Transport Phenomena in Gel.

Masayuki Tokita

doi:10.3390/gels2020017

Abstract

Gel becomes an important class of soft materials since it can be seen in a wide variety of the chemical and the biological systems. The unique properties of gel arise from the structure, namely, the three-dimensional polymer network that is swollen by a huge amount of solvent. Despite the small volume fraction of the polymer network, which is usually only a few percent or less, gel shows the typical properties that belong to solids such as the elasticity. Gel is, therefore, regarded as a dilute solid because its elasticity is much smaller than that of typical solids. Because of the diluted structure, small molecules can pass along the open space of the polymer network. In addition to the viscous resistance of gel fluid, however, the substance experiences resistance due to the polymer network of gel during the transport process. It is, therefore, of importance to study the diffusion of the small molecules in gel as well as the flow of gel fluid itself through the polymer network of gel. It may be natural to assume that the effects of the resistance due to the polymer network of gel depends strongly on the network structure. Therefore, detailed study on the transport processes in and through gel may open a new insight into the relationship between the structure and the transport properties of gel. The two typical transport processes in and through gel, that is, the diffusion of small molecules due to the thermal fluctuations and the flow of gel fluid that is caused by the mechanical pressure gradient will be reviewed.

Highlights

Gel has the characteristics and common structure that consists of the three-dimensional polymer network and gel fluid
K B and T represent the Boltzmann’s constant and temperature. It is clear from the above equation that the diffusion coefficient of the probe molecule is essentially determined by the size of the probe molecule, R h, when it is dissolved in a simple fluid at the constant temperature
We review the probe diffusion of gel and the hydrodynamic friction of the polymer network

Summary

Introduction

Gel has the characteristics and common structure that consists of the three-dimensional polymer network and gel fluid. The detailed studies on the transport of substances by the diffusion and the flow of gel fluid are still required for the full understanding of the volume phase transition of gel and the pattern formation in gel. If the appropriate chemical structure is given to the polymer network of gel, gel shows the volume phase transition phenomena. The correlation length of gel varies with the concentration of the linear chain component, the cross-linking density, and the environmental conditions such as the temperature. The concentration of gel can be changed widely from 0.02 to 0.5 g/mL because of a high affinity with water This is a definite advantage for the present purpose of the study because the correlation length of the polymer network of gel can be changed widely. The critical behaviors of the frictional property can be revealed by studying the frictional property of PNIPA gel in the vicinity of the volume phase transition point

Probe Diffusion in Gel

Friction of Gel

Friction of Non-Uniform Gel

Friction of Colloid Gel

Findings

Conclusions