Slow dynamics in transient polyelectrolyte hydrogels formed by self-assembly of block copolymers

Abstract

Transient polyelectrolyte hydrogels were formed by self-assembly of triblock copolyelectrolytes with a central hydrophilic block, poly(acrylic acid) (PAA), and two hydrophobic end blocks, poly(n-butyl acrylate(50%)-stat-acrylic acid(50%)) [P(nBA(50%)-stat-AA(50%))]. The relaxation of the concentration fluctuations was investigated by dynamic light scattering as a function of the concentration, the pH, the temperature, and the ionic strength. A relatively fast mode was observed at all polymer concentrations caused by cooperative diffusion of the polymers. Above the critical percolation concentration a second slow relaxation mode was observed caused by a linear displacement of small heterogeneities in the network with constant velocity. The relative amplitude of the slow mode was determined by the strength of the electrostatic repulsion. The velocity of the displacement in the transient network is shown to be directly correlated to the terminal relaxation time of the shear modulus and has the same Arrhenius temperature dependence. Both the velocity of the displacement and the mechanical relaxation strongly slow down with decreasing degree of ionization below 0.7 and increasing ionic strength above 0.5 M. A ballistic relaxation process has been reported earlier for colloidal gels, and the present study shows that it can also occur in polymeric networks.

Multiresponsive Hydrogels Formed by Interpenetrated Self-Assembled Polymer Networks

“…The slow mode was not observed in pure tPEO solutions in this concentration range. It was also observed for pure tPAA solutions, but with a smaller amplitude than in the mixtures. The slow mode is possibly caused by relaxation of heterogeneities within the transient network or interdiffusion of the two polymers. − A more detailed investigation is needed to resolve this issue, which was outside the scope of the present investigation.…”

Dynamics in Stimuli-Responsive Poly(N-isopropylacrylamide) Hydrogel Layers As Revealed by Fluorescence Correlation Spectroscopy

“…The origin of the slow diffusion in chemical networks was in dispute until recently . It is now assigned to the relaxation of slow density fluctuations arising either from the aforementioned collapsed dense domains or long correlation lengths between them. − In similar PNiPAAm hydrogel layers, both the fast cooperative diffusion (∼4 × 10 –7 cm 2 /s) and the slow gel diffusion (∼5 × 10 –9 cm 2 /s) were resolved by a microphoton correlation spectroscopy technique . In this gel picture (see Figure below), the slow tracer diffusion of Figure can be associated with the slow gel mode assuming comparable diffusion coefficients.…”

Charge Dependent Dynamics of Transient Networks and Hydrogels Formed by Self-Assembled pH-Sensitive Triblock Copolyelectrolytes

“…An efficient way to decrease γ in a controlled manner is to incorporate hydrophilic units into the hydrophobic blocks. − In recent years, we reported a detailed investigation of the self-assembly of triblock copolyelectrolytes with a poly(acrylic acid) (PAA) A-block and poly( n -butyl acrylate) (P n BA) B-blocks into which 50% AA units were incorporated randomly. ,− Association of these systems was found to be dynamic over a wide range of ionization degrees, whereas the association of pure n BA B-blocks is kinetically frozen. ,− In water, these polymers form above the percolation concentration visco-elastic liquids with a terminal relaxation time that increases sharply with decreasing α for α<0.6 and for α<0.4 they behave as self-supporting hydrogels. The transition between a low viscosity liquid and a hydrogel takes place in the pH range 5.5–4.5.…”