Rheological behaviour linear polyethylenes

Indeed, at very high polymer concentrations enhancement of these effects occurs. Recent studies by Franks et al. 23) on the rheological behaviour and freeze fracture electron microscopical analysis of several synthetic linear flexible polymers, including poly(vinylpyrrolidone) and polyethylene glycol) in concentrated solutions, suggest that these molecules do not form a network mesh but rather exhibit aggregation. Anionic polysaccharides, on the other hand, are known to form an anisotropic packing array in condensed films. These films may be stretched to enhance orientation and be used for X-ray diffraction studies... 

We study here the case of a branched polyethylene (LDPE FN 1010), characterized by long chain branching, whose rheological behaviour has been previously described in Chapter II-l. Compared to linear LLDPE, it exhibits strain-hardening in elongational situations and higher values of first normal stress difference. 

The archetypical example of a branched polymer is low density polyethylene (LDPE), the product of radical polymerization at high temperature and pressure. That LDPE is significantly branched was first suspected because of the influences of polymerization conditions upon the crystallinity of the polymer and upon its rheological behaviour in the melt and in solution. Confirmation was provided by IR analysis which indicated a considerable excess of methyl groups ideally, the maximum number of such groups would be two per molecule, corresponding to the end-groups of a linear alkane. 

Patil. N.. Balzano, L., Portale, G., and Rastogi. S. (2009) Influence of nanopartides on the rheological behaviour and initial stages of crystal growth in linear polyethylene. Macromolecular Chemistry and Physics,... 

It appears, then, that the mechanical degradation process is intimately connected with the molecular structure of the macromolecule and the resulting fluid rheology that arises from this structure. For a flexible coil macromolecule, such as HPAM or polyethylene oxide, the polymer solutions are known to display viscoelastic behaviour (see Chapter 3) and thus a liquid relaxation time, may be defined as the time for the fluid to respond to the changing flow field in the porous medium. It may be computed from several possible models (Rouse, 1953 Warner, 1972 Durst et al, 1982 Haas and Durst, 1982 Bird et al. 1987). The finite extendible non-linear elastic (FENE) (Warner, 1972 Bird et al, 1987a Haas and Durst, 1982 Durst et al, 1982) dumbbell model of the polymer molecule may be used to find the relaxation time, tg, as it is known that this model provides a good description of HPAM flow in porous media (Durst et al, 1982 Haas and Durst, 1982) the expression for fe is ... 

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