PRIMA: Photothermal Resonant Infrared Mechanical Analysis

Understanding the thermomechanical behavior of heterogeneous polymer systems is crucial for designing and tailoring their performances. Most existing techniques provide only bulk-averaged measurements. The ability to characterize these systems in their unaltered state with high resolution remains a key challenge. Herein, we introduce a novel technique that couple AFM nanomechanical measurements with chemistry-selective infrared heating. Using poly(ethylene glycol) films with molecular weights ranging from 1.5k to 100k Da, we demonstrate precise temperature control by varying IR laser repetition rate and power. We observe molecular weight-dependent melting transitions that correlate with bulk measurements. We furthermore demonstrated a decrease in glass transition temperature (T_g) of thickness-confined semi-crystalline poly(lactic acid). The high spatial resolution of AFM combined with rapid, localized IR heating enables non-destructive probing of thermomechanical properties in thin films and nanostructures without perturbing surrounding materials. Since heating is chemistry-selective, this method allows phase-specific thermal characterization, effectively circumventing the bulk material changes associated with conventional heating methods. This technique offers significant promise for characterizing polymer blends, nanoconfined structures, interfacial regions, and other complex heterogeneous materials in their pristine states.