Critical association and thermoreversible gelation of some selected polymers

Abstract

The study of associating polymers is complicated by the fact that at a finite concentration only an a parent molar mass is directly measured. This apparent molar mass deviates From the true one because of thermodynamic interaction. In a good solvent the repulsive interaction can be fully described in terms of the second virial coefficient, which can be measured at very low concentrations, and a correction can be made for the true molar mass. The technique has been successively applied to P-galactosidase in a phosphate buffer and to cellulose 2,Sacetate in acetone. The gel point could be determined, and critical behavior was found in very good agreement with percolation prediction. Two other methods were employed and tested. One is based on the condition that the longest relaxation time must diverge at the gel point, and correspondingly the translational diffusion coefficient should go to zero. The second criterion consists of the prediction that power law behavior should be observed for the time correlation function of dynamic light scattering as well as for the frequency dependent storage and loss moduli.These predictions were indeed found to hold for two polysaccharides which form thermoreversible gels in water at a fairly low salt concentration. The two polysaccharides are the Tamarind seeds polysaccharide in 1M Na,SO and an exopolysaccharide from Rhizobium leguminosarum, strain 8002 in 0.lk NaCl solutions respectively.

Coupling model analysis of polymer dynamics in solution: Probe diffusion and viscosity

“…19,20,22 In these systems, the stretching exponent ␤ was generally found to decrease with increasing polymer concentration. Furthermore, in associating polymer systems in the pre-gel state, from the published data 45,46 ␤ is found to decrease as temperature is lowered toward the gel temperature T gel . These observed dependencies of ␤ on c and T are consistent with the coupling model: increases in c or reductions in T lead to an increase in the dynamic constraints on the diffusing units, thereby increasing the coupling parameter n and decreasing the stretching parameter ␤.…”

“…These observed dependencies of ␤ on c and T are consistent with the coupling model: increases in c or reductions in T lead to an increase in the dynamic constraints on the diffusing units, thereby increasing the coupling parameter n and decreasing the stretching parameter ␤. KWW , whether determined by light scattering from optical probes 13 or from neutron scattering from segmental motion in bulk polymers 46 is found experimentally to have the q-dependence given by Eq. ͑3͒.…”

Superhydrophobicity of polyvinylidene fluoride induced by integrating liquid-exfoliated hexagonal boron nitride nanosheets

“…As reported, the formation of physical gels depends on the cooperative effect of liquid-liquid demixing and solid-liquid demixing. 37 In the current case, the formation of porous PVDF is dominated by a liquid-liquid demixing, while the formation of porous h-BNNS/PVDF is dominated by a solidliquid demixing where h-BNNSs can act as crystallization nuclei (Figure 3(b)(II)), thus promoting solid-liquid demixing. 17,19 Clearly, the surface morphology of the final product can be changed by adding h-BNNSs to PVDF to influence their crystallization.…”

Valence, loop formation and universality in self-assembling patchy particles

“…In our prior paper, we suggested this could be done using light scattering following the approach of Burchard. 46,47 However, we have since found that the required assumptions are incompatible with our simulations, as scattering curves can be extracted from simulation and the predicted molecular mass following Burchard is not consistant with the directly measured mass. An attractive, alternative approach may be sedimentation velocity analytical ultracentrifugation (AUC) 48–51 experiments, which can determine the entire cluster size distribution.…”