Topological restraints

This is not necessarily the case at the molecular level where the efficiency of permanent chain extension can be influenced by factors like chain relaxation, topological restraints, chain size and distribution, dissipation of frictional heat, chain repeating structure, and chain slippage. An expression that accounts for chain slippage and provides a measure of true molecular extension has been proposed by Porter et al. ... 

Following the work of Mishra and co-workers (53), Dean and co-workers (54,55) investigated the SIN syntheses of dimethacrylate- and epoxy-based materials by near-infrared and other methods. The rates and order of the polymerizations were systematically altered. The final conversion of the dimethacrylate was limited by the vitrification or topological restraint of the SIN. Whether one or two glass transitions are observed depends on the relative rates and order of the polymerizations. 

At higher concentrations, where the molecules are substantially entangled, the non-Newtonian behavior is enormously amplified, and 77/770 may fall by several orders of magnitude. This phenomenon, as in undiluted polymers, is associated with topological restraints that can be represented by entanglement coupling or tube constraints, and will be discussed in Chapters 10,13, and 17. 

FIG. 10-13. Models of topological restraints. (I) virtual tube (II) point obstacles (III) slip link.s. (Doi and Edwards. ) Reproduced, by permission, from Journal of the Chemical Society Faraday Transactions II. 

It may be concluded that when entanglements are present the molecular motions do not correspond to the normal modes of the Rouse theory. Indeed, the topological restraints must necessitate very different motions to achieve configurational rearrangements, and other models have been proposed to describe them. 

Although at low shear rates and low frequencies, respectively, the non-Newtonian viscosity v and the dynamic viscosity rf reflect similar characteristic times, it would not be expected that and j/(-y) should be similar functions at large values of their respective arguments, since steady flow at a high shear rate involves very different molecular motions from oscillating flow at low amplitudes where escape from topological restraints is not required. However, it has been frequently observed that 1j7 ( ) closely resembles v y). Here j (< )) = = (7 /(d,... 

The qualitative interpretation of these phenomena has been given here in the vocabulary of entanglements with the alternative tube model of topological restraint, the theory of Doi and Edwards also predicts this phenomenon and is in qualitative agreement with experiment. 

---