SURFACE LAYERS OF POLYMERS AT THE INTERFACE WITH SOLIDS
3.7 MICROHETEROGENEITY OF SURFACE LAYERS
The action of a solid surface leads, as we have seen, to many structural rear-rangements in the surface layers and to formation of a non-uniform or microheterogeneous structure.51In the filled polymer system, two types of het-erogeneity are present: macrohethet-erogeneity, due to the incorporation of solid particles, and microheterogeneity, which is determined by the formation of sur-face layers of a complicated structure. The sursur-face layer, or the interphase re-gion between solid and polymer in bulk, is the rere-gion of microheterogeneity. For polymer systems containing fillers, various levels of microheterogeneity may be distinguished. The first level is determined by the dependence of the surface layer thickness on the property which is investigated. The systems may differ in relation to one property but be similar in relation to another. The second level is determined by the differences in the conformational state of macromolecules in the surface layer and in the bulk, which, in turn, is responsible for differences in the packing density of macromolecules.
Non-monotonous change of molecular properties of the surface layers in di-rection normal to the solid surface indicates the microheterogeneity on the mo-lecular level. The character of this microheterogeneity depends on the properties of both solid and polymer. The transfer of the influence from macromolecules bound to the surface to more remote macromolecules contrib-utes to the microheterogeneity on the molecular level. The solid also affects the conditions of the formation of submolecular structures in the surface layers, and, in particular, the conditions of crystallization and crystallinity degree (see Chapter 4). This factor determines the structure of the polymer in the surface layers and the microheterogeneity on the submolecular level. By analysis of the chemical reactions proceeding at the interface with solid (see Chapter 4), it was established that chemical structure of a polymer (molecular-mass distribution and distribution of the polymer functionality) depended on the distance from the surface. This circumstance leads to another type of microheterogeneity - a chem-ical one, which, in turn, is connected with the appearance of additional molecu-lar and submolecumolecu-lar microheterogeneity.
An important reason for the appearance of the microheterogeneity in the surface layers is the difference in the surface tension of polymer fractions of vari-ous molecular mass. Under the influence of a solid surface, the redistribution,
according to the molecular mass and surface activity between surface layer and polymer bulk (colloid-chemical level of microheterogeneity), occurs. In poly-mer-polymer systems (see Chapter 6), an additional level of microheterogeneity also takes place due to the thermodynamic immiscibility of components and loosening of the interfacial region because of formation of an excess free volume.
In such systems, redistribution of free volume and the emergence of non-homo-geneous distribution take place, which is a direct cause of microheterogeneity at the level of molecular properties, determined by the free volume.
The analysis of the reasons for microheterogeneity in filled polymer sys-tems allows us to give their following classification:51
• Molecular heterogeneity exhibited by changes in physical characteristics in the interfacial layer, determined by the macromolecular structure of polymer chains (thermodynamic properties, molecular mobility, density of packing, free volume, level of intermolecular interactions, etc.)
• Structural microheterogeneity due to the changes in the mutual disposi-tion of macromolecules in reladisposi-tion to each other in the surface and transi-tion layers at different distances from the phase boundary and characterizing the short-range order in amorphous polymers and degree of crystallinity in crystalline polymers
• Microheterogeneity at the supermolecular level, depending on the type and conditions of the formation and packing of supermolecular structures in the surface layer and the bulk;
• Chemical microheterogeneity caused by the influence of the interface on the reaction of the formation of polymer molecules. This type may be an ad-ditional cause of the above three types of microheterogeneity;
• Colloid-chemical microheterogeneity, determined by the difference in the surface energy and surface activity of polymer fractions having different molecular mass.
These types of microheterogeneity are inherent in all polymer systems, filled with particulate and fibrous fillers, in two-phase and multi-phase polymer systems (mixtures of polymers with discrete and continuous distribution of com-ponents), as well as in polymer glues, coatings, fiber-reinforced plastics, i.e., in all polymer composites. However, in polymers with mineral reinforcement, microheterogeneity appears as a result of interfacial phenomena only in the
polymer matrix, whereas for polymeric reinforcement it is typical of polymer filler, polymer matrix, and the transition layer between two polymer compo-nents (Chapter 6). In filled polymer alloys, dissipative structures exist as a re-sult of non-equilibrium process of the formation of the alloy structure. The formation of modulated structures, due to spinodal decomposition and the coex-istence of structures having various wavelength of spinodal decomposition, con-tributes to the microheterogeneity.51
The preceding discussion allows one to address the problem of the phase state of surface and interfacial layers of polymers in composites. Despite the non-uniformity, they can be characterized by their intrinsic dimensions, ther-modynamic functions (entropy, enthalpy, specific volume), and the distinctions of mean local properties from the properties of the polymer in the bulk. In a num-ber of instances these distinctions may be similar to the difference in the proper-ties between amorphous and crystalline regions in semicrystalline polymers.
The redistribution of fractions of different molecular mass in a surface layer, taking account of limited thermodynamic immiscibility of polymer homo-logues,52 provides a basis to consider the transition layer as an independent phase. However, whether the surface and interfacial layers can be considered as an independent phase in the thermodynamical meaning or not is a very impor-tant question.
Let us take into account that the difference in properties, between surface layers and bulk is not indicative of random fluctuations and not described by a statistical distribution. For polymer systems consisting of two phases, the condi-tion of the existence of a sharp phase border is not applicable. The problem of whether the surface layers are an independent phase has been discussed for low-molecular-mass systems by Akhmatov.53He gave two indications of a phase as a volume-extended state: chemical and physical homogeneity, and the avail-ability of a thin transition layer forming at the interface. It is evident that there is no single answer to the problem in view of a great variety of types and states of boundary polymer layers. According to Akhmatov, one cannot speak about the boundary or surface layer as a strictly thermodynamic phase of any size without relating to its characteristics. In some cases, these layers may be considered ei-ther as three-dimensional or two-dimensional. The problem of wheei-ther the sur-face layers are independent phases with their own thermodynamic properties
and structure is similar to the endless discussion of whether semicrystalline polymers are one or two-phase systems. These systems, provided that the inter-face between amorphous and crystalline regions (the required condition of a phase by definition) is absent, are described as a two-phase system from the thermodynamic point of view. Surface and interphase layers would, by analogy, be considered as a phase if they were in equilibrium with the bulk phase. How-ever, from the above it follows that surface layers are not systems that have at-tained thermodynamic equilibrium, although, should it be atat-tained, the properties of the transition or surface layer would still be different from the properties of the bulk. Therefore, we think that surface layers should be consid-ered not as a thermodynamic phase but as a non-equilibrium dissipative system which arises under the influence of the surface force field, causing the deviation of the system from the equilibrium state.
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