Temperature-, pH- and CO₂-Sensitive Poly(N-isopropylacryl amide-co-acrylic acid) Copolymers with High Glass Transition Temperatures

Yeong-Tarng Shieh; Pei-Yi Lin; Tao Chen; Shiao-Wei Kuo

doi:10.3390/polym8120434

Temperature-, pH- and CO₂-Sensitive Poly(N-isopropylacryl amide-co-acrylic acid) Copolymers with High Glass Transition Temperatures

Polymers (Basel). 2016 Dec 14;8(12):434. doi: 10.3390/polym8120434.

Authors

Yeong-Tarng Shieh¹, Pei-Yi Lin^{2

3}, Tao Chen⁴, Shiao-Wei Kuo⁵

Affiliations

¹ Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 81148, Taiwan. yts@nuk.edu.tw.
² Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 81148, Taiwan. m033100007@student.nsysu.edu.tw.
³ Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan. m033100007@student.nsysu.edu.tw.
⁴ Ningbo Institute of Material Technology and Engineering, Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Chinese Academy of Science, Zhongguan West Road 1219, Ningbo 315201, China. tao.chen@nimte.ac.cn.
⁵ Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan. kuosw@faculty.nsysu.edu.tw.

Abstract

A series of poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAAm-co-PAA) random copolymers were synthesized through free radical copolymerization in MeOH. The incorporation of the acrylic acid units into PNIPAAm tended to enhance the glass transition temperature (T_g), due to strong intermolecular hydrogen bonding between the amide groups of PNIPAAm and the carboxyl groups of PAA, as observed using ¹H nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopic analyses. The lower critical solution temperature (LCST) increased upon increasing the pH of the aqueous solution containing PNIPAAm-co-PAA because the COOH groups of the PAA segment dissociated into COO^- groups, enhancing the solubility of the copolymer. In addition, high-pressure differential scanning calorimetry revealed that the LCSTs of all the aqueous solutions of the copolymers decreased upon increasing the pressure of CO₂, suggesting that CO₂ molecules had displaced H₂O molecules around the polar CONH and COOH groups in PNIPAAm-co-PAA, thereby promoting the hydrophobicity of the copolymers in the aqueous solution. In addition, the values of T_g of a film sample increased upon treatment with supercritical CO₂, implying that intermolecular interactions in the copolymer had been enhanced after such treatment.

Keywords: CO2-sensitive; copolymers; hydrogen bonding; pH-sensitive; temperature-sensitive.