The Dependence of the Cloud Point, Clearing Point, and Hysteresis of Poly(N‐isopropylacrylamide) on Experimental Conditions: The Need for Standardization of Thermoresponsive Transition Determinations

Inappropriately, the critical solution temperature ( CST) of lower CST(LCST)- or upper CST(UCST)-type thermoresponsive polymers, measured by one, individual set of conditions, is considered almost exclusively as the LCST or UCST, respectively. These are correctly the minimum or maximum, respectively, of the full phase diagrams. Because the dynamic phase transition depends on the conditions, and no standardized or widely accepted process exists for CST determination, systematic investigations are carried out with the most widely investigated poly(N-isopropylacrylamide) (PNIPAAm) to unveil the effect of data evaluation, measurement conditions on the transmittance-temperature curves, cloud point (T-CP), clearing point (T-CL), and heating-cooling hysteresis. The unusual dependence of the fundamental hysteresis parameters, i. e., width of hysteresis (X-H) and extent of transmittance recovery (Y-H), on a broad range of conditions is revealed for the first time. On the basis of the findings, the inflection point of transmittance(absorbance)-temperature curves as T-CP and T-CL, 0.1 wt% solution, 0.2 degrees C min(-1) heating/ cooling steps with 5 min equilibration between the gradual change of temperature, 488 nm wavelength to obtain data comparable to light scattering at this wavelength, and determination of X-H and Y-H are proposed as standard set of conditions.