Polystyrene craze

Recently, Brown and Kramer have reported a study of the rise in stress after changing the environment of crazed polystyrene specimens under load from methanol, water or their mixtures to air [12]. They derived an equation relating the change in the surface component of the stress to the surface tension and the craze fibril geometry. 

The stress sensitivity of hard elastic polymers to changes in environmental surface tension has been well documented [6,10,11]. Because of their exposure to the environment, the surface contribution to the stress is primarily due to the microfibrils. Brown and Kramer [12], using a cylinder as a model for craze fibrils related the change in the surface component of the stress in crazed polystyrene to the change in surface tension, as shown below ... 

Polystyrene. Polystyrene [9003-53-6] is a thermoplastic prepared by the polymerization of styrene, primarily the suspension or bulk processes. Polystyrene is a linear polymer that is atactic, amorphous, inert to acids and alkahes, but attacked by aromatic solvents and chlorinated hydrocarbons such as dry cleaning fluids. It is clear but yellows and crazes on outdoor exposure when attacked by uv light. It is britde and does not accept plasticizers, though mbber can be compounded with it to raise the impact strength, ie, high impact polystyrene (HIPS). Its principal use in building products is as a foamed plastic (see Eoamed plastics). The foams are used for interior trim, door and window frames, cabinetry, and, in the low density expanded form, for insulation (see Styrene plastics). 

The rubber particles should not be so small that they are completely embedded in a craze. It is interesting to note that in high-impact polystyrene crazes tend to be about 2 p.m thick and the optimum particle sizes observed as a result of experience are quoted in the range 1-10 p.m. For ABS the figures are about 0.5 p.m and 0.1-l.Op.m respectively. 

There are other conditions that result from the frozen-in stresses. In materials such as crystal polystyrene, which have low elongation to fracture and are in the glassy state at room temperature, a frequent result is crazing it is the appearance of many fine microcracks across the material in a direction perpendicular to the stress direction. This result may not appear immediately and may occur by exposure to either a mildly solvent liquid or vapor. Styrene products dipped in kerosene will craze quickly in stressed areas. 

The other effect of having a stretched area is a reduction in resistance to stress cracking. Crazing is a possibility in such areas such as in polystyrenes, and environmental stress cracking caused by solvent substances will occur in the stretched areas. This is a particularly important consideration in vacuum formed products used for packaging food that frequently has some solvent action on the plastics. 

Wellinghoff.S.T., Baer,E. Crazing in ultra-thin films of atactic polystyrene. Unpublished manuscript. 

Fig. 9. Composite transmission electron micrograph of a typical craze formed in polystyrene 26)...

Figure 3. Craze traces in a toughened polystyrene resin (ca. 1100 X) (33)...

Experimental Evidence. Morphology. Figure 3 (33) shows in phase contrast microscopy the development of crack or craze patterns around rubber particles in a toughened polystyrene. The lack of dependence of crack inclination on direction of stress is especially marked in this micrograph, and can be explained only by reference to dynamic branching rather than to crack or craze nucleation by stress raisers. Schmitt and Keskkula refer to the lines as craze cracks and cracks. ... 

Fig. 11 Craze in commercial polystyrene showing the characteristic steps nucleation through void formation in a pre-craze zone, growth of the fibrillar structure of the widening craze by drawing-in of new matrix material in the process zone, and final breakdown of the fibrillar matter transforming a craze into a crack (the crack front is more advanced in the center of the specimen, shielded by a curtain of unbroken fibrils marked by the arrow). The fibril thickness depends—of course—on the molecular variables, the strain rate-stress-temperature regime of the crazing sample and on its treatment (preparation, annealing) and geometry (solid, thin film) for PS typical values of between 2.5 and 30 nm are found [1,60,61]...

Fig. 5 TEM micrographs of thin films a craze in MGIM76 at 0 °C (From [51]), b an active zone at the craze-bulk interface in polystyrene (From [21])...

Under conditions where chain mobility is very low, such as at low temperatures, the loss of entanglements occurs by chain scission this is what happens for polystyrene air crazes. Of course, chain scission crazes (CSC) are MW-independent as soon as chains are long enough to be entangled (at too-low MWs, no craze can be formed, only cracks happen). 

A good dispersion of rubber particles appears to favor the nucleation and growth of a large number of thick crazes uniformly distributed in the polystyrene matrix. This is believed to be an efficient source of energy absorption for the material under mechanical loading. The concepts of stress field overlap and critical volume of stress concentration zone for craze initiation were introduced to explain the observed mechanical behavior of HIPS. 

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