Spherulitic Rings and Cracks in Poly(L-lactic acid) and Its Blends with Poly(3-hydroxybutyric acid)

• Main Authors: Siti Nurkhamidah, 盧絲堤
• Other Authors: Eamor M. Woo
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/10478517618344789329

碩士 === 國立成功大學 === 化學工程學系碩博士班 === 97 === Ring-banded spherulites of Low-molecular-weight poly(L-lactic acid) (LMw-PLLA) were observed by melt crystallization. Crystallization temperature, as well as a top cover, is one of the main factors affecting the formation of the ring-banded spherulites of LMw-PLLA upon melt crystallization. Ring-banded spherulites of LMw-PLLA are only visible under polarized optical microscope (POM) or optical microscope (OM) observation. Only crack patterns can be observed by scanning electron microscope (SEM) or atomic force microscopy (AFM) and no ring bands are seen either in SEM or AFM micrographs. Apparently, there is no periodic variation of thicknesses along the radius and the brightness contrast of the alternating bright and dark bands of LMw-PLLA spherulites is not due to surface topography (height contrast). However, ring-banded spherulites of LMw-PLLA can be observed not only in thin film but also in bulk sample by melt crystallization. When Tc-crystallized samples were cooled down from crystallization temperature (Tc) to ambient, cracks appeared. However, cracks were not observed in the samples held at Tc. Thermal shrinkage may be a factor for the formation of cracks in LMw-PLLA upon cooling and the crack patterns seem to be dependent on the morphology of spherulites. Both ringless and ring-banded spherulites can exhibit cracks upon cooling; whereas, the crack patterns are different in these two types of spherulites. The radial short-segmental voids coinciding with the ridge of ring-bands in spherulites may be related to the aggregation of LMw-PLLA crystals and orientation of lamellae on the ridges. Cracks in crystallized LMw-PLLA are reversible as temperature cycles are induced from Tc to ambient and reversed from ambient to Tc, and back and forth. Correlation between cracks and rings in ring-banded spherulites of LMw-PLLA was attempted in this study. Regular high-molecular weight poly(L-lactic acid) (PLLA) has one type of crack pattern, inter-spherulitic cracks, and occur only during crystallization process. LMw-PLLA was blended with poly(3-hydroxybutyric acid) (PHB) and the miscibility of this blend was investigated by using differential scanning calorimeter (DSC). DSC thermograms for PHB/LMw-PLLA blends show single glass transition temperature (Tg) and the k parameter value of Gordon-Taylor equation is 0.4, indicating that PHB and LMw-PLLA are miscible in the melt at 200 oC. Both LMw-PLLA and PHB are semicrystalline polymers; however, ring-banded spherulites are present in the blend system at Tc ranging from 110 up to 130 oC where PHB is in an amorphous state. After being cooled to ambient temperature, PHB in blends starts to crystallize and formed the same ring-banded spherulites with LMw-PLLA in the blend system. The preformed ring-banded spherulites of LMw-PLLA serve as a growth template and nucleating sites of PHB. In the PHB/LMw-PLLA blends, cracks occur not only during cooling process but also during extended isothermal crystallization process. Cracks in blends occurring during cooling process are caused by thermal shrinkage; whereas, cracks occurring upon extended isothermal crystallization are caused by the volume reduction due to solidifying of PHB. Spherulitic morphology, blend compositions and top cover influence the crack patterns in the PHB/LMw-PLLA blends. The radial short-segmental voids coinciding with the ridge of ring-bands in spherulites are related to aggregation of both LMw-PLLA and PHB crystals and orientation of lamellae on the ridges. The radial short-segmental voids are located between lamellar bundles.