Chemistry Reference
In-Depth Information
simplest molecular theories of rubberlike elasticity. Such networks do not
have the unusually high extensibilities of networks formed in solution
and studied in the dry state.
Networks can also be formed from chains in the deformed state. 71 In
this approach a first network is generally introduced in the undeformed
state, the resulting elastomer is elongated, and a second network is intro-
duced in the stretched state. Release of the stress permits the network to
retract, but the second network of this “double-network” structure pre-
vents retraction down to the original dimensions (figure 7.4). The most
interesting feature of the retracted network is the fact that it is aniso-
tropic in structure and properties.
In some cases, double networks have shown increases in orientability
and strain-induced crystallization, 108 as well as improved fatigue resis-
tance. 109, 110 In fact, some results show that there may be less of a compro-
mise between failure properties in general and the modulus, 111, 112 which
may be due in part to the decreased hysteresis observed for some of these
elastomers. 113 There have even been reports of improved thermal stabil-
it y, 114 although it is hard to visualize how this would occur. Finally, electri-
cal resistivity is more sensitive to strain in carbon-black reinforced double
networks. 115 Better molecular understanding of these observations is
being sought with, for example, extensive studies of residual strains and
birefringence. 116
Results on double networks in the unswollen state generally track the
behavior of single networks. Recent results, however, now indicate that
swelling double networks gives gels that have extremely high mechanical
strength 117-119 and fracture energy. 120
Stretch
Add more
XLs
Retract
Fi g u re 7.4:
Cross linking in the elongated state to prepare networks of simpler topology.
Search WWH ::




Custom Search