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 q_x, 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/R_g>5 clear evidence of the suppression of fluctuations can still be seen in the experimentally available range of q_x. Fluctuations are suppressed for q_x less than a lower wavevector cut-off, q_l,c, which changes with film thickness, though much more weakly than for films of free chains. For values of d/R_g<4, where R_g is the unperturbed radius of gyration of a comparable free chain, q_l,c drops as d increases. For d/R_g>4 q_l,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 T_g substantially below room temperature confirm the trends mentioned above. Further, they give evidence that the value of q_l,c is temperature insensitive above T_g.