Localized Thermomechanical Measurements of Polymers and Blends with AFM-IR

Understanding the thermomechanical behavior of heterogeneous polymer systems is crucial for material design. Herein, we introduce a novel technique that couples chemistry-selective infrared (IR) heating with atomic force microscopy (AFM) nanomechanical measurements. We demonstrate that surface heating of the sample on the AFM-IR can be varied with the IR repetition rate, evidenced by melting poly(ethylene glycol) (PEG) films over a range of molecular weight-dependent melting points. Chemical-selective heating was demonstrated, where heating is dependent on the characteristic IR absorption bands of the material. Coupling of IR laser heating with nanomechanical measurements enables the qualitative detection of its glass transition temperature in thickness-confined semi-crystalline poly(lactic acid) (PLA) films, where an ultra-thin PLA film demonstrated a decrease in modulus to half its initial value with a significantly lower IR repetition rate relative to the IR repetition rate required to induce the same change in a thick PLA film. We further apply this technique to a polymer blend of PLA and uncrosslinked nitrile butadiene rubber to demonstrate phase-specific thermal characterization. This technique minimizes thermal drift, allows for rapid heating with concurrent AFM measurements and circumvents bulk material changes, paving a possible alternative avenue for the probing of thermomechanical properties of heterogenous films.