After forever changing the drug discovery process in the pharmaceutical industry, combinatorial chemistry methodologies are increasingly being applied to the discovery and optimization of more efficient catalysts and materials (see picture). With the advent of new combinatorial synthesis and screening technologies, coupled with integrated data management systems, the application of these technologies to materials science and catalyst research holds tremendous potential and brings high expectations to this new and exciting field.
Macroscopically oriented silicate−surfactant liquid crystals are
produced by slow cooling of lamellar and
hexagonal mesophases through their isotropic−anisotropic phase
transition in an 11.7 T magnetic field. Comparisons
of in situ experimental and simulated 2H NMR
spectra quantify the degrees of orientational order in the
silicate−surfactant liquid crystals. The overall orientations of the
mesophases with respect to the applied magnetic field
depend upon the combined diamagnetic susceptibilities of the individual
molecular species in the multicomponent
surfactant−silicate mixtures. As a result, macroscopic alignment
of these liquid crystalline systems can be controlled
by adjusting their composition: lamellar or hexagonal domains are
shown to adopt different orientations, according
to the diamagnetic susceptibilities of different organic additives
present in otherwise identical mixtures. These
results
demonstrate the utility of liquid crystal processing strategies for
organizing inorganic−organic hybrid materials over
mesoscopic and macroscopic length scales.
The main chain dynamics of amorphous poly(ethyl
methacrylate) (PEMA) and poly(methyl
methacrylate) (PMMA) below and above their respective glass transition
temperatures T
g are analyzed
by two-dimensional solid-state exchange 2H NMR
spectroscopy. In both polymers, a restricted mobility
of the polymer backbone is already present in the glassy state, as is
directly demonstrated and quantified
using samples deuterated at the methyl and methylene moieties of the
polymer main chain. The unusual
main chain mobility below T
g is coupled to the
β-relaxation process, which involves 180° flips of the
carboxyl
side groups. At their respective glass transition temperatures,
the coupling of the β-process to the main
chain motions manifests itself differently in both polymers; the
smaller ester side group reorients
comparatively fast in PMMA, whereas in PEMA, the reorientation of the
bulkier side group remains
anisotropic and the correlation times are slower by about 1 order of
magnitude. Therefore, in PMMA,
the β-relaxation predominantly influences the time scale of the
α-relaxation, leading to a particularly
high mobility of the main chain itself. In contrast, in PEMA, a
slow uniaxial diffusion of the main chain
around its local axis sets in at T
g, the
β-process thus affecting mainly the geometry of backbone
motions,
as is further corroborated by comparing one-dimensional 13C
NMR spectra with two-dimensional exchange
2H NMR spectra at higher temperatures. In summary, the
coupling of the α- and β-processes leads to
longer mean correlation times for the α-relaxation in PEMA than in
PMMA.
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