Temporary network structures

In this Section, the attention is focused on properties of the tramsverse relaxation of protons attached to polymer molecules it is sensitive to the presence of temporary network structures in molten polymers. Any high polymer melt is picttjred as an ensemble of chain segments with temporarily fixed ends. 

n) denote the residual spin Hamiltoniein in the case of dipole-dipole interactions H (p, n) is expressed as 

Equation (1) applies to weakly stretched segments the statistical framework of description appears through the expression of a(n)2. For Gaussian segments  

Considering one chain segment characterized by a normahzed p stretching vector, and a number n of skeletal bonds, the transverse magnetic relaxation function of nuclear spins, attached to this segment, is generally expressed as a product 

The contribution to the relaxation process induced by the presence of residual interactions of spins is given by 

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The plateau region appears when the molecular weight exceeds Mc [(Mc)soln. for solutions], and is taken to be a direct indication of chain entanglement. Indeed the presence of a plateau may be a more reliable criterion than r 0 vs M behavior, especially in solutions of moderate concentration where viscosity may exhibit quite complex concentration and molecular weight behavior. It is postulated that when M greatly exceeds Mc, a temporary network structure exists due to rope-like interlooping of the chains. Rubber-like response to rapid deformations is obtained because the strands between coupling points can adjust rapidly, while considerably more time is required for entire molecules to slip around one another s contours and allow flow or the completion of stress relaxation. 

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 network 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. Section 4 is devoted to the analysis of the effect of entanglement relaxation on NMR properties. 

NMR investigations into properties of the temporary network structure formed in polybutaiene were made by varying both the modulus of temporary elasticity and the polymer concentration ( >. 

The presence of a temporary network structure manifests itself through the first factor in the right hand side of Eq. 17. It comes from the h (t) component of the relaxation function. The second factor results from fast non-isotropic motions of monomeric imits which give rise to the (t r(t) function defined in equations (6) and (9). It must be noticed that the reference temperature Ti = Tg(,xi,2) +50 which appears in NMR properties, is close to the reference temperature Tj introduced from viscoelastic measiurements [11]. The second factor reflects a free volume effect it depends necessarily on an expansion coefficient which is about equal to 10-3K. 

When a high molecular weight HA dissolves at 1%, the solution becomes gel-like and exhibits significant viscoelastic behavior. As previously stated, this is because HA molecules form temporary network structures... 

Rubber-like models take entanglements as local stress points acting as temporary cross finks. De Cloizeaux [66] has proposed such a model, where he considers infinite chains with spatially fixed entanglement points at intermediate times. Under the condition of fixed entanglements, which are distributed according to a Poisson distribution, the chains perform Rouse motion. This rubber-like model is closest to the idea of a temporary network. The resulting dynamic structure factor has the form ... 

Evaluation of the above for a block copolymer with uniform random SI copolymer end-blocks with 60% styrene, leads to an estimate of Tr of about 90 C, well below the typical vulcanization temperatures of 145-160 C. The indication is that the proposed scheme of developing a temporary network which dissociates during the final vulcanization is feasable. This situation would be the epitome of a truely functional use of block polymers since, in this case, the block copolymer structure itself as well as its identity cease to exist after the function has been accomplished. 

Kisperska-Moroh (2010) proposed another definition which is an organization that is subject to constant changes, demonstrating a specific potential when required, and overcoming time and space barriers. VSCs could be described by such attributes such as temporary character, focus on customers, geographical dispersion, and intensive support of IT systems, network structure, and an extensive use of key competencies of their members. 

The strategies presented so far in this chapter rely on a common concept polymers with a network structure based on reversible covalent bonds. Healing is permitted by a temporary transition from a thermoset state to a thermoplastic behavior with enough chains mobility at ambient temperature. Considering this, alternative routes based on the same concept could be... 

A noteworthy finding has been that all the materials show two distinct relaxation dynamics, a fast and a slow relaxation [60]. The fast mode corresponds to relaxation of bulky polymer molecules, while the slow mode is related to relaxation of the filler structure with much longer time scales. As silica particles are physically connected with adsorbed polymer molecules, the formed polymer-particle network is a temporary physical network. On a long time scale, relaxation of this network occurs when immobilized polymer molecules connecting silica particles become free, via dissociation from silica particles or disentanglement from other immobilized polymer molecules. 

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