Advancement of Photospheric Radius Expansion and Clocked Type-I X-Ray Burst Models with the New

We report the first (in)elastic scattering measurement of ^{25}Al+p with the capability to select and measure in a broad energy range the proton resonances in ^{26}Si contributing to the ^{22}Mg(α,p) reaction at type I x-ray burst energies. We measured spin-parities of four resonances above the α threshold of ^{26}Si that are found to strongly impact the ^{22}Mg(α,p) rate. The new rate advances a state-of-the-art model to remarkably reproduce light curves of the GS 1826-24 clocked burster with mean deviation <9% and permits us to discover a strong correlation between the He abundance in the accreting envelope of the photospheric radius expansion burster and the dominance of ^{22}Mg(α,p) branch.

Shape Coexistence at Zero Spin in

The low-spin structure of the semimagic ^{64}Ni nucleus has been considerably expanded: combining four experiments, several 0^{+} and 2^{+} excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0^{+} excitation is located at a surprisingly high energy (3463 keV), with a collective 2^{+} state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N=40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N.

Multifaceted Quadruplet of Low-Lying Spin-Zero States in

A search for shape isomers in the ^{66}Ni nucleus was performed, following old suggestions of various mean-field models and recent ones, based on state-of-the-art Monte Carlo shell model (MCSM), all considering ^{66}Ni as the lightest nuclear system with shape isomerism. By employing the two-neutron transfer reaction induced by an ^{18}O beam on a ^{64}Ni target, at the sub-Coulomb barrier energy of 39 MeV, all three lowest-excited 0^{+} states in ^{66}Ni were populated and their γ decay was observed by γ-coincidence technique. The 0^{+} states lifetimes were assessed with the plunger method, yielding for the 0_{2}^{+}, 0_{3}^{+}, and 0_{4}^{+} decay to the 2_{1}^{+} state the B(E2) values of 4.3, 0.1, and 0.2 Weisskopf units (W.u.), respectively. MCSM calculations correctly predict the existence of all three excited 0^{+} states, pointing to the oblate, spherical, and prolate nature of the consecutive excitations. In addition, they account for the hindrance of the E2 decay from the prolate 0_{4}^{+} to the spherical 2_{1}^{+} state, although overestimating its value. This result makes ^{66}Ni a unique nuclear system, apart from ^{236,238}U, in which a retarded γ transition from a 0^{+} deformed state to a spherical configuration is observed, resembling a shape-isomerlike behavior.

Reversibility of temperature driven discrete layer-by-layer formation of dioctyl-benzothieno-benzothiophene films.

Film forming properties of semiconducting organic molecules comprising alkyl-chains combined with an aromatic unit have a decisive impact on possible applications in organic electronics. In particular, knowledge on the film formation process in terms of wetting or dewetting, and the precise control of these processes, is of high importance. In the present work, the subtle effect of temperature on the morphology and structure of dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) films deposited on silica surfaces by spin coating is investigated in situ via X-ray diffraction techniques and atomic force microscopy. Depending on temperature, bulk C8-BTBT exhibits a crystalline, a smectic A and an isotropic phase. Heating of thin C8-BTBT layers at temperatures below the smectic phase transition temperature leads to a strong dewetting of the films. Upon approaching the smectic phase transition, the molecules start to rewet the surface in the form of discrete monolayers with a defined number of monolayers being present at a given temperature. The wetting process and layer formation is well defined and thermally stable at a given temperature. On cooling the reverse effect is observed and dewetting occurs. This demonstrates the full reversibility of the film formation behavior and reveals that the layering process is defined by an equilibrium thermodynamic state, rather than by kinetic effects.

Neutron reflectivity of supported membranes incorporating terminally anchored polymers: Protrusions vs. blisters.

The effect of terminally anchored chains on the structure of lipid bilayers adsorbed at the solid/water interface was characterized by neutron reflectivity. In the studied system, the inner leaflet, closer to the substrate, consisted of head-deuterated 1,2-distearoyl-sn-glycero-3-phosphorylcholine (DSPC) and the outer leaflet comprised a mixture of DSPC and polyethylene glycol (PEG) functionalized 1,2-distearoyl-sn-glycero-3-phosphoethanolamine. The DSPC headgroups were deuterated to enhance sensitivity and demarcate the bilayer/water interface. The effect on the inner and outer headgroup layers was characterized by w(1/2), the width at half-height of the scattering length density profile. The inner headgroup layer was essentially unperturbed while w(1/2) of the outer layer increased significantly. This suggests that the anchored PEG chains give rise to headgroup protrusions rather than to blister-like membrane deformations.

Symmetric and asymmetric instability of buried polymer interfaces.

We demonstrate using neutron reflectometry that the internal interfaces in a trilayer system of two identical thick polystyrene layers sandwiching a much thinner (deuterated) poly(methyl methacrylate) layer 15 nm thick (viscosity matched with the polystyrene layers) increase in roughness at the same rate. When the lower polystyrene layer is replaced with a layer of the same polymer of much greater molecular mass, two different growths of the interfaces are observed. From the growth of the interface for this asymmetric case in the solid regime using the theoretical prediction of the spinodal instability including slippage at the interface, a value of the Hamaker constant of the system has been extracted in agreement with the calculated value. For the symmetric case the rise time of the instability is much faster.

Interfacial behavior of a hairpin DNA probe immobilized on gold surfaces.

Neutron reflection has been used in combination with electrochemical and quartz microbalance techniques to characterize a mixed monolayer of thiolated hairpin ss-DNA and 4-mercaptobutan-1-ol monolayer self-assembled on gold before and after hybridization with a cDNA target. Neutron reflection has revealed the opening of the stem-loop configuration of the probe associated with helix formation. This change in conformation correlates to the modification of the electron-transfer resistance associated with the [Fe(CN)6](3-/4-) redox marker present in solution.

Neutron reflectivity study of the kinetics of polymer-polymer interface formation.

We have investigated how the interface width between two thin polymer films approaches its equilibrium state. Neutron reflection for different polyolefin bilayers of various degrees of incompatibility as a function of the annealing time was measured. By tuning the interaction parameter, we have probed both an immiscible polymer couple and systems approaching criticality where the interface is wider. Since polymer chains have slow dynamics, we have observed the slow broadening of the interface connected to the growth at long times of the wavelength capillary-wave modes, which involve large-scale hydrodynamic flows.

Primary versus ternary adsorption of proteins onto PEG brushes.

Polyethylene glycol (PEG) brushes are used to reduce protein adsorption at surfaces. Their design needs to allow for two leading adsorption modes at the brush-coated surface. One is primary adsorption at the surface itself. The second is ternary adsorption within the brush as a result of weak PEG-protein attraction. We present a scaling theory of the equilibrium adsorption isotherms allowing for concurrent primary and ternary adsorption. The analysis concerns the weak adsorption limit when individual PEG chains do not bind proteins. It also addresses two issues of special relevance to brushes of short PEGs: the consequences of large proteins at the surface protruding out of a shallow brush and the possibility of marginal solvent conditions leading to mean-field behavior. The simple expressions for the adsorption isotherms are in semiquantitative agreement with experiments.

Early-stage roughening of the polymer-polymer interface approaching the glass transition temperature by real-time neutron reflection.

The early-stage roughening of the interface between thin deuterated poly(methyl methacrylate) (d-PMMA) layers on thick polystyrene (PS) films was studied as a function of the temperature using real-time specular neutron reflectivity. By measuring the growth of the interface roughness as a precursor of the dewetting, the characteristic time constant of the early stages of the process was studied as a function of the temperature approaching the glass transition temperature (T(g)) of the two polymers from above and compared with the prediction of the growth of the interface by the spinodal process. Both solid and liquid regimes were probed, in which the viscosity of the thin film or the substrate dominates respectively. The characteristic growth time of the process also depends on the upper film thickness to a power of 5 or 6 in the solid or liquid regimes, respectively, as predicted by the theory of spinodal dewetting.

Approaching criticality in polymer-polymer systems.

The interfacial width of polyolefins blends has been probed as a function of distance away from the critical point by using neutron reflectivity. For strongly immiscible polymer pairs, the width of the interface increases slowly when the degree of immiscibility is decreased and the interfacial width varies with the interaction parameter chi of the polymers. Closer to the critical point the dependence on the degree of miscibility becomes stronger and the way in which the interfacial width diverges, as criticality is approached, is related to both the chain length and chi. The self-consistent field theory numerical calculations, with the additional contribution due to capillary waves, provides a good description of the width of the interface between two polymer bulk phases in particular at intermediate values of the degree of immiscibility.

Thickness dependence of structural relaxation in spin-cast, glassy polymer thin films.

The isothermal structural relaxation of glassy, spin-cast polymer thin films has been investigated. Specifically, the thickness h of freshly cast poly(methyl methacrylate) thin films was measured over time using spectroscopic ellipsometry. The spin-cast films exhibit a gradual decrease in thickness, which is attributed to structural relaxation of the glass combined with simultaneous solvent loss. In all cases, h was found to be greater than the equilibrium thickness h(infinity) , which is obtained by cooling slowly from the melt. It is observed that both the rate of the volume relaxation and the fractional departure from h(infinity) (referred to as delta(0) ) increase with increasing film thickness. In the limit of very thin films, the initial h is close to h(infinity) , and delta(0) is small, whereas in thick films (>500 nm) , a plateau value of delta(0) of 0.16 is observed, which is close to the volume fraction of the solvent at the vitrification point. This dependence of delta(0) on thickness is observed regardless of the substrate, polymer molecular weight, or angular velocity during spin casting. Enhanced mobility near film surfaces could be leading to greater relaxation in thinner films prior to, and immediately after, the vitrification of the polymer during the deposition process.

Early stages of polymer interdiffusion.

We have examined the wavelength dependence of the diffusion coefficient in a multilayer deuterated polystyrene-hydrogenated polystyrene system. The measurements were performed using neutron reflectivity. The decay of the Bragg peak was also followed in great detail. The measured wavelength diffusion coefficient D(k) was also not found spatially dependent on the distance of the diffusional couples to the substrate. On the basis of our results, it was possible to reconstruct the broadening interface of polymer bilayer sample geometry which had characteristics of a diffuse tail and a sharp core.

Dewetting of thin polymer films at temperatures close to the glass transition.

We present detailed studies on dewetting of thin polystyrene (PS) films which were deposited onto silicon wafers coated with a polydimethylsiloxane (PDMS) monolayer. Experiments were performed at temperatures close to the glass transition temperature of PS. Several significant deviations from the dewetting behaviour of Newtonian liquids were observed. The length of the PS molecules, and thus the viscosity, turned out to be of minor importance in determining the dewetting velocity, in particular for the later regimes. In stark contrast, the geometry of the drying spot had a striking influence on the dewetting velocity. Initially, dewetting from straight contact lines proceeded faster than the opening of circular holes. At later stages, the process slowed down significantly in both cases. Under the conditions at which our experiments were performed, PS cannot flow like a simple liquid. Thus, the observed dewetting has to be the consequence of plastic deformation induced by capillary forces. Our results indicate that under such conditions the energy dissipation process is strongly affected by geometry, which is not the case for viscous liquids.

Structural relaxation of spin-cast glassy polymer thin films as a possible factor in dewetting.

Reiter has recently reported a situation in which the dewetting of quasi-solid films is linked to plastic deformation--rather than viscous flow--resulting from capillary forces. Herein we propose that, in thin films of some glassy polymers--especially poly(methyl methacrylate) (PMMA)--prepared by spin-casting from solvent, structural relaxation might impart sufficient stress to cause plastic deformation. We find that PMMA films decrease in thickness by several percent, which is sufficient to create significant stress in those cases in which the film is attached to a rigid substrate. The floating technique, which can take tens of minutes, might allow most of the structural relaxation to occur prior to dewetting experiments.

Influence of the glass transition on solvent loss from spin-cast glassy polymer thin films.

The interdependence of solvent loss and vitrification in spin-cast poly(methyl methacrylate) thin films is explored. Fast measurements of decreases in film thickness, achieved with ellipsometry, indicate that the rate of solvent (toluene) loss decreases sharply when the solvent volume fraction phi(solv) falls below about 0.15 and the film vitrifies. Ellipsometry and microgravimetry show that solvent is lost from a glassy thin film (150 nm thick) over of a period of more than ten hours, which is much longer than would be required if it was limited by diffusion in the glass. These results support the recently-proposed idea that the compression of the glass creates an energy barrier that slows down solvent loss.

The timescale of spinodal dewetting at a polymer/polymer interface.

We investigate the dynamics of spinodal dewetting in liquid-liquid polymer systems. Dewetting of poly(methyl-methacrylate) (PMMA) thin films on polystyrene (PS) "substrates" is followed in situ using neutron reflectivity. By following the development of roughness at the PS/PMMA interface and the PMMA surface we extract characteristic growth times for the dewetting process. These characteristic growth times are measured as a function of the molecular weight of the two polymers. By also carrying out experiments in the regime where the dynamics are independent of the PS molecular weight, we are able to use dewetting to probe the scaling of the PMMA thin film viscosity with temperature and molecular weight. We find that this scaling reflects bulk behaviour. However, absolute values are low compared to bulk viscosities, which we suggest may be due in part to slippage at the polymer/polymer interface.

Neutron Reflectivity Study of the Adsorption of beta-Lactoglobulin at a Hydrophilic Solid/Liquid Interface.

Adsorption of a globular protein on a hydrophilic solid/liquid interface was investigated. Neutron reflectivity was used to determine structural information on an adsorbed beta-lactoglobulin layer at a hydrophilic silicon surface. The thickness of the protein film was found to be compatible with the diameter of the native protein, indicating that possible conformational changes of the protein during adsorption are not gross enough to alter the shape of the protein. The amount adsorbed is consistent with that derived by our ellipsometry measurements, obtained in similar experiments. Evidence for significant H-D exchange in the adsorbed protein was found. Copyright 1999 Academic Press.