羧甲基纤维素与丙烯酸接枝共聚凝胶的溶胀动力学及过溶胀平衡特性的研究

采用接枝共聚合成了羧甲基纤维素钠、丙烯酸与N,N′-亚甲基双丙烯酰胺的交联凝胶, 研究了这类凝胶在不同pH值的缓冲溶液中的溶胀行为, 发现在酸性介质中凝胶的溶胀动力学行为表现出过溶胀平衡特性(overshooting effect), 即凝胶先发生溶胀到最大值, 然后再逐渐消溶胀到平衡. 这种现象可归因于凝胶溶胀过程中羧基之间通过氢键所产生的协同物理交联. 较之凝胶的组成, 缓冲溶液的pH值对过溶胀平衡现象的影响更为显著. 前者是因为凝胶羧基的总摩尔分数并不随两组分结构单元摩尔数的改变而改变, 羧基之间通过氢键形成的物理交联程度在交联剂摩尔分数接近的条件下变化不大; 后者是由于溶液的pH值显著影响凝胶羧基的质子化程度, 进而影响羧基之间通过氢键形成的物理交联程度. 凝胶在酸性介质中的溶胀过程符合E. Díez-Peña等提出的溶胀动力学定量模型, 理论曲线与实验数据有较好的相关性. 凝胶在pH≥5.0的缓冲溶液中的溶胀不产生过溶胀平衡现象, 这一现象归因于完全离子化的羧基之间不能形成物理交联. 凝胶的溶胀过程遵循Schott二级溶胀动力学.

Study on the Swelling Dynamics of Carboxy Methyl Cellulose-graft- poly(acrylic acid) Hydrogels and the Overshooting Effect

A series of hydrogels were synthesized by graft cross-link copolymerization of carboxy methylcellulose (CMC) and acrylic acid (AA) using N,N′-methylene-bis-(acrylamide) as a cross-linker. The swelling kinetics of the hydrogels in different buffer solutions were studied, and the kinetic curves in acidic medium exhibited a remarkable overshooting effect, viz. the gels firstly swell to a maximum value followed by a gradual deswelling until the equilibrium. The phenomenon was attributed to a cooperative physical cross-linking caused by the hydrogen bond formation between the carboxyl groups of the hydrogels. The effect of pH of buffer solution on overshooting effect is more remarkable than that of the composition of hydrogels. The former reason is that the total molar fraction of carboxyl groups does not change with the molar ratio of the units of carboxy methylcellulose (CMC) and acrylic acid (AA) in hydrogels, thus the degree of physical cross-linking is little or not changed by the composition of hydrogels in the case of the same molar fraction of cross-linkers; while the latter is that the pH change of buffer solution can cause the remarkable change in the extent of protonization of carboxyl groups, consequently resulting in a remarkable change of the degree of physical cross-linking. The swelling processes of the hydrogels under acidic medium follow a quantitative model proposed by E. Díez-Peña et al, and the theoretical curves are in very good agreement with the experimental data. While in pH≥5.0 buffer solutions, the overshoot phenomenon does not appear, and this is attributed to the physical cross-linking not being formed between the ionized carboxyl groups in the media. The swelling processes follow Schott second-order swelling kinetics.