Interfacial Properties of Biomacromolecular Model Systems: Surface Forces and Nanotribology

Abstract: The nanotribology, adhesion and related interfacial properties of biomacromolecular systems have been studied.  The aim was to elucidate the role of physisorbed and chemically grafted bio-polymers implicated as mediators for cellulose based processing and material design.  To that end, model surfaces were prepared and characterized by the versatile techniques quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM) in colloidal probe mode.  A prerequisite for the latter in conducting quantifiable force and friction measurements lies in obtaining accurate values for the cantilever spring constants.  An investigation of various recently proposed calibration methods shows that they return essentially the same result, but that the route of Sader et al. is the least strenuous and indeed most reliable.  A novel approach for determining the required frictional detector sensitivity is proposed.Cellulose model surfaces with varying morphology were evaluated.  For a given surface roughness the adsorption of xyloglucan, a nonionic polysaccharide, leads to a consistent decrease in the friction coefficients for all cellulose substrates, demonstrating that the effects on friction arising from either roughness or chemistry can be decoupled.  Moreover, xyloglucan grafts on gold show biological enzymatic accessibility, and their native affinity towards cellulose is retained, even in an extended brush conformation.Chitosan, a cationic polysaccharide is also found to mediate lubrication and adhesion of cellulose in a solvent dependent manner, though the mechanisms are unlike those of xyloglucan.  At low pH the most efficient lubrication is achieved due to highly charged chitosan species extending out in the aqueous media, forming a highly hydrated cushion.  Conversely, at high pH the deswelling of the layer results in higher friction.  The same trend is observed between synthetic dual-responsive polyionic grafts on gold where the polymer charge is decisive for the friction properties. Above the lower critical solution temperature where the grafts are fully collapsed and display an attractive force, the friction is slightly reduced due to a nanoscopic flattening at the interface.Finally, polymer grafts as matrix compatibilizers for bio-degradable cellulose based nano-composite applications were evaluated.  Adhesion is greatly promoted by chain entanglements.  The effect displays a dynamic diffusion based dependence of which the rate is significantly enhanced at a higher temperature close to the melting point of the polymeric layer.  No such adhesion benefit was obtained between ungrafted cellulose and the matrix material.