K. S. Liang

Structural defects in self‐assembled organic monolayers via combined atomic beam and x‐ray diffraction

We present the results of a combined He atom and x‐ray diffraction study of CH3(CH2)n−1SH monolayers self assembled on Au(111) surfaces. By combining these two complementary probes, we have characterized both the surface and the interior structure of the monolayers. In both cases, we find the same structure containing four molecules per unit mesh. However, we demonstrate that there are significant differences in both the diffraction linewidths and the dependence of the linewidth upon chain length for these two techniques.

Epitaxy and chainlength dependent strain in self-assembled monolayers

We use grazing incidence x-ray diffraction to systematically study the structure of an archetypal self-assembled monolayer as a function of the hydrocarbon chain length, n. The monolayers consists of n-alkyl thiol molecules, CH3(CH2)n−1SH (Cn, 10⩽n⩽30), self-assembled on single crystal Au(111) surfaces. At room temperature, the 2D structure is described by a C(4×2) unit mesh for all chain lengths. However, we demonstrate that there is a systematic dependence of the tilt structure (i.e., the tilt angle and tilt direction) of the hydrocarbon chains as a function of the chain length. Furthermore, we show that the monolayer structures are characterized by distinct “long’’ (n⩾16) and “short’’ (n⩽14) chain length regimes, as well as a smooth variation of the structural parameters within each regime. We associate these systematic structural changes with the conflicting requirements of epitaxy and molecular packing, and argue that the driving force is the changing intra-layer interaction strength (which is propor...

Morphology of miktoarm star block copolymers of styrene and isoprene

The morphologies of several styrene–isoprene miktoarm star copolymers have been examined by small angle x‐ray scattering and electron microscopy. Three of these polymers were of the three‐armed A2B type, and six were four‐armed A3B miktoarms. The ordered microphase‐separated morphologies displayed by these polymers were seen to be highly dependent on the architecture of the chains, and were quite different from those that occur in the corresponding linear block copolymers. These results can be quite well explained by a theory of Milner which is based on the bending energy of the microphase interface and the ability of the chains to stretch away from the interface between the microphases. We speculate that the small differences between the observations and theoretical predictions are due to the effects of the compression of chains in small gaps between adjacent domains.

The structure of n‐octadecane thiol monolayers self‐assembled on Au(001) studied by synchrotron x‐ray and helium atom diffraction

We report on a new, more precise structural characterization of n‐octadecane thiol monolayer self‐assembled on the UHV‐prepared Au(001) surface using a combination of helium atom diffraction, grazing incidence x‐ray diffraction, and x‐ray reflectivity measurements. Our results show that the equilibrated alkane chains form a two dimensional distorted hexagonal structure with lattice constants a=5.77±0.06 A, b=4.81±0.02 A, and α=53.1±0.46°. The alkane chains are tilted 33.5±1.0° from the surface normal and the tilt direction is ∼6.8±1.0° away from the elongated next‐nearest‐neighbor direction. The Au–thiol interface is found to have an unusual p(1×4) structure formed by an excess of Au atoms, which are believed to be produced in the dereconstruction process of the clean hexagonal Au(001) surface during the chemisorption of thiols. We propose that this Au interfacial layer modulates the height of the alkane chains resulting in a highly corrugated top surface, which forms a c(2×8) structure as observed by ato...

Structural study of small catalytic particles using differential anomalous x‐ray scattering

Using the differential anomalous x‐ray scattering (DAXS) technique, the diffuse diffraction pattern associated with small Pt particles has been successfully separated from the strong scattering of crystalline η‐Al2O3 as well as amorphous SiO2 supports. For samples with ≳2.5 wt. % Pt loading, the differential diffraction patterns unambiguously reveal that the Pt clusters are fcc microcrystals. The results demonstrate the feasibility of DAXS technique in structural study of catalytic particles and its advantage over EXAFS in revealing long‐range structure.

Effect of isomer on polymer blends miscibility: a synchrotron small angle X-ray investigation

Abstract We use synchrotron small angle X-ray scattering (SAXS), calorimetry and cloud point measurements to investigate the thermodynamic of mixing in blends of poly(cyclohexyl acrylate) with the ortho and meta isomer of poly(bromostyrene) [P2BrS, P3BrS]. PCHA is miscible with P2BrS and P3BrS with lower critical solution temperature (LCST). Measurements of the absolute scattered intensity enable determination of the Flory–Huggins χ -parameter within the framework of the de Gennes random phase approximation. The following order is observed: χ (PCHA/P2BrS) χ (PCHA/P3BrS).

Use of Synchrotron Radiation in Plastics Analysis

We demonstrate the application of synchrotron small angle X-ray scattering (SAXS) in measuring the thermodynamic properties of polymer blends. The polymers under investigation consist of poly(cyclohexyl acrylate) (PCHA) blended with the ortho and meta isomer of poly(bromostyrene) (P2BrS, P3BrS). Measurements of the absolute scattered intensity enable the determination of the second derivative of the Gibbs free energy of mixing with respect to the concentration, the location of the spinodal temperature, and the Flory-Huggins X-parameter.