共聚分子作用於對排聚笨乙烯之結晶與熔融

Comonomer effects on crystallization and melting of syndiotactic polystyrene copolymers

林詩傑(Shi-Jieh Lin)

指導教授 ： 蘇安仲

國立清華大學/工學院/化學工程學系/博士(2012年)

https://doi.org/10.6843/NTHU.2012.00196

摘要

利用同步輻射中心小角/寬角散射研究聚苯乙烯共聚高分子之結晶結構演化行為。與其均聚物在同樣結晶溫度範圍做比較，摻雜共聚分子傾向於形成alpha結晶相並伴隨著晶板增厚、熔點降低，同時造成共聚分子嵌入結晶中，使得晶格擴張、遠紅外線紅譜特徵峰位移。而在beta結晶相部分，共聚分子未嵌入其結晶，只影響到熔點以及結晶成長。從晶格擴張比例可計算出共聚分子嵌於結晶中的準確含量，常溫下共聚分子比例為0.01~0.03 mol%、高溫時為0.02~0.08 mol%，根據Sanchez-Eby理論帶入共聚分子準確比例，其表面自由能為9~24 mJ m^-2、平衡熔點是291~275度C，並因共聚分子嵌入而帶有一額外能量25 MJ m^-3。在beta相裡，共聚分子未嵌入其中單純造成熔點下降4度C。造成alpha與beta相不同結晶行為的因素主要來自於結晶密度3%的差異。

關鍵字

對排聚苯乙烯 ； 共聚高分子 ； 結晶

並列摘要

Structural evolution of crystalline lamellae (cold- or melt-crystallized isothermally at Tc = 190 to 250 °C) of syndiotactic poly(styrene-stat-4- methylstyrene)s (sPS-4MS, with the mole fraction of 4MS units x ≈ 0.02 to 0.10) and sPS-aPS graft copolymers (based on sPS-10%4MS) upon heating in small steps (2 or 1 °C per 2-min interval) was examined via time-resolved small/wide-angle X-ray scattering (SAXS/WAXS). Compared to the homopolymer crystals prepared in the same Tc range, the incorporation of 4MS units resulted in preferred formation of α crystals (presumably due to its lower packing density and hence better tolerance to 4MS units) with clearly increased lamellar thickness (lc) but comparable melting temperature (Tm), signifying depressed Tm at the same lc; this was accompanied by slight (but measurable) increases in lattice size and significant shifts of characteristic infrared absorption frequency from 857 to 817 cm-1. However, contrary to β crystal, incorporated 4MS units caused identical characteristic absorption peak (FTIR spectra) and diffraction pattern (WAXS profiles) but melting temperature and growth rate depressed due to comonomer units aggregating in chain fold. From lattice expansion, the average mole fraction of 4MS units in the α crystals was estimated to be xc ≈ 0.01 to 0.03 as-prepared and 0.02 to 0.08 near melting, indicating significant fractionation effects due to energetic penalty in the incorporation of 4MS units upon crystallization. On the basis of Sanchez-Eby (SE) theory in the limit of thick crystals where xc → x, Gibbs- Thomson (GT) melting lines (in the idealized absence of comonomer fractionation) were constructed from final points (with solid-liquid equilibration closely approached) of lc−1-T trajectories; these gave surface free energy values of σe ≈ 9 to 24 mJ m-2 (contrary to constant σe assumed in the SE theoretical frame) with a consistent value of penalty (excess) energy ε ≈ 25 MJ m-3 for incorporation of 4MS units as deduced from the depressed equilibrium melting temperature Tm× ≈ 291 to 275 °C (as compared to the corresponding homopolymer value of Tm° ≈ 294 °C) upon extrapolation to lc−1 = 0. Significant increases in σe (as compared to the homopolymer value of 8 mJ m-2) with increasing x were attributed to concentrated comonomer units in the fold loops. Besides, in constant β crystal packing (xc = 0) for Sanchez-Eby theory, the depression of melting temperature was 4 ºC. Also observed was the hysteresis behavior of lc-temperature trajectories in approaching the equilibrium GT melting line due to coexisting thin crystals of low xc and thick crystals of high xc. Complete exclusion of 4-MS units in β crystals and the partial inclusion case of α crystals are attributed to the 3% difference in crystal density, i.e., ρα = 1.067 vs. ρβ =1.033 g mL−1. Grafting of aPS arms resulted in negligible changes in thermodynamic properties and crystal packing from those of sPS-10%4MS except for clearly decreased crystallinity.

並列關鍵字

sPS ； copolymer ； crystallization ； Gibbs-Thomson ； Sanchez-Eby

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