pH Dependence of Macroscopic Swelling and Microscopic Structures for Thermo/pH-Sensitive Gels with Different Charge Distributions

The pH dependence of the microstructure formation of three types of polymer gels that consist of N-isopropylacrylamide (NIPAm) and acrylic acid (AAc) but have different architectures was investigated by means of macroscopic swelling measurements and microscopic small-angle neutron scattering. The first type was an NIPAm/AAc copolymer gel in which NIPAm and AAc monomers were randomly copolymerized by redox polymerization (redox-mono). The second type was an ultraviolet (UV)-cross-linked NIPAm/poly-AAc copolymer gel in which NIPAm monomer was copolymerized with poly(acrylic acid) (UV-poly). The third type was a mixture of poly-NIPAm gel and poly-AAc in which the poly-NIPAm network was prepared by redox polymerization in the presence of cross-linker, whereas poly-AAc was physically entrapped in the poly-NIPAm polymer network (redox-poly). These three types of gels contain the same amount of AAc but different spatial configurations. The following conclusions were obtained: (1) Redox-mono and UV-poly showed swelling behavior in the middle pH range (i.e., pH 6-10) because of the ionization of AAc. Redox-poly did not show such a swelling behavior over all pHs. (2) SANS analyses confirmed that redox-mono and UV-poly, which exhibit similar swelling behaviors. had marked pH dependence in the gel microstructure because of different architectures and spatial distributions of charged AAc groups.