pH/Thermal sensitive copolymers with the various acidic comonomer compositions composed of N-isopropylacrylamide (NIPAAm) with acrylic acid (AAc), 2-acrylamido glycolic acid (AAmGAc), and 2-acrylamido-2-methyl-1-propanesulfonic acid(AMPS) were synthesized by free radical polymerization. In this study, to characterize the effect of different acidic comonomer composition and pH on the lower critical solution temperature(LCST) behaviors of their copolymers, phase transition experiments were performed with a thermo-optical analyzer (TOA). The phase transition temperature (T(p)) of aqueous poly(NIPAAm-co-AAc) solution was lowered with increasing the ionization of the acid group in AAc, that is, the ionized state induced the electrostatic repulsion of ionized groups, In contrast, when AAmGAc was introduced into PNIPAAm, T(p) was little changed at pH 1-3, whereas climbed up significantly from pH 1 to pH 3. In the range of pH 6-10, T-p was lower than that of pH 3-5. This result was considered to be "Ionic Screen Effect" and this effect had been also observed in the case of poly(NIPAAm-co-AMPS).

pH와 온도에 민감한 N-isopropylacrylamide(NIPAAm)계 공중합체인 poly(NIPAAm-co-AAc), poly(NIPAAm-co-AAm-GAc), 및 poly(NIPAAm-co-AMPS)를 산성 공단량체의 조성비를 달리하여 자유라디칼 중합법에 의해 제조하였다. 합성된 공중합체들의 하한임계용액온도거동(LCST)에 대한 pH와 공단량체의 구조 및 함량변화 효과를 열광학분석기(TOA)를 이용하여 측정한 흐림점으로부터 결정하였다. Poly(NIPAAm-co-AAc) 수용액의 상전이온도(T_p)는 공중합체내 AAc의 카르복실기가 이온화될수록, 보다 높은 값을 나타내었는데, 이와 같은 현상은 이온화된 상태에서 공중합체내 이온기들간의 정전기적 반발력이 보다 친수성을 나타나게 한 원인이 되었다. 반면에, 2-acrylamido glycolic acid(AAmGAc)가 공중합체에 도입된 경우, pH 3보다 낮은 pH에서는 T_p의 변화가 거의 없었지만, pH 3에서 5까지는 T_p가 매우 급격히 증가하였다가 pH 6이상에서는 공중합체의 T_p가 ionic screen effect에 의해 pH 3에서 pH 5일 때보다 오히려 더 낮아졌다. 또한 이 같은 ionic screen effect는 강산성 공단량체로 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS)가 도입된 공중합체의 경우에서도 관찰되었다.

Keywords: pH/thermal sensitive polymer; lower critical solution temperature (LCST); poly(N-Isopropylacrylamide); hydrophobic interaction; hydrogen bonding