Normal and Shear Froces Between Adsorbed and Gelled Layers of Chitosan,a Natually Occurring Cationic Polyelectrolyte

摘要

Using a mica surfaceforce balance,we havemeasured the interactions between mica surfaces bearing chitosan(a common,naturally occurring,cationic polysaccharide;the averagemoelcualr weight of our sample was 6 X 10~5,degree of deacetylation 85%) adsorbed from acetic acd solution.We also introduced a polyionic cross-linking agent in order to cause gelation of the adsrobed layer.Both normal and shear interactions (the lateral forces acting between the surfaces as they slide past each other under compresson) wre meansured in the two cases (freely adsorbed and cross-linked chitosan layers).Normal interactions between the adsorbed non-cross-linked chitosan layers wer esimilar to previous reports of interactons between the adsorbed non-cross-linked chitosna layerswere similar to revious reports of interactions betweensuch layers;their shear interactions revealed a very low effective friction coefficient mu_eff=ca.0.003 at low compressions,increasing ot ca.0.07 at pressures of some atmospheres.We attribute the low friction to the weak interpenetration between the layers arising from steric effects and counterion osmotic pressures,together with the presence of hydration sheaths about the charged polyelectrolyte segments,which are known to provide efficient local lubrication.The higher friction on strong compressions is attributed to bridging effects.For the case of the cross-linked layer,normal interactions revealed longer ranged and more repulsive forces,due probably to the network formation resultign in a higher effective modulus of the layers.frictional forces between the rubbing cross-linked layer were much lhigher htkan for the non-cross-linked chitosan,an effect we attribute to increased segmental friction arising from attractive interactions between the cross-linking points;theis is also consistent with our observation that for the cross-linked layers significant hysteresis was observedwhen measuring normal interactions on approach and separation of the layers,an effect that was absent for the case of the non-cross-linked approach and separaton of the layers,an effect that was absent for the case of the non-cross-linked adsorbed layers.refractive index measurements revealed an adsorbance of 1.2 +- 0.4 mg/m~2 of the polymer on each mica surface,consistent with expectations for cationic polymers adsorbed on solid surfaces.