## 1 Introduction  
Chemisorption is the major practical method for post-combustion CO2 recovery due to its process simplicity and treatment capacity. Recently, chemisorption has been applied to the capture and removal of CO2 from the atmosphere (Direct Air Capture: DAC [1; 2; 3]), as have other conventional separation methods. Amine-based chemisorption has been applied practically in post-combustion CO2 recovery; however, the large amount of energy consumption that is associated with the regeneration/desorption treatment, which occurs at high temperatures (>393 K), is a drawback of the process [4]. Tertiary amines and hindered amines have shown promise as CO2 adsorbents; with these adsorbents, energy consumption and process temperature can be significantly reduced. On the premise that waste heat can be utilized, it is required that the chemisorption-based CO2 separation process be driven at temperatures provided by low-grade heat sources. The desorption of CO2 with tertiary amines is less energy-consuming compared to those with other amines. This is because tertiary amines adsorb dissolved CO\({}_{2}\) as bicarbonate ions via electrostatic interactions (r1), without undergoing reactions involving covalent bonding, such as carbamate formation, which occurs for 1 st and 2 nd amines. \[-\mathrm{R_{1}R_{2}N+H_{2}O+CO_{2}\rightleftarrows-R_{1}R_{2}NH^{+}+HCO_{3}^{-}}\] (r1)  
In the present study, thermo-responsive hydrogels with tertiary amines were used as energy-saving adsorbents for the chemisorption-based CO\({}_{2}\) separation and recovery process. Tertiary amines, immobilized in cross-linked hydrogels, exhibit temperature-responsiveness of their pK\({}_{\mathrm{a}}\) in response to the swelling/collapsing of the gels, which is the key property of the gels for the adsorption of CO\({}_{2}\) at the low desorption and regeneration temperature [5; 6; 7]. In this separation and recovery system of CO\({}_{2}\), the adsorption and desorption of CO\({}_{2}\) onto the gels are operated by controlling the swelling of the gels via temperature swing. The influence of the polymer characteristics on the pK\({}_{\mathrm{a}}\) of the amine groups in the gels has been studied in terms of the hydrophobicity of the polymers by neutralization titration [8], pH change in aqueous solution in response to the phase change in the thermo-responsive polymers [9], and temperature dependence of the swelling volume and pH of the gels [10]. The influence of the adsorption of CO\({}_{2}\) on the lower critical solution temperature (LCST) of the polymer in the amine gels has also been reported [11]. In the gel system, CO\({}_{2}\) gas is adsorbed on protonated amines in the gels as bicarbonate ions, when the pH of the gel system is below the pK\({}_{\mathrm{a}}\) of the amines, at a low temperature below the LCST of the gel (the temperature at which the gel swells). Conversely, in principle, the bicarbonate desorbs and is released as CO\({}_{2}\) gas when the pH of the gel system is above the pK\({}_{\mathrm{a}}\) of the amines at a high temperature above the LCST [5; 6]. Therefore, the LCST of the polymer used as the backbone of the gels (polymer networks) is the key index in the CO\({}_{2}\) separation and recovery process, i.e., the CO\({}_{2}\) adsorbed on the tertiary amines in the gels, at a low temperature, is released at a high temperature after the LCST of the gels is attained. The temperature-dependent phase behaviour of the thermo-responsive gels affects the pK\({}_{\mathrm{a}}\) of the amines in the gels, and the pK\({}_{\mathrm{a}}\) change in the amines is most pronounced after the LCST. Therefore, the performance of the CO\({}_{2}\) separation and the recovery process depends heavily on the LCST. We focused on controlling the LCSTs of the gels for the development of an effective, simple, energy-saving CO\({}_{2}\) recovery system.