Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/3519
標題: Crystallization and Melting Behavior of Poly(L-lactide) under spherical microdomain confinement: From hard confinement to soft confinement
聚左旋乳酸於硬相與軟相侷限球形結構中之結晶與熔融行為探討
作者: 鍾才明
Chung, Tasi-Ming
關鍵字: physically confined system
物理侷限系統
出版社: 化學工程學系
摘要: 摘要 本實驗目的是以結晶性高分子聚左旋乳酸 (Poly(L-lactide),PLLA)透過摻合方式來製造物理侷限環境,並探討PLLA均聚合物,其結晶在硬相與軟相物理侷限系統中因外界環境變化之結晶與熔融行為。經由摻合的方式,將聚左旋乳酸 (Poly(L-lactide),PLLA)與具有微觀相分離結構之聚苯乙烯與聚乳酸之雙團聯共聚合物(Polystyrene-b-Poly(D,L-lactide) , PS-PLA)摻合,由穿透式電子顯微鏡之形態觀察,其中未摻合PS-PLA團聯共聚合物之微結構為球形結構(spherical sturcture)平均大小約為20nm,在PLLA/PS-PLA混摻系統中隨PLLA組成比例增加而球形結構變大,當15wt%PLLA混摻系統中有部分發生膨脹現象之球形結構,但仍有部分形態還是維持著原有大小球形結構。然而在30wt%PLLA混摻系統中其每個球形結構都均勻分佈其平均大小約為30-35nm。在混摻系統中PLLA均聚合物與PS-PLA團聯共聚合物中PLA鏈段之數目平均分子量分別為10700g/mol與12000g/mol,依據混摻理論中摻合之均聚合物與共聚合物中相容的團聯鏈段間分子量的比值MH/MB∼1時,此時摻混之均聚合物PLLA於PLA團聯鏈段中呈現出一特殊之局部偏析(localization)現象。由於PLLA均聚合物與PS-PLA團聯共聚合物間並無化學鍵鍵結的因素存在,所以此混摻系統中PLLA的結晶行為乃受限於所謂的物理侷限(physically confinement)。相對於利用具結晶性團聯之共聚合物的化學鍵結方法而自我排組及自我有序形成一微觀相分離系統來探討結晶於侷限空間中之發展與熔融行為即所謂的化學侷限(chemical confinement)將有所不同 。在本實驗中PS-PLA團聯共聚合物系統之有序無序轉化溫度(order-disorder transition temperature, TODT )合理地大於PLLA的結晶溫度範圍,且此結晶溫度範圍(70℃∼120℃)確實跨越PS的玻璃轉化區,若PLLA之結晶溫度高於PS團聯鏈段的玻璃轉化溫度(Tc, PLLA>Tg, PS),在此情形下雙團聯共聚合物的分子鏈處於仍可運動的狀態下所以移動性較好,但相對的對於PLLA的限制力量就相對的降低,則由團聯共聚合物所形成的環境對結晶之行為限制能力不大,在此種狀況下稱為軟相侷限(soft confinement),然而若PLLA之結晶溫度低於PS團聯鏈段的玻璃轉化溫度 (Tg, PS<Tc, PLLA) ,在此情形下 PLLA的分子鏈受到由團聯共聚合物所形成的環境對結晶之行為限制能力大,在此種狀況下稱為硬相侷限(hard confinement),當結晶進行時將受到周圍環境的影響而限制結晶的發展。如此我們探討在一不受化學鍵結影響之物理侷限環境下分析自硬相侷限至軟相侷限其共聚合物分子鏈段移動力的變化對於結晶行為的影響。 PLLA/PS-PLA混摻系統於硬相物理侷限環境(Tc,PLLA<Tg,PS)下進行結晶時,球形結構內PLLA受到外圍PS團聯鏈段的牽制效應,固有的球形結構其最終形態將不受影響;軟相物理侷限環境(Tc,PLLA>Tg,PS)時而外圍PS團聯鏈段的移動力上升,但球形結構內結晶性PLLA分子鏈於三維空間中結晶驅動力還是無法破壞球形結構,推論乃是PLLA結晶溫度遠低於有序無序轉換溫度TODT以下所以此時微觀相分離驅動力大於結晶驅動力。由DSC的熱分析觀察PLLA/PS-PLA混摻系統於不同侷限環境下結晶行為,實驗結果顯示微胞內結晶性PLLA於不同侷限環境下因受制於空間侷限影響,導致結晶速率低於均聚合物PLLA但熔點高於均聚合物PLLA,其中於硬相物理侷限環境受外圍非結晶團聯Tg牽制效應的影響更為明顯,因而造成其結晶速率遠低於均聚合物PLLA。 經由傳統的Avrami的分析法進行PLLA/PS-PLA混摻系統與均聚合物PLLA結晶機構的模擬分析,並搭配偏光顯微鏡的形態觀察來判斷其結晶機構。均聚合物PLLA經由Avrami分析與結晶形態的觀察證實為非依熱成核(athermal),其結晶機構為異相成核(heterogeneous nucleation)機制。而我們將PLLA/PS-PLA物理侷限系統採用Register對於結晶性團聯共聚合物於侷限環境下結晶機構的方式進行分析,由DSC實驗結果,計算出不同時間下在球形結構內PLLA之結晶度隨時間作圖,其結果發現不管在硬相侷限或軟相侷限都呈現sigmoidal曲線趨勢且推算出Avrami數值n>1,但是整個系統為球形結構其中PLLA結晶進行並無破壞其形態所以結晶性鏈段間無相連性,推論其結晶機構乃為均相成核(homogeneous nucleation)機制。
Abstract The competition between the crystallization of crystalline polymer, poly(L-lactide) (PLLA), and the microphase separation of block copolymer, polystyrene-b-poly(D,L-lactide) (PS-PLA), was investigated in this study. The morphological observation demonstrates that the block copolymer of PS-PLA self-assembles to form spherical microdomains. For PLLA/PS-PLA blends containing 30 wt % PLLA, the sphere size is around 30-35 nanometers as determined by TEM micrograph. The size of spheres increases with increasing the added amount of PLLA. When the ratio of number-averaged molecular weights for PLLA and PLA block (M n, PLLA/M n, PLA=10 700/12 000〜0.9) was close to one, unique morphology having PLLA localized in the middle of the PLA microdomains of block copolymer have been observed by TEM. Since the crystallizable chain is not chemically connected to the amorphous microstructure, we name this self-assembly morphology as physically confined system for crystalline polymer so as to distinguish traditional crystallizable block copolymers where the crystallizable block is chemically connected by amorphous block. The crystallization studies of PLLA in the self-assembly system of PLLA/PS-PLA blends thus provide a representative example to explore the crystallization behavior within the nano-scale confinement without the effects of chemical linkages. As predicted by theoretical calculation, the order-disorder transition temperature (TODT) of block copolymer PS-PLA is fairly higher than crystallization temperature for PLLA (Tc, PLLA) so as to generate strong segregation strength for microphase separation. In other words, the crystallization of PLLA is inevitable to encounter the confinement effect from the repulsive PS microdomain. Interestingly, the glassy transition of PS block (Tg, PS) occurs in the range of PLLA crystallization window so that crystallization of PLLA might carry out in different confined environments: soft confinement where TODT >> Tc, PLLA > Tg, PS and hard confinement where TODT >> Tg, PS > Tc, PLLA. This particular blending system thus provides a unique example for studying the crystallization from hard confinement to soft confinement. The glassy PS microdomain effectively constrain the occurrence of PLLA crystallization within the confined dimension while Tg, PS > Tc, PLLA so as to preserve the spherical microstructure. The result is similar to the morphology observed by TEM in soft confined environment. This suggests that the TODT of block copolymer PS-PLA is fairly higher than crystallization temperature for PLLA so as to generate strong segregation strength for microphase separation. The interesting morphological development with respect to crystallization of PLLA at different temperature gave rise to significant change in crystallization rate. In contrast to homopolymer PLLA, confinement effect should decrease the crystallization rate of PLLA in nano-scale spherical microdomain as observed. However, dramatic decrease in crystallization rate has been identified once the crystallization temperature is below Tg, PS, indicating that the chain mobility is indeed critical for crystallization events under confinement. Furthermore, the melting temperature of PLLA crystals in PLLA/PS-PLA system was found to be much higher than neat PLLA. We speculate that the unexpected increase in melting point is attributed to the confined effect where the confinement could induce regular folding for PLLA lamellar crystals, and thus decrease the free energy of folding surface. To understand the crystallization kinetics, Avrami treatment for crystallization exothermic response was conducted. For PLLA homopolymer, typical heterogeneous nucleation has been identified as evidenced by Avrami index of 2.9. By contrast, an average value of 2.2 Avrami index was obtained for PLLA crystallization under confinement where a sigmoidal crystallization kinetics for the plot of crystallinity versus crystallization time was obtained. However, spherical morphology was preserved under both hard and soft confined environment after crystallization. We suggest that the kinetics for PLLA crystallization under confinement is a homogeneous nucleation according to the unchanged spherical microdomain after crystallization.
URI: http://hdl.handle.net/11455/3519
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