Polymers membrane osmometry

ADMET polymers are easily characterized using common analysis techniques, including nuclear magnetic resonance ( H and 13C NMR), infrared (IR) spectra, elemental analysis, gel permeation chromatography (GPC), vapor pressure osmometry (VPO), membrane osmometry (MO), thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The preparation of poly(l-octenylene) (10) via the metathesis of 1,9-decadiene (9) is an excellent model polymerization to study ADMET, since the monomer is readily available and the polymer is well known.21 The NMR characterization data (Fig. 8.9) for the hydrogenated versions of poly(l-octenylene) illustrate the clean and selective nature of ADMET. 

We report here the results of our recent studies of poly(alkyl/arylphosphazenes) with particular emphasis on the following areas (1) the overall scope of, and recent improvements in, the condensation polymerization method (2) the characterization of a representative series of these polymers by dilute solution techniques (viscosity, membrane osmometry, light scattering, and size exclusion chromatography), thermal analysis (TGA and DSC), NMR spectroscopy, and X-ray diffraction (3) the preparation and preliminary thermolysis reactions of new, functionalized phosphoranimine monomers and (4) the mechanism of the polymerization reaction. 

Practically, polymers with molar masses between 2 x 104 and 2 x 106 g/mol can be characterized by membrane osmometry, but measurements of Mn <104 g/mol have also been reported with fast instruments and suitable membranes [16]. The lower limit is set by insufficient retention of short polymer chains. Above M 2 x 106 g/mol, the osmotic pressure, which is proportional to Mr1, is too low for a reasonable signal-to-noise ratio. An advantage of the low molar mass cut-off is that impurities with a very low molar mass can permeate through the membrane and, hence, do not contribute to the measured osmotic pressure. Their equilibration time may, however, be different from that of the solute, leading to complex time-dependent signals. 

Experimental considerations Sample preparation and data evaluation are similar to membrane osmometry. Since there is no lower cut-off as in membrane osmometry, the method is very sensitive to low molar mass impurities like residual solvent and monomers. As a consequence, the method is more suitable for oligomers and short polymers with molar masses up to (M)n 50kg/mol. Today, vapour pressure osmometry faces strong competition from mass spectrometry techniques such as matrix-assisted laser desorption ionisation mass spectrometry (MALDI-MS) [20,21]. Nevertheless, vapour pressure osmometry still has advantages in cases where fragmentation issues or molar mass-dependent desorption and ionization probabilities come into play. 

Randomly - Crosslinked PDMS. The polydimethylsiloxane (PDMS) used to make random networks was obtained from General Electric. Membrane osmometry showed to be 430,000 g/g-mole. The polymer was mixed with various amounts of a free-radical crosslinking agent, dicumylperoxide (Di-Cup R, Hercules Chemical Co.). Samples were then pressed into sheets and crosslinking was effected by heating for 2 h at 150°C in a heated press. Mc values were calculated using equation 2, and are included in Table I. 

In membrane osmometry the two compartments of an osmometer are separated by a semi-permeable membrane only solvent molecule can penetrate through the semi-permeable membrane which is closed except for capillary tubes. The polymer solute remains confined to one side of the osmometer and the activity of the solvent is different in the two compartments. Because of the thermodynamic drive towards equilibrium a difference in liquid level in the two capillaries results. 

Of the preponderance of small ions, the colligative properties of polyelectrolytes in ionising solvents measure counterion activities rather than Molecular weight. In the presence of added salt, however, correct Molecular weights of polyelectrolytes can be measured by membrane osmometry, since the small ions can move across the membrane. The second virial coefficient differs from that previously defined, since it is determined by both ionic and non-ionic polymer-solvent interactions. 

For polydisperse polymer samples, measurements that lead directly to the determination of the molecular weight, such as light-scattering photometry and membrane osmometry, are referred to as absolute molecular weight methods. Techniques such as viscometry are not absolute molecular weight methods because they require calibration using an absolute molecular weight technique. 

A measure of any of the colligative properties involves counting solute (polymer) molecules in a given amount of solvent. The most common technique for polymers is membrane osmometry. The technique is based on the use of a semipermeable membrane through which solvent molecules freely pass, but through which the large polymer molecules are unable... 

Vapor pressure osmometry is slightly less sensitive than membrane osmometry (M < 2 X 10 ) but is not affected by very short chains in the polymer sample which migrate through the semipermeable membrane in the case of membrane osmometry. Therefore, it is in particular valuable for the analysis of oligomeric materials. 

The polymers were characterized by gel permeation chromatography (G.P.C.), membrane osmometry, intrinsic viscosity, and H-NMR spectroscopy. 

G.P.C. as well as the membrane osmometry results. The elution volume behavior for the star-branched materials appear insensitive to the overall star-branched polymer molecular weight, while more dependent upon arm molecular weight. This is what one might expect as the G.P.C. separation process occurs by differences in hydro-dynamic volume not actual molecular weight. As the star-branched arm molecular weight increases so does the hydrodynamic volume, hence earlier elution volumes would be expected with increasing arm molecular weight. 

Since polymer solutions are markedly non-ideal, osmotic pressure data are taken at low concentrations and are extrapolated to infinite dilution (c — 0). In the case of membrane osmometry, the relevant equation is... 

In membrane osmometry, molecular weights above a million are essentially impossible to measure because there are too few particles in a given weight of polymer. On the other hand, polymers with molecular weights less than 25,000 can cause problems by themselves diffusing through the membrane. 

Berth, G., Dautzenberg, H., Lexow, D., and Rother, G. (1990). The determination of molecular weight distribution of pectins by calibrated GPC. Part I. Calibration by light scattering and membrane osmometry. Carbohydr. Polym. 12 39-59. 

Figure 14 shows the plots of the ratio Mw/Mn of the CA fractions prepared by the SSF method 39 42), as a function of their Mw. Mw and Mn values were determined by light scattering and membrane osmometry, respectively. Except for a few fractions of CA(2.46) and CA(2.92), Mw/Mn values of CA fractions lie between 1.2 and 1.5, independently of their Mw values. Most of the fractions of cellulose derivatives reported in the literature were prepared by the SPF method 44) their Mw/Mn values range roughly from 1.2 to 3.7 (most of them 1.5-2.0) and moreover depend markedly on Mw. This indicates clearly that the SSF method is superior to SPF for cellulose derivatives, as it is also the case for synthetic polymers such as polystyrene 38). A computer simulation for a quasi-temary system carried out by K amide and Matsuda also showed the inconditional superiority of the SSF method 34 - 36 ... 

Figure 17 shows the plot of ATs/c vs. c (ATS = temperature difference at steady state c = concentration of polymer) for CA in THF 51 From the intercept of the plot at c = 0, Mn can be evaluated and compared with Mn values estimated by other methods (Table 5). The VPO method seems to yield Mn values only a few percent lower than those estimated by membrane osmometry (MO) or GPC. 

Colligative properties reflect the chemical potential of the solvent in solution. Alternatively, a colligative property is a measure of the depression of the activity of the solvent in solution, compared to the pure state. Colligative properties include vapor pressure lowering, boiling point elevation, freezing point depression, and membrane osmometry. The latter property is considered here, since it is the most important of the group as far as synthetic polymers are concerned. 

The Mark-Houwink relation for polypropylene in o-dichlorobenzene at 130°C was calibrated as follows. A series of sharp fractions of the polymer was obtained by fractionation, and the molecular weight of each fraction was determined by membrane osmometry in toluene at 90"C. The samples were then dissolved in o-dichlorobenzene at I30°C and their intrinsic viscosities ([ ]) were measured. The resulting data fitted an expression of the form... 

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