Chemistry Reference
In-Depth Information
Figure 3.2 A mechanical force causes a loss of an entanglement in a covalent polymeric
network by one of two major mechanisms: process A, chain slippage via reptation; process
B, chain scission.
and they only occur when high stress is placed on the bonds. Chain scission events
lead to an effectively permanent failure in the polymeric material and provide a
molecular-level defect at which forces concentrate and macroscopic cracks initiate.
The supramolecular case can be instructively considered in the same context
(Fig. 3.3). If the entanglements and the polymer chains remain intact under an
applied stress, the SP is elastic. If the SP chain remains intact, but entanglements
are lost through chain slippage, then the polymer is viscous and the viscosity will
depend on the molecular weight of the SP in the same way that it depends on the
molecular weight of the covalent system. In contrast, if entanglements are lost
through chain scission, then the stored energy (elasticity) is lost. For SPs, the
primary mechanism for chain scission is the simple dissociation of the supramole-
cular bond. Unlike covalent bonds, supramolecular associations are such that dis-
sociation often occurs readily in the absence of any stress. Although some
supramolecular interactions are effectively permanent, many have lifetimes that
approach nanoseconds. Chain scission, mediated by supramolecular dissociation,
might therefore make significant contributions to material mechanical properties
that are not observed in covalent polymers.
Supramolecular chain scission differs further from covalent chain scission,
because supramolecular recombination is typically the predominant fate of a ruptured
chain; anthropomorphically speaking, the supramolecular moieties are, by their
very nature, predisposed to reassociation rather than alternative reaction pathways.
This predisposition is not intrinsic to the products of covalent bond rupture,
which might lead either to high-energy intermediates with nonspecific reactivity
or to species that require catalyst or elevated temperature to recombine. The
