Mechanisms of Chemomechanical Reactions

There are several mechanisms which are essentially different in chemomechanical reactions of polymer molecular level structure changes, polymer conformational change, change of polymer-polymer interactions and the change of crosslinked structure in polymer gels. 

As for the change at the molecular level, azobenzene is known to show an isomerization change from trans to cis by photostimulation. This reaction causes a shortening of the molecule and a change of interaction with the neighboring molecules [21]. 

Polyelectrolytes change their conformation with the degree of dissociation which is varied with pH, temperature, polarity of solvent and ionic strength. 

Poly(acrylic acid) is a typical polyelectrolyte and shows a dissociation reaction. When pH is high, the polymer chain is charged negatively and extends itself due to coulombic repulsion and thermal motion of counter ions which pull the charged side chains. 

Flory [3] formalized the equation of state for equilibrium swelling of gels. It consists of four terms the term of rubber-like elasticity, the term of mixing entropy, the term of polymer solvent interaction and the term of osmotic pressure due to free counter ions. Therefore, the gel volume is strongly influenced by temperature, the kind of solvent, free ion concentrations and the degree of dissociation of groups on polymer chains. 

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