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By Gould R.F. (ed.)

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The relaxation will be described from s h o r t to long times following a step strain. It is assumed that, at the end of the step strain, the Rouse chains a n d s u b c h a i n s b e t w e e n e n t a n g l e m e n t p o i n t s are affinely deformed. 1. l , i n e a r c h a i n s The first r e l a x a t i o n process (designated A h e r e a f t e r ) corresponds to a Rouse-like relaxation of chain segments between e n t a n g l e m e n t points. It is a s s u m e d t h a t the e n t a n g l e m e n t points r e m a i n fLxed d u r i n g the time-scale of this relaxation and t h a t no diffusion of monomers t h r o u g h the slip-links is allowed in such short times.

The principle of the NMR approach to semi-local properties of polymeric melts is considered in Section 2 ; it is shown how the existence of a temporary n e t w o r k structure is detected from the relaxation of the transverse magnetisation of protons attached to chains. The observation of segmental motions from the longitudinal relaxation of proton magnetisation is described in Section 3 ; it is also shown how local motions in concentrated polymeric solutions can be probed from the diffusion process of small molecules.

3{2pz 2 - py2 _ px2 } A ( a / a ( n ) ) 2 ~ , (1) where the steady magnetic field direction is along the Z-axis; a is the mean skeletal bond length and c(n) 2 is the mean square end-to-end distance between segment ends [7]. The normalized p vector is defined as p = r/c(n). Orientational correlations of three skeletal bonds, located along one segment, are involved in the definition of the A parameter ; this is called the second order stiffness of the i chain [7]. The H D spin-spin interaction reads i HD = Z Ap,p,Bp,p, , p,p' (2) the sum is extended to all p and p' nuclear spins located on the chain segment; Ap,p, spin operators are supposed to commute with the Zeeman energy and B p,p' numerical factors result from transformations of spherical harmonics from monomeric unit frames to the reference frame ascribed to the r elongation vector of the chain segment.

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Advanced Propellant Chemistry by Gould R.F. (ed.)


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