Structure and mechanical properties of poly(benzyl ether) monodendron Langmuir films

The Langmuir technique combined with a traditional polymer science approach has been used to investigate the monolayer properties of poly(benzyl ether) monodendrons modified with linear oligo(ethylene glycol) chains at the air-water interface. Film balance measurements have been used to probe the relation between the amphiphilic balance of the monodendrons and the stability and structure of the monolayer. Analysis of the surface pressure-area (π- A) isotherms indicates that the linear hydrophilic chain controls the adhesion to the interface, while the hydrophobic monodendron determines the molecular shape, monolayer compressibility, and film cohesion.;A novel interpretation of the π-A isotherm as a stress-strain curve allows for a more detailed analysis of the monolayer mechanical properties. The pseudo-2D monolayers behave in a manner analogous to many bulk polymers, showing a yield point and a region of plastic flow. Within the plastic flow region, the stress-strain behavior can be quantitatively captured using a constitutive law that assumes both a power law stress dependence and an Arrhenius temperature dependence for the strain rate. Constant strain rate and dilational creep experiments have been used to determine the stress exponent and the creep activation energy, respectively.;Analysis of the isotherms suggests the existence of a monolayer glass transition within the temperature range studied. Compressibility experiments provide further evidence of a monolayer glass transition, the temperature for which lies more than 15°C below the bulk value. Dynamic mechanical analysis has been performed using an interfacial stress rheometer to study the viscoelasticity of the monolayer near the glass transition. A master curve of the frequency dependence of the shear modulus produced using time-temperature superposition also implies that the monolayer approaches a glass transition.;As an extension of the work at the air-water interface, we have completed a preliminary study of the organization and surface viscoelasticity of Langmuir-Blodgett films of the monodendrons on hydrophilic silicon substrates using ellipsometry and atomic force microscopy. These films exhibit a viscoelastic transition consistent with the behavior of monolayers at the air-water interface. Furthermore, the Langmuir-Blodgett films assume a number of morphologies that depend on film thickness and may be generated by dewetting from the solid surface.