The water/methanol complexation induced reentrant coil-to-globule-to-coil transition of individual homopolymer chains in extremely dilute solution

Narrowly distributed ultralong linear poly(N-isopropylacrylamide) (PNIPAM) (M-w = 2.63 x 10(7) g/mol and M-w/M-n < 1.1) was successfully prepared by a combination of fractionation and filtration. Using this PNIPAM sample, we have studied the conformation change of individual PNIPAM chains in differe nt methanol/water mixtures at 20 <degrees>C. Both water and methanol are go od solvents for PNIPAM, but not a proper mixture of them. The PNIPAM chain collapsed sharply from a coil to a stable globule when the methanol content (x(methanol)) reached similar to 17 mol %. In the range 17-40 mol %, the P NIPAM chain remained in the globule state. Further addition of methanol fin ally led to a globule-to-coil transition at x(methanol) similar to 50 mol % . The reentrant coil-to-globule-to-coil transition could be attributed to t he formation of different water/methanol complexes, presumably, (H2O)(m)(CH 3OH)(n), which are poor solvents for PNIPAM. We have,for the first time, re vealed that the conformation change of individual polymer chains can be use d as an indicator to probe the complexation, similar to an indicator used i n titration. Our results suggest that when x(methanol) < <similar to> 17 mo l %, the water/methanol molar ratio (m/n) is similar to5, while when X-meth anol > 50 mol %, m/n becomes 1. In the range of similar to 17 mol % < X-met hanol < 50 mol %, different intermediates with 1 less than or equal to m/n less than or equal to 5 coexist. The speculated structures of these complex es could be related to a water pentamer.