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The rheological properties of the sulfated polysaccharide of the red microalga Porphyridium sp., a heteropolymer with a molecular weight of 3-5 x 10(6) Da, indicated that this material might be an excellent candidate for lubrication applications: the viscosity of the polysaccharide is stable over a range of temperatures, pH values, and salinities. In this study, various rheological and lubricant properties of the polysaccharide were evaluated in comparison with those of a widely used biolubricant, hyaluronic acid. The viscosity of the Porphyridium sp. polysaccharide remained essentially unchanged in a temperature range of 25-70 degrees C. In tribology tests on a ball-on-flat ceramic pair, the values for the friction coefficient and wear rate for the pair lubricated with polysaccharide were remarkably lower than those for hyaluronic acid, especially at high loads. In a test on a steel ring/ultrahigh-molecular-weight polyethylene (UHMWPE) block pair, the wear tracks on the surface of the UHMWPE were more pronounced for hyaluronic acid than for the polysaccharide. Atomic force microscopy showed that the polysaccharide was effectively adsorbed onto mica surfaces, forming ultrathin coating layers in the nanometer range. As is required for biolubricant applications, the polysaccharide was not degraded by hyaluronidase. The stability of the Porphyridium sp. polysaccharide to heat and to hyaluronidase combined with its ability to reduce friction and wear indicate its potential as an advantageous biolubricant.

Experimental results concerning end-linked poly(dimethylsiloxane) pregel clusters quenched at different distances from the gelation threshold are reported. Static and quasielastic light scattering experiments and intrinsic viscosity measurements allow us to verify that static properties of this chemical system are properly described by the percolation model. The critical behavior of the rheological properties is investigated with particular attention to the temperature and concentration dependence of the exponent values. It is shown that in this chemical system, local dynamic properties are independent of the distance to the threshold, allowing “real” exponents to be evaluated. One finds that the exponents governing the divergence of the viscosity, s, the emergence of the elastic modulus, t, and the frequency dependence of the elastic modulus, u, are equal to s = 1.1 ± 0.06, t = 1.9 ± 0.15, and u = 0.69 ± 0.005.