- 學位論文
利用正電子湮滅時間光譜技術探討半結晶性之 聚(3-羥基丁酸酯－共－3-羥基戊酸酯)薄膜的結構與自由體積特性
Structure and Free Volume Properties of Semi-crystalline Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Membranes by Positron Annihilation Lifetime Analysis
摘要
本研究主要是利用正電子湮滅時間(positron annihilation lifetime, PAL)光譜技術，討論半結晶性之聚(3-羥基丁酸酯－共－3-羥基戊酸酯) (poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHBV)薄膜的自由體積特性隨環境溫度之變化，包含自由體積的大小、尺寸分布、數量，以及自由體積分率與熱膨脹行為；並藉由薄膜結晶熱歷史的控制，探討高分子結晶結構與自由體積特性之關聯。同時，本研究首次觀察到PHBV薄膜的自由體積在結晶熔融過程中有leveling-off的現象，對應PHBV高分子的FTIR光譜分析結果，推論發生此現象的起始溫度(Tk, knee temperature)為PHBV高分子物理結構產生細微變化的起始點。 研究中利用等溫與非等溫結晶程序，分別在冷結晶(cold-crystallization)與熱熔結晶(melt-crystallization)的條件下完成PHBV薄膜的製備。不同結晶程序可獲得結晶結構各異的薄膜，因而造成薄膜無定形區域的自由體積特性變化，透過PAL量測的結果及結晶結構的分析，得知材料的結晶溫度越高，導致越高的結晶度、較少的自由體積數目以及較低的自由體積分率，並呈現線性關係。針對材料自由體積大小差異，本研究提出材料結晶速率影響自由體積的熱膨脹行為之理論，就PHBV高分子而言，結晶速率越快，形成的結晶結構造成無定形區的自由體積越容易熱膨脹，因此在高溫的環境下，較快的結晶速率下結晶的高分子薄膜具有較大的自由體積尺寸。在此，引用半結晶高分子的三相模型：結晶區(crystalline fraction)、可移動之無定形區(mobile amorphous fraction, MAF)與束縛之無定形區(rigid amorphous fraction, RAF)，以解釋自由體積與結構的變化，根據小角X光散射(small-angle X-ray scattering, SAXS)的結果與薄膜結晶度推論，熱熔結晶條件下成型的PHBV薄膜相對於冷結晶成型薄膜具有較厚的長週期厚度(long period, L)，L為結晶晶板厚度(lamellar thickness, Lc)和無定形層厚度(thickness of amorphous layer, La)的總和，而且此一晶板排列週期厚度呈現雙峰分布，加上較少的RAF含量，導致熱融結晶成型之PHBV薄膜的擁有較廣的自由體積尺寸分布。總結而言，本研究建立一結晶結構與自由體積特性關聯的模型：自由體積大小的熱膨脹行為受到結晶動力學的影響；相對地，自由體積數量和自由體積分率是由薄膜的整體結晶度所控制。另外，經由結晶條件所決定的結晶晶板尺寸與結構則是對應至自由體積大小的分布寬廣。
並列摘要
Free volume properties in the amorphous region of a series of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) membranes, which were prepared by cold- and melt-crystallization processes, were investigated using positron annihilation lifetime (PAL) spectroscopy in this study. From the lifetime parameters, the temperature dependence of free volume size, amount, free volume size distribution, and fractional free volume, and the thermal expansion of free volume were discussed. Furthermore, the knee temperature was first observed in the melting process of the crystallized PHBV membranes. It indicated that there was structural transition of polymer chains during melting as the corresponding results observed with in situ FTIR measurement. A model which assumed that amorphous phase was subdivided into mobile and rigid amorphous fractions (MAF and RAF) in the semi-crystalline polymer was considered to interpret the temperature dependence of those free volume properties. The difference of free volume properties among various PHBV membranes was created according to the crystalline structure of the polymer from different thermal history. The polymer crystallized at higher temperatures resulted in higher crystallinities, less free volume amounts and lower fractional free volumes. Based on the crystallization conditions, the effect of the crystallization rate of PHBV polymer was first proposed to explain the thermal expansion coefficients of free volume size. The faster crystallization rate is, the higher thermal expansion coefficient and the larger free volume size at higher measuring temperatures are. Morphological observation of the semi-crystalline polymer by small-angle X-ray scattering (SAXS) indicated that the cold-crystallized membranes showed a much thinner thickness of the repeating lamellar/amorphous layers and most likely higher amount of RAF, which restrained the chain motion, than the melt-crystallized membranes. Larger dispersion of the free volume size of melt-crystallized membranes was observed as a result of the bimodal distribution of the lamellar periodicity and less amount of RAF than that of the cold-crystallized membranes. In conclusion, the relationship between the crystalline structure and the free volume properties within the amorphous phase of PHBV polymer was well established. Free volume size and thermal expansion of the free volume in PHBV membranes were affected by the kinetic of crystallization; comparatively, the total amount of free volume and fractional free volume were determined by the final crystallinity. The size distribution of free volume was associated with the crystalline lamellar structure which was dominated by the crystallization conditions.