Comprehensive X-ray reflectivity (XR) studies were conducted to characterize the structure of thin polyalkylsilicate films made of a poly(methylsilsesquioxane-co-ethylenylsilsesquioxane) precursor containing a star-shaped poly( -caprolactone) as a pore generator (porogen). The films were deposited on silicon wafer substrates by spin-coating and subsequently cured at various temperatures. Such spin-on glasses have a potential application as a low-dielectric-constant material for advanced semiconductors. Because highintensity synchrotron X-ray radiation was used, the XR data could be measured over 9 orders of magnitude in intensity, which facilitated the observation of fine structural details. A hierarchical fitting procedure for modeling the XR data is given. By evaluation of the critical angle of total reflection of the film material, which was observed at smallest angles, in particular the film electron density could be determined with a high accuracy. The films cured at 420 °C show a lower electron density as compared to those cured at 250 °C. This is explained by the fact that at the higher temperature the porogen is calcined and escapes from the films leaving behind a nanoporous structure. Film porosities could be estimated from the observed changes in the electron densities. From the very large number of high-frequency oscillations observed in the XR curves, it is concluded that the films exhibit a homogeneous, well-defined structure with small interface and surface roughness. The film thickness could be determined with an accuracy of (1 Å. The observation of an additional low-frequency modulation of the XR curves revealed a surface skin layer with a thickness of ca. 45 Å and with a slightly increased electron density as compared to the bulk of the film.

The small-angle X-ray scattering (SAXS) beamline BL4C1 at the 2.5 GeV storage ring of the Pohang Accelerator Laboratory (PAL) has been in its first year of operation since August 2000. During this first stage it could meet the basic requirements of the rapidly growing domestic SAXS user community, which has been carrying out measurements mainly on various polymer systems. The X-ray source is a bending magnet which produces white radiation with a critical energy of 5.5 keV. A synthetic double multilayer monochromator selects quasi-monochromatic radiation with a bandwidth of ca. 1.5%. This relatively low degree of monochromatization is sufficient for most SAXS measurements and allows a considerably higher flux at the sample as compared to monochromators using single crystals. Higher harmonics from the monochromator are rejected by reflection from a flat mirror, and a slit system is installed for collimation. A charge-coupled device (CCD) system, two one-dimensional photodiode arrays (PDA) and imaging plates (IP) are available as detectors. The overall performance of the beamline optics and of the detector systems has been checked using various standard samples. While the CCD and PDA detectors are well-suited for diffraction measurements, they give unsatisfactory data from weakly scattering samples, due to their high intrinsic noise. By using the IP system smooth scattering curves could be obtained in a wide dynamic range. In the second stage, starting from August 2001, the beamline will be upgraded with additional slits, focusing optics and gas-filled proportional detectors.

The phase behavior of deuterated polystyrene-block-poly(n-pentyl methacrylate) copolymers (dPS-PnPMA) was investigated by using small-angle X-ray (SAXS) and neutron (SANS) scatterings and rheology. This block copolymer exhibited a closed-loop type of phase behavior as did hydrogenated PS-PnPMA copolymers. The closed loop consists of two transitions: lower disorder-to-order transition (LDOT) and upper order-to-disorder transition (UODT) occurring at a lower and higher temperature, respectively. The segmental interaction parameter (χ) between dPS and PnPMA blocks at various temperatures was obtained by fitting the incompressible random phase approximation theory to SANS results using a low molecular weight of dPS-PnPMA exhibiting a disordered homogeneous state over the entire temperature range. We found that at higher temperatures χ increased initially with increasing temperature (T), achieved a maximum, and then decreased. Such behavior is in keeping with polymer blends or block copolymers that would exhibit a closed-loop phase behavior. However, at temperatures below LDOT, χ did not decrease monotonically with decreasing T. Rather, with decreasing T, χ decreased initially, achieved a minimum, and then increased again. This strongly suggests that another transition of order-to-disorder transition (ODT) might be expected at lower temperatures.

effects of lead on microtubules in the root meristem. LeadLead is an environmental pollutant that interferes with plant growth. Unfortunately, the mechanisms of lead toxicity in treatment perturbed the alignment of microtubules in a concentration-dependent manner beginning at 10 mM. Microtubules of plants are still poorly understood. In this study, we have investigated both the deposition sites and sources of cellular different regions of the root meristem and in different stages of the cell cycle showed differential susceptibility to lead. These toxicity of lead in maize seedlings (Zea mays L. cv. Golden Cross Bantam). Using atomic absorption spectroscopy and effects do not appear to be general phenomena common to toxic metals, since aluminum and copper, at concentrations that X-ray fluorescence microprobing, we show that lead accumulation is highest in the root meristem, and that the accumulation decreased root growth to a comparable level, did not have the same detrimental effects on microtubules. Based on these occurs both in the apoplast and symplast. Since cells are results, we suggest that the damage to microtubules is partly dividing vigorously in this region and because microtubules play responsible for lead-associated toxicity in plants. an important role in cell division, we have further examined the various pollutants, including heavy metals (Ma et al. 1995, Panda et al. 1997, Kovalchuk et al. 1998, Steinkellner et al. 1998. Because formation of multiple micronuclei results from improper nuclear separation and cytokinetic disorder, we suspect that the components of the processes may be directly susceptible to toxic metals. Microtubules play key roles in both nuclear division and cytokinesis in plants. Thus, in the present work, we examined lead deposition sites in seedlings of maize and the effects of lead on the microtubule organization in the region of root where lead deposition was the highest. Materials and methods Plant materialMaize (Zea mays L. cv. Golden Cross Bantam) seeds were sterilized for 15 min with 0.5% calcium hypochlorite, washed several times with tap water and soaked overnight in

The imidization behavior and structural evolution in a microscaled film of poly(3,4′-oxydiphenylene pyromellitamic acid) precursor are studied by time-resolved synchrotron wide-angle X-ray diffraction and infrared spectroscopy to investigate the relationship between thermal imidization and structural evolution in the precursor. The precursor film displays only short-range order, but its polyimide film shows a crystalline structure based on an orthorhombic crystal lattice unit. When the precursor is heated at 2.0°C/min, it undergoes imidization over the temperature range 124-310°C through a two-step process: (i) decomplexation of the amide linkage from residual solvent molecules and other intra-and intermolecular amic acid groups and (ii) imide-ring closure. The maximum rate of imidization occurs at 148.4°C. Anhydride rings are found to form transiently over the range 93-310°C, which are attributed to the nature of the equilibrium between the precursor and its constituent anhydride-and amino-terminated species. The imidization reaction begins prior to the commencement of structural evolution. The structural evolution takes place over 132-380°C as a three-step process: initiation, primary growth, and secondary growth. In particular, the initiation step requires at least 3.2% imidization. The structural evolution is further influenced by the short-range ordered structure formed in the precursor film in the process of film formation. However, the overall crystallinity in the fully imidized film is limited to only 21.4%.