Hydrophobicity of nanostructured films characterized by a quartz crystal microbalance

The hydrophobicity of two types of nanostructured polymer films were fabricated and characterized with a novel quartz crystal microbalance (QCM) technique to investigate their static and dynamic hydrophobic properties. The nanofibrous films of polymethylmethacrylate(PMMA), PMMA/Polydimethylsiloxane (PDMS) and Polyacrylonitril (PAN) were prepared with an electrospinning process and a PMMA film with nanoscale roughness was fabricated using nanoimprint lithography (NIL) technique. Significantly different static and dynamic hydrophobicities (wettability) were found among these films and the correlation between hydrophobicity and the mechanical impedance of QCM to these films were developed both experimentally and theoretically. It was shown that QCM is capable of quantitatively characterizing the hydrophobicity of these nanostructured polymer surfaces. For nanofibrous films, the double layers - A viscoelastic nanofiber film and a liquid layer result in a nonlinear combination of mechanical impedances of QCM. To simplify the analysis, an apparent viscosity was introduced in the analysis to take into account the interactions between liquid and polymer surfaces. For NIL PMMA film, the hydrophobicity was altered by coating nanoroughened surface with a Teflon layer. The reduction in the mechanical impedance of QCM clearly demonstrates the enhancement of hydrophobicity. The experimental results showed that the hydrophobic surface lead to a small mechanical impedance while the hydrophilic surface resulted in a large mechanical impedance of QCM.