Coil-Globule Transition of PNIPAM in Non-Ionic Surfactant Mesophase

We investigate the Coil-Globule transition of linear poly N-isopropylacrylamide (PNIPAM) in the hexagonal (H$_{1})$ mesophase of a non ionic surfactant, C$_{12}$E$_{9}$ in water, by Turbidimetry, NMR and SAXS. For aqueous PNIPAM, the LCST (coil-globule transition temperature), $T_{PNIPAM}$, is 35\r{ }C, whereas the H$_{1}$ phase transitions to a micellar phase at a temperature, $T_{HI}$, of 45\r{ }C. As PNIPAM is added to the C$_{12}$E$_{9}$/H$_{2}$O system; depending on the ratio of C$_{12}$E$_{9}$:H$_{2}$O,$ T_{PNIPAM}$ changes. It is observed that at 42wt{\%} of C$_{12}$E$_{9}$ (viz. 58wt{\%} water; micellar phase), the $T_{PNIPAM}$ is 33\r{ }C. At 44wt{\%} of C$_{12}$E$_{9}$ (viz. 56wt{\%} water), the H$_{1}$ phase forms and the the coil-globule transition for PNIPAM starts at 28\r{ }C. For 50wt{\%} C$_{12}$E$_{9,}$ the transition starts from 13\r{ }C and for 60wt{\%} C$_{12}$E$_{9}$ in water, PNIPAM does not even shows the phase transition even as below as 5\r{ }C. It is observed using optical microscopy that the PNIPAM is trapped at the domain boundaries of the H$_{1}$ phase. The decrease in the coil globule transition temperature,$ T_{PNIPAM}$, is not only because of the hydrophobic interactions but also due to the competition between polymer and C$_{12}$E$_{9}$ for water in the H$_{1}$ phase. The inability of PNIPAM to become a complete globule at higher temperature may be due to the adsorption of C$_{12}$E$_{9}$.