The performance, reliability, and failure of soft polymeric materials are governed by localized mechanical phenomena that are largely inaccessible with conventional experimental techniques, creating a significant gap between theoretical models and direct validation. This dissertation establishes and applies a quantitative, in-situ mechanochemical mapping platform to bridge this gap. The platform is built upon the integration of nitro-spiropyran (SPN) molecular force probes into a polydimethylsiloxane (PDMS) network, which undergo a force-induced ring-opening reaction to form a fluorescent isomer. By coupling precision micromechanical testing with real-time confocal fluorescence microscopy, this platform enables high-resolution, three-dimensional mapping of internal stress fields as they evolve during deformation.

 

The utility of this mechanochemical approach in failure analysis was demonstrated by deconstructing the micromechanics of short fiber-reinforced composites. In-situ single fiber pull-out tests enabled the quantitative mapping of stress concentrations at fiber ends, revealing the critical role of geometry in managing stress transfer and providing real-time visualization of failure initiation and interfacial debonding. This analysis precisely quantified the stress distributions for various fiber end geometries (flat, cone, round, and sharp) and demonstrated that round ends promote a more gradual and effective stress transfer into the matrix. This quantitative analysis employed a calibration methodology established with a model continuous fiber composite, where a single fiber was subjected to uniaxial tensile loading. That initial system visualized the stress field decaying from the fiber interface and established the robust calibration methodology correlating fluorescence intensity with stresses derived from finite element analysis.

 

The quantitative stress mapping technique was then applied to the flat punch indentation of a confined elastomer. This approach first confirmed that geometric confinement dictates the macroscopic mechanical response, where the apparent stiffness of the system increases substantially with the confinement ratio (probe radius to film thickness). At the molecular level, the in-situ visualization provided direct evidence of the theoretically predicted stress singularity at the contact perimeter under confinement, a critical feature that promotes interfacial failure. The technique further demonstrated its analytical capabilities by decoupling stress components at high-friction and lubricated interfaces and by mapping the residual zones of deformation that act as precursors to material failure.

 

The research then shifted from analyzing externally applied stress to investigating intrinsic material dynamics. Macroscopic adhesion recovery on a fouled surface was used as a novel probe for microscopic chain migration, providing experimental insight into polymer reptation through a fixed network. This approach resolved a kinetic paradox, demonstrating that the rate of recovery is governed not by the network's crosslink density but by the molecular weight of the mobile sol fraction. This insight was achieved by systematically varying the PDMS network architecture to control the concentration and molecular weight of uncrosslinked, mobile polymer, and then quantifying the kinetics of adhesion recovery using Johnson-Kendall-Roberts (JKR) contact mechanics after fouling the surface with graphite particles.

 

Collectively, the central contribution of my dissertation is the establishment of a versatile mechanochemical framework that quantitatively links molecular-level deformation to macroscopic mechanical behavior and failure. This work provides a validated basis for the rational design of durable soft materials, from composites with optimized stress transfer to interfaces with programmable, self-recovering functionality