A. V. Efimov

Structural Aspects of Physical Aging of Polymer Glasses

Published data concerning the problem of the physical aging of glassy polymers are surveyed. Basic attention is given to an analysis of structural rearrangements that accompany the physical aging of glassy polymers. The processes of aging (spontaneous change in the properties of polymer glasses during the storage at a temperature below the glass transition temperature) can be classified into the two following categories: first, the aging of undeformed polymer glasses and, second, the mechanical stress-induced aging of a polymer. It is shown that in the former case, the processes occur throughout the entire body of the polymer and, in the latter case, the aging processes is concentrated in microscopic zones (shear bands) that emerge during polymer deformation. The current concepts of the aging of polymer glasses are discussed.

The effect of cold rolling on crack propagation behavior in high-density polyethylene

Crack propagation behavior in HDPE was studied. The preliminary orientation of the polymer, which is deformed in its isotropic state via necking and breaks down at the neck propagation stage, improves the crack resistance and ductility of the material. The critical crack opening in preoriented HDPE samples dramatically increases at relatively low draw ratios of cold rolling while the speed of transverse crack propagation decreases.

Crazing of isotactic polypropylene in the medium of supercritical carbon dioxide

Uniaxial tensile drawing of films based on semicrystalline isotactic PP in the medium of supercritical carbon dioxide at a pressure of 10 MPa and a temperature of 35°C is studied. The tensile drawing of PP is shown to proceed in the homogeneous mode without necking and is accompanied by intense cavitation. The maximum level of porosity is 60 vol %. The porous structure that develops owing to the tensile drawing of the polymer in supercritical CO2 is provided by formation of a set of crazes that are primarily localized in interlamellar regions. According to small-angle X-ray scattering data, the average diameter of fibrils that bridge craze walls changes slightly with an increase in tensile strain and is ∼10 nm; the specific surface of the craze fibrils is 100–150 m2/cm3.

Effect of orientation on the mechanism of plastic yielding of poly(ethylene terephthalate) in adsorption-active media

The effect of the preliminary orientation on the formation of crazes in poly(ethylene terephthalate) during straining in adsorption-active liquids is studied. Poly(ethylene terephthalate) is oriented by drawing at a temperature of 80°C, which is somewhat higher than its glass-transition temperature (∼75°C). After orientation, samples are tested in tension in organic liquids at room temperature. At low degrees of preliminary drawing, the shear yield stress during straining in air does not increase significantly. However, the stress of craze widening rises in proportion to the degree of preliminary drawing. Thus, the orientation suppresses crazing and leads to the transition to shear flow. A model is proposed to explain the effect of orientation on crazing. According to this model, craze widening and pulling of a nonoriented polymer into the craze volume result from the formation of pores in the bases of fibrils. The formation of fibrils is caused by straining of the polymer between pores.

Visualization of strain-induced structural rearrangements in amorphous poly(ethylene terephthalate)

A direct microscopic observation procedure is applied to study the deformation of amorphous PET decorated with a thin metal layer when stretching is performed at different draw rates and at temperatures below and above the glass transition temperature Tg. Analysis of the formed microrelief allows stress fields responsible for the deformation of the polymer to be visualized and characterized. When tensile drawing is performed at temperatures above Tg, inhomogeneity of stress fields increases with the increasing draw rate; at high draw rates, the stress-induced crystallization of PET takes place. In the case of drawing the polymer at temperatures below Tg, direct microscopic observations make it possible to visualize the development of shear bands that appear in the unoriented part of the polymer specimen adjacent to the neck. The shear bands are oriented at an angle of about 45° with respect to the draw direction. When necking involves the unoriented part of the polymer, shear bands abruptly change their orientation and become aligned practically parallel to the draw axis.

A new type of surface structuring accompanying the rolling of glassy poly(ethylene terephthalate)

158 A popular type of inelastic deformation of glassy and crystalline polymers is rolling (1). At the same time, the mechanism of structural rearrangements accompanying the rolling of polymers has been stud� ied and understood much less than in the case of other, more frequently used types of inelastic deformation, such as uniaxial tension and uniaxial compression. Rolling underlies a popular technological method by which a polymer film is continuously pulled between two rollers rotating in opposite directions. Such a treatment leads to molecular orientation of polymers (2), and also to other significant structural rearrangements (3). By such a treatment, the mechan� ical and strength properties of polymer can be opti� mized. Importantly, most studies of the structural rear� rangements accompanying the rolling of polymers were performed on crystalline polymers with a two� phase structure. In this case, Xray powder diffraction analysis, electron microscopy, and other methods based on phase contrast can be efficiently used. For this reason, investigations of the structural rearrange� ments in the course of the rolling of amorphous poly� mers are much fewer. Nonetheless, there are quite a lot of studies of the effect of rolling on the deformation and strength properties of amorphous glassy polymers (4). These studies showed that rolling of polymers can be considered as a sort of modifying influence on a polymer that can be used for optimizing its mechani� cal properties. Previously (5-9), we described a procedure for visualizing and characterizing the structural rear� rangements accompanying polymer deformations of various types (uniaxial tension, uniaxial compression, rolling, etc.). This procedure is quite simple and con� sists in applying a thin (nanosized) metal coating to the polymer surface. The subsequent deformation (shrinkage) of the polymer leads to structuring of a special kind on the polymer surface. The structures forming thereat contain information on the structural rearrangements of the polymer substrate. In particular, this procedure was used for investi� gating the thermally stimulated shrinkage of amor� phous polycarbonate subjected to rolling in the glassy state (10). It was shown that the polycarbonate sub� jected to deformation of this kind, in contrast loadings of other types (tension and compression), demon� strates a complex pattern of surface structuring. It turned out that the rolled polymer contains quote extensive discrete zones in which the polymer is ori� ented in various directions. The detected structural features of the amorphous polymer deformed under the rolling conditions have not yet been adequately explained. In this work, we made a direct microscopic study of the structural rearrangements accompanying the roll� ing of another amorphous polymer, polyethylene

The effect of orientation on the mechanism of deformation of polymers

The mechanical behavior of HDPE, medium-density PE, and amorphous and amorphous-crystalline PET after their preliminary orientation is studied. The polymers are oriented by rolling at room temperature on lab-scale rolls, tensile drawing at temperatures somewhat higher than their glass-transition temperatures, and extrusion at room temperature. At low degrees of rolling (below 1.5), the tensile yield stress does not actually increase. (In amorphous-crystalline PET, this parameter even decreases.) It seems that the absence of strain hardening at low draw ratios is a common feature of the behavior of polymers below their glass-transition temperatures. In contrast to the tensile yield stress, the engineering strength increases in proportion to the degree of rolling. A new procedure for construction of the dependence of true tensile yield stress on tensile strain is advanced. At low strains, the true tensile yield stress shows practically no increase. This conclusion is verified by theoretical calculations.

Crazing of Polymers in a Supercritical Carbon Dioxide Fluid

ISSN 00125008, Doklady Chemistry, 2009, Vol. 428, Pa rt 2, pp. 238–241.

The influence of preliminary rolling on the mechanical properties of polyethylene terephthalate

The influence of preliminary rolling on the mechanical behavior of amorphous polyethylene terephthalate (PET) has been studied. Samples were oriented via rolling at room temperature on laboratoryscale rolls. After low rolling degrees (λ = 1.15), the tensile yield point decreases; i.e., the effect of strain softening is observed. With time, the yield point recovers gradually, a behavior that is similar to that of polystyrene described in the literature. Rolling suppresses the tendency of polyethylene terephthalate to brittle fracture. This result is due to the decrease in the degree of inhomogeneity of deformation in the neck. At rolling degree λ = 2 necking is completely suppressed. The dependence of the true yield stress of polyethylene terephthalate on the degree of orientation has been obtained. It has been shown that the Konsider diagram describes not the stress of the transition from neck propagation to homogeneous deformation but the degree of orientation at which necking ceases.

Preparation of nanoporous polyolefin films in supercritical carbon dioxide

The peculiarities of uniaxial deformation of semicrystalline polyolefins were studied using high-density polyethylene and isotactic polypropylene as an example in carbon dioxide at 35–75°C and pressures of 0.1–30 MPa. A nanoporous structure (with pore diameters of 3–7 nm) started to form in polymer films by the delocalized crazing mechanism at 4–5 MPa and higher. The increment of the pore volume depends on the parameters (pressure, temperature, and density) of the medium and on the deformation of the polymer, reaching 30–70%. The pore formation by the crazing mechanism was most effective when CO2 was in the supercritical state at a nearly critical temperature (35°C) and its density approached the density of a liquid.