Rigid-chain polymers phase equilibria

Phase Equilibrium in a Rigid-Chain Polymer-Solvent System.81... 

The systems involving rigid-chain polymers and forming both the liquid-crystalline and the crystallosolvate phase are characterized by a complex phase equilibrium. The general principles of constructing phase diagrams (topological analysis) allow us to assume that the sequence of phase transfonnations in such systems has the... 

Phase equilibrium in a rigid-chain polymer-solvent system may be considerably affected by various factors. In particular, this applies to... 

One of the specific features of these polymer systems is a low mobility of the macromolecules and correspondingly slow phase transition rates. This enables one to use, in analyzing such systems, composite phase diagrams showing all the types of phase equilibria inherent in a given system. Extension of this principle to the systems "rigid-chain- polymer-solvent" makes it possible to construct a phase diagram which combines (a) equilibrium with the formation of a crystalline phase, (b) equilibrium with the formation of liquid-crystalline phases, and (c) equilibrium with the formation of amorphous (liquid) phases. 

The paper considers a general type of such a composite phase diagram for a rigid-chain polymer and gives examples of phase transitions with delayed kinetics in setting up the equilibrium state. 

The phase equilibrium in systems containing rigid-chain polymers is characterized by the formation of a liquid-crystalline state, which fact can be illustrated by the diagram due to Flory reproduced in Figure 3. At x values below 0,the polymer-solvent system forms either an isotropic (one-phase) solution mixture of... 

The general phase equilibrium diagram shown in Fig. 5 makes it possible to consider (due account being taken of the phase transition kinetics) many practical cases of separation of rigid chain polymers from solution when obtaining fibers and films. This is particularly important when studying the structural properties of the polymeric materials obtained, and in controlling their properties. 

Fig. 2.5. Theoretical phase equilibrium diagram in V2 % coordinates for the rigid-chain polymer-solvent system (according to Floiy [18]) with x = 1(X) (I isotropic. A anisotropic phases).

Special attention should be turned to the sharp transition from a narrow concentration corridor to a broad heterophase region, mentioned above, which takes place for low positive values of parameter x- It is int esting to compare the appearance of this broad region with the phenomenon of decomposition of solutions of flexible-chain polymers into two phases with the formation of two liquid (amorphous) phases with values of x in the limit (with infinitely high molecular weight of the polymer) of 0.5. TTie phase equilibrium diagrams (in coordinates v-x) for a rigid-chain polymer with an axial ratio of x = 150 and 350... 

Precipitation of the polymer on addition of a nonsolvent or with any changes in the thermodynamic parameters in solutions whose conc tration is below the critical point of the transition to the liquid-crystalline state is the most typical case of the intermediate phase equilibrium in rigid-chain polymer-solvent systems. Instead of the anticipated establishment of isotropic-anisotrqric phase equilibrium, equilibrium of two amorphous (isotropic) phases initially arises if the parameter x attains values greater than +0.5. 

PHASE EQUILIBRIUM DIAGRAMS OF RIGID-CHAIN LINEAR POLYMERS... 

A specific feature of rigid-chain polyamides related to the possibility of dissolution of these polymers only due to the very energetic interaction of the elementary units of the polymer with the solvent molecules is manifested in these systems. At low temperatures, salt compounds crystallize out in the form of crystal solvates [42] with a constant polymer-sulfuric acid molar ratio. At high temperatures, the crystal solvate melts and equilibrium involving a liquid-crystalline phase is attained. The polymer-acid complex in this example is thermally unstable and decomposes at relatively low temperatures. Melting of the compound with decomposition— incongruent melting—results in the appear-... 

For a specific polymer, critical concentrations and temperatures depend on the solvent. In Fig. 15.42b the concentration condition has already been illustrated on the basis of solution viscosity. Much work has been reported on PpPTA in sulphuric acid and of PpPBA in dimethylacetamide/lithium chloride. Besides, Boerstoel (1998), Boerstoel et al. (2001) and Northolt et al. (2001) studied liquid crystalline solutions of cellulose in phosphoric acid. In Fig. 16.27 a simple example of the phase behaviour of PpPTA in sulphuric acid (see also Chap. 19) is shown (Dobb, 1985). In this figure it is indicated that a direct transition from mesophase to isotropic liquid may exist. This is not necessarily true, however, as it has been found that in some solutions the nematic mesophase and isotropic phase coexist in equilibrium (Collyer, 1996). Such behaviour was found by Aharoni (1980) for a 50/50 copolymer of //-hexyl and n-propylisocyanate in toluene and shown in Fig. 16.28. Clearing temperatures for PpPTA (Twaron or Kevlar , PIPD (or M5), PABI and cellulose in their respective solvents are illustrated in Fig. 16.29. The rigidity of the polymer chains increases in the order of cellulose, PpPTA, PIPD. The very rigid PIPD has a LC phase already at very low concentrations. Even cellulose, which, in principle, is able to freely rotate around the ether bond, forms a LC phase at relatively low concentrations. 

Many cellulose derivatives form lyotropic liquid crystals in suitable solvents and several thermotropic cellulose derivatives have been reported (1-3) Cellulosic liquid crystalline systems reported prior to early 1982 have been tabulated (1). Since then, some new substituted cellulosic derivatives which form thermotropic cholesteric phases have been prepared (4), and much effort has been devoted to investigating the previously-reported systems. Anisotropic solutions of cellulose acetate and triacetate in tri-fluoroacetic acid have attracted the attention of several groups. Chiroptical properties (5,6), refractive index (7), phase boundaries (8), nuclear magnetic resonance spectra (9,10) and differential scanning calorimetry (11,12) have been reported for this system. However, trifluoroacetic acid causes degradation of cellulosic polymers this calls into question some of the physical measurements on these mesophases, because time is required for the mesophase solutions to achieve their equilibrium order. Mixtures of trifluoroacetic acid with chlorinated solvents have been employed to minimize this problem (13), and anisotropic solutions of cellulose acetate and triacetate in other solvents have been examined (14,15). The mesophase formed by (hydroxypropyl)cellulose (HPC) in water (16) is stable and easy to handle, and has thus attracted further attention (10,11,17-19), as has the thermotropic mesophase of HPC (20). Detailed studies of mesophase formation and chain rigidity for HPC in dimethyl acetamide (21) and for the benzoic acid ester of HPC in acetone and benzene (22) have been published. Anisotropic solutions of methylol cellulose in dimethyl sulfoxide (23) and of cellulose in dimethyl acetamide/ LiCl (24) were reported. Cellulose tricarbanilate in methyl ethyl ketone forms a liquid crystalline solution (25) with optical properties which are quite distinct from those of previously reported cholesteric cellulosic mesophases (26). 

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