Dipolar Filter Research Articles

The solid-state proton NMR study of bone using a dipolar filter: apatite hydroxyl content versus animal age.

The hydroxyl content of bone apatite mineral has been measured using proton solid-state NMR performed with a multiple-pulse dipolar filter under slow magic angle spinning (MAS). This new method succeeded in resolving and relatively enhancing the main hydroxyl peak at ca. 0 ppm from whole bone, making it amenable to rigorous quantitative analysis. The proposed methodology, involving line fitting, the measurement of the apatite concentration in the studied material and adequate calibration, was proved to be convenient and suitable for monitoring bone mineral hydroxylation in different species and over the lifetime of the animal. It was found that the hydroxyl content in the cranial bone mineral of pig and rats remained in the 5–10% range, with reference to stoichiometric hydroxyapatite. In rats, the hydroxyl content showed a non-monotonic increase with age, which was governed by biological processes rather than by chemical, thermodynamically driven apatite maturation.

Dipolar filtered magic-sandwich-echoes as a tool for probing molecular motions using time domain NMR

We present a simple 1H NMR approach for characterizing intermediate to fast regime molecular motions using 1H time-domain NMR at low magnetic field. The method is based on a Goldmann Shen dipolar filter (DF) followed by a Mixed Magic Sandwich Echo (MSE). The dipolar filter suppresses the signals arising from molecular segments presenting sub kHz mobility, so only signals from mobile segments are detected. Thus, the temperature dependence of the signal intensities directly evidences the onset of molecular motions with rates higher than kHz. The DF-MSE signal intensity is described by an analytical function based on the Anderson Weiss theory, from where parameters related to the molecular motion (e.g. correlation times and activation energy) can be estimated when performing experiments as function of the temperature. Furthermore, we propose the use of the Tikhonov regularization for estimating the width of the distribution of correlation times.

Insights into asphaltene aggregate structure using ultrafast MAS solid-state 1H NMR spectroscopy

Solid-state 1H NMR spectroscopy under ultrafast magic angle spinning (60kHz) is used for the first time to investigate the solid phase aggregate structure of a petroleum asphaltene. The fast spinning speed, combined with high-magnetic field (16.4T, 700MHz 1H) spectral filtering techniques facilitate the observation of signals that generally remain obscured even in solution-state NMR techniques. Signals shielded by aromatic ring currents were observed in both the aromatic and aliphatic regions of the spectrum, indicating a stacked geometry. A homonuclear double quantum correlation experiment reveals molecular interactions, which suggest that asphaltene molecules aggregate through skewed π-π stacking. These nanoaggregates further cluster among themselves through alkyl-alkyl and alkyl-aromatic interactions. Based on the range of chemical shifts and molecular interactions, it is proposed that asphaltenes predominantly consist of large PAHs (∼7 or more). Transverse relaxation times (T2) and dipolar filter experiments reveal two domains in a solid asphaltene cluster: a rigid central core composed of stacked aggregates and a much smaller fraction of mobile peripheral alkyl sidechains. Additionally, the relaxation results are consistent with the presence of small fraction of small PAHs as substituents attached to large central PAHs.

Probing the two-stage transition upon crossing the glass transition of polystyrene by solid-state NMR

A two-stage transition upon crossing the glass transition of polystyrene with increasing temperature was precisely determined and interpreted by using solid-state nuclear magnetic resonance (SSNMR), 1H-1H dipolar couplings based double quantum-filtered (DQF) and dipolar filter (DF) experiments and 13C chemical shift anisotropy (CSA) based centerband-only detection of exchange (CODEX) experiment are used to fully characterize the time scale of molecular motions during the glass transition. While differential scanning calorimetry (DSC) and CODEX experiment predicted the first stage of glass transiton, DQF and DF experiments provided the evidence for the second stage transition during which the time scale of molecular motions changed from very slow (t > ms) to very fast (t < µs). The first stage of glass transition begins with the occurrence of remarkable slow re-orientation motions of the polymer backbone segments and ends when the degree of slow motion reaches maximum. The onset and endpoint of the conventional calorimetric glass transition of polystyrene can be quantitatively determined at the molecular level by SSNMR. In the second stage, a subsequent dramatic transition associated with the melting of the glassy components was observed. In this stage liquid-like NMR signals appeared and rapidly increased in intensity after a characteristic temperature T f (~1.1T g). The signals associated with the glassy components completely disappeared at another characteristic temperature T c (~1.2T g).

Dynamics-based selective 2D (1)H/(1)H chemical shift correlation spectroscopy under ultrafast MAS conditions.

Dynamics plays important roles in determining the physical, chemical, and functional properties of a variety of chemical and biological materials. However, a material (such as a polymer) generally has mobile and rigid regions in order to have high strength and toughness at the same time. Therefore, it is difficult to measure the role of mobile phase without being affected by the rigid components. Herein, we propose a highly sensitive solid-state NMR approach that utilizes a dipolar-coupling based filter (composed of 12 equally spaced 90° RF pulses) to selectively measure the correlation of (1)H chemical shifts from the mobile regions of a material. It is interesting to find that the rotor-synchronized dipolar filter strength decreases with increasing inter-pulse delay between the 90° pulses, whereas the dipolar filter strength increases with increasing inter-pulse delay under static conditions. In this study, we also demonstrate the unique advantages of proton-detection under ultrafast magic-angle-spinning conditions to enhance the spectral resolution and sensitivity for studies on small molecules as well as multi-phase polymers. Our results further demonstrate the use of finite-pulse radio-frequency driven recoupling pulse sequence to efficiently recouple weak proton-proton dipolar couplings in the dynamic regions of a molecule and to facilitate the fast acquisition of (1)H/(1)H correlation spectrum compared to the traditional 2D NOESY (Nuclear Overhauser effect spectroscopy) experiment. We believe that the proposed approach is beneficial to study mobile components in multi-phase systems, such as block copolymers, polymer blends, nanocomposites, heterogeneous amyloid mixture of oligomers and fibers, and other materials.

Solid-state Studies of the Crystalline/Amorphous Character in Linear Poly(ethylenimine hydrochloride) (PEI·HCl) Polymers and Their Copper Complexes

Linear poly(ethylenimine hydrochloride) (PEI·HCl) polymers with different molecular weights (22, 87, and 217 kDa) and their copper complexes (Cu–PEI) were studied with particular attention to the crystalline/amorphous character of the polymeric chains in lyophilized and crystallized samples by using 1D and 2D solid-state nuclear magnetic resonance, X-ray photoelectron spectroscopy, and X-ray diffraction techniques. Different 13C{1H} cross-polarization and magic angle spinning experiments using delayed contact time, 1H dipolar filter and 1H spin-lock filter were crucial to determine the different mobility domains in the crystallized and lyophilized samples of PEI·HCl and Cu–PEI. 13C direct polarization experiments allowed the quantification of the crystalline/amorphous segments in each sample. The lyophilization process produced an increase in the amorphous fraction, even when some ordered domains were present, whereas the coordination of copper/sulfate ions produced a significant decrease in the mobility of polymer chains, leading to homogeneous and ordered materials even when Cu was desorbed.

Thickness Dependence of Glass Transitions Measured by AC-Chip Calorimetry in Films with Controlled Interface

When most prior studies on thin polymer films have shown that glass transition temperature (Tg) decreases under nanoconfinement, the differential alternating current chip (ac-chip) calorimetric method shows little dependence of Tg on thickness for supported film. To reveal this contradiction, we have manipulated a controlled interface by spin-coating polystyrene (PS) with immiscible surfactants such as tetraoctylammonium bromide or citric acid. Since the immiscible surfactants did not show plasticizing effect for PS, there was no observable reduction of Tg from the bulk value ether in powdered blends or in thick films. However, the ultrathin film with thickness h ∼ 25 nm, consisting of 95 wt % PS and 5 wt % surfactants, showed a reduction of Tg by 6–7 °C, as compared to thick film with the same composition. We propose that the surfactant molecules assembled on the interface between thin film and substrate due to phase separation. The molecular mobility of molecules at the interface was dramatically increased, which was detected by 1NMR with dipolar filter sequence. It appeared that the deviation range was not so large as that measured by other methods. But considering that we were measuring Tg at a high frequence (10 Hz), this amount of deviation was quite significant for ac-chip calorimetry. As a result, ac-chip calorimetry measured Tg data unambiguously demonstrate that thickness dependence of Tg is a real property of confined thin film.

Tracking the interdiffusion of polymers at a molecular level by1H dipolar filter solid-state NMR under fast magic angle spinning

In this paper, we characterized the interdiffusion of deuterated polystyrene (PS-D) and hydrogenated poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) by 1H dipolar filter solid-state NMR under fast magic angle spinning (MAS). It is observed that the interdiffusion process of PS-D/PPO is composed of two stages, the wetting stage and the diffusion stage. The characteristic time of the transition from the wetting to diffusion stage is independent of the temperature and PS-D molecular weight. On the other hand, the increment of dipolar filtered 1H NMR signal intensity at the wetting stage depends strongly on the temperature. The PS-D chains can be easily mixed with the PPO chains intimately when the temperature is above the glass transition temperature (Tg) of PPO. However, when the temperature is below the Tg of PPO, the PPO chains are frozen and it is more difficult for the PS-D chains to approach the PPO at a molecular level.

The heterogeneity of segmental dynamics of filled EPDM by 1H transverse relaxation NMR

Residual second moment of dipolar interactions M∼2 and correlation time segmental dynamics distributions were measured by Hahn-echo decays in combination with inverse Laplace transform for a series of unfilled and filled EPDM samples as functions of carbon-black N683 filler content. The fillers–polymer chain interactions which dramatically restrict the mobility of bound rubber modify the dynamics of mobile chains. These changes depend on the filler content and can be evaluated from distributions of M∼2. A dipolar filter was applied to eliminate the contribution of bound rubber. In the first approach the Hahn-echo decays were fitted with a theoretical relationship to obtain the average values of the 1H residual second moment 〈M∼2〉 and correlation time 〈τc〉. For the mobile EPDM segments the power-law distribution of correlation function was compared to the exponential correlation function and found inadequate in the long-time regime. In the second approach a log-Gauss distribution for the correlation time was assumed. Furthermore, using an averaged value of the correlation time, the distributions of the residual second moment were determined using an inverse Laplace transform for the entire series of measured samples. The unfilled EPDM sample shows a bimodal distribution of residual second moments, which can be associated to the mobile polymer sub-chains (M∼2≅6.1rad2s-2) and the second one associated to the dangling chains M∼2≅5.4rad2s-2). By restraining the mobility of bound rubber, the carbon-black fillers induce diversity in the segmental dynamics like the apparition of a distinct mobile component and changes in the distribution of mobile and free-end polymer segments.

Morphology and side-chain dynamics in hydrated hard α-keratin fibres by 1H solid-state NMR

The effect of hydration on phase composition, aminoacids side-chain dynamics, and domain thickness of hard α-keratin was investigated by 1H solid-state NMR. Decomposition of wide-line 1H NMR spectra was used to determine the phase composition and to obtain information on molecular motion. Proton spin-diffusion NMR experiments using a double-quantum dipolar filter were used to estimate the rigid domain sizes for the hydrated Caucasian hair fibres. The relative domain sizes were obtained from the solution of spin-diffusion equation for cylindrical morphologies in the initial-rate approximation by a novel approach. A qualitative model describing the morphological and molecular dynamics changes induced by hydration was developed.

Single and Multichain Association of Polystyrene in Solvents with Various Molecular Sizes

The association and aggregation of polymers in the semidilute solution regime during cooling from above to below the Θ temperature were investigated by the nonradiative energy transfer (NET) and rheological method. The molecular size of the solvent was found to affect the association and aggregation behaviors of the chains. When the normal small-sized solvent, for example decalin or cyclohexane is used, the polystyrene chains first associate among each other, and form aggregates of chains with strong interchain interpenetration at the end of cooling process. However, when the solvent with larger molecular size (also called middle-sized solvent), for example dioctyl phthalate (DOP) is used, polystyrene chains contract separately and form aggregates of collapsed globules at the end of cooling process. The interpenetration between chains for the polystyrene solid sample freeze-dried from different concentrations was detected by a new strategy using dipolar filter 1H solid-state NMR under fast magic angle spinning. We found that when the small-sized solvent was used, the extent of interpenetration in the polystyrene glasses basically kept constant with the concentration of the original solution and it increased suddenly near the critical overlapping concentration ([η]c ∼ 4). However, in the case of middle-sized solvent, even up to [η]c = 10, the extent of the interpenetration kept in a very low level comparable to that from extremely dilute ([η]c ∼ 0.01) small-sized solvent solution. Such results help us to understand the behaviors of fast crystallization and relaxation dynamics of serials of polymer samples freeze-dried from middle-sized solvent solutions discovered in this laboratory.

Investigation of Chain Dynamics in Poly(n‐alkyl methacrylate)s by Solid‐State NMR: Comparison with Poly(n‐alkyl acrylate)s

AbstractPnAMAs exhibit a local nanophase separation associated with intriguing chain dynamics (Macromol. Chem. Phys. 2005, 206, 142). PnAMAs of high molar mass, as determined by SEC and MHKS parameters, were investigated in the melt with a recently‐developed solid‐state NMR method (NOE with dipolar filter; Solid State Nucl. Magn. Res. 2005, 28, 160). The correlation times are assigned to the relaxation of the alkyl nanodomains, as coupled motions of the main chain and hindered local modes in the side chain. Comparison with poly(n‐alkyl acrylates) shows a higher anisotropy of the main chain motions and a better organized local nanophase separation in PnAMAs.magnified image

Characterization of Polymer Compatibility by 1H Dipolar Filter Solid-State NMR under Fast Magic Angle Spinning

Polymer compatibility was investigated by a new strategy reported in our recent communication combining preparation of an isotopically enriched polymer blend with analysis by dipolar filter 1H soli...

Chain Dynamics in Poly(n-alkyl acrylates) by Solid-State NMR, Dielectric, and Mechanical Spectroscopies

A relaxation process was detected and quantified for the first time in various poly(n-alkyl acrylates) in the melt by a solid-state NMR method, NOE with dipolar filter. By comparison with dielectric spectroscopy and dynamic mechanical measurements carried out on the same samples, the relaxation process detected by NMR occurs at a higher temperature than the simple alpha-relaxation and on a longer time scale than the alpha beta-relaxation (cooperative alpha-relaxation). This relaxation process could be the isotropization of the main chain (by analogy with structurally similar polymethacrylates), but it is more probably a local relaxation within the alkyl side chain that is usually detected only at significantly lower temperatures. The fact that this side-chain motion is slower than the cooperative main-chain alpha-relaxation of restricted amplitude can be rationalized in the context of restricted amplitude main-chain motions in the local nanostructure present in the samples, as NMR and dielectric spectroscopy detect motions in different frames.

Dipolar double-quantum filtered rotational-echo double resonance

The homonuclear dipolar coupling of a directly bonded (13)C-(13)C pair has been used to create a dipolar double-quantum filter (D-DQF) to remove the natural-abundance (13)C background in (13)C[(2)H] rotational-echo double-resonance (REDOR) experiments. The most efficient version of this experiment has the D-DQF excitation and reconversion preceding the REDOR evolution period. Calculated and observed (13)C[(2)H]D-DQF-REDOR dephasings were in agreement for a test sample of mixed recrystallized labeled alanines.

The effect of temperature and annealing on the phase composition, molecular mobility and the thickness of domains in high-density polyethylene

Molecular mobility, the thickness of domains and the amount of rigid, semi-rigid, and soft fractions of high-density polyethylene (HDPE) were characterized as a function of temperature and annealing time using time- and frequency-domains proton solid-state NMR. These experiments established the temperature range for which the largest differences are observed in molecular mobility in crystalline phase, semi-rigid crystal–amorphous interface, and soft fraction of the amorphous phase allowing accurate determination of the phase composition and the thickness of these domains. The domain thickness, which was determined by NMR, is in good agreement to those measured by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) on the same sample. Changes in molecular mobility in the amorphous phase upon increasing temperature and annealing are discussed. It is shown that annealing is accompanied by structural reorganizations in the amorphous layer adjacent to the lamella surface causing a continuous shift of the interface towards the inner part of the amorphous regions and thus reducing the thickness of the amorphous layer. A recently introduced method was used for measuring the thickness of domains by a spin-diffusion NMR experiment with a double-quantum dipolar filter. The temperature dependence of the spin diffusivities is reported for the three phases of HDPE. For the first time results of spin-diffusion experiment performed by time-domain low-field NMR and frequency-domain high-field NMR are compared.

Segmental dynamic heterogeneity of short-chain grafted-poly(dimethylsiloxane) by 1H spin-diffusion NMR

Segmental dynamic heterogeneity of short-chain grafted poly(dimethylsiloxane) (PDMS) on pyrogenic silica was investigated using 1H NMR spin-diffusion. A double-quantum dipolar filter was employed for selection of the interface (rigid) region. One-dimensional spin-diffusion equations were solved numerically for a space distribution of spin diffusivity D( x) of the mobile PDMS chains. The degree of heterogeneity can be quantified by the parameters of Gaussian and exponential diffusivity distribution functions which yield similar diffusivities. The rigid and mobile domain sizes and spin diffusivities were correlated with the PDMS chain length, the temperature, and 1H residual dipolar couplings.

Morphology of Thermoplastic Polyurethanes by1H Spin-Diffusion NMR

The morphology and domain sizes are reported for a series of thermoplastic polyurethane (TPU) samples with different content in hard segments and the same molecular weight of the soft segments. NMR spin-diffusion experiments were employed with a 1H double-quantum dipolar filter to establish the dominant dimensionality of the spin-diffusion process, which is shown to take place in two and three dimensions for investigated samples. The correlation between mesoscopic and microscopic properties of the TPU samples is discussed. To this purpose the effective volume of the hard domains is correlated with the TPU content of the hard segments and the segmental orientation of the hard segments obtained from the 1H residual dipolar couplings. A semiquantitative model is developed to explain the functional dependence of the residual second van Vleck moment on the effective volume of the hard segments.

Fluorine-19 solid state NMR study of vinylidenefluoride polymers using selective relaxation filters

Two fluoropolymers, poly(vinylidenefluoride) (PVDF) and a vinylidenefluoride telomer (VDFT), with molecular weights of 1×10 6 and 2×10 3 Da by GPC, respectively, have been analysed by 19F solid-state nuclear magnetic resonance (NMR) spectroscopy. Relaxation-filtered proton-decoupled magic-angle spinning (MAS) experiments, namely T 1 ρ filter, dipolar filter (DF), direct-polarisation delayed acquisition (DPDA) and discrimination induced by variable-amplitude minipulses (DIVAM), allowed signals in the direct polarisation (DP) spectra of PVDF and the VDFT to be discussed in terms of rigid and mobile domains. Both samples showed signals, which were multi-componential, but they differ in the nature of the crystalline form present. Thus, the Vinylidenefluoride (VDF) telomer exhibited a crystalline component corresponding to β PVDF, whereas the PVDF contained crystallites of the α form. Signals relating to end groups and reverse units, plus an anomalous signal displaying long-time transverse relaxation in the DPDA spectrum, were found for both polymers, though they showed diversity in chemical shift and content. Signals related to reverse units and/or end groups were seen between approximately −115 and approximately −117 ppm for both samples. High-speed MAS at higher magnetic field resulted in an increase in resolution so that signals previously attributed to single-phase characteristics are shown to indicate the possibility of several different mobilities. The results are debated with respect to molecular weight and relaxation parameters.

Multiple-quantum NMR on structure, orientation, morphology and dynamics of polymers, biomolecules and ordered tissues

A survey of recent homo- (1H) and heteronuclear (13C–1H) multiple-quantum (MQ) NMR experiments performed in our group at RWTH-Aachen on soft solids is given. MQ build-up curves are used for measurement of residual van Vleck moments and as dipolar filters in spin-diffusion experiments. The orientation studies were made with the help of MQ edited spectra that show Pake doublets. Proton homonuclear as well as 13C–1H heteronuclear residual dipolar couplings were measured site-selectively and for the overall polymer chains and correlated with the cross-link density and network deformation in elastomers and thermoplastic elastomers. The heterogeneity of the segmental dynamics of poly(dimethylsiloxane) chains grafted onto hydrophilic silica was studied by bimodal double- (DQ) and triple-quantum (TQ) build-up curves. The complex morphology of Nylon-6 fibers was characterized with the help of spin-diffusion experiments using a new dipolar filter based on the excitation of DQ coherences. The same procedure was employed for the investigation of the distribution of the residual dipolar couplings of lecithin molecules grafted on the walls of submicrone cylindrical pores. The distribution of the orientation of collagen fibrils in Achilles tendons was estimated from 1H DQ filtered spectra. To cite this article: A. Adams Buda et al., C. R. Chimie 9 (2006). Nous présentons ici les expériences récentes en RMN multi-quanta (MQ) homo- (1H) et hétéronucléaire (13C–1H) réalisées sur des solides mous dans notre groupe à l’université d’Aix-la-Chapelle. Des courbes de montée MQ ont été utilisées pour mesurer les moments résiduels de van Vleck et comme filtres dipolaires dans les expériences de diffusion de spin. L’édition des spectres MQ montrant des doublets Pake a également permis d’étudier les systèmes orientés. Les couplages dipolaires résiduels homo 1H–1H et hétéronucléaires 13C–1H ont été mesurés sélectivement sur certains sites ainsi que pour l’ensemble des chaînes polymères et corrélés avec la densité de réticulation et la déformation du réseau dans les élastomères et les polymères thermoplastiques. L’hétérogénéité dynamique des segments dans les poly(diméthylsiloxanes) greffées sur la silice hydrophile a été visualisée au travers d’une distribution bimodale des courbes de montée à deux et à trois quanta. La morphologie complexe de fibres de nylon-6 a été caractérisée à l’aide d’expériences de diffusion de spin utilisant un nouveau filtre dipolaire basé sur l’excitation de cohérences à deux quanta. La même procédure a été utilisée pour étudier la distribution des couplages dipolaires résiduels de molécules de lécithine greffées sur les parois de pores cylindriques. La distribution de l’orientation des fibres de collagène dans les tendons d’Achille a été estimée à partir de spectres filtrés à deux quanta. Pour citer cet article : A. Adams Buda et al., C. R. Chimie 9 (2006).