Relationship between Segmental Relaxation of Polystyrene Films and Stick-Slip Behavior during Dynamic Wetting of Liquid Droplets on Their Surfaces

Advancements in Novel Mechano-Rheological Probes for Studying Glassy Dynamics in Nanoconfined Thin Polymer Films

The nanoconfinement effects of glassy polymer thin films on their thermal and mechanical properties have been investigated thoroughly, especially with an emphasis on its altered glass transition behavior compared to bulk polymer, which has been known for almost three decades. While research in this direction is still evolving, reaching new heights to unravel the underlying physics of phenomena observed in confined thin polymer films, we have a much clearer picture now. This, in turn, has promoted their application in miniaturized and functional applications. To extract the full potential of such confined films, starting from their fabrication, function, and various applications, we must realize the necessity to have an understanding and availability of robust characterization protocols that specifically target thin film thermo-mechanical stability. Being nanometer-sized in thickness, often atop a solid substrate, direct mechanical testing on such films becomes extremely challenging and often encounters serious complexity from the dominating effect of the substrate. In this review, we have compiled together a few important novel and promising techniques for mechano-rheological characterization of glassy polymer thin films. The conceptual background involved in each technique, constitutive equations, methodology, and current status of research are touched upon following a pedagogical tutorial approach. Further, we discussed each technique's success and limitations, carefully covering the puzzling or contradicting observations reported within the broad nexus of glass transition temperature-viscosity-modulus-molecular mobility (including diffusion and relaxation).

Temperature-dependent soft wetting on amorphous, uncrosslinked polymer surfaces

The wetting of polymer melts at high temperatures is studied by placing a glycerol drop on a poly(n-butyl methacrylate) film and measuring the wetting ridge. The height of the wetting ridge grows continuously over time. These wetting ridge growth rates can be explained by polymer chain dynamics occurring at the molecular level, determined using oscillatory shear rheology of the polymer melt. The shape of wetting ridge profile can be modeled using an equation previously used for elastomers, with a simple modification that incorporates the time-dependent storage modulus of the uncrosslinked melts.

Suppressed Surface Reorganization in a High-Density Poly(methyl methacrylate) Brush

A high-density poly(methyl methacrylate) (PMMA) brush (σ = 0.77 chain/nm²) with a lower molecular weight distribution was prepared onto a silicon wafer by surface-initiated atom transfer radical polymerization. The surface of the PMMA brush chains was characterized upon the process of the environmental change, from air to water, using contact angle measurements in conjunction with sum-frequency generation spectroscopy. The surface structure and properties altered less with the changing environment from air to water for the PMMA brush than for a spin-coated film; that is, the extent of surface reorganization could be suppressed by grafting densely-packed chains onto a substrate. Also, the water penetration into the brush surface was inhibited because of the densely packed chain structure.

The architecture of the adsorbed layer at the substrate interface determines the glass transition of supported ultrathin polystyrene films

To elucidate the mechanism underlying the effect of polymer/solid interfacial interactions on the dynamics of thin polymer films, the glass transition of thin end-functionalized polystyrene films supported on SiO₂-Si, such as proton-terminated PS (PS-H), α,ω-dicarboxy-terminated PS (PS-COOH), and α,ω-dihydroxyl-terminated PS (PS-OH), was investigated. All the PS films exhibited a substantial depression in T_g with decreasing film thickness, while the extent of such depression was strongly dependent on the chemical structure of the end groups and molecular weights. It was found that T - T of the various PS films increased linearly with increasing h_ads/R_g, in which h_ads is the thickness of the interfacial adsorbed layer and R_g is the radius of gyration of PS. The h_ads/R_g is a direct reflection of the macromolecular chain conformation within the adsorbed layer which was affected by its end groups and molecular weights. These findings are in line with the work of Napolitano, and present direct experimental evidence.

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

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.