On the temperature-responsive polymers and gels based on N-propylacrylamides and N-propylmethacrylamides

We studied the behavior in water of polymers, microgels, and macrogels based on the following four monomers: N-isopropylacrylamide (NiPA), N-isopropylmethacrylamide (NiPMA), N-n-propylacrylamide (NnPA), and N-n-propylmethacrylamide (NnPMA). The thermal phase separation of polymers in water as well as of microgels in the aqueous dispersion was examined by a combination of turbidity measurements and differential scanning calorimetry (DSC). The hydrodynamic radius of microgels and the swelling degree of macrogels (fine cylindrical bulk gels) were also examined as a function of temperature using the dynamic light scattering and the microscopic method, respectively. It was found that all the polymers prepared are water-soluble and clearly exhibit the phase separation on heating. The phase separation temperature varies depending on the constituent monomers and becomes higher in the order of NiPMA > NiPA > NnPMA > NnPA. The endothermic enthalpy from the heating DSC curves increases in the order of NnPMA > NnPA approximately NiPMA > NiPA. The same trends were observed in the microgels based on NiPA, NiPMA, and NnPA, which were synthesized via chemical cross-linking with N,N'-methylenebis(acrylamide) (Bis). Although we were unable to synthesize the microgel of NnPMA due to a low water solubility of the monomer, its bulk gel was obtained by gamma-ray irradiation to an aqueous poly(NnPMA) solution at a dose of 10 kGy. An irradiation-cross-linked NiPMA gel was also prepared as a counterpart to the Bis-cross-linked gel. We then studied the gel collapses upon heating by use of the chemically cross-linked gels based on NiPA, NiPMA, and NnPA as well as of the irradiation-cross-linked NnPMA and NiPMA gels. All the gels underwent the collapse transition at a certain temperature which is close to or slightly higher than the phase separation temperature of the corresponding polymer solutions or microgel dispersions. These results indicate that in both the linear and cross-linked polymers there is no difference in the thermally induced interactions between the segments as well as between the segment and the solvent, but these interactions are dependent on the structure of the constituent monomers, i.e., whether the alpha-carbon bears a hydrogen atom or a methyl group and whether the N-propyl group is branched or straight chain. The structure dependence was discussed in terms of amide-amide and amide-water hydrogen bondings as well as of a possible hydrogen bonding of solvent water with the H-C bond of the alkyl groups. Then, water clustering around both the alkyl and the amide groups was considered.