Polyethylene conformational changes

A natural development of this work was to study the effects of structural perturbations like conformational changes on the valence bands of stereoisomers. We have considered the four conformations relevant to the interpretation of the vibrational spectrum of polyethylene (J ). The theoretical calculations on those four conformers of polyethylene T, TG, G and TGTG revealed so important differences in the shape of densities of states that... 

This was expertly demonstrated by Angle Resolved UPS (ARUPS) experiments [39] at the beginning of the 1990s, as illustrated in Fig. 36.6. A natural continuation of this work was to study the effects of structural perturbations such as conformational changes on the valence bands of stereoisomers. To illustrate this approach, a computer experiment has been conducted in the case of polyethylene [40]. A vibrational analysis [41] of crystalline samples suggests the possibility of four conformations of polyethylene trans (T), gauche... 

Gauche-trans equilibrium or other conformational changes which produce an increase beyond the vibrational heat capacity of the solid polymers, as documented with Fig. 2.65 and described in Sect. 5.3 for polyethylene. 

The opposite is tme of cyclic structures in the backbones, as was shown in poly(ethylene terephthalate). Actually, cyclic structures not only inhibit conformational changes but can also make crystallization more difficult. Among the polymers of a-olefins the structures of the pendant groups can influence the melting point [6]. All linear polyethylene melts between 132 and 136°C [7]. Isotactic polypropylene, on the other hand melts at 168°C [8]. 

Many polystyrene fractions are available as probe polymer in nonaqueous systems. For aqueous systems, we can use the dextran-oligosaccharide series and poly(ethy1ene oxide) - polyethylene glycol series (polymers that are substantially neutral and with which there will be little possibility of adsorption or conformational change due to the presence of electrolytes). For the latter polymers, the relations between the Stokes radius and molecular weight have been collected as in Fig. 6. 

Statistical thermodynamics asks how many conformational changes are involved in the melting process. If the polyethylene is crystallized in the all... 

Stoeva, S., Stmctural and conformational changes in sohd-phase chlorinated polyethylene, J. Appl. Polym. Sci. 2004,94 189-196. 

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