The modification of the polymeric substrates bisphenol A poly(carbonate) (PC), poly(vinyl chloride) (PVC) and poly(tetrafluoro ethylene) (PTFE) by the vinylic monomers styrene (S), methyl methacrylate (MMA) and methacrylic acid (MAA) under supercritical conditions was investigated systematically. In a first step, the polymers were impregnated with the monomers and a radicalic initiator, followed by a polymerization inside the swollen substrates to generate a polymer within a polymer. The process parameters were controlled by the radicalic initiator and by the solubility of the monomers in scCO(2). As a result, PTFE shows the least ability to be modified due to its limited swelling capability. With the generation of the water-soluble poly(methacrylic acid) (PMAA) inside the hydrophobic PVC- or PMMA substrates, it was possible to obtain polymer mixtures inaccessible by common techniques such as melt mixing. The modification of PVC with PMAA was characterized in detail and the limits of the modification process evaluated. The comparison with impregnation under common conditions showed the significance of the presence of CO, during both impregnation and polymerization. Carbon dioxide acts as a low-molecular-weight plasticizing agent, which enhances the mobility of the monomer inside the swollen substrate, but does not change the compatibility of the polymers with the monomers. Another peculiarity was the observation of a mixing gradient. There was a linear decrease in the PMAA content from the surface to the inner bulk of the polymer mixtures. The maximal penetration depth found for PMAA during 4h of impregnation was 180 mu m. This means, that polymer blends can readily be obtained with thin films and fibers, but is limited with bulkier substrates. In short, scCO(2) enables the smooth refinement of polymer surfaces and, moreover, has the potential to generate new polymer mixtures. (C) 2000 Elsevier Science B.V. All rights reserved.