Microdomains phase aggregation

Fig. 16. Phase diagram of a mixture containing polystyrene (M = 2400) and styrene/butadiene diblock copolymer (27 % styrene, M = 28000). Liquid phases L, and Lj represent mixtures of disordered block copolymer and polystyrene. Mesophase M, consists of ordered microdomains of the block copolymer swollen with polystyrene. Mesophase Mj probably contains aggregates of block copolymer micelles within the medium of polystyrene. The features on the lower right, drawn in broken lines, are more speculative. (From Roe and Zin...

In this section, we will focus on the solubilization of a substance in water-oil-surfactant samples containing initially three components. These ternary systems include microemulsions (w/o, o/w and bicontinous), lamellar phases and other liquid crystal mesophases. On a microscopic scale, oil microdomains are separated from water microdomains by a surfactant interface. A microdomain is here understood to be an aggregate of at least the order of a hundred self-assembled molecules, although being too small to be considered as a microphase-separated sample. A sample contains separated microphases when domains of micron size of two thermodynamically stable different phases co-exist and do not de-mix even after centrifugation, due to kinetic stability. The solute can then be located at the interface or in the oil or water microdomain (cf. Figure 9.9). Since three environments are available in ternary systems, the interface can be considered as a pseudo-phase or as a surfactant monolayer (37). 

The phase diagrams of solutions of diblock copolymers A-B may be quite complex and depend both on the chemical nature of two blocks and on the solvent. > In a selective solvent, good for the B-block and poor for the A-block, intermolecular aggregates are formed in the dilute regime so that the number of unfavorable contacts A-S is limited. The shape of aggregates (e.g, spherical or wormlike micelles, vesicles), their size and polydispersity depend very much on chain composition and length. In more concentrated solutions, aggregates order in space and form mesophases, i.e., ordered microdomains rich in A (in B) (e.g., lamellar. 

The microdomain structure of novel two-phase hydrogels has been investigated using a variety of analytical techniques. It has been shown that these domains consist of side chains from the starting polymer which aggregate in such a way as to exclude water. In the presence of surfactant, these clusters incorporate surfactant monomer as well as side chain. The composition of these domains, specifically the number of side chains per cluster, influences the rheological properties of the networks. 

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