Microstructure of N-Isopropylacrylamide−Acrylic Acid Copolymer Gels Having Different Spatial Configurations of Weakly Charged Groups

Gel microstructure formation according to spatial configuration of charged groups in gel network has been investigated using macroscopic swelling measurements and microscopic small-angle neutron scattering. Three types of gel consisting of hydrophobic N-isopropylacrylamide (NIPAm) and weakly charged acrylic acid (AAc) copolymers were prepared with different distributions of AAc monomer units in the network. The first type was a NIPAm/AAc copolymer gel in which NIPAm and AAc were randomly copolymerized in monomer units using the redox method. The second type was a UV-cross-linked NIPAm/poly-AAc copolymer gel in which NIPAm was copolymerized with polymer units of AAc. The third type was a NIPAm/poly-AAc polymer gel prepared using the redox method in which poly-AAc was not cross-linked but physically entrapped in the NIPAm polymer network. These three types of gel contained the same amount of charged groups but with different spatial configurations. The differences in spatial and topological distribution of charged groups were reflected in different responses regarding swelling properties and different microscopic structure upon temperature variation that we discussed with the concept of counterion condensation.