Description of structural relaxation of amorphous polymers within the framework of the cluster model

Abstract

The cluster model of amorphous (glassy-like) polymers [1,2] assumes that their structure includes regions of local order (clusters) surrounded by a loosely packed matrix and postulates the concentration of fluctuation free volume in such a matrix [3]. Differences in the structural component of the glassy state cause their different behavior both in the processes of deformation [4] and relaxation. As is known [5,6], the glassy state of polymers includes, in turn, a number of states differing in the temperature dependences of mechanical properties. The characteristic feature of the transition from one state to another is a break in the temperature dependence of the corresponding parameter, e.g., of the yield strength. At the present time, an unambiguous structural identification of these states is lacking. The employment of concepts of the cluster model [l, 2] enables one to consider the possible mechanisms of structural relaxation of glassy polymers and perform this identification.We used amorphous (polyarylate and polycarbonate) and amorphous-crystalline (high-density polyethylene)polymers. Polyarylate and polycarbonate films about 50 ~tm thick were prepared by a method of pouring 5 % solutions of the polymers in chloromethylene on a glass substrate with their further drying in vacuum at 453 K (polyarylate) and 393 K (polycarbonate) lbr 2 days up to the complete removal of moisture and the solvent. High-density polyethylene films about 80 ~tm thick were obtained by pressing under pressure in compliance with GOST 16338-85. Specimens were cut with the use of a template from these films to obtain the shape of a two-sided blade with a base length of 40 mm and an operating length of 5 ram. Stress relaxation tests were perlbrmed within the linear section of the stress-strain curve [7]. A uniaxial tensile test was performed to determine the modulus of elasticity E, yield point Oy, and density of the cluster network of macromolecular engagements V, calculated by a special procedure [l ]. High-density polyethylene specimens for impact tests were prepared by injection moulding (GOST 4647-80, standard size II). In specimens, cuts of length a = 0.5, 0.9, 1.2, and 1.5 mm were made with a sharpened razor blade and their lengths were controlled with a stop in a special device. Impact tests were performed on a pendulum impact testing machine, equipped with a piezoelectric pressure transducer whose signals were transmitted directly to a storage oscillograph in the temperature range 293-473 K for polyarytate, 293-413 K for polycarbonate, and 293-363 K for high-density polyethylene. This unit enabled us to obtain load-time (P-t) diagrams and calculate the modulus of elasticity E [8].In quasistatic tests, we held specimens in the heating chamber of the unit at a fixed temperature for 15 min to reach the heat equilibrium. In impact tests, thermostatic control was performed in conformity with GOST 9454-78.The working capacity of a cluster F (the number of chains going from it) were calculated with the relation [9]F= --2G~176 +2, VkTwh...