Systems cellulose 3,0-acetate+acetone

Reuvers AJ and Smolders CA. Formation of membranes by means of immersion precipitation The mechanism of formation of membranes prepared from the system cellulose acetate-acetone-water. J. Membr. Sci. 1987 34 67-86. 

Figure 11. Three-component phase diagram at 25 C at the systems poly-amide-NMP-water, polyamide-DMAC-water, polyamide-DMSO-water, and cellulose acetate-acetone-water.

V , the polymer-rich phase or the polymer-lean phase separates iRltially from the solution. If v Is smaller than the critical solution concentration, v, the p lymer-rlch phase separates as a small particle, whose slzf, defined by Its radius S., may be almost equal to or slightly larger than the critical radius, S, of the phase separation. Hereafter we call this particle the primary particle" (see Figure 2). As shown In a later section, primary particles have diameters, 2S, of around 20 to 30 nm In the cases of cellulose cuprammonlum soTutlon/acetone and cellulose acetate/ acetone systems. These particles grow Into secondary particles with radii of S.,... 

The dry-cast process for polymeric membrane formation involves dissolving the polymer in an appropriate volatile solvent containing a small amount of nonsolvent to form a single-phase solution. Subsequent evaporation of the solvent causes a phase separation to occur at a sufficiently low solvent concentration. The resulting nonsolvent-rich dispersed phase forms the pores, whereas the polymer-rich phase forms the matrix structure of the membrane. Thus, in the cellulose acetate/acetone/water system, acetone evaporates, cellulose acetate (CA) becomes the continuous phase, and water forms the pores. 

Future needs for instrumentation in the field are many. Longer low-g periods will be needed in order to assess a fuller breadth of problems. Nearly all of the low-gravity observations to date have been limited to the cellulose acetate-acetone-water system. Other polymer solutions will require more or less time to demix. A majority of work has focused on dry casting, although early apparatus was designed around wet casting. Extended MCA designs will be required in order to ... 

Cellulose acetate acetone/DMAc, DCM/methanol, chloroform/meth anol Effect of solvent system on electrospinning of cellulose acetate [130]... 

Figure 8. Phase diagram of the system in cellulose acetate-formamide-acetone...

Fig. 22. Linear expansion coefficient as of cellulose acetate (CA)-solvent systems plotted as a function of Mw 7). The lines are determined by the least-square method. Numbers on the lines denote the total degree of substitution F of CA. O CA(0.49)-DMAc CA(i.75)-DMAc A CA(2.46)-DMAc A CA(2.46)-acetone jL CA(2.46)-THF CA(2.92)-DMAc...

Fig. 40. Plot of the unperturbed chain dimension A against the total degree of substitution for cellulose acetate (CA)-solvent systems. Solid line CA-DMAc chain line asymptotic A at the limit of the dielectric constant s = I broken line A of cellulose at the free rotational state7 asymptotic A value at the limit of s = 1 asymptotic A value at the limit of e = 1 and = 0 (j formamide ) water (j DMAc O- acetone -O THF O TCE...

In systems 5 and 6, this phenomenon is a result of hydrogen-bond formation between the polymer and solvent, which enhances the solubility. As hydrogen bonds are thermally labile, a rise in T reduces the number of bonds and causes eventual phase separation. In solutions, which are stabilized in this way by secondary bonding, the LCST usually appears below the boiling temperature of the solvent, but it has been found experimentally that an LCST can be detected in nonpolar systems when these are examined at temperatures approaching the critical temperature of the solvent. Polyisobutylene in a series of n-alkanes, polystyrene in methyl acetate and cyclohexane, and cellulose acetate in acetone all exhibit LCSTs. 

The basic principles are the same for the preparation of both the cellulose acetate dope and the cellulose triacetate dope with the exception of the particular solvent mixture used for each. The flake, the solvent mixture, and a filtering aid are added to a heavy-duty mixer. The solution is prepared in a fully enclosed system to minimize solvent losses and also to meet strict exposure levels regarding the workers. In the case of cellulose acetate, the main solvent is acetone, which is highly flammable. Therefore, the vapor-air ratio must be maintained at a level that meets safety regulations. Strict fire codes are maintained in the dope-preparation department as well as in the fiber-spinning department. 

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). 

Figure 3.90 are presented the experimental values of UCST and LCST with thc-oretical values for the systems cellulose 3,0-acetatc (CTA) + acetone and cellulose 2,8-acetate (CDA(2,8)) + acetone (Cowie et al., 1971). The authors calculated 7 from the thermal expansion coefficient of acetone and from F.qiiations 28 and 26. The figure demonstrates qualitative agreement between theory and experiment, after the axes for the theoretical and experimental curves T are shifted by 169° relative to each other. 

How does the choice 6f the solvent now influence the membrane structure when water is used as the nonsolvent and cellulose acetate as the polymer The first interesting point is that the slope of the tie lines, which connect the two phases in equilibrium in the two-phase region, is less steep when the mumal affinity (or miscibility) between the solvent and the nonsolvent decreases [35,43]. The binodal and tie lines are depicted in figure in - 43 for the system water/solvent/CA, where the tie lines become steeper as the miscibility with water increases in the order DMF > dioxan > acetone >THE Light transmission measurements conducted on the same water/solvent/CA systems are shown in figure III - 44. When DMSO (e , DMF (d) and dioxan (c) are used as the solvent, instantaneous demixing occurs. Only when the solvent is added to the coagulation bath is... 

Figure 6. Light reflectance vs. time recorded using the system shown in Figure 5 with a filament-lamp illuminator at 1 g. The casting-solution composition was 10% cellulose acetate, 27 % water, 63 % acetone (wt/wt). The initial thickness of the solution was 150 pm. The solid line indicates the drop in reflectance due to demixing (phase separation). Demixing begins after about 3.5 seconds and lasts approximately 3 seconds.

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