摘要

　　90C的稀薄二甲苯溶液 (0.1 wt% xylene solution)，是一個不適合聚左乳酸 ( poly (L-lactide)，PLLA ) 分子成核與進行結晶成長的環境。然而在輕微攪拌的幫助下，聚左乳酸分子可於恆溫 (isothermal course) 過程，在溶液中成核。此晶核於初期發展出a軸較長，兩側鈍角端成圓弧形態的透鏡菱晶 (“a-axis” lenticular crystal)。實驗觀察發現，隨著維持於90℃的恆溫結晶成長，透鏡菱晶銳角端的角度會逐漸的增加。
溶液中透鏡菱晶的數量會隨著時間減少，而同樣是斜方晶系 (orthorhombic)的截角菱晶 (truncated lozenge)，逐漸形成。初形成的截角菱晶，即具有相當或大於透鏡菱晶的大小，並具有一較窄的 {100} 面。隨著時間，{100} 面的寬度逐漸增加，最後 {100} 與 {110} 面寬度比將趨於一定值。於截角菱晶兩側，原本呈現圓弧形貌的鈍角端，也會隨著時間發展出，具有明確120度的鈍角。因此由所觀察到的結晶形態演變過程可知，截角菱晶是由初期的透鏡菱晶轉變形成。
於截角菱晶中，內部區域具有較大的層晶厚度 (lamellar thickness)，外圍環繞著一層晶厚度較薄的窄邊 (narrow rim)，這個窄邊的寬度並不隨著時間而增加。而由低溫析出之結晶薄膜的分佈，也可以發現在初期形成的透鏡菱晶內部，發展出一層晶厚度較大的區域。透鏡菱晶與截角菱晶都可在結晶區內部，發展出層晶厚度較大的區域。因此可以推論，這個結晶形態轉換的過程，是由於透鏡菱晶內部所形成之截角菱形的增厚區域，以較外圍結晶成長 (primary growth) 還快的速度向外擴展 (outward propagation) 而造成。
　　當改變結晶成長階段的溫度時，發現依所選擇的結晶成長溫度不同，透鏡菱晶會在高溫轉變為截角菱晶，而在低溫轉變為菱形結晶 (rhombic lozenge)。但其內部層晶厚度較大的區域，均發展出 {100} 的成長面。因此可以確認於單晶內部發生的增厚重組，在所研究的溫度範圍是非等向性的，並傾向發展出截角菱形的增厚區域。而透鏡菱晶轉變為菱形結晶的觀察表示，有另外一個影響結晶形態的機制，可以在較低的溫度，限制內部增厚重組進一步的擴張。
因此對於經攪拌作用的幫助下，所發生的成核，與初期結晶成長發展出來的透鏡菱晶，由實驗觀察發現，可以有不同的路徑 (ordering process)，來發展至較穩定的狀態。這些結晶形態的發展，均受到成核的限制 (nucleation-controlled process)。所以成核所遭遇的能量障礙 (nucleation barrier)，將決定結晶形態的演變路徑。

Abstract

Morphologic evolution of poly (L-lactic acid) (PLLA) crystals obtained upon crystallized from its 0.1% xylene solutions at 90℃ was examined via transmission electron microscopy. Under a relatively difficult environment for crystallization, the growth of “a-axis” lenticular crystals had been enhanced by applied gentle stirring to the solution during selected isothermal course. With continuing isothermal course, a sudden transition from lenticular to truncated lozenge occurred without involving phase transition.
Within each resulted monolayer of truncated lozenge, an inner thickened region emerged with facet lateral geometry. This thickened region was also recognized within initial lenticular domain, which can be approved by the surface decoration upon quenching and linked to the gradual widening of acute apex. With these morphologic clues, this isothermal morphologic transition was therefore attributed to the outward propagation of secondary reorganization initiated by the central stirring-induced seed within lenticular crystal. Upon varying the transition temperature for confirming transition mechanism, an alternative transition pathway emerged.
The chosen transition temperatures were capable of leading to the formation of either truncated lozenge or rhombic lozenge, which has been linked to the competition between primary and secondary ordering processes. This research thus disclosed a unique and significant example that, as the nucleation barriers of involved ordering processes varied differentially with temperature, the ordering process with relatively lower nucleation barrier became determinative to resulted crystal transformation, and so was the final reached state.

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