Mesophase formation solvent selection

Only a few solvents are known to dissolve cellulose completely, and solid cellulose decomposes before melting. Therefore, it is difficult to study the mesophase behavior of cellulose. Chanzy et al. [32] reported lyotropic mesophases of cellulose in a mixture of jV-methyl-morpholine-Af-oxide and water (20-50%), but were unable to determine the nature of the mesophase. Lyotropic cholesteric mesophase formation in highly concentrated mixtures of cellulose in trifluoroa-cetic acid + chlorinated-alkane solvent [33] and in ammonia/ammonium thiocyanate solutions [34] has been studied, and although poor textures were obtained in the polarizing microscope, high optical rotatory power has been measured in an optical rotation (ORD) experiment, which could be fitted to the de Vries equation [Eq. (3)] for selective reflection beyond the visible wavelength region and was taken as proof of a lyotropic chiral nematic phase. 

Mesophase formation in diblock copolymer solutions has been studied when both blocks arc in a common good solvent or in a highly selective solvent (good solvent for one block and poor solvent for the other block). 

On the other hand, literature data show [16] that different cellulose derivatives which form liquid crystalline solutions in organic solvents may also form cholesteric thermotropic phases in the absence of a solvent—with spontaneous molecular orientation and cholesteric reflection, such as 2-acetoxypropyl cellulose, 2-hydroxypropyl cellulose, the trifluoroacetate ester of hydroxypropyl cellulose, the propanoate ester of hydroxypropyl cellulose, the benzoate ester of hydroxypropyl cellulose, 2-ethoxypropyl cellulose, acetoacetoxypropyl cellulose, trifluoroacetoxypropyl cellulose, the phenylac-etate and 3-phenylpropionate of hydroxypropyl cellulose, phenylacetoxy, 4-methoxy-phenylacetoxy, p-tolylacetoxy cellulose, trimethylsilyl cellulose, trialkyl cellulose, cellulose trialkanoate, the trialkyl ester of (tri-o-carboxymethyl) cellulose, 6-o-a-(l-methylnaphthalene)-2,3-o-pentyl cellulose, etc. Moreover, the suspensions of cellulose crystallites spontaneously form the chiral nematic phase. The formation of mesophase suspension of cellulose crystalHtes varies from one type of cellulose to another, being influenced, in the formation of the chiral nematic phase, by the mineral acid selected... 

Lyotropic mesophases contain at least two chemical components the organic molecule and its solvent. The organic moiety must exhibit some chemical complexity, or otherwise the solvent will simply dissolve the molecule, forming a structureless - and certainly not liquid crystalline - molecular solution of dispersed and disordered molecules. The simplest examples are amphiphilic molecules. The addition of a solvent such as water will selectively hydrate the hydrophilic moiety of each molecule, avoiding the hydrophobic regions. This schizophrenic relationship between the solvent and solute drives the molecules to self-assemble, thereby minimizing the exposure of hydrophobic moieties to the water. (Clearly, the argument holds in reverse if a lipophilic solvent, such as an alkane, is used. Indeed, a combination of hydrophobic and hydrophilic solvents can also lead to the formation of liquid crystalline mesophases.)... 

The final section deals with the problem of intermolecular organization in block copolymer solutions. For dilute solutions in a selective solvent, we discuss micelle formation. For nonselective solvents, we analyze the formation of various organized mesophases. Also in these systems excluded volume interactions lead to nontrivial subtle effects. ... 

Amphiphilic molecules [11-14] consist of mutually incompatible components. Since these components are chemically joined, complete segregation is impossible. It is replaced by various forms of microphase separation. These involve formation of segregated domains such that at least one of their dimensions is comparable to the molecular size. The domains are formed by spontaneous, thermodynamically driven aggregation of the amphiphiles. The process is thus often referred to as self-assembly. The resulting structures, micelles, lamellae, etc. can also form ordered mesophases. The microphase separation can take place in a solvent that selectively solubilizes one component or in a melt of neat amphiphiles. These characteristics are common to both polymeric and monomeric, low molecular weight amphiphiles. For the purposes of our discussion monomeric amphiphiles are defined, somewhat arbitrarily, as those consisting of 10 atoms. Polymeric amphiphiles, on the other hand, can incorporate 10 -10 atoms. The consequences of this difference are the topic of this article. 

In a highly selective solvent intermolecular organization occurs even in the dilute regime. Micelle formation has been sudied using a Flory-type model which gives scaling laws for the critical micelle concentration and the micellar sizes. The critical micelle concentration decays exponentially with molecular weight and occurs thus in an extremely dilute solution that is hardly measurable experimentally. In more concentrated solutions micelles order and mesophases are formed. 

---