Time-resolved SAXS measurements

Grosso, D. Babonneau, F. Albouy, P. A. Amenitsch, H. Balkenende, A. R. Brunet-Bruneau, A. Rivory, J. 2002. An in situ study of mesostructured CTAB-silica film formation during dip coating using time-resolved SAXS and interferometry measurements. Chem. Mater. 14 931-939. 

Time-resolved SAXS studies report the time course of conformational changes following the addition of cations that trigger RNA folding. The ultimate time resolution of these experimental methods is determined both by the type of mixer used and the measurement time. 

Because of the short measurement times of stopped flow, or the small sample volumes associated with continuous flow, a synchrotron source is required for time-resolved SAXS studies of RNA folding. To date studies have been carried out at the APS, at CHESS, and at SSRL. The optics vary by beamline, but all involve intensity enhancement by an insertion device. Monochromatic undulator beam was employed for stopped flow experiments at the 12-ID station at APS (Seifert etal, 2000). Multilayer beam was employed at SSRL beamline 4-2 (Tsuruta et al, 1998), and focused multilayer beam was employed at the CHESS G1 station (Kazimirov et al., 2006). For the continuous flow cell, pink or 3% bandwidth undulator beam was employed at the 8-ID beamline at APS (Sandy et al., 1999). All experiments employed a CCD detector to record a 2D image of the scattering, as illustrated in Fig. 12.1. 

In order to examine, whether a change in ec — Qa takes place during the main crystallization of unoriented amorphous PET, Prieske et al. performed simultaneous measurements of time resolved SAXS and WAXS [41]. The WAXS was registered on a photographic film, which was changed automatically at time intervals of 60 sec. In the center of the films, holes of 2 cm in diameter were provided in order to permit... 

Combination of WAXS and SAXS is a very efficient way of detailed description of crystallization in blends of semicrystalbne polsmiers. Baldrian and co-workers (203,204) has studied isothermal melt crystallization of blends of low molecular weight poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) using time resolved SAXSAVAXS measurements on the ELETTRA synchrotron (205). He has reported the formation of unstable PEO lamellae of nonintegrally folded... 

With high power density and a small divergence of the incident beam, synchrotron X-rays offer improved tools for monitoring in-situ processes (following cryo-or heat treatment) such as crystallization, and the subsequent structural changes that occur in semi-crystalline polymers. Thus, the availability of synchrotron X-ray sources has enabled precise time-resolved SAXS/WAXS measurements to be made, using fast detectors. 

Time-resolved SAXS was observed from a series of the mixed solution of ET-PTMO and TEOS undergoing sol-gel reaction with the SAXES equipment installed at BL-IOC in Photon Factory, Tukuba, Japan. The sample code and the reaction conditions are summarized in Table 1, The SAXS measurement was started immediately after the cell was inserted in the cell holder block kept at 50 C. 98 measurements with an appropriate interval were repeated during the course of hydrosilylation, where each measuring time was 3 minutes. The excess scattering intensity at each reaction time was calculated by subtracting the scattering intensity at time 0. 

In the small-angle X-ray scattering (SAXS) regime the typical nanostructures (in semicrystalline materials, thermoplastic elastomers) are observed. Because of the long distance between sample and detector time-resolved measurements can only be carried out at synchrotron radiation sources (Sect. 4.2.1.2). 

Limits of Time-Resolved and Simultaneous Measurements. Structure evolution studies are based on the ability to carry out time-resolved scattering experiments. The power of this scattering technique is a function of the minimum cycle time during which a scattering pattern with sufficient signal-to-noise ratio can be recorded. As cycle times for anisotropic 2D SAXS patterns have fallen below a value... 

The classical treatment of diffuse SAXS (analysis and elimination) is restricted to isotropic scattering. Separation of its components is frequently impossible or resting on additional assumptions. Anyway, curves have to be manipulated one-by-one in a cumbersome procedure. Discussion of diffuse background can sometimes be avoided if investigations are resorting to time-resolved measurements and subsequent discussion of observed variations of SAXS pattern features. A background elimination procedure that does not require user intervention is based on spatial frequency filtering (cf. p. 140). 

The mainly demanded properties of these detectors are high sensitivity, a wide and linear dynamic range, and the ability to work with high photon rates. This requirements take into account, that especially with SAXS vast intensity differences occur and in the case of fast time resolved measurements, great amounts of photons have to be registered in relatively short time intervals. 

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