Determining Preparation-Induced Differences in Local Forces Acting along Polymer Chains in Spin-Coated Films.

We employed photoluminescence (PL) from mechanochromophores covalently linked into the backbone of a glassy polymer as optical force sensors ("optical springs") to resolve forces acting along polymer chains in spin-coated thin films. From the shift of the peak position of the PL spectrum, we deduced changes in the main force. Changes in the shape of the spectra were interpreted as alterations in populations of polymer chains experiencing lower and higher forces than the main force. Depending on the conditions of spin coating, expressed through a preparation parameter pev, we observed distinct changes in the corresponding PL spectra of the resulting polymer films, reflecting characteristic variations in the distribution of forces acting along polymer chains as a function of pev. Our results allowed to link the forces acting along individual polymer chains in spin-coated polymer films with macroscopically detected residual stresses, with the latter reflecting the combined and cooperative response of a large ensemble of polymer chains. Interestingly, residual stresses and the forces acting along individual polymer chains did not exhibit the same dependence on pev.