Comprehensive Effects\nof Metal Ions on Responsive\nCharacteristics of P(NIPAM-<i>co</i>-B18C6Am)

Abstract

Comprehensive investigations of the effects of species\nand concentrations\nof metal ions on the ion-responsive behaviors of poly­(<i>N</i>-isopropylacrylamide-<i>co</i>-benzo-18-crown-6-acrylamide)\n(P­(NIPAM-<i>co</i>-B18C6Am)) are systematically carried\nout with a series of P­(NIPAM-<i>co</i>-B18C6Am) linear copolymers\nand cross-linked hydrogels containing different crown ether contents.\nThe results show that when the B18C6Am receptors form stable B18C6Am/M^<i>n</i>+ host–guest complexes with special\nions (M^<i>n</i>+), such as K⁺, Sr²⁺, Ba²⁺, Hg²⁺, and Pb²⁺,\nthe LCST of P­(NIPAM-<i>co</i>-B18C6Am) increases due to\nthe repulsion among charged B18C6Am/M^<i>n</i>+ complex groups and the enhancement of hydrophilicity, and the order\nof the shift degree of LCST of P­(NIPAM-<i>co</i>-B18C6Am)\nis Pb²⁺ > Ba²⁺ > Sr²⁺ >\nHg²⁺ > K⁺. With increasing the content of\npendent crown ether\ngroups, the LCST shift degree increases first and then stays unchanged\nwhen the B18C6Am content is higher than 20 mol %. Remarkably, it is\nfound for the first time that there exists an optimal ion-responsive\nconcentration for the P­(NIPAM-<i>co</i>-B18C6Am) linear\ncopolymer and cross-linked hydrogel in response to special metal ions,\nat which concentration the P­(NIPAM-<i>co</i>-B18C6Am) exhibits\nthe most significant ion-responsivity either in the form of linear\ncopolymers or cross-linked hydrogels. With an increase of the content\nof crown ether groups, the value of corresponding optimal ion-responsive\nconcentration increases. Interestingly, there exists an optimal molar\nratio of metal ion to crown ether for the P­(NIPAM-<i>co</i>-B18C6Am) copolymer in response to Pb²⁺, which is around\n4.5 (mol/mol). If the ion concentration is too high, the ion-responsive\nbehaviors of P­(NIPAM-<i>co</i>-B18C6Am) may even become\nsurprisingly unobvious. Therefore, to achieve satisfactory ion-responsive\ncharacteristics of P­(NIPAM-<i>co</i>-B18C6Am)-based materials,\nboth the operation temperature and the ion concentration should be\noptimized for the specific ion species. The results in this study\nprovide valuable guidance for designing and applying P­(NIPAM-<i>co</i>-B18C6Am)-based ion-responsive materials in various applications.