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In this Letter we present first measurements of the radius of gyration ͑R g ͒ for polystyrene in deuterocyclohexane at the critical concentration of the polymer over a temperature range Q $ T $ T c using small-angle neutron scattering and the high-concentration labeling methodology. We do not observe a reduction of R g ͑T c ͒ at the critical temperature of phase demixing T c as compared to R g ͑Q͒ in the Q region, which indicates that the critical polymer solution cannot be considered as an ensemble of partially collapsed noninterpenetrating coils. [S0031-9007(96)02248-X]

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We report the results of an experimental study of the effect of a dilute silica network on liquid-gas critical phenomena in carbon dioxide (CO2). Using small-angle neutron scattering, we measured the correlation length of the density fluctuations in bulk (xi(bulk)) and confined CO2 (xi(conf)) as a function of temperature and average fluid density. We find that quenched disorder induced by an aerogel suppresses density fluctuations: xi(conf) loses the Ising model divergence characteristic of xi(bulk) and does not exceed the size of pores in the homogeneous region.