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Scientists from the MSU studied new liquid-crystalline photochrom


Credit: Alexey Bobrovsky

Members of the Faculties of Chemistry and Fundamental Physical and Chemical Engineering at the Lomonosov Moscow State University in collaboration with foreign partners have synthesized and studied new liquid-crystal photochromic polymers.

The Lomonosov Moscow State University scientists, cooperating with Czech colleagues from the Institute of Physics (Prague), have synthesized and studied new liquid-crystal polymers, combing optical properties of liquid crystals with mechanical properties of polymers. Such polymers could quickly change molecular orientation under the influence of external fields and form coatings, films and details of complex shape. An important advantage of such systems in comparison with low-molecular-mass liquid crystals lies in the fact that at room temperature liquid-crystal polymers exist in the glass-like state, fixing molecular orientation.

Liquid-crystal polymers comprised of molecules with high molecular mass, called macromolecules. They are comb-shaped what implies that photosensitive rigid azobenzene fragments (C6H5N=NC6H5) are attached to the main flexible polymer chain with the help of spacers, consisting of CH2 moieties. These fragments are striving for sequencing and could form a wide variety of "packings" – namely, liquid-crystal phases. When light strikes such polymers, azobenzene groups isomerize, what results in alteration of polymers' optical properties. Such polymers are called photochromic.

The scientists have paid special attention to the processes of photoisomerization and photo-orientation. Photoisomerization is rearrangement of bonds inside a polymer molecule under the influence of light. Photo-orientation stands for alteration of rod-like azobenzene (in our case) fragments' orientation under the effect of plane polarized light, in whose direction of electric field is strictly determined. Being exposed to polarized light, azobenzene fragments change their angle in the course of photoisomerization cycles. This occurs until their orientation becomes perpendicular to the polarization plane of the incident light and the fragments are no more capable of absorbing light. The photo-orientation process not only allows to change the orientation of azobenzene fragments of macromolecules but also causes emergence of dichroism and birefringence. Dichroism is difference of intensity of polarized light absorption in orthogonal directions. Birefringence refers to light beam splitting into two components with orthogonal (perpendicular) polarization; the direction of one of these components doesn't change, while the second beam is refracted.

Alexey Bobrovsky, RAS Professor, Doctor of Chemistry, Chief Research Fellow at the Department of High Molecular Compounds of the Faculty of Chemistry, Lomonosov Moscow State University, and one of the article authors says: "The key idea of our project is to study how the chemical structure of new comb-shaped liquid-crystal photochromic polymers influences their phase behavior and photooptical properties. Photoisomerization and photo-orientation processes allow exactly control phase behavior and optical properties of the elaborated systems."

At first, scientists from the Lomonosov Moscow State University in cooperation with their Czech colleagues from Institute of Physics, the Czech Academy of Sciences synthesized monomers, out of which the Lomonosov Moscow State University scientists obtained liquid-crystal polymers. The authors have studied phase behavior and temperatures of phase transitions of polymers, applying methods of polarizing optical microscopy and differential scanning calorimetry. X-ray structural analysis was used for studying detailed structure of phases at the Faculty of Fundamental Physical and Chemical Engineering of the Lomonosov Moscow State University.

According to the authors' words, the most significant project part was to study photooptical properties and photochromism of the obtained polymers. This stage was divided into two parts: irradiation of the polymer films by unpolarized UV light, during which photoisomerization (namely, rearrangement of intermolecular communications) took place. And the second part implied irradiation by polarized light resulted in photo-orientation.

Alexey Bobrovsky notices that the article relates to a major cycle of projects, devoted to photoinduced processes in photochromic liquid-crystal polymers. This article is research work only: the authors have studied interconnections between the chemical structure and the compound properties.

The scientist concludes: "Photoisomerization and photo-orientation open large perspectives for creation of so called "smart materials". They react to any external stimuli and could be used for information recording, storage and transfer, as well as in optical devices of diverse complexity. These precise polymers will be hardy used in a real-case scenario as they are too expensive and their synthesis is quite complicated. On the other hand, you can predict not in every instance, what systems will find application at what period of time and in what conditions."


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