Segmental attraction

Fig. 1 Anisotropy of the principal elastic interactions in a C-C chain. (1) Bond stretching (2) Bond angle bending (3) Bond rotation (4) Inter-segmental attraction...

For most polymer-solvent combinations i > 0 because of a net segment-segment attraction. This implies that the energy effect opposes the entroplcally driven dissolution of the polymer in the solvent. Such solutions are still thermodynamically stable unless x becomes too high see sec. 5.2e. If > 0 the excluded volume is smaller than the real volume (= ] of a segment. FloryH, Edwards ), De Gennes ) and others have shown that the excluded volume may be written as v . where the dimensionless excluded volume parameter v is defined as... 

The segmental attraction hypothesis It was pointed out in Section 3.2.7 that in dispersion media that are poorer solvents than -solvents for the polymer, the macromolecular segments experience a mutual attraction. This attraction is responsible for the negative deviations from ideal behaviour displayed by polymer chains in worse than d-solvents (see Fig. 3.3). If the solvency for the polymer is made suitably worse than that of a 0-solvent, phase separation ensues (Flory, 1953). 

Evidence favouring the segmental attraction hypothesis Until recently, the evidence in favour of the hypothesis that the segments composing the stabilizing moieties become attractive in slightly worse than 6-solvents, was mainly indirect. Certainly the data presented in Tables 6.2 and 6.3 are too imprecise to establish whether the CFPT lies to the worse solvent side of the 0-point. 

The fact that the second virial coefficient of polymer chains becomes negative in worse than 0-solvents is indicative of mutual segmental attraction (Flory, 1953). Moreover, as discussed in Chapter 8, when sterically stabilized particles coagulate under the action of van der Waals forces, an entirely different pattern of flocculation behaviour ensues. Specifically, flocculation occurs in dispersion media that are better solvents than 0-solvents, i.e. when the segments are undoubtedly mutually repulsive. 

The theta temperatore is a well-defined state of the polymer solution at which the excluded volume effects are eliminated and the polymCT coil is in an unperturbed condition (see Chapter 10). Above the theta tempraature, expansion of the coil takes place, caused by interactions with the solvrait, whereas below 0 the polymCT segments attract one another, the excluded volume is negative, the coils tend to collapse, and eventual phase separation occurs. 

The relation (4.32) relating xi2 to 0 shows that the excluded volume and Y are equal to 0 and F(Y) is equal to 1 at the temperature 0 even in a dilute medium. Thus the chains interpenetrate freely under 0 conditions. At temperatures higher than 0, the excluded volume takes positive values but at temperatures lower than 0, segment-segment attractions prevail and the excluded volume is negative. 

To take the interaction between polymer segments into account, Flory [1320] introduced the excluded volume parameter v in units of m. In good solvents (T > To), the polymer segments repel each other and v is positive. For many calculations, we simply approximate v= The excluded volume decreases with decreasing solvent quality to v = 0 for solvents. It decreases even further and becomes negative for poor solvents (T < T ), where the segments attract each other. 

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