Abstract: The phase behaviors of Poly(N-isopropylacrylamide) (PNIPA) and Poly(N,N-diethylacrylamide) (PDEA) were investigated individually in mixed solvent of water-methanol using UV-Vis spectroscopy. The results show that the low critical solution temperature (TLCS) of PNIPA system undergoes reentrant change, while the TLCS of PDEA system shifts to higher temperature as the volume fraction of methanol (φ) increases. Refractive index measurement shows the formation of water-methanol complexes in PDEA system as well as in PNIPA system. To explain the different phase behavior of PDEA system from PNIPA, the conformational changes of PNIPA and PDEA molecular chains in mixed solvent of water-methanol were studied using the viscometry and Fourier Transform Infrared Spectroscopy. It was found that unlike the PNIPA, which underwent reentrant coil-to-globule-to-coil transition, the PDEA chains experienced homogeneous expansion-to-contraction-to-expansion as φ increased. It is believed that the different phase behavior of PDEA system from PNIPA is due to the more lipophilic property of PDEA structure than that of PNIPA, and consequently, the difference of solvating abilities of water-methanol complexes toward molecules PNIPA and PDEA.

Key words: Poly(N-isopropylacrylamide), Poly(N,N-diethylacrylamide), Phase behavior, Low critical solution temperature, Solution property