Adhesion and friction in polymer films on solid substrates: conformal sites analysis and corresponding surface measurements. Issue 19 (19th April 2017)

• Record Type: Journal Article
• Authors: An, Rong; Huang, Liangliang; Mineart, Kenneth P.; Dong, Yihui; Spontak, Richard J.; Gubbins, Keith E.
• Is Part Of: Soft matter. Volume 13:Issue 19(2017)
• Page Start: 3492
• Page End: 3505
• Publication Date: 2017-04-19
• Subjects: Soft condensed matter -- Periodicals; 530.413
• DOI: 10.1039/c7sm00261k
• Languages: English
• ISSNs: 1744-683X

Abstract

Abstract : Conformal sites theory and atomic force microscopy analysis were presented, relating molecular-level interactions to polymer film adhesion and friction. Abstract : In this work, we present a statistical mechanical analysis to elucidate the molecular-level factors responsible for the static and dynamic properties of polymer films. This analysis, which we term conformal sites theory, establishes that three dimensionless parameters play important roles in determining differences from bulk behavior for thin polymer films near to surfaces: a microscopic wetting parameter, α w x, defined as the ratio of polymer–substrate interaction to polymer–polymer interaction; a dimensionless film thickness, H *; and dimensionless temperature, T *. The parameter α w x introduced here provides a more fundamental measure of wetting than previous metrics, since it is defined in terms of intermolecular forces and the atomic structure of the substrate, and so is valid at the nanoscale for gas, liquid or solid films. To test this theoretical analysis, we also report atomic force microscopy measurements of the friction coefficient ( μ ), adhesion force ( F A ) and glass transition temperature ( T g ) for thin films of two polymers, poly(methyl methacrylate) (PMMA) and polystyrene (PS), on two planar substrates, graphite and silica. Both the friction coefficient and the glass transition temperature are found to increase as the film thickness decreases, and this increase is more pronounced for the graphite than for the silica surface. The adhesion force is also greater for the graphite surface. The larger effects encountered for the graphite surface are attributed to the fact that the microscopic wetting parameter, α w x, is larger for graphite than for silica, indicating stronger attraction of polymer chains to the graphite surface.