Chemistry Reference
In-Depth Information
topic contains examples that span from macroscopic bulk polymers and polymer gels,
to interfaces and thin films, to single molecules. As remains the case with covalent
polymers, an ongoing challenge is to develop hierarchical pictures that unite material
properties to well-defined and specific supramolecular behavior across material
regimes.
3.2. MECHANICAL PROPERTIES OF LINEAR SPs
At the most basic level, mechanical properties are necessarily a matter of action and
reaction, stimulus and response. The actor—a mechanical stress—is a familiar part of
everyday life, but efforts to understand its molecular consequences are only recently
coming to the fore in supramolecular chemistry. We next consider examples of how
covalent polymers respond to a mechanical stress, and then we extend that examin-
ation to the case of SPs. This discussion is not intended to capture all of the
details of a thermodynamically rigorous treatment but is instead focused on providing
a useful conceptual framework for key concepts related to the mechanics of SPs.
The energy applied through a mechanical strain can either be stored as potential
energy or dissipated as heat. In polymers, energy can be stored by the entropic restric-
tion of conformational space available to a given polymer chain. If the polymers are
anchored together, or entangled, at various positions, deformation of the material will
move those positions apart. The conformational freedom of a given chain is, on
average, decreased relative to its equilibrium state; and the collective random,
thermal fluctuations of the polymer chain will, on average, pull the entanglements
toward that equilibrium state. Thus, if the entanglements are fixed, elastic behavior
results; a mechanical stress deforms the material, but the energy is stored in molecular
conformational changes that can be used subsequently to perform work, for example,
shooting a pebble with a slingshot. This case is essentially that of rubber elasticity for
which many excellent reference texts are available.
A complementary case is that in which the entanglements are not fixed but are
lost instead. The competition between two mechanisms of entanglement loss
(chain slippage vs. chain scission; Fig. 3.2) in covalent polymers provides a useful
basis for subsequent discussion of the supramolecular case. Chain slippage typically
occurs via reptation, the process by which a polymer chain is constrained to a tube by
surrounding polymer chains (Fig. 3.2, process A). Movement within this tube is
driven by the diffusion of stored length in a snakelike motion, and the rate of the
reptative relaxation drops exponentially as the molecular weight of the polymer
increases. The physical consequence of reptation, or other chain slippage mechan-
isms, is that an entanglement that once anchored an elastic chain is lost, and the
energy stored in that elastic chain is lost (dissipated as heat) along with it.
An alternative pathway for entanglement loss is chain scission (Fig. 3.2, process
B), in which a covalent bond along the polymer main chain is broken and a
stress-bearing, otherwise elastic, chain is lost. Chain scission reactions, for
example, homolytic carbon-carbon cleavage, have obviously high activation ener-
gies. The stress-free rates of these reactions are therefore typically extremely low,
