ALBERT

Notes: We demonstrate that x-ray reflection standing wave fluorescence spectroscopy is a highly sensitive technique for the determination of the distribution of metal nanoparticles inside polymer thin films. We investigate both the depth profile and the in-plane spatial correlation of gold nanoparticles in an asymmetric polystyrene-b-poly(2-vinylpyridine) block copolymer film. These copolymer films self-assemble into alternating, nanometer-sized domains that are, upon annealing, selectively decorated by thermally evaporated metal nanoparticles to form metal/polymer composites. © 1999 American Institute of Physics.

Notes: This paper reports the results of experimental studies of the dynamics of particles in a dense quasi-two-dimensional colloidal liquid. We find that at high density, near close packing but still in the liquid phase, the spatial configurations of the particles in the colloidal liquid consist of small ordered domains separated by disordered boundaries. There are frequent exchanges of particles between the ordered and disordered domains, so the lifetime of a particular ordered domain is short and the state of the system is ergodic. The motion of a particle in an ordered domain is constrained but fully two dimensional. The motion of a particle in a disordered boundary has considerable one-dimensional file-server character. By virtue of exchanges of particles between the ordered and disordered domains, the time dependence of the particle displacement has mixed character. We find that the particle dynamics in the dense quasi-two-dimensional colloidal liquid can be characterized with three simultaneous competing relaxation processes, each of which contributes to the mean square displacement of a particle, W(t). For an interval that is shorter than the time between collisions, W(t) is linear in time; this behavior results from particle displacements within a cage of fluctuating neighbors. At somewhat longer time W(t) has a sublinear dependence on time. We associate this temporal behavior with a file-server-like contribution to W(t), arising from correlated motion in the disordered boundary regions. At very long time we find that there are contributions to W(t) from infrequent large displacements, of the order of a particle diameter in length. The large displacements are associated with density fluctuations that fluidize the surroundings of a particle. These large displacements are not ballistic; rather the particle diffuses rapidly enough to generate a large displacement per unit time. The common occurrence of structural heterogeneity in simulation studies of systems with different particle–particle interactions and different equations of motion suggests that fluctuations that generate local order in a dense two-dimensional liquid may be a ubiquitous feature of that class of systems. The results of the experiments reported in this paper suggest that current theories of relaxation processes in a dense quasi-two-dimensional liquid do not adequately take into account the class of fluctuations that generates transient ordered domains in the liquid; nor do these theories adequately account for the cooperative relaxation of those domains. © 2001 American Institute of Physics.

Notes: We report the results of studies, using digital video microscopy, of the spatial distribution of particles in a quasi-one-dimensional colloidal liquid. The system studied consists of a water suspension of silica particles with diameter σ=1.58 μm confined in a 3 μm×3 μm×2 mm silicone elastomer channel. The measured spatial distributions have been characterized by the nearest-neighbor distribution function and the pair correlation function, and the pair correlation function has been inverted, using the hypernetted chain approximation, to yield the effective colloid–colloid interaction. The effective colloid–colloid interaction is found to have an attractive potential well with depth of ∼0.3kBT at a colloid-colloid separation R(approximate)1.18σ. Molecular-dynamics simulations of the nearest neighbor and pair correlation functions carried out using the empirically determined colloid–colloid interaction are in very good agreement with the experimental data. The experimental data obtained rule out any important charge–charge contribution to the measured colloid–colloid interaction. It is suggested that because the host liquid does not wet the channel wall, and the colloid particles are tightly confined in the long narrow channel, capillary forces determine the effective colloid–colloid interaction. This suggestion is consistent with the determination (also reported) that the effective colloid–colloid interaction in a quasi-two-dimensional suspension of the same particles is different from that in the quasi-one dimensional suspension. © 2002 American Institute of Physics.

Notes: The in-plane angular distribution of light scattered from a water supported monolayer of a diblock copolymer of polystyrene and polymethyl methacrylate has been measured. These data lead to the inference that the polymer molecules aggregate into discs with diameter 3800 A(ring), sensibly independent of surface coverage in the range 6.0–18.0 A(ring)2/monomer. There is no sign of this aggregation in the monolayer isotherm.

Notes: We report the results of a grazing incidence x-ray diffraction study of monolayers of F(CF2)11COOH and F(CF2)10CH2COOH on water at several temperatures. It is found that in both monolayers the amphiphile molecules have a very small collective tilt angle (relative to the normal to the surface). For the temperature range studied (∼4–30 °C) the data for both monolayers indicate the existence of only a first-order transition between an ordered condensed phase and a disordered dilute phase; there is no evidence, in this temperature range, for the existence of a continuous tilt transition such as has been observed in monolayers of arachidic acid. The experimental data suggest that the critical point for the transition observed is at a lower temperature for a monolayer of F(CF2)11COOH than for a monolayer of F(CF2)10CH2COOH.

Notes: We report the results of static and dynamic evanescent wave light scattering studies of a monolayer of a diblock copolymer, polystyrene-b-polymethylmethacrylate (PS-b-PMMA) with weight averaged molecular weights (Mw) of 880 000:290 000 supported at the air/water interface. Our studies probe the interfacial structural and dynamic properties of the monolayer on a length scale which is a fraction of the wavelength of light. The static light scattering studies were carried out as a function of polymer surface coverage and temperature; we also report some preliminary data for the dependence of the static structure function on the relative molecular weights of the PS and PMMA blocks. The complementary dynamic light scattering studies were carried out only as a function of surface coverage. Our data suggest that, upon spreading in the air/water interface, PS-b-PMMA (880:290 K) copolymers form thin disklike aggregates containing about 240 molecules. These data are consistent with a model in which each such aggregate is a "furry disk'' with a dense core consisting of a layer of collapsed PS blocks atop a thin layer of extended PMMA blocks on the water surface and a brushlike boundary of extended PMMA blocks. The data show that the furry disks diffuse freely when the surface coverage is small, but when the surface coverage is large, they are immobile. Our data also suggest that the furry disks can aggregate to form even larger "islands'' of disks with an extension greater than 20 μm. The static structure function of the assembly of furry disks is well described, over a wide range of surface coverage, by the structure factor of a two-dimensional hard disk fluid modulated by a two-dimensional hard disk form factor.

Notes: We present results of an extensive x-ray diffraction study of a monolayer of C10F21CH2COOH spread on water (pH2) at 19.1 °C. Lever-rule analysis of the in-plane scattering is used to show that there is a coexistence region between ordered condensed islands and a dilute disordered phase. The coexistence region is found to be bounded by molecular areas of 29 A(ring)2 and 2000±600 A(ring)2, in agreement with the pressure-area isotherm. The molecular tilt of the ordered phase remains unchanged from closest packing (near collapse of the monolayer) throughout the coexistence region, and has a value of 2±3 degrees with respect to the normal to the liquid surface. These results are contrasted with those for hydrocarbon monolayers in which the onset of order in the coexistence region is close to the condensed phase boundary, and the ordered phase is compressible with a continuously variable tilt angle ranging from 30 to 0 deg at closest packing. The differences are attributed to the enhanced chain stiffness of the fluorinated chain. To illustrate this interpretation, preliminary scattering results are presented for a monolayer of C8F17(CH2)4COOH on water, which has some of the features of the aliphatic lipid monolayers. Recent molecular dynamics simulations have been found to reproduce all the important qualitative features of these systems [S. Shin, N. Collazo, and S. A. Rice, J. Chem. Phys. 96, xxxx (1992)].

Notes: X-ray speckles due to scattering of partially (transverse) coherent x rays from an optical reflection grating are observed. The speckles indicate the presence of surface inhomogeneities of the grating that are otherwise undetectable with either visible laser light or transversely incoherent x-ray scattering. Qualitative analysis of the speckle patterns provide information on the surface morphology of the grating. The underlying order due to the periodicity of the grating enhances the detection of the speckles. © 1998 American Institute of Physics.