Chitosan adsorption on hydroxyapatite and its role in preventing acid erosion.

Polymer adsorption onto an artificial saliva (AS) layer is investigated using quartz-crystal microbalance with dissipation (QCM-D) and chitosan as the model polymer. QCM-D is utilized in an innovative manner to monitor in situ adsorption of chitosan (CH) onto a hydroxyapatite (HA) coated crystal and to examine the ability of the adsorbed layer to "protect" the HA upon sequential exposure to acidic solutions. After deposition of a thin AS layer (16 nm), the total thickness on the HA substrate increases to 37 nm upon exposure to CH at pH 5.5 for 10 min. Correspondingly, the surface charge changes from negative (i.e., AS) to positive, consistent with the adsorption the polycationic CH onto or into the AS layer. Upon exposure to an oxidizing agent, the chitosan cross-links and collapses as noted by a decrease in thickness to 10 nm and an increase in the shear modulus by an order of magnitude. Atomic force microscopy (AFM) is used to determine the surface morphology and RMS roughness of the coated and HA surfaces after citric acid challenges. Both physisorbed and cross-linked chitosan are demonstrated to limit and prevent the erosion of HA, respectively.

Tunable wetting of nanoparticle-decorated polymer films.

In this paper, amine-modified silica nanoparticles (NPs) with diameters (d) from 15 to 230 nm are covalently linked to poly(styrene-random-acrylic acid) (P(S-ran-AA)) films, and wettability is studied as a function of diameter and NP surface coverage. During attachment, films swell and exhibit long and short scale roughness, consisting of a ridged, honeycomb structure, approximately 1 mum wide and 45-50 nm deep, which encircles nanoscale features 10-15 nm high and approximately 50 nm apart. A maximum NP coverage of approximately 70% was achieved for d less than or nearly equal to the nanoscale roughness induced by surface swelling. For d several times greater than this nanoscale roughness, the maximum coverage was limited by interparticle repulsion and reached only approximately 30%. For NPs with diameters of 15-106 nm, the water contact angle increased from 75 degrees to 120 degrees as NP coverage increased from 0 to 70%. At low and high NP coverage, the Wenzel and Cassie models, respectively, accurately describe the data. However, at intermediate NP coverage, neither model is satisfactory. An increase in surface roughness alone cannot account for this discrepancy. Atomic force microscopy (AFM) studies show that the NPs partially embed into the swollen P(S-ran-AA) surface, suggesting that the amine-coated NPs are wet by the copolymer, exposing low surface energy styrene. These studies demonstrate that control over surface properties of coatings, such as wetting, can be achieved by selecting NP sizes that complement film roughness.