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Eighteen years ago, Fredrickson et al. [Macromolecules 1992, 25, 2882-2889] predicted that a molten polymer brush possesses a shear modulus that would cause the surface structure to deviate from that of a liquid. But their predictions have been largely unchecked. Recently, two groups reported that molten polystyrene (PS) monolayer films deposited on Si demonstrated surface structures indicating that the films possessed a shear modulus even the polymer should already be in the terminal flow regime given the annealing temperatures and times used. This remarkable observation was interpreted to arise from the PS chains being strongly pinned to the substrate surface, rendering the molten polymer films the properties of a Guiselin brush, i.e., one in which the brush chains have the same conformation as that of molten polymers. This suggestion is illuminating, but not without uncertainty. In particular, it implies that the shear modulus of a Guiselin brush can be up to ~1/10000 times that predicted by Fredrickson et al.'s theory. In the past two years, we had been carrying out experiments to verify Fredrickson et al.'s theory and to understand the aforementioned anomalous observations found in molten PS films. The main results of our studies are as follows:

(1) We confirmed the validity of Fredrickson et al.'s theory for uniform polymer brushes.
(2) Fredrickson et al.'s theory is, however, not applicable to a Guiselin brush. Specifically, the shear modulus of a brush mimicking a Guiselin brush was found to be two orders of magnitude smaller than that predicted by the theory.
(3) We observed for the first time that PS formed an irreversibly adsorbed monolayer on Si when we subjected the PS/Si films to extensively annealing. This provides a foundation to the suggestion that these films can form a brush so possess a shear modulus in the molten state.
(4) However, we also found that the initial observation, showing molten polymer films to possess a shear modulus, could as well be accounted for if the films were not fully equilibrated. We estimated that the time for a monolayer PS film to equilibrate could be more than 10 years if the molecular weight of the polymer was more than 500 kg/mol even at 165 ^oC, i.e., 65 ^oC above the T_g of the polymer. This calls for careful interpretation of the data in this sort of measurements. 

The project investigations have led to four publications, including three in Macromolecules and one in Applied Physics Letter as well as several invited talks by the PI at scientific conferences and academic institutions. Of equal importance is the training and development of personnel. The postdoctoral fellow whom this award had supported predominantly was recruited to take on an assistant professorship this March. This PRF award had also enabled the PI to engage one graduate student and one undergraduate in research. These students have since given oral presentations at scientific conferences and co-authored a paper with the PI. All of the above scientific and educational attainments would not have been realized without the support of PRF. We are grateful to be given the award.