WATER BINDING AND IRREVERSIBLE DEHYDRATION PROCESSES IN CELLULOSE-ACETATE MEMBRANES
WATER BINDING AND IRREVERSIBLE DEHYDRATION PROCESSES IN CELLULOSE-ACETATE MEMBRANES
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DOI：
10.1002/app.1973.070170725
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发表时间：
1973-01-01
影响因子：
3
通讯作者：
MESSALEM, RM
中科院分区：
文献类型：
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作者：
FROMMER, MA;SHPORER, M;MESSALEM, RM
通过核磁共振技术，从自由感应衰变中水组分的初始高度（MNR强度）确定了各种水湿醋酸纤维素（CA）膜中冷冻水和非冷冻水的相对含量。结果表明：(1)各种湿CA膜中相当一部分水不冻结，可能是因为与聚合物的强相互作用；(2)非冻结水的弛豫时间st2在毫秒量级，表明它们与冰相比仍然具有很强的流动性；(3)致密CA膜或相对湿度为0.93的平衡膜中所含的所有水在将膜从室温冷却到- 60℃时不冻结；(4)膜中不冻结的结合水的量高于同一聚合物的致密膜从液态水中吸收的总水量。然而，从“相对核磁共振强度”中计算出的各种CA膜中非凝结水的数量，在假设膜中水的熔合热与纯水的熔合热相同的情况下，大大低于从DSC熔融吸热计算出的数量。讨论了造成这种差异的各种可能原因。对第一次脱附-吸附循环的测量也进行了weca膜。他们认为，在第一次脱水过程中，膜结构发生了不可逆的变化，导致聚合物与水相互作用的可及性显著降低。膜结构中这些不可逆变化的程度取决于脱水过程的细节，在更高的温度下更明显。
The relative amounts of freezing and nonfreezing water in various water‐wet cellulose acetate (CA) membranes were determined by NMR techniques, from the initial heights of the water component in the free induction decay (MNR intensity). The results suggest that (1) a significant fraction of the water in various wet CA membranes does not freeze, probably because of strong interaction with the polymer; (2) the relaxation timesT2of the nonfreezing water are of the order of milliseconds indicating that they are still highly mobile compared with ice; (3) all the water contained in dense CA films or in membranes equilibrated at relative humidity of 0.93 does not freeze upon cooling the membranes from room temperature to −60°C; (4) the amounts of nonfreezing bound water in membranes is higher than the total amount of water absorbed from liquid water by a dense film of the same polymer. However, the amounts of nonfreezing water in various CA membranes as calculated from the “relative NMR intensities” is substantially lower than those calculated from DSC melting endotherms by assuming the heat of fusion of water in membranes to be identical to that of pure water. Various possible reasons for this discrepancy are discussed. Measurements on the first desorption‐adsorption cycle ofwetCA membranes have also been performed. They suggest that during the first dehydration process, irreversible changes are induced in the structure of the membrane which result in a significantly lower accessibility of the polymer to interact with water. The extent of these irreversible changes in membrane structure is dependent on the details of the dehydration process being more pronounced at higher temperatures.