Hexagonal mesophases

Figure 13.21 Contact region at 78 °C showing the phase sequence smectic-A (fan texture), M (unidentified mesophase), hexagonal channelled layer phase (ChLhex), square columnar phases p4gm and p4mm (from left to right) upon increasing concentration of 13.18 (reproduced by permission of The Royal Society of Chemistry). See plate section for colour version of this image.

Serrette and Swager reported the mesomorphic properties of a series of oxovanadium(IV) complexes of dimeric Schiff bases 85, and 86 (Y=—CH2—CH2— —CH2—CH2—CH2— —CH2—C(Me)2—CH2— R = OCi4 H29) [163]. Despite the non-discoidal shape of these complexes, they all displayed columnar mesophases, some of the complexes showing several columnar-to-columnar phase transitions. The mesophases (hexagonal and rectangular) existed over very wide temperature-range, some of them being stable at ambient temperature. 

The zwitterionic surfactant IVA -dimethyldodecylamine oxide (C12DMAO) form lyotropic mesophases with La(N03)3-6H20 in water-decanol mixtures (Selivanova et al., 2010b). Different mesophases (hexagonal, lamellar, nematic) are formed depending on the molar ratio between the surfactant... 

Twelve oxyethylated fatty alcohols with various lengths of alkyl chain and ethylene oxide were used in this investigation. In view of numerous literature data available, physicochemical tests were limited to measurements of surface tension, wettability and viscosity. Microscope photographs were taken in polarized light in order to confirm the appearance of liquid crystalline structures. As expected, formation of micelles was observed at low concentrations, whereas mesophases (hexagonal and lamellar) were identifled at concentrations of about 50% to 70%. 

Figure C2.3.13. Nonnal (H, left) and inverse (H, right) hexagonal mesophases composed of rodlike micelles.

In the mesophase, the molecules exist in a discotic hexagonal columnar ordered stmcture, schematically shown in Figure 4. 

Using the so-called "block copolymers (a block of Na A-monomers at one end is covalently bonded to a block of Nb B-monomers) one can also realize the analogy of order-disorder phenomena in metallic alloys with polymers one observes transitions from the disordered melt to mesophases with various types of long range order (lamellar, hexagonal, cubic, etc ). We shall not consider these phenomena here further, however... 

The discotic mesophases are classified in two types columnar, and nematic discotic. The structure of the nematic discotic mesophase (Np, Figure 8.3, left) is similar to that of rod-like molecules, but constituted by disk-like units. In columnar mesophases, the molecules are stacked in a columnar disposition and, depending on the type of columnar arrangement, several columnar mesophases are known. The most common lattices of the columnar phases are nematic discotic (No), columnar nematic (Ncoi), columnar hexagonal (Coin), and columnar rectangular (Col,) mesophases. 

Figure 8.3 Schematic representation of the molecular arrangement in the nematic discotic (Nq), columnar nematic (Ncoi), columnar hexagonal (Coin), and columnar rentangular (Coir) mesophases. A tablet represents a molecule with disk-like shape.

Kim, S.J., Kang, S.H., Park, K.-M., Kim, H Zin, W.-C., Choi, M.-G. and Kim, K. (1998) Trinuclear Gold(l) Pyrazolate Complexes Exhibiting Hexagonal Columnar Mesophases with Only Three Side Chains. Chemistry of Materials, 10, 1889-1893. 

We will then examine other flexible polymer crystallization instances which may be interpreted, at least qualitatively, in terms of the bundle model. We will concentrate on crystallization occurring through metastable mesophases which develop by quenching polymers like isotactic polypropylene, syndiotactic polypropylene etc. In principle also hexagonal crystallization of highly defective polymers, and order developing in some microphase-separated copolymer systems could be discussed in a similar perspective but these two areas will be treated in future work. A comparison between the bundle approach and pertinent results of selected molecular simulation approaches follows. 

These phases very often share, aside from a relatively high disorder, another important feature with the bundle-like pre-crystalline entities that we discussed up to this point. This feature is hexagonal or pseudo-hexagonal packing of the polymer chains [61]. In this respect molecular order at a local level is similar, although in the thermotropic mesophases we are now discussing disorder is even more dynamic than in previous cases. Indeed they are essentially characterized by a high entropy. 

Table 1 Thermal and geometrical data of selected semiflexible polymers giving rise to thermotropic hexagonal mesophases or main-chain disordered crystalline phases (adapted from [11])... 

We examine briefly some specific instances starting with syndiotactic polypropylene (sPP). Aside from the already discussed hexagonal mesophase which can be obtained both drawing fibers and under quiescent conditions, this polymer presents four crystalline forms phases I [73] and II [74-76] where chains adopt the (T2G2)n helical conformation, forms III [30] and... 

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