Tilted Structures

Note 4 The biaxiality of the phase does not result from tilted structures as, for example, in... 

Crystallization from the melt often leads to a distinct (usually lamellar) structure, with a different periodicity from the melt. Crystallization from solution can lead to non-lamellar crystalline structures, although these may often be trapped non-equilibrium morphologies. In addition to the formation of extended or folded chains, crystallization may also lead to gross orientational changes of chains. For example, chain folding with stems parallel to the lamellar interface has been observed for block copolymers containing poly(ethylene), whilst tilted structures may be formed by other crystalline block copolymers. The kinetics of crystallization have been studied in some detail, and appear to be largely similar to the crystallization dynamics of homopolymers. 

The experimental observations could be consistently explained if the general tilt structure (SmCo) inside the layers is assumed. For most bent-core smectics the polar vector is perpendicular to the tilt plane, defined by the layer normal and averaged long axis direction, just as polarization in the ferroelectric rod-like liquid crystalline systems. However, since in the bent-core liquid crystals the polar order is decoupled from the tilt order, the polar director can in general have any direction in space thus it can also have a non-zero component along the layer normal. This can be achieved by a combination of tilting (rotation around the polar director) and leaning (rotation around the direction perpendicular to the polar director) of... 

Fig. 11 XRD pattern of modulated SmCG general tilt structure with the synclinic tilt of molecules in the neighboring layers. The pattern corresponds to a structure with the body center oblique crystallographic unit cell note that for this phase the tilt of the unit cell is correlated to the tilt of the molecules...

Numerical results show that the reduction of the cone angle in the region of the unfavorable splay is smaller in the general tilt structure than in the structure with polarization being perpendicular to the tilt plane. The cone angle reduction is coupled to the undulation of layers. So in the general tilt structure the undulation is less pronounced than in the regular Blrev phase (Fig. 13), as was indeed observed experimentally [25]. 

Fig. 13 In the general tilt structure stronger lamellarization is predicted by the theoretical model and also observed experimentally. Left, the layer structure in the Blrev-type phase right, the lamellar structure in the higher temperature general tilt structure...

Gorecka E, Pociecha D, Vaupotic N, Cepic M, Gomola K, Mieczkowski J (2008) Modulated general tilt structures in bent-core liquid crystals. J Mater Chem 18 3044-3049... 

The shifting of all the diffraction maxima to lower 26 was proportional to the size of alkyl chain of the fatty acids and indicated a linear relationship between basal spacing and the number of carbon atoms in the fatty acid alkyl chain, which is depicted in Figure 8.6(b). A rather good linear correlation (R = 0.94) was observed between basal layer distance and the number of carbon atom (ajc) of the fatty acid. It is also interesting to note that the slope of the linear fit (0.10 nm) was very close to the expected value of the C-C bond length of fatty acids (0.15 nm). The small deviation was possibly attributed to the tilted structure of the fatty add in the interlayer space of clay minerals. 

Then the SAM formation was studied. To this end, the orientation of the molecule with respect to the surface and the size of the aliphatie ehain were incrementally changed. A transition from perpendicular to tilted structures was calculated for n = 7, after which the tilt angle to the surface is 35° (see Fig. 5.13). This angle optimizes the lateral-lateral interactions. The adsorption energy decreases with increasing chain length (to reach the value of -2.77 eV for n = 9). This was attributed to the stabilization by van der Waals forces through lateral interactions. 

Figure 5.16b shows a broken fan texture, which results from cooling down the sample from the focal conic fan texture of the lamellar L phase. The continuity of the fans suggests that the lamellar structure persists at the phase transition. However, the breaking of the fans again indicates the formation of a tilted structure. 

The textures presented already dehver strong evidence that the lyotropic phase discussed possesses a lamellar, fluid and tilted structure. Further proof that this phase is indeed a lyotropic analog of the thermotropic SmC phase was obtained by more detailed stractural investigations via X-ray diffraction cf. Sect. 5.3). 

Hydrocarbon-bridged [2]-ferrocenophanes (51) possess strained ring-tilted structures (tilt-angles = ca 21°) and these species have been foimd to yield poly-ferrocenylethylenes (52) via ROP at 250-300°C (eq. 33) (172). As a consequence of the presence of a more insulating bridge, these polymers show much smaller interactions between the iron atoms compared to polyferrocenylsilanes. 

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