| Abstract
| - We have used an interfacial stress rheometer to study the changes in the mechanical shear propertiesof a Langmuir monolayer of a linear polymer with reactive side chains. The polymer investigated had alinear 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 theair−water interface. The sorbyl moiety in the side chain was photopolymerizable at 254 nm. Upon exposureof 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 networkby reaction of side chains on different polymer backbones. Rubber-like properties were deduced from thefrequency dependence of the moduli and from creep experiments, where elastic recoil could be observedupon removing the applied stress. Such a network could be formed over a wide range of surface pressuresranging from 5 to 25 mN/m. We investigated the cross-linking kinetics upon UV exposure as a functionof temperature, initial surface pressure, and light intensity. We have developed a lumped kinetic modelthat agrees with the transient behavior of the storage modulus, our measure of the density of cross-linkpoints. The reaction was independent of temperature (over 10−30 °C) and exhibited a first-order dependenceon light intensity. Since the reactive side groups must be in close enough proximity for the interpolymerside chain reaction to occur, a threshold side chain density is required to form junction points. Afterextensive UV exposure, the elasticity of the network degraded, and the rate of degradation was faster forfilms polymerized at lower initial surface pressures.
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