Spherulitic Rings and Cracks in Poly(L-lactic acid) and Its Blends with Poly(3-hydroxybutyric acid)
裂縫型態 ； 環帶狀球晶 ； 聚乳酸 ； 相容性 ； PHB/LMw-PLLA混摻 ； crack patterns ； ring-banded spherulites ； miscible ； PHB/LMw-PLLA blends ； LMw-PLLA
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低分子量聚乳酸(LMw-PLLA, Mw=2000 g/mol)經由熔融結晶的方式可觀察到具有明暗相間的環帶狀之球晶，影響其環帶狀球晶的主要因素為結晶溫度(Tc)以及蓋上上蓋(top cover)。存在於LMw-PLLA的環帶狀球晶僅能以偏光顯微鏡 (POM) 觀察，若使用電子顯微鏡(SEM)或原子力顯微鏡(AFM)進行觀察，僅觀察到cracks的存在，而未能觀察到環帶狀球晶，是由於環帶狀球晶並非晶板表面起伏(高度差)所造成，故難以觀察。此外，使用熔融結晶的方式製備薄膜狀及bulk的LMw-PLLA試樣，亦可觀察到具有環狀帶的球晶。 LMw-PLLA試樣由結晶溫度冷卻到室溫時，會可觀察到cracks，其生成主要是由於試樣冷卻時的熱收縮所造成；此外，無論是否具有環帶狀球晶，試樣於結晶溫度冷卻到室溫的過程中所生成的cracks，其型態亦與球晶型態有關。此外，若crack生成於ring-bands的波峰處，則與晶板的排列及方向有關。而PLLA(Mw=152,000 g/mol) 於結晶過程中，僅存有inter-spherulitic cracks。在本研究中探討cracks與環帶狀(ring)球晶之關連性。 LMw-PLLA與PHB兩成份混摻系統之相容性以DSC來做說明。由DSC結果分析下，可發現在各組成皆為單一的玻璃轉移溫度(Tg)，並藉由Gordon-Taylor equation 計算k值為0.4，說明此摻合系統為相容。利用POM觀察球晶型態的變化，由於在結晶溫度為110~130oC時，PHB為不定型態(amorphous stae)，因此其主要呈現LMw-PLLA之環狀帶的球晶。當試樣降溫到室溫後，PHB的球晶會成長且形成與LMw-PLLA型態相同的球晶，意即較先結晶出的LMw-PLLA球晶提供給PHB成長的骨架及成核處。在PHB/LMw-PLLA摻合系統中，cracks不僅發生在冷卻至室溫的過程中，亦於在結晶過程時生成，其原因可能是由於PHB結晶造成體積減少所致。此外，生成於ring-band波峰處的crack，則與 LMw-PLLA及PHB晶板的排列及方向有關。
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.
工程學 > 化學工業