Reentrant Coil-to-Globule-to-Coil Transition of a Single Linear Homopolymer Chain in a Water/Methanol Mixture

Both water and methanol are good solvents for poly( $N$-isopropylacrylamide) (PNIPAM) at room temperature, but strangely not a mixture of them. Using narrowly distributed linear PNIPAM chains ( ${M}_{w}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.6\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{0ex}{0ex}}\mathrm{g}/\mathrm{mol}$ and ${M}_{w}{/M}_{n}<1.1$), we have, for the first time, observed a coil-to-globule-to-coil transition of a single PNIPAM chain when methanol was gradually added into water. This novel reentrant transition leads to insight into the complexation between methanol and water. We also found that the chain was crumpled in its globule state and the globule still contained $\ensuremath{\sim}80%$ of solvent in its hydrodynamic volume.