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Trends in Physical Chemistry   Volumes    Volume 6 
Abstract
Polymer dynamics and rheology in aqueous solutions and gels of a poloxamer
Harald Walderhaug, Bo Nyström
Pages: 89 - 106
Number of pages: 18
Trends in Physical Chemistry
Volume 6 

Copyright © 1997 Research Trends. All rights reserved

ABSTRACT
 
A triblock copolymer of the type poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (Pluronic F68) in water has been investigated at three concentrations of 10, 20 and 35 wt % and at various temperatures in the range 23 to 40° C by dynamic light scattering. The 35 wt % system has also been investigated in the same temperature range by using rheological methods and pulsed gradient NMR. The 35 wt % sample forms a gel when heated to ca. 37°C, and exists as a micellar solution at lower temperatures. The two other concentrations do not form gels at elevated temperature, but exist in the form of micellar solutions. The autocorrelation function can be described by a single exponential decay at short times, followed by a  stretched exponential decay at longer times. At higher temperatures and/or concentration a power-law decay enters between the other two modes. The power-law decay mode is present also in the sol state, depending on the combination of temperature and concentration.The initial single exponential decay is shown to be diffusive, whereas the two other modes exhibit a more complex behaviour. From rheological measurements on the 35 wt % sample, the stress relaxation modulus, calculated from shear oscillation data or measured directly, is found to show the same type of profile as the autocorrelation function. An extended power-law decay is observed in the vicinity of the gel point. The power- law exponent for the rheology data is around 0.5, while the corresponding values of the power-law exponent obtained from dynamic light scattering are lower. From pulsed field gradient NMR the mean square displacement of the polymer chains is measured is a function of time. In the sol state this quantity is demonstrated to increase linearly with time, i.e. the self-diffusion process exhibits an ordinary Fickian behaviour. As the gel zone is entered the mean square displacement follows a power-law in time, with an exponent less than 1, i.e. an anomalous diffusion process is detected. At the highest temperature investigated, this power-law exponent is ca. 0.5. Different models are discussed in order to explain the findings from dynamic light scattering, rheology and pulsed field gradient NMR. Complex dynamical and rheological features are observed for the Pluronic system. A possible structure for the gel phase is discussed, based on the present experimental findings and recent results from other binary Pluronic/water systems.
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