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Kargin lectures
Polymer Science U.S.S.R. Vol. 32, No. 10, pp. 2138-2140, 1990 Printed in Great Britain.
0032-3950/90510.00+ .00 © 1991 Pergamon Press plc
CHRONICLE KARGIN LECTURES* THE 20th Kargin Lectures were organized on 23 January 1990 in the Large Chemical Lecture Hall at the M. V. Lomonosov Moscow State University. P. V. Kozlov, Chairman of the Organizing Committee, noted in his opening statement that 165 lectures, dealing with almost all aspects of macromolecular science practised in this country, had appeared on the agenda since the foundation of the Lectures in 1971. The hope expressed during the 1st Kargin Lectures, namely that regular meetings of this type would create a tradition in the Soviet science reflecting a tribute to the outstanding scientist V. A. Kargin, has been fully substantiated. The first lecture by V. A. Kabanov was devoted to fundamental sources of scientific and technical progress in the field of polymer materials. An analysis of the sources of scientific progress in this area during the last decades reveals the unique role of exploratory and fundamental chemical research. For the present industrial production of high-density polyethylene, polyolefins, and stereoregular elastomers, amounting to about 25% of worldwide production of polymer compounds, we are indebted to a serendipic discovery made by K. Ziegler in the middle of the 1950s. Investigations that led to the discovery were by no means motivated by a wish to find new ways of polymer synthesis, but involved chemistry of organometallic compounds and represented an attempt to elucidate the mechanism of a novel organic reaction. Interestingly, the chemical reaction in question has never found any serious practical application, although it has found its way into the bulk of organic chemistry. The most important scientific and technical results that have led to the discovery of basic classes of construction polymeric materials and composites lying behind the progress of modern technology also stem from exploratory and fundamental investigations carried out over the last decades. They include, for example, theoretical prediction of the existence of liquid crystalline state in macromolecules exhibiting a certain limiting chain stiffness (P. Flory, U.S.A., 1955). The prediction was soon confirmed experimentally. Several groups of investigators, including our colleagues at the Institute of Macromolecular Compounds, U.S.S.R. Academy of Sciences, were successful in elucidating the nature of the new state and the character of the corresponding phase transitions in polymeric systems. However, more than 15 years were necessary for these studies to exceed purely academic ground; the discovery made independently in the U.S.A. (Dupont) and in the Soviet Union (Scientific-Production Union "Khimvolokno") that fibres manufactured from polymers in the LC state exhibit ultrahigh tensile strength made it clear that this research ranks among the main priorities of polymer technology and materials science. It soon emerged that polymers that exhibit liquid crystalline behaviour--in addition to forming the basis of ultra-high-strength and ultra-highmodulus reinforcing fibres---can also act as excellent binders for composites and high-quality plastics, in many respects surpassing known polymer- and metal-based materials. In this connection it is necessary to continue stressing that exploratory and fundamental investigations, although by no means promising immediate practical advantages, must precede the formulation of all scientific priorities. Making sure that this work will continue on a sufficiently broad front and scientific level ranks in itself among top priorities in the Academy of Sciences, a priority which must be subsidized and financed. Only scientific knowledge, intellectual potential of investigators and the available instrumental basis can provide criteria for deciding what research to * Vysokomol. soyed. A32: No. I0, 2224-2226, 1990.
2138
Chronicle
2139
support. Sufficiently broad scope of fundamental research and effective exploratory investigation require substantial means, invested mostly to modern instrumentation. Also in this country we must find them, or else the scientific knowledge, necessary for technical progress, will drop very rapidly below a critical level; all future attempts to restore it will require years if not decades. Yu. K. Godovskii delivered a lecture (co-authored by N. N. Makarova and V. S. Papkov) dealing with one of the most important and topical problems in physical chemistry of polymers: self-organization of long macromolecular chains. The lecture generalized the results, obtained by many authors over the last approximately ten years, and concerned with the unique ability of linear and cyclolinear polyorganosiloxanes to exist in a stable mesomorphic state, intermediate between the crystalline and amorphous states but different from the common liquid crystalline state. Linear and cyclolinear polyorganosiloxanes characterized by high chain flexibility, not containing any traditional stiff mesogenic group, represent (along with another group of elemento-organic polymers, polyorganophosphazenes) a new class of polymers capable to exist in a thermotropic mesomorphic state. The basic structural and morphological features of the mesomorphic state, the kinetics of its formation, and molecular dynamics were presented on examples of poly(diethylsiloxane) and poly(dipropylsiloxane). Their structure is characterized by two-dimensional order in a plane perpendicular to the chain axis and by high molecular mobility which underlies the possibility of intramolecular rotation of monomer units and longitudinal displacements. By studying thermodynamics and kinetics of formation and disappearance of the mesomorphic structure the authors were able to prove the mesomorphic structure to be an equilibrium phase and the isotropization process to be a first-order phase tradition. It has been also shown that elongation facilitates deformation of the mesomorphic state. An analysis of structural features and of molecular dynamics has led the authors to the conclusion that--in contrast to liquid crystalline mesophases---energy factors originating in specific interactions between macromolecules play an important role in the formation of the thermotropic mesomorphic state. The importance of effects of geometry and chain stiffness for the formation of the mesomorphic state was demonstrated on the example of a new class of mesomorphic polysiioxanes---cyclolinear polyorganosiloxanes, many of which can exist in the mesomorphic state. It has been found that the ring size, the symmetry of incorporation of the ring into the macromolecule in the para-position, and the chemical nature of substituents attached to the silicon atoms are factors responsible for the appearance of the mesomorphic state. By varying the nature of side substitutents one can control the chain stiffness over broad limits, from flexible (methyl substituents) to semirigid (phenyl substituents). Stereoregularity and molar mass decide on the width of the temperature interval of existence of the mesophase: high-molecular mass polymers with transtactic structure exist in the mesomorphic state over the broadest temperature interval. Of particular interest is the fact that the mesophase can exist also in atactic polymers regardless of their amorphous character. Thus, an increase of chain stiffness raises the stability of the mesophase. The lecturer mentioned the possibility of preparing novel polymeric materials--mesomorphic elastomers, block copolymers and mesomorphic glasses. The lecture by V. I. Gerasimov dealt with mechanical properties and structure of loaded, oriented crystalline polymers. Of special importance is the finding that oriented samples in this state exhibit moduli of elasticity several times higher than those corresponding to non-loaded specimens. Crystalline polymers under load can thus represent models of high-modulus systems, at least from the structural point of view.
2140
Chronicle
On the basis of experimental results a structural model has been proposed for orientated samples under load, represented by a fibrillar structure with alternating amorphous and crystalline regions. It has been hypothesized that the tie molecules in the amorphous regions exhibit a certain distribution of length and, moreover, that under load the ends of taut tie molecules can extricate themselves from the crystallites. The model enables one to explain the specific features of the stress-strain curve of the oriented polymer, of its derivative, and also the variation of modulus in samples under static load. The model also proved to be a useful tool facilitating the understanding of processes that take place during preliminary drawing or thermal treatment of oriented specimens. V. A. Izumrudov dealt with reactions between polyelectrolytes. He started by reminding the audience of the attention V. A. Kargin always devoted to that topic and stressed that the study of such reactions can reveal the basic properties and functional principles of natural biological systems. Synthetic polyelectrolytes were characterized as the most suitable objects of such investigations. The lecturer expounded the history of the discovery of soluble, non-stoichiometric polyelectrolyte complexes and the role they played in elucidation of interpolyelectrolyte reactions, i.e. reactions characterized by a transfer of polyions between particles formed by polyelectrolyte complexes. Factors that influence the equilibria and the rate of such reactions were discussed in detail. New results enable one to consider solutions of polyelectrolyte complexes as systems that encompass two, at first sight contradictory properties: high stability and, on the other hand, mobility and ability to undergo interpolyelectrolyte reactions. Experiments with a broad range of systems have proved that a number of states including extreme cases can exist in solutions containing polyions with unequal charge. Thus, polycations are distributed uniformly in a medium consisting of polyanions of the same kind. If, however, the polycation associates preferentially with a certain type of polyanion, the system may exhibit almost complete selectivity. It has been shown on two typical systems how small are the differences in interaction energy per pair of oppositely charged units that suffice for selective binding. The first system where polymethacrylate and polyphosphate anions compete for poly(N-ethyl-4-vinyl-pyridine) cation proved to be extremely sensitive to the nature of the inorganic counterion: in LiCl solutions the polycation binds exclusively to the polysphosphate chain, whilst the reverse is true in KCI solutions. The second example served for demonstrating how the introduction of a single hydrophobic pyrene unit per 1400 polymethacrylic building blocks leads to preferential binding of polycations to these labelled anions; separation of chains containing one label per 400 methacrylate units proceeds almost quantitatively. It has been stressed that the three principles---stability, reactivity, and selectivity--form the basis of concepts which must be actively pursued in attempts to elucidate the relationship between oppositely charged polymeric objects. The same principles underlie the interaction of globular proteins with linear polyions, polyelectrolytes with micelles of surfactants, and polyions with oppositely charged latex particles. It is only natural that they should involve interactions in systems containing biopolymers: protein-protein, protein-linear polyion, and polyion--charged cell membrane. In conclusion the lecturer discussed problems connected with the design of enzymes immobilized inside soluble polyelectrolyte complexes and demonstrated their unique properties. P. V. KOZLOV
Translated by M. Kuai~
0032-3950/90510.00+ .00 © 1991 Pergamon Press plc
CHRONICLE KARGIN LECTURES* THE 20th Kargin Lectures were organized on 23 January 1990 in the Large Chemical Lecture Hall at the M. V. Lomonosov Moscow State University. P. V. Kozlov, Chairman of the Organizing Committee, noted in his opening statement that 165 lectures, dealing with almost all aspects of macromolecular science practised in this country, had appeared on the agenda since the foundation of the Lectures in 1971. The hope expressed during the 1st Kargin Lectures, namely that regular meetings of this type would create a tradition in the Soviet science reflecting a tribute to the outstanding scientist V. A. Kargin, has been fully substantiated. The first lecture by V. A. Kabanov was devoted to fundamental sources of scientific and technical progress in the field of polymer materials. An analysis of the sources of scientific progress in this area during the last decades reveals the unique role of exploratory and fundamental chemical research. For the present industrial production of high-density polyethylene, polyolefins, and stereoregular elastomers, amounting to about 25% of worldwide production of polymer compounds, we are indebted to a serendipic discovery made by K. Ziegler in the middle of the 1950s. Investigations that led to the discovery were by no means motivated by a wish to find new ways of polymer synthesis, but involved chemistry of organometallic compounds and represented an attempt to elucidate the mechanism of a novel organic reaction. Interestingly, the chemical reaction in question has never found any serious practical application, although it has found its way into the bulk of organic chemistry. The most important scientific and technical results that have led to the discovery of basic classes of construction polymeric materials and composites lying behind the progress of modern technology also stem from exploratory and fundamental investigations carried out over the last decades. They include, for example, theoretical prediction of the existence of liquid crystalline state in macromolecules exhibiting a certain limiting chain stiffness (P. Flory, U.S.A., 1955). The prediction was soon confirmed experimentally. Several groups of investigators, including our colleagues at the Institute of Macromolecular Compounds, U.S.S.R. Academy of Sciences, were successful in elucidating the nature of the new state and the character of the corresponding phase transitions in polymeric systems. However, more than 15 years were necessary for these studies to exceed purely academic ground; the discovery made independently in the U.S.A. (Dupont) and in the Soviet Union (Scientific-Production Union "Khimvolokno") that fibres manufactured from polymers in the LC state exhibit ultrahigh tensile strength made it clear that this research ranks among the main priorities of polymer technology and materials science. It soon emerged that polymers that exhibit liquid crystalline behaviour--in addition to forming the basis of ultra-high-strength and ultra-highmodulus reinforcing fibres---can also act as excellent binders for composites and high-quality plastics, in many respects surpassing known polymer- and metal-based materials. In this connection it is necessary to continue stressing that exploratory and fundamental investigations, although by no means promising immediate practical advantages, must precede the formulation of all scientific priorities. Making sure that this work will continue on a sufficiently broad front and scientific level ranks in itself among top priorities in the Academy of Sciences, a priority which must be subsidized and financed. Only scientific knowledge, intellectual potential of investigators and the available instrumental basis can provide criteria for deciding what research to * Vysokomol. soyed. A32: No. I0, 2224-2226, 1990.
2138
Chronicle
2139
support. Sufficiently broad scope of fundamental research and effective exploratory investigation require substantial means, invested mostly to modern instrumentation. Also in this country we must find them, or else the scientific knowledge, necessary for technical progress, will drop very rapidly below a critical level; all future attempts to restore it will require years if not decades. Yu. K. Godovskii delivered a lecture (co-authored by N. N. Makarova and V. S. Papkov) dealing with one of the most important and topical problems in physical chemistry of polymers: self-organization of long macromolecular chains. The lecture generalized the results, obtained by many authors over the last approximately ten years, and concerned with the unique ability of linear and cyclolinear polyorganosiloxanes to exist in a stable mesomorphic state, intermediate between the crystalline and amorphous states but different from the common liquid crystalline state. Linear and cyclolinear polyorganosiloxanes characterized by high chain flexibility, not containing any traditional stiff mesogenic group, represent (along with another group of elemento-organic polymers, polyorganophosphazenes) a new class of polymers capable to exist in a thermotropic mesomorphic state. The basic structural and morphological features of the mesomorphic state, the kinetics of its formation, and molecular dynamics were presented on examples of poly(diethylsiloxane) and poly(dipropylsiloxane). Their structure is characterized by two-dimensional order in a plane perpendicular to the chain axis and by high molecular mobility which underlies the possibility of intramolecular rotation of monomer units and longitudinal displacements. By studying thermodynamics and kinetics of formation and disappearance of the mesomorphic structure the authors were able to prove the mesomorphic structure to be an equilibrium phase and the isotropization process to be a first-order phase tradition. It has been also shown that elongation facilitates deformation of the mesomorphic state. An analysis of structural features and of molecular dynamics has led the authors to the conclusion that--in contrast to liquid crystalline mesophases---energy factors originating in specific interactions between macromolecules play an important role in the formation of the thermotropic mesomorphic state. The importance of effects of geometry and chain stiffness for the formation of the mesomorphic state was demonstrated on the example of a new class of mesomorphic polysiioxanes---cyclolinear polyorganosiloxanes, many of which can exist in the mesomorphic state. It has been found that the ring size, the symmetry of incorporation of the ring into the macromolecule in the para-position, and the chemical nature of substituents attached to the silicon atoms are factors responsible for the appearance of the mesomorphic state. By varying the nature of side substitutents one can control the chain stiffness over broad limits, from flexible (methyl substituents) to semirigid (phenyl substituents). Stereoregularity and molar mass decide on the width of the temperature interval of existence of the mesophase: high-molecular mass polymers with transtactic structure exist in the mesomorphic state over the broadest temperature interval. Of particular interest is the fact that the mesophase can exist also in atactic polymers regardless of their amorphous character. Thus, an increase of chain stiffness raises the stability of the mesophase. The lecturer mentioned the possibility of preparing novel polymeric materials--mesomorphic elastomers, block copolymers and mesomorphic glasses. The lecture by V. I. Gerasimov dealt with mechanical properties and structure of loaded, oriented crystalline polymers. Of special importance is the finding that oriented samples in this state exhibit moduli of elasticity several times higher than those corresponding to non-loaded specimens. Crystalline polymers under load can thus represent models of high-modulus systems, at least from the structural point of view.
2140
Chronicle
On the basis of experimental results a structural model has been proposed for orientated samples under load, represented by a fibrillar structure with alternating amorphous and crystalline regions. It has been hypothesized that the tie molecules in the amorphous regions exhibit a certain distribution of length and, moreover, that under load the ends of taut tie molecules can extricate themselves from the crystallites. The model enables one to explain the specific features of the stress-strain curve of the oriented polymer, of its derivative, and also the variation of modulus in samples under static load. The model also proved to be a useful tool facilitating the understanding of processes that take place during preliminary drawing or thermal treatment of oriented specimens. V. A. Izumrudov dealt with reactions between polyelectrolytes. He started by reminding the audience of the attention V. A. Kargin always devoted to that topic and stressed that the study of such reactions can reveal the basic properties and functional principles of natural biological systems. Synthetic polyelectrolytes were characterized as the most suitable objects of such investigations. The lecturer expounded the history of the discovery of soluble, non-stoichiometric polyelectrolyte complexes and the role they played in elucidation of interpolyelectrolyte reactions, i.e. reactions characterized by a transfer of polyions between particles formed by polyelectrolyte complexes. Factors that influence the equilibria and the rate of such reactions were discussed in detail. New results enable one to consider solutions of polyelectrolyte complexes as systems that encompass two, at first sight contradictory properties: high stability and, on the other hand, mobility and ability to undergo interpolyelectrolyte reactions. Experiments with a broad range of systems have proved that a number of states including extreme cases can exist in solutions containing polyions with unequal charge. Thus, polycations are distributed uniformly in a medium consisting of polyanions of the same kind. If, however, the polycation associates preferentially with a certain type of polyanion, the system may exhibit almost complete selectivity. It has been shown on two typical systems how small are the differences in interaction energy per pair of oppositely charged units that suffice for selective binding. The first system where polymethacrylate and polyphosphate anions compete for poly(N-ethyl-4-vinyl-pyridine) cation proved to be extremely sensitive to the nature of the inorganic counterion: in LiCl solutions the polycation binds exclusively to the polysphosphate chain, whilst the reverse is true in KCI solutions. The second example served for demonstrating how the introduction of a single hydrophobic pyrene unit per 1400 polymethacrylic building blocks leads to preferential binding of polycations to these labelled anions; separation of chains containing one label per 400 methacrylate units proceeds almost quantitatively. It has been stressed that the three principles---stability, reactivity, and selectivity--form the basis of concepts which must be actively pursued in attempts to elucidate the relationship between oppositely charged polymeric objects. The same principles underlie the interaction of globular proteins with linear polyions, polyelectrolytes with micelles of surfactants, and polyions with oppositely charged latex particles. It is only natural that they should involve interactions in systems containing biopolymers: protein-protein, protein-linear polyion, and polyion--charged cell membrane. In conclusion the lecturer discussed problems connected with the design of enzymes immobilized inside soluble polyelectrolyte complexes and demonstrated their unique properties. P. V. KOZLOV
Translated by M. Kuai~