Molecular Adhesion and Friction at Elastomer/Polymer Interfaces
We have studied the contact interface between elastomeric poly(dimethyl siloxane) (PDMS) lenses with various solid surfaces during adhesion and friction using IR-visible sum frequency generation spectroscopy (SFG). SFG in total internal reflection (TIR) geometry can be used to determine molecular structure at the polymer/solid and polymer/polymer contact interfaces. It is a nonlinear optical technique, which detects the orientation and density of molecules at interfaces. In this study, we have designed a novel approach to couple SFG with adhesion and friction experiments. The solid surfaces were chosen to be octadecytrichlorosilane monolayer (OTS), poly(vinyl n-octadecyl carbamate-co-vinyl acetate) (PVNODC), polystyrene (PS), poly(n-butyl methacrylate) (PnBMA), and poly(n-propyl methacrylate) (PnPMA). In the first part of the research, we have concentrated on the importance of characterizing the static contact interface in relation to adhesion. Our results for the OTS in contact with oxygen plasma treated PDMS show surprising surface restructuring, which results in adhesion hysteresis. The short PDMS chains generated during plasma treatment are locally confined and are as strongly ordered as OTS. SFG spectra from other surfaces (sapphire substrates and fluorinated monolayers (FC)) indicates that short PDMS chains require not only confinement but also an ordered template provided by the methyl groups of OTS. In the second part, we have studied the sliding contact interfaces of various polymers with PDMS. The friction forces between PDMS lenses and glassy PS are about four times higher than PDMS sliding on crystalline well-packed PVNODC surfaces. This cannot be explained by the difference in adhesion energy or hysteresis. The in-situ SFG measurements indicate local interdigitation during contact, which is evident from the change in orientation of PS phenyl groups upon mechanical contact and during sliding compared to that at the PS surface. Such a local penetration is unexpected at room temperature (T R) that is much below T g. For comparison, we have also studied PnBMA and PnPMA having T gbelow and above T R, respectively. Preliminary studies at the contact and sliding interfaces with PDMS exhibited similar interpenetration based on adhesion, friction and SFG results regardless of the bulk T g. In addition, our results indicate that the adhesion energy and hysteresis of surfaces are not sufficient to predict their friction properties, which makes the characterization of the molecular structure at the static and dynamic contact essential. Finally, we have reported a fabrication process of constructing polymer surfaces with multiwalled carbon nanotube (MWNT) hairs. The force measurements with scanning force microscope (SPM) indicated strong nanometer level adhesion forces, 200 times higher than those observed for Gecko foot-hairs. These forces are a combination of van der Waals forces and energy dissipation during the elongation of the carbon nanotubes which comes from their material properties. In the velocity region of 0.50Hz up to 14Hz there is no detectable change in the energy dissipation and contact area. In conclusion, we have demonstrated that structures found in Gecko’s feet can be fabricated on polymer surfaces by using multiwalled carbon nanotubes.
School:The University of Akron
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:adhesion friction and of elastomer polymer interfaces sum frequency generation spectroscopy sfg molecular gecko foot hair mimicking synthetic adhesives
Date of Publication:01/01/2006