Thermal and nuclear magnetic relaxation studies of water–sodium polystyrene sulfonate systems

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Abstract
The phase transition behavior of water–sodium polystyrene sulfonate (NaPSS) systems was investigated by differential scanning calorimetry (DSC), 1H and 23Na nuclear magnetic resonance spectroscopy (NMR) in a temperature range from 150 to 330 K, and in a water content (Wc = grams of water/gram of dry sample) range from 0 to 2.5. With slow cooling from the isotropic liquid state, the system formed the liquid crystalline state and the crystalline state. The temperature range of the liquid crystalline state became broader with deceasing Wc. When the system contained an excess amount of free water, the liquid crystalline state was not observed. The 1H longitudinal relaxation time (T1) of the system showed a minimum value in the temperature range where the liquid crystalline state was observed. At the same time, the 1H transverse relaxation time (T2) dropped at the transition temperature from the isotropic liquid to the liquid crystalline state (T*). The correlation time (τc) was calculated using modified Woessner's equations. The variation of τc as a function of temperature agreed well with the phase diagram obtained from DSC. The 23Na T1 and T2 values were correlated with Wc and temperature. The activation energy was calculated to be from 20 to 25 KJ/mol depending on Wc. The formation of the liquid crystalline phase of NaPSS is affected by 10–20 water molecules, which can be categorized as freezing bound water.