Probing the rheological properties of supported thin polystyrene films by investigating the growth dynamics of wetting ridges

Abstract

Despite its importance in the processing of nanomaterials, the rheological behavior of thin polymer films is poorly understood, partly due to the inherent measurement challenges. Herein, we have developed a facile method for investigating the rheological behavior of supported thin polymeric films by monitoring the growth of the "wetting ridge"-a microscopic protrusion on the film surface due to the capillary forces exerted by a drop of ionic liquid placed on the film surface. It was found that the growth dynamics of the wetting ridge and the behavior of polystyrene rheology are directly linked. Important rheological properties, such as the flow temperature (Tf), viscosity (η), and terminal relaxation time (τ0) of thin polystyrene films, can be derived by studying the development of the height of the wetting ridge with time and the sample temperature. Rheological studies using the proposed approach for supported thin polystyrene (PS) films with thickness down to 20 nm demonstrate that the PS thin film exhibits facilitated flow, with reduced viscosity and lowered viscous temperature and a shortened rubbery plateau, when SiOx-Si was used as the substrate. However, sluggish flow was observed for the PS film supported by hydrogen-passivated silicon substrates (H-Si). The differences in enthalpic interactions between PS and the substrates are the reason for this divergence in the whole-chain mobility and flow ability of thin PS films deposited on SiOx-Si and H-Si surfaces. These results indicate that this approach could be a reliable rheological probe for supported thin polymeric films with different thicknesses and various substrates.

Relaxing nonequilibrated polymers in thin films at temperatures slightly above the glass transition

“…Confinement may affect rheological properties of thin glassy polymer films, such as elastic modulus, creep compliance, and viscosity . In this context, the glass transition temperature ( T g ) of thin polymer films has been the central focus of many studies for the last twenty years.…”

Suppressed Chain Entanglement Induced by Thickness of Ultrathin Polystyrene Films

“…The viscosity of the PS films was examined based on the time-dependent evolution of the wetting ridge. 1,41 When a liquid droplet was put on the polymer film, the region near the three-phase contact line could be deformed owing to the vertical component of γ, as shown in panel (a) of Figure 1. The deformed region denotes the wetting ridge.…”

“…The height of the wetting ridge was examined in the following. 1 A heating stage with an accuracy of ±1 K (INSTEC, USA) was used to control the sample temperature. A supported PS film was mounted onto the heating stage after the temperature reached a constant.…”

“…The viscous flow temperature (T f ), effective viscosity (η), and reptation (disentanglement) time (τ d ) of thin PS films could be extracted from the time evolution of the wetting ridges in the liquid flow region at various temperatures. 1 In this paper, the characteristic time τ*, corresponding to the transition from the elasticity-to viscosity-dominated regime, was obtained by monitoring the time dependence of the wetting ridges in the rubbery plateau region. The characteristic time τ*, which was demonstrated to be related to disentanglement time τ d by investigating the relationship between the τ* of PS in bulk and its molecular weight, was used to probe the variation in the chain entanglements of supported ultrathin PS films.…”

“…Chain entanglements are the basis of lots of the typical physical properties of macromolecules, such as rheological characteristics, , melt viscosities, , diffusion behaviors, and mechanical behaviors. , Specifically, the high strain and fracture properties of glassy polymers are also profoundly influenced by chain entanglements. , With the rapid development of nanotechnology, the chain conformation and entanglement in nanoconfined systems have recently attracted increasing interest in both experiments ,, and simulations. − For instance, Si et al reported that the effective molecular weight between two entanglement points ( M e ) in a thin free-standing polystyrene (PS) film was markedly larger than that of the bulk by using the thickness ratio of the shear deformation zone to the un-deformed film. Rathfon et al claimed that the chain conformation would be significantly affected in ultrathin films.…”