Developing a Nano-Mechanical Probe for Polymer Thin Films using Photoactive Azo Dyes

Understanding the mechanical behavior of polymers in thin films and nanoconfined systems is crucial for applications from flexible electronics to nanocomposites. Interfaces in these systems impart gradients in local properties that can extend up to hundreds of nanometers. Existing local methods to measure mechanical properties often rely on contact-based techniques that probe only the free surface layer, limiting access to buried layers. There is a need for new experimental approaches that can access the mechanical response of polymers at the nanoscale and be adapted as selective probes to map the mechanical properties at specific depths within multilayer structures. Towards this end, here we present a non-contact optical approach using Disperse Red-1 (DR1) dyes that undergo light-driven isomerization to produce internal stress within poly(methyl methacrylate) (PMMA) films. Laser excitation of the DR1 dye is combined with in-situ ellipsometry to monitor real-time film thickness and refractive index changes during repeated on/off laser cycles for various blends of DR1-labeled PMMA mixed with unlabeled PMMA. As the DR1 relaxation is comparatively fast, the polymer film’s relaxation from this reproducible and nondestructive photoexpansion reflects the polymer's viscoelastic behavior. A spring-dashpot model is used to quantify the strain and resulting time-dependent polymer compliance, demonstrating this approach’s potential as a non-contact method for probing nanoscale mechanics.