Molecular weights from osmotic pressure

EXAMPLE 3.1 Molecular Weights from Osmotic Pressure Measurements. To what molecular weights do the limiting reduced osmotic pressures obtained from Figure 3.5 correspond The data in Figure 3.5 are presented in such a way that unit problems do not enter the picture. We are not usually so lucky Consider some possible combinations of units for tt, V, and R that are compatible with the units of the ordinate in Figure 3.5. 

Molecular weight from osmotic pressure measurements Degree of polymerization and molecular weight distribution Excluded volume from osmotic pressure measurements Theta temperature from second virial coefficient data Evaluation of charges on macroions from osmotic pressures... 

Example 14-14 Molecular Weight from Osmotic Pressure... 

Theta temperature n. (1) (0, Flory temperature). d is also the temperature for the polymer solution at which the second virial coefficient in the equation giving molecular weight from osmotic pressure approaches zero. (2) The critical temperature of mixing for a polymer of infinitely high degree of polymerization. 

EXAMPLE 16.2 Determining molecular weights from osmotic pressure. 

Figure 8.8 Osmotic virial equation and the estimation of molecular weight from osmotic pressure data. The polymer is PMMA (poly(methyl methacrylate)) in various solvents at 20 °C. Toluene is a good solvent, acetone is a poor solvent, and in acetonitrile the polymer forms globular structures. From the y-axis intercept on the figure (c = 0) the molecular weight can be estimated to be about 348 000 g mot . Adapted from Hamley (2000), with permission from John Wiley Sons, Ltd...

The number average molecular weight is required. This is obtained directly from measurements of a colligative property, such as the osmotic pressure, of dilute polymer solutions (see Chap. VII). It is often more convenient to establish an empirical correlation between the osmotic molecular weight and the dilute solution viscosity, i.e., the so-called intrinsic viscosity, and then to estimate molecular weights from measurements of the latter quantity on the products of polymerization. 

Equation 3.27 forms the basis for determination of Molecular weight from light scattering data. Like Osmotic pressure measurements, it is essential to consider the non-ideality of solutions and the concentration dependence. Following Debye, eq. 3.27 gets modified to... 

EXAMPLE 3.3 Excluded Volume of Bovine Serum Albumin from Osmotic Pressure Measurements. A plot of 7r/c versus c for an aqueous solution of the bovine serum albumin molecule at 25°C and pH = 5.37 is shown in Figure 3.6. The molecule is known to be nearly spherical and uncharged at this pH. Evaluate the molecular weight and the excluded volume of this protein from the intercept and slope of this line, 0.268 torr (g kg-1)-1 and 1.37 10 3 torr kg2 g 2, respectively. From the particle mass and volume, estimate the partial specific volume of the solute in solution. The specific volume of the unsolvated protein is about 0.75 cm3 g 1 does the solute appear to be solvated ... 

From osmotic pressure measurements, the molecular weight of amylose has been determined as approximately 10,000-50,000. This value is well in line with the value estimated from the determination of the terminal functional groups. 

Calculated from the amino acid composition. The molecular weights were calculated from sedimentation and difiiision measurements, unless otherwise stated. Determined by sedimentation equilibrium studies. Determined by light-scattering. Value from osmotic-pressure measurements, 15,400. minimum value determined by gel permeation. Value from osmotic-pressure measurements, 59,500. Extrapolated from published data. 

Evidence that glycogen has a high molecular weight was published in 1923. Samec and Isajevic found that a sample of dog-liver glycogen had a molecular weight of 1.1 X 10, from osmotic-pressure measurements. ... 

The occurrence in a-casein, the protein with 0.99% phosphorus (95), of 40 % of this element as monoester, 40 % as diester, and 20 % as pyrophosphate requires a minimum of at least ten atoms of phosphorus per molecule. This would correspond to a minimum molecular weight of 31,000, a value in agreement with that of 33,000 reported from osmotic pressure measurements (12) or any multiple thereof. 

Problem 4.8 Show that the molecular weight determined from osmotic pressure measurements is the number average molecular weight. 

The initial number average molecular weights (Mn) for the poly-(amic-acids) were between 10,000 and 20,000, and no attempt was made to optimize them. Dimethylacetamide solvent was distilled from calcium hydride under vacuum and stored under nitrogen. The resin solutions were stored also under nitrogen. The water content of the solvent was measured with a Porapak Q column in a Hewlett Packard 5750 gas chromatograph. Unless otherwise stated, the water concentration in the solvent remained below 0.05 vol %. Mn was determined from osmotic pressure measurements on a Hewlett Packard-Mechrolab 502 osmometer using ArRo Lab gel cellophane 600D membranes. 

Equation (11-210) is used in the determination of the molecular weights of high polymers from osmotic-pressure measurements. 

Using molecular weight values from osmotic pressure data, Stau-dinger evaluated the K , constant for partially degraded potato starch triacetate as 1.02 X 10 in chloroform, 0.69 X 10 in acetone, and... 

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