Characterizing polymer brushes via surface wrinkling

We apply surface wrinkling to measure the mechanical properties of poly (2-hydroxyethyl methacrylate) polymer brush layers tethered to the surface of a flexible poly(dimethylsiloxane) (PDMS) substrate. A facile modification scheme based on hydrochloric acid treatment was employed to introduce hydroxyl groups to the surface of PDMS, which served to covalently attach initiator groups for subsequent polymerization. Upon mechanical compression of the brush layer on PDMS, a wrinkling instability occurs whose wavelength yields a Young's modulus for the brush layer that is comparable to the corresponding polymer in the bulk. Moreover, we show that the wrinkling wavelength can provide an accurate measure of the brush thickness, which is often difficult to assess on transparent, flexible substrates. When using thermal strain to generate wrinkled surfaces, the patterns are stable at room temperature but can be erased by solvent treatment, which relaxes the applied strain and thus imparts reversibility to the wrinkled surfaces.