Photopolymerization-assisted self-assembly as a strategy to obtain a dispersion of very high aspect ratio nanostructures in a polystyrene matrix

Abstract

Very high aspect ratio nano-objects dispersed in a polymer matrix impart a number of desired characteristics to the material, such as barrier properties to the diffusion of small molecules. Commonly, these dispersions are obtained via top-down procedures that require several time-consuming steps of purification, surface modification, and processing. In this article, we show that it is possible to produce a dispersion of crystalline nano-objects of very high aspect ratio through an in situ approach based on photopolymerization-driven self-assembly. For this purpose, a synthesized poly(styrene-block-ε-caprolactone) (PS-b-PCL) block copolymer was dissolved in styrene (St) monomer, and the solution was slowly photopolymerized at room temperature. Under such conditions, long crystalline nanoribbons dispersed in a PS matrix were obtained after four days of irradiation. The nanostructuration process was investigated by in situ small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (XRD), and transmission electron microscopy (TEM). Obtained results allowed us to conceptualize the formation mechanism of nanoribbons as a stepwise micellization-crystallization-growth process. When photopolymerization at room temperature was replaced by thermal polymerization at 90 °C, a dispersion of amorphous nanorods was obtained. This demonstrated that crystallization of PCL blocks played a dominant role in determining the morphology of the very high aspect ratio nano-objects obtained by photopolymerization.