NMR Results (nmr + result)

Distribution by Scientific Domains

Selected Abstracts

Synthesis and crystallization behavior of acetal copolymer/silica nanocomposite by in situ cationic ring-opening copolymerization of trioxane and 1,3-dioxolane

Lanhui Sun
Abstract The acetal copolymer/silica nanocomposite was prepared by in situ bulk cationic copolymerization of trioxane and 1,3-dioxolane in the presence of nanosilica. The crystallization behavior of acetal copolymer/silica nanocomposite was studied by AFM, DSC, XRD, and CPOM, and the macromolecular structure of acetal copolymer/silica nanocomposite was characterized by FTIR and 1H-NMR. The 1H-NMR results showed that the macromolecular chain of acetal copolymer had more than two consecutive 1,3-dioxolane units in an oxymethylene main chain, while that of acetal copolymer/silica nanocomposite had only one 1,3-dioxolane unit in an oxymethylene main chain. There existed interaction between the macromolecular chains and nanoparticles (such as hydrogen bonds and coordination). On one hand, nanoparticles acted as nucleation center, which accelerated the crystallization rate but reduced the crystallinity. The spherulite sizes also decreased with addition of nanoparticles attributed to the nucleation effect. On the other hand, the presence of nanoparticles interrupted the spherical symmetry of the crystallite. In conclusion, the high surface energy and small scale of nanoparticles have a prominent impact on the polymerization mechanism and crystallization behavior of nanocomposite. 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source]

Synthesis and characterization of injectable bioadhesive hydrogels for nucleus pulposus replacement and repair of the damaged intervertebral disc

J. Vernengo
Abstract Bioadhesive polymers are natural or synthetic materials that can be used for soft tissue repair. The aim of this investigation was to develop an injectable, bioadhesive hydrogel with the potential to serve as a synthetic replacement for the nucleus pulposus of the intervertebral disc or as an annulus closure material. Branched copolymers of poly(N -isopropylacrylamide) (PNIPAAm) and poly(ethylene glycol) (PEG) were blended with poly(ethylene imine) (PEI). This three component injectable system can form a precipitated gel at physiological temperature due to the phase transition of PNIPAAm. The injection of glutaraldehyde into the gel core will adhere the implant to the surrounding tissues. 1H NMR results indicated the successful physical incorporation of PEI into the PNIPAAm-PEG network by blending. In addition, the covalent crosslinking between the amine functionalities on the PEI and the aldehyde functionalities on the glutaraldehyde was verified using FTIR difference spectroscopy. Mechanical characterization of these blends showed a significant increase (p < 0.05) in compressive modulus following glutaraldehyde injection. The in vitro bioadhesive force studies with porcine skin showed a significant increase (p < 0.05) in the mean maximum force of detachment for PNIPAAm-PEG/PEI gels when glutaraldehyde was injected into the gel core. The results of this study indicate that the reactivity between amines and aldehyde functionalities can be exploited to impart bioadhesive properties to PNIPAAm-PEG/PEI copolymers. 2010 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2010 [source]

Computational studies of the cone and 1,2,3 alternate calix[6]arene bis-crown-4 isomers: structures, NMR shifts, atomic charges, and steric compression

Meghan N. Lawson
Abstract The cone and 1,2,3 alternate isomers of calix[6]arene bis-crown-4 were investigated computationally. Structural optimizations, energies, bond distances, and Mulliken charges were calculated by the application of the B3LYP/6-31g(d) method/basis, followed by NMR calculations via both B3LYP/6-31g(d) and HF/6-31g(d). Calculations were completed at three different levels of imposed symmetry, and two calculations investigated the chloroform solvent effects. Better NMR results were obtained from HF/6-31g(d) calculations that did not impose molecular symmetry constraints. Consideration of solvent effects improved ground state energies, but other improvements were minimal and not significant enough to justify the added computational expense of solvent calculations. Overall results are consistent with known experimental assignments and were valuable for assigning previously unknown NMR peaks. Net charges, electrostatic forces, and local dipoles , but not bond lengths , are strongly correlated to spectroscopic manifestations of steric compression. Copyright 2009 John Wiley & Sons, Ltd. [source]

Synthesis and characterization of sulfonated-fluorinated, hydrophilic-hydrophobic multiblock copolymers for proton exchange membranes

Xiang Yu
Abstract Hydrophilic/hydrophobic block copolymers as proton exchange membranes (PEMs) has become an emerging area of research in recent years. These copolymers were obtained through moderate temperature (, 100 C) coupling reactions, which minimize the ether-ether interchanges between hydrophobic and hydrophilic telechelic oligomers via a nucleophilic aromatic substitution mechanism. The hydrophilic blocks were based on the nucleophilic step polymerization of 3,3,-disulfonated, 4,4,-dichlorodiphenyl sulfone with an excess 4,4,-biphenol to afford phenoxide endgroups. The hydrophobic (fluorinated) blocks were largely based on decafluoro biphenyl (excess) and various bisphenols. The copolymers were obtained in high molecular weights and were solvent cast into tough membranes, which had nanophase separated hydrophilic and hydrophobic regions. The performance and structure-property relationships of these materials were studied and compared to random copolymer systems. NMR results supported that the multiblock sequence had been achieved. They displayed superior proton conductivity, due to the ionic proton conducting channels formed through the self-assembly of the sulfonated blocks. The nano-phase separated morphologies of the copolymer membranes were studied and confirmed by atomic force microscopy. Through control of a variety of parameters, including ion exchange capacity and sequence lengths, performances as high, or even higher than those of the state-of-the-art PEM, Nafion, were achieved. 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1038,1051, 2009 [source]

Investigation of the Structural Characterization of Mesoporous Molecular Sieves MCM-41 from Sepiolite

Shengming Jin
Mesoporous molecular sieves, MCM-41, were synthesized from sepiolite using acid leaching, followed by hydrothermal reconstruction and then calcinations at 540C for 5 h. The structures and the porosity of MCM-41 were investigated by means of small-angle X-ray diffraction patterns, Brunaer-Emmett-Teller (BET), 29Si MAS NMR, Fourier transform infrared (FTIR), and high resolution transmission electron microscope (HRTEM) methods. The results showed that the hexagonal MCM-41 was formed in an alkaline solution of pH 12, when crystallization was carried out at 100C for 24 h. The specific surface area, pore diameter, and pore volumes of MCM-41 from sepiolite were 1036 m2/g, 2.98 nm, and 1.06 cm3/g, respectively. 29Si MAS NMR results revealed that amorphous silica decomposed into Si,O chains consisting of two layers of Si atoms, with Q3 configurations resulting in an increase in the fraction of Q3 configuration during the crystallization of post-Mg-extraction sepiolite. The IR results illustrated that the complex of ,,SiO,,CTA+ was formed during the synthesis of MCM-41 from post-Mg-extraction sepiolite. [source]

Applications of 13C NMR to predict low temperature viscosity performance of base oils

B.L. Papke
Average structural parameters as determined by 13C nuclear magnetic resonance (NMR) spectroscopy, coupled with average base oil carbon number values from mass spectroscopy, were used to predict low temperature base oil viscosity performance. The advantages of this technique are that it provides quantitative carbon type distribution without any need for prior sample separation and can differentiate between various normal and iso-paraffinic structures. The 13C NMR technique also provides detailed information on the nature of branching in the iso-paraffinic hydrocarbons associated with waxy base oil components. Correlations between 13C NMR results and ASTM D 2983 Brookfield viscosities are reported for API Group II and Group III base oils, with R2 (adjusted) coefficients of 0.96 and 0.94, respectively. Results from this study also provide an indication of how specific base oil compositional features may affect low temperature viscosity performance. Copyright 2006 John Wiley & Sons, Ltd. [source]

Double-Grafted Cylindrical Brushes: Synthesis and Characterization of Poly(lauryl methacrylate) Brushes

Youyong Xu
Abstract Double-grafted cylindrical brushes with poly(lauryl methacrylate) (PLMA) as the side chains were synthesized using the grafting-from strategy via atom transfer radical polymerization (ATRP). The polyinitiator poly[2-(2-bromoisobutyryloxy)ethyl methacrylate] (PBIEM) with ,=,240 and 1,500 served as the backbone. The PLMA side chains of the brushes carry long alkyl chains. GPC and 1H NMR measurements confirmed the successful formation of the PLMA cylindrical brushes. The side chains were cleaved from the cylindrical brushes by transesterification. GPC and 1H NMR results indicate that the initiating efficiency of the bromoester groups on the backbone for the bulky monomer was in the range of 0.34,,,f,,,0.67. Static and dynamic light scattering show that the ratio of the radius of gyration to the hydrodynamic radius, Rg/Rh, is in the range of 1.2,1.3, indicating that the LMA cylindrical brushes are semiflexible in solution. Atomic force microscopy (AFM) measurements show that short PLMA brushes exhibit a spherical morphology while the long brushes exhibit a worm-like structure. DSC displayed melting peaks at around ,30,C, indicating the alkyl side chains of the PLMA chains in the double-grafted cylindrical brushes are crystallizable. [source]

Exothermal Process in Miscible Polylactide/Poly(vinyl phenol) Blends: Mixing Enthalpy or Chemical Reaction?

Ester Zuza
Abstract Summary: In a previous paper (Macromolecules, 2005, 38, 9221), the enthalpy of mixing in poly(DL -lactide) /poly(vinyl phenol) blends was directly measured by DSC. The first DSC scan for solution/precipitation blends showed phase separation, but miscibility was observed in the second DSC scan. Hence, miscibility was achieved after thermal treatment, an unusual behavior in polymer blend current research. However, the exothermal event observed during the first heating scan could also be the result of a chemical reaction. In this work, a new research study conducted to elucidate the nature of the exothermal heat observed in PDLA/PVPh during the first DSC heating scan. Since the single-phase PDLLA/PVPh blend obtained after thermal treatment can be redissolved and reprecipitated, results obtained in the consecutive DSC scan prove that the process is completely reversible. Furthermore, GPC and 13C NMR results provide evidence that there is no change in the chemical structure of the studied polymers before and after the thermal treatment, which evidences the absence of transesterification reactions. Therefore, it can be concluded that the exothermal heat is the result of a mixing process, and miscibility is a consequence of specific interactions. New structure expected for transesterfication reactions. [source]

Elucidation of spermidine interaction with nucleotide ATP by multiple NMR techniques

Zhiyan Song
Abstract Interaction of polyamines with nucleotides plays a key role in many biological processes. Here we use multiple NMR techniques to characterize interaction of spermidine with adenosine 5,-triphosphate (ATP). Two-dimensional 1H- 15N spectra obtained from gs-HMBC experiments at varied pH show significant shift of N-1 peak around pH 2.0,7.0 range, suggesting that spermidine binds to N-1 site of ATP base. The binding facilitates N-1 deprotonation, shifting its pKa from 4.3 to 3.4. By correlating 15N and 31P chemical shift data, it is clear that spermidine is capable of concurrently binding to ATP base and phosphate sites around pH 4.0,7.0. The self-diffusion constants derived from 1H PFG-diffusion measurements provide evidence that binding of spermidine to ATP is in 1:1 ratio, and pH variations do not induce significant nucleotide self-association in our samples. 31P spectral analysis suggests that at neutral pH, Mg2+ ion competes with spermidine and shows stronger binding to ATP phosphates. From 31P kinetic measurements of myosin-catalyzed ATP hydrolysis, it is found that binding of spermidine affects the stability and reactivity of ATP. These NMR results are important for advancing the studies on nucleotide,polyamine interaction and its impact on nucleotide structures and activities under varied conditions. Copyright 2009 John Wiley & Sons, Ltd. [source]

Mechanical properties of Al2O3/polymethylmethacrylate nanocomposites

Benjamin J. Ash
Alumina/polymethylmethacrylate (PMMA) nanocomposites were produced by incorporating alumina nanoparticles, synthesized using the forced gas condensation method, into methylmethacrylate. The particles were dispersed using sonication and the composites were polymerized using free radical polymerization. At an optimum weight percent, the resulting nanocomposites showed, on average, a 600% increase in the strain-to-failure and the appearance of a well-defined yield point when tested in uniaxial tension. Concurrently, the glass transition temperature (Tg) of the nanocomposites dropped by as much as 25C, while the ultimate strength and the Young's modulus decreased by 20% and 15%, respectively. For comparison, composites containing micron size alumina particles were synthesized and displayed neither phenomenon. Solid-state deuterium NMR results showed enhanced chain mobility at room temperature in the nanocomposites and corroborate the observed Tg depression indicating considerable main chain motion at temperatures well below those observed in the neat polymer. A hypothesis is presented to relate the thermal and mechanical behavior observed in the composites to the higher chain mobility and Tg depression seen in recent ultrathin polymer film research. [source]