1H T1ρ Research Articles

Sensitivity-enhanced static 15N NMR of solids by 1h indirect detection.

A method for enhancing the sensitivity of 15N spectra of nonspinning solids through 1H indirect detection is introduced. By sampling the 1H signals in the windows of a pulsed spin-lock sequence, high-sensitivity 1H spectra can be obtained in two-dimensional (2D) spectra whose indirect dimension yields the 15N chemical shift pattern. By sacrificing the 1H chemical shift information, sensitivity gains of 1.8 to 2.5 for the 15N spectra were achieved experimentally. A similar sensitivity enhancement was also obtained for 2D (15)N-(1)H dipolar and 15N chemical shift correlation spectroscopy, by means of a 3D 1H/15N-1H/15N correlation experiment. We demonstrate this technique, termed PRINS for proton indirectly detected nitrogen static NMR, on a crystalline model compound with long 1H T(1rho) and on a 25-kDa protein with short 1H T(1rho). This 1H indirect detection approach should be useful for enhancing the sensitivity of 15N NMR of oriented membrane peptides. It can also be used to facilitate the empirical optimization of 15N-detected experiments where the inherent sensitivity of the sample is low.

Phase Behavior and Segmental Mobility in Binary Blends of Polystyrene and Poly(vinyl methyl ether)

The phase behavior and segmental mobility in binary blends of polystyrene (PS) and poly(vinyl methyl ether) (PVME) were investigated. Two nearly monodisperse PSs having weight-average molecular weights (Mw) of 57 000 and 95 000 with polydispersity indexes (PDI) of 1.06 and 1.09, respectively, and PVME having Mw = 99 000 with a PDI of 2.13 were used for this study. Two sets of PS/PVME binary blends with varying compositions were prepared by solvent casting from toluene. Thermograms from differential scanning calorimetry showed that each blend has a single, yet very broad glass transition temperature. Cloud point measurements via He−Ne laser light scattering were taken to determine phase equilibria in each blend, which exhibited lower critical solution temperature (LCST) behavior. Solid-state nuclear magnetic resonance (NMR) spectroscopy was used to examine segmental mobility and component domain sizes. 1H T1ρ experiments were run at temperatures ranging from −40 to 140 °C. We observed only small differences in 1H T1ρ values of PS and PVME at temperatures below 45−80 °C (depending on blend composition) and a large divergence of 1H T1ρ values at higher temperatures. 13C T1ρ and wide-line separation (WISE) experiments were run at room temperature on untreated and heat-treated samples. WISE experiments revealed that heterogeneities from 3.5 nm to greater than 30 nm existed within the blends, depending on the temperature of heat treatment. Since it has been found that 1H T1ρ measurements can give ambiguous domain information, 1H-NOESY NMR was used to examine several blend compositions at 100 °C. We conclude from this study that nanoheterogeneities exist in these PS/PVME blends at temperatures below the binodal curve determined by cloud point measurements and that a broad, single glass transition should not be construed as evidence of miscibility at the molecular level. It has been shown that nanoheterogeneities exist on a segmental level and that there are large changes in mobility at temperatures above 45−80 °C. However, the blend does not phase separate until the critical temperature (LCST), determined by cloud point measurements, is reached.

The effects of molecular orientation on the physical aging and mobility of polycarbonate?solid state NMR and dynamic mechanical analysis

Solid-state NMR and dynamic mechanical (DMA) measurements were performed on a series of uniaxially hot-drawn bisphenol-A polycarbonate samples in order to determine the effects of stretching on the structure, mobility, and local orientation environment. Proton spin-lattice relaxation times, 1H T1ρ, for the phenylene carbon protons were fitted to a biexponential decay function, and both the long and short relaxation times initially increased with stretching. Intensity data indicated an increase in the number of short relaxation time protons and a decrease in the number of long relaxation protons with orientation. Similarly, DMA spectra showed that the β-relaxation strength also increased with drawing, which implied an increase in the number of localized segmental relaxations. It is theorized that the long and short 1H T1ρ relate to protons within tightly packed “cooperative domains,” and to those with greater localized free-volume, respectively. Stretching is known to distort the free-volume distribution, causing a decrease in the mean free-volume but an increase in the number of larger, more elliptical holes. This is expected to cause a decrease in the α-transition mobility (due to larger cooperative domains) and an increase in the β-mobility (due to the increase in the number of β-relaxing segments associated with the larger free-volume holes). These predictions are consistent with results recently reported by Shelby and Wilkes on the physical aging and creep behavior of these samples (M. D. Shelby & G. L. Wilkes, Polymer 1998, 39, 6767; M. D. Shelby & G. L. Wilkes, J Polym Sci Part B: Polym Phys 1998, 36, 2111). © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 32–46, 2001

Direct Detection of Miscibility in Saturated Polymer Blends

We have used variable-temperature solid-state NMR methods to determine that amorphous polypropylene (a-PP) is miscible with blends of poly(ethylene)ethylethylene (PEE) and poly(ethylene-co-octadecene) (PE-OD). Rothwell−Waugh line-broadening effects and selective 1H T1ρ measurements indicate that the frequency of reorientational chain dynamics increases for a-PP upon blending with PEE and PE-OD but remains unchanged for blends with poly(ethylene-co-dodecene) (PE-DD). We infer partial miscibility on the basis of converged but incompletely averaged relaxation curves for the a-PP/PEE and a-PP/PE-OD blends, while the a-PP/PE-DD blends were found to be immiscible. Dipolar filter experiments in which spin-diffusion was used to monitor polarization transfer provided direct evidence for miscibility in the a-PP/PE-OD blends and immiscibility in the a-PP/PE-DD blends. From these data, domain sizes were calculated to be 5.1 nm (i.e., on the order of chain dimensions) for the miscible regions of the PP/PE-OD blend.

Erosion of a 1,3-Bis(p-carboxyphenoxy)propane−Sebacic Acid Poly(anhydride) Copolymer by Water Vapor Studied by 1H and 13C Solid-State NMR Spectroscopy

The erosion of a 1,3-bis(p-carboxyphenoxy)propane−sebacic acid poly(anhydride) copolymer, p(CPP:SA), by water vapor has been studied by 1H wide-line and 13C MAS solid-state NMR spectroscopy. In contrast to erosion by phosphate buffer, no mobile absorbed water was detected. 1H T1, T1ρ, and line width data indicated that the polymer matrix became more rigid on erosion due to the formation of crystalline degradation products. From the 13C spectra, the hydrolysis of the anhydride links into SA and CPP carboxylic acid groups was determined quantitatively. The rate of hydrolysis by water vapor was comparable to that by phosphate buffer. The SA−SA and SA−CPP links were hydrolyzed at comparable rates, and both were hydrolyzed more rapidly than CPP−CPP links.

Rotator Phase in Dodecylammonium Chloride Studied by 1H and 2H Solid NMR and Thermal Measurements

Abstract 1H NMR spin-lattice relaxation times, T1 and T1ρ, the second moment M2 of 1H NMR linewidth, 2H NMR spectra, powder X-ray diffraction, electrical conductivity, and thermal measurements have been performed on C12H25NH3Cl and C12H25ND3Cl in the range between room temperature and the melting point. From these measurements, the presence of the rotator phase was revealed above ca. 345 K, where a highly disordered structure characterized by the rapid 2-D self-diffusion as well as the axial rotation of the cations taking place in the lamellar-type double layer structure is formed. These molecular motions, the crystal structure and the thermal nature found in this phase indicate that the rotator phase is close to the plastic crystal and can be described as “a low-dimensional plastic phase”. The determined dynamic parameters of molecular motions in this phase were compared with those in the values reported in the rotator phases of short alkyl chain analogues.

Phase inversion membranes with an organized surface structure from mixtures of polysulfone and polysulfone?poly(ethylene oxide) block copolymers

Phase inversion precipitation of a polysulfone and polysulfone—poly(ethylene oxide) block copolymer solution yields a membrane with an organized surface structure. The poly(ethylene oxide) block of the polysulfone—poly(ethylene oxide) block copolymer segregates to the surface of the membrane. Measurement of the 1H T1ρ of the component materials, X-ray photoelectron spectroscopy, and differential scanning calorimetry reveal the organized surface structure of the membrane. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1353–1358, 1997

Phase inversion membranes with an organized surface structure from mixtures of polysulfone and polysulfone—poly(ethylene oxide) block copolymers

Phase inversion precipitation of a polysulfone and polysulfone—poly(ethylene oxide) block copolymer solution yields a membrane with an organized surface structure. The poly(ethylene oxide) block of the polysulfone—poly(ethylene oxide) block copolymer segregates to the surface of the membrane. Measurement of the 1H T1ρ of the component materials, X-ray photoelectron spectroscopy, and differential scanning calorimetry reveal the organized surface structure of the membrane. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1353–1358, 1997

Cross‐polarization for quadrupolar nuclei—proton to molybdenum‐95

AbstractHigh‐field, 95Mo, solid‐state nuclear magnetic resonance spectra of (NH4)6Mo7O24 · 4H2O and (Bu4N)2Mo2O7 were obtained using solid‐echo techniques together with 1H cross‐polarization and dipolar decoupling. The results show that reliable second‐order powder line shapes can be obtained with excellent cross‐polarization enhancements of 66–86% of maximum theoretical enhancement. The lower than maximum enhancements are due to the relatively long TIS values, which are similar in magnitude to the TH1ρ values. Experiments were performed which reveal the dynamics involved in the cross‐polarization process, including measurement of the cross‐relaxation time constant and the 1H and 95Mo rotating‐frame spin‐lattice relaxation times. It was found that the principal relaxation process causing low gain in the observed signal was the 1H T1ρ, which was found to be on the order of 23–24 ms. The experimental results obtained indicate that selective cross‐polarization dynamics occur in molybdenum cluster species with inequivalent molybdenum sites. The 95Mo rotating‐frame spin‐lattice relaxation rate is long in comparison with the other relaxation processes, making molybdenum a good cross‐polarization candidate but precluding the use of multiple contact cross‐polarization.

1H NMR Studies on Cationic Dynamics in Solid Methylguanidinium, 1,1‐Dimethylguanidinium, 1,2,3‐Trimethylguanidinium, and 1,1,3,3‐Tetramethylguanidinium Chlorides

AbstractThe temperature dependence of 1H NMR second moments and spin‐lattice relaxation times, T1 and T1ϱ in the laboratory and rotating frames, respectively, were observed for solid methylguanidinium (MeguH+), 1,1‐dimethylguanidinium (1,1‐Me2guH+), 1,2,3‐trimethylguanidinium (1,2,3‐Me3guH+), and 1,1,3,3‐tetramethylguanidinium (1,1,3,3‐Me4guH+) chlorides. Phase transitions were found for (1,1‐Me2guH)Cl and (1,2,3‐Me3guH)Cl at 400 and 409 K, respectively, which were confirmed by differential thermal analysis experiments. Below room temperature, rapid C3 reorientational motions of CH3 groups were detected for the all cations in the above crystals. The activation energy for the motion of each CH3 group in the cations was evaluated. Above room temperature, the temperature dependences of T1ϱ were determined against T−1 for (1,1‐Me2guH)Cl, (1,1,3,3‐Me4guH)Cl, and their partially deuterated (NH2 → ND2) analogs. For these salts, 1H T1ρ minima were observed and could be interpreted as arising from the 180° flip motions of the NMe2 group and also of the whole cation for (1,1‐Me2guH)Cl and its deuterated analog about the C+—NMe2 axis of the cations, and the whole cationic 180° flip motion about the C+—NH2 or C+—ND2 bond axis of the cation for (1,1,3,3‐Me4guH)Cl or its deuterated analog, respectively, in addition to the NMe2 180° flip motions.

Self-Diffusion, Overall Rotation of Molecules, and Ring Motions in the Three Solid Phases of Formylferrocene, Fe(C5H5) (C5H4CHO), as Studied by 1H NMR and Differential Thermal Analysis

The temperature dependences of static and rotating frame 1H spin-lattice relaxation times, T1 and T1ϱ, respectively, and also of 1H spin-spin relaxation time, T*2 were studied for formylferrocene Fe(C5H5)(C5H4CHO). Broadline 1H NMR was investigated over a wide range of temperature. Differential thermal analysis revealed a new second-order phase transition at 294 K. At 77 K, formylferrocene was found to form a rigid lattice. Above 130 K, the onset of the Cs reorientation of the cyclopentadienyl ring in the molecule about its C5 axis was observed to take place with the average activation energy of 14.5 kJ mol−1. In the plastic phase above 317 K, the uniaxial as well as overall rotations and self-diffusion of the molecule as a whole were detected to occur. Other motional parameters of the unsubstituted cyclopentadienyl rings and the molecules in each phase were evaluated.