Elastic properties of well-defined, high-density poly(methyl methacrylate) brushes studied by electromechanical interferometry

We have investigated the elastic properties of dry polymer brushes comprising low-polydispersity poly(methyl methacrylate), (PMMA) chains with high graft densities over 0.4 chains/nm(2) by means of electromechanical interferometry. End-grafting on a glass plate by living radical polymerization yielded high-density brushes composed of the highly stretched graft chains, and the layer thickness of the brushes is 4-5 times as large as the unperturbed dimension of the graft chains. The layer thickness change induced by an applied electric field (electrostriction) is measured by a Nomarski optical interferometer as a function of temperature. The analysis of the electromechanical and dielectric data yields the plate compressibility (K-p) of the brushes in the glassy and molten states. Comparison of the results for the high=density PMMA brushes and a reference spin-coated PMMA layer reveals the differences in the elastic and dielectric properties between a brush composed of highly stretched graft chains and a layer formed by the equivalent chains in the random-coiled state. The temperature dependence of the dielectric loss suggests that the glass transition temperature of the high-density brushes is ca. 10degreesC higher than that of the spin-coat layer. In the glassy state, there is' no appreciable difference in Kp between the brushes and the spin-coat layer, whereas in the molten state, Kp of the brushes is definitely (ca. 30-40%) lower than that of the spin-coated layer. This proves that the molten high-density PMMA brushes are more resistant against compression relative to the PMMA melt. The low compressibility of the molten high-density brushes is interpreted on the basis of rubber elasticity of entanglement network in the. stretched state. This model successfully explains the magnitude of the plate compressibility of the brushes, which suggests that the low compressibility is mainly, attributed to a strain-hardening effect of the highly stretched entanglement chains and that there exists a considerable amount, of entanglements of different graft chains contributing to elastic modulus.