Nematic Phase of Plate-like Semicrystalline Block Copolymer Single Crystals in Solution Studied by Small-Angle X-Ray Scattering

Abstract

Crystalline block copolymers have been used to prepare plate-like colloidal systems with well-controlled size, shape, and size distribution. The isotropic-to-nematic (I–N) phase transition of the novel plate-like colloidal particle suspensions has been reported previously. In this work, we focus on the characterization of the solution structure of the crystals and the N-phase using small- and ultrasmall-angle X-ray scattering techniques (SAXS/USAXS). The system has polystyrene-block-poly­(l-lactide) (PS-b-PLLA) block copolymer single crystals (BCSCs) with different sizes dispersed in p-xylene. These crystals are truncated lozenge in shape and have effective diameters ranging from 550 to 4000 nm with a uniform dry thickness of 18.0 nm. Scattering of the individual crystal in solution can be simplified using a disc model with a core layer of 9–10 nm due to the lower contrast of the tethered PS layer. BCSC suspensions filled in thin quartz capillaries are prepared for monitoring the structural information. SAXS measurements of the isotropic phase show a strong face-to-face correlation, indicating that platelets form small stacked clusters in solutions. The isotropic phase is thus a coexistence of single crystals and the stacked multiple-layered clusters. The face-to-face spacing, d, in the N phases is around 75–90 nm, which increases slightly upon increasing the size of crystals. For a given system, the spacing does not change with increasing concentration under the current experimental conditions. Finally, the possible formation of lamellar domains within the N phase is also discussed due to the lateral attraction of this system. These results demonstrate the importance of the lateral attraction between the polar crystalline PLLA blocks on the formation of the N phase: the BCSCs self-assemble into larger sheets via the lateral attraction, which further enhances the I–N transition.

Single crystals and two‐dimensional crystalline assemblies of block copolymers

“…He et al studied the LC phase behaviors in the suspension of a series of PLLA-b-PS BCPSCs and PLLA/PLLA-b-PS HSCs. 91,173,174 They found that the critical volume fraction of phase transition was obviously smaller than the theoretical value, which resulted from the lateral attraction between the lamellae and external gravity. They also determined the critical volume fraction of the phase transition as a function of the size of the crystals and verified the size-dependence via the self-seeding procedure to obtain lamellae of different sizes.…”

“…Nanocomposite hydrogels with platelet micelles showed better mechanical properties, and a self‐healing ability between two freshly cut surfaces of the same gels, which is promising for use in living organisms. He et al studied the LC phase behaviors in the suspension of a series of PLLA‐ b ‐PS BCPSCs and PLLA/PLLA‐ b ‐PS HSCs 91,173,174 . They found that the critical volume fraction of phase transition was obviously smaller than the theoretical value, which resulted from the lateral attraction between the lamellae and external gravity.…”