Quantifying Strain via Buckling Instabilities in Surface Modified Polymer Brushes
Cassandra M. Reese, Wei Guo, Brittany J. Thompson, Phillip K. Logan, Christopher M. Stafford, Derek L. Patton
We present a quantitative method of measuring the compressive strain of wrinkled polymer films and coatings with knowledge of the skin thickness, wrinkle wavelength, and wrinkle amplitude. The derived analytical expression is validated with a well-studied model system, e.g., stiff, thin film (PS) bonded to a thick, compliant substrate (PDMS), comparing the applied strain and calculated strain for a series of PS film thicknesses. After validation, we use the analytical expression to quantify the applied swelling strain of previously reported wrinkled poly(styrene-alt-maleic anhydride) brush surfaces after surface modification, where the swelling strain was determined to increase with anhydride conversion in the skin layer of the modified brush. Finally, the applied strain is used to rationalize the observed persistence length of aligned wrinkles created during AFM lithography and subsequent solvent exposure. The ability to accurately determine the applied strain in wrinkled systems adds another tool to the metrology of instabilities in polymer thin films and coatings.