Title

Dynamics of Semirigid Rod Polymers from Experimental Studies

Document Type

Contribution to Books

Publication Date

2006

Publication Title

Soft Condensed Matter: New Research

Abstract

Experimental measurements of static and dynamic properties of non-ionic polymer solutions provide good testing grounds for existing and new models of polymer solution dynamics. We extensively studied aqueous solutions of the high molecular weight, rod like, semi flexible polymer hydroxyl propyl cellulose (HPC). Here we present our systematic analysis of measurements of: a) low shear viscosity η, b) quasi-elastic light scattering spectra of HPC solutions, including mode structure analysis at a range of temperatures and concentrations, c) quasi-elastic light scattering spectra of optical probe particles diffusing through HPC solutions, including careful characterization of modes of diffusive relaxation for tracer particles of different sizes, and d) static light scattering. We found a variety of novel phenomena. (i) The functional dependence of η on concentration has a transition at c+, with disparate small and large concentration dependences being seen. For c c+, η depends on c via a power law η∼ c x with x≈ 4 (“melt like” behavior). The viscometric crossover at c+ is echoed by the probe diffusion data, confirming the physical reality of the solution-to-melt like transition. (ii) Optical probe spectra and polymer spectra are both strongly non-exponential and can be decomposed into two or (at larger polymer concentrations) three spectral modes. Mode structure analysis reveals that probe relaxations and polymer relaxations in the same solution sample different aspects of polymer dynamics. (iii) Except for the largest (0.76μm) probes, diffusion coefficients of tracer particles in polymer solutions are not determined by the solution macroscopic viscosity. (iv) At concentrations above c+, light scattering spectra of HPC in solution have an ultraslow relational mode. This mode exhibits properties expected for long-lived dynamic structures but not the properties expected for local equilibrium regions of elevated polymer concentration. Properties of the slow mode are consistent with predictions from some models of glass formation. Finally, (v) polymer solutions have a characteristic dynamic length, which is approximately the hydrodynamic radius of the polymer.