Review of Scientific Activity

Surface Phenomena at the Interface Polymer — Solid

Adsorption of Polymers from Solutions and Structure of Adsorption Layers. The investigations of adsorption of polymers from solutions done by Yu.S. Lipatov and his coworkers (L.M. Sergeeva, T.T. Todosiychuk, V.N. Chornaya) are distinguished from the majority of experimental works in this field because they are not restricted by the region of dilute polymer solutions, but include also the region of semi-diluted solutions where macromolecular coils begin to overlap and form the macromolecule aggregates and the region of concentrated solutions where due to coil overlapping the entanglement network is formed. The aggregation of macromolecules in solutions was investigated to understand better the adsorption peculiarities.

The experimental data on adsorption have shown that the experimentally found adsorption value is much higher than if it were determined only by the adsorption of isolated macromolecules. The adsorption value goes through the maximum at a definite solution concentration. These data have enabled to formulate the molecular-aggregate mechanism of adsorption of polymers from semi-diluted and concentrated solutions (1965). This mechanism may be presented as follows. By adsorption from solutions where aggregation of macromolecules proceeds, macromolecular aggregates transit preferentially onto the adsorbent surface, being less “soluble” as compared with isolated macromolecules. Really, in solution in the presence of an adsorbent, after establishing the adsorption equilibrium, the aggregates are practically absent.

Some time is needed for their formation (establishing equilibrium between aggregated and isolated molecules) after which the equilibrium concentration of aggregates is restored in solution corresponding to the given concentration. These data prove the transit of aggregates onto the adsorbent surface and allow explaining the main features of adsorption from semi-diluted and concentrated solutions (high adsorption value, adsorption maximum at adsorption isotherms and inversion of the solvent effect on adsorption). Decreasing adsorption after maximum was explained by enhancement of the entanglement network in solution that makes it difficult and then fully prevents from the transit of macromoleculesonto the adsorbent surface. This effect explains full absence of adsorption after the concentration exceeding a critical one. Another possible explanation is that with concentration increase the aggregation constant is diminished. It was shown that in any case the simultaneous adsorption of both aggregates and isolated macromolecules occurs. The aggregation constant change affects the ratio between adsorbed aggregated and isolated macromolecules. The establishing of the equilibrium between aggregated and isolated macromolecules and small values of aggregation constant show that in solution the equilibrium is shifted in favor of adsorption of aggregated macromolecules. The calculations have shown that with increasing the solution concentration, the number of isolatedmolecules increases faster as compared with aggregate ones. Due to this effect, the adsorption of isolated chains is imposed on the adsorption of aggregates, the conformation of isolated macromolecules being different as compared with that one in dilute solutions due to the coil overlapping. This effect manifests itself in the phenomenon impossible for adsorption of low molecular weight compounds where an isotherm of adsorption does not depend on the adsorbent/solution ratio.

The peculiarities of adsorption of polymers, connected with aggregate formation, determine that at each solution concentration a new equilibrium between isolated and aggregated macromolecules is established and as a result another structure of adsorbed entities corresponds to each point on isotherm (from isolated coils of different dimension up to their aggregates). As a result, the maximum of adsorption may have its origin from the change on the ratio of the number of adsorbed aggregates and isolated macromolecules. In this case the isotherm shape depends on the adsorbent/solution ratio (1970).

For adsorption from semi-diluted and concentrated solutions one more principal feature was established. It consists in various degree of bonding of adsorbed macromolecules with the adsorbent surface depending on the adsorbent/solution ratio. All the changes take place in the concentration region where aggregation proceeds and equilibrium between aggregated and isolated molecules is established that depends on solution concentration, solvent nature and adsorbent/ solution ratio.

The molecular-aggregate mechanism of adsorption determines that at each concentration, as distinct from classical adsorption, we are dealing with adsorbing entities of various shape and composition that depend on solution concentration and solvent nature. The structure of adsorbing entities leads to the appearance of peculiarities in the structure of adsorption layers obtained by adsorption from solutions of different concentrations. In those cases, when molecular aggregate mechanism is operating, the structure and thickness of adsorption layer are determined by the dimensions and structure of molecular aggregates. Due to this effect, the number of segments bonded directly with the surface is much less as compared with adsorption from dilute solutions. Most part of adsorbed segments entering aggregate does not interact with the surface directly. The fraction of bonded segments changes with increasing the solution concentration in the same nonmonotonous way as the value of adsorption, being dependent on the solvent thermodynamic quality. The higher is the adsorption of aggregates, the less is the fraction of bonded segments. As a result of indirect interaction of the most part of adsorbed segments with the surface, their bonding with the surface is rather weak as compared with the bonding of isolated molecules. Adsorption of aggregates determines that the adsorption layer has the thickness essentially exceeding the thickness of monomolecular layers. At the same time the interaction between macromolecules in solution affects the conditions of formation of the adsorption layer, whose properties are strongly dependent on the adsorption conditions.

Since the 1980’s a great deal of works was dedicated to adsorption of polymer mixtures from solutions, this problem being important for the theory of reinforced polymer alloys. In such systems, the surface layers are formed simultaneously by both alloy components. At the same time, since the majority of polymer pairs are immiscible, in transition layers at the interface the intermediate region may appear between two adsorbed polymers. The selective adsorption of one of the components leads to the changes in structure of adsorption layers as compared with equilibrium solution. As a result, nonuniform distribution of concentration (concentration profile) arises in the surface adsorption layers.

The application of the molecular-aggregate adsorption concept to adsorption from polymer mixtures gave a possibility to establish characteristic features of adsorption. The complicated adsorption behavior is determined by the differences in thermodynamic state of components in a common solvent due to their immiscibilily. Solution of component A may be considered as a solvent for polymer B and vice versa, and thermodynamic quality of such “polymeric” solvent changes both with the change of solution concentration and component ratio. Worsening of a “polymeric” solvent for component B leads to the transfer of B macromolecules and their aggregates to the adsorbent surface. The formation of aggregates of both components also depends on the thermodynamic quality of such a mixed solvent and depends in such a way on the component ratio and their total concentration in solution. It was established that the adsorption behavior depends on the values of critical concentration of overlapping of both component and total critical concentration of the mixture that depends on the component ratio. The effect of the component ratio and its concentration may be described in the framework of the effect of the solvent nature on adsorption.

Thus the isotherm shape, adsorption value and selectivity of component adsorption depend on many factors and determine the complicated structure of adsorption layers by adsorption from polymer mixtures.

The series of works was dedicated to the investigation of adsorption heats for simultaneous adsorption of two polymers. Using IR spectroscopy the shifts in the position of bands responsible for the vibration of free and bound OH-groups

were found anc the energies of adsorption interaction Q between OH-groups of adsorbent and functional groups of polymers were calculated. The heats of adsorption were calculated as a product of Q by p, fraction of the bound segments determined from IR spectroscopy. It is important that for adsorption from mixtures, the heats of adsorption were estimated simultaneously and separately for each polymer component and in a wide concentration interval. Essential changes were observed in the adsorption energy in the region of semi-dilute solutions where macromolecular clusters, instead of isolated macromolecular coils, are adsorbed. Correspondingly, the adsorption of clusters is characterized by smaller values of

the adsorption heat because of a smaller fraction of the bound segments. Thus, such an important characteristic as the adsorption heat is not constant value but depends on the fraction of bound segments and adsorption regime. For understanding the structure of adsorption layers in a mixed system of great importance is isosteric heat of adsorption which was found from the temperature dependence of adsorption according to the Clapeyron — Clausius equation. The values of isosteric heat of adsorption found simultaneously for each polymer of the mixture show that the transition of macromolecules from solution onto the adsorbent surface leads to an increase in their enthalpy, i.e. polymers transit in energetically less favorable state. Such an effect is more pronounced for adsorption from ternary systems as compared with binary systems. That means that the presence of one polymer affects the conformation state of another. The effect depends on the degree of surface coverage. The higher is adsorption value, the higher is the change in enthalpy. All these factors determine the features of adsorption layers under adsorption from mixture.

One more very important factor determining the structure of adsorption layers has been studied. The adsorption values depend on the ratio of the adsorbent surface S to the volume of solution V from which adsorption proceeds. For various systems it was established that the ratio S/V depends on the solution concentration. These effects are connected to the changes in molecular mass distribution of both polymers by adsorption. In such a way changing the value of S/V allows to regulate the structure of adsorption layers. The reason for this effect consists in the polydispersity of polymers, which predetermines the establishing of multiple equilibria between the surface and macromolecules differing in molecular masses. The experimental data on adsorption from ternary systems allowed the development of a new approach to the description of adsorption thermodynamics. In some works done with A.E. Nesterov the expression for the free energy of adsorption has been derived which includes not only the traditional parameters

of thermodynamic interactions between polymer molecules and a solvent and between them and a surface, but interaction parameter between two dissolved species. In such a way the new parameter characterizes the miscibility of two polymers in solution. For adsorption from binary solution there is valid

the expression:

dUS = (UPS - ULS) = -cskT

where UPS, and ULS, are the energy of interaction between polymer and surface and between solvent and surface, dU, is the difference in the interaction energy between segment in contact with surface and solvent molecule in contact with surface. Being expressed in such a way, a negative value of parameter cs, favors adsorption. For adsorption from ternary solution another relation should be valid:

dUAS = (UAS - ULS - UAB) = -cskT

dUBS = (UBS - ULS - UAB) = -cskT

the thermodynamic compatibility between polymers is taken into account and a new factor, determining adsorption, is the compatibility of two polymers in solution. The expression for the free energy of adsorption includes also the volume

fraction of an adsorbent (fraction of active sites on its surface) and in such a way explains the dependence of adsorption on the adsorbent amount in solution (effect S/V).

The experimental data on adsorption allowed to calculate the thickness of adsorption layers formed under the assumption that the whole adsorbent surface is saturated at low solution concentration and that the increase of adsorption at higher solution concentrations is associated with the adsorption of macromolecular clusters and with the rearrangement of the adsorption layer structure. The structure of adsorption layers for various systems was also estimated from the data of atomic

force microscopy (AFM) (V.N. Bliznyuk). It was discovered that in the case of adsorption from mixtures of immiscible polymers, the adsorption layer is formed by clusters and has mosaic structure with a characteristic domain size of approximately 200 nm for each component. AFM studies revealed that the adsorption layers are fractal structures whose fractal dimensions depend on the polymer type and on the adsorption regime.

Adhesion of Polymers to Solid Surfaces. Adhesion of polymers to the solid surface is a determining factor of polymer reinforcement. Estimation of the polymer adhesion to solids may be done on the basis of the values of surface tensions of a solid and polymer and interfacial tension between them. Since the early seventies the thermodynamics of adhesion was studied jointly with A.E. Fatnerman. As a result the rule of interfacial interaction was established according to which to calculate the thermodynamic work of adhesion one has to use in corresponding equations not the value of surface tension ofa pure polymer but the surface tension at the interface with a solid of a given nature. It was experimentally proved that the surface tension of a polymer at the interface depends on the substrate nature due to the effects of polarization and depolarization. Because of it, the surface tension of a polymer experimentally found from he wetting angles depends on the surface tension of the wetting liquid. Theoretically correct estimations of the thermodynamic work of adhesion may be done only if to take these effects into account. Jt was shown that in the zone of adhesion contact the surface tension of a polymer and, therefore, the cohesion energy, is determined by the action of the surface field forces. The role of the interfacial layers in adhesion was established (1980) that is connected with the microheterogeneity of these layers. It was established that from thermodynamic point of view, the thermodynamic work of adhesion is determined by the cohesion energy of the weakest component, i.e. of the polymer.

For the case of polymer alloys, the thermodynamic equation was proposed for calculating the adhesion work (1990). This equation includes the values of the surface tension of two phases evolved by formation of alloys and their ratios. The phenomenological model of adhesion of polymer alloys was proposed that enables to predict the adhesion strength. The peculiarities of blend adsorption were established that are connected with the selective adsorption and selective wetting

of the surface by one of the blend components (the investigations done jointly with VF. Babich).

It was established that the changes of the structure and properties of the surface layers of polymers at the interface with a solid (see below) contribute to the adhesion joint strength, if to consider it from the point of view of J.J. Bikerman

about weak boundary layers. The concept was put forward (1974) according to which the origin of weak boundary layers is not only technological defects, but mainly physicochemical factors accompanying the formation of adhesion joints. The main reason for the appearance of the weak boundary layersconsists in the effect of the interface on the formation conditions of the structure of boundary layers and appearance of various levels of microheterogeneity of the surface layers (see below). The theoretical approach to the adhesion and structure of the interphase has allowed developing various ways of improving adhesion, that are based on the introduction into the adhesive of various additives, fillers, surfactants,

application as adhesive of interpenetrating polymer networks etc.

During last 10 years the concept of adhesion of polymer blends has been formulated. It was based on the estimation of the thermodynamic work of adhesion of blend expressed using the values of thermodynamic work of cohesion of the blend components. It was established the important role of the selective wetting of the surface by the melts of the constituent components and formation of mixed surface layers. In some works done in cooperation with G.V. Kozlov beginning from 2000 for the first time there were investigated fractal characteristics of adhesion. It was theoretically shown that the adhesion strength depends on the difference in the fractal dimensions between solid surface and polymer. It was also shown that the important role belongs to the fractal properties of interfacial layers of polymer formed at the interface with solid.

Filled Polymers (Polymer Composites)

Surface and Interphase Layers at Polymer — Solid Interface. In some works done in 1970-1980 the concepts were developed concerning the conditions of the formation of the boundary layers and interphase at the interface polymer — solid. These layers are the important factor of reinforcing action of fillers in filled polymers. When analyzing properties of the surface layers the distinction was made between the adsorption layers where macromolecules directly interact with the surface and boundary layers whose properties differ from the properties of a polymer in bulk. The adsorption layer consists of macromolecules with higher surface activity and is formed by applying a polymer melt or solution or reaction mixture to the solid. Its thickness depends on the layer formation conditions. Boundary or interphase layers are those with the properties changed as compared with bulk due to surface effect. These layers are characterized by the effective thickness that depends on the property under consideration. The function describing the change in properties of a polymer at various distances from the surface is determined by three fundamental variables: polymer cohesion energy, surface energy of a solid and polymer chain flexibility.

The effect of the surface forces on the properties of boundary layers is the following. Adsorption interaction of polymer molecules with a solid results in the redistribution of intermolecular bonds acting between macromolecules and in the formation of additional junction points with the surface.

Due to these effects the molecular mobility of chains in the layers near the interface diminishes and relaxation processes of the structure formation proceeding under other conditions as compared with the polymer bulk. In the works done beginning from 1972 jointly with F.G. Fabulyak and V.F. Rosovitsky using the mechanical, dielectric and NMR spectroscopy and volume relaxation, it was shown that due to the restriction of molecular mobility at the interface one can observe the increase in glass transition temperatures of a polymer and relaxation time. Due to superimposing of properties of the boundary layers and polymer in bulk, the relaxation spectra of filled polymers are also changed. The main contribution to the diminishing molecular mobility rather belongs to the conformational restrictions imposed by the surface, than to energetic interactions of polymer molecules with a solid. In order to explain the long-range surface effect, the “relay-race” mechanism was proposed, ascending to which the influence of the surface on molecules that interact directly with the surface owing to intermolecular forces and formation of these or other supramolecular structures, extends to neighboring macromolecules that are not in contact with the solid surface directly. This point of view, following from the analysis of polymer adsorption, is confirmed by the fact that the long-range effect is the greater, the higher is the energy of cohesion of a polymer, i. e. the stronger are the intermolecular interactions. The “relay-race” mechanism includes both intermolecular interactions and chain entanglements. One of the most important consequences of molecular mobility restrictions in the boundary layers is the hindering of relaxation processes and formation of more loose structure with lower packing density. The loose packing facilitates the movements of the side groups and short segments of chains, responsible for the group losses. The non-equilibrium state of the surface layers may be shifted to more stable state by heat

treatment.

By special measurements, it was established that the boundary layers are characterized by the non-uniformity of their properties and by a gradient of properties towards the surface. The surface influence depends not only on the surface energy of a solid, but also on the phase state of a polymer, i.e. is different for amorphous and crystalline polymers, linear and branched polymers and for three-dimensional network polymers.

The results of investigations done jointly with V.P. Privalko have enabled to give thermodynamic definition of what is the highly loaded polymer (1983). According to this definition, such polymers are the systems where the entirc polymer component has transformed into the state of boundary layer, i.e. all the polymer volume is under the action of the surface forces; the thickness of such layers in highly loaded polymers is of the order of the gyration radius of macromolecular coil.

For polymers filled with polymeric fillers it was established the mutual influence of the interface on the molecular mobility of the both components in the boundary layers. For the blends of thermodynamically incompatible polymers, their formation is accompanied by the formation of an interphase diffuse layers (works done jointly with E.V. Lebedev, 1975). In the interphase region there is possible both the formation of the border surface and coexistence of two border layers on both sides of the interface. These effects are due to thermodynamic incompatibility and to features of microrheological processes by the system formation and by the values of the interfacial tension.

Microheterogeneity of Surface Layers. The influence of the solid surface on the properties of the polymer surface layers is an important factor determining the emergence of a nonhomogeneous or microheterogeneous structure. In disperse polymer systems the heterogeneity of structure, determined by introduction of the filler particles into polymer matrix, is superimposed with microheterogeneity caused first of all by the appearance of the surface layers and interphase and by the difference in structure and properties of the interphase region and polymer in bulk at the molecular and supramolecular level. The interfacial layer itself is the region

of the heterogeneity.

For filled polymeric systems the concept was developed concerning various levels of microheterogeneity (1974). The first level appears because the surface layer thickness depends on the property to be determined. As regards one characteristic the properties of the system differ from those of the polymer in bulk and as regards the other one they do not differ. The second level is determined by differences in the conformation set of macromolecules. Changes in the packing of macromolecules contributed to this level of microheterogeneity. The nonmonotonous change of surface layer properties in a direction normal to the surface leads to the emergence of microheterogeneity on the molecular level. Its character and distance from the surface at which it is revealed, depend on the properties of the solid and polymer and especially on its cohesion energy. For polymers having low cohesion energy the transition to the state of the surface layer has little effect on properties. At high cohesion energy, the surface effect is transferred due to the “relay-race” mechanism and leads to the long-range  influence of the surface.

The effect of the surface leads also to the change of crystallization condition in the surface layer and degree of crystallinity. Thus the microheterogeneity on the structure level arises. For polymers that were formed from monomers in the presence of fillers, the surface affects the reaction conditions and chemical structure and molecular masses of the forming polymers at different distances from the surface. This level ofheterogeneity, a chemical one, in its turn, leads to the appearance of all heterogeneities mentioned above.

In polymer-polymer systems some additional levels of microheterogeneity appear due to formation of two boundary layers at the interface between two polymers and diffuse interphase. The colloid-chemical reason of the microheterogeneity is the existence of the difference in the surface tension of two components and fractions of various molecular masses. As a result the redistribution of the components in the interface zone proceeds in such a way as to

minimize the free interfacial energy. An additional level of microheterogeneity is connected with thermodynamic incompatibility of two polymers which results in the appearance of an additional free volume in the interphase, that determines the changes in properties connected with freevolume. Thus three main types of microheterogeneity may be present in filled systems:

1) molecular heterogeneity determined by the macromolecular structure of polymer chains (thermodynamic properties, molecular mobility, density packing);

2) structural microheterogeneity due to changes in the ordering of macromolecules near the interface at different distances from it;

3) microheterogeneity on the structural level (difference in the crystalline ordering, crystallinity degree, morphology of amorphous polymers);

4) chemical microheterogeneity that is determined by the effect of the surface on the conditions of polymer formation in the filler presence;

5) colloid-chemical microheterogeneity connected with the difference in the surface properties of polymers and their fractions.

The further development of the investigation of the surface layers enabled to establish the surface effect on the processes of microphase separation in polymer blends and interpenetrating polymer networks proceeding in the filler presence and elucidate the role of the dissipative structures on the microheterogeneity level in polymer alloys and blends (works done jointly with V. V. Shilov, 1980). The formation of modulated structures in the course of spinodal decomposition in presence of fillers and formation of structures with different wave length of spinodal decomposition contributes very much to the microheterogeneity of filled polymeric systems.

Viscoelastic Properties of Filled Polymers. The concepts of surface layers of polymers at the interface with filler surface allowed the generalized model of the structure of the filled polymer to be proposed (1975). According to this model, between polymer matrix and filler particles there exists an intermediate region (interphase) with properties that are different as compared with those in the bulk. Depending on the share of the matrix with changed properties (that is determined by both polymer and surface nature and filler concentration) the interphase contribution to the system properties will be changed. The proposed model generally used now allows estimating this contribution using mechanical models where the thickness of the interphase and its glass transition temperature are taken as a variable. Such a consideration is underlain by the data on thermodynamics of

filling, packing density and glass transitions. It was found that in the majority of cases with increasing the filler content, the glass transition of a polymer increases due to the changes in molecular mobility in the interphase. However, due to “relay-race” mechanism, only one glass transition is usually observed, i.e. the transition in the interphase and in matrix is very diffuse one.

Jointly with V.F. Rosovitsky, the conditions of the resolution of transition temperatures were established theoretically (1980). The effects of the change in glass transition temperature are connected with changes of the free volume of the interphase and matrix. On this basis the methods of calculation of the viscoelastic functions of filled polymers were proposed. The experimental investigations of temperature and frequency dependencies of viscoelastic functions of polymers with

various degree of loading, done jointly with V.F. Rosovitsky and V.F. Babich, allowed establishing the principal concept of concentration (filler)-frequency and concentration-temperature superposition (1974-1975). Its essence consists in the idea that the filler introduction and the interface appearance, as a result of diminishing the molecular mobility, is equivalent either to the decrease of temperature or to the increase of frequency. The investigations of the rheological properties of filled polymers have shown that they are determined by the mechanical properties of the structural frame formed due to interactions between the filler particles covered with adsorption layers. The regulation of the rheological properties of the filled polymers may be achieved by means of surface modification of the filler particles that determines the interaction between particles and with dispersion medium. The concepts developed allowed the mechanism of reinforcing action of fillers in polymeric systems to be considered from nontraditional point of view. The determining role belongs here to the changes of the matrix properties under the action of the filler surface and formation of an interphase with changed, as compared with bulk, properties. An important role in reinforcing belongs also to the changes in conditions of the binder curing on the interface with the filler surface. These changes are determined by various factors — changes in the reaction rates, changes in elementary constant of polymerization, distortion of stoichiometry due to selective adsorption of components with filler surface, changes in viscosity of the reaction system etc. These factors determine the changes of the matrix properties formed in filler presence and the degree of conversion in the reacting system.

Polymer Alloys and Hybrid Matrices

On the Polymer Alloy and Hybrid Matrix. The investigations of polymer composites formed by a polymeric matrix and dispersed fillers have led to a new approach in creating composite materials based on polymer alloys. The essence of

this approach is the following. When preparing polymer blendsor alloys, depending on the thermodynamic conditions of their mixing and type of critical solution temperature, the processes of phase separation are determined by thermodynamic incompatibility (immiscibility) of linear components of the alloy. Phase separation in the polymer systems, as a rule, is uncompleted. The analysis of the behavior of the blends of polymers of linear structure, cross-linked polymers and interpenetrating polymer networks allows proposing the thermodynamic definition of polymer alloys (1990) and formulating the concept about a new type of binders for composite materials, the so-called hybrid binders (1985). These binders themselves may be considered as multiphase heterogeneous composite materials formed by two or more polymers. Various polymeric systems may be referred to hybrid binders (blends of linear polymers, semi- and full interpenetrating polymer networks and all other multicomponent polymer systems with thermodynamic incompatibility of components under the definite conditions). These systems are characterized by one common feature: by changing the polymer mixture temperature or in the course of the curing reaction there appears thermodynamic incompatibility of the system components and as a consequence the system appears with incomplete phase separation. The result of incomplete phase separation is the forrnation between two phases evolved of transition regionsor interphase with properties that differ from those of the phases or pure components. The hybrid matrix is not in the state of true thermodynamic equilibrium but is only stable kinetically.

As a consequence of the processes of phase separation proceeding according to different mechanism (nucleation or spinodal decomposition) the segregated structure is formed with a complex of specific properties: appearance of the regions with different density, composition and mechanical properties, appearance of the internal interphase boundary etc. Hence, the definition was given: a hybrid matrix, wherein segregation of microvolumes of constituent components or phases has occurred owing to incomplete microphase separation, may be considered as self-reinforced (filled) or disperse-reinforced system, wherein the size, properties and distribution of the regions of microphase separation (virtual particles of a filler) are determined by the phase diagram of the binary or multicomponent system, conditions of the system transition through the binodal and spinodal and by the mechanism of phase separation. In polymeric systems like hybrid matrices,

two groups of interfacial phenomena should be considered — at the interface between two regions of microphase separation and at the interface with reinforcement.

The principal distinction of these systems from the polymer  blends consists in one-phase state of the initial system (melts of linear polymers or reaction systems capable of curing). Microphase separation in such systems occurs by changing thermodynamic conditions. For the mixtures of linear polymers, the transition from one-phase system to two-phase one may occur either by decreasing or by increasing temperature depending on the type of critical solution temperature. For reactionable systems the phase transition proceeds after the system reaches a definite conversion degree and critical value of thermodynamic interaction parameter, corresponding to the system transition to two-phase state.

The investigations of structure by means of SAXS and of viscoelastic and thermodynamic characteristics of such systems have allowed characterizing them quantitatively by two main parameters: a parameter of thermodynamic interaction and that of segregation degree (1985) (works done jointly with V.V. Shilov and V.F. Rosovitsky). Using these parameters a great number of complicated multicomponent systems were characterized, including the filled ones. It was established that the physical and mechanical properties may be adequately described by segregation degree, whereas the degree of nonequilibrium is described by the thermodynamic parameter of interaction. Experimentally found interaction parameter consists of three parts that are determined by the conditions of the phase separation and degree of approaching equilibrium.

Filling of Linear Polymer Alloys. Beginning from 1985 a t deal of investigations in the field of filled polymer alloys with various fillers have shown that the introduction of the “fatter changes both the shape and position of the phase diagram. ‘Theoretically and experimentally it was established (jointly with A. E. Nesterov and T. T. Todosiychuk) that the reasons for the effect of a solid on the position of the curves of phase equilibrium and on thermodynamic stability are as follows:

1) the difference in changes of free energy while mixing each

alloy component with a filler,

2) selectivity of interaction of any component with the filler surface, that leads to the change of the alloy composition near the interface,

3) change of conditions for the interaction of two different polymers near the interface due to conformational restrictions,

4) effect of a solid on the kinetics of phase separation by transition through binodal and spinodal and on the final result of non-equilibrium separation.

It was shown that the introduction of fillers might lead both to the “forced” compatibility of components and to “forced state” of incompatibility (microphase separation). The conditions of these “forced” effects depend on the kinetics of phase separation and peculiarities of interaction of components with the surface. The experimental data allowed the conclusions to be drawn as for the thermodynamic state of filled alloys. The concept was introduced concerning the compatibilizing effect of fillers and on the equilibrium and non-equilibrium compatibilization (1991). Under equilibrium compatibilization the increasing Stability of the system is determined by changes in thermodynamic interaction between components ncar the interface and by symmetric interactions of components with the surface. Under non-equilibrium compatibilization the apparent improving of compatibility is determined by the hindering phase separation due to interaction of components with the surface, as a result the structure is “frozen” corresponding to the state of the system at more favorable

conditions.

There were also found the conditions of formation of the interphase layers in filled polymer alloys and of nonequilibrium surface segregation of components by the alloy formation in the presence of a solid. It was shown that the surface enrichment in one of the alloy components as a result of selective adsorption changes the interphase properties and as a result the properties of the whole system. Surface segregation depends on the ratio of thermodynamic parameter of interaction between two polymers and each polymer and a solid. It was established that the surface affects the processes of phase separation and compositions of evolved phases.

Segregation effects depend also on the molecular mass of the components and their distribution. Because of the selectivity of interaction the segregation degree near the surface is lower as compared with the bulk, i. e. the surface layer is in the state of forced compatibility. In real systems the segregation proceeds by changing temperature of the alloy, i. e. proceeds in non-equilibrium conditions and stops on reaching glass transition temperature or melting point. If the temperature of the phase separation is higher than glass transition point, surface segregation can proceed simultaneously from two phases evolved if there are no conditions for the selective wetting of the surface by one of the phases. The experimental data allowed the model of filled polymer alloy to be proposed (1995). As distinct from the model of a filled polymer, this model supposes the existence of a gradient of composition normal to the surface due to surface segregation. Since the compatibility depends on the component ratio, its degree is also different at various distances from the surface. According to the model the redistribution of alloy composition takes place in the system and the matrix composition does not correspond to its composition for unfilled system. That means that the changes in the component compatibility depending on filling are extended to the whole volume of the polymer alloy.

In a series of works done together with A.E. Nesterov and T.D. Ignatova the processes of phase separation in filled polymer blends were investigated. The effect of the filler surface on the distribution of components in separated phases, kinetics of phase separation and activation energies of the process was established. The increasing thermodynamic compatibility of incompatible polymers in blends was discovered and described in terms of differences of thermodynamic interaction parameters between components and components and filler surface. The theoretical grounds of the influence of the interface with a solid on the phase behaviour of polymer blends and their thermodynamic stability are the following:

- the difference in the change of the free energy of mixing of each component with the filler surface and the free energy of interactions between components in the surface layers;

- different interaction parameters between the components in the surface layer and in the bulk; the selectivity of interactions between the surface and polymeric components that determines the selective adsorption or surface segregation at the interface in the melt state, the latter being dependent on the filler content or on the thickness of the polymer layers at the interface.

The surface segregation of the blend components at the interface and formation of the surface layers leads to the difference in the compositions of the surface layer and the bulk. The redistribution according to the molecular masses of two components contributes to the different compositions of the surface layers and the bulk. Phase separation in filled blends proceeds in the surface layer and in the bulk at various temperatures due to the difference in their composition. As a result in the system at least four regions of the phase separation appear: two in the surface layer and two in the bulk. The border between them is very diffusive due to the formation of an intermediate region in the course of the phase separation. The shape and position of the phase diagram of filled blends depends on the filler concentration. The dynamics of the phase separation in the filled blends is determined by the retarding action of the filler surface due to adsorption interactions of the components with the surface. For the blends formed in situ (reaction mixing of two linear polymers or formation of the interpenetrating polymer networks) the thermodynamic incompatibility develops in the reaction course at a certain stage of conversion (before gel point). The process of phase separation develops under condition of the continuous change in chemical composition of the system. The superposition of the chemical processes of the polymer formation and physical processes of phase separation determines non-equilibrium conditions of the material formation. The phase separation dynamics in filled systems is connected with the filler effect on the kinetics of the reaction

and thus on the time of the phase separation onset. Besides, adsorption at the interface retards the rate of the phase separation.

Thus the experimental data available allow the conclusion to be drawn that all the processes of the phase separation in the filled blends proceed under the non-equilibrium conditions, which, in its turn, determines the non-equilibrium state of the materials obtained.

Interpenetrating Polymer Networks.

Cross-Linked Polymer Alloys. Since the 1970’s Yu.S. Lipatov paid great attention to the kinetics of reactions leading to the formation of semi- and full interpenetrating networks (IPN) and investigation of their viscoelastic properties and structure. The main investigation objects were polyurethane networks and polybutylmethacrylate, oligoesteracrylates, styrene-divinyl-benzene copolymer and others. It was established, that in the case of sequential IPNs the host-network and in the case of simultaneous IPNs the network formed earlier, change the conditions of reaction of formation of the guest- or second network in the kinetic region due to changes in the ratio of the rates of propagation and termination and due to interaction with the matrix-network (host-network) chains. Simultaneously, the reaction parameters are also changed in the diffusion region. It was established that under formation of both sequential and simultaneous IPNs in the course of reaction of curing of the networks, the microphase separation proceeds due to the arising thermodynamic incompatibility of growing chains of constituent networks.

Detailed investigations, done together with T.T. Alekseeva, have shown that the microphase separation is tightly connected with reaction kinetics. The rates of the formation of two constituent networks are interconnected and there exists the

mutual influence between the reaction of formation of one network and the reaction of the formation of the second network. The kinetic parameters depend on the ratio of constituent networks. It was discovered that the microphase separation begins long before the gel point and the time of the onset of microphase separation does depend on the reaction rates and network ratio. The degree of microphase separation (segregation degree) also depends on the reaction kinetics. At equal conversion degrees, the time of the microphase separation onset is longer at lower reaction temperatures when the compatibility is lower (for the system with upper critical solution temperature) and viscosity of the reaction medium is higher. The principal feature of the IPN formation consists in the superposition of the chemical processes of the network formation and physical processes of microphase separation.

In the works done since the 1980’s together with V.V. Shilov and A.E. Nesterov, it was established that the process of separation in IPNs proceeds according to the spinodal mechanism (1985). Depending on the reaction rate, the microphase separation may be suppressed at various conversion degrees. In such a way the kinetic parameters of the reaction and the way of formation (simultaneously or sequentially) being changed it becomes possible to change the structure. Generally, the microphase separation is a function of the reaction rates and the ratio of the constituent networks depending on their compatibility or incompatibility. Microphase separation, proceeding simultaneously with chemical reaction, takes place under the non-equilibrium conditions (1984).

In the late 1980’s the concept of self-organizaticn in IPN was put forward and developed. The self-organization in the non-equilibrium systems is the appearance and development of the structure in initially uniform medium. Since the formation of IPNs proceeds in the non-equilibrium conditions due to superposition of chemical and physical processes, the concept of self-organization was applied to hybrid matrices (1990). The main processes accompanying the selforganization are the formation of the cross-linked network structure and microphase separation. The experimental data allow the conclusion to be drawn as for the general conditions of the self-organization in reacting systems like IPNs.

Under synthesis of IPN, the initial system is a homogeneous one-phase solution of the reaction components for in situ processes and homogeneously swollen network for sequential process. On reaching a definite conversion, far from the gel point, the thermodynamic incompatibility arises in the system,

which takes place on reaching a critical value of the interaction parameter. The initial condition of the transition of the system from one- to two-phase state is the transition of the interaction parameter from negative values (miscibility region) to positive values (immiscibility region). In the majority of cases this transition is realized at rather low conversion degrees in the kinetic region of the reaction. However, the phase separation from the very beginning proceeds under the non-equilibrium conditions because of the developing chemical reaction that changes the parameter value and conditions of phase separation. The rate of reaching the critical parameter value depends on the reaction rates of two reactions leading to the cross-linking. At the initial period of the reaction, the onset of the phase separation is determined only by the reaction rates, i.e. by the rate of the molecular mass increasing. However, the crosslinking and the formation of the three-dimensional network structure prevents from phase separation that may, in such a

way, occur only in the definite conversion interval and definite time. In this time interval the composition and ratio of two evolved phases change continuously. The type of structures developing on the various stages of the reaction is determined both by phase separation and chemical reaction. As a result, the final structure of IPNs is the result of the coexistence of three types of the regions of incomplete phase separation that are dissipative structures. Two microregions are formed as a result of phase separation. Each of them preserves the state of component mixing corresponding to earlier stages and may be considered as an independent IPN with forced molecular level of mixing. In these IPNs the molecular level of mixing is preserved. The coexistence of two evolved phases of various compositions predetermines the system heterogeneity. Together with these two phases, the third region of incomplete phase separation exists that is the result of non-equilibrium separation. This region ts the transition zone between two evolved phases that, in its turn, also may be considered as an independent IPN. It may be supposed that the composition of the transition zone corresponds to the initial ratio of two networks. Each of three regions has different composition and is non-equilibrium. After the phase separation ceasing at a definite conversion degree, the further cross-linking and formation of the final chemical structure occurs in these three regions. Thus, the complexity of the self-organization andformation of IPNs consists of superposition of the chemical reaction and sol — gel transition and of phase separation. The sequence of transitions is determined by the reaction kinetics.

The latter may also change the mechanism of phase separation. Thus, the general conditions of self-organization are determined by 1/ the ratio of the reaction rate of formation of two networks, 2/ sol — gel and liquid — liquid transitions occurring in the course of the reaction.

The following conclusions about IPN structure have been made. As distinct from the accepted point of view, the absence of molecular mixing and interpenetration only on the level of evolved phases, the structure of IPN is considered as consisting of three regions of molecular mixing, that form three independent regions of incomplete microphase separation.

Composition of these regions is determined by the state of forced compatibility. IPN being generally non-equilibrium structure consists of the regions that are in the state of quasiequilibrium and represent the dissipative structures (1990).

This concept meets experimental data on the viscoelasticity of IPNs and relaxation spectra. Investigations, done together with V.F. Rosovitsky and T.T. Alekseeva, beginning from the middle of the eighties, dealt with filling IPNs by dispersed and fibrous fillers. It was established that the fillers exert the compatibilizing action (1991) and increase the thermodynamic stability of IPNs due to hindering the processes of microphase separation. The filler aiso affects the kinetic conditions of the reaction and in such a way the conditions of microphase separation. Thus, the filler plays not only the traditional role of reinforcement, but also the role of the agent regulating microphase structure.

The investigations of the compositions of the IPNs near the interface and in the bulk of the system enabled to establish the enrichment of the boundary layer with one of the IPN components. The obtained result allowed the conditions of non-equilibrium surface segregation in IPN to be formulated.

It was established (1995) that the interface with a solid is enriched in the component that faster reaches the higher surface tension as compared with other component, i. e. the kinetic factor plays the decisive role in segregation.

A great role in developing the problems of IPNs belonged to the permanent discussions with outstanding world leaders in this field Prof. Kurt C. Frisch and Prof. L. Sperling. Since 2000 the processes of compatibilization of IPNs have been investigated for the first time (T.T. Alekseeva, S.I. Grishchuk, N.V. Babkina, N.V. Yarovaya). As distinct from compatibilization of the blends of liner polymers, where compatibilizing agents are introduced into the melt of two immiscible polymers, in the case of IPNs these compounds are introduced into one-phase reaction mixture before curing. The investigation of the effects of some additives, which, according to their chemical constitution, could be considered as potential compatibilizers for IPNs, allows establishing two different types of their compatibilizing action.

In one case, the additive introduced into initial reaction system prevents from microphase separation of the system. It may mean that the ternary miscible system arises where thermodynamic interactions between components lead to the

diminishing of total thermodynamic interaction parameter between components down to the negative value typical of thermodynamic stable system. In this case the viscoelastic properties are changed in such a way that instead of two loss maxima characteristic of the phase-separated system, only one maximum is present, that is a possible result of the formation of one-phase ternary system. In another case, the additives exert some effect on the reaction kinetics and thus accelerate microphase separation in the system, which proceeds at lower conversion degrees as compared with pure IPNs. Under such conditions, thermodynamic compatibility of IPN components in the presence of additives decreases. The acceleration of the reaction in the presence of additives only shortens the time for the onset of phase separation. However, in this case there proceeds also the transition from the systems with two relaxation maxima (two-phase system) to the system with one, broad or narrow, maximum. This may be the result of the additive concentration in the interfacial region between two phases. It is evident that the morphology of such a system also undergoes changes, and thus, instead of discrete phases, the diffuse structure or structure of the matrix — inclusion type is formed. In such structures the sizes of phase domains (in clusions) of component present in smaller amount are rathersmall to be detected by the methods of dynamic mechanical spectroscopy. In fact, the detection of a single 7, only signifies that the size of the blend domains is very low.The effect of additives on the kinetics of formation of the INP components and on the time of onset of phase separation may serve as a test for the compatibilization mechanism. The additive is introduced into the homogeneous reaction mixture and so it is distributed uniformly throughout the whole mixture volume. If the additive acts as the third component improving miscibility, the phase separation onset should be shifted by the time scale to higher times or should be fully absent. On the contrary, if the additive has no effect on miscibility or has negative effect (increasing the thermodynamic interaction parameter of the ternary system) one should suppose that this additive is concentrated at the interface between two phases, changing the morphology of the system that is formed in the course of IPN curing. It follows from the above experimental data that, as a rule,

introduction of compatibilizers increases the fraction of an interphase, which may be interpreted as apparent increasing compatibility. However, the physical essence of this effect is not connected with real improving the compatibility, i.e. with increasing thermodynamic stability of the system. Generally, the formation of an interphase is the result of uncompleted phase separation at which some part of the system remains in an unseparated (mixed) state partially corresponding to the state of the system before the onset of phase separation (‘‘frozen” compatibility). Thus the interphase is the region of thermodynamic instability that increases the thermodynamic instability of the system. It is evident that the introduction of compatibilizing agents prevents from phase separation and increases that part of the system, which is not phase-separated. Compatibilizer transfers the system to the less stable state by increasing the fraction of the system which is not separated.

The currently available experimental data on the structure of IPNs allow the following conclusion to be drawn. The uncompleted phase separation during the reactions of IPN formation and formation of the thermodynamic nonequilibrium

system results in evolving two phases of variable composition. Each evolved phase may be considered as quasiequilibrium phase with a molecular level of mixing. The IPN as a whole is the system where one can observe no interpenetration on the molecular level over the entire volume of the system. One may only speak about interpenetrating of various phase regions. The most distinguishing feature of IPNsconsists of the existence of the interfacial region, which may conventionally be considered as non-equilibrium third quasiphase.

Its appearance is the result of the spinodal mechanism of phase separation at which there is no sharp interface between coexisting regions of phase separation. The interfacial regions preserve the level of molecular mixing of the system before the onset of phase separation (“frozen” compatibility). The existence of interfacial region strongly affects the viscoelastic properties of IPNs. Thus in the IPNs in the course of their formation there appears the hierarchy of structures which, due to

kinetic and thermodynamic reasons, hold the thermodynamically non-equilibrium state. At the same the kinetic stability of all these system is very high. An important conclusion is that the correlation between compositions of the separated phases (after full conversion) and initial conditions of the phase separation shows that the final structure of the material is put at the early stages of the reaction. Phase separation proceeds in a four-component system (two initial components and two various network fragments) according to the spinodal mechanism. Full conversion of functional groups at the latest stages of reaction or the cross-linking after reaching the gel point takes place after the phase separation and has no essential influence on the initially formed structure. This also explains the fact that

the regularities of the reactions and phase separation are practically the same for the formation of IPNs (in the presence of cross-linking agent) and in blends of linear polymers formed in situ according to various mechanisms.

Thus the final structure of phase-separated IPNs is characterized by the presence of three conventional phase regions which differ in composition, each region being an IPN with molecular level of mixing. All three regions are thermodynamically non-equilibrium, but the degree of deviation from the equilibrium is different for each part of the system.

The reaction kinetics in IPNs formation plays a very important role in the formation of the final structure. It determines both the segregation degree and the interfacial region fraction. In some of works the detailed study of the effect of kinetic parameters on the microphase structure has been investigated for full and semi-IPNs based on various cross-linked polyurethanes and styrene, butyl methacrylate and methyl methacrylate. For various concentrations of an initiator of radical polymerization and urethane formationcatalyst the following characteristics of the processes were determined: the ratio of components, the time of the onset of microphase separation (by light scattering), the conversion degrees of both components at the onset of phase separation, the segregation degree a and fraction of the interfacial layer (1 -F).

The segregation degree and a fraction of the interfacial region depend on many factors, the conversion at the point of the onset of phase separation being the most important. The general conclusion can be drawn that the value of segregation

degree is determined by:

~— the time of the onset of phase separation, i.e. the time of the

reaching the critical molecular masses at which the

incompatibility arises;

— conversion degree at which phase separation begins;

— reaction rates of the formation of both components;

— molecular masses of both components at the onset of phase

separation;

— the ratio of components in the starting reaction mixture.

The problem is that the reaction rates of the formation of both components in the reaction mixture depend on the presence of a second component and are not equal to the reaction rates for pure components. In this case one network is formed earlier and the formation of the second one proceeds in the matrix of the first. At a higher reaction rate at least of one of the networks, the critical molecular mass for the onset of phase separation is reached faster.

It is worth noting that the concentration of the cross-linking agent plays almost no role in segregation because the latter proceeds before reaching the gel point. In a series of special investigations the formation in situ of the blends of two linear polymers which are formed according to various mechanisms (principle of the IPN formation) have been studied. It was found that the regularities of this process are the same as for IPNs, this testifying to the similar mechanism of the phase separation in the absence and in the presence of the crosslinking agent (L.F. Kosyanchuk).

Molecular masses at the onset of the phase separation are the determining factors in the appearance of the thermodynamic incompatibility of the network fragments. At the same time, up to now there are no experimental data on the molecular masses of both components at various reaction stages and at the onset of phase separation. For the blends of linear polyurethane and poly(methyl methacrylate) synthesized in situ the molecular masses of both components after completion of the reaction have been determined. Mutual influence of two simultaneously proceeding reactions of polymer formation on the molecular masses and molecular mass distribution of both components has been established.

Microphase separation depends also on the ratio of components in the starting reaction mixture. This ratio determines the reaction kinetics and time of the onset of phase separation. Dependence of α on composition is conditioned by the thermodynamic reasons.

It is also worth noting that the general rule is valid: the higher is the segregation degree o the less is the fraction of an intermediate region (1 — F), which is natural, as the latter is the measure of the unseparated part of the system.

All the kinetic parameters influence the time of the phase separation onset and molecular masses reached by that time, i.e. the appearance of the critical conditions for phase separation.

In fact the segregation degree and fraction of the interfacial region are governed only by the thermodynamic parameters, and kinetics is only the tool to reach the necessary thermodynamic conditions. Segregation depends also on the type of the system (full or semi-IPN).

When considering the effect of the reaction kinetics on the microphase separation in reaction systems of IPNs three cases may be selected:

1. If one network is formed much faster than the other, its formation proceeds in the liquid medium of components of the second network. In such a case the phase separation may reach a high degree of completeness without any steric hindrances. The swollen phase of this network may evolve in the liquid phase formed by the second network components. The latter forms the network at the latest stages. As a result the microphase structure with a high degree of segregation is formed. It is evident that the higher is the segregation degree the lower is the fraction of interfacial region.

2. If both reactions proceed at high rates the microphase separation has no time to proceed and the structure is “frozen” as is typical of one-phase state. Such a system is thermodynamically non-equilibrium with the “quenched” structure of initial reaction mixture. “Freezing” of the onephase structure implies the absence of any interfacial regions. The most typical case is when the reaction rates are comparable. The microphase separation begins at earlier reaction stages and then slows down with increasing the system viscosity and cross-linking up to the gel point. The microphase structure with uncompleted phase separation is formed with two phases, whose composition depends on the reaction rates of the formation of two networks. The composition of two phases evolved depends on the reaction rates and on the rate of the phase separation (superposition of two processes determines the fraction of the interfacial region).

The achievements reached by Yu.S. Lipatov in various fields of physical chemistry of polymers are the result of the efforts made by him together with many coworkers at the Department of Physical Chemistry of Polymers, Institute of Macromolecular

Chemistry. At various stages of Yu.S. Lipatov’s service to science, his scientific activity has been shared by

T.T. Alekseeva, V.F. Babich, N.V. Babkina, L.I. Bezruk, V.N. Bliznyuk, V.N. Chornaya, G.V. Dudarenko, F.G, Fabulyak, A.E. Fainerman, I.P. Getmanchuk, T.D. Ignatova, L.V. Karabanova,   Yu. Yu. Kercha, L.F. Kosyanchuk, A.N. Kuksin, T.S. Khramova,

E.V. Lebedev, T.E. Lipatova, Yu. V. Maslak, G. Ya. Menzheres, E.G. Moisya, A.E. Nesterov, L.N. Perepelitsyna, V.P. Privalko, V.F. Rosovitsky, L.M. Sergeeva, G.M. Semenovich, V.V. Shilov, V.F. Shumsky, T.T. Todosiychuk, N.V. Yarovaya.