Glass Transition Behavior in Ultra‐Thin Polystyrene Films

Abstract

Abstract 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 films in which we find a T g depression of 10 K. We tentatively ascribe the smaller value of T g depression for these grafted chains to the constraining effect of the anchor.