Vitrification of Thin Polymer Films: From Linear Chain to Soft Colloid- like Behavior
E Glynos and B Frieberg and A Chremos and G Sakellariou and DW Gidley and PF Green, MACROMOLECULES, 48, 2305-2312 (2015).
We show that the vitrification of star-shaped polystyrene (PS), of functionality f and molecular weight per arm M-w(arm), thin films supported by silicon oxide, SiOx, is strongly dependent on M-w(arm) and f. When f is small, the vitrification behavior is similar to that of linear-chain PS where the average glass transition, T-g, decreases with decreasing film thickness (Delta T-g < 0). However, for sufficiently large f and small M-w(arm), T-g becomes independent of film thickness (Delta T-g < 0). In this region, where Delta T-g approximate to 0, the star-shaped macromolecules self-assemble into ordered, periodic structures, similar to that of soft spheres or colloids, as revealed by simulations and experiments. This is identified as the soft-colloidal region. The transition from the linear-chain-like to the soft-colloidal- like region occurs over an intermediate range of functionalities and arm lengths; throughout this transition range Delta T-g > 0. We show that the overall vitrification behavior of these thin film star-shaped polymers is due to competing entropic interactions associated with changes in f and M-w(arm). The vitrification behavior of thin star- shaped PS films on SiOx is summarized in terms of a diagram of states.
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