Thermotropic behavior

T. E. (1978). Studies on anomalous thermotropic behavior of aqueous dispersions of dipalmitoyl phosphatidylcholine-cholesterol mixtures. Biochemistry. H, 1984-1989,... 

Effect of the Polymer Backbone on the Thermotropic Behavior of Side-Chain Liquid Crystalline Polymers... 

A review of the literature demonstrates some trends concerning the effect of the polymer backbone on the thermotropic behavior of side-chain liquid crystalline polymers. In comparison to low molar mass liquid crystals, the thermal stability of the mesophase increases upon polymerization (3,5,18). However, due to increasing viscosity as the degree of polymerization increases, structural rearrangements are slowed down. Perhaps this is why the isotropization temperature increases up to a critical value as the degree of polymerization increases (18). 

The thermotropic behavior and phase diagrams of binary mixtures of copper and gold complexes of the type [MX(CNR)] (X = anionic ligand, R = p-alkoxyaryl group) have been studied [32] and the main results are ... 

Once bearing some substituents, the decrease of polarity of the sucrose derivatives makes them soluble in less-polar solvents, such as acetone or tert-butanol, in which some lipases are able to catalyze esterifications. Unlike proteases, which necessitate most often the use of an activated acyl donor (such as vinyl or trifluoroethyl esters), lipases are active with simple esters and even the parent carboxylic acids in the presence of a water scavenger. The selectivity of the lipase-catalyzed second esterification is specific for OH-6 allowing the synthesis of mixed T,6 -diesters.123,124 For some lipases, a chain-length dependence on the regiochemistry was observed.125 Selectively substituted monoesters were thus prepared and studied for their solution and thermotropic behavior.126,127 Combinations of enzyme-mediated and purely chemical esterifications led to a series of specifically substituted sucrose fatty acid diesters with variations in the chain length, the level of saturation, and the position on the sugar backbone. This allowed the impact of structural variations on thermotropic properties to be demonstrated (compare Section III.l).128... 

Percec et al.130bl have employed 1 — 2 C-branching and ether-type connectivity for the preparation of nonspherical, thermotropic liquid crystalline dendrimers. Observed thermotropic behaviors are predicated on mesogenic monomers that are capable of conformational isomerism. 

As described above, the presence of one or two terminal alkoxy chains in the chemical architecture of the mesogenic units induces a drastic modification in the thermotropic behavior of the dendrimer (from lamellar to columnar structures). The introduction of both types of mesogenic units (Fig. 28, Ll and L2) in different concentrations within the same dendritic structure, i.e. random co-dendrimers, should present a greater possibihty of tuning the symmetry of the mesophase exhibited by the material. A series of such G3 co-dendrimers was synthesized, in which both types of mesogenic... 

Experimental evidence for a core localization of hydrocarbon was obtained from NMR, calorimetry, and SAXRS studies (Katagiri et al., 1985, 1987). Thus, from the similarity in thermotropic behavior of pure hydrocarbons isolated from locust lipophorin, and the same hydrocarbons in the native lipoprotein, it was concluded that hydrocarbons are partially segregated from the other lipophorin components forming a hydrocarbon-rich cluster. The presence of an internal region of low electronic density was observed by SAXRS in lipophorins containing hydrocarbons, suggesting that hydrocarbons form part of an inner lipid core, which is not exposed to the aqueous environment. 

The transition of a given phospholipid from mesophase Lp to L<, is kno-wn as main transition temperature (Tm) and it characterizes the thermotropic behavior of the lipid membrane in the range of temperatures that the phenomenon takes place. 

Differential Scanning Calorimetiv has been used for studying the interaction of labdanes 1 (labd-13(E)-ene-8a,15-diol), 2 (labd-13(E)-cne-8a-oI-15-yl-acetate) and 3 (Iabd-14-ene-8,13-diol (Sclareol)) Fig. (12) [76-77] with DPPC model lipid bilayers. The thermotropic changes caused by the labdanes in DPPC bilayers could be used for evaluating the incoiporation efficiency of labdanes in DPPC liposomes. Furthermore in these studies [29, 76], a comparison with the structurally similar molecule to cholesterol was carried out based on their thermotropic behavior. The development of liposomal formulations incorporating 1 and 2 has been achieved, wliile the stability studies of the liposomal formulations based on ,-potential and on liposomal size have been evaluated. 

Polymer characterization. The thermotropic behaviors of the polyester-carbonates prepared were examined under a polarized hot stage microscope (Sybron Co.). Normally the sample became softer at 120-150°C and melted with birefringence afterwards, depending on the molecular weight and composition of the polymer. The samples stayed bire-fringent until 350°c or higher. 

A benzyl bromide-type initiator (MI-27) was utilized for the copper-catalyzed synthesis of 3-armed poly(acrylate)s with mesogen units as side-chain groups.324 The effect of molecular architecture on the thermotropic behavior was compared with the corresponding linear polymers in both living and conventional polymerizations. 

All the examples cited so far involve the formation of side chains by ionic com-plexation. Sallenave and Bazuin studied the selectivity of hydrogen-bond complex-ation in the presence of both ionic moieties and highly polar groups and also investigated the effect of these potentially interacting functions on the thermotropic behavior of SCLCP complexes [28]. A scheme of the compounds studied by these authors is 45 ... 

One of the main features of nonionic water-soluble cellulose derivatives is that they exhibit, like some other polyethers, an inverse solubility-temperature behavior, i.e. there is phase separation on heating above the so-called lower critical solution temperature (LCST). The temperature at which a polymer-rich phase separates is normally referred to as the cloud point (CP). For ideal solutions, this temperature corresponds to the theta-temperature. Actually, for some derivatives, the cloud point may be preceded, if the concentration is not too low, by a sol-gel transformation with an increase in viscosity and possibly formation of liquid crystals (see Sect. 3.5). As it will be seen later, this reversible thermotropic behavior may be detrimental to the performance of the derivatives or can be advantageneously utilized to develop applications. 

Table 16 Thermotropic behavior of the ortho-metaiiated compiexes 95-98 ...

Watanabe J, Ono H, Uematsn I, Abe A (1985) Thermotropk pdypeptides. 2. Molecular packing and thermotropic behavior of poly(L- utamate)s with low n-alkyi side chains. Macromolecules 18 2141... 

Thermotropic behavior of phospholipids in the presence of blood clotting factors. The changes in the transition temperature (Tm) of phospholipid mixtures induced by the presence of the proteins were monitored by fluorescence polarization of the hydrophobic probe, DPH, inserted in the bilayers. These changes are generally interpreted as indicative of phase separation, as already demonstrated for the effect of calcium, at concentrations higher than 10 mM in PS-rich bilayers (23), and in the case of some proteins (24). 

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