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Summary of Work Plan

This project addresses fundamental questions regarding complex fluid aggregation by measurements of dynamics sucgh as small-angle light scattering.  A key method of the plan is our hypothesis that the aggregate structure of dilute solutions of high molar mass poly(ethylene oxide) can be manipulated by the addition of chaotropic salts that disrupt the hydrogen bond structure of water.  These methods will be applied to address: (i) What is the effective size of PEO aggregates in water, as judged by the decay of intensity time correlation functions at ultra-low wavevectors?  (ii) Does this effective size explain anomalous rheological measurements, such as enhanced shear thinning and large viscoelastic relaxation times?

Project results: Year 1, U-Michigan (1 January 2009 – 31 August 2010).

The project work plan identified the aggregate structure of complex fluids such as poly(ethylene oxide) for investigation by use of an ultra-low angle light scattering device.  In the first year of the grant, Abhi Shetty constructed and used scattering and microscopy instrumentaion to study the dynamics of single-walled carbon nanotubes, the dilute structures of poly(ethylene oxide) aggregates, and the aggregation and vitrification behavior of polymer rods comprised of polyamide.  We here briefly summarize each activity.

With collaborators Dr. Georgina Wilkins and Dr. Jagjit Nanda, Abhi Shetty completed a project in which he introduced the method of multiangle depolarized dynamic light scattering (MA-DDLS) to characterize the length and diameter of covalently functionalized single-walled carbon nanotubes (SWCNTs). MA-DDLS yields simultaneous characterization of the mean translational and rotational diffusivities of dilute solutions of SWCNTs. By using an anisotropic rigid rod model, he uniquely determined the length and diameter of the SWCNTs from the independent measurements of rotational and translational diffusion. The multiangle depolarized light scattering technique is found to be a fast, noninvasive, and reproducible method for identifying the average length and diameter of SWCNTs in solution.  This work was was published in a peer reviewed publication.

In the second activity, Abhi Shetty designed and built an ultra-small angle scattering device to investigate the aggregate dynamics of poly(ethylene oxide).  The performance of the device  was assessed by comparison to the theoretical result for scattering from a pinhole (data not shown).  Our plan is to use the ultra-small angle light scattering device, which have previously been developed for soft condensed matter physics, to interrogate colloidal gel structure and dynamics at very low scattering vectors.  (Typical small-angle light scattering devices use a pinhole and optics to collimate scattered light from a localized spot within the sample onto a CCD chip.  The scattering volume is located at the focal point of the collimating lens.  The minimum scattering angle of these devices is, at best, about 2° because of pinhole and scattering volume sizes.  In the more recent designs we favored for this project, the CCD detector images the back focal plane of a magnifying lens, much like a microscope.  Here, the minimum scattering angle we have achieved is about 0.4°, nearly an order of magnitude improvement.)  After designing the instrument and aligning it, we observed the q-resolved autocorrelation of speckles and assessed the performance of small-angle DLS systems with experiments with colloidal spheres.  We then performed initial measurements of PEO aggregate size with this instrument.  Our conclusion from the measurements was that the scattering intensity of the polymeric species was too low for resolvable measurements under conditions of both no salt and of chaotropic salt.  Even when using an intensified CCD camera, the scattering is too low relative to the colloidal species that have been previously studied with a device like such as this one.

In the third activity, Abhi Shetty used scattering measurements, confocal microscopy and rheology to study the effect of aspect ratio on the slow dynamics of polymer rods. Time series of confocal microscopy images showed that arrested dynamics in these rod suspensions occurred at a critical volume fraction that depended sensitively on the aspect ratio of the rod suspension. The arrest volume fraction was identified by measuring the volume fraction dependence of an average correlation coefficient that compares intensity values between two different images in a confocal microscopy time series as a function of the delay time between images. This method is analogous to image correlation methods used in ultra-small angle light scattering, and was chosen for that reason.  The arrest volume fractions obtained for the different aspect ratios was lower than theory and simulation predictions of the minimum percolation volume fraction in a random homogenous network of rods. Abhi Shetty also investigated the elasticity and yielding behavior of these rod suspensions above their critical volume fraction by means of oscillatory shear experiments. He found that for all the rod suspensions studied, the elastic modulus monotonically increased with rod volume fraction and followed a power law, independent of the aspect ratio of the rod. This work is currently being prepared for peer reviewed publication.

Future Plans

Youngri Kim, a new chemical engineering Ph.D. student, has joined the project to study the effect of poly(ethylene oxide) on the aggregation and gelation of polyamide rods in the system studied by Abhi Shetty in the first year of the grant.  She will focus on how poly(ethylene oxide) leads to gelation by affecting the local, bundled structure of the rods.  Techniques she will use include the dynamical correlation methods developed in the first year of the grant.

Personnel Supported

University of Michigan graduate student Abhishek Shetty was supported by the grant in its first year. At the conclusion of the year, Abhi Shetty successfully defended his dissertation and began employment in Research and Development of a large U.S. corporation.