Spacers textures

Figure 9. Top Two tetraurea calixarene monomers connected by a rigid spacer at their bottom rim display diverging hydrogen-bonding sites ideally suited for polymerization. The polymer (53) bears capsules of ca. 1.6 nm x 2.2 nm dimensions like beads on a string. Bottom Photomicrographs of typical Schlieren textures of 53 in chloroform (top row, left) and p-difluorobenzene (top row, middle) as viewed...

Figure 2. DSC trace of methyl-substituted poly(ester) with pentamethylene spacer, which displayed both smectic and nematic textures.

Insofar as small crystals of nonreducible oxides dispersed on the internal interfaces of the basic structural units (platelets) will stabilize the active catalyst surface Fe(lll), the paracrystallinity hypothesis will probably hold true. But the assumption that this will happen on a molecular level on each basic structural unit is not true. The unique texture and anisotropy of the ammonia catalyst is a thermodynamically metastable state. Impurity stabilization (structural promotion) kinetically prevents the transformation of platelet iron into isotropic crystals by Ostwald ripening [154]. Thus the primary function of alumina is to prevent sintering by acting as a spacer, and in part it may also contribute to stabilizing the Fe(lll) faces [155], [156], [298],... 

The polymers with diethylene oxide spacers (PEO dimer) reportedly had, in addition to a nematic mesophase, a monotropic smectic mesophase that was detected by X-ray diffraction. No confirmation was available by microscopy because almost all polymers developed a homogeneous texture that gave no real detail. 

Blumstein and coworkers studied the cholesteric behavior of polyesters with azoxybenzene mesogenic units and the same chiral spacer, (+) 3-methyladipic acid. They could clearly observe oily streak textures, which are typical of low molecular weight cholesterics, for the following homopolymer and copolymers ... 

Cholesteric oily streak textures could again be clearly observed for these LC polymers containing chiral spacers. In general, the pitch of the helical packing increased in a regular manner not only with temperature, as judged by an iridescent color, but also with the amount of achiral component in the copolymers. 

Recently we studied several polymers of structure 36 in Table 1 that were identified by texture observation as being smectic X-ray diffraction patterns showed a spacing at approximately 5 A and others at 28, 31, and 29 A for the polymers with 9,10, and 12 methylene groups in the spacer, respectively. Such repeat lengths are not consistent with the presence of fully extended chains in the smectic A mesophase for these polymers. Instead, the polymer with a nonamethylene spacer appears to form a smectic C phase in which the polymer chain is tilted with respect to the layers, as suggested by the small interlayer distances. 

Several fundamental studies have shown the importance of monomer sequence distribution on mesophase behavior (26). Simply changing the direction of ester linkages in a chain affects the transition temperatures, the range of the mesophase stability and, in some cases, even the mesophase texture (2Z). Polyester chains are susceptible to transesterification, which raises the question of which sequence structure is actually responsible for the properties observed for a given polymer. A recent study of aromatic LC polymers by neutron scattering indicates that transesterification occurs in the mesophase at rates twice that in poly(ethylene terephthalate) (28). Such behavior has also been observed to occur in other aromatic polyesters where rapid sequence redistribution was detected by nmr, see for example, the chapters by Jin and Economy et al. The temperature dependence of this effect has not been fully explored, and it may not be as pronounced in those polymers which exhibit mesophase behavior at lower temperatures, for example, those with aliphatic spacers. 

Senthil and Kannan prepared ferrocene-based liquid crystalline polymers containing phosphate groups in the backbone.191,192 The majority of these polymers showed grainy nematic textures, whereas polymers with 8 and 10 methylene groups in the backbone displayed clear nematic textures. Liquid crystalline behavior was not observed for the polymer incorporating both ethylene spacers and a pendent phenyl group. 

Cholesteric PEIs may also be obtained by extension with chiral spacers - a methyl substituent placed on the alkyl chain of the aforementioned thiophenol on the 3 carbon [20]. Photo-reactive cholesteric PEIs with this type of structure have subsequently been reported by the same author [23] formed from the condensation of 1,4-phenylene di-acrylic acid and N-(4-carboxyphenyl) trimellitimide with a chiral spacer prepared from (S)-3-bromo-2-methylpropanol and 4-mercaptophenol. The resulting polymer had enantriotropic cholesteric melt with grandjean texture upon slight shearing (6). 

The texture of polymeric chiral liquid crystalline phases. The chiral liquid crystalline phases include the chiral smectics and the chiral nematic or cholesteric phase. Poly(7-benzyl-L-glutamate) and derivatives of cellulose are popular examples of polymers that form a chiral mesophase. Side-chain type copolymers of two chiral monomers with flexible spacers of different, lengths and copolymers of one chiral and the other non-chiral mesogenic monomers may also form a cholesteric phase (Finkelmann et al., 1978 1980). In addition, a polymeric nematic phase may be transformed to a cholesteric phase by dissolving in a chiral compound (Fayolle et al., 1979). The first polymer that formed a chiral smectic C phase was reported by Shibaev et al. (1984). It has the sequence of phase transition of g 20-30 Sc 73-75 Sa 83-85 I with the Sc phase at the lower temperature side of Sa- More examples of Sc polymers are given by Le Barny and Dubois (1989). 

The Rh complex [Rh(cod)(diphosphine)], with diphosphine ligands functionalized with hydroxyls groups, was immobilized onto several ACs and carbon cloth with different textural chemistry previously modified with the spacer naph-toic acid the materials were characterized by gas adsorption, TPD, and TG-DSC... 

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