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

A lamellae-forming poly(ethylene oxide)-b-polystyrene (EOS) has been blende d 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, r espectively). Both blends exhibit cylindrical microphase morphologies, with the PEO volume fractions being 0.32. The order-disorder transition tempera tures (T-ODT) of both blends are 175 and 84 degreesC, respectively, as dete rmined by temperature-dependent small angle X-ray scattering experiments. T he glass transition temperature of the PS matrix (T-g(PS)) for the EOS/PS-3 2 blend is 64 degreesC as determined by differential scanning calorimetry ( DSC), while that for the EOS/PSO-32 blend is only 16 degreesC. Thus, by con trolling the crystallization temperatures (T-c(PEO)), two kinds of nano-con fined PEO crystallizations have been achieved in these blends: when T-ODT m uch greater than T-g(PS) > T-c(PEO) in the EOS/PS-32 blend, the PEO-block c rystallization is confined under a hard PS confinement, while for T-ODT > T -c(PEO) greater than or similar to T-g(PS) in the EOS/PSO-32 blend, the PEO -block crystallization is confined under a soft PS confinement. DSC and wid e-angle X-ray experiments show that the crystallizations of the PEO blocks in these two confinement environments behave differently. The PEO-block cry stallization kinetics in the hard confinement is much slower than that in t he soft confinement. The DSC kinetics studies show that for T-c < 30 degree sC, the In K 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 t he 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 cry stals in the soft confinement environment can continuously increase their t hermodynamic stability through a crystal thickening process, by which the s oft confinement environment is partially destroyed. (C) 2001 Published by E lsevier Science Ltd.