Small-angle neutron scattering studies of polyethylene crystallized at high pressure

The technique of small-angle neutron scattering (SANS) has been used to study the chain configuration in pressure crystallized polyethylene. Two narrow molecular weight fractions of deuterated molecules (PED) of M_w 23 000 and 54 000 were solution blended with a protonated matrix polymer of M_w 81 500. Although pressure crystallization was shown to have produced a clustering of the PED molecules, the radii of gyration $\text{〈}\text{S}{}^2\text{〉}{}_{\mathrm{z}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ were, nevertheless, shown to be consistent with a model in which the PED molecules possessed rod-like configurations. The predicted rod lengths were in close agreement with the molecular stem lengths of the PEH matrix polymer, which were independently determined by nitric acid etching. Furthermore, a doubling of the PED molecular weight produced no change in the value of $\text{〈S}{}^2\text{〉}{}_{\mathrm{z}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$. This was interpreted in terms of a chain folding mechanism in which a molecule is bounded by the surfaces of a lamellar block and is therefore unable to increase its' rod length.     

Conformation of the molecules in drawn polypropylene revealed by neutron scattering studies

It has not been possible previously to do a complete study of drawn polyolefins using SANS techniques because of void scattering. In the case of polypropylene this can be reduced sufficiently to permit measurements with the Q-vector perpendicular and parallel to the draw direction. A complete analysis has been carried out with a draw ratio 8:1 in which the PPD molecules of narrow molecular weight distribution and molecular weights in the range 390 000 to 50 000 are molecularly dispersed in a drawable PPH matrix. The dimensional changes on drawing are to extend the molecule by a factor of 2 with a corresponding reduction in diameter to a maximum of 40%. We have interpreted the data at high Q on the basis that the molecule is not uniformly distributed but the majority is folded into sub-units and the number of these is proportional to the molecular weight of the polymer. A further feature of the sub-units structure is that its length is approximately two lamellae thick, this dimension is similar to the SANS scattering length (I_n) identified in the study of isotropic material. The double stems belonging to the same molecule have been concentrated by the drawing process some 16 times and a consequence of this is to markedly change the Kratky curves from the isotropic case. Notwithstanding, this increase in the packing of the molecule, the stems are still, on average, some 40 Å apart and surrounded by many stems belonging to other macromolecules. Similarly, each sub-unit is surrounded by sub-units belonging to neighbouring molecules.     

Small-angle neutron scattering study of amorphous polycarbonate

Experiments have been performed on deuterated polycarbonate chains with five different molecular weights, randomly dispersed in a protonated polycarbonate matrix. There is evidence for some ester interchange in the melt. This requires that the values of M_w obtained from the SANS data be used. The measured radii of gyration were found to follow the relationship: R_w = βM^a_w, with β = 0.457(9) and α = 0.5. The high value for β compared with that for polystyrene (β = 0.27) is a reflection of the stiffer backbone in polycarbonate.     

Two-dimensional neutron scintillation detector with optimal gamma discrimination

For a two-dimensional scintillation neutron detector based on position-sensitive photomultipliers a digital differential discrimination unit has been developed which minimizes the gamma sensitivity of the detector and was necessary because of position- dependent PM gain. The digital upper and lower discrimination levels are set position-dependent due to the position-dependent photomultiplier gain which is obtained from calibration measurements. Narrow discriminator windows can be used to reduce the gamma background drastically without affecting the homogeneous neutron sensitivity of the detector. The new discrimination method is described and its function is tested with neutrons.     

Properties of the flux line lattice in hysteretic type II superconductors. I. Neutron diffraction experiments

A detailed study of neutron rocking curves on flux line lattices was made in single-crystalline hysteretic type II superconductors with Ginzburg-Landau parameters between 1.0 and 2.5. Information was obtained about the symmetry and the quality of the flux line lattice as a function of crystal orientation, sample purity, and magnetic field. The flux line lattice is always observed to form a single crystal. The width of the rocking curves, which is a measure of the misorientation of flux line crystallites in the plane perpendicular to the external field, is found to decrease strongly with increasing field along the initial magnetization curve, and to be much smaller upon decreasing the field from aboveH _c2 . This width varied between 8 and 1.5°. The minimum size of a coherently scattering flux line crystallite was estimated to be about 2–3 µm. Along the initial magnetization nearH _c1 the flux is found to form flux line bundles of constant lattice spacing. Although large flux density gradients have been observed earlier with field probes nearH _c1 ^1,2 , the lattice parameter measured with neutrons remained constant in this field range.     

Neutron diffraction studies of the morphology of flux line crystals in type II superconductors

Neutron diffraction measurements on flux line single crystals in different type II materials are reported. It is shown that the type, the perfection, and the orientation of the vortex lattice with respect to the metal crystal mainly depend on the symmetry of the crystal axis which is parallel to the magnetic field. Results on samples with twofold, fourfold, and sixfold symmetry axes are represented and compared with theoretical predictions. We further studied the dependence of the widths of rocking curves on temperature and flux density and show that there is a close connection between the mosaic spread of the vortex lattice and its elastic properties.     

Maximum-likelihood principle for determining positions in neutronscintillation detectors

Pulse height division is generally used to determine a position channel for storing detection events in position-sensitive detectors (PSDs). An alternative method based on the maximum likelihood principle is discussed. This principle is modified in order that the storage of events can be done online. Analytic expressions for the spatial resolution are derived and evaluated. Both detection methods are simulated for linear PSDs. Results on uniformity, linearity, stability and spatial resolution are presented. The major specific advantage of the proposed method is an improved uniformity of the response along the entire detector     

Small-angle neutron scattering studies of isotropic polypropylene

Small-angle neutron scattering studies have been made of molten and crystalline polypropylene using samples containing small amounts of deuterated polypropylene in a protonated polypropylene matrix. The specimens were characterized by small- and wide-angle X-ray scattering to determine the d-spacing and the degree of crystallinity χ and by gel permeation chromatography to determine molecular weight, M_w, and molecular weight distribution. The degree of crystallinity was varied from 0.5 to 0.7, the d-spacing from 120 to 250 Å and the molecular weight from 34 000 to 1 540 000. Clustering was not observed. The radius of gyration $\text{〈s}{}^2\text{〉}{}_{\mathrm{w}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of the tagged molecules was approximately proportional to $\text{M}{}_{\mathrm{w}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and almost independent of d and χ. In the melt similar values were obtained which are, within experimental uncertainties, the same as in a θ-solution. For 〈s²〉_wk²? 1 the scattering law approaches a k^?2 dependence. The results are discussed with reference to the chain-folded model but a fit cannot be obtained over all molecular weights. A simple random coil model fits the neutron scattering data partly but this does not explain the origin of the d-spacing.     

Magnetic refinement of tips for magnetic force microscopy

Silicon cantilever probes with monolithically integrated tips are commercially available and are routinely used for atomic force microscopy (AFM). For such probes, amagnetic refinement of the silicon tip has been developed and results in a deposition of ferromagnetic material such as nickel or CrCoTa in the top area of the tip. The method consists of essentially three steps: (1) A broad-area sputter deposition of a ferromagnetic material; (2) a selective electron beam-induced carbon deposition at the top of the tip; (3) a broad-area ion-beam sputter etching, which removes the magnetic layer everywhere except underneath the carbon cap. The method allows to control the total amount and extension of the magnetic material left at the tip. It is applicable to all kinds of ferromagnetic materials which can be deposited as a thin layer by sputter deposition or evaporation. Experiments indicate that the method is reliable and improves the resolution of magnetic force microscopy (MFM). With such magnetically refined tips on silicon cantilevers, MFM measurements have been performed in contact mode as well as in dynamic and static noncontact modes. In this paper, the method for magnetic tip refinement is described and MFM measurements with these tips are presented.