Osmotic pressure rubber elasticity

We can express the free energy of gel in terms of the above quantities, and calculate from it the osmotic pressure 7t acting on the network. For a gel to be in equilibrium with the outer solvent, n must be zero. Several different mechanisms are known to contribute to 7t, the mixing process, the rubber elasticity, and counter ions, etc. The osmotic pressure due to mixing is expressed as... 

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. 

The osmotic pressure of the polystyrene solution is balanced by the elastic retractive forces of the more or less densely crosslinked rubber lamellae osmotic pressures in the swollen rubber particles are rather high because of the high positive virial coefficient of polystyrene in toluene (A2 = 5 X 10"4 mole cm3 g"2) (14). For SI = 10 (corresponding to c = 0.1 g X cm3) and M = 5 X 104, one... 

The present short treatise, which is intended merely to illustrate general principles of internal molecular statistics and to indicate their application to rubber elasticity, was chosen in view of this latter point and of the possibility of explaining certain features of the viscous flow of high polymeric substances (compare page 290). It may, however, be remarked that many other characteristic constants of high polymeric substances— osmotic pressure, double refraction etc.—are related to the internal mobility of long chain or reticulate structures. 

This chapter is about semidilute solutions, c > c. We learn both thermodynamics and dynamics. The properties of semidilute solutions are drastically different from those of dilute solutions. With a mere tenfold increase in the concentration, the osmotic pressure can easily increase by a factor of several hundred. In the ideal solution, in contrast, the osmotic pressure is proportional to c. Furthermore, the overall chain motion is slow in semidilute solutions because the chains are entangled semidilute solutions of a high-molecular-weight polymer can barely flow. The solutions are highly viscous and may even behave like elastic rubber. 

If water-soluble substances are introduced when compounding the rubber, moisture will eventually diffuse to the water-soluble particles and form solution droplets within the rubber matrix. The difference in the vapour pressure of the internal solution and the vapour pressure of any external water phase then becomes the driving force for further infusion of water. When the rubber matrix is vulcanised, the elastic modulus of the rubber network exerts pressure on the internal solution, and when this pressure equals the osmotic pressure of the internal solution, equilibrium is established and absorption ceases.Consequently, when compounding for minimum water absorption it is important to minimise the introduction of electrolytic substances into the compound and to select the grade of butyl rubber and curing system which will give the highest elastic modulus consistent with other performance needs. 

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