The effect of strain rate on solvent crazing of poly(ethylene terephthalate) in solutions of poly(ethylene oxide) of various molecular masses

The effect of strain rate on the behavior of PET during its tensile drawing in highly viscous liquids, such as liquid PEG (M 400) and semidilute solutions of PEO (M = (4 × 10⁴)−(1 × 10⁶)), is studied. With an increase in strain rate, the mechanism of tensile drawing of PET in PEG changes from solvent crazing to shearing; at the same time, over the selected interval of strain rates, tensile drawing of PET in semidilute solutions of PEO proceeds via the mechanism of solvent crazing. During tensile drawing of PET in PEO solutions, the behavior of PET is almost the same as the mechanism of tensile drawing in a pure solvent. This result indicates that, in the course of flow of the polymer solution through the formed porous structure, PEO is filtered off in the local tip region of the growing craze.     

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.     

Environmental stress cracking of poly(vinyl chloride) in alkaline solutions

The environmental stress cracking (ESC) effects on PVC of various high pH sodium hydroxide environments have been studied. The behaviour of PVC specimens in air and pH 12, 13, 13.5 and 14.39 sodium hydroxide solutions has been examined under three-point bend, tensile and creep conditions. Two parameters were used in three-point bend testing to determine the effect of an applied strain and high pH environment on the stability of PVC, namely time to craze initiation and width of crazing. It was found that, in general, crazing occurred sooner and to a greater degree with increasing strain and pH, although there was some evidence that craze growth was most rapid at pH 13.5. The results also indicated a critical strain value of 1.5–1.6%, below which crazing was not observed in any of these alkaline environments. Creep and tensile testing revealed that the time for which a PVC specimen was immersed in the environment was very important in determining the severity of the environmental effect. Creep tests at elevated temperatures showed that the time for the effects to be manifest decreased with increasing temperature. Creep rates were highest in pH 13.5 sodium hydroxide solution indicating that this was the most hostile of the environments considered.     

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.     

SEM OBSERVATION OF THE CRAZING FOR POLYPHENYLQUINOXALINE FILMS DURING IN SITU STRETCHING*

The crazing of polyphenylquinoxaline (PPQ-E) films during in situ stretching has been observedby SEM. The crazing phenomena and craze morphology of PPQ-E films were interpreted. The strain values atcritical crazing and yielding and the craze stability of PPQ-E samples depend on the thermal-dealingcondition for the samples. From the point of view of cohesional entanglements and energy absorbed bysamples, the experiment results were explained.     

物理老化对聚苯基单醚喹啉薄膜银纹化的影响

物理老化实质上是聚合物材料在 Tg 以下存放过程中 ,其凝聚态结构通过链段或更小运动单元的运动 ,从热力学非平衡态向平衡态过渡的一个结构弛豫过程[1] .在这一过程中 ,聚合物的密度、自由体积、焓、熵和力学性质随温度和时间产生变化 .因为银纹化是聚合物的特性 ,所以银纹化也将随结构回复过程而产生变化 .有关物理老化对聚合物银纹化的影响尚未得出一致的结论 [2～ 4 ] .聚苯基单醚喹啉 (结构见 Scheme1 )是一种高性能的芳杂环聚合物 [5] ,它可以在比较苛刻的条件下作为绝缘材料和膜材料使用 .有关这类高性能的芳杂环聚合物的物理老化…     

Mechanical properties of glass continuous poly(cyclohexylethylene) block copolymers

The bulk mechanical properties of linear triblock and pentablock copolymers that self‐assemble into hexagonally packed cylinders with glassy, unentangled matrices of poly(cyclohexylethylene) (PCHE for a homopolymer, C for a block copolymer) with rubbery poly(ethylene‐alt‐propylene) (P) and semicrystalline polyethylene (E) minority components are examined. The tensile properties of high C content CEC triblock copolymer could not be quantified; however, CPC can plastically deform under uniaxial strain, unlike brittle PCHE. Both CECEC and CPCPC pentablock copolymers exhibited ductile tensile behavior, but the tensile properties of blends of these two pentablock copolymers show that the addition of crystallinity in the minority phase prevents strain softening after yielding and necking, which indicates that these samples deform only via crazing. On the other hand, the white gage region of CPCPC and the ability of CPCPC to neck indicate that high C content materials deform via shear yielding and crazing when the minority component is a rubbery material. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Solvent-induced embrittlement in a crystallizable polyarylate

The effects of solvent-induced crystallization on the micromechanical properties of thin films of polyarylate (PAr) were studied. Under uniaxial extension amorphous polyarylate was observed to deform exclusively by shear deformation with no evidence of crazing. Upon exposure to methylethyl ketone, vapor, or liquid, PAr crystallizes and is subsequently embrittled. Our transmission electron microscopy results clearly show that this embrittlement results from a transition in plastic deformation mechanism from shear yielding to crazing. A detailed examination of the samples revealed that the crazes formed preferentially within the noncrystalline regions and that the craze tips followed a complex trajectory around the crystallites. In some cases the craze tip advance deviated by as much as ±30 from a direction normal to the tensile axis. Because crazes are inherently more susceptible to forming cracks than shear deformation zones, crystallization reduces the fracture toughness of the polymer. This type of embrittlement, via a transition in plastic deformation mechanism, is believed to be a general behavior for solvent-crystallizable thermoplastics.     

Visualization of structural rearrangements during annealing of solvent-crazed poly(ethylene terephthalate)

A direct microscopic procedure is used for studying structural rearrangements during the annealing of PET samples after solvent crazing. Even at room temperature, solvent-crazed PET samples experience shrinkage which is provided by processes taking place in crazes. This shrinkage is observed at temperatures up to the glass transition temperature of PET and proceeds via drawing together of crack walls. Once the glass transition temperature is attained during annealing, the spontaneous self-elongation of the polymer sample occurs. The mechanism of this phenomenon is proposed. The low-temperature shrinkage of the polymer sample is related to the entropy contraction of highly dispersed material in crazes that has a lower glass transition temperature than that of the bulk polymer. This shrinkage cannot be complete, owing to crystallization of the oriented polymer in the volume of the crazes. As a result of crystallization, the oriented and crystallized polymer in the crazes coexists with the regions of the unoriented initial PET. As the annealing temperature approaches the glass transition temperature of the bulk PET, its strain-induced crystallization takes place. As a result, the regions of the unoriented polymer between crazes are elongated along the direction of tensile drawing and the sample experiences contraction in the normal direction.     

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.     

Energy absorption in polymer crazing

The total energy absorbed by a craze during its development in creep is analyzed and calculated on the basis of a time-dependent theory of crazing. Experimental measurements of the craze length have been utilized in the energy calculations. For polystyrene the initial energy absorption in the craze region is found to be several hundred times that in the uncrazed medium. This ratio decreases sharply in a short period of time to about 50 to 1 and less and remains low afterward. For polycarbonate, somewhat similar behavior has been found. The initial strain energy absorption by crazing is about 200 times that in the uncrazed region. The energy ratio reduces rapidly to about 55 to 1 and tends to level off thereafter. However, in general, the amount of strain energy absorbed does increase as a function of time, as it should.     

Cyclic stress–strain behavior of the dry polycarbonate craze

Equipment and methods have been developed which allow photomicrographic determination of the stress–strain properties of the individual craze. Serial cyclic tensile tests on polycarbonate crazes are described. Under stress the typical dry polycarbonate craze thickens solely by straining; no adjacent polymer of normal density is converted to craze material. The craze exhibits a yield stress followed by a recoverable flow to roughly 40–50% strain at 6000–8000 psi. On return to zero stress the craze exhibits creep recovery at a decelerating rate. The yield stress and loss factor of each cycle decrease with increasing initial strain and cycles initiating at 50% strain or more show completely Hookean behavior. Creep recovery results in recovery of yield stress and loss factor also. Craze tensile behavior is suggested to be essentially an extension of the craze formation process. Decrease in elastic modulus and yield stress with increasing strain are rationalized in terms of strain-produced decrease in density and resultant increase in stress concentration factor on the microscopic polymer elements of the craze. Polymer surface tension and the large internal specific surface area of the craze are suggested to be important factors in the large creep recovery rates of the craze.     

The deformation behavior of polyethersulfone and polycarbonate

In thin films of polyethersulfone and polycarbonate it is shown that crazing becomes increasingly likely to occur as the temperature is raised. The transition temperature depends on strain rate and molecular weight. Three regimes of behavior can be identified. In both the low-temperature (shear deformation) and high-temperature (crazing) regimes the strain to craze is independent of molecular weight. However, in the intermediate crazing regime chain length is important, indicating the importance of disentanglement over this temperature range. It is thought that at the highest temperatures disentanglement is still occurring, but the rate-determining step is now general plastic flow into the craze fibrils.     

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.     

Progress and challenge in polymer crazing and fatigue

In this review on polymer crazing and fatigue three aspects have been treated more explicitely: the molecular rearrangements preceding and provoking craze initiation, the competition between disentanglement and chain scission during lateral craze growth, and the distinct fatigue failure mechanism occurring in cyclically loaded PET and PA fibers. An overview on other aspects is given including references to work in progress.     

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 theory for environmental craze yielding of polymers at low temperatures

It has been recently discovered that polymers craze at low temperatures in the presence of nitrogen or argon. A quantitative theory has been developed which explains (1) the critical temperature above which the phenomenon disappears, (2) the critical stress for nucleating a craze, (3) the effect of strain rate on the yield point and size of crazes, (4) the drop in the load during craze yielding, and (5) the increase in strength of the polymer in N₂ or Ar at high strain rates so that the ultimate strength may exceed that in He or vacuum. The crazing action of the gases is described qualitatively at the molecular level.     

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 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.     

Microstructure and mechanical properties of hot‐air drawn poly(ethylene terephthalate) fibers

Hot‐air drawing method has been applied to poly(ethylene terephthalate) (PET) fibers in order to investigate the effect of strain rate on their microstructure and mechanical properties and produce high‐performance PET fibers. The hot‐air drawing was carried out by blowing hot air controlled at a constant temperature against an as‐spun PET fiber connected to a weight. As the hot air blew against the fibers weighted variously at a flow rate of about 90 ℓ/min, the fibers elongated instantaneously at a strain rate in the range of 2.3–18.7 s⁻¹. The strain rate in the hot‐air drawing increased with increasing drawing temperature and applied tension. When the hot‐air drawing was carried out at a drawing temperature of 220°C under an applied tension of 27.6 MPa, the strain rate was the highest value of 18.7 s⁻¹. A draw ratio, birefringence, crystallite orientation factor, and mechanical properties increased as the strain rate increased. The fiber drawn at the highest stain rate had a birefringence of 0.231, degree of crystallinity of 44%, tensile modulus of 18 GPa, and dynamic storage modulus of 19 GPa at 25°C. The mechanical properties of fiber obtained had almost the same values as those of the zone‐annealed PET fiber reported previously. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1703–1713, 1999