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A pinhole small-angle x-ray scattering (SAXS) instrument was constructed at the SUNY X3A2 beamline, National Synchrotron Light Source, Brookhaven National Laboratory. The three pinholes were mounted in a thick-walled stainless steel pipe and prealigned by using a portable laser source and a charge-coupled device (CCD) area detector. After the prealignment, incorporation of the collimator to the synchrotron x-ray source required only maximization of the incident x-ray intensity passing through the pinholes, which could be done easily by using a scintillation counter after proper attenuation. The entire synchrotron SAXS instrument setup took only a few hours even without stepping motor control for the pinhole collimator unit. By combining this collimator with a CCD-based x-ray area detector which could be assembled by using commercially available components, the SAXS instrument showed good performance for structural scales up to an order of 100 nm. The CCD-based x-ray area detector used a computer- (or manually) controlled intensified unit with a variable gain setting in order to accommodate the changing x-ray flux and to protect the detector from over exposure, a necessary feature for operation of an area detector at a synchrotron light source.

SYNOPSISPolyurethanes were synthesized from polyester and butanediol with three different diisocyanates, i.e., 4,4'-diphenylmethane diisocyanate (MDI), m-xylene diisocyanate (XDI), and 2,4-toluene diisocyanate (TDI). The effect of chemical structures of diisocyanate compounds on the degree of crystallinity and the thermal stability were observed. Differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS) were used to determine the degree of crystallinity of the hard segment. The thermal degradation of polyurethanes was studied by the thermogravimetric method. It has been shown that the polyurethane hardsegment crystallinity decreases in the following order: MDI > XDI > TDI. The experimental results also indicated that polyurethanes with aralkyl diisocyanates, i.e., XDI, had the best thermal stability. The polyurethanes synthesized from aromatic diisocyanates, i.e., MDI and TDI, had worse thermal stability than from XDI. However, owing to the higher degree of hard-segment crystallinity for polyurethanes from MDI, these polyurethanes had a better thermal stability than those based on TDI. 0 1996 John Wiley & Sons, Inc. I NTRO DUCT10 NProperties of segmented polyurethanes depend on the structure of hard and soft segments, their length, and concentration, as well as the interaction of both phases. Usually the soft segment is based on polytetramethylene oxide or aliphatic polyester and the hard segment on butanediol (BD) and diisocyanate compounds such as diphenylmethane diisocyanate (MDI), xylene diisocyanate (XDI), and toluene diisocyanate (TDI). Being of different chemical nature from the soft segments, the hard segment tend to separate in the matrix due to immiscibility of both phases and tend to associate into "domains" forming physical crosslinks.'-6 While hard segment is glassy or semicrystalline at room temperature, the soft segment phase is in the rubbery state. Polyesters synthesized from adipic acid and low molecular weight glycols are crystalline homopolymers having melting points around 50°C. However, polyester soft segments in polyurethanes lose the ability to crystallize for short segment length, due to steric hindrances. Because crystallization of the soft segments is an undesirable property, it can be avoided by copolymerization or, as in some commercial prepolymers, by inserting TDI or other irregularities into the soft segments. Generally speaking, polyurethanes are thermally very stable polymers. The onset degradation temperature of the urethane bond depends on the type of isocyanate and glycol used. It is a general rule that the more easily formed urethanes are less stable, i.e., more easily dissociated than are the more difficult ones. Thus, the highest degradation temperature, above 250°C, is observed for a urethane formed from alkyl isocyanate and alkyl alcohol, followed by aryl isocyanate-alkyl alcohol, alkyl isocyanate-aryl alcohol, and an aryl isocyanate-aryl alcohol corn bin at ion^.^,^ In this work, crystallinity and thermal stability of segmented polyurethanes based o...

Employing methods of computational fluid dynamics, we investigated the physical phenomena and fluid dynamics of a microfluid during ejection of a droplet with a designed system of a nozzle plate connected to a flat-plate piezoelectric material. A comparison between experimental measurements and numerical simulations was devised to validate the theoretical model. The volume-of-fluid piecewise linear-interface construction (VOF-PLIC) interface-capturing method was adapted to represent the fluid domain and to track the evolution of its free boundaries whereas the continuous surface force (CSF) mode was chosen to model the interfacial physics. The results show that the curvature of the flow channel affects the velocity, period before disintegration, volume of the droplet and number of satellite drops. Increasing the diameter of the orifice increases the volume and decreases the velocity of the droplet. Increasing the amplitude or frequency of the nozzle plate raises the input energy, so increasing the velocity, decreasing the volume and hastening the disintegration of the droplet, but an increased amplitude or frequency increases the number of satellite drops. At the hydrophobic boundary, the velocity increases, the droplet volume decreases and the period before disintegration is abbreviated because of the decreased adhesive force between the fluid and the boundary surface.