K.O. Prins

An NMR spin-echo study of self-diffusion in xenon

An experimental investigation has been made of the self-diffusion coefficient in xenon, using the NMR spin-echo technique. This experiment in xenon is the first systematic investigation of self-diffusion in a monatomic gas over a wide range of density and temperature. The data have been obtained using an NMR high-pressure probe in a superconducting magnet. The experimental results are compared with the results of computer simulations on systems of hard spheres and on systems of Lennard-Jones molecules.

MOBILITY OF POLYETHYLENE CHAINS IN THE ORTHORHOMBIC AND HEXAGONAL PHASES INVESTIGATED BY NMR

Abstract This is a report on a study of polyethylene, in and near the hexagonal phase, using NMR. From proton and deuteron NMR lineshapes and spin–lattice relaxation rates, we determine the rate and type of motion of the polyethylene chains both in the orthorhombic and hexagonal phase. In the hexagonal phase, we find fast axial local motion in combination with fast chain diffusion, which is in marked contrast to the more restricted motion we observe in the orthorhombic phase.

Low‐temperature, high‐pressure apparatus for nuclear‐magnetic‐resonance experiments

A description is given of a liquid‐helium cryostat containing a high‐pressure NMR probe, suitable for hydrostatic pressures up to 15 kbar. The tail of the cryostat is designed to fit between the poles of a conventional electromagnet. Hydrostatic helium gas pressure is generated by means of a commercially available two‐stage gas compressor. The apparatus is designed for performing nuclear‐magnetic‐resonance (NMR) experiments on samples subjected to pressure, in a temperature range from 2 to 100 K.

NMR study of chain dynamics in the hexagonal high pressure phase of polyethylene

At high pressure (>3300 bar) polyethylene shows a hexagonal phase between the orthorhombic phase and the liquid phase. This first NMR investigation of polyethylene in its hexagonal phase shows that the mobility of the molecular chains is very high. Deuteron quadrupole echo spectra have been used to determine the character and rate of local reorientations of the molecular chains. The chains perform rapid axial reorientations, similar to those observed in the rotator phases occurring in the long normal alkanes. From proton rotating frame relaxation data we derive a high rate of translational diffusion in the chain direction. These observations contribute to the understanding of the important role of the hexagonal phase in the growth of chain-extended polyethylene crystals.

NMR spin-echo study of self-diffusion in xenon and ethene

Abstract The NMR spin-echo technique has been used in an experimental study of the density dependence of the self-diffusion coefficient in gases and liquids. The experiment in xenon is the first systematic investigation of self-diffusion in a monatomic gas over a wide range of densities and temperatures. By using an improved NMR probe we have also succeeded in extending earlier high density measurements in ethene down to 5 Amagat. Comparison is made with the molecular dynamic results found by several authors.

High‐pressure probe and cryostat for pulsed NMR in a cryomagnet

The construction of a probe suitable for pulsed NMR for the investigation of fluids and solids at hydrostatic pressures up to 3 kbar is reported. The probe fits in a cryostat constructed inside the room‐temperature bore of a superconducting magnet. The temperature can be controlled in the range from 80 to 380 K. The apparatus has been used successfully in a NMR study of self‐diffusion in gaseous and liquid xenon, using 129Xe and 131Xe spin echoes at 75 and 22 MHz, respectively.

NMR investigation of phase transitions in polyethylene in the vicinity of the hexagonal high pressure phase

Abstract The location of the hexagonal crystalline phase of polyethylene in a pT-diagram, occurring at high pressure and high temperature, was determined from proton-NMR spin–lattice relaxation rates and found to be in good agreement with the results obtained with other techniques. Hexagonal and orthorhombic crystalline components as well as the amorphous part in a sample at high pressure can clearly be distinguished by deuteron NMR spectra. The changes in these components were followed as a function of temperature at various pressure values, showing the coexistence of orthorhombic and hexagonal crystalline materials during crystallization.

Phase transitions of solid CH4 at pressures up to 9 kbar determined by NMR

Abstract Conclusive evidence for a new phase transformation in solid CH4, with a hysteresis of 20 K, is presented. At pressures above 5 kbar this new phase IV is definitely stable while at lower pressures, even down to P = 0 bar, phase IV is stable for at least several days. Previous high-pressure results which were not completely understood are re-examined and re-interpreted in terms of this new phase transition. The spin-lattice relaxation time T1 has been measured in the phases I, II, III and IV up to 9 kbar and from 2.5 to 90 K. In addition, the triple point of the I–II and II–III phase lines is established at 3.9 kbar and 32.1 K.

Phase transitions of solid methane at pressures up to 8 kbar determined by NMR: CH2D2 and CD4

Abstract The phase diagrams of solid CH 2 D 2 and CD 4 have been determined up to 8 kbar. The transformation to the new phase IV, which has been shown previously to occur in solid CH 4 , also occurs in the deuterated species. At normal pressure, phase IV is stable for at least several days. The T 1 data in the phases III and IV suggest the existence of several inequivalent sublattices with low site symmetries. The triple points of the I–II and II–III phase-lines of CH 2 D 2 and CD 4 have been established.

NMR probe for high pressure and high temperature

A description is given of a probe which is designed for nuclear magnetic resonance (NMR) experiments at 180 MHz on samples at hydrostatic pressure up to 10 kbar. Temperature control has been designed to allow experiments from room temperature up to 600 K, regulated with an accuracy of 0.1 K. Radio frequency feedthroughs can sustain high‐voltage pulses of considerable length. The probe has been used successfully in a proton spin relaxation study at 180 MHz of polyethylene near the melting line.