B. Chu

Solution behavior of buckminsterfullerene (C60) in benzene

Solution characterization of buckminsterfullerene C60 in benzene was carried out by laser light scattering. Static and dynamic light scattering (SLS and DLS) experiments were accomplished by using an incident laser beam of 790 nm wavelength, and a fiber optic probe which was coupled to a RCA C31034 photomultiplier tube sensitive to the near ir wavelength. The effective hydrodynamic radius of C60 in benzene solution determined by using DLS was 6.4±0.1 A, in close agreement with the distance measured from the center of the solvated benzene to the center of the benzene‐solvated C60 (C60⋅4C6H6):6.7∼6.8 A as evaluated from the single crystal structure by x‐ray diffraction. C60 was found to aggregate slowly even in fairly dilute solution concentrations ranging from 0.78 to 1.39 mg/mL at temperatures 23∼27u2009°C. Based on the high specific refractive index increment of C60 in benzene, the molar mass of the C60 aggregates could be estimated. Together with the radius of gyration, the hydrodynamic radius, and its size...

A fiber‐optic light‐scattering spectrometer

A new fiber‐optic light‐scattering spectrometer, which uses fiber‐optic detector probes each comprising of an optical fiber and a graded index microlens is described. The fiber‐optic detectors form an integral part of the scattering cell, which no longer requires transparent windows for the exit (or entrance) light beam. This feature eliminates the need for a goniometer which is one of the most bulky and expensive components of a conventional light‐scattering spectrometer. Thus, the spectrometer, unlike all of its predecessors, has no moving parts. Our fiber‐optic light‐scattering spectrometer can utilize a reaction vessel of whichever shape or composition as the scattering cell for light‐scattering measurements because the fiber‐optic detector probes are directly immersed in the scattering medium. Furthermore, with the miniaturization using the optical fiber/microlens combination, the light‐scattering spectrometer can be held in the palm of one hand and the signals processed by standard optical fiber remote sensing techniques.

Intensity and Linewidth Studies of the System Polystyrene–Cyclohexane in the Critical Region

The cloud‐point curve and critical opalescence of the system polystyrene (Mnu2009=u2009392 000)–cyclohexane have been investigated. The critical point, as determined by the phase‐volume ratio method, is at a higher concentration than the maximum in the cloud‐point curve. Experiments are reported on the angular and the spectral distribution of scattered light for a solution of composition corresponding to the maximum in the cloud‐point curve and on the spectral distribution for a solution of critical concentration. The temperature dependence of the extrapolated zero‐angle scattered intensity obeys a relation lim K→0 IC−1∝(Tu2009−u2009Tsp)γ with γu2009=u20091.26u2009±u20090.08. The long‐range correlation length ξs obeys a relation ξs∝(Tu2009−u2009Tsp)−νs with νsu2009=u20090.62. The extrapolated linewidth obeys a relation lim K→0 (Γu2009/u2009K2)∝(Tu2009−u2009Tsp)γ* with γ*u2009≈u20091 at the concentration exhibiting the maximum phase separation temperature (Tp,max), and γ*u2009≈u20090.77 at the critical solution concentration (Tspu2009=u2009Tc). The long‐range correlation length of the critica...

Density of methoxybenzylidene butylaniline about the isotropic–nematic phase transition

We have measured the density and determined the coefficient of expansion of MBBA about its isotropic−nematic transition temperature using a Mettler/Paar density meter. Our results show essential agreement with those of Press and Arrott using a buoyancy method. The small difference (<0.3°C) between the actual transition temperature and the virtual critical temperature differs from that determined by light scattering studies which yield values of about 1°C. Further examination on the nature of the virtual critical temperature is suggested.

Laser light scattering study of a ternary liquid mixture: Ethanol‐water‐chloroform

The intensity and Rayleigh linewidth of light scattered by concentration fluctuations in a ternary ethanol‐water‐chloroform system have been studied as a function of angle and temperature. Critical exponents γx, νx, and η obtained from the angular distribution of scattered intensity measured at constant compositions and different temperature distances from the plait point gave values of 1.50, 0.765, and 0.077, respectively. The results demonstrate the critical exponent renormalization, show first evidence of the nonzero critical exponent η in a three‐component fluid system, and are not inconsistent with the scaling relations. In view of the renormalization which provides us with a stronger temperature dependence on the correlation length ξ, the critical region (K ξ> 1) can be reached even at T − Tc = 0.5°C. Thus, we are able to compare our experimental linewidth and hydrodynamic shear viscosity data with the Kawasaki theory at very large values of K ξ ≃ 50. Our experimental observations agree very well wi...

Kratky block-collimation small-angle x-ray diffractometer for synchrotron radiation

A block slit collimation system of the Kratky design concept has been modified and adapted for synchrotron radiation. The building block small‐angle x‐ray diffractometer (SAXD) not only retains the essential advantages of the Kratky camera, i.e., accessibility at small values of q[=(4π/λ)sin(θ/2) with λ and θ being the x‐ray wavelength and the scattering angle, respectively] and ease of alignment, but also provides portability and low construction cost. With our SAXD operating at the SUNY X21A beamline of the National Synchrotron Light Source, we were able to reach θ∼1 mrad corresponding to q∼0.04 nm−1 (or a Bragg spacing of 150 nm) using an incident slit width (d) of 0.5 mm and λ=0.15 nm. At d=0.5 mm, ∼10% of the monochromatic synchrotron x‐ray radiation passed through the block collimation system and a parasitic intensity to main beam intensity ratio of ∼10−5 at θ=1 mrad was achieved. With d∼0.1 mm, it is anticipated that q ∼0.01 nm−1 can be accessible. By changing the vacuum tube length between the sam...

Dynamics of polyisoprene in star block copolymers confined in microstructures: A dielectric spectroscopy study

Dielectric spectroscopy has been employed to study selectively the local and global dynamics of polyisoprene (PI) in three microphase separated star diblock copolymers (SI)4, where PI and polystyrene (PS) form the core and corona, respectively. The thermally induced order-to-disorder transition (ODT) has been identified by rheology and small-angle x-ray scattering (SAXS). All dielectric measurements were made at temperatures well below the ODT and the polystyrene glass transition temperatures, where the four PI chain ends are tethered by the glassy polystyrene domains. The ordered state morphology, studied by SAXS, revealed the formation of PS spheres (fPS=0.25, where fPS is the polystyrene volume fraction), polyisoprene cylinders (fPS=0.68), and a lamellar structure (fPS=0.41) in the three copolymers. In contrast to the local segmental motions, the chain orientational dynamics associated with the amplitude and the characteristic relaxation time are strongly influenced by the spatial confinement. The main...

Least‐squares integration method in intensity fluctuation spectroscopy of macromolecular solutions with bimodal distributions

This letter presents a new least‐squares integration method of analysis for time‐dependent correlation functions consisting of two exponentials. It also contains an investigation of the effect of sample time on the accuracy that can be achieved in a given intensity correlation measurement by means of 24‐ and 48‐channel single‐clipped photon‐correlation spectrometers. Comparison is made with the least‐squares normal equation method using experimental as well as simulated results.

Low-density polymer thin film formation in supercritical carbon dioxide

We report a method for producing stable low-density polymer films by using supercritical carbon dioxide (scCO2). Two different molecular weight polystyrene films with various thicknesses were exposed to scCO2 along the density fluctuation ridge in P–T phase diagram. The swollen structures could be then frozen by flash evaporation of CO2 without forming additional voids. X-ray reflectivity data clearly showed that exposure to scCO2 could be used to produce uniform low-density films of about 2Rg thick or less, where Rg is radius of polymer gyration.

Prism laser light‐scattering spectrometer

A prism light‐scattering spectrometer capable of measuring both the angular distribution of scattered intensity and Rayleigh linewidth is described. The spectrometer employs a unique prism light‐scattering cell capable of making small‐angle self‐beating dynamic light‐scattering measurements down to a scattering angle of 2° and is able to achieve an optical coherence factor β∼0.9 routinely, with the theoretical limit for β being 1. The prism light‐scattering cell also permits refractive index measurements in the 1.33–1.60 range to a precision of 8×10−7 using a single‐axis lateral position sensor. It is a flow cell with potential as a detector for other analytical techniques once the cell volume can be minimized. The apparatus can be controlled by a computer for measurements, as well as acquisition, display, computation, and storage of data.