The effect of temperature on crazing mechanisms in polystyrene

At room temperature scission is the dominant mechanism for the modification of the entanglement network required for craze formation in polystyrene, but as the temperature is increased towardsT _g, there is the possibility that disentanglement processes may contribute. These will be most important for short chains. If disentanglement can occur, a molecular weight dependence of the crazing stress as a function of temperature will result. This prediction is tested by straining thin films of a range of monodisperse samples of polystyrene at temperatures between 40 and 90° C. The nature of the ensuing deformation has been characterized by transmission electron microscopy. It is observed that whereas only crazing occurs over the entire temperature range for the lowest molecular weight sample, shear processes become important for higher molecular weight materials. For the longest chains, crazing is almost entirely suppressed at 80° C, with the preferential formation of shear deformation zones occurring. These observations are consistent with the idea that disentanglement is playing a significant role in craze formation at sufficiently high temperatures.     

Methanol crazing of coarse shear bands in polystyrene

A specimen with coarse shear bands produced at stress concentrations by compression was immersed in methanol to observe craze formation. Thin crazes were initiated at shear bands and joined together while propagating to form thick crazes. Crazes were formed only on the tension side of shear bands with the craze planes perpendicular to the shear bands. When a craze propagated through a shear band, each displaced the other at intersections. Some secondary shear bands were transformed partly into crazes resulting in about a factor of ten increase in thickness. This transformation was achieved by a tensile deformation of fibrous sheets in the shear band with simultaneous production of fine fibres.     

The effect of temperature on the deformation and fracture of polystyrene

Tensile and compression tests have been carried out over a range of temperatures between 78 and 360 K to determine the micromechanisms of deformation and the mode of fracture. In compression, deformation occurred by shear band propagation whereas, in tension, deformation was restricted to crazing followed by fracture. Anomalously low crazing stresses were observed for tests on specimens immersed in liquid nitrogen. Analysis of the mechanism of fracture confirmed previous work relating crazing and fracture and the fracture model has been modified slightly to take account of additional features observed in failure of crazes at high temperatures.     

The competition between shear deformation and crazing in glassy polymers

Whereas thin films of some polymers such as polystyrene readily form crazes when strained in tension, thin films of other polymers such as polycarbonate rarely exhibit crazing under the same testing conditions; the polymers that rarely craze tend to form regions of shear deformation instead. Polymers such as polystyrene-acrylonitrile which lie between these two extremes of behaviour may exhibit both modes of deformation. Thin films suitable for optical and transmission electron microscopy (TEM) of six such co-polymers and polymer blends have been prepared. After straining, the nature of the competition between shear deformation and crazing is examined by TEM. It is found that in these polymers many crazes have tips which are blunted by shear deformation. This process leads to stress relaxation at the craze tip, preventing further tip advance. In this way short, but broad, cigar-shaped crazes are formed. Examination of the deformation at crack tips in the same polymers shows more complex structures, the initial high stress levels lead to chain scission and fibrillation but as the stress drops, shear becomes the dominant mechanism of deformation and the stress is relieved further. Finally, at long times under stress, chain disentanglement may become important leading to fibrillation and craze formation again. The nature of the competition is thus seen to be both stress and time dependent. Physical ageing of these polymers, via annealing below T _g, suppresses shear leading to the generation of more simple craze structures.     

Environmental shear deformation zones and crazes in crosslinked polystyrene and poly(para-methylstyrene)

Thin films ( 1m) of polystyrene and poly (para-methylstyrene) were bonded to annealed copper grids and crosslinked using electron radiation to increase, the network strand density. At each separate films were strained in air and in Freon 113 until the critical strain, _c, for plastic deformation was reached. Transmission electron microscopy observations reveal that both sets of films undergo a transition from crazing to localized shear deformation zones (DZs) as is increased. The critical strains for both crazing and DZ formation are markedly decreased in Freon 113. The DZs formed in Freon 113 show abrupt changes in extension ratio,, at their interfaces while those formed in air show more diffuse changes in. A mechanism for environmental DZ formation is proposed in which the diffusion rate of Freon 113 into the DZ is strongly enhanced by the ongoing plastic deformation. This mechanism is suggested by Rutherford backscattering measurements of Freon 113 diffusion into PS which show that such diffusion in the absence of plastic deformation is much too slow to allow the observed rate of growth of the environmental DZs.     

The effect of varying the polyethylene content and the co-polymer content on crazing in polystyrene—low-density polyethylene blends

The effect of varying the low-density polyethylene content and the polystyrenepolyethylene block co-polymer content on the rates of craze initiation and craze growth in polystyrene/low-density polyethylene blends has been studied. it was found that the parameters in the Eyring rate coefficients for craze initiation and craze growth are not dependent on the low-density polyethylene content. However, the rates of craze initiation increased with increasing low-density polyethylene content. This is explained tentatively by a new model for craze initiation. It is argued that effective crazes are only formed within clusters of low-density polyethylene particles that have overlapping stress-concentration fields. The dependence of the rate of craze initiation on the volume-fraction of dispersed phase that follows from this cluster model is qualitatively in agreement with experimental results. PS-PE co-polymer addition gives rise to changes in the Eyring parameters of the rate coefficients of craze initiation and craze growth. This may be a consequence of changes in morphology near the interface and of the different stress state at the interface.     

The effect of cross-linking on crazing in polyethersulphone

Cross-links have been introduced into thin films of PES (polyethersulphone)/1 wt% sulphur by heating them in air at 350 °C. The effect of this is to suppress crazing in favour of shear deformation in high-temperature regimes where disentanglement crazing dominates for uncross-linked films of the same composition. We argue that light cross-linking (one or two cross-links per chain) is sufficient to give rise to a finite gel fraction in the films which, because it effectively forms an infinite network, cannot disentangle. Thus for crazing to occur, chains which form part of the gel fraction must always break rather than disentangle. This has the effect of raising the crazing stress relative to the yield stress in the weakly temperaturedependent regime of crazing at high temperature, where disentanglement is normally considered sufficiently rapid for entanglement loss not to contribute to the crazing stress. Hence as the gel fraction is increased by increasing the heat-treatment time, crazing is suppressed at the highest temperatures with respect to shear deformation, leading to a second transition, this time from crazing back to shear.     

The effect of temperature on the fracture of rubber modified polystyrene

Brittle fractures were obtained in rubber modified polystyrene over the temperature range −120 to 20° C using the surface notch method. When compared with single edge notch data, a thickness effect was apparent and this was described in terms of plane stress and plane strain fracture toughness values. The plane strain value agreed closely with that of the polystyrene matrix indicating that the constrained region showed no toughening effect. The relaxation process of the rubber was apparent in the plane stress toughness.     

The effect of the environment on the stress crazing of polycarbonate

The effect of a number of different liquids on the rate of crazing of polycarbonate was studied. It was found that liquids either had no effect on the mechanical response, caused immediate cracking and fracture or caused a measurable rate of crazing, depending on the solubility parameter of the liquid. The rate of crazing in those liquids that caused crazing was found to vary inversely with the square root of time, indicating that diffusion through the end of the craze controlled the craze growth. Pre-immersion in one of the solvents, ethanol, had a strong effect on the subsequent rate of crazing under a constant load. The pre-immersion of pre-cracked samples initially caused an increase in rate of crazing. After a pre-immersion time of roughly 24 to 72 hours, the subsequent rate of crazing decreased with an increase in pre-immersion time. When the pre-immersion was conducted on an uncracked specimen and the crack was generated after the pre-immersion, the subsequent rate of crazing increased steadily with pre-immersion time. An explanation for the observed results is given.     

The effect of temperature and shear rate on platelet aggregation

Samples of whole blood were obtained from male volunteers and exposed to combinations of shear rates and temperatures representative of cardiopulmonary bypass (CPB) in a modified computer-controlled concentric cylinder rotational viscometer for a period of 100 s. Blood sampled from the chamber was fixed in paraformaldehyde, stained with CD41 and analysed by flow cytometry. Only platelet-positive particles were acquired, each individual cell, or aggregate of cells, identified by analysis of its fluorescence and forward light scatter characteristics. Little platelet aggregation was observed at shear rates of less than 4000 s-1 for temperatures of greater than 24°C, but large numbers of aggregates were formed at all temperatures at 4000 s-1 (p<0.05), with more aggregates forming at 24 and 30°C than at 37 and 42°C (p<0.05). We conclude that the process of aggregation is dependent on both temperature and shear rate. We note that a large number of platelets become involved in aggregates under conditions of temperature and shear-rate typical of CPB.     

The effect of specimen size on the mechanical behaviour associated with crazing

Since crazes generally nucleate at the surface it is expected that the size of the specimen, as described by the ratio of surface area to volume, should affect the mechanical behaviour of polymers which deform primarily by crazing. The stress relaxation curves and the stress-strain curves of PS, PMMA, PTFE, and PC were measured in liquid nitrogen for specimens of different size which were machined from the same rod. The predicted size effect was observed in that the smaller (6.4mm diameter) specimens stress-relaxed faster and the stress to produce a given amount of craze deformation was lower than for the larger (12.7 mm diameter) specimens. The range of the tensile strength from 0 to size is also presented based on the stress to nucleate the first craze and on the tensile strength that is observed when no crazing occurs     

Effect of deformation ratio on fibril deformation in fatigue of polystyrene

Small-angle X-ray scattering has been used to measure the deformation of craze fibrils during mechanical fatigue of polystyrene. The maximum deformation of the sample in the fatigue cycle was kept constant while the minimum deformation was varied. When the minimum deformation was 50% or more of the maximum, the load on the craze fibrils remained tensile. When the minimum deformation was reduced below this, the load on the fibrils became compressive and they buckled. The main effect of minimum sample deformation on fatigue life occurred in the regime where the fibrils remained straight. In this regime a decrease in minimum sample deformation caused a considerable decrease in fatigue life. At low minimum sample deformations the effects of minimum deformation on fatigue life were not large. These effects probably stem from the fibril strains involved in the deformation processes.     

Slip processes in the deformation of polystyrene

Two slip processes are characterized in the deformation of atactic polystyrene by compression. In the optical microscope, one appears as intensive shear bands and the other as diffuse shear zones. But in the electron microscope, the latter reveals itself in the form of two sets of numerous, fine, discontinuous shear bands intersecting at nearly right angles. In addition to their differences in appearance, the coarse slip band propagates fast along a localized path, inclines at less than 45 with the compression axis, and invariably produces shear fracture when it extends across the specimen. On the other hand, the fine slip bands spread slowly by multiplication mainly along the maximum shear stress direction, contribute to almost all the macroscopic strain and cause shape changes of the specimen. Hence the coarse band process is a brittle mode and the fine band process a ductile mode. The relative abundance of these bands depends on the thermal history of the specimen, the loading condition, and the deformation temperature. The average shear strain inside either band is about 1.5 and is recoverable upon annealing.     

Combined effect of shear and large deformation on stress intensity factors

An attempt has been made to study the influence of large deformation on the stress intensity factor in a cracked plate subjected to bending including shear deformation. It is assumed that singular terms for stress resultants and strains in the case of large deformation have the same angular distribution and order of singularity as in the case of a linear problem. With this in view the small deformation singular element has been used at the crack tip region surrounded by large deformation plate bending elements. The finite element analysis, based on total Lagrangian formulation combined with the modified Newton–Raphson technique, has been used to get numerical results. Several examples connected with large deformation of cracked plates subjected to bending are studied. Using the above technique stress intensity factors for linear and non-linear cases have been compared.     

Structural transition in poly(methyl methacrylate) due to large deformation at temperatures below the equilibrium second-order transition temperature

To analyse the structural change in glassy polymers under large deformation at low temperatures, poly(methyl methacrylate) specimens were uniaxially compressed at temperatures below the equilibrium second-order transition temperature T ₂ with varying strain rates. The state of steady plastic flow which appeared in the lower yield range of the stress-strain curve was analysed using the Eyring equation in a novel way. This analysis provided the following results: at low temperatures, the volume of a flow unit decreased with temperature, probably approaching the least critical value; and a functional relation between the activation enthalpy H and the activation entropy S for the glass deviated gradually from that for the melt derived from the WLF equation with decreasing temperature. This deviation could be attributed to the structural transition of the glass into liquid-like structures of the melt at elevated temperatures above T _g + 100 K where the WLF equation is no longer available.