J. Perlich¹, V. Köstgens¹, L. Schulz¹, R. Georgii^2,3, P. Müller-Buschbaum^1
1 Physics Department E13, Technische Universität München, 85747 Garching, Germany
² Physics Department E21, Technische Universität München, 85747 Garching, Germany
³ ZWE FRM-II, Technische Universität München, 85747 Garching, Germany

The detection of remaining solvent in thin polymer films is of importance due to its effect on chain mobility and film homogeneity. Moreover, it gives an estimate on possible aging effects caused by the reduction of the solvent content, which typically yield an increased brittleness. Here we use a well controlled model system, which consists of protonated polystyrene (PS) with different molecular weights M_w of 7, 27, 207, 514, 908, 1530 kg/mol, spin-coated out of protonated or deuterated toluene (solvent) onto silicon (Si) wafer substrates. Directly after spin-coating the thin PS films were investigated with neutron reflectivity (NR) at the MIRA instrument at a wavelength of 16 Å. A narrow q_z range (0 Å⁻¹ to 0.02 Å⁻¹) around the critical edge was probed with high resolution. The experiment focuses on two different key parameters which influence the solvent content: the molecular weight of PS and the film thickness investigated in the range of 10 to 100 nm. Focussing on the molecular weight, thin PS films with a fixed thickness of 50 nm are investigated: The expected shift of the critical edge position, which is observed in neutron reflectivity simulations on this model system, is verified by the MIRA measurements (Figure 1). In direct comparison the reflectivity of the sample prepared out of deuterated and protonated solvent is plotted. The data show a shift of the critical edge with increasing molecular weight, although the measured critical edges of much higher M_w shift not completely but rather indicate a slightly changed slope of the critical edge. For the investigation of the film thickness as the second key parameter, PS with a molecular weight of M_w = 207 kg/mol is dissolved in toluene-d8. The thickness of the thin PS film is depending on the viscosity of the solution and thus the concentration of PS in the solution. Therewith the concentration is chosen in such a way to achieve a desired film thickness. Figure 2 shows the obtained reflectivity data for films with thickness of 10, 30 and 100 nm at fixed molecular weight. The data of the critical edge indicate an influence on the film thickness, but prevent a definite conclusion about the behaviour in dependence of the film thickness without fitting the data. In summary, the experiment was very successful. A direct comparison indicates a clear distinction between thin films prepared of PS dissolved in protonated or deuterated toluene. This directly transforms into the amount of solvent remaining in the polymer film.