Study of the effect of propylene glycol on the properties of poloxamer 338 solutions

The aim. Study of the characteristics of 20 % solutions of poloxamer 338 (P338) in water and mixed solvents water – propylene glycol (PG) at various temperatures using rotational viscometry and the spin probe method. Materials and methods. 20 % m/m solutions of P338 in water and water – PG mixtures were the objects of research. The solutions were studied by rotational viscometry at 25 °С, 32 °С і 37 °С; the flow behaviour, low-yield stress (t0), hysteresis area (SN) and dynamic or apparent viscosity (η) were determined. Spin probes based on fatty acids, which differ in molecular structure, solubility, and radical localisation, were added to the solutions. Electron paramagnetic resonance (EPR) spectra was obtained to determine their type and parameters. Results. Depending on the content, PG affects the rheological properties of 20 % P338 solution. The ability of this solution to undergo thermally induced sol-gel transitions, resulting in the formation of gels with plastic flow behaviour at temperatures of 32 °C and 37 °C, is maintained at PG content of up to 20 %. At 37 °C and a 30 % PG content, an atypical thixotropic gel is formed. The rheological characteristics of gels containing 10-20 % PG at 32 °C and 37 °C are higher than those of gels without PG. The increase in the PG concentration from 0 to 40 % generally has little effect on the rotational correlation times (τ) and values of the order parameter (S) of the spin probes. In the case of the ammonium salt of 5-doxylstearic acid (5-DSA NH4 salt), the anisotropic EPR spectra at a PG concentration of 40 % undergoes a transformation, becoming a triplet. This coincides with the loss of the ability of 20 % P338 solutions to thermally induced sol ↔ gel transitions. An increase in the concentration of PG (in contrast to ethanol) does not lead to the solvation of P338 micelle cores by the dispersion medium. The transformation of the EPR spectrum of the 5-DSA NH4 salt into a triplet is probably the result of the interaction between PG and the hydrophilic shell of micelles through the formation of hydrogen bonds. Conclusions. The rheological properties of 20 % P338 solution are affected by the PG, depending on its content. The P338 solutions can undergo a thermally induced sol ↔ gel transition, provided that the PG content does not exceed 30 %. A correlation has been identified between alterations in the rheological properties of 20 % P338 solution and the corresponding change in the types of EPR spectra observed for the 5-DSA NH4 salt, namely a transition from anisotropic spectra to triplet. As the PG content in the P338 solution increases up to 40 %, the solvation of micelle cores by the dispersion medium does not occur. It may be posited that the alteration in the structure of P338 micelles is a consequence of the interaction between PG and their hydrophilic shell