Monodispersed polystyrene with different molecular weights, M
w, from below the entanglement molecular weight, M
e (∼20K Da), to well above was studied for effects of chain ends and chain
entanglement on the glass transition temperature, T
g, of polymer thin films. The relative importance
between these two effects is expected to reverse as M
w crosses over M
e. We found that the T
g generally
decreases with decreasing film thickness, t, consistent with previous findings. The difference between
results from the M
w = 13.7K and the 550K samples was negligible, suggesting that entanglement has
little significance on the thickness dependence of the T
g of polymer under confinement in thin films. On
the other hand, the M
w dependence of T
g was markedly weakened compared to that of the bulk when t
was reduced to 15 nm. According to Fox and Flory's theory, our result reveals that some of the chain
ends in the film have been segregated to the surface, but they have little effect on the T
g of the film. Our
findings undermine a surface mobile layer where the surface chain ends localize being the cause for the
reduction of T
g observed.
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