Pluronic block copolymers (Pluronics) are produced on a commercial scale to enable wide range of novel applications as amphiphilic macromolecules from emulsification and colloidal stabilization to nanotechnology and biotechnology. While the Pluronic block copolymers offer the advantages of being readily available for such applications, they contain different levels of low molecular weight (MW) “impurities” that would interfere with the self-assembly of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO, triblock copolymers, particularly in aqueous solutions. For example, the eluent gel permeation chromatography technique provided a strong evidence that commercially available Pluronics contain the low MW impurities that do not participate in the micellization of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) PEO-PPO-PEO triblock copolymers in water even at temperatures above critical micellization temperature.
In this presentation, we will highlight two major advances in studying Pluronic block copolymers. First, we have developed a large scale (e.g. gram scale) purification technique by taking advantages of competitive adsorption between low MW impurities and triblock copolymers in the silica slurry upon tuning the solvent quality. Adsorption-based HPLC technique was used to confirm that we can maintain the content of the low MW impurities less than 2 wt.% upon the large scale purification using silica slurry, while the as-received Pluronic samples can contain about 20 wt.% impurities. Second, we have investigated the impact of the low MW impurities on the micellar packing and solution phase behavior of Pluronic F108 and F127 samples. Small angle x-ray scattering (SAXS) experiments on as-received and purified Pluronics revealed that the inter-micellar distance upon micellar ordering depends not on the polymer concentration, but on the triblock copolymer concentration. When the Pluronic solutions develop ordered structures, the presence of non-micellizable impurities in as-received Pluronics make it difficult for the micelles to form ordered structures, such as body-centered-cubic (BCC) and face-centered-cubic (FCC). For example, compared to purified Pluronics, higher triblock concentrations were needed for the as-received samples to develop the ordered structures in solution. Finally, the purified Pluronic block copolymer exhibited rather distinct BCC-to-Disorder or FCC-to-Disorder transition on heating at high temperatures. This behavior correlates with the slight decrease of hydrodynamic size in dilute solutions of Pluronics at high temperatures based on dynamic light scattering (DLS) studies.