Lyotropic polymer cubic phases

The photoinitiated polymerization of divinylbenzene (DVB) within four separate cubic phases of the system DVB/didodecyl dimethyl ammonium bromide (DDAB) is reported to yield retention of the lyotropic order during the course of the reaction [48], although the structure of the pure polymer matrix after removal of the template was not investigated. Similarly, polymerization of acrylamide within lamellar, hexagonal, and cubic phases of the surfactant Aerosol OT led to preservation of the parental mesostructure [49]. SAXS measurements showed similar diffractograms before and after polymerization, but again there was no report of characterization of the polymer matrix after surfactant removal. Hence, at least in these cases, the formation of a polymer phase within a lyotropic mesophase does not prevent the formation of lyotropic surfactant phases. 

Recently the polymerization of styrene within lamellar and cubic phases of the surfactant DODAB (dioctadecyldimethylammonium bromide) was studied [50]. After polymerization, the polystyrene/water/DODAB system showed the same phase behavior as the binary water/DODAB system, a result suggesting a phase separation during polymerization into a polymer-rich (with M 400,000) and a lyotropic phase. 

Mesophases can be locked into a polymer network by making use of polymerizable LCs [59]. These molecules contain moieties such as acryloyl, diacety-lenic, and diene. Self-organization and in situ photopolymerization under UV irradiation will provide ordered nanostmctured polymers maintaining the stable LC order over a wide temperature range. A number of thermotropic liquid crystalline phases, including the nematic and smectic mesophases, have been successfully applied to synthesize polymer networks. Polymerization of reactive lyotropic liquid crystals also have been employed for preparation of nanoporous polymeric materials [58, 60]. For the constmction of nanoporous membranes, lyotropics hexagonal or columnar, lamellar or smectic, and bicontinuous cubic phases have been used, polymerized, and utilized demonstrated in a variety of applications (Fig. 2.11). 

Although the phase structures presented by thermotropic and lyotropic liquid crystal polymers are usually different this is not always true. For example, cubic phases are more abundant in lyotropic liquid crystals, but some thermotropic phthalocyanine derivatives [39,40] also exhibited this kind of phase... 

In a solvent, block copolymer phase behaviour is controlled by the interaction between the segments of the polymers and the solvent molecules as well as the interaction between the segments of the two blocks. If the solvent is unfavourable for one block this can lead to micelle formation in dilute solution. The phase behaviour of concentrated solutions can be mapped onto that of block copolymer melts [95]. Lamellar, hexagonal-packed cylinder, micellar cubic and bicontinuous 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 1.6. 

As for low molecular weight surfactants, the superstructures are assumed to be formed by micellar aggregates [126], But it seems that the formation of lyotropic liquid crystals is supported by the additional presence of thermotropic mesogens [87,122-124,126], Lamellar, hexagonal, cubic and even nematic and cholesteric mesophases were reported for binary systems, the latter being exceptional. Lyotropic mesophases were also observed in non-aqueous solvents [240,400,401,405], If polymerizable surfactants are studied, not only the phase diagram but also the types of mesophases observed for the monomer and the polymer may be different. 

Lee et al. demonstrated the synthesis of nanostructured cubic polymer gels by copolymerization of dienyl substituted lipids [66]. No phase transitions, or changes in dimensions, were observed with temperature changes for the polymerized sample. Furthermore, the polydomain square lattice of the gel was visualized by TEM of ultramicrotomed samples after extraction of the template (Fig. 7). In contrast, copolymerization of monoacylglycerol and 1,2-diacylglyc-erol in a cubic lyotropic state did not result in a continuous gel structure. Linear polymer chains were obtained instead, and the cubic morphology was destroyed by addition of organic solvent [67]. Similar polymerizations in the inverted... 

The series of lyotropic phases that are characteristic of the binary water-amphiphile system are well described for the most part by Ginzburg-Landau models [50,51] and polymer chain models [29] (see Fig. 2). The most complicated lyotropic phase, the cubic gyroid phase, was the last phase to be determined theoretically, and it was found first [77] in models of long ffexible diblock copolymers, as described in Sec.V.C.1. Quite recently, it has also been obtained in simulations of the short polymer chain model [78]. 

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