Craze initiation in glassy polymers: Quantifying the influence of molecular orientation

A study has been made of crazing stress in oriented glassy polystyrene. The aim was to develop a methodology for prediction of crazing stress in glassy polymers with frozen-in molecular orientation. Oriented specimens of two grades of monodisperse polystyrene (PS) and one grade of polydisperse PS were produced by uniaxial melt-drawing and subsequent quenching of compression-moulded bars. Birefringence and crazing stress parallel to the draw direction (in the presence of diethylene glycol) were measured on miniature beam specimens cut from them. The crazing stress increased substantially with orientation, and the magnitude of the increase relaxed approximately on a timescale associated with the longest Rouse time. Specifically, a linear correlation was found, to within experimental scatter, between the increase in crazing stress and the orientation expressed in terms of frozen-in conformational stress, as predicted by the theory of Maestrini and Kramer [13]. The inverse gradient (constant β in the theory) was found to be 0.059 ± 0.002, when inferring the conformational stress from the measured birefringence. Crazing was found to be suppressed in favour of yielding in the most highly oriented specimens, and this could be explained in terms of the differing sensitivities of crazing and yield to molecular orientation.     

Surface crazing in PVC and other polymers

Initiation and growth of surface crazes in rigid PVC, subjected to tensile creep loads, has been studied experimentally for a wide variety of conditions. The tests were carried out at room temperature, at stress levels ranging from 10 to 50 MPa, in air or in the vapour of natural gas condensate (a crazing agent). PVCs with different molecular weights (K-values) and degrees of gelation were tested. Physical aging of the sample was considered by comparing quenched and aged samples. The behaviour under continuous loading was compared with that under cyclic loading. In cyclic loading, crazing behaviour differs markedly from that observed in continuous loading. The defects grow linearly with the number of load periods, i.e. linearly with the loading time, and they appear to be associated with cracks. In continuous loading, the craze length grows linearly with the logarithm of the loading time, after some initiation time has been passed. This logarithmic growth in constant loading is bounded by two processes of growth cessation. The first, at high stresses, coincides with gross yielding of the sample; the second, at low stress levels, is less well understood. The effects of stress level, crazing agent, K value, degree of gelation and physical aging on the processes of initiation and cessation of crazing under constant stress are discussed. The growth rate was shown to depend only on the stress; the crazing agent, molecular weight and degree of gelation have little influence. For other polymers, such as PS, PMMA and PC, the growth kinetics are essentially the same as for PVC. For these materials, the growth rate for crazing sharply decreases with increasing stress, according to the inverse second to inverse third power of the stress.     

Measurement of the residual mechanical properties of crazed polycarbonate. I: Qualitative analysis

A new technique to quantify the bulk craze density of transparent plates was used to characterize the craze growth behavior of polycarbonate at various stress levels. The craze growth rates were found to exponentially increase with an increase in stress, obeying the Eyring equation for thermally activated processes in the presence of an applied stress. The residual mechanical properties of crazed polycarbonate were then correlated to the crazing stress, relative craze density and strain rate. The results show that increasing the bulk craze density does not affect the yield stress but decreases both the failure stress and ductility of polycarbonate. Also, a crazing stress of 40 MPa was found to cause a much larger degree of degradation of failure properties than a crazing stress of 45 MPa. Correlating the crazing stress to the craze microstructure revealed that fewer, larger crazes form at the lower crazing stress. Therefore, flaw size has a greater effect on the failure properties of polycarbonate than flaw quantity.     

Different deformation mechanisms of two modified‐polystyrene bimodal systems

It is well known that the dominant toughening mechanism of rubber‐modified polystyrene is multiple crazing. Some researchers have investigated polystyrene that can be modified by rubbers with dual particle sizes, leading to better mechanical properties. That is, the way to absorb energy during the deformation process is crazing and cavitation induced by rubber particles. Two types of polybutadiene‐graft‐polystyrene (PB‐g‐PS) rubber modifiers which have core‐shell structures were synthesized via an emulsion graft polymerization using redox and oil‐soluble initiators, respectively. To balance the yield strength, general‐purpose polystyrene was blended with the PB‐g‐PS modifiers, as well as commercial high‐impact polystyrene. Blends were defined as R‐bimodal and O‐bimodal corresponding to dispersed PB‐g‐PS particles formed using the redox and oil‐soluble initiators, respectively. The impact strength of R‐bimodal was improved significantly by altering the ratio of core to shell. However, little change of impact strength was observed for O‐bimodal. Transmission electron microscopy images of fracture surfaces indicated that the deformation mechanism of R‐bimodal is shear‐yielding induced by multi‐crazing. Moreover, PB‐g‐PS particles dispersed in O‐bimodal can form a ‘cluster’ structure, leading to crazing to absorb energy. Scanning electron microscopy images also showed obvious distinctness between the R‐bimodal and O‐bimodal systems due to different deformation mechanisms. Copyright © 2010 Society of Chemical Industry     

Schädigungskriterien glasiger polymer im hinblick auf deren schlagzähmodifizierung

In rubber-modified, stiff and brittle polymers an applied stress generates multiaxial stress concentrations in the matrix material at the rubber interface and initiates a large number of limited regions of plastic deformation. In order to understand this toughening mechanism, one needs a certain knowledge of the kind of plastic flow which is initiated within a glassy polymer according to the type and amount of stress, the so-called failure criteria. Unlike criteria which apply to the onset of shear flow, craze criteria are still contro-versially discussed in the literature. Argon has suggested a theoretical criterion of bulk crazing. Sternstein et al. have studied experimentally the surface crazing of PMMA. Based on experimental results of Rehage and Goldbach and ourselves, calculations have given the following results: The Sternstein criterion, measured and formulated for surface crazing under the condition of plain stress has till now been incorrectly transformed for use in principal stress space. Correctly transformed, it deviates strongly from Argon's criterion in the range of states of stress with large portions of hydrostatic tension. The Sternstein criterion obviously lends itself to describing multiaxial corrosion. For rubber-modified systems, however, one needs bulk craze criteria, for which no experimental results exist. Preliminary results show that these can more easily be estimated for PS than for PMMA. These two polymers therefore show markedly different shapes of the failure surfaces.     

The pressure and temperature effects on brittle-to-ductile transition in PS and PMMA

Tensile experiments in polystyrene (PS) and poly(methyl methacrylate) (PMMA) conducted at constant strain rate over a wide range of pressure and temperature have shown that a brittle-to-ductile transition is induced in these amorphous polymers by the superposition of hydrostatic pressure as well as by the raise of the experimental temperature. A detailed stress–strain analysis permits explanation of the mechanism for the brittle-to-ductile transition in terms of interaction between two competing processes of plastic yielding—crazing and shear banding phenomena. The crazing and shear banding processes respond quite differently to changes of pressure or temperature, causing shifting of the brittle-to-ductile transition point to where the craze initiation stress and shear band initiation stress again become equal. The evidence that the brittle-to-ductile transition pressure becomes lower with increasing temperature refutes a previously suggested concept that the transition relates primarily to mechanical relaxation phenomena.     

Effect of compound mineral powders on workability and rheological property of HPC

The compounding effect of silica fume (SF) with phosphorus slag (PS) or limestone (LS) powder is investigated in this paper. It is found that such a compounding can achieve various rheological behavior of HPC. The compound powders of PS with SF lower plastic viscosity and yield stress of fresh concrete, but increase the slump and promote continuous flowability of concrete greatly. However, the compounding of LS with SF increases the yield stress, but decreases both slump and slump flow of concrete, although the viscosity remains broadly unchanged compared with the concrete containing LS only. It is demonstrated that rheological property can be highly correlated with the surface characteristic of each component of the compound powders. Based on the experimental work, the appropriate fractions of the components in these compounds, especially the optimum content of SF, have been suggested for improving rheological property of HPC.     

Synthesis of sub‐micrometer core–shell rubber particles with 1,2‐azobisisobutyronitrile as initiator and deformation mechanisms of modified polystyrene under various conditions

BACKGROUND: Sub‐micrometer core‐shell polybutadiene‐graft‐polystyrene (PB‐g‐PS) copolymers with various ratios of polybutadiene (PB) core to polystyrene (PS) shell were synthesized by emulsion grafting polymerization with 1,2‐azobisisobutyronitrile (AIBN) as initiator. These graft copolymers were blended with PS to prepare PS/PB‐g‐PS with a rubber content of 20 wt%. The mechanical properties, morphologies of the core‐shell rubber particles and deformation mechanisms under various conditions were investigated. RESULTS: Infrared spectroscopic analysis confirmed that PS could be grafted onto the PB rubber particles. The experimental results showed that a specimen with a ‘cluster’ dispersion state of rubber particles in the PS matrix displayed better mechanical properties. Transmission electron micrographs suggested that crazing only occurred from rubber particles and extended in a bridge‐like manner to neighboring rubber particles parallel to the equatorial plane at a high speed for failure specimens, while the interaction between crazing and shear yielding stabilized the growing crazes at a low speed in tensile tests. CONCLUSION: AIBN can be used as an initiator in the graft polymerization of styrene onto PB. The dispersion of rubber particles in a ‘cluster’ state leads to better impact resistance. The deformation mechanism in impact tests was multi‐crazing, and crazing and shear yielding absorbed the energy in tensile experiments. Copyright © 2009 Society of Chemical Industry     

METHODS FOR TESTING CRAZING OF GLAZES CAUSED BY INCREASES IN SIZE OF CERAMIC BODIES1

Investigations to determine the causes of a type of crazing which develops on certain glazed ceramic wares, some time after firing, revealed the fact that in many cases such crazing is caused by an expansion of the body. This is probably due to the combination of water with the body, while the glaze remains practically constant in size. A body may be tested for this type of crazing by placing the glazed specimens in an autoclave and subjecting them to a steam pressure of 150 to 175 pounds per square inch for one hour. Bodies showing the least tendency to develop this type of crazing are those which have a low porosity, are not very soluble in sulphuric acid, and have a low ignition loss above 110°C.     

Polystyrene and poly(methyl methacrylate) interfaces reinforced with diblock carbon nanotubes

An asymmetric double cantilever beam test was used to determine the ability of carbon nanotubes with varying chemistry along their lengths, that is, diblock nanotubes, to compatibilize the polystyrene/poly(methyl methacrylate) (PS/PMMA) interface. PS molecules were grafted primarily to one of the blocks to cause that block to migrate to the PS phase since otherwise both blocks would prefer to reside in PMMA. Fracture toughnesses increased monotonically with increasing diblock carbon nanotube concentration and maximum values were like those for block copolymer-reinforced interfaces while single-chemistry nanotubes showed no reinforcing effect. However, the abrupt increase in fracture toughness with added compatibilizer indicative of a transition to crazing was not found consistent with nanotubes suppressing crazing in homopolymers. Scanning electron microscopy images of the fractured surfaces show agglomerates of carbon nanotubes present which are likely limiting the efficacy of carbon nanotubes at toughening the interface.     

Fatigue crack propagation behavior of cellulose esters

The effect of plasticizer concentration on fatigue crack propagation (FCP) rate in cellulose acetate-propionate (CAP) was determined. Compact tension specimens were machined from 6.2 mm-thick injection molded plaques and tested on an MTS servohydraulic testing machine using a sinusoidal waveform with a frequency of 1 Hz. Two FCP mechanisms were identified: a crazing mechanism, which dominated at low values of stress intensity factor range, ΔK, and a shear yielding mechanism, which dominated at high values of ΔK. The value of ΔK at the onset of the transition from the crazing mechanism to the shear yielding mechanism was a function of plasticizer concentration, and therefore yield strength of the CAP. The transition in crack propagation mechanism created a V-shaped feature on the fracture surface, which could be used to weight the contributions from the two crack propagation mechanisms to the overall FCP rate.     

Effects of environment on the mechanical properties of plastics under high pressure

Polystyrene (PS), high-impact polystyrene (HIPS), and polyethylene (PE) have been investigated studying the pressure dependence of stress-elongation behavior in tension over the range from atmospheric pressure to four kilobars at room temperature. The effect of strain rate was also observed for PS specimens. Tensile deformation of PS and HIPS has shown that the pressure-transmitting fluid (silicon oil) acts as a stress crazing and cracking agent. Non-sealed specimens of PS showed a brittle-to-ductile transition at 2.95 kbar while specimens sealed from the environment showed the same transition at only 0.35 kbar. Scales HIPS and PE specimens exhibited ductile behavior at all pressures. The extent of plastic deformation for PE was affected when specimens where exposed to the silicon oil environment. Surprisingly, HIPS exposed to the oil exhibited two transitions as the applied hydrostatic pressure was raised: a ductile-to-brittle followed by a brittle-to-ductile transition. Analysis of the stress-elongation curves for sealed PS and HIPS specimens indicated that the pressure dependency of craze-initiation stress differs from that of shear band initiation stress. The brittle-to-ductile transition occurred when the initiation stresses of both processes became equal. The principal stress for craze initiation showed almost no pressure dependency, suggesting that crazes initiate when the principal stress level of the tensile specimen reaches a critical value irrespective of the applied hydrostatic pressure.     

ABS树脂拉伸性能及形变机理的研究

研究了丙烯腈丁二烯苯乙烯共聚物（ABS）树脂在不同温度和不同拉伸速率时的拉伸行为以及物理老化对其拉伸行为的影响。结果表明,屈服强度随测试温度的升高而下降,断裂伸长率并不随着测试温度的升高而提高,直到测试温度升高到接近ABS树脂塑料相的玻璃化转变温度时,断裂伸长率才显著提高;断裂伸长率随拉伸速率的增加而降低,在不同的拉伸速率下,ABS的形变区内均可观察到银纹现象;在较高的拉伸速率下,形成的银纹数量较多,但银纹较短,银纹的扩展得到了有效抑制;ABS树脂经物理老化后断裂伸长率明显降低,银纹数量增加并出现了空洞成串现象。     

Crazing studies of polystyrene. I. A new phenomenological observation

This paper reports a new phenomenological observation regarding the stress crazing of glassy polystyrene. It was found that the applied stress to initiate a craze, often called the critical crazing stress, is independent of molecular weight. Further, the gross structure of the craze does depend on molecular weight, and other phenomenological aspects previously reported have been reaffirmed. These observations are interpreted in the light of knowledge from the literature and provide a better understanding of the crazing process.     

Measurement of the residual mechanical properties of crazed polycarbonate. II: Design of experiments analysis

The Design of Experiments (DOE) approach was used to build quantitative empirical models of the residual mechanical properties of crazed polycarbonate as functions of relative craze density, crazing stress, and strain rate. Crazing did not affect the yielding behavior of polycarbonate, but increasing the strain rate increased the yield stress according to the Eyring theory. The Eyring activation volume for yielding of crazed polycarbonate was measured to fall between reported values for conformational changes of a dilute solution of polycarbonate chains and yielding of uncrazed polycarbonate. Also, with as little as 1% relative craze density, the failure stress was approximately 10% lower and the ductility was, on the average, 50% lower than for uncrazed polycarbonate properties. It was also found that increasing the crazing stress from 40 to 45 MPa increased the failure stress and ductility for a given magnitude of relative craze density.     

Microdeformation mechanisms in thin films of amorphous semi-aromatic polyamides

Optical and transmission electron microscopy have been used to investigate the microdeformation mechanisms in thin films of amorphous semi-aromatic polyamides with different chemical structures and chain lengths. In the chosen range of test temperatures (between −120 °C and the principal mechanical relaxation temperature, T_α), three successive microdeformation mechanisms were identified: chain scission crazing (CSC) at the lowest temperatures, shear deformation zones (SDZs) at intermediate temperatures and chain disentanglement crazing (CDC) at the highest temperatures. The critical stress for SDZ formation was identified with the experimental yield stress, whereas the critical stresses for CSC and CDC were derived from model expressions, using experimental data for the molecular mass between entanglements, the monomeric friction coefficient and the plastic flow stress. Variations in the transition temperatures among the different polymers were attributed to differences in the temperature dependence of the yield stress, and hence to variations in chain mobility, which could be accounted for in terms of the chemical structure.     

Cold rolled ABS. Part 1: The effect of rubber particle size on the tensile properties of ABS before and after cold rolling

The effect of rubber particle size on the tensile properties of rolled and unrolled acrylonitrile-butadiene-styrene has been studied by considering model systems consisting of mixtures of a small particle (0.1 micron diam) rubber, S, and a large particle (0.56 micron diam) rubber, L, in an SAN matrix. Before rolling, tensile toughness is characterized by crazing. While both rubber induce matrix crazing, ABS systems containing only the S rubber exhibits early failure due to crack formation, before crazing is propagated very far along the tensile axis. The inefficiency of the small particle rubber is interpreted in terms of high composite yield stress and insufficient distance between particles to allow craze branching. The efficiency of the small particle rubber is improved via the addition of a small amount of large particle, L, rubber to the composite or by a slight degree of cold rolling, both of which enhance craze propagation in the tensile direction. With further rolling, the tensile deformation mode changes from one of localized crazing, which is propagated, to one of uniform deformation, which occurs without crazing.     

Particulate-filled glassy polymers

Glass beads, NaCl crystals, or other imperfections act as stress concentrators in glassy polymers causing early crazing upon stressing. Stress–strain curves of glassy thermoplastic polymers filled with uncoupled glass beads or NaCl crystals exhibit a knee, or a slope-discontinuity point. The characteristics of the slope change, related to crazing, are studied in the present work. Foams were produced from the polymer/salt samples by soaking them in water resulting in complete leaching of the salt. This preparation procedure gives open-cell foams and there are strong indications that these polymeric foams have a highly cracked structure formed during the extraction step.     

Fracture behavior of poly(vinyl chloride)/methyl methacrylate/butadiene/styrene polymer blends

The fracture mode of poly(vinyl chloride)/methyl methacrylate/butadiene/styrene (PVC/MBS) polymer blends can change from ductile to brittle in accordance with the changes in shape of the test specimen or test conditions. Therefore, the mechanisms of impact energy absorption and the main cause of stress whitening are complicated. The following results on PVC/MBS blends were obtained by carrying out fracture experiments at different test speeds and temperatures:

• (1) The ductile/brittle fracture mode of the PVC and PVC/MBS blends can be explained by σ (the craze initiation stress)/σ_y (the shear yield initiation stress), which depends on the strain rates and temperature.
• (2) The fracture behavior of the PVC/MBS blends can be classified into the following types from the standpoints of fracture mode and whitening degree: Fracture I: ductile fracture without whitening; Fracture II: ductile fracture with whitening; and Fracture III: brittle fracture without whitening.
• (3) The following concepts can be estimated from the measurements of yield stress, specific gravity and SEM, TEM and visual observations. In Fracture I, shear yield occurs mainly. In Fracture II, both shear yield and crazing occur. In Fracture III, deformation of the rubber and local crazing occur.
• (4) The main cause of stress whitening in PVC/MBS blends is light scattering by cavities in the rubber particles.
• (5) In Fracture II, at first, crazes with cavities in the rubber particles occur. Then, shear yield occurs. Finally, crazes are healed by the heat, and only the cavities in the rubber remain.

     

The detergent stress-cracking of polythene

Whenever the fracture of a polymer is preceded by crazing or other types of cavity formation, energy must be supplied both for local yielding and for the generation of a new surface. However previous calculations for rigid glassy polymers have shown that the surface energy contribution required for the formation of a craze, compared with that associated with plastic and elastic strains is small. In order to study this problem further, we have now made a finite element analysis of the cavitation of an elastic-plastic solid introducing a surface energy term. This shows that the ratio of the plastic energy requirement to surface energy depends, not unexpectedly, on (I) the ratio of yield stress/ surface tension and (II) cavity size. Thus for polymers with a low yield stress such as polythene, the surface tension becomes significant if the hole diameter is below 100 nm. In these circumstances crazing and cracking is predictably accelerated by dilute aqueous detergents which do not need to penetrate the polymer, provided of course that the detergent has access to the cavities. Hole growth also depends on the extent of orientation hardening during plastic strain and this contributes to the greater stress crack resistance of high molecular weight polythene. The occurrence of a larger amount of plastic deformation on the fracture surfaces of high molecular weight polymer has been demonstrated by Harman for high density polythene and is confirmed here in scanning electron micrographs of low density material. Other electron micrographs show that very small cacities are indeed formed when high density polythene is broken in the coventional detergent stress cracking test, thus accounting for the accelerating effect of the detergent.