Relationship between compressive yield and tensile behavior in glassy thermoplastics

The effect of temperature and strain rate on the compressive yield behavior of polystyrene is compared with the effect of the same variables on crazing in tension. The results support the conclusion of other, more extensive work, which shows that crazing involves the same types of molecular processes as those which occur during deformation under compression and shear. An improved method of measuring compressive stress–strain curves is then described, and the compressive yield stress is also compared with an extrapolated tensile yield stress. The difference between the two is in line with concepts which assume a dependence of yield stress on the state of hydrostatic tension (or compression). It can be adequately described by the Mohr-Coulomb yield criterion. Application of this criterion also enables a theoretical stress strain curve in tension to be derived from other results in compression. Comparison of the tensile stress–strain curve so obtained with those which can be directly measured with other plastics, supports the hypothesis that crazing is favored by a marked decline in engineering stress during tensile elongation (plastic instability).     

Nanocomposites on the Basis of Crazed Polymers

Analysis of published data on the mechanism of structural rearrangements in solid polymers on their crazing in liquid media is presented. The experimental evidence characterize crazing not only as a kind of spontaneous polymer dispersion under joint action of a mechanical stress and an active liquid medium, but also as the method of colloidal dispersion of low-molecular substances in a polymer. In the process of crazing, active liquid fills the porous structure of crazes, thereby transporting various low-molecular substances to the polymer volume. Crazing is believed to open the ways for preparing various nanocomposites on the basis of a wide variety of glassy and crystalline polymers, on the one hand, and target additives on the basis of practically any low-molecular substances, on the other.     

Craze initiation in poly(methyl methacrylate) under biaxial stress

Criteria for craze initiation in poly(methyl methacrylate) have been investigated under various combined loadings including biaxial tension and torsion-compression at 65°C in air and at room temperature in a crazing agent, kerosene. Environmental crazes are observed even under torsion-compression loading when air crazing does not occur, and the stress locus for environmental crazing is very different from that for air crazing. A theoretical model analogous to the Cottrell model in crystal plasticity is proposed. Theoretical crazing loci derived from the model are compared with the experimental results.     

Crazing and fracture in crystalline,isotactic polypropylene and the effect of morphology,gaseous environments,and temperature

Stress crazing is studied in three forms of crystalline, isotactic polypropylene (PP): (1) smectic/nonspherulitic, (2) monoclinic/nonspherulitic, and (3) monoclinic/spherulitic PP. Optical and scanning electron microscopy as well as stress—strain measurements are used to characterize crazing behavior in these three forms as a function of temperature (?210 to 60°C) and of the gaseous environment (vacuum, He, N₂, Ar, O₂, and CO₂). Forms 1 and 2 are found to craze much like an amorphous, glassy polymer in the temperature range between ?210 and ?20°C, irrespective of environment. The plastic crazing strain is large close to the glass-transition range (ca. ?20°C) of amorphous PP and in the neighborhood of the condensation temperature of the environmental gas. Near condensation, the gas acts as a crazing agent inasmuch as the stress necessary to promote crazing is lower in its presence than in vacuum. A gas is the more efficient as a crazing agent, the greater is its thermodynamic activity. Spherulitic PP (form 3) crazes in an entirely different manner from an amorphous, glassy polymer, showing that the presence of spherulites influences crazing behavior much more profoundly than the mere presence of a smectic or monoclinic crystal lattice. Below room temperature, crazes are generally restricted in length to a single spherulite, emanating from the center and going along radii perpendicular, within about 15°, to the direction of stress. They never go along spherulite boundaries. Gases near their condensation temperature act as crazing agents much as in nonspherulitic PP. Above room temperature the crazes are no longer related to the spherulite structure, being extremely long and perfectly perpendicular to the stress direction. Apparently the crystals are softened enough by thermally activated segmental motion to permit easy propagation of the craze. The morphology of the fracture surfaces and its dependence on temperature and environment is described and discussed. Concerning the action of gases as crazing agents it is argued that the gas is strongly absorbed at the craze tip, where stress concentration increases both the equilibrium gas solubility and the diffusion constant. Hence, a plasticized zone is formed having a decreased yield stress for plastic flow. This is considered to be the main mechanism by which the gas acts as a crazing agent. In addition, reduction of the surface energy of the polymer by the adsorbed gas eases the hole formation involved in crazing.     

The combined effect of photodegradation and stress cracking in polystyrene

This work aims to analyze the effects of photodegradation on the stress cracking resistance of polystyrene. Injection moulded samples were exposed to the ultraviolet light for various times in the laboratory prior to solvent contact. The bars were then stressed in a tensile testing machine under the presence of butanol. During this period the stress relaxation was monitored and the ultimate properties were evaluated after selected periods of stress cracking. Complementary tests were done by size exclusion chromatography and by scanning electron microscopy. The results indicated that butanol causes significant modification in polystyrene, with extensive surface crazing as well as reduction in mechanical properties. This is intensified under higher mechanical stress. The previous degraded samples showed a higher level of stress relaxation and a greater loss in tensile strength in comparison to the undegraded ones. The synergist action of photodegradation and stress cracking in polystyrene may be a consequence of the chemical changes caused by oxidation like the formation of polar chemical groups and the reduction in molecular weight.     

Crazing of polymers in the presence of hyperbranched poly(ethoxysiloxane)

The crazing of various polymers (PET, isotactic PP, and HDPE) in the presence of branched poly(ethoxysiloxane) and its low-molecular-mass analog—tetraethoxysilane—has been studied. The hyperbranched poly(ethoxysiloxane) is shown to be an effective adsorptionally active medium for crazing of various solid polymers and development of nanoporous structures with a volume porosity of up to 60%. Depending on the nature of polymers, two mechanisms of crazing (either classical or delocalized crazing) can take place. The reactions of hydrolysis (basic and acidic) within the pores leading to formation of solid silica have been performed. Electron microscopic observations provide evidence that the transformation of a viscous adsorptionally active liquid into a solid compound directly within the volume of a polymer matrix leads to the stabilization of a highly dispersed polymer structure that arises in the course of crazing.     

Effect of the Nanoparticle Functionalization on the Cavitation and Crazing Process in the Polymer Nanocomposites

The nanoparticle(NP) functionalization is an effective method for enhancing their compatibility with polymer which can influence the fracture property of the polymer nanocomposites(PNCs). This work aims to further understand the cavitation and crazing process, hoping to uncover the fracture mechanism on the molecular level. By adopting a coarse-grained molecular dynamics simulation, the fracture energy of PNCs first increases and then decreases with increasing the NP functionalization degree α while it shows a continuous increase with increasing the interaction ε_(pA) between polymer and modified beads. The bond orientation degree is first characterized which is referred to as the elongation. Meanwhile, the stress by polymer chains is gradually reduced with increasing the α or the ε_(pA) while that by NPs is enhanced.Furthermore, the percentage of stress by polymer chains first increases and then decreases with increasing the strain while that by NPs shows a contrast trend. Moreover, the number of voids is quantified which first increases and then decreases with increasing the strain which reflects their nucleation and coalescence process. The voids prefer to generate from the polymer-NP interface to the polymer matrix with increasing α o r ε_(pA).As a result, the number of voids first increases and then decreases with increasing α while it continuously declines with the ε_(pA). In summary, our work provides a clear understanding on how the NP functionalization influences the cavitation and crazing process during the fracture process.     

Effect of cyclic stress on enthalpy relaxation in polycarbonate

Possible effects of cyclic stress on physical aging in polycarbonate were investigated using differential scanning calorimetry (DSC) measurements. When the enthalpy overshoot by DSC of specimens of different previous thermophysical aging histories is measured as a function of the cyclic stress amplitudes, two characteristic regimes are observed. By correlating with optical microscopic observations, these regimes are identified as the incubation and crazing stages (denoted regimes I and II, respectively). The enthalpy relaxation behavior in Regime I is similar to thermophysical aging, indicating that the glassy structure as a whole is initially shifted to one where molecular mobility is retarded by relatively low amplitude cyclic stress. A strong interaction is also seen between the enthalpy overshoot and previous physical aging. That is, the more the material is previously aged, and the shorter the incubation period, the longer the crazing region is. As a result, brittle failure occurs over a wider load range compared with less aged specimens.     

Amorphous fraction controlled mechanical and optical properties of polylactic acid below glass transition temperature

A negative relation between strength and crystallinity is observed in polylactic acid below glass transition temperature. Study indicates that entangled molecules in amorphous regions act as load bearing structures and are responsible for stress induced crazing. A three-phase model is proposed to explain how amorphous fraction changes with heat treatments and contributes to polymer modulus below glass transition temperature. Incorporation of carbon quantum dots into amorphous fraction dominated PLA creates a new type of luminescent composites that can be used for food labelling, tracking, packaging and production of origin.     

The effect of strain rate on craze yielding,shear yielding,and brittle fracture of polymers at 77°K

The tensile strength of poly(methyl methacrylate) (PMMA), polycarbonate (PC), polychlorotrifluoroethylene, and polysulfone was measured in liquid nitrogen over the strain rate range of 2 × 10^?4 to 660 min^?1. These polymers deformed by crazing which was induced by the liquid nitrogen. The stress versus log strain rate curve was sigmoidal in that its slope increased and then decreased with strain rate. Above a critical strain rate of about 200 min^?1, which varied somewhat with the polymer, crazing was not observed with the optical microscope; the behavior became brittle, and the tensile strength became constant. The nonlinear behavior of stress versus log strain rate at low strain rates was associated with a decrease in activation volume with increasing strain rate whereas the nonlinear behavior at high strain rates was associated with an increase in density and decrease in length of the crazes with strain rate. The strain rate effect was the basis for calculating the diffusion coefficient of nitrogen into the polymers at 77°K. The shear deformation mode of PC was measured under compression and under tension. The compressive strength versus log strain rate was linear throughout the entire range giving a compression shear activation volume of 360 ų. The shear tensile strength of PC varied only slightly with strain rate when compared to the compressive strength. The brittle fracture stress of PMMA, in the absence of crazing, in compression and in tension, did not vary with strain rate.     

Crazing of glassy polymers in alcohols and hydrocarbons

In the crazing of glassy amorphous polymers, wetting ability and penetration of the fluid are the important practical parameters governing the activity of the fluid. Higher molecular weight and the presence of polar groups in the fluids result in an increase in the critical stress for craze initiation in polystyrene and polycarbonate. The Eyring treatment of the craze process can describe fairly well the temperature and strain rate dependence of the critical stress. The parameters involved in the Eyring theory suggest that the crazing takes place by a molecular motion of lower energy than does macroscopic yielding.     

A test method for the determination of the stress cracking effects of liquid environments on thermoplastics by measurement of critical strain

The stress cracking effect of liquids on thermoplastic materials can be quantified by measuring the critical strain to cause cracking or crazing. The critical strain is ideally defined as the value of applied strain, for a given material and liquid combination, below which no cracking or crazing occurs. A method is described for the determination of critical strain using a simple straining jig. A strain gauge extensometer attached directly to the specimen is used to accurately monitor the applied strain while visual observation of the sample is used to record the time to crack or craze.Critical strain values measured by this technique are quoted for various alcohols and ketones in contact with polycarbonate and compared with literature values.     

Craze initiation of glassy polymers under the action of crazing agent

This paper deals with the formation of crazes that may be caused by an external load on glassy polymers wetted with kerosene. First, the orientation of crazes has been determined when applying a uniaxial tension to a specimen of cold-rolled polyvinyl chloride sheet at various angles to the rolling direction. The critical stress for craze initiation in poly(methyl methacrylate) and polyvinyl chloride rods has been investigated under combined tension–torsion loading. It is shown that: (1) in an anisotropic, as well as an isotropic polymer, the direction of crazes is perpendicular to that of the maximum strain calculated by taking into account the internal stress due to rolling; and (2) under the action of a crazing agent, crazing may occur even under the pure torsional load, i.e., in the absence of dilatational stress.     

The effect of molecular properties on mechanical behaviour of PMMA—III. Environmental stress crazing in methanol

The correlation between the stress-relaxation curves of methanol-equilibrated poly(methyl methacrylate) (PMMA) and environmental fracture behaviour indicates the role of molecular entanglements in determining if the fracture process of PMMA in methanol occurs via stress-cracking (Regime II) or stress-crazing (Regime I and Transition Regime). The fracture data for high molecular weight (MW) PMMA (Regime I) and medium MW PMMA (Transition Regime) were analyzed by linear, elastic fracture-mechanics (LEFM) and Williams-Marshall (WM) theory. High MW PMMA absorbed more energy and had a wider crack-opening-displacement (COD) than medium MW PMMA, due to a thicker primordial craze thickness. Craze matter in the high MW sample gave a slower crazing stress decay and a better stretching capability with time than that of medium MW sample. The craze-initiation and the craze-growth obeyed a flow-controlled mechanism and relaxation-controlled mechanism, respectively.     

Effects of polar group incorporation on crazing of glassy polymers: Styrene–acrylonitrile copolymer and a dicyano bisphenol polycarbonate

The degree of swelling and attendant reduction in the glass transition temperature have been determined for a 70% styrene–30% acrylonitrile copolymer in a large number of organic liquids. The critical strain ?_c for crazing or cracking has been determined also in air and in each agent. Limited crazing data have been obtained also on a dicyano bisphenol polycarbonate in which the CN groups take the place of the methyl groups in bisphenol A (BPA) polycarbonate. The two resins are compared with polystyrene and BPA polycarbonate, respectively, in terms of crazing resistance, swelling, and other properties. In both systems CN incorporation raises ?_c (air) and reduces susceptibility to liquids of low solubility parameter δ; T_g and shear yield stress are raised in the polycarbonate but not in the styrene system. The volume efficiency of CN in raising ?_c (air) is greater in the polycarbonate system than the styrene system; for the rise in polymer solubility parameter, CN efficiency is apparently reversed. These changes are discussed in terms of the differences in molecular architecture of the two systems. For glassy polymers, ?_c (air) is shown to depend in semiquantitative fashion on polymer T_g, δ, and resistance to shear deformation.     

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.     

Nanovoid relaxation in a series of copolyester glasses under cyclic loading using synchronous PALS

This study examines if correlation between a specific molecular motion and the growth and relaxation of nanovoids, which are precursors to crazes, can be established. A novel technique, positronium annihilation lifetime spectroscopy (PALS) synchronized with cyclic stress, is reported. In this technique the positronium annihilation signal was accumulated in sixteen channels corresponding to sixteen phases of the sinusoidal load, which allowed statistically significant data for nanovoid generation and relaxation due to the fluctuating stress to be accumulated. This technique was applied to a series of copolymers of poly(ethylene terephthalate) (PET) and poly(1,4-cyclohexylenedimethylene terephthalate) (PCT). Previous studies have shown that the cyclohexylene rings in the main chain of PCT actively undergo chair-boat-chair conformational transitions in the glassy PCT at around room temperature. The PET-co-PCT series was also chosen for this study because the crazing stress increases systematically with the cyclohexylene content. The synchronized PALS data are consistent with the hypothesis that transient nanovoids generated by the cyclic stress in polymers containing more cyclohexylene rings relax more readily than those with fewer rings. The results also correlate well with the crazing stress in this series of copolymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1410–1417, 2007     

The influence of sub-Tg annealing on environmental stress cracking resistance of polycarbonate

To explore the effect of physical aging on environmental stress cracking (ESC) behavior of polycarbonate (PC), sub-T_g annealing was utilized as a method for accelerated aging. Injection molded samples were annealed at 130 °C for different time varying up to 96 h. A three point bending apparatus was used to evaluate critical stress for crazing and to record the variation of stress with immersion time at constant strain. The ESC results indicated that the critical stress for crazing initiation of PC in ethanol is increased by sub-T_g annealing. However, the resistance of annealed PC to ESC with immersion time during the stress relaxation test depends on the level of initial stress. When a relatively low initial stress was used, a short time (24 h) of sub-T_g annealing reduced the stress relaxation rate and decreased the number of cracks on the surface of PC. However, under higher initial stress, the stress relaxation rate of PC had a slight change only when the annealing time was prolonged about threefold (72 h). This can be explained by the formation of cohesional entanglement sites during the sub-T_g annealing process, which was demonstrated by the thermal and dynamic mechanical tests.     

Crazing of polystyrene containing two rubber balls: A model for ABS plastics

In order to study the crazing behavior in rubber-toughened glassy polymers, polystyrene samples containing two rubber balls of the same diameter with varying separations have been prepared. They were subjected to simple tension, and their crazing behavior was observed. When the two balls are close together, the craze-initiation stress is considerably lower than that of single-ball samples. With increase in the distance between the two balls the craze-initiation stress increases at first almost linearly and levels off when l/d reaches about 1.45, where l and d are the center-to-center distance and the diameter of the balls, respectively. When l is sufficiently small, the crazes are seen to develop extensively at the inner surfaces of the balls and finally bridge with each other. The crazes bridged between the balls expand largely in the plane perpendicular to the applied load.     

Microvoid formation during deformation of high-impact polystyrene

Small-angle x-ray scattering (SAXS) has been used to study the formation of microvoids in polymers which craze or stress-whiten extensively. Specimens are subjected to a stepwise uniaxial strain, with scattering curves being obtained at each step. The increase in scattering intensity upon crazing is attributed to the formation of microvoids, and the relative size, shape, and concentration of the scattering elements are determined by a Porod analysis of the SAXS curves. The major portion of our work has been on high-impact polystyrene which shows a large increase in SAXS intensity as crazing occurs. We are able to follow the changes in void size and concentration during craze initiation and growth. Effects of temperature, molecular orientation, and matrix molecular weight have also been studied. The results add to the information on craze growth and microstructure known from electron microscopy and dilatometry. In addition, a qualitative physical model for microvoid nucleation is proposed, and the implications for toughness are discussed.