Pseudo-Equilibrium Mechanical Properties

In gels with permanent chemical tetrafunctional cross-links, the equilibrium shear elasticity would be expected to be given by equation 33 of Chapter 10, 

In other cases, the changes with time are slower, as for polyacrylonitrile in dimethyl acetamide, cellulose trinitrate in isophorone, and poly(vinyl chloride) in di-2-ethylhexyl phthalate. In the latter, and in other polymers with structural irregularity due to lack of tactic order or copolymerization, the degree of crystallinity is presumably limited and the increase in Gg with time eventually be- 

Pseudoequilibrium shear modulus plotted against time for 20% gels of polyacrylonitrile in dimethyl formamide, aged at the three temperatures indicated. (Bisschops. )- 

Pseudoequilibrium Young s modulus plotted logarithmically against polymer concentration by volume for gels of poly(vinyl chloride) in di-2-ethylhexyl phthalate near room temperature, 

The concentration dependence of the pseudoequilibrium modulus can be expressed by a power relation over a moderate range of concentration, as illustrated for gels of poly (vinyl chloride) in Fig. 17-42. The slope of the linear portion of this doubly logarithmic plot is 3.47. Slopes between 3 and 4 are obtained for poly(vinyl chloride) gels in many other solvents. For gelatin gels in water, on the other hand, the slope of this plot is usually close to 2. 

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