Swelling retraction

Some theories have been put forward relating muscle contraction with swelling retraction but it seems rather doubtful whether the assumption of so a simple analogy is justified. 

If stretched model filaments showing a permanent extension are allowed to swell in a suitable swelling agent, considerable retractions at once occur. The filaments become shorter and simultaneously their volume increases. This phenomenon was termed swelling retraction. Though a complete reversal of the extension can be only reached under extreme and difficuldy realisable conditions, the contraction, as far as it goes, actually represents a true partial reversal of the process of elongation. ... 

It has been shown in Chapter XI that the force of retraction in a stretched network structure depends also on the degree of cross-linking. It is possible therefore to eliminate the structure parameter ve/Vo) by combining the elasticity and the swelling equations, and thus to arrive at a relationship between the equilibrium swelling ratio and the force of retraction at an extension a (not to be confused with the swelling factor as). In this manner we obtain from Eq. (XI-44) and Eq. (39)... 

This equation calls attention to the well-established inverse relationship between the degree of equilibrium swelling of a series of rubber vulcanizates in a given solvent and the forces of retraction, or moduli, which they exhibit on stretching. The indicated approximate dependence of qm on the inverse three-fifths power of the modulus has been confirmed. 

Fig. 135.—The relationship between the equilibrium retractive force T(x (in lbs./in.2) at 241 °C for various multilinked poly (e-caproamides) at the extensions (a) indicated, and their equilibrium swelling ratios in m-cresol at 30°C. O, tetralinked polymers octa-linked polymers. The lines have been calculated according to Eq. (41), with appropriate revision for the octafunctional case (broken lines), an arbitrary value being assigned to the parameter Xi for each elongation. (Schaefgen and Flory.33)...

Rubber A maferial fhaf is capable of recovering from large deformations quickly and forcibly and can be, or already is, modified fo a sfafe in which if is essentially insoluble (but can swell) in boiling solvent such as benzene, methyl ethyl ketone, and ethanol-toluene azeotrope. A rubber in its modified state, free of diluents, retracts within 1 min to less than 1.5 times of its original length after being stretched at room temperature to twice its length and held for 1 min before release. 

As pointed out earlier, an elastomer cross-linked above its gel point will not dissolve in a solvent, but will absorb it and swell. The swelling will continue until the forces of swelling balance the retractive forces of the extended chains of the network. The cross-link density can then be calculated from the degree of swelling using the Flory-Rehner Equation ... 

If the freely swollen gel is subjected to a unidirectional strain, its degree of swelling will change from q to q and a retractive force will arise, similar to the dry case given by Eq. (III-ll)... 

Both copolymers were cross-linked by copolymerization with (1-2%) ethylene dimethacrylate the effect of irradiation was followed by measuring the change of elastic retractive force at constant elongation. In the first case the contraction upon irradiation of the gel is mainly due to changes of chain conformation and swelling equilibrium induced by trans-cis photoisomerization. In the second case by comparing the retractive force for irradiated and unirradiated samples the heat effect was evaluated, and the photoinduced contraction estimated to 1%. 

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