Interfacial Behaviors of Polymer and Metal-Polymer Thin Films under Contact and Solvent Stimuli

Abstract

Polymeric thin films and metal-polymer bilayers are foundational components in flexible electronics, adaptive surfaces, and emerging interface technologies, valued for their mechanical compliance, ease of processing, and tunable functionality. Unlike homogeneous rigid or bulk elastic materials, polymer thin films exhibit inherently display scale-dependent mechanical and optical properties, particularly at interfaces, where deformation behavior is governed by factors such as film thickness, cross-link density, and free liquid content, often resulting in nonlinear and time-dependent viscoelastic phenomena. The integration of nanoscale metallic coatings introduces an additional layer of complexity: the rigid-soft coupling further alters both mechanical and optical properties, giving rise to hybrid contact behaviors that go beyond the assumptions of classical contact theories developed for either purely stiff or bulk soft materials. Moreover, the interfacial deformations are typically confined to micro- or even nanometer scales, where stress localization, modulus gradients, and geometric confinement jointly govern the contact behaviors and its evolution. This multiscale, multiphysics coupling presents significant challenges for experimental characterization. Most conventional techniques isolate mechanical and optical measurements, rely on model-specific assumptions, and involve multi-step or complicated procedures. Thus, these techniques present certain challenges in precisely characterizing the real micro-scale structural properties and deformation behavior under actual loading or environmental conditions. Despite growing interest in these bilayer systems, a clear, experimentally accessible framework for quantitatively probing their contact behaviors and underlying properties remains lacking. Addressing this gap is essential not only for advancing the fundamental understanding of thin film mechanics, but also for enabling the rational design of high performance, multifunctional soft interfaces for next generation technologies. To address this gap, this dissertation applies spectroscopy and microscopy techniques: confocal Raman spectroscopy and dual-wavelength reflection interference contrast microscopy (DW-RICM), to systematically investigate the interfacial behaviors of polymeric and metal-polymer bilayer thin films under contact and solvent stimuli. The resulting insights aim to inform the rational design of multifunctional soft interfaces with enhanced performance and broader applicability in next generation surface and device technologies. The dissertation first highlights a parameter independent framework for characterizing soft contact deformation using in-situ confocal Raman spectroscopy. To achieve this, a calibration platform was constructed using a five-glass sphere probe assembly, with a glass slide placed on top to establish a reliable measurement protocol. This setup was then adapted for soft contact analysis by replacing the upper glass slide with a PDMS coated glass slide (P10), enabling localized deformation under controlled spherical contact. Raman mapping was performed in three spatial dimensions by tracking the intensity distribution of the 2905 cm⁻¹ Raman peak, which serves as the -CH3 stretching of PDMS. The resulting contour maps enabled imaging of the deformation region across multiple planes, allowing the extraction of physical parameters such as contact radius and indentation depth. The obtained results were broadly consistent with Hertz predictions, while revealing local deviations indicative of non-conformal contact behavior. The proposed technique and framework offers a unique method to observe the formation and change of contact deformation, leading to a deeper insight into the soft contact system. Building upon this molecular insight, a methodology was developed to simultaneously extract the optical and mechanical properties of polymer and metal-polymer bilayer thin films using dual-wavelength reflection interference contrast microscopy (DW-RICM). Nanoscale gold (Au) and silver (Ag) layers were deposited on PDMS substrates with varying elasticity and coating thickness. By analyzing interference patterns obtained at two wavelengths (488 nm and 561 nm) during contact with a glass probe of known geometry, the effective refractive indices and elastic modulus of the bilayer system were quantitatively determined. The refractive index was found to decrease with increasing metal deposition time, decreasing PDMS elasticity and increasing coating thickness, consistent with UV/Vis spectroscopy measurement. Elastic moduli were derived using Hertz theory based on the measured contact radii. This integrated optical-mechanical approach simplifies current multi-step characterization procedures, offering insights into fundamental properties of the metal-polymer bilayers. To further investigate the metal thin layer coating on the contact behavior of metal-polymer bilayers, a black-ink-coated probe was incorporated with DW-RICM to suppress unwanted reflections at the probe-air interface. This modified configuration enabled visualization of deformation features including contact deformation region and contact ridge formation with nanoscale resolution. Experimental results show that increasing the metal deposition time (i.e., metal coating thickness) leads to a reduction in both contact radius and contact ridge height. Moreover, long-term contact analysis of the gold coated bilayers showed a steady increase in ridge height over time, unlike the gradual decrease observed in the bare counterparts until reaching a steady state, suggesting altered interfacial viscoelastic behavior driven by the presence of the metal coating layer. Building on insights into the role of free liquids in thin film contact behavior, the final phase of this dissertation investigates out-of-equilibrium interfacial mechanic of solvent-induced surface instabilities on solid supported soft substrates using DW-RICM. The morphological evolution of PDMS and silicone gel substrates were monitored over prolonged hexane extraction and drying. A consistent transition was observed from shallow circular depressions to highly ordered triradial (Y-shaped) surface patterns driven by internal stress accumulation and elastic modulus gradients between the surface and bulk. Comparative analysis across formulations with varying crosslinking densities revealed that while the pattern formation mechanism is broadly conserved, it remains sensitive to material properties such as free liquid content and elasticity. These findings shed light on the mechanisms of solvent-mediated patterning and underscore solvent processing as a promising strategy for engineering programmable surface architectures in soft materials. Overall, this dissertation constructs a framework for soft interface characterization, integrating mechanical deformation under contact and surface morphological transformation under solvent extraction. The findings highlight how vibrational spectroscopy and optical interferometry can be used in tandem to probe complex contact phenomena, offering new tools and insights for the design of adaptive, compliant surfaces in applications such as soft robotics, flexible electronics, and interfacial patterning.