Dynamic heterogeneity is an active field of glass-transition research. The length scale of this heterogeneity is called the characteristic length. It can be calculated from complex heat capacity curves in the equilibrium liquid or from dynamic calorimetry curves corrected with regard to nonequilibrium. No molecular parameters or microscopic models are necessary for obtaining the length. We report the characteristic length near glass temperature for about 30 glass formers including small-molecule liquids, polymers, silicate glasses, a metallic glass, a liquid crystal, and a plastic crystal. The lengths are between 1.0 and 3.5 nm with certain cumulations between 1.0 and 2.0 nm and between 2.5 and 3.5 nm. To try a correlation to other properties, we find that at least two should be included, e.g., Angell's fragility and the distance of T g from the crossover temperature, T c .
The formation of temperature-, concentration-, and pH-responsive hydrogels composed of the symmetric long-chain bolaamphiphile dotriacontane-1,1'-diyl bis[[2-(dimethylammonio)ethyl]phosphate] (Me(2)PE-C32-Me(2)PE) was investigated by rheological, scattering, and spectroscopic techniques. At pH 5, this bolaamphiphile is known to form a dense network of helically structured nanofibers (Köhler et al. Soft Matter 2006, 2, 77-86). Rheological measurements and dynamic light scattering were used to describe the macroscopic behavior of the hydrogels. Small-angle neutron scattering (SANS) and time-resolved static light scattering were applied to get information about the morphology of the self-assembled aggregates. Finally, solid-state 31P NMR spectroscopy was used to gain insight into the mobility of the bolaamphiphile molecules within the fiber aggregates. In comparison with the previously examined trimethylammonio analogue PC-C32-PC, which forms temperature-dependent hydrogels, Me(2)PE-C32-Me(2)PE exhibits additional concentration- and pH-dependent gelling properties. The significantly higher stability of the Me(2)PE-C32-Me(2)PE hydrogel is supported by the SANS data, which indicate the presence of fiber aggregates up to 50 degrees C.
Helical jumps in poly(ethylene oxide),
which are the molecular
processes underlying the intracrystalline chain diffusion, are studied
on the microseconds to milliseconds time scale by means of NMR. Using
a simple proton time-domain technique, a wide range of melt-crystallized
morphologies is investigated ranging from extended-chain crystals
of short chains to crystals with disordered fold surfaces of longer
chains up to 190 kg/mol. From variable-temperature data we directly
determine the Arrhenius activation parameters and find that the activation
energy is always around 65 kJ/mol. At a given temperature, average
correlation times vary from sample to sample over about 1 decade and
increase approximately linearly with the lamellar thickness. The observed
linear relation is reproduced by a generic Monte Carlo simulation
model implementing a mechanism of diffusing defects. The experimental
results are compared to 1D carbon-13 MAS exchange NMR (CODEX) and
proton rotating-frame relaxation (R
1ρ) data, for which we highlight the challenges and significant bias
effects arising from the significant distribution of correlation times.
Effective spin-diffusion averaging of the proton R
1ρ demonstrates that monomers with different jump
dynamics are spatially close; i.e., they coexist in neighboring stems.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.