Stability of liquid crystalline state

THE EFFECT OF THE CHEMICAL STRUCTURE ON THE STABILITY OF LIQUID CRYSTALLINE STATE... 

At first,we studied the effect of the chemical structure of polymer chain on the stability of liquid crystalline state.The liquid crystalline state of polyether-esters derived from substituted hydroquinone and substituted 1,2-bis(phenoxy) ethane-4,4 -dicarboxylic acid could be controled by the... 

We clarified the reason of the lower Tm of Me-HQ/BB than that of Me-HQ/TA by investigating the stability of liquid crystalline state of the model compounds derived from BB and TA as shown in Fig. l.The reason that Me-HQ/BB showed lower Tm is probably the lower enthalpy of Me-HQ/BB due to the twisted chemical structure of biphenylene unit as reported by K.Tashiro (4). ... 

PEC or tBu-HQ/PEC showed the moduli of 26 and 36 GPa, respectively which were lower than those of Me-HQ/PEC or Cl-HQ/ PEC because of their decreased stability of liquid crystalline state by the presence of the bulky substituent as shown in Fig. 

In order to obtain high modulus as-spun fibers,the stability of liquid crystalline state and the rigidity of polymer chain are influential factors.Therefore,we expected that the polyarylates derived from BB would show higher modulus than those from PEC or Cl-PEC.As-spun fiber of the polyarylate derived from Cl-HQ and BB modified with TA(Cl-HQ/BB/TA) showed the modulus of 95 GPa(m/n= 7/3) which was higher modulus compared with that of Me-HQ/Cl-PEC. However,as-spun fiber of Cl-HQ/BB/TA showed the modulus of only 11 GPa(m/n=8/2) in spite of the rigid chemical structure,and this is referred to the decreased elongational flow orientation as shown in Fig. 4(3). 

FIGURE 2.5 The stacked bilayers of the skin barrier are envisioned as composed of crystalline domains separated by fringes of lipids in the liquid crystalline state.38 The fringe zones may actually oscillate in a very small time scale between a liquid crystalline state and a crystalline (gel) state. Such a tentative idea would mean that the barrier is open just temporarily at a certain location since penetration must occur in the liquid crystalline areas. Thus, the action of a penetration enhancer would be to stabilize a liquid crystalline state or transform it into another type of structure, for example, a cubic phase. 

The influence of substituents on the solubility of molecules in water can be due to their effect on the properties of the solid or liquid (for example, on its molecular cohesion) or to the effect of the substituent on its interaction with water molecules. It is not easy to predict what effect a particular substituent will have on crystal properties, but as a guide to the solvent interactions, substituents can be classified as either hydrophobic or hydrophilic, depending on their polarity (see Table 5.4). The position of the substituent on the molecule can influence its effect, however. This can be seen in the aqueous solubilities of o-, m- and p-dihydroxy-benzenes as expected, all are much greater than that of benzene, but they are not the same, being 4, 9 and 0.6 mol dm, respectively. The relatively low solubility of the para compound is due to the greater stability of its crystalline state. The melting points of the derivatives indicate that is so, as they are 105°C, 111°C, and IZO C, respectively. In the case of the ortho... 

The formation of miscible rubber blends slows the rate of crystallization (Runt and Martynowicz, 1985 Keith and Padden, 1964) when one of the components is crystallizable. This phenomenon accounts for data that show lower heats of fusion that correlate to the extent of phase homogeneity (Ghijsels, 1977) in elastomer blends. Additionally, the melting behavior of a polymer can be changed in a miscible blend. The stability of the liquid state by formation of a miscible blend reduces the relative thermal stability of the crystalline state and lowers the equilibrium melting point (Nishi and Wang, 1975 Rim and Runt, 1520). This depression in melting point is small for a miscible blend with only dispersive interactions between the components. 

The formation of bilayer vesicles was also observed for dialkyldimethylammonium bromide 33 in ether-containing ionic liquids 26, 27 (Figure 24) [124]. Ionic amphiphiles intrinsically possess solvophilic (or ionophilic) groups and electrostatic repulsions between them are highly screened in ionic liquids. In addition, the presence of ether-linkages in ionic liquids increases their net polarity. These factors contribute to enhance the association force of ammonium amphiphiles in ionic liquids and concurrently increase the thermal stability of gel(crystalline) state bilayers [124]. 

The greater stability of the crystalline state of networks formed from unoriented but crystalline chains compared with networks formed from amorphous polymers, can be explained in the same way as for networks formed from axially oriented natural rubber. Although prior to network formation the crystallites are randomly arranged relative to one another, portions of chains are still constrained to lie in parallel array. The cross-linking of the predominantly crystalline polymer cannot, therefore, involve the random selection of pairs of units. The units that can be paired are limited by the local chain orientation imposed by the crystalline structure. An increase in the isotropic melting temperature of such networks would therefore be expected. It can be concluded that orientation on a macroscopic scale is not required for partial order in the liquid state to develop. Concomitantly a decrease in the entropy of fusion will result, which reflects the increase in molecular order in the melt. This is an important concept that must be kept in mind when studying the properties of networks formed in this manner. This conclusion has important implications in studying the properties of networks formed from unoriented crystalline polymers. 

Polyarylates prepared from cyclohexyl-HQ (Ch-HQ) and PEC (Ch-HQ/PEC) did not show liquid crystallinity due to the more bulky substituent on the HQ unit compared to those on f Bu-HQ and Ph-HQ. As-spun fibers of Ch-HQ/PEC exhibited lower moduli than those of fBu-HQ/PEC and Ph-HQ/PEC. Therefore, in order to obtain high-modulus as-spun fibers, the stability of the liquid crystalline state (7j — 7j,) is an influential factor, as shown in Table 19.1. 

Therefore, we expected that the polyarylates synthesized from substituted HQs and BB would show higher stabilities of the liquid crystalline state and higher moduli than those produced from substituted HQs and substituted PEC. 

Thus, the influential factors in obtaining high moduli for injection molded specimens are as follows (1) the stability of the liquid crystalline state, (2) the rigidity of the polymer chain, and (3) the packing density of the polymer chain. 

Figure 1 demonstrates the drastic influence on the stability region of a lamellar liquid crystalline phase when an aromatic hydrocarbon is substituted by an aliphatic one. The lamellar phase formed by water and emulsifier is stable between 20 and 60 wt % water. Addition of an aromatic hydrocarbon (p-xylene) to the liquid crystalline phase increased the maximum amount of water from 45 to 85% (w/w) (Figure 1 left). Inclusion of an aliphatic hydrocarbon (n-hexadecane) gave the opposite result the maximum water content in the liquid crystalline state was reduced (right). Some of the factors which govern the association behavior of these surfactants and cause effects such as the one above are treated below. 

The best vesicle stabilization effect was obtained by polymerization of buta-diyne units within the hydrocarbon chains of amphiphiles after the vesicle was formed. This polymerization produces red or blue polyenes and occurs only if the vesicle membrane is in the liquid crystalline state. No polymers formed... 

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