Micellar triblock polymers

NAN Nandni, D., Vohra, K.K., and Mahajan, R.K., Study of micellar and phase separation behavior of mixed systems of triblock polymers, J. Colloid Interface Sci., 338, 420, 2009. 

The molecular structure of L64 is shown in Figure 7.2. The triblock polymer is EO13PO30EO13, first a hydrophilic block, then a long hydrophobic block and then again the hydrophilic block. The mesoscale simulations proceed by calculating the lyotropic phases in a narrow concentration range 50-70% [21]. From accurate experimental data [22-24], in this interval four phases are known, and the structure factors have been measured micellar phase, cylindrical or hexagonal phase, a bicontinuous phase and a lamellar phase. 

Very well known commercially available block copolymers showing micellar behavior are polymers of the Pluronic type. Pluronic-type copolymers are symmetric triblock copolymers with the central block of hydrophobic polypropylene... 

Fig. 52 Schematic illustration of different modes of self-organization of PE- -PPO- -PEO triblock terpolymer in water. PE segments, A PPO segments, o PEO segments. Ordered phases are denoted as follows a normal (water-continuous) hexagonal. Hi, b lamellar, L, c reverse (polymer-continuous) hexagonal, H2 and d reverse micellar cubic, I2. From [164]. Copyright 2003 EDP Sciences, Societa Italiana di Fisica, Springer...

The studied triblock copolymer PS-PVP-PEO was purchased from Polymer Source (Dorval, Canada). The number-average molar masses of PS, PVP, and PEO blocks were 2.1 x 10 , 1.2 x 10 , and 3.5 x 10 g mol , respectively, and the poly-dispersity index of the sample was 1.10. The copolymer is insoluble in aqueous media, but the micelles can be prepared indirectly both in acidic and alkaline aqueous solutions by dialysis from 1,4-dioxane-methanol mixtures [88]. The micelles can be transferred from acidic to alkaline alkaline solutions and vice versa, but the addition of a base together with intense stirring promotes aggregation. Two factors contribute to the destabilization of micelles after the pH increase (a) In alkaline media, the PVP blocks become insoluble, collapse and form an upper layer of the core. Since the cores of micelles are kinetically frozen, the association number does not change. The mass of insoluble cores increases, while the length of soluble shellforming chains decreases, which results in a deteriorated thermodynamic stability of micellar solutions, (b) The PVP middle layer shrinks and PEO chains come close to each other, which worsens the solubility due to insufficient solvation of PEO blocks. 

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