XRD and NMR characterization of synthetic hectorites and the corresponding surfactant-exchanged clays

in Clay Minerals (2008), 43

Motional heterogeneity of intercalated species in modified clays and poly(ε-caprolactone)/clay nanocomposites

Conference (2006, November 17)

Motional heterogeneity of intercalated species in modified clays and poly(epsilon-caprolactone)/clay nanocomposites

in Langmuir (2006), 22(10), 4818-4824

Modified laponites and synthetic saponites are used as precursors for the preparation of poly(epsilon-caprolactone) (PCL)/clay nanocomposites. The structure and dynamics of species intercalated in the ... [more ▼]

Modified laponites and synthetic saponites are used as precursors for the preparation of poly(epsilon-caprolactone) (PCL)/clay nanocomposites. The structure and dynamics of species intercalated in the modified clays and the corresponding nanocomposites are characterized by X-ray diffraction and magic-angle spinning NMR. The influence of the headgroup, the hydrocarbon chain length, and the loading of the surfactant on the nanocomposite formation are discussed. The yield of PCL intercalation is related to the probability of direct polymer-clay interactions and to the size of the clay platelets. Relaxation times in the laboratory and rotating frames that allow characterization of fast and slow molecular dynamics in these systems are discussed, showing a motional heterogeneity of the intercalated species. [less ▲]

The step-by-step Robinson annulation of chalcone and ethyl acetoacetate - An advanced undergraduate project in organic synthesis and structural analysis

in Journal of Chemical Education (2006), 83(8), 1225-1228

The Robinson annulation is a three-step process involving a Michael addition followed by an internal aldol condensation and a dehydration. Under appropriate experimental conditions, it is possible to stop ... [more ▼]

The Robinson annulation is a three-step process involving a Michael addition followed by an internal aldol condensation and a dehydration. Under appropriate experimental conditions, it is possible to stop the reaction after every step and to isolate the three products separately. This feature is particularly attractive in the frame of an organic chemistry course. It allows students to confirm experimentally the validity of the stepwise mechanism and to get a more thorough understanding of the whole process. It also permits them to synthesize a rich set of related molecules that can be compared and characterized through various analytical techniques. Thus, a stoichiometric mixture of chalcone and ethyl acetoacetate was reacted in ethanol. Depending on the quantity of barium hydroxide monohydrate used as catalyst, the reaction time, and the temperature, three different products were obtained. Their full IR, 1H, 13C, COSY, NOESY, and HETCOR NMR spectra are supplied. Examination of the spectroscopic data helps uncover many challenging structural analysis problems. Among them, the diastereoselective formation of chiral centers during the annulation process, the distinction between axial and equatorial substituents on a cyclohexane ring, and the possibility of a keto-enol tautomerism are extensively discussed. [less ▲]

Solid-state NMR study of intercalated species in poly(epsilon-caprolactone)/clay nanocomposites

in Langmuir (2004), 20(22), 9828-9833

The structure and dynamics of surfactant and polymer chains in intercalated poly(epsilon-caprolactone)/clay nanocomposites are characterized by (31)P magic-angle spinning (MAS) and (13)C cross ... [more ▼]

The structure and dynamics of surfactant and polymer chains in intercalated poly(epsilon-caprolactone)/clay nanocomposites are characterized by (31)P magic-angle spinning (MAS) and (13)C cross-polarization MAS NMR techniques. To obtain hybrid materials with the low polymer content required for this study, in situ intercalative polymerization was performed by adapting a published procedure. After nanocomposite formation, the chain motion of the surfactant is enhanced in the saponite-based materials but reduced in the Laponite ones. Compared to the starting clay, the trans conformer population of the surfactant hydrocarbon chain in the nanocomposite decreases for the saponite systems. Mobility of the polymer chain is higher in the nanocomposites than in the bulk phase. The charge of the modified saponite does not significantly influence chain mobility in the nanocomposites. [less ▲]

Self-assembly of poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers in aqueous solution

in Langmuir (2004), 20(20), 8442-8451

The associative behavior of monodisperse diblock copolymers consisting of a hydrophilic poly(ethylene oxide) block and a hydrophobic poly(epsilon-caprolactone) or poly(gamma-methyl-epsilon-caprolactone ... [more ▼]

The associative behavior of monodisperse diblock copolymers consisting of a hydrophilic poly(ethylene oxide) block and a hydrophobic poly(epsilon-caprolactone) or poly(gamma-methyl-epsilon-caprolactone) block has been studied in aqueous solution. Copolymers have been directly dissolved in water. The solution properties have been studied by surface tension, in relation to mesoscopic analyses by NMR (self-diffusion coefficients), transmission electron microscopy, and small-angle neutron and X-ray scattering. The experimental results suggest that micellization occurs at low concentration (approximately 0.002 wt %) and results in a mixture of unimers and spherical micelles that exchange slowly. The radius of the micelles has been measured (ca. 11 nm), and the micellar substructure has been extracted from the fitting of the SANS data with two analytical models. The core radius and the aggregation number change with the hydrophobic block length according to scaling laws as reported in the scientific literature. The poly(ethylene oxide) blocks are in a moderately extended conformation in the corona, which corresponds to about 25% of the completely extended chain. No significant modification is observed when poly(gamma-methyl-epsilon-caprolactone) replaces poly(epsilon-caprolactone) in the diblocks. [less ▲]

Solid-state NMR study of poly(epsilon-caprolactone)/clay nanocomposites

in Langmuir (2004), 20(5), 2039-2041

Freezing of water bound in lichen thallus as pbserved by H-1 NMR. 2. Freezing protection mechanisms in a cosmopolitan lichen Cladonia mitis and in Antartic lichen species at different hydration levels

in Colloids and Surfaces B : Biointerfaces (2003), 28

Na-23 2D 3QMAS NMR and Si-29, Al-27 MAS NMR investigations of Laponite and synthetic saponites of variable interlayer charge

in Clay Minerals (2003), 38

Surfactant molecules intercalated in laponite as studied by C-13 and Si-29 MAS NMR

in Langmuir (2002), 18(16), 6159-6163

The structure and dynamics of surfactant molecules intercalated in Laponite have been characterized by solid-state C-13 NMR relaxation times and C-13- and Si-29-detected proton relaxation times. These ... [more ▼]

The structure and dynamics of surfactant molecules intercalated in Laponite have been characterized by solid-state C-13 NMR relaxation times and C-13- and Si-29-detected proton relaxation times. These results are compared to similar data obtained for a surfactant intercalated in montmorillonite and for different mesoporous materials. As compared with montmorillonite clay, a higher mobility and a greater percentage of the gauche conformation of the surfactant molecules are found in Laponite. The Si-29-detected proton relaxation times in the rotating frame are sensitive to the amount of intercalated surfactant. [less ▲]