The effect of preliminary orientation of polymers via tensile drawing at elevated temperature on solvent crazing

We studied how the preliminary orientation of an amorphous glassy PET via its uniaxial tensile drawing above the glass transition temperature affects the deformation behavior during subsequent tensile drawing in the presence of adsorptionally active environments. The tensile drawing of the preoriented PET samples with a low degree of preliminary orientation (below 100%) in the presence of liquid environments proceeds via the mechanism of solvent crazing; however, when a certain critical tensile strain is achieved (150% for PET), the ability of oriented samples to experience crazing appears to be totally suppressed. When the tensile drawing of preoriented samples is performed at a constant strain rate, the craze density in the sample increases with increasing degree of preliminary orientation; however when the test samples are stretched under creep conditions, the craze density markedly decreases. This behavior can be explained by a partial healing and smoothening of surface defects during preliminary orientation and by the effect of entanglement network. The preliminary orientation of polymers provides an efficient means for control over the craze density and the volume fraction of fibrillar polymer material in crazes.     

Creep deformation caused by crazing

Creep deformation caused by crazing is analyzed using an analog of dislocation dynamics. Results are compared with experiments on polycarbonate and poly(vinyl chloride) subjected to the action of kerosene as a crazing agent. Good agreement between the theory and experiment shows the value of the dislocation analog approach. The theoretical craze creep strain is found to be numerically approximated by the commonly used law similar to the Nutting equation, and may provide a physical foundation for this widely used equation formally derived from rate theory.     

Effect of molecular relaxations on the fracture behavior of poly(vinyl chloride)

The fracture toughness of PVC has been measured by using three-point bend specimens tested over a wide range of strain rate and temperature. A method has been described of deriving fracture load indirectly by measurement of stiffness from a preliminary “low blow” test in instrumented impact testing. Some limitations of the method, when used with semi-ductile material of low stiffness, have been discussed. Fracture toughness results have also been evaluated by an alternative energy method, which is however more suitably applied to the lower speed impact test. The curves of K_1c versus temperature for PVC contain a weak maximum at a temperature below T_g, the location of which varies with testing speed. The position of the maximum in the time–temperature field has been compared with the locus of the β damping peak derived from mechanical (flexural vibrations, torsion pendulum) and dielectric loss measurements, with reasonable agreement. Static toughness was higher than dynamic, and this correlated with fracture surface appearance, thereby indicating a real difference in toughness probably associated with crack tip craze development.     

凝聚条件对聚甲基丙烯酸甲酯薄膜银纹化现象的影响

<正> 当玻璃态聚合物受张力作用时,常可以观察到银纹形成,它是聚合物材料内部断裂的先兆,银纹是垂直于拉伸方向的平面缺陷,与正常裂纹不同,银纹的质量不是零,而是由沿拉伸方向排列的高度应变的微纤及孔穴组成,银纹的典型尺寸约为长100μm,宽1—2μm,微纤的直径约5—15nm,微纤的总体积分数约占20—50％,微纤能支撑负荷,因此银纹在控制聚合物屈服以后的力学行为中的作用一直是个重要的研究领域。近年来从聚合物分子缠结网对银纹生长,微纤断裂及银纹稳定性的影响方面做了一些有意义的工作。     

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.     

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.     

Molecular fracture in natural rubber during tensile testing at low temperatures

Sulfur-cured natural rubber and other elastomers subjected to tensile tests at low temperatures and low strain rates are found to swell and “foam” after testing when brought to room temperature. The conditions under which this phenomenon can occur are established and related to load-extension curves. Free radicals formed during tensile testing are studied by electron spin resonance (ESR) techniques. It is found that the free radicals observed at the low temperatures are stable below the glass transition temperature of the material, and it is suggested that these radicals arise from mainchain fracture occurring during yielding of the material. The subsequent swelling at higher temperatures is found to be due to the expansion of environmental gases absorbed during tensile testing and to the release of hydrogen in certain cases from the materials tested. It is also suggested that yielding of the material which gives rise to these characteristics occurs by crazing of the material; the voids in the craze bands absorbing the environmental gases which subsequently cause the foaming at higher temperatures.     

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.     

Fracture energy measurements in polycarbonate and PMMA

Fracture energies in polycarbonate and PMMA were measured using different test techniques. Results from instrumented impact tests and slow three-point bend tests on Charpy specimens were compared with tensile test results on Compact Tension (CT) specimens. The Charpy specimens were provided with a blunt Charpy V (0·25 mm radius) notch and sharp (25 μm radius) notch. It is shown that impact testing of Charpy specimens with blunt notches gives values of the fracture energy which are two to six times higher than those obtained from the CT tensile tests. Only by using a sharp, 25 μm notch and performing the three-point bend test at a low rate were similar results obtained.     

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.     

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.     

Transition from crazing to shear deformation in ABS/PVC blends

ABS/PVC blends were prepared over a range of compositions by mixing PVC, SAN, and PB‐g‐SAN. All samples were designed to have a constant rubber level of 12 wt % and the ratio of total‐SAN to PVC in the matrix of the blends varied from 70.5/17.5 to 18/80. Transmission electron microscope and scanning electron microscope have been used to study deformation mechanisms in the ABS/PVC blends. Several different types of microscopic deformation mechanisms, depending on the composition of blends, were observed for the ABS/PVC blends. When the blend is a SAN‐rich system, the main deformation mechanisms were crazing of the matrix. When the blend is a PVC‐rich system, crazing could no longer be detected, while shear yielding of the matrix and cavitation of the rubber particles were the main mechanisms of deformation. When the composition of blend is in the intermediate state, both crazing and shear yielding of matrix were observed. This suggests that there is a transition of deformation mechanism in ABS/PVC blends with the change in composition, which is from crazing to shear deformation. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 687–695, 2006     

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.     

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…     

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.     

聚合物银纹化的分子机理

本文综述了近年来有关聚合物银纹化分子机理方面研究的进展情况。内容包括银纹的起源、生长、断裂过程以及各种结构关系对银纹的影响，并对几种典型的聚合物银纹形态进行了描述。     

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.     

Kinetics of craze initiation in polystyrene in N-alcohols

The kinetics of craze initiation has been investigated for unmodified and rubber-modified polystyrenes in n-alcohols. The dependence on time and temperature of the critical strain at which crazes could be detected visually was determined with a Bergen elliptical strain device. Sorption studies were also conducted at room temperature on films exposed to the saturated vapor of n-alcohol. The analysis of crazing data in terms of the Eyring model gave activation energies from 9.4 to 17.4 kcal/mole, increasing with increasing chain length of n-alcohol and increasing rubber content. The activation volume multiplied by a stress concentration factor decreased with increasing rubber content and was nearly independent of the chain length of the n-alcohol. The larger the diffusion coefficient, which we measured by sorption experiments, the smaller was the activation energy for craze initiation. The values of diffusion coefficients, estimated from the experimental data on craze initiation, were found to be comparable with those from the sorption experiments. It was concluded that the rate of craze initiation on exposure to liquids is controlled by the diffusion of the molecules of liquid into polymer.     

可交联PVC的合成及其交联反应

N,N-二乙基二硫代氨基甲酸钠(铜试剂,简称NasR)取代PVC链中的部分氯原子,制得不同官能度的功能高分子PVC-SR.在热的作用下,实现了PVC-SR的自交联和PVC-SR/NBR共混体系的共交联.研究了温度、铜试剂用量对取代反应的影响及PVC-SR官能度对交联反应硫化曲线、产物的凝胶含量、玻璃化温度、拉伸性能和压缩永久变形的影响,并初步探讨了交联反应机理.