In situ vapor sorption apparatus for small-angle neutron scattering and its application

Abstract

An in situ vapor sorption apparatus has been constructed for use in small-angle neutron scattering (SANS) measurements. The apparatus adapts two independent operating mechanisms, with and without a carrier gas, to control relative or absolute pressure, respectively, in the SANS sorption cell. By controlling the absolute pressure, a target relative vapor pressure between 0% and 90% can be reached within 1–2 min. The short response time makes it possible to correlate diffusion kinetics and/or sorption/desorption isotherms with structure evolution during wetting/drying, which is not possible in gravimetric methods. Also, one can extract diffusion coefficients and interaction parameters. Other uses include the enhancement of scattering contrast in the study of semicrystalline polymers by the preferential vapor sorption of deuterated vapor into the amorphous regions. Thus, one can obtain the same structural information as small-angle x-ray scattering measurements on dry samples. Also, the apparatus has the capability to inject a pore-masking liquid into the sample cell while under vacuum to ensure the filling of all open porosities in a sample. The capability to mix two vapors in various ratios facilitates the determination of a contrast-matching point using a single sample, which also eliminates a major source of systematic error. The performance of the apparatus is demonstrated using a polyelectrolyte membrane and semicrystalline polyethylene. Additionally, technical points relating to controlling the relative vapor pressure, relative humidity, and regarding the vapor distribution in the cell are discussed.