Columnar mesophase behavior of Poly(Di-n-alkylsiloxane)s

Effect of pressure on the dielectric behavior of a bent-core liquid crystal

We report the effect of applied pressure on the dielectric properties of the B2 phase of a bent-core liquid crystal. This study on bent-core banana-shaped molecules shows that while the dielectric anisotropy hardly varies with pressure, the relaxation parameters associated with the rotation around the long axes of the molecules are significantly influenced. These studies also bring out the fact that there are additional phases between the B2 phase and the true crystalline solid. Indeed, the existence of another variant of the B2 phase (labeled B2'), is revealed only in dielectric studies but not seen in x-ray and calorimetric measurements. Employing the dependence of the relaxation frequency along isobaric as well as isothermal paths, different activation parameters are determined and their behavior is compared with those of rodlike systems. The influence of dc bias on the dielectric behavior obtained at atmospheric pressure is also presented, which exhibits features similar to chiral antiferroelectric smectics, and further shows an additional relaxation over a small window of dc voltages.

Tris-cis-tris-trans-dodeca[organo(dimethylorganosiloxy)]cyclododecasiloxanes {RSi(O)[OSiMe2R']}12. Self-ordering features

The molecular order and thermotropic transitions of tris-cis-tris-trans-dodeca- [organo(dimethylorganosiloxy)]cyclododecasiloxanes {RSi(O)[OSiMe(2)R']}(12) (R = Ph, R' = Me, CH(2)Cl, Vi; R = Me, Et, Vi, R' = Me) have been investigated using differential scanning calorimetry, thermogravimetric analysis, and X-ray scattering. The cyclododecasiloxanes with phenyl side groups (R = Ph) can form mesomorphic structures within a very wide temperature range. Compounds with R = Me and Vi are liquids and exhibit microphase separation above their glass transition temperature because of the different nature and structure of the organic R and trimethylsiloxy OSiMe(3) side groups. When the side group R = Et, a mesomorphic structure is formed in a substantially more narrow temperature region than that for cycles containing phenyl groups. Thus, the type of side group R in organocyclododecasiloxanes determines their ability for self-ordering into mesomorphic structures and the thermal stability of the mesomorphic state.

Crystals and crystallinity in polymeric materials

A new view of the concepts of crystallinity and crystals in synthetic macromolecules is discussed. In polymeric materials, crystallinity may be present with the concomitant occurrence of large amounts of structural disorder and in the absence of true three-dimensional long-range order. The structures of semicrystalline polymeric materials are discussed in terms of idealized limit models of crystals, where long-range order may be achieved for structural features that are not necessarily coincident with single atoms and are not necessarily point-centered. Typical examples of non-point-centered structural features are the straight lines corresponding to the chain axes of polymer molecules. This view explains the unusual ability of polymers to crystallize even in the presence of a high degree of structural disorder.