Cellulose triacetate, fractionation

Seven primary cellulose triacetate fractions were Isolated by this method. The first primary fraction rich in hemlcellulose was redissolved and reprecipitated into three subfractions in the same way as described above. The refractionation of the first fraction was necessary to isolate the hemlcellulose material for subsequent analysis and characterisation. 

A total of 50 injections were made. Fractions were recovered by removing dichloromethane under vacuum at low temperature. The cuts ware characterized in the same way as described previously for cellulose triacetate fractions. 

Narrow molecular weight cellulose triacetate fractions... 

Polystyrene standards. Solutions of the monodisperse polysty-renes (Waters, Mass., USA) in N-methylpyrrolldone (0.5 m/V) and dichloromethane (0.125 m/V) were used as calibrants. Figure 1 shows a plot of log (tj) vs. log Mn for cellulose triacetate fractions in dichloromethane at 21 C. 

Figure 1. Log [n] versus log Mjj relationship for cellulose triacetate fractions.

Narrow Molecular Weight Triacetate Fractions. Narrow molecular weight cellulose triacetate fractions were obtained by both fractional precipitation and preparative GPC as described above. The number average molecular weight (1 ) of the various fractions and cuts was determined by high speed membrane osmometry. A linear dependence of GPC elution volume on log molecular weight for all cellulose triacetate fractions was found in both methylpyrroli-done and dichloromethane. 

Universal Calibration. A function of the hydrodynamic volume [r ] M was plotted against the elution volumes of cellulose triacetate fractions and polystyrene standards run in dichloromethane have all indicated slight deviation from linearity as shown in Figure 2. 

The structural homogeneity of the various cellulose triacetate fractions obtained by fractional precipitation was established by both Infrared and nuclear magnetic resonance spectroscopy. 

Narrow Molecular Weight Triacetate Calibration. A linear relationship was found when log against the elution volumes of various cellulose triacetate fractions was plotted. For narrow molecular weight distribution triacetate fractions, the GPC experimental average molecular weight, termed can be expected to conform... 

Fig. 4a. Resolution of racemic (67) by column chromatography on microcrystalline cellulose triacetate (column B) and by recrystallization 35) C = crystals M = mother liquor the index following C or M gives the number of recrystallizations the fraction has undergone. The value of [ot] ° is given, followed by the quantity obtained...

Fractionation and Characterization of Commercial Cellulose Triacetate by Gel Permeation Chromatography... 

Cotmnercial cellulose triacetate samples were fractionated by both fractional precipitation and preparative gel permeation chromatography (GPC). The triacetate fractions were characterized by vlsco-metry, high speed membrane osmometry (HSMO) and GPC. A fair agreement has been found between the molecular weights of various triacetate fractions determined by the three procedures. 

All unfractionated cellulose triacetate samples and high molecular weight fractions showed a shoulder on the high molecular weight side of the GPC distribution. Material Isolated from this region was found to be highly enriched in mannose and xylose, attributed to the presence of a hemicellu-lose derivative. Cellulose triacetate from cotton linters did not show this behavior. 

A literature survey ( 1 - 11) on the fractionation of cellulose triacetate by precipitation indicates that in most cases it has been unsuccessful due to the possibility of hydrogen bonding between polymer and solvent in solutions (10. 12). GPC has been applied to the fractionation of cellulose derivatives by many workers. Segal (13), Meyerhoff (14 - 16), Muller and Alexander (17) have reported the fractionation of cellulose nitrate by GPC. Muller and Alexander (17), Brewer, Tanghe, Bailly and Burr... 

Preparative GPC of Cellulose Triacetate Sample. A 1% (m/V) solution of cellulose triacetate (medium) prefiltered through porosity 3 glass sinter was fractionated by repeated injection through the column set described above. Seven cuts covering the entire elution curve were collected. The flow rate, injection time and the experimental conditions were identical to those stated above. 

Fractional Precipitation of Cellulose Triacetate. The reported partial or non-fractionation of cellulose triacetate from chlorinated hydrocarbons or acetic acid may be explained in terms of the polymer-solvent Interaction parameter x (1-11) The x values for cellulose triacetate-tetrachloroethane and cellulose triacetate-chloroform systems are reported (10,21) as 0.29 and 0.34 respectively. The lower values of x for such systems will result in a smaller or negative heat of mixing (AHm) and therefore partial or non-fractionation of the polymer in question results. 

Linear polymers, polystyrene and cellulose triacetate exhibit differences in hydrodynamic behavior in solution. Cellulose and its derivatives are known to have highly extended and stiff chain molecules below a Dp of about 300, but as the Dp Increases above 300 the chain tends to assume the character of a random coll (27,28). The assumption that hydrodynamic volume control fractionation in GPC may not be true for polystyrene and cellulose triacetate, though it has been found satisfactory for non-polar polymers in good solvents (29). 

Table 1. Analytical Data of Various Cellulose Triacetate Samples and Fractions...

Fig. 3. Typical TLC chromatograms of cellulose triacetate (CTA) fractions and whole polymer (Ac w = 61.0wt%) having various Mw 12) solid lines fractions broken line whole polymer numbers on curves represent 10 4 Mw. (Rf = rate of flow.)...

The chromatograms of cellulose triacetate (CTA) whole polymer (Ac w = 61.0 wt %, dotted curve) and its fractions (solid curves) are illustrated in Fig. 3. For the cellulose diacetate (CDA) and CTA fractions, the TLC becomes apparently sharp with an increase in Mw. The double-peaked form of the chromatograms is characteristic of the CTA samples, although their gel permeation chromatography (GPC) curves have been found to be single-peaked. This fact implies that the peak at the lower end of Rf corresponds to fully substituted CTA and the peak at the higher end is obviously due to the existence of not-fully substituted acetate. In this sense, real CTA is a binary mixture of ideal CTA and CDA. 

Addition of solvents as solubilizers is important in the dissolution of polymers. Poly(vinyl acetate) is insoluble in pure ethanol, but dissolves fairly readily in ethanol containing 3% water. Cellulose triacetate is sparingly soluble in pure trichloromethane, but is readily soluble in trichloromethane containing 3% methanol. Many paint resins dissolve more readily in aromatic-containing naphtha fractions than in pure naphtha. Poly(vinyl chloride) is sparingly soluble in acetone and carbon disulfide, but is more readily soluble in a mixture of the two solvents. 

To illustrate, chloroparalEns are compatible with cellulose triacetate up to 50% but do not render any plastieiziug actioa On heating these plasticized films become fragile. On immersion in water, the ehloroparafBns are washed away completely. Chloroparaffins are extracted from the other derivatives of eellnlose in a similar way. The increase in the chloroparaffin content in PVC has httle effect on Tg. Low temperature performance is degraded as the chloroparafBn fraction increases in a mixture with phthalate plasticizers. 

The desulfurization of liquid fuels using pervaporation has been increasingly investigated over the last few years [84]. As middle distillates contain mainly aromatic sulfur compounds, desulfurization membranes tend to make use of developments in aromatic-aliphatic separation. The most frequently used membrane materials investigated for the desulfurization of liquid hydrocarbon mixtures are polyurea-polyurethane, polysiloxane. Nation, cellulose triacetate, and poly-imide [84]. In addition to a range of processes for the desulfurization of naphtha fractions patented by ExxonMobil, Transionics, and Marathon Oil, only the S-Brane process developed by W. R. Grace and Sulzer has been tested beyond the laboratory scale [84]. 

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