Thermal transitions in associating aqueous block copolymers: light scattering, rheology and spectroscopy

Solutions containing a polyoxy-ethylene/polyoxy-propylene/polyoxy-ethylene (PEO-PPO-PEO) block copolymer, indicated as F68, in water were investigated as a function of composition and temperature. Hydrogen nuclear magnetic resonance (H-1 NMR) line width, chemical shift, self-diffusion, spin-lattice relaxation times, laser light scattering and rheological methods were used. The monomer-micelle equilibrium and the micelle-liquid crystalline phase transitions depend on the F68 content in the mixture and temperature. Significant changes in light scattering intensity and apparent hydrodynamic radius are associated to micelle formation above the critical micellar temperature (CMT). According to a Contin analysis, this behaviour is reflected in the presence of two populations in the intensity-intensity autocorrelation functions. The contributions due to molecules and micelles can be evaluated separately. No such effects are observed below the CMT. Micelle onset is also associated to variations in H-1 NMR spectra, affecting the chemical shift, line width and spin-lattice relaxation time of the PPO methyl protons and self-diffusion, as well. Spin-lattice relaxation times of PEO chains, conversely, change significantly at temperatures close to the micelle-liquid crystalline thermal transition. Similar results were obtained from the line width of H-2 NMR spectra as a function of T. Significant changes in both viscous and elastic modulus were also observed and ascribed to PPO dehydration, at the CMT, as well as to squeezing and dehydration of PEO units in liquid crystal formation, respectively.