Flexible-chain polymers phase equilibria

Special attention should be turned to the sharp transition from a narrow concentration corridor to a broad heterophase region, mentioned above, which takes place for low positive values of parameter x- It is int esting to compare the appearance of this broad region with the phenomenon of decomposition of solutions of flexible-chain polymers into two phases with the formation of two liquid (amorphous) phases with values of x in the limit (with infinitely high molecular weight of the polymer) of 0.5. TTie phase equilibrium diagrams (in coordinates v-x) for a rigid-chain polymer with an axial ratio of x = 150 and 350... 

However, it was recently found that many flexible-chain polymers containing no mesogenic groups are capable of forming thermotropic ordered phases in thermodynamic equilibrium and intermediate in structure and properties between crystalline and amorphous without any additional orienting effect of external fields. 

Because of the profound influence that the overall shape of a polymer chain undoubtedly has on any orientation process, which involves rotation of the whole molecule, it is worthwhile digressing on the subject of equilibrium conformation. Flexible, linear polymers tend to be somewhat coiled-up in the liquid phase or in a solution. In a good solvent, polymer-solvent contacts are preferred energetically to polymer-polymer contacts and so the coils... 

It occurred that the term phase extended onto microscopic objects has a rather conditional meaning. Because of this the main criterion used to separate colloidal systems and polymer solutions, i.e., viewing true solutions as thermodynamically equilibrium systems, was no longer significant. The studies on polymer solutions indicated that the latter often contained molecular aggregates and that ideal solutions obeying the statistical theory of dissolution of flexible chains, were rarely encountered. 

The first group includes liquid-crystalline systems based on polymers containing mesogenic groups in the main chain wh they alternate with flexible firagments, or in the side chains where they are joined to the main chain by flexible spacers. Phase equilibrium between these systems is due to thermal transitions from the crystalline state to some type of liquid-crystalline phase and subsequently to an isotropic melt. 

The crystallization process of flexible long-chain molecules is rarely if ever complete. The transition from the entangled liquid-like state where individual chains adopt the random coil conformation, to the crystalline or ordered state, is mainly driven by kinetic rather than thermodynamic factors. During the course of this transition the molecules are unable to fully disentangle, and in the final state liquid-like regions coexist with well-ordered crystalline ones. The fact that solid- (crystalline) and liquid-like (amorphous) regions coexist at temperatures below equilibrium is a violation of Gibb s phase rule. Consequently, a metastable polycrystalline, partially ordered system is the one that actually develops. Semicrystalline polymers are crystalline systems well removed from equilibrium. 

Casassa, ET. Equilibrium distribution of flexible polymer chains between a macroscopic solution phase and small voids. J. Polym. Sci. Polym. Lett. Ed. 1967, 59 (9), 773-778. 

Flexible polymers in poor solvent show a quasi-ideal behavior at a certain compensation point 0. At a slightly lower solvent quality, phase separation occurs, and there is an equilibrium between a nearly pure solvent phase (containing a few chains, each being severely contracted) and a polymer-rich phase. The latter can be described by the Floiy-Huggins theory. At the onset of phase separation (at the critical point) each polymer coil behaves like one individual argon atom at the liquid-gas transition of argon, and the Flory-Huggins approximation is not valid. 

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