  Borosilicones (e.g., Silly Putty) have been known for 70 years, but their
peculiar behaviors have remain unexplained. In this work, experiment and theory
are used to show that they are network liquids---dynamic macromolecules that
appear elastic on short timescales but exhibit flow on longer timescales. Each
borosilicone is a vast covalent network of silicone polymer chains joined by
trifunctional boron crosslinks. At any instant, the borosilicone is a
highly-crosslinked elastic material. Because the boron crosslinks are
temporary, however, the network evolves with time and the borosilicone exhibits
liquid behavior. A simple borosilicone exemplifies a classic transient network
model and behaves as a simple (Lodge) elastic fluid. Its measured moduli and
viscosities fit those predicted by the transient network model and the Maxwell
viscoelastic model: a spring in series with a dashpot, including the observed
exponential relaxation processes. When a borosilicone includes permanent
crosslinks, however, it no longer behaves as a simple elastic fluid. Its
measured moduli and viscosities fit those predicted by the Fractional Maxwell
viscoelastic model: a spring in series with a the spring-pot, including the
observed slower-than-exponential relaxation processes. Beyond the gelation
threshold, a borosilicone becomes a viscoelastic silicone rubber (VSR). With a
permanent network that spans the material coupled to a temporary network that
also spans the material, the VSR is a network liquid piggybacking on a network
solid. The Fractional Zener viscoelastic model: an elastic spring in parallel
to the Fractional Maxwell model, accurately predicts the measured moduli of
VSRs. The temporary nature of boron crosslinks is due to exchange reactions.
Because the mean lifetime of temporary crosslinks is a borosilicone's only
significant timescale, it exhibits thermo-rheological simplicity.
