT-IR microspectroscopy and differential scanning calorimetry

The present study is aimed at investigating structure, dispersibility, and crystallinity of poly(3-hydroxybutyrate) (PHB) and poly(L-lactic acid) (PLLA) blends by using FT-IR microspectroscopy and differential scanning calorimetry (DSC). Four kinds of PHB/PLLA blends with a PLLA content of 20, 40, 60, and 80 wt % were prepared from chloroform solutions. Micro-IR spectra obtained at different positions of a PHB film are all very similar to each other, suggesting that there is no discernible segregated amorphous and crystalline parts on the PHB film at the resolution scale of micro-IR spectroscopy. On the other hand, the micro-IR spectra of two different positions of a PLLA film, where spherulite structures are observed and they are not observed, are significantly different from each other. PHB and PLLA have characteristic IR marker bands for their crystalline and amorphous components. Therefore, it is possible to explore the structure of each component in the PHB/PLLA blends by using micro-IR spectroscopy. The IR spectra of a position of blends except for the 20/80 blend are similar to that of pure PHB. On the other hand, the IR spectra of another position of the blend consist of the overlap of those of pure PHB and PLLA. For the 20/80 blend, it is difficult to find a position whose spectrum is similar to that of pure PHB. However, a crystalline peak due to the C=O stretching band is observed at 1718 cm(-1). This means that PHB crystallizes as very small spherulites or immature spherulites under such blend ratio. DSC curves of the blend show that the heat of crystallization of PHB varies with the blending ratio of PHB and PLLA. The recrystallization peak is detected for PLLA and the 20/80 blend respectively at 106.5 and 88.2 degrees C. The lowering of recrystallization temperature for the 20/80 blend compared with that of pure PLLA suggests that PHB forms small finely dispersed crystals that may act as nucleation sites of PLLA. The results for the PHB/PLLA blends obtained from IR microspectroscopy indicate that PHB crystallizes in any blends. However, crystalline structures of PHB in the 80/20, 60/40, and 40/60 blends are different from those of the 20/80 blend.