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Turbidimetric Monitoring of Phase Separation in Aqueous Solutions of Thermoresponsive Polymers

A method for the experimental determination of turbidity in thermoresponsive polymer solutions is presented. The method is based on measuring the intensity of light streams: weakened and backscattered from the sample, which is in the region of the volume phase transition. The turbidimeter developed by us, unlike industrial devices of this type, is equipped with a thermostat that allows to study the dependence of the solution turbidity on the temperature. The device operates automatically, the values of luminosity and temperature from digital sensors are processed by the microcontroller and transmitted via the RS-232 protocol to USB ports of a personal computer. The error of luminosity measurement does not exceed 0.1 lux, the temperature measurement accuracy is ± 0.1 °C. This article demonstrates an example of using the designed turbidimeter to study the first-order phase transition in an aqueous solution of hydroxypropylcellulose (2 wt. %) in the presence (absence) of NaCl ions. It is assumed that the sol-gel transition in the investigated solution is due to swelling of polymer coils and their subsequent aggregation. In our opinion, the displacement of the phase transition temperature is due to additional swelling of polymer coils owing to shielding of the electrostatic interaction of monomers in the presence of NaCl ions. It is shown that the intensity of backscattered light corresponds to the kinetics of the formation of the gel network, which makes it possible to establish the dimensions of formed aggregates in the network nodes.