Repulsive segment

Poor R = Repulsions segments-solvent collapse of the coils... 

AT=Tci-T = 8 °C where T is the temperature of the measurements and Tci is the doud point. DOP is a neutral solvent for PS and PB and weakens repulsive segmental interactions between PS and PB. ° The system can be approximately treated as a pseudo-binciry system where a phase separation between PS and PB occurs in the medium of DOP and the phase separation between the polymers and the solvent is insignificant. The pseudo-binary system is regarded to be equivalent to bulk systems when the segments of polymers in bulk are replaced by the blobs, as already pointed out at the beginning of Section 2.30.2.1. 

Hard-sphere models lack a characteristic energy scale and, hence, only entropic packing effects can be investigated. A more realistic modelling has to take hard-core-like repulsion at small distances and an attractive interaction at intennediate distances into account. In non-polar liquids the attraction is of the van der Waals type and decays with the sixth power of the interparticle distance r. It can be modelled in the fonn of a Leimard-Jones potential Fj j(r) between segments... 

Altematively, tire polymer layers may overlap, which increases tire local polymer segment density, also resulting in a repulsive interaction. Particularly on close approach, r < d + L, a steep repulsion is predicted to occur. Wlren a relatively low molecular weight polymer is used, tire repulsive interactions are ratlier short-ranged (compared to tire particle size) and the particles display near hard-sphere behaviour (e.g., [11]). 

Another important feature of some random copolymers is the abihty to achieve miscibility in either a homopolymer or a second random copolymer. This "copolymer effect" has been shown empirically for quite some time, eg, PVC is miscible with random copolymers of ethylene and vinyl acetate (52). Such systems are effective because repulsions between the dissimilar segments in the copolymer are enough to overcome the repulsions between these segments and those of the second component in the mixture. In other words, in the above example, the ethylene units "hate" vinyl acetate units more than either of them "hate" PVC. Thus there is a net negative interaction energy and the two materials are miscible (53). 

Fig. 2. Sketch of the interaction potential between segments m and n. The potential can be decomposed into a hard core repulsive potential Unm (hard) and a weak attractive potential Unn, (attr)...

In good solvents, the mean force is of the repulsive type when the two polymer segments come to a close distance and the excluded volume is positive this tends to swell the polymer coil which deviates from the ideal chain behavior described previously by Eq. (1). Once the excluded volume effect is introduced into the model of a real polymer chain, an exact calculation becomes impossible and various schemes of simplification have been proposed. The excluded volume effect, first discussed by Kuhn [25], was calculated by Flory [24] and further refined by many different authors over the years [27]. The rigorous treatment, however, was only recently achieved, with the application of renormalization group theory. The renormalization group techniques have been developed to solve many-body problems in physics and chemistry. De Gennes was the first to point out that the same approach could be used to calculate the MW dependence of global properties... 

Two segments of a given polymer molecule cannot occupy the same space and, indeed, experience increasing repulsion as they move closer together. Hence the polymer has around it a region into which its segments cannot move or move only reluctantly, this being known as the excluded volume. The actual size of the excluded volume is not fixed but varies with solvent and temperature. 

---