Temperature-Enhanced Ordering in Plate-like Semicrystalline Block Copolymer Single-Crystal Suspensions Studied by Real-Time SAXS/WAXS

Self-assembly of hard platelet colloids into liquid crystalline phases is typically driven by entropy, making them less sensitive to temperature changes. However, soft interaction potentials often exist in real colloidal systems, which can lead to temperature-sensitive phase transitions. Despite significant progress in understanding phase behavior in the past 2 decades, studies on temperature-dependent phase behavior remain rare, and there is limited knowledge about how soft interactions influence phase transitions upon temperature changes. In this work, we investigated a platelet colloid system in isotropic and nematic phases using small- and wide-angle X-ray scattering techniques (SAXS/WAXS) and polarized optical microscopy (POM). The system consisted of polystyrene-block-poly(l-lactide) (PS-b-PLLA) block copolymer single crystals (BCSCs) with varying sizes dispersed in p-xylene. These crystals were truncated lozenge-shaped with effective diameters of 500 and 1000 nm and a uniform dry thickness of 18.0 nm. The ordering behaviors of BCSC500 in the isotropic phase and BCSC1000 in the N phase were monitored through SAXS/WAXS during heating, quenching, self-seeding, crystal growth, and final quenching. Enhanced ordering, specifically face-to-face correlation, was observed during heating prior to crystal melting. For BCSC500, ordering emerged at 105 °C during heating. In the case of BCSC1000 in the N phase, ordering was enhanced with increased heating and reached up to the ninth order of correlation peaks, indicating the formation of lamellar domains within the N phase. After seeding and crystal growth, both systems exhibited ordering. However, during the final cooling to room temperature, ordering disappeared for BCSC500 but persisted for BCSC1000. POM observations revealed that for both systems, initial heating resulted in a decrease in overall brightness; however, enhanced nematic domains or tactoids emerged prior to melting. Subsequent thermal treatments did not induce noticeable changes in the observed order. While both techniques revealed increased order, discrepancies were noted. SAXS indicated intensified short-range correlations, while POM showed the formation of local nematic domains or tactoids. We propose three distinct ordering regimes to reconcile these observations: large-scale nematic order, enhanced short-range order, and short-range clusters. We attributed the temperature-enhanced ordering phenomenon to lateral interactions between the BCSCs, annealing, and memory effects during melting and crystallization.