Topological AFM characterization of graft polymerized silica membranes

The topology of porous silica membranes (500 Å native pore size) that were surface modified by silylation with vinyl trimethoxysilane (VTMS) and graft polymerization with poly(vinyl acetate) (PVAc) was investigated by atomic force microscopy (AFM). The polymer-modified membrane surfaces were populated by terminally anchored polymer chains with the depth of modification of about 200 nm as assessed from scanning electron microscopy (SEM) images. Surface roughness increased upon silylation but decreased as the result of surface graft polymerization. AFM imaging of surrogate silicon wafers, silylated or grafted in a manner similar to the membranes, revealed that silylation increased the root-mean-square (RMS) surface roughness by about 1.0 nm, while poly(vinyl acetate) modification reduced the RMS surface roughness by approximately 0.4 nm. The distributions of surface heights for the modified surfaces were asymmetric with the tails of the distribution representing the sparse larger features. Surface roughness analysis revealed that the tails make a significant contribution to the measured RMS surface roughness. Although the RMS surface roughness values of the polymer-modified surfaces were essentially similar (0.81–0.85 nm), skewness of the distribution decreased while the width of the Gaussian portion of the distribution increased with decreasing initial monomer concentration.