No abstract

Pulsed-field gradient NMR spectroscopy was used to study the diffusion of three different poly(propyleneimine) dendrimers with hydrophilic triethylenoxy methyl ether terminal groups (generations 2, 4, and 5) in poly(vinyl alcohol) aqueous solutions and gels. The effects of the diffusant size, polymer concentration (from 0 to 0.26 g/mL), and temperature on the self-diffusion coefficients have been studied, and the model of Petit et al. [Macromolecules 1996, 29, 6031] was used to fit the experimental data. The Stokes−Einstein hard-sphere radii were also calculated in the zero concentration limit and were compared with those of the linear poly(ethylene glycol)s under the same conditions. The proton NMR relaxation times (T 1 and T 2) were measured to study the mobility of the dendrimer core part and terminal group as a function of the dendrimer size.

A molecular necklace of polypseudorotaxanes was prepared by threading β-cyclodextrins (β-CD) onto biodegradable and thermoresponsive polyurethanes derived from bile acids. These polyurethanes were synthesized via a simple step condensation of bile acid-based dicarbonate with poly(ethylene glycol)-diamine. The β-CD rings slide onto the poly(ethylene glycol) segments and selectively recognize the bile acid units of the polyurethane chains, whereas the poly(ethylene glycol) segments remain crystalline with a lower crystallinity. This bio-compound-derived molecular necklace can be visualized by scanning tunneling microscopy. The polypseudorotaxanes show thermosensitivity in water and the phase transition temperature may be fine-tuned by varying the molar ratios of β-CD to the bile acid units. Such an interesting necklace model of polypseudorotaxane constructed from natural compounds may lead to the further exploration of their applications, such as as an enzyme model, due to their biological nature.

Pd(II)-directed self-assembly of a 3-pyridyl grafted Lindqvist hexavanadate led to the formation of a unique trimeric species, as confirmed by a variety of techniques, including pulsed-field gradient NMR spectroscopy and high-resolution ESI mass spectrometry.

The local dynamics of three poly(propylene imine) dendrimers with hydrophilic triethylenoxy methyl ether terminal groups were studied in D2O by the measurement of the 1H NMR relaxation times, which were treated with the Lipari–Szabo model‐free approach. The results showed that the overall mobility increased with temperature and decreased with increasing dendrimer size. An Arrhenius trend was observed for both overall and local motions. The activation energy of overall tumbling increased from 11.3 to 17.5 kJ/mol with the dendrimer size. The local mobility decreased from the outer part to the inner part of the dendrimer and with the dendrimer size. The spatial restriction of local motions decreased with increasing temperature up to 55 °C and remained constant above 55 °C. Local motions were more restricted when the dendrimer size increased. The results showed that the hydrophilic end groups of the dendrimers were located preferentially at the periphery of the molecules and were extended in the aqueous environment. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2969–2975, 2003

Pharmaceutical tablets made of modified high-amylose starch have a hydrophilic polymer matrix into which water can penetrate with time to form a hydrogel. Nuclear magnetic resonance imaging was used to study the water penetration and the swelling of the matrix of these tablets. The tablets immersed in water were imaged at different time intervals on a 300 MHz NMR spectrometer. Radial images show clearly the swelling of the tablets and the water concentration profile. The rate constants for water diffusion and the tablet swelling were extracted from the experimental data. The water diffusion process was found to follow case II kinetics at 25 degrees C. NMR imaging also provided spin density profiles of the water penetrating inside the tablets.

Solid-state NMR spectroscopy is an established experimental technique which is used for the characterization of structural and dynamic properties of materials in their native state. Many types of solid-state NMR experiments have been used to characterize both lithium-based and sodium-based solid polymer and polymer–ceramic hybrid electrolyte materials. This review describes several solid-state NMR experiments that are commonly employed in the analysis of these systems: pulse field gradient NMR, electrophoretic NMR, variable temperature T1 relaxation, T2 relaxation and linewidth analysis, exchange spectroscopy, cross polarization, Rotational Echo Double Resonance, and isotope enrichment. In this review, each technique is introduced with a short description of the pulse sequence, and examples of experiments that have been performed in real solid-state polymer and/or hybrid electrolyte systems are provided. The results and conclusions of these experiments are discussed to inform readers of the strengths and weaknesses of each technique when applied to polymer and hybrid electrolyte systems. It is anticipated that this review may be used to aid in the selection of solid-state NMR experiments for the analysis of these systems.

No abstract