No abstract

Results are presented of neutron incoherent scattering experiments on isotropic linear polyethylene samples of high (80%) and low (48%) crystallinity in the temperature range between −180°C and +85°C for values of the scattering vector between 0.29 Å−1 and 1.81 Å−1 obtained with a high resolution backscattering spectrometer (Δħω = 0.25 − 1.0 μeV) and between 0.57 Å−1 and 2.4 Å−1 with a time‐of‐flight spectrometer (Δħω = 420 μeV). From a comparison of the results on these samples one concludes that relaxation takes place predominantly in the noncrystalline regions. This motion cannot be adequately accounted for by any of the existing models for the γ‐process. Therefore, a more liquidlike motion is suggested. Diffusion of shorter chain segments has also been ruled out since it is too slow to be observed. A simplified model of protonic jumps between equidistant sites located on the periphery of a circle of radius 2.5 Å reproduces the experimental results well. For the average time between successive CH2‐group reorientations one obtains τ1 = τ0 exp(EactRT) with τ0 = (2.0 ± 1.5) × 10−13 sec and Eact = (4.5 ± 1.0) kcal/mole. The values join up well with those for the γ‐process observed by NMR. It has been concluded that 60–90% of the protons in the noncrystalline regions participate in this motion.

In this paper, the principle of a new neutron diffractometer is presented. In this diffractometer are combined the advantages of a high‐resolution time‐of‐flight (TOF) diffractometer and some essential characteristics of a classical double‐axis spectrometer (DAS), specific disadvantages of both instruments being eliminated. Its name, MARTIN (multiple‐angle high‐resolution time‐of‐flight instrument for neutrons), is derived from the particular instrumental set‐up. With the MARTIN diffractometer, a neutron pulse from a narrow wavelength band (0.2–0.5 Å) impinges on the sample. The diffracted radiation is registered and time‐analyzed by detectors located at several scattering angles. The angular arrangement of the detectors is such that each detector samples a different portion of the diffractogram, with the portions detected by adjacent detectors partially overlapping. These partial diffractograms can be conjoined to yield the complete diffractogram of interest. This new machine has all the advantages of a TOF diffractometer: no contamination of the primary beam by higher orders, higher counting rates because of the possible utilization of the entire Debye–Scherrer ring, higher flexibility because no monochromators are needed and less strict requirements of mechanical precision as a result of the fixed detector arrangement. In addition, the use of a narrow wavelength band reduces the problem of absorption and extinction corrections in TOF diffractometry to one solvable by the well known data treatment of two‐axis diffractometry. A prototype of the MARTIN diffractometer was designed for powder diffraction and installed as an extension of the TOF diffractometer at the Munich Research Reactor (FRM) in Garching, where its characteristics were tested on a powder sample of Al2O3.

C-376 13. NEUTRON DI FFRACTION displacement of the oxygen atoms from the linear O-Re-O chains of cubic Re03 to a "hinged" arrangement at the oxygens.This necessitates a doubling of the lattice and a change from simple cubic to a body centered geometry.