The relationship between ionic conductivity, morphology, and rheological properties of polystyrene-block-poly(ethylene oxide) copolymers (SEO) doped with a lithium salt, Li[N(SO2CF3)2], is elucidated. We focus
on lamellar samples with poly(ethylene oxide) (PEO) volume fractions, φ, ranging from 0.38 to 0.55, and PEO
block molecular weights, M
PEO, ranging from 16 to 98 kg/mol. The low-frequency storage modulus (G‘) at 90 °C
increases with increasing M
PEO from about 4 × 105 to 5 × 107 Pa. Surprisingly, the conductivity of the SEO/salt
mixtures with the molar ratio of Li to ethylene oxide moieties of 0.02 σ, also increases with increasing M
PEO,
from 6.2 × 10-5 to 3.6 × 10-4 S/cm at 90 °C. We compare σ with the conductivity of pure PEO/salt mixtures,
σPEO, and find that σ/[φσPEO] of our highest molecular weight sample is close to 0.67, the theoretical upper limit
for transport through randomly oriented lamellar grains.
Micelles of cetyltrimethylammonium bromide (CTAB), when doped with increasing levels of 4-ethylphenol, show microstructural transitions from spherical micelles to elongated wormlike micelles, disks, and subsequently to globular and then to tubular vesicles. Wormlike micelles are observed at a dopant-to-CTAB molar ratio of 1:3. At higher dopant ratios (1:1), globular vesicles are observed which transition to tubular vesicles when the dopant becomes the predominant species at a ratio of 3:1. These transitions are reflected in small-angle neutron scattering analysis and, interestingly, can be directly observed through cryo-transmission electron microscopy. The para-substituted phenol is interfacially active and modulates interfacial curvature of the micelles. The observations of microstructure modifications have relevance to the synthesis of mesoporous materials using CTAB as the template.
Highly aligned stringlike silica nanostructures are obtained through templated synthesis in the columnar hexagonal structure of a rigid crystalline surfactant mesophase. A two-step procedure is used to first shear-align the surfactant mesophase and then conduct synthesis under quiescent conditions in the mesophase. The mesophase retains its alignment for extended periods, allowing materials synthesis to be decoupled from the application of shear. The observations have significant implications in the control of ceramic microstructure morphology and transitions from nonaligned to aligned nanowire type structures.
An optically clear, crystalline, gel-like mesophase is formed by the addition of water to a micellar solution
consisting of a mixture of 0.85 M anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and
a 0.42 M zwitterionic surfactant phosphatidylcholine (lecithin) in isooctane. At 25 °C and water to AOT
molar ratio of 70, the system has a columnar hexagonal microstructure with randomly oriented domains.
The shear-induced orientation and subsequent relaxation of this structure were investigated by rheological
characterization and small-angle neutron scattering (SANS). The rheological response implies that the
domains align under shear, and remain aligned for several hours after cessation of shear. Shear-SANS
confirms this picture. The sheared gel mesophase retains its alignment as the temperature is increased
to 57 °C, indicating the potential to conduct templated polymer and polymer−ceramic composite materials
synthesis in aligned systems.
The concept of reverse templating of an organogel to form imprinted porous divinylbenzene polymer films with submicrometer channels is demonstrated. The organogel comprising a 1:1 molar ratio of two organogelators, that is, bis(2-ethylhexyl) sodium sulfosuccinate and 4-chlorophenol, was formed in divinylbenzene. The gel was cast as a thin film before UV polymerization of the solvent, and the organogelators were later removed by simple washing with water and isooctane. The integrity of the fiber bundles of the organogel was preserved during polymerization, and an exact hollow replica was obtained after the organogelators were leached away. It is easily possible to imprint gel fiber bundle structures into polymeric films through this technique. The gel can also be formed on macroporous substrates to yield supported thin porous polymeric films. With the incorporation of functional nanoparticles in AOT inverse micelles and hence the organogel, nanoparticle-containing porous polymer films exhibiting luminescence or magnetic properties are envisioned.
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