Surface Shear Rheology of a Polymerizable Lipopolymer Monolayer

Abstract

We have used an interfacial stress rheometer to study the changes in the mechanical shear properties of a Langmuir monolayer of a linear polymer with reactive side chains. The polymer investigated had a linear hydrophilic backbone consisting of poly(ethylene imine-co-ethyl oxazoline) and contained 20% hydrophobic sorbyl side chains, making it amphiphilic and capable of forming stable monolayers at the air−water interface. The sorbyl moiety in the side chain was photopolymerizable at 254 nm. Upon exposure of the film to UV light, the surface dynamic shear modulus, Gs‘, which is a measure of the elasticity, increased 1000-fold to ∼2 mN/m. This large increase in elasticity arises from the formation of a network by reaction of side chains on different polymer backbones. Rubber-like properties were deduced from the frequency dependence of the moduli and from creep experiments, where elastic recoil could be observed upon removing the applied stress. Such a network could be formed over a wide range of surface pressures ranging from 5 to 25 mN/m. We investigated the cross-linking kinetics upon UV exposure as a function of temperature, initial surface pressure, and light intensity. We have developed a lumped kinetic model that agrees with the transient behavior of the storage modulus, our measure of the density of cross-link points. The reaction was independent of temperature (over 10−30 °C) and exhibited a first-order dependence on light intensity. Since the reactive side groups must be in close enough proximity for the interpolymer side chain reaction to occur, a threshold side chain density is required to form junction points. After extensive UV exposure, the elasticity of the network degraded, and the rate of degradation was faster for films polymerized at lower initial surface pressures.