Solid State Nuclear Magnetic Resonance 22, 409–422 (2002) doi:10.1006/snmr.2002.0069 Sorption Isotherm Measurements by NMR Johannes Leisen,* ,1 Haskell W. Beckham,* and Michael Benham y *School of Textile and Fiber Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0295; and y Hiden Analytical, Warrington WA5 7UN, UK FESTSCHRIFT IN HONOR OF PROFESSOR HANS-WOLFGANG SPIESS ON THE OCCASION OF HIS 60TH BIRTHDAY An experimental setup is described for the automated recording of sorption isotherms by NMR experiments at precisely defined levels of relative humidity (RH). Implementation is demonstrated for a cotton fabric; Bloch decays, T 1 and T * 2 relaxation times were measured at predefined steps of increasing and decreasing relative humidities (RHs) so that a complete isotherm of NMR properties was obtained. Bloch decays were analyzed by fitting to relaxation functions consisting of a slow- and a fast-relaxing component. The fraction of slow-relaxing component was greater than the fraction of sorbed moisture determined from gravimetric sorption data. The excess slow-relaxing component was attributed to plasticized segments of the formerly rigid cellulose matrix. T 1 and T * 2 sorption isotherms exhibit hysteresis similar to gravimetric sorption isotherms. However, correlating RH to moisture content (MC) reveals that both relaxation constants depend only on MC, and not on the history of moisture exposure. # 2002 Elsevier Science (USA) Key Words: cotton; cellulose; sorption isotherm; hysteresis; NMR. INTRODUCTION Environmental moisture adsorbed by hygroscopic materials often impacts a variety of physical properties. For instance, the reliability of electronic circuit boards is adversely affected by moisture absorbed into the epoxies used as packaging materials [1]. Mechanical properties of paper are a strong function of the relative humidity (RH) [2]. Cotton apparel is generally appreciated as comfortable clothing in part due to its tendency to absorb perspiration. On the other hand, cotton fabrics are not used in high-performance sports wear since absorbed moisture leads to increased weight. These examples demonstrate the importance of measuring and understanding environmental moisture uptake and release by hygroscopic materials. Moisture uptake is typically measured with gravimetric methods. A sample is weighed after equilibration in an environment well defined by its temperature and RH. For a given temperature, the amount of moisture sorbed by a material depends on the RH, but also on the history of moisture exposure [3–5]. Beginning from a 1 To whom correspondence should be addressed. Tel.: 404-894-9241. Fax: 404-894-9766. E-mail: johannes.leisen@tfe.gatech.edu 409 0926-2040/02 $35.00 # 2002 Elsevier Science (USA) All rights reserved.