Four different cellulose model surfaces, and one silica surface, have been studied by means of atomic force microscopy (AFM). The normal interactions have been found to consist of a longer range double layer force with a short range steric interaction, the nature of which is extensively discussed. Both the surface charge and range of the steric force depend on the type of cellulose substrate used, as does the magnitude of the adhesion. Studies of friction reveal that surface roughness is the determining factor for the friction coefficient, with which it increases monotonically. The absolute value, however, is determined by the surface chemistry. All studied cellulose surfaces show similar behavior in response to xyloglucan addition.

Cellulosic model surfaces functionalized with chitosan, a naturally occurring cationic biomacromolecule, by in-situ adsorption have been studied with an atomic force microscope (AFM) in colloidal probe configuration.  The interaction forces on approach and separation, as well as the nanotribological properties, were shown to be highly pH-dependent and a significant difference in the behaviour was seen before and after chitosan adsorption.  In general, all forces on approach showed a highly repulsive interaction at shorter distances due to deformation of the probe.  At high pH, before chitosan adsorption, a long-range electrostatic repulsion was observed, consistent with DLVO theory.  However at low pH no electrostatic contribution was found before adsorption probably due to charge neutralization of carboxyl groups.  After chitosan adsorption, repulsive forces acting over a much longer distance than predicted by DLVO theory were present at low pH.  This effect was ascribed to chain extension of the chitosan species of which the magnitude and the range of the force increased dramatically with higher charge at low pH.  In all cases, a typical saw-tooth patterned adhesion was present with pull-off events occurring at different separations.  The frequency of these events after chitosan adsorption was greatly increased at longer distances.  Additionally, the adsorbed chitosan markedly reduced the friction where the largest effect was a 7-fold decrease of the friction coefficient observed at low pH.

This study focuses on the manufacture and characterisation of model surfaces consisting of end grafted xyloglucan (XG), a naturally occurring polysaccharide, onto a gold substrate.  The now well-established XET-technology was utilised for enzymatic incorporation of a thiol moiety at one end of the xyloglucan backbone.  This functionalised macromolecule was subsequently top-down grafted to gold, forming a thiol-bonded xyloglucan brush-like layer. The grafting was monitored in-situ with QCM-D and a significant difference in the adsorbed/grafted amount between unmodified xyloglucan and the thiol-functionalised polymer was observed.  The grafted surface was demonstrated to be accessible to enzyme digestion using the plant endo-xyloglucanase TmNXG1.  The nanotribological properties towards cellulose of the untreated crystal, brush modified surface and enzyme exposed surfaces were compared with a view to understanding the role of xyloglucan in friction reduction.  Friction coefficients obtained by the AFM colloidal probe technique using a cellulose functionalised probe on the xyloglucan brush showed an increase of a factor of two after the enzyme digestion and this result is interpreted in terms of surface roughness.  Finally, the brush is shown to exhibit binding to cellulose despite its highly oriented nature.

This study reports on a direct approach of quantitatively probing the nanotribological response of chemically end-grafted polyions.  A combination of quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM), in the now well established colloidal probe mode was utilized to investigate the stimuli-induced lubrication behavior between poly-2-(dimethylamino)ethyl methacrylate (PDMAEMA) grafts on gold.  Force and friction measurements showed reversible transitions of up to an order of magnitude difference induced by varying the solvent conditions.  The greatly enhanced lubrication observed at low pH was attributed to the formation of a repulsive, highly charged, hydrated cushion.  At high pH the friction was significantly increased.  The system turned attractive above the lower critical solution temperature (LCST) with a small friction reduction interpreted as nanoscopic flattening at the interfacial boundary.

The efficiency of poly(ε-caprolactone) (PCL) as a matrix polymer for cellulose nano-composites has been investigated at the macromolecular contact level using atomic force microscopy (AFM) in colloidal probe configuration.  Model cellulose micro-spheres grafted with PCL were prepared via ring-opening polymerization (ROP).  Force measurements between the functionalized particles revealed the adhesion to be highly dependent on contact time due to a diffusion controlled mechanism.  Moreover, an increase of the temperature to 60 °C (close to T_m for the PCL-graft) greatly enhanced the adhesion at the polymer-polymer interface demonstrating the importance of entanglements in the annealing of composite materials.

A number of atomic force microscopy cantilevers have been exhaustively calibrated by a number of techniques to obtain both normal and frictional force constants to evaluate the relative accuracy of the different methods. These were of either direct or indirect character-the latter relies on cantilever resonant frequencies. The so-called Sader [Rev. Sci. Instrum. 70, 3967 (1999)] and Cleveland [Rev. Sci. Instrum. 64, 403 (1993)] techniques are compared for the normal force constant calibration and while agreement was good, a systematic difference was observed. For the torsional force constants, all the techniques displayed a certain scatter but the agreement was highly encouraging. By far the simplest technique is that of Sader, and it is suggested in view of this validation that this method should be generally adopted. The issue of the photodetector calibration is also addressed since this is necessary to obtain the cantilever twist from which the torsional force is calculated. Here a technique of obtaining the torsional photodetector sensitivity by combining the direct and indirect methods is proposed. Direct calibration measurements were conducted in liquid as well as air, and a conversion factor was obtained showing that quantitative friction measurements in liquid are equally feasible provided the correct calibration is performed.