Structure and Dynamics of Soft Condensed Matter

Soft Condensed Matter Soft condensed matter describes a class of materials where weak intermolecular interactions are typical. The materials are flexible and are susceptible to thermal fluctuations, and they tend to form from self-assembly of molecular components. Our research focuses on structure and dynamics in systems such as polymers, gels, vesicles, and colloids. We use static and dynamic light scattering and related techniques to study properties of these systems. Current projects: Crystallization in Colloid-Polymer Mixtures Polymeric Materials for Fuel Cells Yeild-Stress Fluids Lipid Vesicles Other Projects: Soft Colloidal Particles Particle Membranes

Crystallization in Colloid-Polymer Mixtures

Photo of BCAT 5 apparatus taken before launch containing 10 colloidal samples. Samples 6, 7, and 8 are the SFU samples.

Sequence of photos taken of sample SFU-2 as it phase separates in the lab. The latter photos show crystals are forming in the lower phase.

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Polymeric Materials for Fuel Cells

AB Di-Block Copolymer Membranes

Our group of interesting model materials that can be used to study transport properties are polymers made up of two blocks, one proton-conducting and one hydrophobic. Different structure can be achieved by varying the relative block sizes.

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We use Small Angle Neutron Scattering to study the structure of these materials. This graph shows scattering spectra for three samples in a series consisting of a poly(vinylidene difluoride-co-hexafluoropropylene) block and a sulfonated polystyrene block where the sulfonation rate is increased from 22% (1A) to 40% (1C).

Yeild-Stress Fluids

In collaboration with John de Bruyn (MUN), we are studying properties of yield-stress fluids from both a macroscopic and microscopic perspective. Yield-stress fluids are materials that are solid at low stress but will flow above a well-defined stress threshold. Examples include mayonnaise, shaving cream and paint. We are studying diffusion and viscosity in a variety of systems in order to uncover universal aspects of this interesting class of materials. Currently, we are investigating the properties of Carbopol solutions, where Carbopol is a material used as a thickener in many personal care products, using both light scattering and light microscopy to track movement of probe particles inside the material

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Lipid Vesicles

Vesicles are quasi-spherical shells formed from lipid bilayers. They occur naturally in cells where they are used for intra- and inter-cellular transport but they can also be produced in the lab where they are used as model membranes and in pharmaceutical applications, specifically for drug delivery. One aspect of our work focuses on understanding the extrusion process by which they are made. Vesicle formation by extrusion involves using an applied pressure to push lipid solution through a membrane filter consisting of cylindrical pores. During this process, large structures consisting of many lipid bilayers are broken up into quasi-spherical shells approximately the same size as the pores, and consisting of only a single bilayer. The technique is used extensively, but the actual process by which the vesicles are produced is not well understood. Results from systematic investigation of the properties of lipid vesicles have been used to develop a method of measuring the lysis tension of lipid vesicles and to develop models of the extrusion process. We are also working on a project studying the effect of sterols on lipid membrane properties with Jenifer Thewalt and Martin Zuckermann and on a project studying the aggregation of vesicles with Rosemary Cornell.

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Particle Membranes

We experimented with proton-conducting membranes made from particles with charged shells and hydrophobic cores. Membranes were formed first by drying solutions of polymer particles and then incubating the dried film above the glass transition temperature of the polymer, so that the polymer chains from adjacent particles entangled to form a stable film.

The conductivity of the particle films is higher than other films of the same composition showing the impact of structural design.

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Soft Colloidal Particles

We investigated the properties of colloidal gel particles made from poly(N-isopropylacrylamide) [PNIPAM]. These particles are very swollen at low temperatures but collapse above a well defined transition temperature. We studied the heterogeneity of the particles as a function of crosslinking density as well as viscosity and packing of the particles in solution as a function of concentration. We have also used them as a test particle in investigations of the formation of porous membranes from colloidal particles.

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