In Situ UV Torsional Force Spectroscopy for Real‐Time Mapping of Cross‐linking Kinetics and Mechanical Properties in Polymeric Films

An advanced in situ ultraviolet (UV) torsional force spectroscopy (TFS) technique is introduced for real-time mapping of in-plane nanomechanical properties in UV-cross-linked poly(1,4-butadiene) (PB) films on the nanoscale. This approach combines UV illumination and torsional force spectroscopy, offering high-resolution insights into in-plane shear stress, storage shear modulus, and dissipated energy throughout thiol-ene cross-linking reactions. Complementary swelling experiments provide a direct correlation between increasing UV dose, cross-linking density, and mechanical stiffness. Ex situ measurements confirm the reliability of this in situ method with only minor deviations attributed to UV scattering effects and experimental variations. The reduced stiffness and increased heterogeneity observed by TFS in PB films containing phase-separated polystyrene (PS) domains are associated with changes in the spatial distribution of the cross-linking agent and the resulting mechanical response of the PB matrix, consistent with complementary X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) analyses. Overall, this work establishes in situ UV torsional force spectroscopy as a predictive and quantitative method to optimize UV curing conditions and additive concentrations for tailoring the mechanical properties and performance of polymeric materials.