臺灣博碩士論文加值系統

在先前關於聚左旋乳酸 (PLLA) 經由特定溶劑處理的研究之中，可以發現在低溫環境之下會形成溶劑所引導出的ε結晶以及少量的α結晶，但關於升溫程序時相轉換的細節、ε與α相的結晶行為以及溶劑對於PLLA的效應還不清楚。在這裡我們選擇PLLA加環戊酮 (CPO) 的系統來進行分析並且記述我們嘗試釐清這些問題的結果。
藉由連續式的小角度/廣角度X光散射儀及熱重量分析儀，我們可以在30到90 °C的溫度範圍以3 °C/min的升溫速率來監控相轉換。廣角度X光散射儀的觀察輔以小角度X光散射儀的結果顯示在46 °C之上相轉換可分成三個階段：(1) 相轉換的開始，在這階段ε相開始進行轉換並同時轉換成中間相及α相，(2) α相從非晶相區域成核且成長，同時ε相的轉換加速，(3) 剩餘的ε相進行熔化，象徵著相轉換的終止。最重要的是主導的相轉換機制為ε相直接轉換成α相，伴隨著額外的α相從非晶相成核並成長。再者，從TGA所得到的結果指出當溫度高於46 °C之後重量損失速率變快，可以發現相轉換的進行加速溶劑的揮發速率。在ε相轉換成中間相及α相之後因為被束縛在晶格中的溶劑分子被釋放出來，所以溶劑的揮發速率加快。
為了更嚴密地檢驗相轉換也進行了恆溫的WAXS/SAXS及TGA實驗。恆溫於46 °C的結果顯示主要的相轉換機制是ε相直接轉換成α相，同時少量的α相是從非晶相所形成，與升溫程序的結果一致。此外，由於溶劑緩慢的轉移在相轉換發生之前存在一段潛伏期。藉由比較持溫於46及48 °C的WAXS結果，可以發現導致相轉換的臨界溫度確實在46 °C附近，這也解釋了相轉換發生之前存在一段潛伏期。
藉由製備浸泡CPO配合不同時間條件的樣品，WAXS的量化分析揭露了ε相與α相的結晶行為。隨著浸泡時間的增加，ε相的組成隨之增加同時α相則是維持固定的組成，表明了ε相及α相都是從非晶相分別獨立生成出來的結晶相。
藉由比較經由溶劑處理或不經溶劑處理的樣品的WAXS結果，可以發現再升溫過程PLLA經過特定的溶劑處理會傾向形成α (而不是α′) 相，意味著存在溶劑會增強高分子鏈的移動性，因此會傾向直接生成排列較為緊密的α相。

In previous studies of poly(l-lactic acid) (PLLA) with specific solvent, the solvent-induced ε crystals with a small amount of α crystals at low temperature (＜ 0 °C) had been identified, but details of the phase transition upon heating process, crystalline behavior in both ε and α phases, and solvent effect in PLLA are unclear. Here we select the PLLA with cyclopentanone (CPO) as a system to analyze and report the results of our attempt to clarify these questions.
Phase transition during programmed heating from 30 to 90 °C at a rate of 3 °C/min was monitored via simultaneous small/wide-angle X-ray scattering (SAXS/WAXS) and thermogravimetric analysis (TGA). WAXS observations along with supporting SAXS results show that the phase transition above 46 °C involves three stages: (1) beginning of phase transition, in which displays that ε phase starts to transform into meso- and α phases at the same time, (2) nucleation and growth of α phase from amorphous matrix, accompanied with faster transformation of ε phase, (3) final melting of the residual ε phase, representing the end of phase transition. Most importantly, the dominant mechanism of phase transition is the direct transformation from ε to α phases, with additional nucleation and growth of α phase from amorphous phase. Moreover, result obtained from TGA indicates that the rate of weight loss is faster when the temperature is above 46 °C, implying the progress of phase transition accelerates the evaporation rate of solvent. As more solvent molecules trapped in crystal lattice are released after the ε phase transforms into meso- and α phases, the rate of solvent evaporation increases.
Isothermal experiments of WAXS/SAXS and TGA were also conducted to examine the phase transition more closely. Results at 46 °C show that the primary mechanism of phase transition is direct transformation from ε to α phases, with a small amount of α phase formed from amorphous phase, which is consistent with the results obtained from heating process. However, there is an incubation period before phase transition due to slow solvent transport. By comparing the time-dependent WAXS 1D profiles of isothermal experiments at 46 and 48 °C, it can be observed that the critical temperature of phase transition is indeed around 46 °C, which also interprets why there is an incubation period before phase transition.
By preparing samples immersed into CPO for different periods of time, quantitative analysis for WAXS measurement reveals the relation between ε to α phases. As immersion time increases, the amount of ε phase rises moderately while that of α phase remains largely unchanged, indicating that ε and α phases are formed from amorphous phase and independent from each other.
By comparing the WAXS results of sample with or without solvent treatment, it is found that PLLA with specific solvent treatment tends to give α (rather than α′) phase upon heating, implying that the presence of solvent enhances chain mobility, hence favoring direct formation of the α phase of higher packing density.

摘要 I
ABSTRACT II
誌謝 III
List of Figures V
1. Introduction 1
1.1 Background 1
1.2 Objective and Approach 4
2. Experimental Details 5
2.1 Materials and Specimen preparation 5
2.2 Instruments 5
2.3 Data Analysis 5
3. Results and Discussion 7
3.1 Phase transition in PLLA/CPO ε crystals upon heating process. 7
3.2 Phase transition in PLLA/CPO ε crystals upon isothermal process. 13
3.3 Solvent behavior in heating and isothermal processes. 17
3.4 Crystal behavior with solvent treatment at low temperature 19
4. Conclusion 22
References 23

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