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

Abstract

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 ∼ R g); (3) contraction in film thickness, followed by expansion for the films with h ∼ 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.

Combined X-ray Reflectivity and Infrared Study of the Effect of Hydrogen Bonding of the OH Group on the Relaxation Behavior in Ultrathin Polyvinylphenol Films on SiO2

“…Based on the results provided in this study, as well as the results of PS and PMMA from previous studies, , it can be suggested that at least two competing relaxation processes are taking place in ultrathin glassy polymers. One dominates in the glassy state, giving rise to NTE, while the other takes over in the melt state, resulting in thickness contraction even at temperatures higher than T g bulk .…”

“…Such an intriguing and unusual behavior of thickness expansion in the glassy state and shrinkage in the melt state were also seen in well-annealed PS films supported on SiO 2 and PMMA on SiO 2 and SiOH substrates in our previous studies. 41,42 Thickness expansion in PMMA was observed for films with thickness less than their R g values; above R g , data showed a competing behavior between thickness expansion and contraction, and finally for the thickness greater than 2.5R g or 3R g , the thickness decreased with time. In contrast to PMMA films, 41 which experienced thickness expansion for films with an initial thickness of < R g , PVPh films exhibited thickness expansion for all of the films ranging from 30 Å (∼0.75R g ) to 140 Å (∼3.5R g ).…”

“…41,42 Thickness expansion in PMMA was observed for films with thickness less than their R g values; above R g , data showed a competing behavior between thickness expansion and contraction, and finally for the thickness greater than 2.5R g or 3R g , the thickness decreased with time. In contrast to PMMA films, 41 which experienced thickness expansion for films with an initial thickness of < R g , PVPh films exhibited thickness expansion for all of the films ranging from 30 Å (∼0.75R g ) to 140 Å (∼3.5R g ). However, as the thickness increased above R g , the magnitude of the thickness expansion decreased by approximately half, suggesting that the expansion in thickness is a confinement effect.…”

“…In our previous studies, we have proposed a preliminary model to understand the thickness-dependent relaxation measurements in PMMA films supported on different substrates, 41 as well as temperature-dependent studies on PS films. 42 Glass-Transition Temperature.…”

“…A detailed discussion of the thickness and temperature-dependent relaxation (physical aging) of PS and PMMA has been presented in our previous studies. 41,42 The purpose of the present study is to explore several important properties of the technologically important polymer polyvinylphenol (PVPh). PVPh is utilized as a dielectric layer in many electronic devices, such as in organic thin-film transistors.…”