<i>T</i>_g Confinement Effect of Random Copolymers of 4-<i>tert</i>-Butylstyrene and 4-Acetoxystyrene with Different Compositions

Abstract

We observe that the <i>T</i>_g confinement effect of polymer films can saturate with polymer-substrate interaction. Thickness dependences of the glass transition temperature, <i>T</i>_g(<i>h</i>₀), of random copolymer films of 4-<i>tert</i>-butylstyrene (TBS) and 4-acetoxystyrene (AS) supported by silica (SiOx) were measured for different TBS concentrations, <i>X</i>_TBS. For 0 ≤ <i>X</i>_TBS ≤ 0.47, <i>T</i>_g(<i>h</i>₀) displays identical enhancements, independent of <i>X</i>_TBS. For <i>X</i>_TBS > ∼0.66; however, <i>T</i>_g(<i>h</i>₀) decreases steadily with <i>X</i>_TBS. The <i>X</i>_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 <i>T</i>_g, and thereby <i>T</i>_g reduction. We propose that saturation in <i>T</i>_g(<i>h</i>₀) found for <i>X</i>_TBS ≤ 0.47 is caused by the maximization in polymer-substrate-specific bond formation. Further experiments and a calculation support this proposition.