2 research outputs found
Entanglement Effects in Elastomers: Macroscopic vs Microscopic Properties
This
Perspective highlights how entanglement effects on rubber elasticity
can be unveiled by a combination of different macroscopic and microscopic
methods, taking advantage of new developments in proton low-field
NMR spectroscopy as applied to bulk and swollen rubbers. Specifically,
the application of a powerful yet routinely applicable double-quantum
method, combined with a back-extrapolation procedure over results
measured at different degrees of swelling, allows one to characterize
the recently introduced “phantom reference network”
state, which only reflects contributions of actual cross-links and
topologically trapped entanglements. We further present an assessment
of the qualitative yet popular Mooney–Rivlin analysis of mechanical
data, where the influence of entanglement contributions on the fitted,
purely empirical parameters <i>C</i><sub>1</sub> and <i>C</i><sub>2</sub> is reconsidered in the context of different
tube models of rubber elasticity. We also review the impact of
entanglements on results of equilibrium swelling experiments and address
the validity of the common Flory–Rehner approach, where we
stress its qualitative nature and the need to use NMR observables
for a correct estimation of the relevant volume fractions. We discuss
semiquantitative estimations of the cross-link density from these
macroscopic experiments with its microscopic determination by NMR
on the example of lowly cross-linked synthetic and natural poly(isoprene)
rubber prepared by a novel UV-based curing protocol of dried latex
based upon thiol–ene chemistry, which in contrast to previously
studied thermally peroxide-cured natural rubber contain only small
amounts of short-chain defects. We find that the entanglement effects
in these samples can best be described by the Heinrich–Straube
tube model with positive scaling exponent ν > 0.3 as well
as by the slip-link model of Ball et al./Edwards–Vilgis with
a slip parameter η > 0.1. A comparison with literature results
demonstrates that these findings are not universal in that the apparent
entanglement contribution depends significantly on the sample (in)homogeneity,
i.e., of the NMR-determined fraction of inelastic defects and spatial
cross-linking inhomogeneities. This means that conclusions on the
validity or invalidity of specific tube theories cannot be drawn without
careful consideration of the network microstructure
Outer-Sphere Control of Catalysis on Surfaces: A Comparative Study of Ti(IV) Single-Sites Grafted on Amorphous versus Crystalline Silicates for Alkene Epoxidation
The effect of outer-sphere
environment on alkene epoxidation catalysis
using an organic hydroperoxide oxidant is demonstrated for calix[4]arene-Ti<sup>IV</sup> single-sites grafted on amorphous vs crystalline delaminated
zeotype (UCB-4) silicates as supports. A chelating calix[4]arene macrocyclic
ligand helps enforce a constant Ti<sup>IV</sup> inner-sphere, as characterized
by UV–visible and X-ray absorption spectroscopies, thus enabling
the rigorous comparison of outer-sphere environments across different
siliceous supports. These outer-sphere environments are characterized
by solid-state <sup>1</sup>H NMR spectroscopy to comprise proximally
organized silanols confined within 12 membered-ring cups in crystalline
UCB-4, and are responsible for up to 5-fold enhancements in rates
of epoxidation by Ti<sup>IV</sup> centers