Study of the Polydispersity of Grafted Poly(dimethylsiloxane) Surfaces Using Single-Molecule Atomic Force Microscopy

Single-molecule atomic force microscopy (AFM) was used to study the statistical distribution of contour lengths (polydispersity) of polymer chains grafted to a surface. A poly(dimethylsiloxane) (PDMS) monolayer was grafted on a flat silicon substrate by covalently bonding Cl-terminated PDMS (M w = 15000−20000) to an OH−silicon surface and characterized using contact angle measurements, ellipsometry, and single-molecule AFM. A model for the single-chain dynamics is presented. The statistical distributions of the polymer contour lengths were found to depend on the concentration of the PDMS polymer used in the grafting solutions. Shifts of the statistical distributions toward higher contour lengths indicated preferential adsorption of longer chains with increasing PDMS:CH2Cl2 volume ratios of 0.005−0.16. The gel permeation chromatography (GPC) profile was found to correlate with the most dilute (0.005 volume ratio) AFM data. The polydispersity index (PI) calculated using AFM data was found to be 1.56 compared to 1.62 by GPC. A surface grafted with two PDMS polymer samples of average molecular weights, 3000 and 15000−20000, was found to have a bimodal distribution of contour lengths, with peaks corresponding to the two grafting samples.