Theyencheri Narayanan

Analysis of thermosensitive core–shell colloids by small-angle neutron scattering including contrast variation

We present an investigation of thermosensitive core–shell particles by small-angle neutron scattering (SANS). The particles consist of a solid poly(styrene) core and a shell of crosslinked poly(N-isopropylacrylamide) (PNIPA) chains. These latex particles are dispersed in water and have a diameter of ca. 150 nm. At ambient temperature the PNIPA-network in the shell is swollen but at higher temperature water is expelled and the shell undergoes a continuous volume transition. The radial extension of the shell is investigated as a function of temperature by use of SANS. The analysis by SANS is performed at different contrasts using appropriate mixtures of H2O and D2O. It demonstrates that the shell has a well-defined compact structure above the n volume transition. The swelling of the shell upon cooling can be described in terms of an affine expansion of the n network. This is followed by a slight decrease of the volume fraction with increasing distance to the surface of the cores. The analysis by SANS demonstrates that the phase behavior of the network in the shell may be undertaken in terms n of average volume fractions. It thus supplements the previous analysis by SAXS in a decisive manner.

Analysis of the correlation of counterions to rod-like macroions by anomalous small-angle X-ray scattering

We present a study of a rod-like polyelectrolyte by anomalous small-angle X-ray scattering (ASAXS). The polyelectrolyte consists of a stiff poly(p-phenylene) backbone with attached positive charged groups that are balanced by bromine counterions. The scattering data are taken far below the absorption edge (13u2006473.7 eV) and in its immediate neighborhood. The decrease of the measured intensity predicted by theory is directly observed. A new analysis of these ASAXS-data leads to three partial intensities in a numerically self-consistent fashion. In particular, the scattering intensity that is solely due to the cloud of the counterions could be determined and compared to the prediction of the Poisson–Boltzmann cell model. Quantitative agreement is found. ASAXS is thus shown to be a new and highly effective tool for the analysis of polyelectrolytes.

High-dynamic range SAXS data acquisition with an X-ray image intensifier

Data with a wide dynamic range of intensity can be collected with a pinhole high-brilliance small-angle X-ray scattering (SAXS) camera using an image-intensified charge-coupled device (CCD) detector. The point spread function (PSF) of this detector has a narrow peak with a broad low tail such that a high level of scattered intensity at small angles can cause a significant background in the detector elements at higher angles. A correction scheme for the long tail of the PSF of the detector is needed when this integrating area detector is used for measuring intensity that spans a dynamic range of four to five orders of magnitude. A procedure is described for measuring the PSF contribution by masking a small part of the detector from the scattered radiation with an absorbing material. In order to measure the PSF, it is necessary to use a high-intensity spot, which is readily achieved by using a sample that scatters strongly at small angles. Although this intensity is spread over many pixels, the sharp features in the scattering from the silica sample chosen for this study permit one to obtain simultaneously both the narrow and the broad parts of the PSF. The data are compared with the actual scattering function, which has been measured exactly with a point-geometry Bonse–Hart camera. The advantages of this procedure are discussed.