We show that thin film star-shaped macromolecules exhibit significant differences in their average vitrification behavior, in both magnitude and thickness dependence, from their linear analogs. This behavior is dictated by a combination of their functionality and arm length. Additionally, the glass transition temperature at the free surface of a star-shaped molecule film may be higher than that of the interior, in contrast to their linear analogs where the opposite is true. These findings have implications for other properties, due largely to the origins, entropic, of this behavior.

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, SiO x , 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 (ΔT g < 0). However, for sufficiently large f and small M w arm , T g becomes independent of film thickness (ΔT g ≈ 0). In this region, where ΔT g ≈ 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 ΔT 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 SiO x is summarized in terms of a "diagram of states".

Time-dependent changes of thermodynamic properties due to structural relaxations and physical aging occur in all glasses. We show that the physical aging of thin supported films of star-shaped macromolecules, with f arms of length N(arm), exhibits average aging dynamics that are sensitive to f and N(arm). Regions of the films in proximity to interfaces age at substantially different rates than the interior of the film; this is also true of linear chain systems. This behavior may be reconciled in terms of a universal picture that accounts only for changes in the local T(g) of the films.

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Time-dependent structural relaxations, physical aging, of films with thicknesses in the range 0.4 μm < H < 2 μm of star-shaped polystyrene (SPS) macromolecules were investigated. Our studies reveal that the aging rates of star-shaped PS macromolecules are appreciably slower than their linear chain analogs. The magnitude of the difference between the aging rates of the linear and star-shaped macromolecules increases with increasing functionality, f, and decreasing molecular weight per arm, M n arm, of the stars. Our results are consistent with the notion that constraints imposed due to the architecture of the macromolecule suppress relaxations associated with and accommodate the reduction of the free volume of the system.