Water content, relative permittivity, and ion sorption properties of polymers for membrane desalination
Water content, relative permittivity, and ion sorption properties of polymers for membrane desalination
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DOI：
10.1016/j.memsci.2018.12.048
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发表时间：
2019-03-15
影响因子：
9.5
通讯作者：
Geise, Geoffrey M.
中科院分区：
文献类型：
--
作者：
Chang, Kevin;Luo, Hongxi;Geise, Geoffrey M.
使用交联聚甲基丙烯酸缩水甘油酯 (XL-pGMA) 聚合物研究了聚合物水含量、介电相对介电常数和离子吸附性能之间的相互作用。通过聚合物侧链上环氧化物环的部分水解，增加基础XL-pGMA聚合物的含水量，制备一系列水解XL-pGMA材料。在 45 MHz 至 26.5 GHz 范围内进行的微波介电谱显示，与 Nafion (R) 117 相比，在给定的水含量下，其相对介电常数较低。这一观察结果表明，仅了解聚合物水含量可能不足以估计水合聚合物的相对介电常数特性。水状态分析表明（对于吸附水总量相似的聚合物）与 Nafion (R) 117 中的水相比，水解 XL-pGMA 中更多的水与聚合物相互作用，这一结果可能与观察到的两种材料相对介电常数特性的差异有关。与文献中报道的许多不带电材料相比，水解聚合物具有更高的水/离子吸附选择性，这对于海水淡化应用很重要，但与 Nafion (R) 117 相比选择性较低。膜相平均离子活度系数表明，Nafion (R) 117 比水解 XL-pGMA 材料在热力学上更理想（即，平均离子活度系数更接近统一），因此，离子排除Nafion (R) 117 主要是由于 Donnan 排除所致。测量的静态相对介电常数值用于通过静电理论估计每种材料中离子吸附的自由能垒，并且这些值与使用测量的离子吸附数据确定的值定性一致。这项研究表明，聚合物化学（而不仅仅是水含量）会影响水合聚合物的相对介电常数特性，并且相对介电常数测量可以提供对离子吸附特性的定性洞察，这对于设计先进的海水淡化膜非常重要。
The interplay of polymer water content, dielectric relative permittivity, and ion sorption properties was studied using a cross-linked poly(glycidyl methacrylate) (XL-pGMA) polymer. The water content of the base XL-pGMA polymer was increased, by partial hydrolysis of the epoxide ring on the polymer side chain, to prepare a series of hydrolyzed XL-pGMA materials. Microwave dielectric spectroscopy, performed from 45 MHz to 26.5 GHz, revealed lower relative permittivity at a given water content compared to Nafion (R) 117. This observation suggests that knowledge of polymer water content alone may be insufficient for estimating relative permittivity properties of hydrated polymers. State of water analysis suggested that (for polymers that contain similar total amounts of sorbed water) more water in hydrolyzed XL-pGMA interacts with the polymer compared to that in Nafion (R) 117, and this result may be related to the observed differences in the relative permittivity properties of the two materials. The hydrolyzed polymers were more water/ion sorption selective, important for desalination applications, compared to many uncharged materials reported in the literature but less selective compared to Nafion (R) 117. Membrane phase mean ionic activity coefficients suggest that Nafion (R) 117 is more thermodynamically ideal (i.e., the mean ionic activity coefficients are closer to unity) than the hydrolyzed XL-pGMA materials, and thus, ion exclusion in Nafion (R) 117 is primarily due to Donnan exclusion. Measured static relative permittivity values were used to estimate, via electrostatic theory, the free energy barrier for ion sorption in each material, and these values were qualitatively consistent with values determined using measured ion sorption data. This study suggests that polymer chemistry, not water content alone, influences the relative permittivity properties of hydrated polymers and that relative permittivity measurements can provide qualitative insight into ion sorption properties, which is important for designing advanced desalination membranes.