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.