Chemistry Reference
In-Depth Information
fibrils
voids
FIGURE 4.26
Sketch of a craze in polystyrene
[9]
. The upper figure shows a craze, with connecting
fibrils between the two surfaces. The lower figure is a magnification of a section of the
craze showing voids and fibrils. Actual crazes in this polymer are about 0.1
2
μ
m thick;
this figure is not to scale.
viscous flow of polymer segments. Although crazes appear to be a fine network
of cracks, the surfaces of each craze are connected by oriented polymeric struc-
tures and a completely crazed specimen can continue to sustain appreciable
stresses without failure. Crazing detracts from clarity, as in poly(methyl meth-
acrylate) signage or windows, and enhances permeability in products such as
plastic pipe. Mainly, however, it functions as an energy sink to inhibit or retard
fracture.
The term
crazing
is apparently derived from an Anglo-Saxon verb
krasen
,
meaning “to break.” In this process polymer segments are drawn out of the
adjoining bulk material to form cavitated regions in which the uncrazed surfaces
are joined by oriented polymer fibrils, as depicted in
Fig. 4.26
. Material cohesive-
ness in amorphous glassy polymers, like polystyrene, arises mainly through entan-
glements between macromolecules and entanglements are indeed essential for
craze formation and craze fibril strength in such polymers
[9]
.
Glassy polymers with higher cohesiveness, like polycarbonate and cross-
linked epoxies, preferentially exhibit shear yielding
[10]
, and some materials,
such as rubber-modified polypropylene, can either craze or shear yield, depending
on the deformation conditions
[11]
. Application of a stress imparts energy to a
