Chain uncoiling

Chain uncoiling, and the converse process of coiling, is conveniently considered as a unimolecular chemical reaction. It is assumed that the rate of uncoiling at any time after application of a stress is proportional to the molecules still coiled. The deformation Dhe(0 at tinie t after application of stress can be shown to be related to the equilibrium deformation Dhe( ) by the equation... 

Figure 3.2. Application of stress to a highly elastic body. Rate of chain uncoiling with time...

If we consider the total deformation (T>totai) occurring during flow to be almost entirely composed of a viscous flow (Dyisc) and a high elastic deformation due to chain uncoiling then we may write... 

A deformation due to chain uncoiling which is not instantaneous and whose rate depends on temperature (high elastic deformation, Dhe)-... 

Quite large elastic strains are possible with minimal stress in TPEs these are the synthetic rubbers. TPEs have two specific characteristics their glass transition temperature (7 ) is below that at which they are commonly used, and their molecules are highly kinked as in natural TS rubber (isoprene). When a stress is applied, the molecular chain uncoils and the end-to-end length can be extended several hundred percent, with minimum stresses. Some TPEs have an initial modulus of elasticity of less than 10 MPa (1,500 psi) once the molecules are extended, the modulus increases. 

The thermodynamic analysis of conformational and structural transformations in the melt at high pressures34 showed that the free volume and free energy minimum required for hydrostatic compression is attained as a result of the transition of the molecules in the melt into a more extended conformation (gauche —> trans transitions) since the extended molecules ensure a more compact packing of the chains at compression. Chain uncoiling leads to a decrease in their flexibility parameter f with increasing pressure p ... 

The conformations adopted by polyelectrolytes under different conditions in aqueous solution have been the subject of much study. It is known, for example, that at low charge densities or at high ionic strengths polyelectrolytes have more or less randomly coiled conformations. As neutralization proceeds, with concomitant increase in charge density, so the polyelectrolyte chain uncoils due to electrostatic repulsion. Eventually at full neutralization such molecules have conformations that are essentially rod-like (Kitano et al., 1980). This rod-like conformation for poly(acrylic acid) neutralized with sodium hydroxide in aqueous solution is not due to an increase in stiffness of the polymer, but to an increase in the so-called excluded volume, i.e. that region around an individual polymer molecule that cannot be entered by another molecule. The excluded volume itself increases due to an increase in electrostatic charge density (Kitano et al., 1980). 

Conformation depends on the degree of ionization and concentration of the polyion, the type and concentration of the counterion and the interaction between counterion and polyion. Extension is favoured by low concentrations of counterion and polyion. Conformational change is also affected by the extent of the charge on the polyion. As the charge on a polyion increases, the chain uncoils and expands under the influence of repulsive forces. Thus, the neutralization of a polyacid is accompanied by... 

For real elastomers, however, the internal energy term (dU jdl )t cannot be exactly zero, since chain uncoiling would require that the bond rotational energy barriers are overcome. 

When a rubber is deformed, its entropy S changes. The long chains tend to adopt a most probable configuration this is a highly coiled configuration. When the material is stretched, the chains uncoil, resulting in a less probable chain configuration. When the force is removed, the system wants to return to the more probable coiled-... 

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