Density profiles in thin PMMA supported films investigated by X-ray reflectometry

We have investigated the electron density profiles normal to the substrate of 20-80 nm thick films of stereoregular poly(methyl methacrylate) (PMMA) spin-cast on silicon wafers by X-ray reflectometry. The reflectivity curves were analyzed using a least-squares procedure based on the distorted-wave Born approximation that is capable of modeling slight density fluctuations. From the calculated electronic density profiles, an increase in the density in the vicinity of the solid SiOx surface can be observed regardless of PMMA film thickness and tacticity. This result is discussed in terms of local molecular packing near surfaces. Moreover, small periodic density fluctuations appeared for film thicknesses above 30 nm. These are tentatively attributed to a layering formation due to both fast solvent evaporation and polymer autophobicity during the spin-coating process. This layering effect is compared to that observed after freeze-drying of a semidilute solution where fast evaporation limits the interpenetration of polymer coils in the thin film.