According to De Gennes polymer solution theory, it was reported that the depletion layer has been developed around the surface of the particle while some mesoscopic particles are dispersed in polymer. Here the depletion layer means some region where the polymer chains are excluded from the surface of the particle and thus mainly solvent molecules exists within this layer''s thickness. It is clear that when the particle having this layer is moving faster around the polymer solution than other one without such layer, because this layer makes local viscosity near the moving particle drop a lot. However not much work has been done so far about this very interesting topic. Recently, there has been reported some noticeable papers about theoretical approaches for calculation of this thickness. First, there are two theoretical methods such as Vincent''s viscosity-depletion layer theory and Vincent''s pragmatic theory. Using the former equation, the depletion layer can be experimentally calculated from the diffusion coefficient of the particle experimentally obtained from dynamic light scattering. The later one shows us only how the depletion layer theoretically depends on the concentration. On this subject, Eom etc had been carried out the diffusion experiments of polystyrene latex probe particle at the system of poly(vinyl alcohol)/dimethyl sulfoxide(DMSO) which had some advantage for lowering by 10 times the scattered intensity from the polymer/DMSO matrix compared to polymer/water system. Even in this matrix system, the scattered intensity from the matrix could not eliminated perfectly. Thus at high polymer concentration, the contribution of the fast motion of polymer chain severely contaminated the time correlation function(TCF). It must need another step to ged rid of this effect. Through searching process of more suitable system in which the refractive indices of polymer and that of polymer match perfectly, poly(vinyl acetate)(PVAc)/DMSO system was found and there dn/dC of PVAc/DMSO system was estimated around -0.0037±0.001 mL/g which was ~17 times lower than that of PVA/DMSO system.
In this study, diffusion of PS probe particle in PVAc/DMSO matrix system was investigated by means of dynamic light scattering. All the reduced diffusion coefficients of particle, D/Do measured at various matrix concentration have fallen on the one master curve. When -ln(D/Do) values of these data were double-logarithmically plotted against the reduced concentration C[η], the relation was appeared as a linear one with the slope of 0.95. To check applicability of Stokes-Einstein equation to our system, the physical quantity of was plotted as a function of the reduced concentration and it had maximum value of 1.35 at C[η]=1.5, and then began to decrease as increasing concentration. This trend seemed to be totally different from probe''s behavior in Eom''s system of PVA/DMSO, where the quantity was increasing monotonically as increasing concentration. Based upon this result, it was found that the probe particle did not follow Stokes-Einstein equation in the system of PVAc/DMSO solution. Next regarding to the ratio of effective viscosity to bulk viscosity, it was continuously decreased down to 0.75 around C[η]~1 and then turned to increase at C[η]~2 and reached the value of =1.2 at C[η]~6. This abnormal behavior is attributed to the fact that some attractive force plays important roles to the probe particles dispersed in polymer solution. It seems that the attractive force is not yet reached to the level of particles'' aggregation but that it can give some constraints to a certain extent of its moving in the polymer solution. Finally also this effect is disclosed again as the thin thickness of the depletion layer decreased abruptly above C[η]>1. Over-all, although the very simple TCF was obtained through the perfectly matching system of the refractive index, and its analysis become much easier than before, yet only apparent depletion layer was obtained owing to the inevitable Oosawa type attractive interaction.