John A. Pople

Collagen orientation and crystallite size in human dentin: A Small angle X-ray scattering study

The mechanical properties of dentin are largely determined by the intertubular dentin matrix, which is a complex composite of type I collagen fibers and a carbonate-rich apatite mineral phase. We performed a small angle X-ray scattering (SAXS) study on fully mineralized human dentin to quantify this fiber/mineral composite architecture from the nanoscopic through continuum length scales. The SAXS results were consistent with nucleation and growth of the apatite phase within periodic gaps in the collagen fibers. These mineralized fibers were perpendicular to the dentinal tubules and parallel with the mineralization growth front. Within the plane of the mineralization front, the mineralized collagen fibers were isotropic near the pulp, but became mildly anisotropic in the mid-dentin. Analysis of the data also indicated that near the pulp the mineral crystallites were approximately needle-like, and progressed to a more plate-like shape near the dentino-enamel junction. The thickness of these crystallites, approximately 5 nm, did not vary significantly with position in the tooth. These results were considered within the context of dentinogenesis and maturation.

Intrafibrillar Mineral May be Absent in Dentinogenesis Imperfecta Type II (DI-II)

High-resolution synchrotron radiation computed tomography (SRCT) and small-angle x-ray scattering (SAXS) were performed on normal and dentinogenesis imperfecta type II (DI-1I) teeth. The SRCT showed that the mineral concentration was 33% lower on average in the DI-II dentin with respect to normal dentin. The SAXS spectra from normal dentin exhibited low-angle diffraction peaks at harmonics of 67.6 nm, consistent with nucleation and growth of the apatite phase within gaps in the collagen fibrils (intrafibrillar mineralization). In contrast, the low-angle peaks were almost non-existent in the DI-II dentin. Crystallite thickness was independent of location in both DI-II and normal dentin, although the crystallites were significantly thicker in DI-II dentin (6.8 nm [SD = 0.5] vs. 5.1 nm [SD = 0.6]). The shape factor of the crystallites, as determined by SAXS, showed a continuous progression in normal dentin from roughly one-dimensional (needle-like) near the pulp to two-dimensional (plate-like) near the dentin-enamel junction. The crystallites in DI-II dentin, on the other hand, remained needle-like throughout. The above observations are consistent with an absence of intrafibrillar mineral in DI-II dentin.

Effect of shear on cubic phases in gels of a diblock copolymer

The effect of shear on the orientation of cubic micellar phases formed by a poly(oxyethylene)–poly(oxybutylene) diblock copolymer in aqueous solution has been investigated using small-angle x-ray scattering (SAXS) and small-angle neutron scattering (SANS). SAXS was performed on samples oriented in a Couette cell using steady shear, and SANS was performed on samples subject to oscillatory shear in situ in a rheometer with a shear sandwich configuration. A body-centered-cubic (bcc) phase observed for gels with concentrations greater than 30 wt % copolymer was found to orient into a polydomain structure, with the close-packed {110} planes both parallel and perpendicular to the shear plane. For gels with 30 wt % copolymer or less, a face-centered-cubic (fcc) phase was observed, and this was also observed on heating the more concentrated gels that formed a bcc phase at room temperature. The fcc phase could be oriented to form a highly twinned structure, with a significant deviation from the ABCABC… stacking se...

Electron density in Mo/ller–Plesset theory

The Mo/ller–Plesset theorem states that for the electronic Hamiltonian in the form H = JpFp+V, where the eigenfunctions of Fp are the Hartree–Fock molecular spin orbitals and the eigenvalues are the one‐electron energies, by treating V as a perturbation the first order term in the expansion of the electron density vanishes. In spite of this, some one‐electron properties are not given at the H–F level. This Note presents the expression for the second order correction to the electron density.

An integrated Couette system for in situ shearing of polymer and surfactant solutions and gels with simultaneous small angle x-ray scattering

A Couette cell designed for in situ small angle x-ray scattering (SAXS) studies of polymeric systems under shear is described. Representative results are presented for the effect of shear on the lamellar phase of a concentrated poly(oxyethylene)–poly(oxybutylene) diblock copolymer in aqueous solution. It is shown that the application of shear resulted in a reduction in the lamellar spacing and of the defect density. The former was observed as a shift in the principal peak position in the SAXS pattern, and the latter as an increase in the associated correlation length. It was further shown that the cessation of shear led to a relaxation of both shear induced effects, over a time period of hours.

Orientation effects in monodomain nematic liquid crystalline polysiloxane elastomers

A series of monodomain liquid crystalline (LC) elastomers based on a polysiloxane were synthesised. These elastomers were prepared either with one or two cross-linking agents in the presence of a mechanical field. By using the real-time X-ray facility at the University of Reading (AXIS), we have shown that the nematic order parameter 〈P2 〉 is dependent on both the extension λ value and the degree of cross-linking. We have also shown that the monodomain elastomers, exhibiting permanent alignment and 〈P2 〉 values of about 0.5, can be prepared by using only one cross-linking agent making the synthesis of these monodomain LC elastomers much more simple and cost effective than that proposed by Kupfer.

In-situ time-resolving wide-angle X-ray scattering study of crystallization from sheared polyethylene melts

The effect of simple shear flow on molten polyethylene is studied using in situ time-resolving wide-angle X-ray scattering procedures. A small degree of global molecular orientation is observed under shear flow which is linearly related to the logarithm of the shear rate. The system rapidly relaxes to an isotropic state upon cessation of shear flow. A sharp temperature drop coupled with the cessation of shear flow leads to the development of crystallinity, and it is found that the shear flow prior to crystallization has a strong effect on the degree of orientation in the recrystallized state. In fact, above a critical prior shear rate, the recrystallized state changes from globally isotropic to globally anisotropic.

Shear-induced orientational order in the hexagonal phase of oxyethylene/oxybutylene diblock copolymer gels

Shear-induced orientational ordering in diblock copolymer gels of E40B10 in 0.2 mol dm−3 aqueous K2SO4, where E denotes oxyethylene and B denotes oxybutylene, was studied in the hexagonally packed rod phase at 85°C for a number of diblock copolymer concentrations. Large amplitude (λ = 50%) oscillatory shear at a shear rate of ω = 10 rad s−1 was applied to the gels and the degree of orientation was quantified in terms of global orientational order parameters extracted from small-angle X-ray scattering (SAXS) patterns. Shearing the gel produced a rapid drop in the dynamic shear moduli and a slower increase in orientation; up to a second rank orientation parameter of P2 = 0.26 for the highest concentration (38 wt%) diblock gel. Time-resolved SAXS measurements showed that the steady state level of orientation was established over a time-scale of 3 min. Lower concentration gels yielded lower orientation parameters from the same deformation process, and a 25 wt% gel remained unoriented. Upon cessation of shear the dynamic shear moduli recovered rapidly to their initial values, although the shear-induced anisotropy in the SAXS patterns relaxed very slowly to zero over a time-scale of over an hour.

Self-Assembly of Asphaltene Aggregates: Synchrotron, Simulation and Chemical Modeling Techniques Applied to Problems in the Structure and Reactivity of Asphaltenes

Increased understanding of the structure and chemistry of asphaltenes is essential to developing ways of mitigating the effects of asphaltenes, destroying them or finding new uses for them. The chemical structure and physical structure of the asphaltenes are unique and much has been learned about their physics and chemistry.1 However, there are still fundamental questions regarding the origin and structure of asphaltenes that remain to be answered. In this report, new synchrotron WAXS (wide angle x-ray scattering data) and SAXS (small angle x-ray scattering data) for Venezuelan and Mexican asphaltenes are reported showing the ubiquitous presence of the “asphaltene particles” with sizes in the 3–5 nm ranges. The particles exist both as correlated packets in the precipitated asphaltene and in the parent crude oil as individual particles. Furthermore, in the second section of this report the self-assembly of the “asphaltene” particles from model compounds is reported. That the “asphaltene particles” can self-assemble indicates the basic stability of the particles and generates interesting questions regarding the origins of petroleum. Increasingly, heavy crudes are becoming a major source of petroleum hydrocarbons as lighter crudes become scarce. One major difference between a light crude and a heavy crude is the asphaltene content of the crude. The asphaltene fractions contain most of the metals in the crude and generally more sulfur and

Langmuir Monolayers of Straight-Chain and Branched Hexadecanol and Eicosanol Mixtures

Langmuir monolayers of straight-chain and branched hexadecanol and eicosanol mixtures were previously studied using surface pressure- area isotherms, Brewster angle microscopy, and interfacial rheology. In this paper, we investigate the structure of these fatty alcohol mixtures using these previous results together with X-ray diffraction and reflectivity measurements, which provide a better understanding of the structure of the monolayer in terms of the phase segregation and location of branched chains. For eicosanol below 25 mN/m, the branched chains are incorporated into the monolayer, yet they are phase-separated from the straight chains. At higher surface pressures, the branched chains are expelled from the monolayer and presumably form micelles or some other aggregate in the subphase. In contrast, the hexadecanol branched chains are not present in the monolayer at any surface pressure. These behaviors are interpreted with the help of the X-ray measurements and density profiles, and are explained in terms of straight-chain flexibility. We will discuss the effect of the monolayer structure on the surface shear viscosity. These studies provide a deeper understanding of the structure and behavior of amphiphilic mixtures, and will ultimately aid in developing models for lipids, micelle formation, and other important biological functions.