Templating Polymer/Chromophore Crystallization in a Gyroid Matrix

Abstract

Blending the small molecule nonlinear optical chromophore, 2-chloro-4-nitroanaline (CNA), with a semicrystalline block copolymer, polystyrene-poly(ethylene oxide) (PS-PEO), drives the formation of a bicontinuous cubic microstructure, known as gyroid (GYR), demonstrating Ia3̅d cubic space group symmetry. The morphology transition is due to the selective partitioning of CNA into the PEO domains, which happens to be the majority phase while PS forms the GYR network. Furthermore, PEO and CNA cocrystallize together, forming a single crystalline phase that exhibits a different crystal structure from the starting PEO and CNA materials. At room temperature, PEO crystals and PEO/CNA cocrystals coexist and display different melting temperatures, Tm,PEO = 45 °C and Tm,PEO/CNA = 78 °C, respectively. Interestingly, although the PEO domain is the matrix, the GYR network templates the crystallization of the PEO/CNA cocrystal by precisely controlling the cocrystal long period. Furthermore, circular dichroism (CD) measurements indicate that the crystallized sample contains a chiral component when the PEO/CNA cocrystal is present, and upon melting of the PEO/CNA cocrystal (Tm,PEO/CNA = 78 °C), the CD signal vanishes. The results shown here indicate that adding a small molecule to semicrystalline block copolymer in which one block will form a cocrystal with the small molecule opens the possibility of controlling hierarchical structure and material functionality.