Polysaccharide nano crystal reinforced nanocomposites

Abstract

There are numerous examples of animals or plants that synthesize extracellular high-performance skeletal biocomposites consisting of a matrix reinforced by nano sized crystalline domains. Cellulose and chitin are classical examples of these reinforcing elements, which occur as whisker-like microfibrils that are biosynthesized and deposited in a continuous fashion. In many cases, this mode of biogenesis leads to crystalline microfibrils that are almost defect-free, and whose axial physical properties therefore approach those of perfect crystals. During the last decade we have attempted to mimic biocomposites by blending cellulose or chitin whiskers from different sources with polymer matrices. Aqueous suspensions of such nano crystals can be prepared by acid hydrolysis of the substrate. The object of this treatment is to dissolve away regions of low lateral order so that the water-insoluble, highly crystalline residue may be converted into a stable suspensoid by subsequent vigorous mechanical shearing action. The resulting nano crystals occur as rod-like particles or whiskers, whose dimensions depend on the nature of the substrate. They are typically a few hundred nm long and between 5 and 20 nm in diameter. Starch can also be used as a source for the production of nano crystals. The constitutive nano crystals appear as platelet-like nano particles with a length ranging between 20 and 40 nm, a width ranging between 15 and 30 nm, and a thickness ranging between 5 and 7 nm. Since the first announcement of using cellulose whiskers as a reinforcing phase, they have been used extensively as model fillers in several kinds of polymeric matrices, including synthetic and natural ones. Casting mixtures of polysaccharide nano crystals and lattices led to the production of nano composite materials with drastically enhanced mechanical properties, especially at T > Tg of the matrix, by virtue of the formation of a whiskers network, even when the whisker volume fraction was only a few percent. The formation of this rigid network, resulting from strong interactions between whiskers, was assumed to be governed by a percolation mechanism. This hydrogen-bonded network induced a thermal stabilization of the composite up to 500 K, the temperature at which polysaccharides start to decompose. Any factors that perturb the formation of this percolating network directly affect the reinforcing effect of polysaccharide nano crystals. In addition to some practical applications, the study of these nano composite materials can help researchers understand such physical properties as the geometric and mechanical percolation effect.Key words: nano composites, polysaccharide, polymer, cellulose, nano crystal.