V. I. Gerasimov

Gelation in Solutions of High-Molecular-Mass Polyacrylonitrile under Shear Deformation

The effect of shear fields on the rate of physical gelation in semidilute solutions of ultra-high-molecular-weight polyacrylonitrile in propylene carbonate and on the structural features of the resulting gels has been studied. At 30–70°C in the uniform shear field, the induction period of gelation decreases from several hours to several seconds with an increase in the shear stress. The patterns of these dependences are rather close at shear stresses above 20 Pa, while significant differences are observed at these temperatures only at low shear stresses (below 15 Pa). Depending on the shear stress, two types of gelation can be distinguished at 70°C. At stresses below 15 Pa, the polymer phase emerging in the course of shear deformation is aligned along shear lines and the sample is macroscopically nonuniform. At higher shear stresses, the structure of the sample is uniform throughout its bulk.

Heterogeneous chemical crosslinking and high-voltage polarization in a ferroelectric copolymer of vinylidene fluoride and tetrafluoroethylene

The low-temperature chemical crosslinking of a copolymer of vinylidene fluoride and tetrafluoroethylene was studied through various physicochemical methods. The reaction was conducted in solution in the presence of diethylenetriamine as a crosslinking agent. The penetration of the mixture molecules and the crosslinking agent only in the amorphous phase of the copolymer was provided via selection of the ratio between a good solvent (dimethylformamide) and a poor solvent (ethanol). Owing to this, the crosslinking reaction occurred in the amorphous phase and hardly involved the crystals. This outcome was confirmed by wide-angle X-ray diffraction data. The structural and chemical changes in the amorphous phase during crosslinking were recorded with the use of IR spectroscopy and differential scanning calorimetry. It is shown that crosslinking is accompanied by the formation of C=C bonds in the copolymer chains. The study of high-voltage polarization and conductivity during exposure to bipolar rectangular pulses suggested that crosslinking leads to an increase in the carrier concentration. It was found that the surface potentials in the films increase with an increase in the number of high-voltage pulses applied to the sample. This circumstance is attributed to the fact that the double bonds formed in the copolymer chains can effectively trap negatively charged carriers.

Kinetic and structural aspects of gelation in polyacrylonitrile-propylene carbonate system

Structural rearrangements in solutions of high-molecular-mass polyacrylonitrile in propylene carbonate upon phase separation were studied and quantitatively characterized by means of turbidity spectroscopy and oscillation rheometry with the use of previous data obtained via the light transmission technique. The kinetic features of separation of the new phase and the particle size growth were established. It was shown that there is a time lag between the onset of physical-gel formation in the system and the onset of phase separation. The average particle size that corresponds to the transition of the system into gel and determines the gel morphology at the subsequent further stages was found for different temperatures.

Use of polyacrylonitrile as promising material for matrix of liquid crystal composite

Liquid crystal composites consisting of high porous polymeric matrices based on polyacrylonitrile filled with liquid crystal compounds MBBA, cholesteryl oleyl carbonate and a commercial mixture of ZhK-1282 were prepared and studied. Obtained LC-composites strongly scatter visible light at room temperature, when the LC is in the mesophase, but when the LC is in the isotropic phase the samples are transparent. It is shown by differential scanning calorimetry and optical polarizing microscopy that it is possible to change the temperature interval of existence of the mesophase by varying the size of pores in a PAN matrix. The electro-optical researches have shown that the reorientation of MBBA in composites does not occur in a wide interval of frequencies and voltage irrespective of the sizes of pores. The similar researches have shown that X-16 in composites also is not exposed to action of electric field. The electro-optical researches of samples with ZhK- 1282 have shown that the electrical field does not render influence on LC in matrix with small pores, while in samples with large pores the reorientation of a part of the LC is appreciable.

Structural Aspects of Complexation Between CDs and Poly(Alkylene Oxides)

The mixing of di- and threeblock copoiymers of ethylene and propylene oxides (proxanols, pluronics, polaxomers) with CDs results in the formation of crystalline complexes — molecular necklaces. The regularities of complexation between CDs and these copoiymers are studied. The stoichiometry of complexes based on diblock copoiymers corresponds to one CD molecule per two monomer units of poly(alkylene oxide) as for corresponding homopolymers. At the same time the composition of complexes based on threeblock copoiymers depends on the concentration of the solution and varies from completely filled to half filled molecular necklaces. Thus the complexation reaction result in the production of new types of block copolymes of diverse architecture.