Microcalorimetric detection of lower critical solution temperatures in aqueous polymer solutions

A sensitive solution microcalorimeter was used to study lower critical solution temperatures (LCSTs) of aqueous polymer solutions. Endotherms with enthalpies on the order of the strength of hydrogen bonds were observed at temperatures concurring with LCSTs detected by classical cloud-point measurements for solutions of poly(N-isopropylacrylamide) (PNIPAAM), poly(vinyl methyl ether) (PVME), poly(propylene glycol) (PPG), and hydroxypropylcelluloe (HPC). The LCST of PNIPAAM was found to be dependent on the chain length, generally increasing with decreasing M-bar_n in the range M-bar_n = 1.6 X 10^5 to 5.4 X 10^3. Treatment of the calorimetric endotherm according to a two-state transiton model afforded cooperative unit sizes of the order of the chain length for PNIPAAM, PVME, and PPG. The apparent cooperative unit for the demixing transition of HPC was found to be larger than a single chain, consistent with previous observations of aggregation of HPC below the LCST. Depression of the LCST by added salts was observed.