The attenuated total reflection infrared analysis of surface crystallinity of polyhydroxyalkanoates

The surface crystallization of three different polyhydroxy alkanoates (PHAs), poly(3-hydroxybutyrate) (PHB), PHB-co-5 weight percent 3-hydroxyvalerate (PHBV5), and PHB-co-12 weight percent 3-hydroxyvalerate (PHBV12), under different processing conditions is analyzed with depth profiling attenuated total reflection spectroscopy (ATR) and differential scanning calorimetry (DSC). The results of ATR indicate that there is a crystallinity distribution on the outer 2 microns thick surface layer. The degree of crystallinity in different penetration depths follows the order of 0.92 mu > 0.41 mu > 1.70 mu for all three hot-pressed PHAs studied and the outer surfaces have a much greater crystallinity than the bulk. The surface crystallinity of specimens is in the order of PHB > PHBV5 > PHBV12 regardless of the different penetration depths analyzed which is consistent with the DSC results. It is also obtained that the effect of annealing in enhancing the crystallization of PHAs is more significant in the surface layer. The surface crystallinity of PHAs obtained after annealing follows the trend of Petri-dish side of solvent-cast PHB > hot-press PHB > air side of solvent-cast PHB > PHBV5 > PHBV12. The difference in crystallinity between the surface and the bulk of PHAs decreases with the addition of 3-hydroxyvalerate but increases after annealing. It is confirmed that the film forming method and the substrate contacted affect the surface crystalline state of PHAs which is attributed to the changes in crystallization conditions in the surface materials.