Page 190 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN012c-598 July 26, 2001 15:59

Polymers, Mechanical Behavior 699

the more flexible the chain is and the easier it will be for it
to undergo changes in its conformation. As expected, the
bond rotational potential energies will be a function of the
steric hindrance by side groups extending from the chain
backbone. In the case of the presence of a double bond
within the backbone, there is of course no rotation allowed
about this type of bond. As we shall learn, temperature is
a particularly important variable in the discussion of the
mechanical properties of polymers; higher temperatures
provide higher thermal energy (higher kT ) and therefore
FIGURE 1 General schematic showing how a coiled polyethy- permit higher degrees of thermal Brownian motion that
lene molecule might be extended into a rodlike shape. may promote changes in the inter- and intramolecular en-
ergy states of the molecules.
a strand of spaghetti, which is often used as an analogy The mechanical behavior of a material extends from
to linear polymer molecules. This long threadlike nature its stress–deformation response, in which it is particularly
or high aspect ratio is an important feature, for it leads important to define the mode of deformation (uniaxial, bi-
to the physical entanglement of a given molecule with axial, etc.) and the loading profile and environment under
many of its neighbors through the intertwining or inter- which a given test is carried out. Often times in the appli-
weaving of one molecule with several others including cation of polymeric materials, the mode of failure may be
itself. The consequences of these entanglements and the induced by a more complex loading scheme than is easily
time-dependent behavior of these molecules becoming applied within a testing laboratory. However, it is impor-
unentangled or changing their entanglement density are tant to develop, where possible, a basic understanding of
discussed in Section V. It should be recognized, however, the properties of any new polymeric material through a
that in the above-calculated estimate of aspect ratio, a well-defined loading profile and to learn how these prop-
rather “skinny” chain was utilized. If one were to carry erties depend upon molecular variables and the external
out the same calculation for a 100,000 molecular weight variables mentioned above. In this article, the general me-
polystyrene molecule that contains bulky phenyl groups chanical or stress–deformation responses of materials are
on alternate carbons, the same calculation would lead often illustrated in this article in the form of a schematic
to a much smaller aspect ratio, this being about 240. diagram showing how this behavior may change with spe-
However, even this number is still greater than the typical cific variables rather than by presenting actual experimen-
aspect ratio for a strand of spaghetti! tal data.
Another important feature of molecular systems is that Due to the macromolecular or chainlike nature of the
their behavior under a given set of loading conditions is components, the orientation of polymer chains may well
very dependent on a balance of three types of energy: occur during loading or may already exist within the sys-
(1) intramolecular, which is related to energy changes in- tem to be tested due to previous orientation inducement
volved with bond rotations within the backbone and other caused by such common fabrication schemes as fiber spin-
types of intramolecular interactions such as changes in ning, film drawing, and injection molding. Hence, it is im-
secondary bonding that occurs between atoms or groups portant to recognize whether the material can be viewed as
within the same molecule; (2) intermolecular, which re- isotropic, that is, possessing equal properties in all direc-
sults from the energies concerned with secondary bonding tions, or whether it is anisotropic in that the properties are
butisnowbetweengroupsoratomsordifferentmolecules; directionally dependent such as would occur if one were
and (3) thermal energy, which is dictated by the product to test a previously oriented system, two common exam-
kT , where k is the Boltzmann constant and T is absolute ples being a drawn monofilament or a deformed film. For
temperature. When T is ∼300 K (ambient), the value of the purposes of our discussion, emphasis will be placed on
RT , where R is the molecular gas constant, is of the or- the mechanical behavior of isotropic systems; anisotropic
der of 0.6 kcal/mol. This may be contrasted with van der systems will be discussed in Section IX.
Waals energies involved with secondary bonding, which
areoftheorderof 2to3kcal/mol,whilestrongerhydrogen
bonding may reach levels of the order of 7 to 11 kcal/mol. II. TYPES OF DEFORMATION
The intramolecular bond rotation energies or more specifi-
cally the difference between various rotational isomeric There are three principal modes by which systems un-
states may be as little as a fraction of a kilocalorie per dergo deformation: (1) tensile or extensional deforma-
mole to many kilocalories per mole. The lower this value tion, (2) shear deformation, and (3) bulk or hydrostatic