Fractionation of Various Polymers

Other studies reported in the literature involve the fractionation of a high molecular weight silicone oil, a poly(dimethyl)siloxane (Krukonis, 1983c). In the early 1980s, the material was being tested for an electronics application. 

As a pictorial comparison of the peak widths of these fractions, figure 9.37 superposes the chromatograms of fraction 1 and fraction 6 onto the chromato- 

Tabie 9.26 Molecular Weight Distribution in Parent Polydimethylsiloxane and in Fractions Separated by Supercritical CO2 

Fraction Number Average Molecular Weight Weight Average Molecular Weight Parent (wt%) 

Supercritical ethylene is a better solvent and propane and propylene still 

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Table I. Fractions of Various Polymers and Copolymers Grafted...

Fractionation of Various Polymers 2Ti Table 9.31 Polymers Tested for Fractionation... 

Reciprocals of the critical temperatures, i.e., the maxima in curves such as those in Fig. 121, are plotted in Fig. 122 against the function l/x +l/2x, which is very nearly 1/x when x is large. The upper line represents polystyrene in cyclohexane and the lower one polyisobutylene in diisobutyl ketone. Both are accurately linear within experimental error. This is typical of polymer-solvent systems exhibiting limited miscibility. The intercepts represent 0. Values obtained in this manner agree within experimental error (<1°) with those derived from osmotic measurements, taking 0 to be the temperature at which A2 is zero (see Chap. XII). Precipitation measurements carried out on a series of fractions offer a relatively simple method for accurate determination of this critical temperature, which occupies an important role in the treatment of various polymer solution properties. 

If the e of a two-component solvent mixture is increased progressively, the character of the polystyrenes formed in it by various HA changes abruptly near e, ca. 6, from a polymer of low DP with a monomodal DPD to one with a bimodal DPD containing an appreciable fraction of high polymer [45]. 

The radiation-induced cross-linking of polyethylenes in the presence of CTFE/butadiene mixture with various compositions are shown in Figure 6. The overall degree of co-grafting decreased gradually with the increase in the mole fraction of butadiene in the mixture, while the gel fraction of the polymer was increased rapidly by the addition of a small amount of butadiene to CTFE and then decreased with the increase in butadiene mole fraction in the mixture. The maximum of the gel fraction was found at about 0.1-0.2 of butadiene mole fraction in the mixture. 

In a series of papers, Diphant has been used to probe the microviscous properties of various polymer oils, and free volume parameters have been extracted. 4,88 90) In a comparative study of Excimer and TICT probes, it could be shown that the response of these probes is frozen out at lower temperatures, as can be expected from the large reaction volume necessary, whereas the TICT reaction still shows sizable rates at these high-viscosity conditions. (26) Moreover, this study also showed that the free volume fraction measured by the TICT probes is larger than that measured by Excimer probes. 

Table 1.3 summarizes molecular weights (Mn) and second virial coefficient (A2) data for various fractions of this polymer. 

In later studies on the homopolymerization of E-l,3-pentadiene with NdO/ TIBA/DEAC crystalline polymers with cis- 1,4-contents in the range 84-99% and a high isotacticity were obtained. It was found that the cis- 1,4-content increases when the polymerization temperature is decreased from room temperature to -30°C. The polymerization of E-2-methyl-l,3-pentadiene resulted in polymers which almost exclusively comprised cis- 1,4-units and no dependence of the cis- 1,4-content on polymerization temperature was observed. The obtained poly(2-methyl-l,3-pentadiene) was composed of various polymer fractions with different stereo regularities [165,166]. 

The coil-globule transition can also be observed by PL (from changes in the IMM of the polymer), when polymers are precipitated in organic solvents. These data were obtained for PMMA in toluene precipitated with octane or other non-solvents. Figure 9 shows the dependence Tw on the concentration of octane used as the precipitant for PMMA fractions of various molecular weights. Naturally, for PMMA in... 

Results related to drug micronization by GAS are summarized in Table 2, which also includes some data dealing with biocompatible polymers often used as carriers in sustained delivery devices (91-95). Unless otherwise noted, the antisolvent used in these studies is carbon dioxide. The GAS technique was successfully carried out to achieve the fractionation of various mixtures (96,97). 

Measurements of osmotic pressure were carried out [F. Danusso, G. Moraglio, and G. Gianotti, J. Potym. ScL, 51, 475 (1961)] at different concentrations of cii-l,4-polybutadiene in isobutyl acetate at various temperatures, using different molecular weight fractions of the polymer. 

Van Krevelen [20] examined various equations for solution viscosity. He recommended the use of the following two equations [24,25] for the r ss of dilute polymer solutions (at polymer concentrations above the limit of c—>0 but with low values of c, especially c<0.01 g/cc). In these equations, Solv denotes the solvated volume fraction (see Section l.D for conversions between volume, weight and mole fractions) of the polymer, and p is the polymer bulk density. 

McHugh and coworkers describe the fractionation of various poly(ethylene-co-methyl acrylate) copolymers using supercritical propane, propylene, 1-butene, and chlorodifluoromethane (Meilchen, Hasch, and McHugh, 1991 Pratt, Lee, and McHugh, 1993). The objective of their work was to extend the work of Krukonis and coworkers (Scholsky et al., 1987 Watkins and Krukonis, 1991) who demonstrated the ability of fractionating by chemical composition of the polymer as well as molecular weight. This concept is also described with other polymer-SCF solvent systems in the next few sections of this chapter. 

Since the cell walls from most cereal endosperm contain little or no pectic substances, extraction with a chelating agent could be omitted. Various workers have used different schemes for extraction (Mares and Stone, 1973a Anderson and Stone, 1973 Fincher, 1975 Ballance and Manners, 1978), so it is difficult to make firm recommendations. However, the following sequential treatments of the CWM (1 g) may prove useful to effect partial fractionation of the polymers (1) The loosely held polysaccharides could be solubilized by incubation with 100 ml of 10 mM NaOAc, pH 5, at 40°C for 2 h three times. If the CWM is not completely free of... 

They are easily separated by various relaxation experiments [10,96]. Figure 20 shows partially relaxed NMR spectra of polymer 4 (a-CDj) at four different pulse separations ti in a saturation recovery and a quadrupole echo sequence (see Fig. 6). The spectra refer to the same temperature and parallel orientation of alignment axis and magnetic field. Again two spectral components are observed. The central peaks refer to the mobile fraction of the polymer while the outer peaks correspond to the... 

The results of numerical solution of Eq. (13) with such experimentally obtained B(X) values in the form of unoriented volume fraction of a polymer matrix vs.draw ratio curves for various filler contents are shown in Fig. 4. As could be expected, unoriented volun fraction of a polymer matrix for a given draw ratio X increases with... 

Table 5-7. Various Free-Volume Fractions of Amorphous Polymers Calculated from Crystalline Density at 0°C°...

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