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We have used neutron reflectivity to measure the concentration profile of carboxylic acid-terminated polystyrene at the liquid/solid interface. Two isotopic systems were studied: (1) deuterated polystyrene (DPS) in cyclohexane and (2) polystyrene (PS) in deuterated cyclohexane. Although measured at 24°C (a poor solvent condition), the high grafting density of PS tethered chains causes the chains to weakly stretch to 3-5 times its unperturbed dimension. Although of similar molecular weight, the height of the DPS layer is about 50% higher than that of the PS. In both systems, the volume fraction of polymer near the wall is ∼ 0.60. For the DPS system, reflectivity profiles can be simulated using a parabolic profile with a depletion layer at the silicon-liquid interface. However, in the PS system, no depletion is observed. Here, the shape. of the PS concentration profile is parabolic with a rounded tail.
Microemulsions are isotropic mixtures of water, oil, and surfactant which self-assemble to form interfacial monolayers with a characteristic length scale on the order of hundreds of Angstroms. Small angle neutron scattering was used to determine the bulk structure of a wateroctane-C10E4 bicontinuous microemulsion. The mean curvature of the surfactant film was verified to be near zero by independently measuring the scattering contributions from the water-surfactant and oil-surfactant interfaces through hydrogen-deuterium contrast variation. The exponentially damped, oscillatory scattering length density profile of the microemulsion near a hydrophobic silicon surface was also observed with neutron reflectivity. The surface reflectivity results as well as the bulk small angle scattering measurements are explained using a simple Ginzburg-Landau theory.
Simultaneous measurements of neutron reflectivity and prompt gamma ray emission, from samples with buried Gd layers, are shown to be of significant aid in determining the depth profile of the entire sample. Because of resonant enhancement of the neutron standing waves in the sample, the gamma ray signals are considerably enhanced making these experiments possible. A possible application of this technique to study grazing angle neutron diffraction is also mentioned.
The results of specular and diffuse x-ray scattering studies of multilayers are discussed. We show here that such studies can yield detailed statistical information about the interfacial roughness and morphology. Results on a GaAs/AlAs multilayer are presented and the data is analyzed within the Born approximation.
The order-disorder transition in thin films of symmetric diblock copolymers of polystyrene and polymethylmethacrylate has been investigated by neutron reflectivity. At temperatures above the order-disorder temperature, TMST, a surface induced oscillatory segment density profile with an exponential decay length, χ, is observed. The inverse of decay length 1/≤ TMST a lamellar morphology oriented parallel to the surface propagates through the entire specimen.
The specular reflectivity of neutrons has been used to characterize quantitatively the microphase separated morphology of symmetric, diblock copolymers of polystyrene, PS and polymethylmethacrylate, PMMA, as a function of the total molecular weight of the copolymer where either block is perdeuterated. It is shown that the hyperbolic tangent function, as opposed to a linear or cosine squared function, most closely describes the concentration gradient at the interface between the lamellar copolymer microdomains. The effective width of the interface is found to be independent of the molecular weight of the copolymer blocks and has a value of 50 ± 3Å. This interface is also found to be identical to that of PS and PMMA, homopolymers. However, using measured values of the Flory-Huggins interaction parameter for PS and PMMA current theoretical treatments cannot describe the observed widths of the interface.
X-ray and neutron reflectivity was used to determine the kinetics of adsorption and characterize the concentration profile of a low molecular weight trichlorosilane endfunctionalized polystyrene adsorbed onto polished silicon wafers. Higher adsorbed amounts were obtained from a cyclohexane solution of the polymer rather than toluene, with kinetics that followed a stretched exponential behavior. For moderately high grafting densities the polymer concentration profile in deuterated toluene (a good solvent) was best modelled using a small tailed parabola. In deuterated cyclohexane (a poor solvent) the brush profile steepened but was substantially smoother than a step, while in D20 (a non-solvent) it became step-like.
The thermal expansion of thin deuterated polystyrene (dPS) films supported on energetically repulsive, fluorinated polyimide (PI) substrates (PI/dPS bilayer) was measured via neutron reflectometry as a function of initial dPS film thickness. Film thickness was measured before and after capping with a top layer of the same repulsive, high glass transition polyimide that comprised the substrate layer (PI/dPS/PI trilayer) in an attempt to observe any effects of the dPS free surface in the bilayer geometry. Bulk thermal expansion behavior, characterized by a discontinuous change in coefficient of thermal expansion (CTE) at the glass transition (Tg), is observed in films with thickness > 70 rim. For thicknesses between 70 nm and 40 nm a transition is seen from bulk behavior in bilayer films to glassy thermal behavior in the trilayer films persisting up to 20 °C above the bulk Tg. In films with thickness < 40 nm the bulk glassy CTE persists well above the bulk Tg for both bilayer and trilayer films
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