Chemistry Reference
In-Depth Information
body which can be dissipated either by complete recovery (on removal of the
load), by catastrophic rupture, or by polymer flow in the stress application region.
The latter process, called shear yielding, or shear banding, is a useful mechanism
for absorbing impact forces.
A few comments on the distinction between crazing and shear yielding may
be appropriate here. A material which undergoes shear yielding is essentially elas-
tic at stresses up to the yield point. Then it suffers a permanent deformation.
There is effectively no change in the volume of the material in this process. In
crazing, the first craze initiates at a local stress less than the shear stress of the
bulk material. The stress required to initiate a craze depends primarily on the
presence of stress-raising imperfections, such as crack tips or inclusions, in the
stressed substance, whereas the yield stress in shear is not sensitive to such influ-
ences. Permanent deformation in crazing results from fibrillation of the polymer
in the stress direction.
Craze formation is a dominant mechanism in the toughening of glassy poly-
mers by elastomers in “polyblends.” Examples are high-impact polystyrene
(HIPS), impact poly(vinyl chloride), and ABS (acrylonitrile-butadiene-styrene)
polymers. Polystyrene and styrene-acrylonitrile (SAN) copolymers fracture at
strains of
10
2
2
, whereas rubber-modified grades of these polymers (e.g.,
HIPS and ABS) form many crazes before breaking at strains around 0.5.
Rubbery particles in polyblends act as stress concentrators to produce many
craze cracks and to induce orientation of the adjacent rigid polymer matrix.
Good adhesion between the glassy polymer and rubbery inclusion is important
so that cracks do not form and run between the rubber particles. Crazes and
yielding are usually initiated at the equators of rubber particles, which are the
loci of maximum stress concentration in stressed specimens, because of their
modulus difference from the matrix polymer. Crazes grow outward from rubber
particles until they terminate on reaching other particles. The rubber particles
and crazes will be able to hold the matrix polymer together, preventing forma-
tion of a crack, as long as the applied stress is not catastrophic. The main fac-
tors that promote craze formation are a high rubber particle phase volume, good
rubber-matrix polymer adhesion, and an appropriate rubber particle diameter
[12,13]
. The latter factor varies with the matrix polymer, being about 2
B
m for
polystyrene and about one-tenth of that value for unplasticized PVC. It is intui-
tively obvious that good adhesion between rubber and matrix polymer is
required for transmission of stresses across phase boundaries. Another interest-
ing result stems from the differences in thermal expansion coefficients of the
rubber and glassy matrix polymer. For the latter polymers the coefficient of lin-
ear expansion (as defined by ASTM method D696) is
μ
10
2
6
K
2
1
, while the
B
10
2
2
K
2
1
. When molded samples of
rubber-modified polymers are cooled from the melt state the elastomer phase
will undergo volume dilation. This increases the openness of the rubber struc-
ture and causes a shift of the
T
g
of the rubber to lower values than that of unat-
tached elastomer
[14]
.
corresponding value for elastomers is
B
