Influence of poly(ε-caprolactone) end-groups on the temperature-induced macroscopic gelation of Pluronic in aqueous media

Abstract

Temperature-induced gelation of amphiphilic copolymers in aqueous media has attracted a great deal of interest in recent years. We have investigated phase behavior, gelation, rheology, and structure of modified versions of the water-soluble copolymer of type PEO-PPO-PEO (Pluronic, F127) with short (PCL(5)) or long (PCL(11)) poly(caprolactone) blocks at both ends. By visual inspection of the studied semidilute and concentrated polymer samples at various temperatures, gelation and phase separation could be mapped and the length of the PCL blocks had a strong impact on the observed features. By using oscillatory shear experiments, the gel points of the systems were determined and an interesting anomaly of the complex viscosity was observed at high frequencies where the moieties were kinetically arrested. This irregularity was strengthened for the PCL(11) copolymer and this was ascribed to bridging of the micelles due to the long PCL blocks. The mesoscopic structure of the systems was studied by using small angle neutron scattering. The position and amplitude of the observed correlation peak, which discloses vital intermicellar correlations in the scattering function depends on temperature, concentration, and the length of the PCL blocks. Frequently the scattering function could be portrayed by a spherical core-shell micelle model with hard sphere interaction between them. The estimated aggregation number (Nagg) was substantially affected by the PCL content of the polymer and a significant influence of polymer concentration and temperature on the value of Nagg was found.