Aliphatic urethane polymers have been synthesized and characterized, using monomers with high molecular symmetry, in order to form amorphous networks with very uniform supermolecular structures which can be used as photo-thermally actuable shape memory polymers (SMPs). The monomers used include hexamethylene diisocyanate (HDI), trimethylhexamethylenediamine (TMHDI), N,N,N',N'-tetrakis(hydroxypropyl)ethylenediamine (HPED), triethanolamine (TEA), and 1,3-butanediol (BD). The new polymers were characterized by solvent extraction, NMR, XPS, UV/VIS, DSC, DMTA, and tensile testing. The resulting polymers were found to be single phase amorphous networks with very high gel fraction, excellent optical clarity, and extremely sharp single glass transitions in the range of 34 to 153°C. Thermomechanical testing of these materials confirms their excellent shape memory behavior, high recovery force, and low mechanical hysteresis (especially on multiple cycles), effectively behaving as ideal elastomers above T g . We believe these materials represent a new and potentially important class of SMPs, and should be especially useful in applications such as biomedical microdevices.

Metal−organic frameworks (MOFs) offer an attractive alternative to traditional hard and soft templates for nanocluster synthesis because their ordered crystalline lattice provides a highly controlled and inherently understandable environment. We demonstrate that MOFs are stable hosts for metal hydrides proposed for hydrogen storage and their reactive precursors, providing platform to test recent theoretical predictions that some of these materials can be destabilized with respect to hydrogen desorption by reducing their critical dimension to the nanoscale. With the MOF HKUST-1 as template, we show that NaAlH4 nanoclusters as small as eight formula units can be synthesized. The confinement of these clusters within the MOF pores dramatically accelerates the desorption kinetics, causing decomposition to occur at ∼100 °C lower than bulk NaAlH4. However, using simultaneous thermogravimetric modulated beam mass spectrometry, we also show that the thermal decomposition mechanism of NaAlH4 is complex and may involve processes such as nucleation and growth in addition to the normally assumed two-step chemical decomposition reactions.

We present a robust and general method for embedding nanoparticles, such as quantum dots (QD) or colloidal gold (Au) nanocrystals, into a highly water-soluble thin silica shell doped with paramagnetic gadolinium (Gd3+) ions without negatively impacting the optical properties of the QD or Au nanoparticle cores. The ultrathin silica shell has been covalently linked to Gd3+ ions chelator, tetraazacyclododecanetetraacetic acid (DOTA). The resulting complex has a diameter of 8 to 15 nm and is soluble in high ionic strength buffers at pH values ranging from approximately 4 to 11. For this system, nanoparticle concentrations exceed 50 μM, while most other nanoparticles might aggregate. In magnetic resonance imaging (MRI) experiments at clinical magnetic field strengths of 1.4 T (1H resonance frequency of 60 MHz), the gadolinium−DOTA (Gd−DOTA) attached to SiO2-coated QDs has a spin−lattice (T 1) particle relaxivity (r 1) and a spin−spin (T 2) particle relaxivity (r 2) of 1019 ± 19 mM-1s-1 and 2438 ± 46 mM-1 s-1, respectively, for a 8-nm QD. The particle relaxivity has been correlated to the number of Gd3+ covalently linked to the silica shell. At 1.4 T, the Gd−DOTA ion relaxivities, r 1 and r 2, respectively, are 23 ± 0.40 mM-1s-1 and 54 ± 1.0 mM-1s-1. The sensitivity of our probes is in the 100-nM range for 8−10 nm particles and reaches 10 nM for particles approximately 15 nm in diameter. Preliminary dynamic contrast enhancement MRI experiments in mice revealed that silica-coated MRI probes are cleared from the renal system into the bladder with no observable affects on the health of the animal. This current approach may offer numerous advantages over other similar approaches, , including greater relaxivity and greater simplicity for the synthesis process of dual modality contrast agents that allow both MRI and optical detection as well as applicability to other nanoparticles.

Sodium aluminum hydrides have gained attention due to their high hydrogen weight percent (5.5% ideal) compared to interstitial hydrides, and as a model for hydrides with even higher hydrogen weight fraction. The purpose of this paper is to investigate the Ti-compounds that are formed under solution-doping techniques, such as wet doping in solvents such as tetrahydrofuran (THF). Compound formation in Ti-doped sodium aluminum hydrides is investigated using X-ray diffraction (XRD) and magic angle spinning (MAS) nuclear magnetic resonance (NMR). We present lattice parameter measurements of crushed single crystals, which were exposed to Ti during growth. Rietveld refinements indicate no lattice parameter change and thus no solubility for Ti in NaAlH 4 by this method of exposure. In addition, X-ray diffraction data indicate that no Ti substitutes in NaH, the final decomposition product for the alanate. Reaction products of completely reacted (33.3 at.%-doped) samples that were solvent-mixed or mechanically milled are investigated. Formation of TiAl 3 is observed in mechanically milled materials, but not solution mixed samples, where bonding to THF likely stabilizes Ti-based nano-clusters. The Ti in these clusters is activated by mechanical milling.

Thermal degradation of a filled, cross-linked siloxane material synthesized from poly(dimethylsiloxane) chains of three different average molecular weights and with two different cross-linking species has been studied by (1)H multiple quantum (MQ) NMR methods. Multiple domains of polymer chains were detected by MQ NMR exhibiting residual dipolar coupling () values of 200 and 600 Hz, corresponding to chains with high average molecular weight between cross-links and chains with low average molecular weight between cross-links or near the multifunctional cross-linking sites. Characterization of the values and changes in distributions present in the material were studied as a function of time at 250 degrees C and indicate significant time-dependent degradation. For the domains with low , a broadening in the distribution was observed with aging time. For the domain with high , increases in both the mean and the width in were observed with increasing aging time. Isothermal thermal gravimetric analysis reveals a 3% decrease in weight over 20 h of aging at 250 degrees C. Degraded samples also were analyzed by traditional solid-state (1)H NMR techniques, and off-gassing products were identified by solid-phase microextraction followed by gas chromatography-mass spectrometry. The results, which will be discussed here, suggest that thermal degradation proceeds by complex competition between oxidative chain scissioning and postcuring cross-linking that both contribute to embrittlement.

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Bacteria often reside in communities where the cells have secreted sticky, polymeric compounds that allow them to attach to surfaces. This sessile lifestyle, referred to as a biofilm, affords the cells within these communities a tolerance of antibiotics and antimicrobial treatments. Biofilms of the bacterium Pseudomonas aeruginosa have been implicated in cystic fibrosis and are capable of colonizing medical implant devices, such as heart valves and catheters, where treatment of the infection often requires the removal of the infected device. This mode of growth is in stark contrast to planktonic, free floating cells, which are more easily eradicated with antibiotics. The mechanisms contributing to a biofilm's tenacity and a planktonic cell's susceptibility are just beginning to be explored. In this study, we have used a metabolomic approach employing nuclear magnetic resonance (NMR) techniques to study the metabolic distinctions between these two modes of growth in P. aeruginosa. One-dimensional 1H NMR spectra of fresh growth medium were compared with spent medium supernatants from batch and chemostat planktonic and biofilms generated in continual flow system culture. In addition, 1H high-resolution magic angle spinning NMR techniques were employed to collect 1H NMR spectra of the corresponding cells. Principal component analysis and spectral comparisons revealed that the overall metabolism of planktonic and biofilm modes of growth appeared similar for the spent media, while the planktonic and biofilm cells displayed marked differences. To determine the robustness of this technique, we prepared cell samples under slightly different preparation methods. Both techniques showed similar results. These feasibility studies show that there exist chemical differences between planktonic and biofilm cells; however, in order to identify these metabolomic differences, more extensive studies would have to be performed, including 1H-1H total correlated spectroscopy.

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