Patrick David Gallagher

Physical gels and microphase separation in multiblock copolymers

We present computer simulation results of a solution of multiblock copolymers, and show that a network can form under certain conditions. This network formation is due to a bridging effect, which exists for multiblock but not for diblock copolymers. Because of this special property, we suggest that multiblock copolymers may be used quite generally to describe physical gels. Further, we find that separation of block types from each other hinders phase separation of polymers from solvent when the solvent is poor for both block types.

Small-angle X-ray scattering studies of semidilute polystyrene-cyclohexane solutions

Small-angle X-ray scattering was used to study semidilute polystyrene-cyclohexane solutions at temperatures from below the coexistence curve to above the Θ point. Deviations from Θ point concentration scaling were observed as the coexistence line was approached. A new scaling form is suggested which combines the concentration scaling near the Θ point with the temperature scaling near a spinodal; it collapses the experimental data. This approach assumes that the “bare” correlation length ge0 scales as c−1. The observed low-angle “excess scattering” is best described by the Debye-Bueche random heterogeneity model, rather than the attractive potential model of Medjahdi and co-workers. The “excess scattering” is virtually independent of temperature, suggesting that the inhomogeneities which cause it are not affected by the approaching phase transformation.

Neutron reflectivity study of a chemically end-grafted polystyrene brush in a binary solvent mixture

We report neutron reflectivity measurements on a deuterated polystyrene brush chemically end-grafted onto a silicon surface and immersed in a mixture of cyclohexane and methanol. The solvents by themselves form a binary liquid mixture which phase separates upon lowering the temperature. Here the results for a solvent composition rich in methanol are reported. At high temperatures, the brush is collapsed as compared to its height measured in pure cyclohexane. However, upon lowering the temperature, the brush height increases substantially, indicating that excess cyclohexane is adsorbed within the brush as the solvent phase separation temperature is approached. By varying the deuterium content of the cyclohexane, we have determined that the solvent composition within the brush layer becomes nearly pure cyclohexane as the phase separation temperature is approached.

Neutron Reflection Studies of Swelling of Chemically End-Grafted Polymer Chains

Neutron reflection is used to compare the swelling of two chemically end-grafted polystyrene brushes having different grafting densities exposed to a theta solvent cyclohexane and to a good solvent toluene. The relative swelling of the brushes with a variation of solvent quality becomes smaller and the temperature dependence becomes weaker with an increase of grafting density. This type of swelling is very similar to polymer networks where an increase of cross-linking density leads to a decreased range of swelling and diminished dependence of the swelling. The model density profile fits to the body of both brushes is parabolic in a good solvent so that some fluctuation effects, as found generally in lower density brushes, are obtained as the brush expands.

Resonance Enhanced Neutron Standing Waves in Thin Films

Simultaneous measurements of neutron reflectivity and prompt gamma ray emission, from samples with buried Gd layers, are shown to be of significant aid in determining the depth profile of the entire sample. Because of resonant enhancement of the neutron standing waves in the sample, the gamma ray signals are considerably enhanced making these experiments possible. A possible application of this technique to study grazing angle neutron diffraction is also mentioned.