Effects of molecular weight and tacticity on the Tg of poly(methyl methacrylate) films supported by silica

The glass transition temperature ($T_{\mathrm{g}})$ of poly(methyl methacrylate) (PMMA) films supported by silica is studied as a function of film thickness at different molecular weights ($M_{\mathrm{w}})$ for different polymer tacticities. The $T_{\mathrm{g}}$ confinement effect is found to depend on the $M_{\mathrm{w}}$ and tacticity. For the films with a low $M_{\mathrm{w}}$ of 2.5 kg/mol,$ T_{\mathrm{g}}$ is depressed for the atactic films, consistent with previous results. In contrast, the films with a higher syndiotactic content exhibit $T_{\mathrm{g}}$ enlargement as thickness decreases. We tentatively suggest this to be caused the influence of chain stiffness on the $T_{\mathrm{g}}$ that dominates at low $M_{\mathrm{w}}$ and varies with tacticity. For sufficiently high $M_{\mathrm{w}}$, the effect of chain stiffness is expected to be small. At $M_{\mathrm{w}} \quad =$ 50 kg/mol, the $T_{\mathrm{g}}$ confinement effect of the atatic and more syndiotactic films reverses from that observed of the low-$M_{\mathrm{w}}$ counterpart films. We suggest the chain stiffness effect to be negligible at this $M_{\mathrm{w}}$, and attribute the opposite $T_{\mathrm{g}}$ confinement effect to be caused by a competition between the surface $T_{\mathrm{g}}$ and the substrate $T_{\mathrm{g}}$ in these films. The $T_{\mathrm{g}}$ found of bilayers made of the atatic and more syndiotatic PMMAs with this $M_{\mathrm{w}}$ supports our attribute.