Dry processes for surface modification of a biopolymer: Chitosan

Standard dry surface modification reactions have been applied to partially deacetylated chitosan without affecting its bulk properties. Chitin, extracted from shells of Penaeus vannamei, yielded chitosan with a degree of acetylation of 70% and molecular weight of 250 000 D. The copolymer consists of (beta-(1-4)-2-2-acetamido-D-glucose) units linked to (beta-(1-4)-2-amino-D-glucose) units. Since the main interest of this work was to study the surface properties of films on substrates, a method to cast this material onto Al-coated silicon wafers had to be developed. X ray photoelectron spectroscopy (XPS) has been used to determine the surface composition of the unmodified films and to follow modification changes. The films were treated in either an oxygen plasma environment or under UV/ozone irradiation. Water advancing contact angle measurements and infrared spectroscopy (FTIR) were used to complement XPS measurements. The films appeared to orient on the silicon wafer surface in the type II chitin structure. The rates of oxidation are faster for the plasma process but they result in similar changes to those induced by UV/ozone treatment. Atomic force microscopy (AFM) clearly shows the advantage of the milder modification reaction without much change in surface morphology. The oxidation processes, as detected by XPS, proceed without much alteration of the amine nitrogen atoms but carbonyl containing moieties are formed as a function of treatment time. Specific reactions with a fluorosilane to measure the activity of hydroxyl groups indicate that at short treatment times, these groups are essentially inactive. The resulting surfaces can also serve as a potential way to induce silica-like domains that can function as diffusion barriers. Irradiation of chitosan solution shows that UV/ozone induces depolymerization. In both cases, i.e. plasma and UV/ozone reactions, the main active component to surface modification appears to be UV irradiation with a wavelength below 360 nm.