Comonomer effects on crystallization and melting of syndiotactic polystyrene copolymers

• Main Authors: Lin, Shi-Jieh, 林詩傑
• Other Authors: 蘇安仲
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/76727618326997240836

博士 === 國立清華大學 === 化學工程學系 === 100 === 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.