A molecular recognition ion gating membrane opens and closes its pores using the volume phase transition of PE (polyethylene)-g-N-isopropylacrylamide (NIPAM)-co-benzo [18]crown-6-acrylamide (BCAm), which recognizes specific ions with its BCAm receptors and changes its volume by swelling and shrinking. In this study, we clarify the mechanism of the molecular recognition response of the PE-g-NIPAM-co-BCAm. The complex formation constant (log K) of the crown ether receptors contained in BCAm units was determined to be an order of magnitude lower than that of benzo[18]crown-6. Calculating from log K, we estimated the quantity of the complex of the crown ether receptor and the ion; the ratio of the BCAm units forming the complex to total monomer units that consists of NIPAM and BCAm (X) was below 5%. Although X was less than 5%, the complex affected the swelling and the shrinking of NIPAM units. Poly-NIPAM is well-known to change its volume at the lower critical solution temperature (LCST), being accompanied by the change of the enthalpy (DeltaH). The differential scanning calorimetry (DSC) data of PE-g-NIPAM-co-BCAm showed that LCST increased and DeltaH decreased with increasing X, regardless of ion species. DeltaH is generated to compensate for the breakage of hydrogen bonds between the water molecules that surround the hydrophobic moieties of polymer chains; therefore, formation of the complexes of the crown ether receptors and the ions broke the hydrogen bonds and then shifted the LCST higher. In addition, the LCST determined by DSC predicts the temperature at which the membrane pores open. Therefore, this suggested mechanism contributes to both the molecular design of grafted stimuli-responsive copolymers and the design of the gating membrane function.