Scientists Synthesize and Study New Liquid-Crystal Photochromic Polymers
New liquid-crystal photochromic polymers have been synthesized and studied by members of the Faculties of Chemistry and Fundamental Physical and Chemical Engineering at the Lomonosov Moscow State University in association with foreign partners.
The Lomonosov Moscow State University Scientists teamed up with Czech colleagues from the Institute of Physics (Prague) to synthesize and study new liquid-crystal polymers, combing optical properties of liquid crystals with mechanical properties of polymers. Such polymers are capable of quickly changing molecular orientation under the influence of external fields and form coatings, details of complex shape and films.
When compared with low-molecular-mass liquid crystals, an advantage of such systems refers to the fact that at room temperature liquid-crystal polymers are present in the glass-like state, fixing molecular orientation.
Liquid-crystal polymers are made up of molecules with high molecular mass, known as macromolecules. They are comb-shaped pointing out that photosensitive rigid azobenzene fragments (C6H5N=NC6H5) are fixed to the main flexible polymer chain with the help of spacers, comprising of CH2 moieties. These fragments attempt sequencing and are capable of forming a wide range of “packings”, such as, liquid-crystal phases. Azobenzene groups isomerize when light strikes such polymers, resulting in modification of the optical properties of the polymers. These polymers are known as photochromic.
The Scientists have specifically concentrated on the processes of photo-orientation and photoisomerization. Photoisomerization refers to the rearrangement of bonds within a polymer molecule under the influence of light. Photo-orientation refers to modification of rod-like azobenzene (in this case) orientation of fragments under the effect of plane polarized light, in whose direction of electric field is firmly determined.
Azobenzene fragments, when exposed to polarized light, change their angle in the course of photoisomerization cycles. This takes place until their orientation becomes perpendicular to the polarization plane of the incident light and also until the fragments are no longer capable of absorbing light.
The photo-orientation process changes the orientation of azobenzene fragments of macromolecules and also brings about emergence of dichroism and birefringence. Dichroism refers to the difference of intensity of polarized light absorption in orthogonal directions. Birefringence is the splitting of light beam into two components with orthogonal (perpendicular) polarization; the direction of one of these components does not change, despite the fact that the second beam is refracted.