A relationship between the amount of thermal shrinkage and the structural parameters of polyethylene eztruded in the solid state and polymerization-filled compositions based on it is studied. It is shown that thermal shrinkage is determined by the fractaI dimension of the polymer structure and the degree of stretching of the molecular chain.Investigation of thermal-shrinkage characteristics gives valuable information on the structural changes caused by orientation processes in polymers [1]. However, most papers devoted to this problem have studied only qualitative regularities of these processes. In recent years, fractal analysis [2] has been successfully employed for quantitative description of the relationship between the structure and properties of polymers. We used this approach to establish a relationship between the thermal shrinkage and structural parameters of specimens of superhigh-molecular-weight polyethylene (SHMWPE) and polymerization-filled compositions based on it, oriented by solid-state extrusion [3]. The processing method of [3], which combines transformation of the starting powder to a monolithic state and its orientational extrusion, yields high values of rigidity and strength of the articles produced [4].In our experiments, SHMWPE with a molecular weight of ,,,106 and the polymerization-filled compositions SHMWPE-AI and SHMWPE--bauxite are used. The particle size of the filler is 10 #m, and the concentrations (by mass) are 70 and 45%, respectively.Test specimens were prepared by solid-state extrusion by the following scheme [4]: preliminary compaction of a powder specimen in a cylindrical mold, free heating of the compacted specimen to 403 K (SHMWPE) or 393 K (compositions based on SHMWPE), extrusion through a die heated to the same temperatures using a high-pressure container. At the indicated temperatures, extrusion of a polymer workpiece through a die with an orifice diameter smaller than the diameter of the workpiece ensured production of monolithic specimens with a typical anisotropic structure oriented along the axis of extrusion [5]. The degree of extrusion elongation was varied using dies of different diameters and was calculated from the formula A = ~/~, where ds and do are the diameters of the specimen and the die opening, respectively. For comparison, specimens produced by pressing (pressing temperature 433 K and pressure 100 MPa) were tested.Thermal shrinkage was studied on cylindrical extrudates of diameters 5-12 mm and length 15 mm after heating them in glycerin with exposure to each test temperature for 15 min. Since, upon heating, the length of such an oriented polymer specimen decreases with simultaneous increase in its diameter (recovery of the shape preceding extrusion), ~b = (d2 -dl)/d2 (dl and d2 are the extrudate diameters before and after exposure to the given temperature) was used as the parameter that reflects the shrinkage process.Mechanical properties were measured for cylindrical specimens of diameter 4.5 mm and working length 30 mm under three-point bending. The ...

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A quantitative model is proposed for flow and forced high elasticity of cross-linked polymers based on the cluster model of amorphous state structure. The flow process is seen as the loss of displacement stability by the clusters, while forced high elasticity is related to the mecham'cal devitrification of the sofrpack matrix. Fractal analysis was used to show a turbulent regime for the forced high elasticity of cross-linked polymers.The mechanisms for fluidity and cold flow have been studied intensively in light of their theoretical and practical importance. However, despite the vast number of investigations in this area, a complete understanding of these processes has not yet been achieved. To a large extent, this is the case for amorphous and, especially, cross-linked polymers. Various approaches from a dislocation-disclination model [1] to a molecular kinetics model [2] have been employed to describe these processes. Such a broad range of approaches is a function of the nature of amorphous glassy polymers, which are intermediate in structure between liquids and true solids [3]. In our view, further progress in this field for cross-linked polymers is inhibited by at least two circumstances: 1) an extreme exaggeration of the role of the network of chemical cross-links and 2) the lack of a quantitative structural model.At present, the idea of the existence of regions of local order in amorphous polymers first proposed by V. A. Kargin is accepted almost without discussion although direct experimental evidence for such regions has not been obtained due to their small dimensions. The cluster model [4] presupposes that the structure of amorphous polymers entails regions of local order (clusters) surrounded by a softpack matrix. In turn, the clusters consist of several collinear dense-packed segments of various macromolecules, whose length is equal to the length of the statistical segment. Furthermore, two types of clusters are assumed to exist in the softpack matrix: clusters with a small number of segments and, thus, low thermal stability and clusters with a large number of segments and high thermal stability [4]. Evidence for two types of clusters is found in the two-step glass formation process [5], which occurs at the glass transition temperature of the softpack regions T s' and the glass transition temperature of the polymer itself Tg. In this case, Tg' is about 50 K below Tg. In the devitrification (thawing) process, which is seen as decomposition of the regions of "frozen" local order [6], the more stable clusters are lost at Tg', while the more stable clusters decompose at Tg. By analogy with crystallites, the less stable clusters should have smaller dimensions, i.e, contain a smaller number of segments. The value of T s' clearly corresponds to T 2 in the model of Filyanov [2], in which T2'divides two temperature ranges characterized by different behavior of the mechanical properties. This f'mding was attributed by Filyanov [2] to different deformation mechanisms. In the present work, a mechanism is propos...

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