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R. Hołyst



Mesophases in polymer systems: structure and phase transitions (in English)


Polimery 1998, No 9, 523


DOI: dx.doi.org/10.14314/polimery.1998.523


Summary

The system of AB diblock copolymers forms many ordered structures, e .g ., hexagonal, lamellar, cubic, and gyroid. These structures are formed owing to the incompatibility of the homopolymers A and В that constitute the copolymer. The A-В covalent bonds can prevent the incompatibility that results in macrophase separation. The system can separate locally (microphase separation) into ordered А-rich and В-rich spherical (cubic phase), layered (lamellar phase) or cylindrical (hexagonal phase) domains. The gyroid structure is special: it is bicontinuous and the А-rich and B-rich domains form channels of the Ia3d symmetry. Three methods used to study the systems are the mean-field model, self-consistent one-loop approximation, and the self-consistent field theory; each can be developed from the Edwards Hamiltonian. The single chain statistics in the disordered phase of a diblock copolymer is shown to deviate from the Gaussian statistics on account of fluctuations. In the one-loop approximations, the diblock copolymer chain is shown to stretch at the point where two incompatible blocks meet; each Block shrinks close to the microphase separation transition. Stretching outweighs shrinking and the net result is the increase in the radius of gyration about the Gaussian value. Another example of the ordered structure is provided by liquid crystalline (LC) polymers. The LC polymer main chains (usually very stiff) form primarily nematic phases which are characterized by the orientational order. The long chains are ordered in one direction, breaking the rotational symmetry. Finally we show the general features of the Landau- Ginzburg model, which is the simplest model for the study of ordered polymer systems.


Keywords: ordered structures, Landau-Ginzburg model, bond configurations, monomer interactions, density operators


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