2020同步年報

Soft Matter 039 Relation Between Melting Behavior and Phase Transi- tions in Polymer Crystallization A relation between the crystalline lamellar thickness and the melting temperature is revealed by small- and wide-angle X-ray scattering. T he equilibrium melting temperature of a crystallisable polymer, defined as the melting temperature of an infinitely thick crystal, is an important thermodynamic prop- erty for polymers, which acts as the reference temperature to define the degree of supercooling, i.e ., the driving force of crystallization. In general, the melting temperature is depressed with the crystallization temperature range, when an A/B random copolymer is introduced as non-crystalliz- able comonomer units (B) into the crystallizable homopoly- mer (A). There are two conditions for the distribution of the comonomer units in the crystalline structure: one is that the comonomer units are fully excluded from the crystalline structure, raising the concentration of comonomer units in the amorphous phase; the second is that the comonomer units are nondiscriminatingly incorporated into the crystal- line lattice, resulting in a uniform composition in the crystal- line and amorphous phases. To clarify how the lamellar thickness affects the crystal melt- ing in the copolymer systems, An-Chung Su (National Tsing Hua University) constructed the melting behavior for α and β crystals of syndiotactic poly(styrene-stat-3-methylstyrene) (sPS-3MS) and poly(styrene-stat-4-methylstyrene) (sPS-4MS) random copolymers with varied comonomer proportions, using simultaneous synchrotron small- and wide-angle X-ray scattering (SAXS/WAXS) heating profiles in situ . 1 These measurements were made at TLS 23A1 . Figures 1(a)–1(c) show the simultaneous SAXS/WAXS heating profiles of the cold-crystallized sPS-5%3MS spec- imen (for varied crystallization temperatures, T c ) at rate 2 °C min −1 . The lamellar peak position shifts from q c = 0.036 to 0.027 Å −1 , corresponding to a long period ( L p ) increased from L p = 17 to 23 nm, i.e ., a specimen cold-crystallized at higher temperature exhibits a greater interparticle distance between nanograins. Upon heating, the lamellar peak in each profile shifts successively to smaller q positions and Fig. 1 : (a−c) SAXS heating profiles of sPS-5%3MS cold-crystallized specimens at rate 2 °C min −1 . Representative deconvoluted WAXS profiles for sPS- 5%3MS cold-crystallized specimens at temperatures (a) 220, (b) 230 and (c) 240 °C. [Reproduced from Ref. 1] (a) (b) (c) (d) (e) (f) Intensity (cm -1 ) Intensity (cm -1 ) Intensity (cm -1 ) Intensity (arb. units) Intensity (arb. units) Intensity (arb. units)