Pluronics triblock structure

Pluronics, also known as poloxamers, are a class of synthetic block copolymers which consist of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(propylene oxide) (PPO), arranged in an A-B-A triblock structure, thus giving PEO-PPO-PEO (Fig. 11.7) (Batrakova and Kabanov 2008). They can be found either as liquids, pastes or solids (Ruel-Gariepy and Leroux 2004). Due to their amphiphilic characteristics (presence of hydrophobic and hydrophilic components), pluronics possess surfactant properties which allow them to interact with hydrophobic surfaces and biological membranes (Batrakova and Kabanov 2008). Being amphiphilic also results in the ability of the individual block copolymers, known as unimers, to combine and form micelles in aqueous solutions. When the concentration of the block copolymers is below that of the critical micelle concentration (CMC), the unimers remain as molecular solutions in water. However, as the block copolymer concentration is increased above the CMC, the unimers will self-assemble and form micelles, which can take on spherical, rod-shaped or lamellar geometries. Their shapes depend on the length and concentration of the block copolymers (i.e. EO and PO), and the temperature (Kabanov et al. 2002). Micelles usually have a hydrophobie eore, in this case the PO chains, and a hydrophilic shell, the EO ehains. 

In a solvent, block copolymer phase behavior is controlled by the interaction between the segments of the polsrmers and the solvent molecules as well as the interaction between the segments of the two blocks. If the solvent is unfavorable for one block, this can lead to micelle formation in dilute solution. The phase behavior of concentrated solutions can be mapped onto that of block copolymer melts (97). Lamellar, hexagonal-packed cylinder, micellar cubic, and bicontinu-ous cubic structures have all been observed (these are all lyotropic liquid crystal phases, similar to those observed for nonionic surfactants). This is illustrated by representative phase diagrams for Pluronic triblocks in Figure 6. 

PEO-PPO, and later on PEO-PBO copolymers, represent the link between classical low molecular weight non-ionic surfactants and polymeric surfactants. These commercially available products (formerly known as POLOXAMERS, PLURICARE, PLURONICS, SYN-PERONICS), mainly with di- and triblock structures can form, depending on temperature and concentration, true solutions, micelles of different shapes and physical gels. Their micellization behavior has been studied quite extensively and the experimental as well as the theoretical results were summarized in the review articles of Nace [10], Chu and Zhou [116], Ahngren etal. [147], Hamley [11], Booth and co-workers [79,148] and Wanka etal. [149]. 

There is a vast body of diblock copolymer studies since block choice can be such that they resemble amphiphilic surfactants. For the sake of brevity, we will skip them. Instead, we present an interesting case of triblock copolymers of poly(ethylene oxide), PEO, and poly(propylene oxide), PPO, commonly known by one of its trade names, Pluronics [117]. They have been used as non-ionic surfactants for a variety of applications such as in emulsification and dispersion stabilization. In aqueous solutions, these copolymers form micelles, and there exists a well-defined critical micelle concentration that is experimentally accessible. Several groups have investigated colloidal suspensions of these polymers [118-122], The surface properties of the adsorbed monolayers of the copolymers have been reported with respect to their structures and static properties [123-126]. 

Rather sophisticated structures of the Ti02 porous film were introduced by Coakley et al. [69,70]. Based on a titanium(lV) tetraethoxide (TEOT) titania precursor in combination with a pluronic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (P123) as the structure-directing agent, regular hexagonal structmes (honeycomb) with pore sizes around 10 nm were prepared (Fig. 60). 

A solution containing deionized water (42 mmol) in anhydrous sec-butyl alcohol (10 mL) was added very slowly to a stirred homogeneous solution containing Pluronic 64L a triblock copolymer of composition (PEO) 13(PPO)3o(PEO) 13 (2.1 mmol) and aluminum sec-butoxide (25 mL). The resulting gel was stirred for 3 h and then diluted with sec-butyl alcohol to react for additional 16 h. The solid product was washed with ethanol and dried at room temperature for 16 h and at 100 °C for 6 h. The calcination was carried out at 500 °C for 4 h. To recover the structure-directing agent (Pluronic 64L) from the as-synthesized solid, extraction with ethanol can be successfully used [65],... 

The group of Goldfarb and coworkers have in recent years explored how (spin-labeled) thermoresponsive triblock copolymers of the Pluronic -type (PEO-PPO-PEO, poly(ethylene oxide)-poly(propyleneoxide)-poly(ethyleneoxide)) can be used to build templates, e.g., for the formation of mesoporous frameworks [93, 94]. These structures bear great potential as carrier materials for catalysts and hence could aid societal needs in energy and sustainability. 

One of the few reports on the preparation of mesoporous sihca nanofibers with a soft template other than triblock copolymers of the Pluronic type deals with the synthesis of sihca nanofibers with high aspect ratios containing linear arrays of mesopores by a solution-induced self-assembly process (Fig. 23), as previously reported for thin-film configurations [184]. To this end, PS-fo-PEO diblock copolymers were employed as structure-directing agents in sol solutions containing toluene/ethanol mixtures. For a Dp-value of 35 nm, a single line of mesopores formed, for a Dp-value of 60 nm two parallel rows of mesopores were obtained [185]. 

---