T g Confinement Effect of Random Copolymers of 4- tert -Butylstyrene and 4-Acetoxystyrene with Different Compositions.

We observe that the T g confinement effect of polymer films can saturate with polymer-substrate interaction. Thickness dependences of the glass transition temperature, T g ( h 0 ), of random copolymer films of 4- tert -butylstyrene (TBS) and 4-acetoxystyrene (AS) supported by silica (SiOx) were measured for different TBS concentrations, X TBS . For 0 ≤ X TBS ≤ 0.47, T g ( h 0 ) displays identical enhancements, independent of X TBS . For X TBS > ∼0.66; however, T g ( h 0 ) decreases steadily with X TBS . The X TBS > 0.66 result is in keeping with expectations that TBS interacts less strongly with SiOx than AS does, and weaker polymer-substrate interaction renders greater dominance of the air surface over substrate surface on T g , and thereby T g reduction. We propose that saturation in T g ( h 0 ) found for X TBS ≤ 0.47 is caused by the maximization in polymer-substrate-specific bond formation. Further experiments and a calculation support this proposition.