Collective Cohesion Forces in Polymers

Abstract

Collective cohesion forces are a set of normal cohesion forces the origins of which exhibit a regularity in their spatial distribution. Atoms, molecules or segments that undergo their action display collective coupled motions where the neighbours do not change or identity. Such particular kind of motion can be called “rigidity”. The repetitive character of the polymeric chain renders the segments particularly susceptible to generate collective cohesion forces but this tendency is not the same for each of them. Rigid segments have a lower chemical potential than fluid segments. This is the case for the crystalline parts of the semi-crystalline polymers. The tendency of some sequences to generate collective cohesion forces is so great that in most cases rigid domains are spontaneously formed where the spatial regularity does not extend to the three dimensions, what makes them less detectable by diffraction methods. But these rigid domains being connected with other zones deprive the last ones from macroscopic mobility and no crystallization can take place. Moreover, as evidenced by dipolar measurements, even in true solutions given sequences of segments are regularly involved in ephemeric rigid domains, and have therefore a lower chemical potential. Phase separation occurs when, as a consequence of the change of solvent or temperature, some rigid domains become permanent. As a consequence of this inherent inequality between the segments, the criterion of phase separation in polymer solutions is the existence of two states α and β in which the lowest chemical potential or the segments is the same. This fundamentally differs from the liquid-liquid treatment of Flory.