Thickness and Substrate Dependences of the Relaxation of SpinCoated Polymethyl Methacrylate Ultrathin Films Supported on SiO2 and SiOH Substrates

The relaxation of ultrathin polymethyl methacrylate (PMMA) films, which were spin-coated on SiO2 and SiOH substrates, was examined with different film thicknesses. In the glassy state, several relaxation processes revealed temporal variations in thicknesses. In sharp contrast to bulk PMMA in which the monotonic volume reduction was expected, the ultrathin films exhibited thickness variations, which would be classified into at least four types: (1) a slow increase in the thickness with time, where the thickness, h, is less than the radius of gyration, R-g (h < R-g); (2) scattered, fluctuating temporal variation (h similar to R-g); (3) contraction in film thickness, followed by expansion for the films with h similar to 2.6R(g); and (4) scattered temporal variation eventually leading to bulk relaxation (h > 3R(g)). Such a complex relaxation phenomenon could have originated from the interaction of the substrate with the heterogeneous out-of-equilibrium configurations of the polymer molecules that were confined to a quasi-2-D geometry.

Automated summary

This study examined the relaxation of ultrathin PMMA films on SiO2 and SiOH substrates with varying thicknesses. It revealed complex thickness variations over time, which were attributed to the interaction between the substrate and out-of-equilibrium polymer configurations in a quasi-2-D geometry.