GLASS-TRANSITION BEHAVIOR IN ULTRA-THIN POLYSTYRENE FILMS

We have used ellipsometry to measure the glass transition temperature (T-g) of ultra-thin films of polystyrene (PS) (less than 20 nm thick) obtained by spin-casting from solution onto silicon substrates. We find that T-g in these ultra-thin films is depressed from the bulk value in qualitative accord with our earlier results on thicker films of PS. In films as thin as 8 nm, the depression from the bulk value of T-g is 35 K. We have also prepared ultra-thins by grafting PS-COOH on the native oxide of Si and by spin-casting PS-COOH onto Si. Here we have been able to measure the T-g of 5-nm firms in which we find a T-g depression of IO K. We tentatively ascribe the smaller value of T-g depression for these grafted chains to the constraining effect of the anchor.