Neutral polymer slow mode and its rheological correlate

Abstract

Quasi‐elastic light scattering spectroscopy intensity–intensity autocorrelation functions [S(k,t)] and static light scattering intensities of 1 MDa hydroxypropylcellulose in aqueous solutions were measured. With increasing polymer concentration, over a narrow concentration range, S(k,t) gained a slow relaxation. The transition concentration for the appearance of the slow mode (ct) was also the transition concentration for the solution‐like/melt‐like rheological transition (c+) at which the solution shear viscosity [ηp(c)] passed over from a stretched exponential to a power‐law concentration dependence. To a good approximation, we found ct[η] ≈ c+[η] ≈ 4, [η] being the intrinsic viscosity. The appearance of the slow mode did not change the light scattering intensity (I): from a concentration lower than ct to a concentration greater than ct, I/c fell uniformly with increasing concentration. The slow mode thus did not arise from the formation of compact aggregates of polymer chains. If the polymer slow mode arose from long‐lived structures that were not concentration fluctuations, the structures involved much of the dissolved polymer. At 25 °C, the mean relaxation rate of the slow mode approximately matched the relaxation rate for the diffusion of 0.2‐μm‐diameter optical probes observed with the same scattering vector. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 323–333, 2005

Spectral time moment analysis of microgel structure and dynamics

“…HPC is readily available, FDA‐approved nontoxic food additive used in soups and ice creams as an emulsifier9 and encapsulator 10. Because of its electrical neutrality, semiflexible nature, and solubility in a wide range of solvents, HPC has been widely used for studies of polymer dynamics 11–24…”

Fluorescent Labeling Can Alter Polymer Solution Dynamics

“…The slow mode was attributed to the center of mass displacement of chains in response to their direct and hydrodynamic interactions. The ultraslow mode was believed to arise from multiple-chain, loosely structured, temporal aggregates or perhaps polydispersity fluctuations, subject to the constraint given by static light scattering 26 that the local concentration of polymers within the temporal aggregates could not be significantly different from the average concentration of polymers in the solution. Such vitrified regions might readily be more apparent to DLS than to FPR because scattering methods are exquisitely sensitive to large objects but also because those objects do not need to last very long to contribute to the relaxation of the DLS signal.…”

“…Our second major finding is that comparison of DLS and FPR on the very same labeled samples gives new information on the physical interpretation of the observed mode structure. Prior studies based on DLS, static light scattering, and optical probe diffusion proposed that each spectral mode corresponds to a single physical process. ,, In this model, the fast mode was attributed to the consequences of fundamentally local motions, such as bending and structural relaxation of polymer chain segments, as they relaxed overall concentration fluctuations. The slow mode was attributed to the center of mass displacement of chains in response to their direct and hydrodynamic interactions.…”

“…35,36 Systematic reanalysis of these studies leads to a coherent phenomenological image of relaxational modes, dynamic transitions, and fundamental length scales in HPC solutions. 25,26,37 DLS spectra are multimodal, containing at least two stretched-exponential relaxations. The critical concentration c + was determined.…”

“…The extensive viscosity measurements of Phillies and Quinlan show that at smaller concentrations the viscosity η­( c ) is a stretched exponential in c , but at some critical concentration c + has a sharp, analytic (first derivative continuous) crossover to a power-law concentration dependence at larger c . This “solution-like–melt-like” crossover was a primary motive for many of our later studies of this system by other methods. ,,− Scattering experiments include (a) static light scattering studies, , (b) DLS measurements of polymer mutual diffusion in HPC:water, and (c) optical probe diffusion studies using DLS of polystyrene spheres of many sizes in HPC:water. ,,− Extending the probe diffusion studies to smaller probe sizes, FPR was used to measure D s of dextrans and sodium fluorescein through HPC solutions. , …”