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Salt-induced microphase separation of amorphous dendritic poly(ethylene oxide)-block-linear polystyrene copolymers

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Abstract

We prepared two block copolymers 1 and 2 consisting of a third-generation dendron with poly(ethylene oxide) (PEO) peripheries and a linear polystyrene (PS) coil. The PS molecular weights were 2000 g/mol and 8000 g/mol for 1 and 2, respectively. The differential scanning calorimetry (DSC) data indicated that neither of the block copolymers showed glass transition, implying that there was no microphase separation between the PEO and PS blocks. However, upon doping the block copolymers with lithium triflate (lithium concentration per ethylene oxide unit = 0.2), two distinct glass transitions were seen, corresponding to the salt-doped PEO and PS blocks, respectively. The morphological analysis using small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) demonstrated that a hexagonal columnar morphology was induced in salt-doped sample 1-Li+, whereas the other sample (2-Li+) with a longer PS coil revealed a lamellar structure. In particular, in the SAXS data of 2-Li+, an abrupt reduction in the lamellar thickness was observed near the PS glass transition temperature (Tg), in contrast to the SAXS data for 1-Li+. This reduction implies that there is a lateral expansion of the molecular section in the lamellar structure, which can be interpreted by the conformational energy stabilization of the long PS coil above Tg. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2372–2376, 2010

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