Gelation behavior of PEO–PLGA–PEO triblock copolymers in water

Aqueous solutions of poly(ethylene oxide-b-(dl-lactic acid-co-glycolic acid)-b-ethylene oxide) with two different molecular weights (550–3.4K–550 and 750–3.5K–750) show the change in turbidity as a function of temperature. The variation in turbidity is also found to be independent of block copolymer concentration. By increasing the molecular weight of the hydrophilic PEO end block with the molecular weight of the hydrophobic PLGA middle block kept almost the same, it is found that the phase boundary is shifted to a higher temperature and only soft gels are observed. Large aggregates or clusters of micelles with sizes larger than 5000 nm are observed with dynamic light scattering at the temperatures of turbid region and the size distribution of the aggregates is also found to depend only on temperature, not on concentration. The storage moduli (G′) of aqueous solutions of the PEO–PLGA–PEO do not show the abrupt change at the sol-to-(hard) gel boundary, as normally seen in the case of PEO–PPO (poly(propylene oxide))–PEO block copolymers and the behavior of G′ as a function of temperature is qualitatively the same and independent of polymer concentration except the absolute values of G′, which are proportional to the copolymer concentration. This implies that micelle–micelle clusters are formed due to hydrophobic attraction between core phases.