Molecular weight critical

Among the complications that can interfere with this conclusion is the possibility that the polymer becomes insoluble beyond a critical molecular weight or that the low molecular weight by-product molecules accumulate as the viscosity of the mixture increases and thereby shift some equilibrium to favor reactants. Note that we do not express reservations about the effect of increasing viscosity on the mobility of the polymer molecules themselves. Apparently it is not the migration of the center of mass of the molecule as a whole that determines the reactivity but, rather, the mobility of the chain ends which carry the reactive groups. 

To a large extent, the properties of acryUc ester polymers depend on the nature of the alcohol radical and the molecular weight of the polymer. As is typical of polymeric systems, the mechanical properties of acryUc polymers improve as molecular weight is increased however, beyond a critical molecular weight, which often is about 100,000 to 200,000 for amorphous polymers, the improvement is slight and levels off asymptotically. 

The first process begins as initiator decomposes in the diluent phase and polymerizes the monomer to form oligomers which precipitate from solution upon reaching a critical molecular weight. 

This critical molecular weight increases with the solubility of the polymer and is low enough so that all the oligomers are captured or nucleate particles before their radicals are terminated. As a result, nearly all polymerization takes place in the particles and the polymer concentration in the diluent phase is low. 

Although polymers exhibit both viscous and elastic responses at all temperatures, the elastic response is particularly strong at temperatures less than 50°C above the glass transition temperature, particularly for polymers well above their critical molecular weight. Polymers are often considered to have dominant viscous rheological responses if they are stressed at temperatures over 100 °C above the glass transition temperature for amorphous polymers or 100°C above the crystalline melting point for semicrystalline resins. 

The polyethylene linewidths, as illustrated in Fig. 1, clearly indicate that a constant value is attained in the melt between 1-8 X 10. Proton T2 measurements at 150°C indicate that there is a change in slope in this quantity as a function of molecular weight at about = 6 x 10. ( ) The critical molecular weight as determined from bulk viscosity measurements for this polymer has been given as 2 x 10 ( ) and 3.8 x 10. (5 ) These values are very close to those which would be deduced from the linewidth measurements. For polydimethyl siloxane the break in the proton T2-molecular weight curve occurs at about M = 5 x 10. (37) is about 2.5 x 10 from viscosity measurements. ( ) Fig. 5... 

For all the cases cited above, which represent those data for which a comparison can be presently made, there is a direct connection between the critical molecular weight representing the influence of entanglements on the bulk viscosity and other properties, and the NMR linewidths, or spin-spin relaxation parameters of the amorphous polymers. Thus the entanglements must modulate the segmental motions so that even in the amorphous state they are a major reason for the incomplete motional narrowing, as has been postulated by Schaefer. ( ) This effect would then be further accentuated with crystallization. 

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