Chain Connectivity And Adhesion At Polymer Solid Interfaces

Author : Betul Buehler
Publisher :
ISBN 13 :
Total Pages : 234 pages
Book Rating : 4.:/5 (911 download)

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Download or read book Molecular Adhesion and Friction at Elastomer/polymer Interfaces written by Betul Buehler and published by . This book was released on 2006 with total page 234 pages. Available in PDF, EPUB and Kindle. Book excerpt: We have studied the contact interface between elastomeric poly(dimethyl siloxane) (PDMS) lenses with various solid surfaces during adhesion and friction using IRvisible sum frequency generation spectroscopy (SFG). SFG in total internal reflection 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 noctadecyl 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 to understand adhesion. Our results for the contact between two macroscopic rough surfaces, OTS and oxygen plasma treated PDMS, show surprising surface restructuring, which results in adhesion hysteresis. The short PDMS chains generated during plasma treatment are locally confined between two flattened asperities and are as strongly ordered as OTS. SFG spectra from other surfaces (sapphire substrates and fluorinated monolayers) indicate 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 a factor of four 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 interpenetration 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[subscript R]) that is much below the T[subscript g] of PS. For comparison, we have also studied PnBMA and PnPMA having T[subscript g] below and above T[subscript 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[subscript g]. 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 hairs. The force measurements with scanning force microscope 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.