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Multifunctional Materials 329

UCLA have created a strong, tough polymer that forms a high degree of thermally reversible,
covalent cross-links. Mechanical failure of this polymer occurs preferentially along these cross-
links, and due to the reversible nature of this bond it may be repaired by application of moderate
pressure and heat.

12.2.3.1 Polymer Healing

Polymers offer many attributes that allow for healing of damage within the material. Polymers
4 6
consist of long chain molecules with molecular weights ranging from 10 to 10 . These long chains
are made up of a string of monomers, which are the molecular repeat unit identifying that particular
polymer. Linear, branched, and the other nonnetworked polymers generally form amorphous or
semicrystalline polymers with thermoplastic character. Interaction between chains occurs through
noncovalent bonding such as hydrogen bonding and chain entanglements. As a thermoplastic, the
polymer may be heated to its melting temperature and solidified repeatedly with little change in the
properties. In contrast, thermosets consist of cross-linked networks formed by covalent bonding and
will degrade, rather than melt, upon heating.
Healing damage within a polymer is most often associated with the softening and flow of
material across a damaged interface that occurs upon heating a thermoplastic polymer. This
technique is commonly known as thermoplastic welding. Healing in thermoplastic polymers occurs
largely due to the restoration of entanglements in the polymer interface. Secondary bonding
between chains, in the form of van der Waals or London dispersion forces, is also critical to the
healing process. Other bonding, such as hydrogen bonding and chemisorption, can play an
important role. Chains are able to diffuse across the interface when heated above the glass transition
temperature, T g . The rate of crack healing is strongly time dependent, as the once separated
molecular structures diffuse across the interface to form an equivalent bonding state to that of the
virgin material. Crack healing in thermoplastic polymer surfaces seldom results from the reforma-
tion of broken bonds (primary bonds) in the polymer backbone. Typically the polymer chains at the
crack interface have been irreversibly damaged through bond breakage which results in an average
molecular weight significantly lower than that of the bulk polymer. Furthermore, the catalyst for
polymerization (through addition or condensation reactions) is not present to repolymerize the
material. The crack healing and welding in thermoplastics has been widely studied in the literature,
particularly by Wool and Kausch and their co-workers (Wool, 1978, 1979, 1995; Jud and Kausch,
1979; Jud et al., 1981; Wool and O’Connor, 1981a,b, 1982; Kausch and Jud, 1982; Kausch, 1983;
Kausch et al., 1987; Kausch and Tirrell, 1989; Wool et al., 1989)
Healing damage within thermoset polymers is typically not possible due to the cross-linked
nature of these materials. The cross-link network prevents the polymer chains from diffusing
through the material when heated above the T g . Rather the material begins to thermally degrade
when heated excessively, and in contrast to thermoplastics, cannot be returned to its original state.
Until recently, there has been no evidence of repairing damage in highly cross-linked materials. An
interesting repair scheme has been proposed by White et al. to embed a healing system within an
epoxy polymer (White et al., 2001) Liquid monomer is microencapsulated and embedded with
dispersed catalyst in an epoxy polymer, such that a propagating crack intersects a microcapsule and
releases the healing agent to effectively glue the crack shut. A schematic of the healing process
is given in Figure 12.25. Another system, proposed by Wudl and coworkers, uses a novel reversible
bonding approach to repair cross-links (Chen et al., 2002). It is this polymer that has been utilized in
the multifunctional material under development at UCSD.

12.2.3.2 Thermo-Reversibly Cross-Linked Polymer

In 2002, Wudl and coworkers published work on a polymer with the ability to repair internal
cracking (Chen et al., 2002). Until that time, there had been no highly cross-linked polymers that