Surface quasi-elastic light scattering from spread monolayers of a poly(methyl methacrylate) poly(ethylene oxide) block copolymer

A linear diblock copolymer of poly(methyl methacrylate) and poly(ethylene oxide), with a mole ratio of methyl methacrylate to ethylene oxide units of 1:1, has been spread at the air-water interface. The surface visco-elastic parameters of this spread film, surface tension, gamma(0), transverse surface shear viscosity, gamma ', dilational modulus, epsilon(0), and dilational viscosity, epsilon ', have been determined using surface quasielastic light scattering. For a fixed value of the scattering vector parallel to the water surface, the surface visco-elastic parameters have been determined as a function of surface concentration of block copolymer. From the surface tension and transverse shear viscosity and using a simple Maxwell fluid model of the spread polymer, a relaxation time has been obtained which, from its concentration dependence at fixed angle of incidence for the incident light beam, suggests only a single relaxational process. Use of dilational parameters produces relaxation times of the same order which suggests that the same molecular process is involved in the relaxation process. For a fixed surface concentration of block copolymer the frequency dependence of the surface visco-elastic parameters has been investigated by selecting different scattering vectors. A capillary wave frequency from circa 1 x 10(4) s(-1) to 2 x 10(5) s(-1) has been explored here. A Maxwell model can be fitted to these data to provide a single relaxation time but the value is considerably different from that obtained using concentration dependent data at a single scattering vector. A Cole-Cole plot of these frequency dependent surface visco-elastic parameters clearly shows that a single relaxation time cannot prevail. A distribution in relaxation times is evident with the distribution apparently skewed to the high frequency side of the spectrum, but the frequency range we are able to explore is too limited to be able to define any parameters of this distribution.