Estimation of incoherent backgrounds in SANS studies of polymers

Abstract

Small-and intermediate-angle neutron scattering studies have been undertaken on mixtures of deuterated and protonated poly(ethylene terephthalate) (PET) molecules with a wide range of concentration. The samples were converted to random copolymers by transesterifying in the melt, thus removing the coherent component of the intensity associated with single-chain scattering. The transesterified samples were used to determine the level of incoherent scattering associated with each isotopic mixture which was compared with estimates from established background-correction techniques based on extrapolating from the scattering of fully protonated or deuterated blanks. Scaling via the measured sample transmission works well for predominantly protonated mixtures but fails when the fraction of deuterated polymer exceeds ~0D --0"6. Scaling via the component of the sample transmission attributable to the incoherent scattering from H nuclei works well over the whole concentration range. This method involves calculations of the sample transmission via the bound-atom cross section and this is a good approximation at the wavelength employed (A -4-75 A), where the hydrogen incoherent cross section is close to the bound-atom cross section (o'H -80 x 10 -28 m2). However, oH is a strong function of wavelength and the method cannot be used where the measured and calculated transmissions diverge. A recently developed theory describing the scattering from random copolymers was used to analyze the residual coherent cross section of the copolymers and led to a statistical segment length b = 14-3 +1 A, in good agreement with existing values for PET.

A Small‐Angle Neutron Scattering Study of γ‐Crystallins Near Their Isoelectric Point

“…The transmission of the sample was measured in a separate experiment (Dubner et al, 1990) by collimating the beam with slits (irises) approximately 1 cm in diameter, separated by a distance of about 7.5 m. A strongly scattering sample, porous carbon, was placed at the sample position to spread the beam over the whole detector. placed at a sample-detector distance of about 10 m. Without the carbon in position, the beam would either be blocked by the beam stop or concentrated in a few detector cells, with the possibility of saturating or damaging the detector.…”

Structural Characterization of a Polymer Substituted Fullerene (Flagellene) by Small Angle Neutron Scattering

“…Although the contribution of this component is subtracted from the Q = 0 cross section [via equation (4)], this limitation may be the cause of the slight differences in the shapes of the measured and fitted scattering at finite Q [e.g. fig.…”

“…The transmission of the sample was measured in a separate experiment as described previously', 4 . Measurements were performed at room temperature (T = 23°C) and typical values of the empty quartz cell and sample transmissions were T=0.95 and T=0.68 respectively for a 5 mm path length.…”

Analysis of neutron scattering data: Visualization and parameter estimation

“…A traditional evaluation [Dubner et al (1990); Chen and Lin (1987)] of the macroscopic SANS crosssection for a sample requires the measurement and correction for contributions to scattered intensity from the following instrument components or a sample container: the so-called empty container signal Ie; the ambient background, lb, present during the measurement from external sources when a sheet of cadmium (Cd, a strong neutron absorber) is blocking the beam; and a correction for the efficiency of the SANS camera detector, which is itself evaluated by measurement of a scattering sample. Also required is the evaluation of the transmission oft he sample (and, in most cases, the empty sample container) to correct for attenuation, caused by either absorption or scattering: of the incident neutron beam in traversing the sample.…”