Local stiffness approximation

Another interesting feature is displayed by the molecular dynamics simulation concerning the local flexibility of intrinsically flexible polyion chains. On a local scale where the OSF theory introduces the local stiffness approximation (see Sect. 4.1.3) the simulations reveal quite a large flexibility of the polyion beads originating from entropic fluctuations. It is only at larger distances that these locally flexible blobs become correlated to expanded structures. This observation more or less confirms the basic criticism [252] of the OSF model already discussed in Sect. 4.1.3. 

As it is known [2, 12], within the frameworks of cluster model the elasticity modulus E value is defined by stiffness of amorphous polymers structure both components local order domains (clusters) and loosely packed matrix. In Fig. 13.3, the dependences E(v J are adduced, obtained for tensile tests three types with constant strain rate, with strain discontinuous change and on stress relaxation. As one can see, the dependences E(yJ are approximated by three parallel straight lines, cutting on the axis E loosely packed matrix elasticity modulus E different values. The greatest value E is obtained in tensile tests with constant strain rate, the least one - at strain discontinuous change and in tests on stress relaxation E = 0 [1]. 

Figure 6.10. Sketch of force response of free random coil and adsorbed chain in dependence on the coil strain related to the maximum elongation, 100% 0.81 Nh. Whilst free coil exhibits Gaussian response under given low strain approximately up to 20%, adsorbed loops and bridges, represented by A = 10 and 100, are highly Langevineau with incremental stiffness considerably increased. Strain induced perturbation of interphase accompanied by desorption of trains causes significant local stress and strain relief and, consequently, a lower stiffness...

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