Thermokinetics of formation of ordered structures in polymer solutions and gel
ABSTRACT A general approach is proposed allowing the reduction to a ‘common denominator’ of different forms of formation of ordered structures in polymer–solvent systems. This ‘common denominator’ results from the combination of the equivalence principle of thermokinetics and the concept of diphility of macromolecules. In the first sections the discussion deals with a precise definition of order and with the foundation of the equivalence principle and related physical formalisms. It is shown that the consideration of any mode of structure formation can be reduced to the analysis of phase diagrams and free energy–temperature diagrams, the possibility of such reductions being connected with two main assumptions (supported by all known experimental data): i Due to inherent diphility the Flory–Huggins parameter χ 1 can change continuously or discontinuously with the concentration of components leading to the occurrence of preferential chain conformations or abrupt intramolecular conformational transitions. In cases of pronounced explicit diphility (in random or block copolymers) the preference of interactions also leads to intramolecular segregation which on increasing polymer concentration develops as a supermolecular order ii One can connect these conformational transformations with the dependence of Flory's flexibility parameter f on χ 1 . Taking into account this dependence makes it possible to interpret identically ‘antiplastification’ phenomena, the formation of liquid crystalline mesophases with different morphologies and the formation of binary crystals. Hence, various manifestations of molecular and supermolecular forms of ordering in solutions and gels can be treated by analogy with the usual polymorphism in polymers, in the same way as in the Flory–Mandelkern crystallization theory. The equivalence principle allows us to consider from quite similar positions the nucleation and growth of crystals or liquid crystals in the presence of external fields. In these cases the flexibility parameter f turns out to be dependent on the external field and all further considerations can be reduced to the already analysed situation with variable and interdependent χ 1 and f parameters. A more profound analysis shows that this equivalence is not formal but reflects the real state of events. The uncoiling of macromolecules in a shear or longitudinal field and the increase of their interactions means in reality the increase of the interaction parameter χ 1 and ultimately the loss of solubility, or desolvation. The analysis from those positions of a large amount of experimental data shows that in some cases organized structures may exist only in the presence of a solvent which stabilizes them thermodynamically rather than only stimulates kinetically their formation. When organized structures are formed due to external fields they can persist on the elimination of the field and removal of the solvent, on condition that the energy ‘pumped over’ in the system from the external field is compensated for as the latent crystallization heat (i.e. is simply used for crystallization in the strict sense of the word). In the opposite case (in an atactic polymer) the oriented state is unstable due to developing internal stresses. In special cases these stresses lead to an ‘orientational catastrophe’—the self-annihilation of the sample.