Selected Article



Crystal Structure and Morphology of Syndiotactic Polystyrene Nanocomposites.


[[abstract]]對位聚苯乙烯(syndiotactic polystyrene, sPS)為一結晶性高分子材料,其晶型態相當複雜,隨著結晶環境的不同而有α、β、γ及δ四種不同的結晶形態產生,其中α-form和β-form的分子鏈之排列為平面鋸齒狀結構,一般於熔融結晶時形成,結晶型態的不同主要是與其所受的熱歷史有關,在不同的熱處理條件下其α-form和β-form所佔比列也會跟著改變。而γ-form及δ-form的分子鏈排列為立體螺旋結構,此兩種結晶型態僅在溶劑誘導下才會出現產生。 高分子奈米複合材料,係指分散相之粒徑在奈米尺度均勻混合於高分子連續相中所形成的新材料,可充分發揮分子層級之結構特性,如粒徑小、高長寬比、層狀結構、離子鍵結等特性,兼具高強度、高剛性、高耐熱性等高功能性質,在材料的開發應用上極具潛力。本論文主要在利用界面改質與表面處理的技術,將奈米級尺度蒙脫土均勻分散於對位聚苯乙烯基材中,製備成對位聚苯乙烯/蒙脫土之奈米複合材料。藉由X光繞射結果得知蒙脫土經表面改質後其層間距離被撐開至3.81 nm,以穿透式電子顯微鏡觀察,蒙脫土在對位聚苯乙烯基材中以成束狀均勻分散。將此奈米複合材料,經由不同結晶條件(如熔融溫度、結晶溫度、降溫速率…)處理後,製成sPS/MMT薄膜,利用分析XRD之圖譜鑑定其結晶結構,再配合偏光顯微鏡觀察其晶體結構之變化。由熔融結晶行為與等溫結晶實驗結果可知,高分子量的對位聚苯乙烯(HM-sPS)比低分子量(LM-sPS)具較大的α-β相轉換(phase transformation)溫度區間,介於265?290℃,且其單一α-form存在最高之熔融溫度(即轉換起始溫度, Ti)與單一β-form存在最低之熔融溫度(即轉換終止溫度, Tf)皆較高溫,而LM-sPS之Ti與Tf較低,相轉換的溫度區間範圍較小介於263?278℃,即LM-sPS的相轉換情形對溫度的敏感度較高,變化較明顯。而將LM-sPS加入奈米尺度蒙脫土(MMT)後會造成連續相的高分子其結晶行為與結晶型態發生改變,且改變情形在低含量時影響最大。0.5 wt%之sPS/MMT奈米複合材料其Ti較LM-sPS提高2℃,Tf也增加了12℃,α-β相轉換溫度區間範圍共增加了10℃。但隨著MMT含量的增加其Ti與Tf逐漸降低,且相轉換溫度區間也會變小。

[[abstract]]Abstract Syndiotactic polystyrene (sPS) has received considerable attention due to its high melting temperature, fast crystallization rate, low dielectric constant and permeability to gases and excellent chemical resistance. Structural studies by XRD, FTIR and NMR have revealed a complex polymorphic behavior. The various crystalline structures, namely α, β, γ, δ, differ with respect to the chain conformation as well as for the chain packing within unit cell. The α and β forms, containing trans planar zigzag chains, can be obtained from the melt or the glassy state of sPS under different thermal conditions, while the γ and δ forms are formed under conditions where the solvent induced. Polymer nanocomposites defined by the particle size of the dispersed phase containing at least one dimension less than 100 nm have relatively high aspect ratio and increase much interest due to their excellent physical, mechanical and thermal behavior over their conventional microcomposites. The preparation of synthetic nanocomposites is the intercalation of monomers or polymers into swellable layer silicate hosts. In this thesis, we have used montmorillonite as the dispersed phase to prepare sPS/clay nanocomposites. Both x-ray diffraction data and transmission electron microscopy of sPS/clay nanocomposites show the layered montmorillonite are intercalated with sPS at least in the scale of 3.81nm. X-ray data also indicates the presence of polymorphism in sPS/clay nanocomposites. This polymorphic behavior is dependent on the thermal history of sPS/clay nanocomposites from the melt and on the content of clay in sPS/clay nanocomposites. The quenching from the melt induces the crystallization into the α crystalline form for pure sPS as well as sPS/MMT nanocomposites. The effect of crystallization temperature, melting temperature and cooling condition on the temperature range of α-β crystalline transformation is also discussed.

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