Michael Solomon , University of Michigan
This project addresses fundamental questions regarding complex fluid aggregation by measurements of dynamics. 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? Methods for this project include techniques to measure dynamics, including small-angle light scattering and particle tracking with confocal microscopy.
Brief Resume of Project results from 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). This work 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, as assessed by comparison to the theoretical result for scattering from a pinhole (data not shown). 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.
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. 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.
Summary of Project results: Year 2, U-Michigan (1 September 2010 – 31 August 2011).
Youngri Kim, supported by a fellowship, and Heekyoung Kang, supported by the ACS-PRF grant, joined the project to study the effect of flows on the dynamics of aggregated complex fluids using the model system of colloidal gels.
We extended the work from the first year so as to develop dynamical correlation methods to apply to complex fluids such as aggregated polymers and gelled colloidal suspensions. We continued to emphasize confocal laser scanning microscopy (CLSM) characterization of dynamics over light scattering techniques, because the CLSM techniques can be used to resolve the motion of every particle in the suspension, whereas light scattering produces a characterization of the ensemble averaged dynamics. For CLSM, refractive index and density matching of suspensions is important, so Youngri Kim used the synthesis method reported in Z.K. Zhang, et al, “Synthesis and Directed Self-Assembly of Patterned Anisometric Polymeric Particles,” J. American Chemical Society, 133, 392 (2011) to produce colloidal rods suitable for gelation studies. Current results include CLSM imaging of the structure of aggregated colloidal gels produced from these rods due to the addition of polymeric depletant. These CLSM image volumes undergo image processing according to the method of Mohraz and Solomon Langmuir, 21, 5298 (2005) so as to quantify how structure changes with aspect ratio in colloidal rods.
In addition, we have applied our image processing methods for dynamics to understand new features of colloidal gel yielding. Previous research has indicated that colloids aggregated with short-range attractions are an excellent model of a wide range of aggregated systems including associating polymers. The elasticity of these gels is highly correlated with their colloidal dynamics. For example, mode coupling theory suggests an inverse relationship between a gel’s elasticity and the square of its localization length (Y.-L. Chen, and K.S. Schweizer, J. Chem. Phys., 120, 7212 (2004).). Here the localization length is defined at the maximum mean-squared displacement of the bonded colloid. One idea about the strain-dependent rheology of aggregated systems is that the elasticity of the gel decreases with strain because the particles become increasingly delocalized due to the flow-induced breakage of bonds V. Gopalakrishnan, and C.F. Zukoski, Langmuir, 23, 8187 (2007).
In the last year, Heekyoung Kang tested this idea by applying step-strain deformation to colloidal gels and measuring their single-particle dynamics immediately after the deformation. The step-strain deformation induces yielding. Kang observed and quantified a concomitant increase in the localization length of the colloids. However, this increase was insufficient to explain the change in elasticity of the gel. Instead, Kang found that a certain subpopulation of bonded colloids control the gel elasticity, and it is the delocalization of these colloids that is predictive of the modulus. This important result points out new directions for theorical development in the rheology of aggregated systems. The data and their analysis are currently being written for peer-reviewed publication.
Plans for no-cost extension
We have received a no-cost extension to allow us to prepare the results of our study of the relationship between aggregated system dynamics and gel elasticity for peer reviewed publication.