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The microstructure of the plasma‐polymerized methylmethacrylate (ppMMA) films is characterized using neutron reflectivity (NR) as a function of the plasma reaction time or film thickness. Variation in the crosslink density normal to the substrate surface is examined by swelling the film with a solvent, d‐nitrobenzene (dNB). In the presence of dNB, uniform swelling is observed throughout the bulk as well as at the air surface, and silicon oxide interfaces. The results indicate that the MMA film prepared by plasma polymerization (ppMMA) has a uniform crosslink density from air surface to substrate surface. Additionally, the scattering length density of the plasma‐polymerized MMA film (SLD ≈ 0.750 × 10−6 Å−2) is much lower than that of a conventional PMMA film (SLD = 1.177 × 10−6 Å−2). The increase in film thickness following dNB sorption is 7.5% and at least 36% for the ppMMA and PMMA films, respectively. This suggests that the films formed by plasma polymerization are different from conventional polymers in chemical structure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2522–2530, 2004

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