Impact of chain architecture (branching) on the thermal and mechanical behavior of polystyrene thin films

The modulus and glass transition temperature (Tg) of ultrathin films of polystyrene (PS) with different branching architectures are examined via surface wrinkling and the discontinuity in the thermal expansion as determined from spectroscopic ellipsometry, respectively. Branching of the PS is systematically varied using multifunctional monomers to create comb, centipede, and star architectures with similar molecular masses. The bulk-like (thick film) Tg for these polymers is 103 +/- 2 degrees C and independent of branching and all films thinner than 40 nm exhibit reductions in Tg. There are subtle differences between the architectures with reductions in Tg for linear (25 degrees C), centipede (40 degrees C), comb (9 degrees C), and 4 armed star (9 degrees C) PS for 5 nm films. Interestingly, the room temperature modulus of the thick films is dependent upon the chain architecture with the star and comb polymers being the most compliant (approximate to 2 GPa) whereas the centipede PS is most rigid (approximate to 4 GPa). The comb PS exhibits no thickness dependence in moduli, whereas all other PS architectures examined show a decrease in modulus as the film thickness is decreased below similar to 40 nm. We hypothesize that the chain conformation leads to the apparent susceptibility of the polymer to reductions in moduli in thin films. These results provide insight into potential origins for thickness dependent properties of polymer thin films. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012