C.Y.J Lim, Z. Bao, L.F. Michalek
Stanford University,
United States
Keywords: banomechanics, thermomechanical, infrared
Summary:
Understanding the thermomechanical behavior of heterogeneous polymer systems is crucial for designing and tailoring their performances. However, most existing techniques provide only bulk-averaged measurements, masking important local variations that govern material response. The ability to characterize these systems in their unaltered state with high spatial resolution remains a key challenge. Advancing characterization methods that resolve phase-specific properties, interfacial mechanics, and nanoscale heterogeneities will enable a more comprehensive understanding of structure–function relationships and guide the rational design of next-generation materials. Atomic force microscopy (AFM) has become an essential tool to enable spatially resolved mechanical characterization of soft materials at the nanoscales. Herein, we introduce a novel technique that couple AFM nanomechanical measurements with chemistry-selective infrared (IR) heating. Leveraging the bond-specific nature of IR absorption, this approach enables targeted heating of individual components within heterogeneous materials. Using poly(ethylene glycol) (PEG) 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 differential scanning calorimetry (DSC) measurements. Furthermore, we confirm the chemistry-selective nature of heating by showing that the required heating power varies inversely with IR absorbance at different wavenumbers. At the onset of the glass transition temperature, the modulus of polymer decreases sharply owing to the increased segmental mobility of the polymer chains. Through coupling IR heating with nanomechanical measurements, such thermomechanical phase transitions could be measured. We demonstrated a decrease in glass transition temperature (Tg) of thickness-confined semi-crystalline poly(lactic acid) (PLA) through the observation that the thickness-confined film required a lower IR laser frequency to induce the significant decrease of its modulus. 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.