Page 583 - Carrahers_Polymer_Chemistry,_Eighth_Edition

546 Carraher’s Polymer Chemistry

or Chl a) serve as primary electron acceptors and three Fe S centers (ferredoxins) as secondary
4 4
acceptors. A quinone molecule may or may not serve as an intermediate carrier between primary
electron acceptor (Chl) and secondary acceptor (Fe-S centers).
A large number of chlorophyll antennas are used to harvest the solar energy, which in turn are
used to excite the special pair P . The P donor will in turn transfer an electron to a primary
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acceptor (A , phyophytin) and in less than 100 ps to a secondary acceptor (A , a phylloquinone).
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The electron received by A is in turn transferred to an iron–sulfur cluster and then to the terminal
1
iron–sulfur acceptor.
16.10 MECHANISMS OF PHYSICAL ENERGY ABSORPTION
Let us consider a force, stress, acting on a material producing a deformation. The action of this force
can be described in terms of modeling components—a Hookean spring and a Newtonian dashpot.
In the Hookean spring, the energy of deformation is stored in the spring and may be recovered by
allowing the spring to act on another load or through release of the stress; in either case, the site
is returned to zero strain. A Newtonian dashpot is pictorially a frictionless piston and is used to
describe chains flowing past one another. The energy of deformation is converted to heat. In actual-

ity, the deformation of most plastics results in some combination of Hookean and Newtonian behav-
ior. The Newtonian behavior results in net energy adsorption by the stressed material, some of this
energy producing the work of moving chains in an irreversible manner while some of the energy is
converted to heat.
There are three major mechanisms of energy absorption: shear yielding, crazing, and cracking.
The latter two are often dealt with together and called normal stress yielding.
We can distinguish between a crack and a craze. When stresses are applied to polymeric materi-

als, the initial deformation involves shear flow of the macromolecules past one another if it is above
T , or bond bending, stretching, or breaking for glassy polymers. Eventually, a crack will begin to
g

form, presumably at a microscopic flaw, which will then propagate at high speed, often causing
catastrophic failure. The applied stress results in a realigning of the polymer chains. This results
in greater order, but decreased volume occupied by the polymer chains, that is, an increase in free
volume. This unoccupied volume often acts as the site for opportunistic smaller molecules to attack,
leading to cracking and crazing and eventually property failure.

A crack is an open fi ssure, whereas a craze is spanned top to bottom by fibrils that act to resist
entrance of opportunistic molecules such as water vapor. Even here, some smaller molecular inter-
actions can occur within the void space, and eventually the specimen is weakened.
Crazing and cracking can be induced by stress or combined stress and solvent action. Most typical
polymers show similar features. To the naked eye, crazing and cracking appear to be a fi ne, micro-
scopic network of cracks generally advancing in a direction at right angles to the maximum princi-
ple stress. Such stress yielding can occur at low stress levels under long-term loading. Suppression
of stress yielding has been observed for some polymers by imposition of high pressure.
o
In shear yielding, oriented regions are formed at 45 angles to the stress. No void space is pro-
duced in shear yielding. Crazing often occurs before and in front of a crack tip. As noted before, the

craze portion contains both fibrils and small voids that can be exploited after the stress is released or
if the stress is maintained. Materials that are somewhat elastic are better at preventing small stress
related crazing and cracks. Most plastics are not ideal elastomers and additional microscopic voids
occur each time a material is stressed.
All three mechanisms result in a difference in the optical properties of the polymeric material
because of the preferential reorientation, with realignment of the polymer chains resulting in a
change in optical properties such as refractive index, allowing detection through various optical

methods including visual examination, microscopy, and infrared spectroscopy of films and sheets.
Crazed and cracked sites of optically clear materials appear opaque, whereas shear-yielded sites
may appear to be “wavy” when properly viewed by the naked eye employing refracted light.

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