Laboratory Frame Spin-lattice Relaxation Research Articles

Conformation transition and molecular mobility of isolated poly(ethylene oxide) chains confined in urea nanochannels

Inclusion compounds formed from host small molecules and guest polymers have provided a novel platform to study the behavior of isolated polymer chains confined in nanochannels. In this article, the PEO chain conformation in the metastable poly(ethylene oxide) (PEO)–urea inclusion compound (IC) and its transition was characterized via a combination of different analytical methods. Based on the FTIR and Raman spectroscopy results, PEO chains in the metastable tetragonal IC are tentatively assigned to the tgg′ conformation. The structural changes of the metastable tetragonal IC to the stable trigonal form were observed via in situ FTIR and ex situ WAXD. The transformation is a kinetic solid–solid process and can even occur at room temperature. The activation energy of about 222kJ/mol indicates that the transition occurred via cooperative disruption of several hydrogen bonds. Measurement of the laboratory frame spin-lattice relaxation time T1 (13C) shows that molecular motions of the nanoconfined PEO chains are more intensive than the neat crystalline PEO but weaker than those of the neat amorphous PEO. Second harmonic generation microscopy demonstrates that the trigonal IC exhibits stronger nonlinear optical activity than the tetragonal IC. The intermolecular hydrogen bonding is attributed to the driving force for the transformation of the metastable tetragonal IC into the stable trigonal form.

Distinct critical fluctuations and molecular motions manifest in a model biomembrane

Lattice dynamics in bis-( n-C 16H 37NH 3) 2SnCl 6, where the hydrocarbon part is analogous to lipid membrane, was investigated by means of 200 MHz 1H nuclear magnetic resonance. As a result, critical fluctuations and molecular dynamics associated with the phase transitions, an order–disorder and a conformational phase transition, were distinguished in a wide temperature range. The dynamical origin of the critical fluctuations, observed in the long-chain compounds but not in the short-chain compounds, by the laboratory frame spin–lattice relaxation measurements, is revealed and discussed in this work.

NMR evidence for change in the local structure ofZrCr2Hx

We report here measurements of ${T}_{1\ensuremath{\rho}}^{\ensuremath{-}1},$ the spin-lattice relaxation rate in the rotating frame, for protons from 4 to 300 K in two samples of ${\mathrm{ZrCr}}_{2}{\mathrm{H}}_{x},$ with $x=0.2$ and 0.5. We find, especially for $x=0.2,$ that the maximum rate ${(T}_{1\ensuremath{\rho}}^{\ensuremath{-}1}{)}_{\mathrm{MAX}}$ is substantially larger than expected from the laboratory frame spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ by comparison with a model that successfully fits the ${T}_{1}^{\ensuremath{-}1}$ data. Further, a dimensionless parameter R is formed that expresses the relative strength of ${(T}_{1}^{\ensuremath{-}1}{)}_{\mathrm{MAX}}$ after correction for the different measurement frequencies. The measured values of R for $x=0.2$ are larger by a factor of 2 or more than that of any simple model, indicating that the dipolar second moment ${M}_{2}$ (primarily proton-proton) increases as temperature decreases, a surprising result. The increasing second moment is confirmed by direct measurement from dipolar echoes (magic echoes) at low temperature. The increased second moment reflects either decreasing average H-H distances below 150 K (partial clustering, a change in the local structure) or the progressive freeze-out of motional averaging of ${M}_{2}$ by high-frequency local motions.

Temperature Dependence of Segmental Mobility in Poly(ethylene oxide) Complexed with Ba(ClO4)2: a Carbon-13 NMR Study

The local polymer chain mobility of low molecular weight poly(ethylene oxide) (PEO) complexed with Ba(ClO 4 ) 2 has been investigated with 13 C NMR techniques over the temperature range 297-463 K. The carbon line width and the rotating frame spin-lattice relaxation rate provide information of segmental motion in the mid-kilohertz region, as shown by their temperature dependencies'with clear minima at ∼327 K. The laboratory frame spin-lattice relaxation rate minimum is observed at ∼378 K, consistent with the progressive changes in correlation times of motion above the glass transition temperature. When comparing the present relaxation data for PEO-Ba(ClO 4 ) 2 with those from the literature for PEO, we find that salt complexation dramatically slows down the segmental motions in the amorphous phase

13C CP-MAS NMR study of compatibility in blends of poly(3-octyl-thiophene) and poly(phenylene oxide)

Blends of undoped poly(3-octyl thiophene) and poly(phenylene oxide) have been investigated using solid-state 13C cross-polarization magic-angle spinning (CP-MAS) NMR. Proton spin-lattice relaxation times in the rotating frame (T 1ϱ(H)) are unchanged in the blends as compared with the homopolymers. This shows that the blends are incompatible on a molecular level. The averaging of laboratory frame spin-lattice relaxation times (T 1(H)) indicated that the maximum domain size is approximately 100 nm for POT-concentrations below 30%.

An investigation of the structures and molecular dynamics of natural waxes. III. Montan wax

For pt.II see ibid., vol.21, p.1429 (1988). A detailed NMR investigation, supported by DSC and X-ray diffraction measurements, of montan wax is reported. The solid-state high-resolution 13C spectrum showed that it has a very high number of carbon atoms occurring in methyl and methylene groups and a relatively low ester, acid and carbonyl content. Laboratory frame spin-lattice relaxation results showed that the methyl groups have a high degree of mobility and that branching of the chains is of minor importance. A reliable value of 34.6+or-2.2 kJ mol-1 was obtained for the activation energy of the reorientational-translational motion of the chains. The temperature at which T2, the spin-spin lattice relaxation time, starts to increase with increasing temperature yields tau 0=(8.7+or-0.6)*10-14s for this motion. The average chain length of montan wax is 31 carbon atoms. The chain-end motion as revealed by spin-lattice relaxation measurements in the rotating frame is less hindered in montan wax than in the other natural waxes which have been investigated.

An NMR study of the molecular mobility in BeSO 4·4H 2O

Molecular and ionic mobility in the solid hydrate BeSO 4·H 2O has been studied by 1H-NMR spectroscopy. Spin-lattice relaxation rates in the laboratory and rotating frames, T −1 1 and T −1 1π, have been measured as a function of temperature for polycrystalline samples, and as a function of orientation for a single crystal. One dynamical process has been clearly identified as being responsible for the rotating frame relaxation: an H 2O flip motion with an activation energy of 58 kJ mol −1. The pre-exponential factor of the Arrhenius relation for the correlation time of this motion is 4.3 × 10 −16 s. A second process, responsible for the laboratory frame spin-lattice relaxation is observed, but cannot be identified unambiguously.

High‐power 13C and 1H nuclear magnetic resonance in dry gluten

AbstractHigh‐resolution, solid‐state 13C NMR spectra of wheat gluten obtained under a variety of acquisition conditions are reported. These spectra are compared with the results from a 1H relaxation study of dry gluten in which the transverse relaxation times, the spin–lattice relaxation times in the rotating frame and the spin–lattice relaxation times in the laboratory frame at three different Larmor frequencies are characterized. It is concluded that 13C experiments, the proton transverse relaxation measurements and the rotating frame results distinguish between protein and lipid motional environments, whilst the laboratory frame proton spin–lattice relaxation results detect two separate protein environments. The relationship between the laboratory frame spin–lattice relaxation time constants and Larmor frequency is consistent with a model of macromolecular motion in which the polymer chains are held between cross‐link points. Observed changes in the various proton relaxation times on removal of lipid or disruption of disulphide cross‐links are interpreted in terms of the frequency and anisotropy of molecular motions in the gluten system.

1H nuclear magnetic resonance and spin–lattice relaxation in solid, high-density polyethylene

1 H spin–lattice relaxation, in both laboratory and on-resonance rotating frames, are reported for five samples of a high-density, linear polyethylene prepared so as to produce a range of lamellar crystalline widths. The samples are also characterised by measurements of density, Raman longitudinal acoustic modes (LAM) crystallinity (DSC) and small-angle X-ray scattering (SAXS). The laboratory-frame spin–lattice relaxation (LFSLR) is well represented by a single process, and the relaxation rates are linearly related to the crystalline fraction (DSC), which is consistent with the prediction of the fast-diffusion limit of the two-region, spin-diffusion-coupled model of relaxation in these systems. The rotating-frame relaxation (RFSLR) is, as found in previous studies, a multi-exponential process. It is shown that a fit to four components is optimum in terms of quality of fit and signal-to-noise ratio and that this leads to accurate values for the amplitude and time constant of the dominant, longest relaxation-time component. It is also shown that the use of a solid-echo refocussing pulse, following the spin-locking substantially increases the total signal observed but has a much smaller effect on the amplitudes and time constants. The values of the relaxation times and amplitudes for the two longest relaxation time components for each sample are compared with the predictions of the slow-diffusion limit of the spin-diffusion model. The longest relaxation-time component is found to be proportional to the square of the lamellar thickness determined from Raman LAM, and this is used to deduce a value for the spin-diffusion coefficient in the crystalline region, D2≈ 2 × 1016 m2 s–1. Assuming the slow-diffusion limit, the amplitudes of the long-time components in the RFSLR are used to derive values for the size of the disordered region, l1, using both the Raman and NMR derived values of l2, the size of the crystalline region. The l1 and l2 values obtained from the RFSLR are used to derive densities for regions 1 and 2 from the measured sample densities. The values of (l1+l2) from NMR are compared with those obtained from SAXS, and good agreement is found. The validity of the spin-diffusion model is further supported by the data reported here. The relaxation processes (both LF and RF) seem to imply a higher degree of ‘crystallinity’ than revealed by DSC or X-ray diffraction. The LFSLR and RFSLR are internally self-consistent, and this suggests that for SLR the crystalline component includes a fraction of the material characterised as non-crystalline by other methods (XRD, DSC).

T1 ρ study of pitch softening

Proton rotating frame spin-lattice relaxation time ( T 1 ρ ) measurements have been made in 10 samples of pitch from room temperature to 673 K to obtain information about the mechanisms of pitch softening. The T 1 ρ minimum of pitch was found to occur at approximately the same temperature as the softening point. This result suggests that the softening phenomenon of pitch can be detected at the molecular level by T 1 ρ measurements. Laboratory frame spin-lattice relaxation time ( T 1) measurements were also made on pitches to obtain information about the molecular motion of pitch in the high frequency ranges.

CP/MAS Carbon-13 NMR Study of Spin Relaxation Phenomena of Cellulose Containing Crystalline and Noncrystalline Components

Abstract Cross-polarization, 13C rotating frame spin-lattice relaxation and C laboratory frame spin-lattice relaxation processes have been studied for different cellulose samples by CP/MAS 13C NMR spectroscopy. It was found that the CP process can be described by a simple thermodynamic model and relative intensities of the respective resonance lines are consistent with the atomic ratios for the spectra obtained at a contact time of about 1 ms. The observed rotating frame spin-lattice relaxation times TC 10 were dominantly dependent on the time constant TD CH by which 13C nuclei were coupled to the 1H dipolar spin system. It was, therefore, impossible to obtain information about molecular

Pure-Dipolar Nuclear Spin-Lattice Relaxation in Nematic Liquid Crystals

Abstract A technique is described for exploiting the large nuclear dipole-dipole interactions in nemaic liquid crystals to study the details of the spin-lattice relaxation mechanisms, including cross relaxation effects. This technique provides a means for selectively preparing the magnetization in such a way as to enhance the relative contribution of a selected group of spins. Data illustarating this technique are presented for the nematic phase of MBBA. Further, the orientational dependence of the pure-dipolar spin-lattice relaxation time T1D for MBBA is shown to be similar to that of the laboratory frame spin-lattice relaxation time T1 and markedly different from the orientational dependence of the rotating-grame spin-lattice relaxation time T1a.

Nuclear magnetic relaxation study of solid sulfolan

Sulfolan has been studied by pulsed NMR measurements of laboratory frame spin–lattice relaxation T1 and rotating frame spin–lattice relaxation times T1ρ in the temperature range from 186 to 339 K. The previously reported phase transition at 288 K has been observed, but the data also show the presence of two additional, as yet unreported, phase transitions. The two crystalline phases are characterized by twofold rotations of the sulfolan molecules while the two plastic phases are both characterized by isotropic rotation and translational diffusion. It is suggested that the lower temperature plastic phase involves monovacancy diffusion while the higher temperature one involves relaxed vacancy diffusion. The relaxation times at most temperatures in the solid phases are dependent on the previous thermal history of the sample, with the transitions being sluggish and displaying hysteresis effects.