Coil shrinkage

The explanation for this is that with increasing solvent power intermolecular repulsion becomes decisive. Enhancement of the polymer concentration leads therefore to coil shrinkage [64], whereas in the limiting case of a 0-solvent no concentration-induced shrinkage is to be expected. Raising c further leads to a critical concentration, c, at which the coils begin to overlap and interpenetrate. 

The range of semi-dilute network solutions is characterised by (1) polymer-polymer interactions which lead to a coil shrinkage (2) each blob acts as individual unit with both hydrodynamic and excluded volume effects and (3) for blobs in the same chain all interactions are screened out (the word blob denotes the portion of chain between two entanglements points). In this concentration range the flow characteristics and therefore also the relaxation time behaviour are not solely governed by the molar mass of the sample and its concentration, but also by the thermodynamic quality of the solvent. This leads to a shift factor, hm°d, is a function of the molar mass, concentration and solvent power. 

Simha and Zakin (126), Onogi et al (127), and Comet (128) develop overlap criteria of the same form but with different numerical coefficients. Accordingly, flow properties which depend on concentration and molecular weight principally through their effects on coil overlap should correlate through the Simha parameter c[ /], or cM , in which a is the Mark-Houwink viscosity exponent (0.5 < a < 0.8). If coil shrinkage, caused by the loss of excluded volume in good... 

A concomitant of coil shrinkage would seem to be a macroscopic shrinkage of the gel structure, called syneresis, if the crosslinking were carried out in solution. Syneresis is indeed observed at the later stages of crosslinking in highly diluted systems [see Refs. (17) and (257) for examples]. Some evidence of localized network inhomogeneities has also been adduced in polycondensation networks... 

Mahabadi and O Driscoll also studied the effect of dissolved polymer upon the termination rate coefficient [82, 83]. They derived a theoretical relationship for the dependence of kt upon conversion [82], based on a previously derived segmental diffusion model [100] that allowed for a concentration dependent linear expansion coefficient and polymer-solvent interactions. Also Mahabadi and O Driscoll pointed out that the rate of segmental diffusion would increase if the segment density gradient was increased as a result of coil shrinkage. In a simplified form, the dependence of kt upon polymer concentration, C, could be represented by ... 

From the arguments presented above we are led to consider with some confidence that the fractional power dependence of <(c) is a flexible polymer effect in semidilute solutions originated from the coil shrinkage due to interchain interactions. As the transition in the observed data are rather sharp the value of c could be located with good confidence. For the transition from a semidilute to a concentrated solution the value of c has been found to be in the order of 10 g/dl for all polymers examined, being roughly independent of molar mass. As an amorphous polymer has the same density value irrespective of its molar mass it is reasonable to... 

The fractional power dependence of < in the semidilute solution of flexible chain polymers is the manifestation of the polymer coil shrinkage with increasing concentration. This is true also for a flexible chain polymer in a 0-solution. 

We can imagine the coil tightening up as this point is approached from better conditions. This is not a shrinking to the vanishing point as suggested by the u = 0 criterion, but a contraction to the point where intramolecular exclusion effects are offset by shrinkage. 

Some effects due to higher concentrations have been discussed in the literature. For example, increasing concentration will, in general, lead to a shrinkage of the molecular coils, as increasing numbers of them occupy a given volume (44), Theories of this effect have been presented (45). 

Excimer formation in flexible chain polymers such as poly(ethylene terephthalate) is controlled by the shrinkage of the polymer coils which increases with increasing polymer concentration . Stacking... 

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