Mechanical properties of methanol crazes in poly(methyl methacrylate)

Methanol crazes are grown from sharp cracks in poly(methyl methacrylate) (PMMA). The craze thickness profile is measured using a replica technique after the craze opening displacement profile of the growing craze has been measured with holographic interferometry. The craze strain profile is then computed from these data. The craze surface stress profile is determined by two methods: (1) from the uniaxial strain profile of regions adjacent to the craze as measured from the fringe spacing on the reconstructed hologram and (2) from the craze opening displacement profile using the Fourier transform method of Sneddon. From the surface stress and craze-strain profiles a true stress-strain curve for the craze fibrils has been constructed. The extrapolated fibril yield stress is in good agreement with the yield stress of bulk PMMA plasticized with methanol indicating that surface tension effects do not contribute importantly to craze fibril mechanical properties at room temperature. The craze strain increases from 0.4 near the craze tip to 1.4 near the craze base implying that methanol crazes in PMMA thicken by further straining of the existing craze fibrils and not by drawing new material into the craze from the craze surfaces. The primordial craze thickness, i.e., the original thickness of polymer which fibrillates to form the craze fibrils, is approximately 1 μm and is constant over most of the craze length. This thickness may be determined by diffusion of methanol normal to the craze surfaces in a process zone just behind the craze tip.     

Micro and nano layered polymers

The crazing behavior of coextruded microlayer sheets consisting of alternating layers of polycarbonate (PC) and styreneacrylonitrile copolymer (SAN) was investigated as a function of PC and SAN layer thicknesses. In this study, the total sheet thickness remained essentially constant and the PC and SAN layer thicknesses were changed by varying both the total number of layers from 49 to 1857 and the PC/SAN volume ratio.^[1,2] Photographs of the deformation processes were obtained when microspecimens were deformed under an optical microscope. Three different types of crazing behavior were identified: single crazes randomly distributed in the SAN layers, doublets consisting of two aligned crazes in neighboring SAN layers, and craze arrays with many aligned crazes in neighboring SAN layers. The transition from single crazes to doublets was observed when the PC layer thickness was decreased to 6 microns. Craze array development was prevalent in composites with PC layer thickness less than 1.3 microns. It was concluded that SAN layer thickness was not a factor in formation of arrays and doublets; formation of craze doublets and craze arrays was dependent only upon PC layer thickness.     

The optical interference pattern in a polystyrene craze layer

In the interference pattern seen in monochromatic light reflected from a craze layer in polystyrene, the bright fringes are of alternating intensity. The phenomenon is explained in terms of a four-beam interference. Apart from the two reflections from the outer surfaces of the craze layer others arise from a thin layer of approximately constant thickness within the craze layer at its median plane. The phenomenon provides independent evidence to support the electron microscopic observations of the microstructure of crazes in polystyrene.     

Nucleation and growth of crazes in amorphous polychlorotrifluoroethylene in liquid nitrogen

The density of mature crazes initially increases linearly with stress and then more rapidly at higher stresses. Once the crazes become observable then density was independent of time. The lowest stress at which an appreciable density of crazes was produced corresponds to the proportional limit. The average velocity of mature crazes was constant for a given stress and varied exponentially with the stress. The velocity depended on stress in the same way that the post-yield point stress depended on strain rate, whereas the yield point varied differently being a nonlinear function of the logarithm of the strain rate. The density of crazes was quantitatively related to the concentration of surface defects at which the crazes nucleate. The craze velocity was directly related to the diffusion coefficient of N₂ into the polymer. The analysis indicates that bulk diffusion of the N₂ governs the craze velocity and that plasticization of the tip of the craze is most important for the nucleation and growth of a craze in PCTFE.     

The use of small-angle electron scattering to compare the structure of craze found in thin films with that found in bulk materials

Small-angle electron scattering (SAES) has been used to examine the structure of crazes in polystyrene. It has been shown theoretically that the analysis of SAES is similar to the equivalent x-ray patterns (SAXS) except perhaps for the higher-angle scattering. A direct comparison of the SAES patterns from crazes in films of ca. 400 nm thickness with SAXS patterns from crazes in films of ca. 1 mm thickness has shown that the craze structures are similar in form for the thin and thick films but the fibrils are about three times larger in the thin films.     

用透射电镜研究PS薄膜银纹的微观结构、变形特性和破坏过程

本文介绍了用透射电镜观察PS薄膜银纹的微观结构、变形特性和破坏过程。实验观察到PS银纹质完整的网络结构和取向银纹质断裂以后的松弛特性。实验发现PS银纹质断裂转变成微裂纹的过程类似有机玻璃慢裂纹区的“撕布”模式,裂端银纹的外形符合Dugdale模型。     

Crazing in thin films of styrene–butadiene–styrene block copolymers

The mechanism of craze initiation and growth and its relationship to mechanical properties has been studied in thin films of styrene–butadiene–styrene (SBS) block copolymers. Optical microscopy and transmission electron microscopy were used to examine three copolymers which has a spherical rubber domain morphology but varied in rubber content from 20 to 50%. With increasing rubber content, the crazes became longer and less numerous. Widening of the crazes was at least partially responsible for the higher strains achieved in the copolymers, especially for the composition with the highest rubber content where the crazes widened to form micronecks. Transmission electron microscopy revealed that craze initiation and growth at the craze tip occurred by cavitation in the polystyrene phase. Cavitation of the continuous phase rather than the rubber domains was attributed to the concentration of chain-end flaws in the polystyrene. Crazes in the block copolymers followed a meandering pathway and the boundaries between crazed and uncrazed material were indistinct. Incorporation of fibrillated rubber particles into the craze fibrils strengthened the craze. At higher rubber content, the craze widened in the stress direction by voiding and fibrillation, which produced a cellular morphology.     

Craze growth and healing in polystyrene

The kinetics of craze growth and craze healing were studied by dark-field optical microscopy in monodisperse molecular weight polystyrene (PS) that varied in molecular weight from 88,000 to 1,334,000. The following observations were made. (1) G₁ the virgin growth rate, decreased rapidly with increasing molecular weight until M_n ～ 200,000 and then remained constant. (2) G₁ decreased with increasing craze density. (3) The growth rates of approaching craze tips decreased when the craze tips overlapped, and the effect was less for crazes whose parallel growth paths were greater than 40 μm apart. (4) Complete craze healing was observed by comparison of the nucleation times, τ₂, and growth rates, G₂, of healed individual crazes with the craze kinetics of the virgin sample. (5) The extent of healing was characterized using four cases in which τ and G were measured as a function of healing time, temperature, constant stress, and molecular weight. (6) Craze healing times were found to increase with molecular weight and were analyzed in terms of the modified molecular weight of the craze zone. (7) Significant bond rupture was determined to occur during crazing by comparison of healing times with stress relaxation and diffusion data. (8) Craze healing studies provide insight into both crack healing and fracture of glassy polymers.     

Stress in polyimide coatings

The effect of film thickness on in-plane molecular orientation and stress in polyimide films prepared from pyromellitic dianhydride with 4,4′-oxydianline was investigated using a prism coupling technique to measure the refractive index. Film thickness was controlled by varying both solution concentration and spinning conditions. Birefringence, the difference between the in-plane and out-of-plane refractive indices, was used to characterize the in-plane molecular orientation. The observed birefringence is a combination of the birefringence resulting from molecular orientation and the birefringence induced by the residual stress present in the films. The birefringence decreases with increasing film thickness over the range of thicknesses studied (3–20 μm) indicating that the molecular orientation decreases with increasing film thickness. The in-plane coefficient of linear thermal expansion (CTE), controlled by the level of orientation in the film, increases from 18 to 32 × 10^?6/°C over the same thickness range. The birefringence of free-standing films was lower than that of adhered films due to the release of residual stress in the film once the film is removed from the substrate. The residual film stress arises primarily from the mismatch in CTEs between the polyimide film and the substrate to which the film is adhered. Thus, since the film anisotropy decreases with increasing thickness, the film stress increases with increasing thickness. Residual stress calculated by integrating the product of the film modulus and the CTE mismatch assuming temperature-dependent properties is comparable to experimentally measured film stress. Ignoring the temperature dependence of the film properties leads to an overestimation of stress. Moisture uptake was used to study the stress dependence of the optical properties. Moisture uptake increases both the in-plane and out-of-plane refractive indices by equal amounts in free-standing films due to an isotropic increase in the polarizability. In adhered films, an increase in moisture uptake leads to a decrease in the birefringence due to a swelling-induced decrease in the residual film stress. © 1994 John Wiley & Sons, Inc.     

A SAXS study of a single crack and craze in plasticized polystyrene

Small-angle x-ray scattering (SAXS) was used to study the structure along a single craze that had broken down to form a crack along part of its length. This study was made possible by use of radiation from the synchrotron source CHESS which is sufficiently intense to permit examination of just a single craze. The total scattering from the craze was in excellent agreement with that expected from a knowledge of its dimensions and fibril volume fraction and width. This fact adds confidence to the interpretation of the scattering pattern of the craze as part diffraction, part reflection, and demonstrates that SAXS is a technique that may be used to measure craze volume within a sample. The craze was shown to grow in width by surface drawing with a constant structure, and then the fibrils broke to form a crack. The broken fibrils contracted and their diameters increased but they appeared to stay parallel with a constant fibril-axis radial distribution function.     

The effect of plasticization on craze microstructure

The structure of crazes in plasticized polystyrene has been studied by means of small-angle x-ray scattering and optical interference microscopy. Addition of plasticizer causes a rapid increase in the mean fibril diameter D and a slow decrease in the craze fibril volume fraction v_f. The crazing stress σ_c was also measured and it was found that the product D σ_c is independent of plasticizer concentration. These results are shown to be consistent with the entanglement model for controlling v_f and the meniscus instability model of craze thickness growth.     

Electron microscopic investigations of initiation and growth of crazes in polystyrene

The crazes in polystyrene (PS) were investigated by using a high voltage electron microscope (HVEM, accelerating voltage of 1000 kV). The early stages of the formation and the growth of the crazes were studied in detail.The smallest deformation structures visible are weak domains or microvoids with diameters of 10–15 nm and distances of a few 10 nm between them. They act as craze nuclei and are located in narrow, long pre-craze zones. Conclusions are drawn on the processes of initiation and propagation of the crazes; both are based on molecular heterogeneities and on an increasing heterogeneity of deformation. In particular, the transformation of the closed cell structure of the voids into the open cell structure of the craze fibrils is described.Growth of crazes in thickness definitely occurs by drawing new polymeric material from the craze boundaries into the craze.     

高抗冲聚苯乙烯中银纹的引发与终止

银纹是由孔穴和断裂面间相联结的原纤维组成的微小裂纹,其中原纤维的体积分数可达40%.银纹的体积分数与材料的韧性成正比.银纹化是高抗冲聚苯乙烯(ＨＩＰＳ)在脆化温度以下,抵抗破坏而消耗外界能量的主要方式.银纹的产生与材料内部不均一性所导致的应力集中有关.ＨＩＰＳ中的橡胶粒子能够控制银纹在本体中均匀地发展,这是ＨＩＰＳ高韧性的原因[1].ＨＩＰＳ的分散相是由聚丁二烯(ＰＢ)为连续相,ＰＳ为分散相构成的细胞结构粒子.通常ＨＩＰＳ中ＰＢ的含量为7%～8%,而细胞结构粒子的体积分数可高达23%,可见细胞结构粒子内部ＰＳ的含量为ＰＢ的…     

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.     

Development of crazes in polycarbonate,investigated by ultra small angle X-ray scattering of synchrotron radiation

The development of crazes in polycarbonate is investigated with the method of ultra small angle X-ray scattering of synchrotron radiation. Measurements at T = 130°C are discussed. The two-dimensional scattering patterns are analysed by means of a simple fibrillar model of the crazes. The geometrical parameters of the crazes as a function of the macroscopic draw ratio λ_d are determined using a curve-fitting procedure. The craze fibril volume fraction ν_f shows a complex dependence on λ_d.     

Low-angle electron diffraction from high temperature polystyrene crazes

Low-angle electron diffraction (LAED) was used to study the microstructure of crazes produced at different temperatures T and strain rates in thin films of monodisperse polystyrene (PS). At a slow strain rate of 4.1 × 10^?6 s^?1 both the fibril diameter D and the fibril spacing D₀ of crazes in 1800k molecular weight PS remained constant with temperature up to T ≈ 70°C and then sharply increased as T approaches T_g. At a higher strain rate of ～ 10^?2 s^?1, both D and D₀ increase only slightly with T. The values of D and D₀ over a range of temperature are in very good agreement with those values obtained in bulk samples using small-angle x-ray scattering. The crazing stress was measured as a function of temperature in the thin films of the 1800k molecular weight PS strained at the same slow strain rate used for the LAED measurements. These measurements were analyzed using a simple model of craze growth to reveal the temperature and strain rate dependence of the craze surface energy Γ. At room temperature Γ ≈ 0.076 J/m² (versus Γ ≈ 0.087 J/m² predicted) and was observed to remain constant up to T ≈ 70°C and then decrease by approximately a factor of two at T = 90°C. This decrease in Γ is believed to result from chain disentanglement to form fibril surfaces at sufficiently high temperatures and occurs in the same temperature range in which the craze fibril extension ratio λ was observed to increase.     

Form birefringence in biaxially oriented polypropylene

The birefringence of several biaxially oriented polypropylene films swollen with a number of fluids has been measured and found to exhibit a minimum when plotted against the fluid refractive index, as predicted by the theory of Wiener. However, a discrepancy in the form birefringence behavior is observed when samples of different degrees of crystallinity but the same total birefringence are compared. These results are interpreted in terms of Bullough's theory and suggest that this discrepancy arises because of different morphologies. A refractometric technique was employed that makes possible the simultaneous determination of birefringence and the volume fraction of fluid.     

Electron microscopy of crazes in glassy polymers: Use of reinforcing impregnants during microtomy

Attempts to prepare undamaged microtomed sections of crazes without reinforcement have failed. Several methods of reinforcing crazes in glassy polymers with impregnants prior to microtomy have been tried. Generalized characteristics of successful impregnant systems are suggested on the basis of this experience. The most successful system has involved the infusion of liquid sulfur into crazes in poly(2,6-dimethyl-1,4-phenylene oxide). After quenching, the solid sulfur reinforces the crazes successfully during microtomy but subsequently sublimes away under vacuum. The resultant, largely undamaged craze structure is seen by transmission electron microscopy to resemble an open-cell foam, the holes and polymer elements of which uniformly average ～200 Å in diameter. A moderate degree of orientation in the original tensile stress direction is observed. Implications drawn from craze structure for the existence of order in the glassy state are discussed.     

Effect of molecular entanglements on craze microstructure in glassy polymers

Thin films of ten glassy polymers are bonded to copper grids and strained in tension to produce crazes, which are then examined in the transmission electron microscope. The average craze fibril extension ratio λ for each polymer is determined from microdensitometer measurements of the mass thickness contrast of the crazes. The extension ratio λ is found to increase approximately linearly with the chain contour length l_e between entanglements, as determined from melt elasticity measurements of the entanglement molecular weight of these polymers. These results are analyzed by comparing them with λ_max, the maximum extension ratio of an entanglement network in which polymer chains neither break nor reptate (i.e., permanent entanglement crosslinks are assumed). The values of λ_max are given by l_e/d where d, the entanglement mesh spacing in the unoriented glass, is computed from d = k(M_e)^1/2 with k determined either from small-angle neutron scattering results on isolated chains in the glass or from coil size measurements in dilute solutions of a θ solvent. The craze extension ratios fall somewhat below λ_max at low λ but increase to well above λ_max for polymers with high l_e. This comparison suggests a significant contribution due to chain breakage (or reptation) in the higher-λ crazes of large-l_e polymers, which may arise from the higher true stresses in the craze fibrils (which for a given applied stress increase proportionally to λ). The results also imply that a useful way to increase the “brittle” fracture stress and decrease the ductile-to-brittle transition temperature of a glassy polymer is to decrease its entanglement contour length l_e.     

A model of environmental craze growth in polymers

A model of environmental craze growth has been developed based on the customary meniscus (or Rayleigh-Taylor) instability model of craze propagation but allowing for the possibility that environmental plasticization may cause the active layer of material adjacent to the craze to be of significant thickness with respect to the fibril spacing. Initially, as the active layer thickness increases, the fibril growth rate increases at constant fibril spacing, but eventually the fibril spacing comes to be controlled only by the active layer thickness and not by the surface tension and stress. This model of craze growth has been coupled to a model of stress-enhanced case II diffusion that is itself based on the Thomas-Windle model. Two main regimes of craze thickness growth are distinguished. In one the craze growth rate is controlled by the velocity of the diffusion front, the meniscus instability growth rate is assumed to be relatively slow, so that a significant plasticized active layer exists whose thickness assures that the meniscus instability front travels at the same speed as the diffusion front. In the other regime the propagation of the craze front is sufficiently fast that it also forms the diffusion front, so the growth rate is controlled by a combination of the two processes: diffusion and meniscus instability.