High-throughput studies of the effects of polymer structure and solution components on the phase separation of thermoresponsive polymers

This paper describes a high-throughput method for characterization of the temperature-dependent folding of poly(N-isopropylacrylamide), PNIPAM, using dark field microscopy to measure this thermoresponsive polymer's lower critical solution temperature (LCST). The effect of ionic solution components (halide and alkali metal ions) on the polymer precipitation temperature follows the Hofmeister series whereas solution isotopic effects using D₂O give rise to a roughly linear increase in the LCST with the mole fraction of D₂O added. The polymer structure itself was also varied through the synthesis of N-alkylacrylamide copolymers. It was found that replacement of the isopropyl N-alkyl pendant groups of PNIPAM with varying amounts of N-n-propyl groups results in a monotonic decrease in the LCST. Repeated analyses of the same sample and of separately prepared samples show that this type of analysis is quite precise and that investigations of subtle effects of polymer microstructure and solvation are feasible even when the difference in LCST temperatures is < 1°C. Such effects may be difficult to study by other methods.