Surface fluctuations of polymer brushes probed by diffuse X-ray scattering

The diffuse scattering from the surfaces of melt and glassy polymer brushes has been studied systematically for the first time using polystyrene (PS) and poly(n-butylacrylate) (PnBA) brushes synthesized by free radical polymerization. The data show unambiguously that the diffuse scattering behavior varies systematically with brush thickness for both types of brushes. We attribute a cross-over in scattering with qx, the in-plane scattering vector, to the presence of surface thermal fluctuations and their suppression for longer wavelengths, a phenomenon already reported for films of untethered chains. Long wavelength fluctuations are suppressed more strongly on the surface of a PS brush than on the surface of a film of untethered ('free') PS chains of comparable thickness, so that even in films of thickness, d, such that d/Rg>5 clear evidence of the suppression of fluctuations can still be seen in the experimentally available range of qx. Fluctuations are suppressed for qx less than a lower wavevector cut-off, ql,c, which changes with film thickness, though much more weakly than for films of free chains. For values of d/Rg<4, where Rg is the unperturbed radius of gyration of a comparable free chain, ql,c drops as d increases. For d/Rg>4 ql,c begins to increase with brush thickness, in qualitative agreement with theory, indicative of a transition to a true 'brush' state in which stretching of the chains makes longer wavelength fluctuations at the surface unfavorable. Measurements with PnBA brushes having Tg substantially below room temperature confirm the trends mentioned above. Further, they give evidence that the value of ql,c is temperature insensitive above Tg.