W. P. Slichter

The Distribution of Solute in Crystals Grown from the Melt. Part I. Theoretical

The incorporation of solute elements into single crystals of germanium grown from the melt is examined in terms of a simple model. The theory takes account of the contribution of solute transport in the melt, owing to diffusion and fluid motion, to the over‐all process of impurity incorporation during steady‐state crystallization. The analysis is extended to treat the transient inclusion of solute which results when the composition of the melt is abruptly changed.

Distribution of Solute in Crystals Grown from the Melt. Part II. Experimental

Experiments have been performed on the distribution coefficients of a number of solute elements in germanium crystals grown from the melt. The variation of distribution coefficient with conditions of crystallization is examined in the light of the theory of Part I. The incorporation of solute elements into the crystal is shown to depend critically upon the transport processes occurring in the melt.

Molecular Motion in Polypropylene, Isotactic and Atactic

Molecular order and motion in isotactic and atactic polypropylene molecules have been studied by proton magnetic resonance methods over the temperature range 77–400°K, and by x‐ray diffraction. It is shown that some motion persists at the lowest temperatures, and becomes pronounced in the temperature interval 77–110°K. This behavior is ascribed to motion of the methyl groups about the threefold axis. The resonance undergoes further narrowing near room temperature for the atactic polymer, presumably owing to rotational and translational motions of chain segments. The isotactic compound possesses a narrow resonance superposed on a broad absorption over a wide temperature range. The former, ascribed to the amorphous regions, narrows at higher temperatures than for the atactic polymer, presumably owing to constraints imposed by the crystalline regions. The chains of the isotactic compound are less mobile than in polyethylene, as shown by the resonance studies and by x‐ray diffraction comparisons of the rate o...

Effect of Hydrostatic Pressure on Molecular Rotation in Solids

Measurements of the NMR spin—lattice relaxation time T1 have been carried out at various temperatures and pressures for eight solids that demonstrate rotational degrees of freedom. Apparatus is described which permits NMR pulse experiments to be performed over the temperature interval between 169° and 400°K under applied hydrostatic pressures between 1 and 680 atm. The intramolecular rotation of groups within molecules is insensitive to pressures within this range, and the rotation of entire molecules in the solid state is pressure dependent in varying degrees. The observed pressure coefficients of T1 in these solids are related to the thermal expansion coefficient of the lattice by a simple model that postulates a localized lattice expansion as an important step in the reorientation process.

Nuclear Magnetic Relaxation in Camphor

Solid solutions containing d‐ and l‐camphor have been investigated by nuclear magnetic resonance and x‐ray powder diffraction between 77° and 473°K. Striking differences are observed among the various solutions in the vicinity of the second‐order phase transition. The transition temperature is found to vary with composition, passing through a minimum at the 3:1 d:l solution. The width of the λ transition, reflected in the NMR second moment, increases going from the pure isomer to the racemic mixture. Methyl group reorientation persists at 100°K, although molecular tumbling has been arrested at this temperature in all samples. The x‐ray powder patterns of the pure isomer, the 1:1, and 3:1 mole ratio solids differ at 77°K, but are identical at 298°K. Plots of the NMR spin—lattice relaxation time versus reciprocal absolute temperature yield an activation energy of 2.35 kcal/mole for methyl‐group reorientation in d‐camphor below the λ point. Above the rotational transition, the x‐ray and NMR data are independ...

Nuclear Magnetic Resonance Studies of Molecular Motion in Natural Rubber

Nuclear magnetic resonance measurements have been made on natural rubber to examine how frequency, temperature, and crystallinity influence the nuclear relaxation. Molecular motions were studied by observing NMR linewidths and spin‐lattice relaxation times at temperatures between −100° and 100°C, and at radio frequencies between 2 and 60 Mc. The influence of crystallinity was seen in measurements on stark rubber. The relation between frequency and temperature in the spin‐lattice relaxation process is examined in terms of the Arrhenius equation and the WLF expression. The importance of using frequency as a variable in NMR studies of molecular motion is stressed.

Defects in Polyethylene Crystals

Nuclear magnetic resonance measurements have been made of the development of chain motion in linear polyethylene and polymethylene crystallized from solution. It is shown that segmental mobility is produced at room temperature when the compounds are heated below the melting point. The changes are rapid and irreversible, and are independent of crystallization temperature and concentration, except at high concentration. The effects are ascribed to the development of defects in the crystalline structure.

Nuclear Magnetic Resonance in Poly (Normal α‐Olefins)

The spin‐lattice nuclear magnetic resonance relaxation has been measured in a series of poly(normal α‐olefins) in which the side chains ranged in length from methyl to hexadecyl. The studies were carried out as a function of temperature, from −160° to 150°C, and as a function of the radiofrequency, specifically at 10, 20, 30, and 50 Mc/sec. The dependence of the NMR relaxation upon temperature and frequency shows that increase in the length of branches contributes to increased segmental mobility. Effects of thermal history are examined. The rate of side‐chain crystallization of polydodecene can be measured from the time dependence of the spin‐lattice relaxation.

Molecular Motion in Disordered Regions of Solid Polyethylene

Proton magnetic resonance studies have been made to compare the molecular motions ascribable to the disordered regions of solution‐grown and melt‐grown specimens of solid polyethylene. Studies have been made of both the spin‐lattice and spin‐spin relaxations. Restricted motion attributable to defect regions develops in solution‐grown crystals as the temperature is raised, but the number of protons in motion changes reversibly with heating and cooling, provided the maximum temperature is below ∼100°C. With higher temperatures the motion is greater in abundance and broader in frequency distribution. The increase is permanent despite annealing. Segmental motion attributed to molecules within the crystal is seen in solution‐grown crystals in the interval 50–100°C, but occurs to a much smaller degree in melt‐grown and heat‐treated samples. A model for the observed behavior is proposed.

Heteronuclear Spin—Lattice Relaxation in Some Crystalline Aromatic Compounds

Nuclear spin—lattice relaxation has been studied in some crystalline aromatic compounds, the toluene derivative C6H5CF3, and the xylene derivates m‐ and p‐CF3C6H4CF3, over the temperature interval between −185°C and the respective melting points. The decay of both the 1H and 19F nuclear polarization was investigated by pulse techniques. The relaxation data were supplemented by measurements of the proton and fluorine NMR second moments. Between −160°C and the melting point, the 1H second moment of each compound indicated that the protons were immobile. The 19F second moments indicated the effective reorientation of the CF3 group in all three compounds throughout the same temperature range. The relaxation of the 19F polarization was found to occur more rapidly than the corresponding 1H relaxation. Moreover, the relative rates of 1H and 19F relaxation also varied quite considerably with temperature. These results were analyzed by Solomons treatment of heteronuclear relaxation. Semiquantitative agreement between theory and experiment was achieved for the C6H5CF3 relaxation data.