Poly(N-isopropylacrylamide) (PNIPAM) is an important polymer with stimuli-responsive properties, making it suitable for various uses. Phase behavior of the temperature-sensitive PNIPAM polymer in the presence of four low-molecular weight additives tert-butylamine (t-BuAM), tert-butyl alcohol (t-BuOH), tert-butyl methyl ether (t-BuME), and tert-butyl methyl ketone (t-BuMK) was studied in water (D2O) using high-resolution nuclear magnetic resonance (NMR) spectroscopy and dynamic light scattering. Phase separation was thermodynamically modeled as a two-state process which resulted in a simple curve which can be used for fitting of NMR data and obtaining all important thermodynamic parameters using simple formulas presented in this paper. The model is based on a modified van't Hoff equation. Phase separation temperatures T p and thermodynamic parameters (enthalpy and entropy change) connected with the phase separation of PNIPAM were obtained using this method. It was determined that T p is dependent on additives in the following order: T p(t-BuAM) > T p(t-BuOH) > T p(t-BuME) > T p(t-BuMK). Also, either increasing the additive concentration or increasing pK a of the additive leads to depression of T p. Time-resolved 1H NMR spin-spin relaxation experiments (T 2) performed above the phase separation temperature of PNIPAM revealed high colloidal stability of the phase-separated polymer induced by the additives (relative to the neat PNIPAM/D2O system). Small quantities of selected suitable additives can be used to optimize the properties of PNIPAM preparations including their phase separation temperatures, colloidal stabilities, and morphologies, thus improving the prospects for the application.

Department of NMR Spectroscopy Institute of Macromolecular Chemistry AS CR v v i Heyrovsky Sq 2 Prague 6 162 06 Czech Republic

Faculty of Mathematics and Physics Department of Macromolecular Physics Charles University 5 Holešovičkách 2 180 00 Prague 8 Czech Republic

National Institute for Materials Science 1 1 Namiki Tsukuba Ibaraki 305 0044 Japan

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