Fluorescence studies on the non-ideal mixing in triblock copolymer binary mixtures under the effect of temperature: A block hydration effect

The fluorescence and viscosity measurements have been carried out for mixed triblock copolymers (TBP) binary mixtures such as P103 + F127 and P103 + P123 over the whole mole fraction range in the temperature range of 25–40 °C. From the fluorescence measurements mixed critical micelle concentration, micropolarity, Stern–Volmer constant, and excimer formation were determined. A variation of these parameters for pure P103, F127, and P123 components indicated that the micelles of these TBPs acquire stronger hydrophobic environment with the increase in temperature and that is proportional to the degree of hydration. Among these pure components, F127 micelles are more hydrated due to the presence of greater number of poly(ethylene oxide) units than that of P103 and P123 and hence get dehydrated at higher temperature. The components of the mixed micelles of P103 + F127 show attractive interactions at 25 °C which decrease and approach ideality as the temperature increases to 40 °C. On the contrary, the components of P103 + P123 mixtures show ideal mixing at 25 °C and the mixing becomes increasingly unfavorable with the increase in temperature. The results have been fully supported by viscosity measurements and have been explained on the basis of dehydration of oxyethylene groups with increasing temperature. The latter effect results in the unfavorable mixing of unlike components of both mixtures, which takes place at lower temperature range in the case of P103 + P123 while at higher temperature range in the case of P103 + F127.