Hard and soft confinement effects on polymer crystallization in microphase separated cylinder-forming PEO-b-PS/PS blends

Abstract

A lamellae-forming poly(ethylene oxide)-b-polystyrene (EOS) has been blended with a polystyrene homopolymer (PS) and a PS oligomer (PSO), respectively, to obtain miscible polymer blends (denoted as EOS/PS-32 and EOS/PSO-32, respectively). Both blends exhibit cylindrical microphase morphologies, with the PEO volume fractions being 0.32. The order–disorder transition temperatures (TODT) of both blends are 175 and 84°C, respectively, as determined by temperature-dependent small angle X-ray scattering experiments. The glass transition temperature of the PS matrix (TgPS) for the EOS/PS-32 blend is 64°C as determined by differential scanning calorimetry (DSC), while that for the EOS/PSO-32 blend is only 16°C. Thus, by controlling the crystallization temperatures (TcPEO), two kinds of nano-confined PEO crystallizations have been achieved in these blends: when TODT≫TgPS>TcPEO in the EOS/PS-32 blend, the PEO-block crystallization is confined under a hard PS confinement, while for TODT>TcPEO≳TgPS in the EOS/PSO-32 blend, the PEO-block crystallization is confined under a soft PS confinement. DSC and wide-angle X-ray experiments show that the crystallizations of the PEO blocks in these two confinement environments behave differently. The PEO-block crystallization kinetics in the hard confinement is much slower than that in the soft confinement. The DSC kinetics studies show that for Tc<30°C, the lnK values in the Avrami equation for both blends appear similar, while the n parameter for the EOS/PSO-32 is higher than that for the EOS/PS-32 blend. The melting temperature and weight percent crystallinity of the PEO crystals in the soft confinement environment are higher than those in the hard confinement environment, indicating that the PEO crystals developed in the soft confinement environment possess higher thermodynamic stability than in the hard confinement environment. Furthermore, heating the PEO crystals in the soft confinement environment can continuously increase their thermodynamic stability through a crystal thickening process, by which the soft confinement environment is partially destroyed.