Structure and Dynamics of Soft Materials

Soft Matter Physics

Soft materials include gels, polymers, colloidal materials, and of course biological materials. They are characterized by weak intermolecular interactions, which results in them being flexible and susceptible to thermal fluctuations. They tend to form from self-assembly of molecular components. Our research focuses on relationships between structure and material properties, where we use scattering techniques to study structure at a wide range of length scales.

Current projects:

Competition Between Phase Separation and Crystallization in Colloid-Polymer Mixtures
Polymeric Materials for Fuel Cells
Yeild-Stress Fluids

Other Projects:

Lipid Vesicles
Soft Colloidal Particles
Particle Membranes

Competition Between Phase Separation and Crystallization in Colloid-Polymer Mixtures

Video made by CSA Astronaut Chris Hadfield about BCAT-C1.

Phase separation and crystallization play crucial roles in the development of materials including plastics and metal alloys. We are investigating the interplay between these phenomena in colloid-polymer systems where the interaction potential can be precisely tuned by changing the size and concentration of polymer and colloid. In particular, we are studying domain growth and crystallization in colloid-polymer samples that demonstrate gas-crystal and gas-liquid-crystal coexistence. In addition to ground-based studies, we are participating in experiments on the International Space Station. Three samples demonstrating gas-liquid-crystal coexistence were studied as part of BCAT5, a glove-box apparatus installed in the International Space Station. A further 7 samples are currently on the ISS and experiments are being planned for January 2013.

Photo of BCAT 5 apparatus on the International Space Station. The apparatus contains 10 colloidal samples; Samples 6, 7, and 8 are the SFU samples.

Sequence of photos taken of a three-phase colloid-polymer sample as it phase separates in the lab. The latter photos show crystals are forming in the lower phase.

Sequence of photos taken of a three-phase colloid-polymer as it phase separates on the International Space Station. Phase separation is arrested by formation of crystals within the liquid phase. For further details, please see Phys. Rev. Lett. 109, 195701 (2012).

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Polymeric Materials for Fuel Cells

The heart of the Proton Exchange Membrane Fuel Cell is a proton-conducting polymer membrane. In collaboration with Steven Holdcroft (Chemistry), we work with model membranes to study the relationship between structure and conductivity.

AB Di-Block Copolymer Membranes


One 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.

We use Small Angle X-Ray and 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). For further details, please see Macromolecules 39, 720-730 (2006).

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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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