Smectic crystalline form

Brosse et al. [41] modified isotactic polypropylene and other polyolefins by a cold plasma. In isotactic polypropylene, plasma treatment results in a polypropylene crystallization of paracrystalline or smectic form into a a-crystalline form. Further, the active films are susceptible to react with monomers in a postgrafting reaction. 

In 1978, Bryan [11] reported on crystal structure precursors of liquid crystalline phases and their implications for the molecular arrangement in the mesophase. In this work he presented classical nematogenic precursors, where the molecules in the crystalline state form imbricated packing, and non-classical ones with cross-sheet structures. The crystalline-nematic phase transition was called displacive. The displacive type of transition involves comparatively limited displacements of the molecules from the positions which they occupy with respect to their nearest neighbours in the crystal. In most cases, smectic precursors form layered structures. The crystalline-smectic phase transition was called reconstitutive because the molecular arrangement in the crystalline state must alter in a more pronounced fashion in order to achieve the mesophase arrangement [12]. 

Crystalline forms presenting large amounts of disorder of the kind (ii) or (iii) are generally called mesomorphic modifications (Section 3.6), in analogy with the ordered liquids (smectic and nematic). In these cases the lack of periodicities in one or two dimensions (e.g., along the chain axes or along the directions normal to the chain axes) prevents the definition of a unit cell. Typical features in the X-ray diffraction patterns of mesomorphic forms are diffuse halos on the equator or on the layer lines depending on the kind of disorder present. 

Figure 11.1(c) shows the smectic crystalline of i-PP. This spectrum is similar to that of the a-form. This suggests interchain packing in smectic crystalline is similar to that of /3-form. 

All these PEIs derived from BiTCA proved to form a broad smectic-A type LC-phase and a smectic crystalline solid state (for definition see Sect. 7). The details of their syntheses and properties will be published in the near future [78-80]. 

A further modification of the PEI 82a-i consisted of the incorporation of ether groups into the aliphatic spacers [85]. Again, the PEIs 86a,b form a smectic crystalline state and a smectic A phase, but the temperature range of the smectic LC-phase is smaller than in the case of 80f-i. When hydroquinone or 2,6-hydroxynaphthalene were used as building blocks instead of 4,4 -dihydroxy-biphenyl, no LC-phases were formed at all. Hence, these results indicate that ether groups in the aliphatic spacers of PEIs destabilize the LC-phase. A further confirmation of this hypothesis will be presented below. 

Another interesting structural variation of the PEIs 81a-i and 82a-i was obtained by using methyl substituted diamines as spacers (PEIs 87a, b and 88a, b) [86]. Again, a destabilization of the layer structures was found. The PEIs 86a and 86b still formed as smectic crystalline state, but the melt of 86a was nematic and that of 86b a combination of smectic and nematic. The solid state of 88a, b turned out to be a smectic glass or in other words a frozen smectic-A phase. No... 

Another set of problems was studied in connection with the PEIs 179,180 [136]. It was found that all these PEIs do not show an LC-phase,but form a smectic crystalline solid state with upright mesogens (smectic-E like). However, the PEIs can in principle adopt two different kinds of chain packing and layer structures as illustrated in Fig. 29. The first is that the mesogens pack in an antipar-... 

Finally, it should be emphasized that the tendency of polyimides containing aliphatic spacers to form stable layer structures of various dimensions and degrees of order can be utilized for an even wider variety of studies. A recent example is a study of the epitaxial growth of polyethylene on smectic crystallites of PEI [140]. Most likely smectic crystallites adopt a lamellar form with the large surface covered by loops of the aliphatic spacers. This hypothesis still need detailed studies and confirmation. Anyway, smectic crystalline polymers are interesting substrates for studies of epitaxial crystallizations. Furthermore, layer structures derived from long aliphatic spacers (alkanes or oligoethers) may play... 

Isotactic polypropylene is known to crystallize when cooled quickly from the melt into a crystal form which was called a smectic, mesomorphic form. The X-ray diffraction pattern resembled clearly that of a smectic material. Later, this structure was also called para-crystalline to indicate the poor crystalline order. It was found that this structure is metastable below 335 K. At room temperature it has been reported to persist for over 18 months so that it should be called a CD-glass. The proposed transition mechanism to the stable crystal form involves intramolecular helix... 

As previously stated in Chapter 3, isotaetie polypropylene is a polymorphic material with four basic crystalline forms, namely the monoelinie (a), trigonal (P), orthorhombic (y) and mesomorphic smectic (intermediate state between ordered and amorphous phase) forms. The a-form exhibits excellent modulus and tensile strength but poor fracture toughness. ... 

Isotactic polypropylene can crystallize in different forms (modifications), such as a, P, y, and smectic, which differ by their unit cell type and thus by their packing density." The most common are the a-form and the y-form. The a-modification is the preferred crystalline form of polypropylenes synthesized by conventional Ziegler-Natta catalysts." High molecular weight isotactic polypropylenes prepared by metallocene catalysts preferentially crystallize in the y-form. The different polymorphic behaviors of metallocene and Ziegler-Natta samples can be related to the... 

Addition of talc into isotactic polypropylene (PP) induces two types of crystalline forms, i.e., a and forms [89,92,93 ]. Some reported that PP can form 4 different crystalline forms, i.e., a,jS,y,and(5 (smectic) forms [93]).Ferrageetal. [94] showed crystalline onset temperature shift by addition of talc and an a nucleating agent (sodium 2,2 -methyl-bis-(4,6-di-fert-butylphenyl phosphate) in polypropylene. Addition of triphenodithiazine, pimelic acid with calcium stearate, or quinacridone dye permanent red can increase thephase [94 to 96]. 

Thisayukta J, Nakayama Y, Kawauchi S, Takezoe H, Watanabe J (2000) Distinct formation of a chiral smectic phase in achiral banana-shaped molecules with a central core based on a 2,7-dihydroxynaphthalene unit. J Am Chem Soc 122 7441-7448 Tschierske C, Dantlgraber G (2003) From antiferroelectricity to ferroelectricity in smectic mesophases formed by bent-core molecules. Pramana J Phys 61 455-481 Ungar G, Percec V, Zuber M (1992) Liquid crystalline polyethers based on conformational isomerism. 20. Nematic-nematic transition in polyethers and copolyethers based on l-(4-hydroxyphenyl)2-(2-R-4-hydroxyphenyl)ethane with R=lluoro, chloro and methyl and flexible spacers containing an odd number of methylene units. Macromolecules 25 75-80 Urayama K (2007) Issues in liquid crystal elastomers and gels. Macromolecules 40 2277-2288 Vorlander D (1908) About transparently clear, crystalline liquids. Rep Ger Chem Soc 41 2033-2052... 

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