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 ...
ABSTRACT:The possibility of applying the models of the irreversible aggregation and the fractal analysis for the description of curing kinetics of haloid-containing epoxy polymers was shown. There are two different modes of curing (homogeneous and nonhomogeneous), responding to conditions D ϭ const and D ϭ variant as a function of reaction time (D is the fractal dimension of microgels). The first condition corresponds to one dimension of the formed microgels, and the second one corresponds to the distribution of these dimensions. The mode of curing is determined by the level of fluctuations of density in the reaction medium. It was also shown that the fractal reactions at curing can be of the two following classes: reactions of fractal objects and reactions in fractal space. The basic difference of the two mentioned classes of reactions is the dependence of their rate on the fractal dimension of reaction products. The application of the methods of the fractal analysis and the theory of percolation allows us to find out that the first gelation point by crosslinking polymers is a structural transition, which is realized at filling by microgels of the whole reaction space. The physical nature of the autoacceleration (autostopping) effect in curing reactions is determined.
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