BEHAVIOR OF FREE LINEAR-CHAINS OF POLYSTYRENE IN A NETWORK OF METHYL-METHACRYLATE IN TOLUENE

We have used small-angle neutron scattering (SANS) to study the spatia l distribution of free chains of deuterated polystyrene (PSD), in a ne twork of methyl methacrylate (MMA) in toluene, and compared it to PSD chains dispersed in a semidilute solution of PMMA chains of the same P MMA concentration. First, we just studied the gels with no free chains of PSD inside them. Gels show increased scattering compared to the so lutions, contrary to the predictions of the classical theories. Second , linear PSD chains were introduced in the system: again increased sca ttering is observed from chains in the gel compared to the chains in a n equivalent semidilute solution. These results imply that when the tr anslational entropy of free PSD chains is weak (especially for the lar ger ones), they would tend to mix less efficiently with the gel than w ith the solution. At low q, the scattering increases with molecular we ight and progressively tends to a master curve varying as q-2.5. We di scuss an explanation of this effect in terms of the percolation model of heterogeneities. The larger molecular weight PSD chains define the contour of the large percolation clusters in the gel and thus would re veal them. Only in the case where we remove the solvent does the syste m show strong segregation effects which override the effects of the he terogeneities of the network.