The story started with the establishment in 1948 of the Leningrad Institute of High-Molecular Compounds. The "architect" of the project-Corresponding Member of what was then the USSR Academy of Sciences, M. Volkenstein - attached to the Lab of Polymer Structure a small team of specialists in theoretical studies. Their main task was the establishment of quantitative links between the chemical structure of polymers, their flexibility and physical properties.
By Tatyana BIRSTEIN, Yuri GOTLIB, Anatoly DARINSKY, Doctors of Science (Phys. & Math.), RAS Institute of High-Molecular Compounds
Experts in the field were already aware by that time of the fact that the flexibility of macro-molecules is explained by what they called internal rotation around single, or unitary bonds of the chain frame. And the main idea, which suggested approaches to the solution of the central problem, was a concept formulated by Professor Volkenstein in 1951. It can be briefly summed up as follows: in flexible polymer chains there exist not just any turns of links but discrete sets of different states (rotational isomers, or conformers). Their geometrical and energy characteristics are determined by the chemical structure of a concrete polymer. And experts developed methods of assessment of these characteristics.
As had been earlier believed, rotational isomerism is a feature of low-molecular compounds alone. The specifics of this phenomenon in chain macromolecules turned out to be the interdependence of rotational isomerism states of neighboring links in the chain. This suggested to researchers of Prof. Volkenstein's team the idea about the possibility of using the so-called unidimensional Izing model that had been earlier developed in the physics of magnetic phenomena. Researchers worked out a mathematical apparatus for calculating the characteristics of macromolecules as a single whole: dimensions, dipolar momenta, optical anisotropy, etc. on the basis of characteristics of some rotational isomers.
These studies provided the basis for a whole new area of research-statistical physics of macromolecules. This was described in a generalized form in the early "priority" monographs published in 1959 and 1964, which were translated into English almost there and then and brought out in the international High Polymers series. These studies received broad international recognition, and in his Nobel Lecture in 1974 the US physico-chemist, Prof. Paul Flory, stressed that his own studies on the theory of polymer flexibility were based on the works of the Leningrad school of Prof. Volkenstein. The ideas and methods used as the basis of statistical physics of macromolecules have since withstood the test of time, and are still being used in the works of many scientists.
Further development of the theory of equilibrium properties of polymers proceeded along several lines. First, methods of quantum chemistry were used for the calculation of conformations and energies of rotational isomers, and among significant contributions in this field one should mention, above all, the works of Prof. V. Zubkov and Prof. I. Milevskaya. Another trend of research initiated by the works of Prof. Volkenstein and Prof. Ptitsyn was linked with the development of molecular biophysics. This included studies of conformational transitions of the type: spiral- knot, folded structure-knot, globula-knot in separate macromolecules present in solution. Processes of this kind are typical of vitally important nucleic acids and proteins.
Another area of research was based on the interaction of polymers with diverse surfaces-something which made it possible to change membranes permeability, control colloidal particles interactions, etc.
Also studied were regularities of adsorption of these compounds on surfaces of different curvature, and molecular systems representing mono-layers of polymer chains "attached" to the surface by tail groups. Such systems are usually known as polymer grids.
One should also mention studies related to development of the theory of supermolecular structures which appear in di- and try-blockcopolymers. The latter are macromolecules consisting of blocks of different chemical nature. When these are incompatible, there occurs separation and regular supermolecular superstructures at nanometer level are formed. By changing the relative sizes of the blocks, one can obtain their modifications, such as flat ones and also those containing cylindrical or spherical domains. Studies of researchers of the Volkenstein school demonstrated that along with the "chemical" methods of management of supermolecular structures there can also be a "physical" one- by mixing copolymers with blocks of different size. As has been shown in theory, single superstructures are produced in mixtures of this kind.
Started almost simultaneously with the development of the theory of balanced flexibility of macromolecules were studies of the theory of their dynamic properties in the
works of Prof. Volkenstein and his pupil Dr. Yu. Gotlib and the pupils of the latter: K. Salikhov, Yu. Svetlov and A. Darinsky. In the studies of the 1960s methods of kinetics of cooperative processes (Izing dynamic model) were extended to the analysis of dynamics of polymers with rotational isomeric mechanism of flexibility. At the same time modeling was started of relaxation processes of polymer grids (in connection with the problem of ultrasound absorption in corresponding solutions).
Put forward for the first time in the 1960s- 1970s was a theory taking into account the interconnection of different dynamic processes in solid polymers which explains the peculiarities of the temperature-frequency dependence of different relaxation characteristics.
At the same time considerable attention was given to local relaxation processes in polymers and their manifestations in dielectric relaxation, nuclear magnetic resonance and polarized luminescence. Institute specialists studied links between the reactivity of macromolecules and molecular mobility. They established among other things, how the rotational isomeric mechanism of mobility is manifested in the above phenomena. One important achievement of these studies consisted in the discovery of the concrete mechanism of the related restructurings in macromolecules.
From 1985 one of the main directions of research was the development of a theory of molecular mobility in anisotropic and partially ordered polymer systems, such as liquid crystals, or under the effect of strong non-linear external fields. Researchers are also studying the regularities of multicomponent cooperative processes in polymer grids taking into account multi-component cooperative processes in such grids and intra- and inter-chain interactions and topological limitations (polymer lattice points).
The advent and progress of electronic computer technologies made it possible to tackle problems which could not be dealt with by simple analytical methods. In the 1960s, parallel with studies in the West, our researchers headed by Prof. O. Pti-tsyn began modeling polymer chains by the Monte-Carlo method. These studies proved to be very effective. For example, Prof. A. Kron suggested a new method of generation of what is known as the ensemble of conformations of macromolecules which is still being used today. Prof. A. Emelyashev was one of the first to use this method for studies of properties of charged polymers (polyelectrolytes); working together with Dr. L. Morgenstern and Dr. T. Birtein, he used the method for modeling knot-globula transitions. In the mid-1970s it was used in a cycle of studies and experiments by the team of Prof. Birstein for solving the problem of which conformations are possessed by flexible polymet chains in amorphous substrate. These studies confirmed the prediction of Prof. P. Flory that intermolecular interactions do not provide for additional orderliness in this chains status.
In the late 1970s, in cooperation with the Computer Center of the USSR Academy of Sciences (Push-chino-on-the Oka), researchers headed by Prof. A. Darinsky initiated a new field of research-computer modeling of dynamic properties of polymers. Successfully used for the first time was the molecular dynamics method (used before for low-molecular systems). In their initial studies, and because of limited computer capacity, researchers investigated sufficiently simple models reflecting universal properties of polymers. Subsequent development and perfection of the computer base made it possible to pass on to more complex systems.
In the 1980s researchers in the USSR started using the Brownian dynamics method in computer modeling of polymer dynamics. This makes it possible to increase appreciably the time-scale of the processes under investigation, register changes of polymer chains with retarded internal rotation and establish the mechanism of rotational-isomeric transitions in them.
The St. Petersburg school of polymer theory is keeping up its vigorous studies as a leader in this field and continues to develop its cooperation with the international scientific community.
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T. Birstein, Yu. Gotlib, A. Darinsky, THEORY OF POLYMERS IS FIFTY // New-York: Libmonster (LIBMONSTER.COM). Updated: 09.09.2018. URL: https://libmonster.com/m/articles/view/THEORY-OF-POLYMERS-IS-FIFTY (date of access: 22.09.2023).
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