Vapor phase osmometer

Vapor Phase Osmometry. A Wescan Model 233 vapor phase osmometer was used to obtain number average molecular weights. The lignin solutions were made up with HPLC grade tetrahydrofuran (THF) and shaken manually until the solutions were clear. The experiments were conducted at 30°C. Number average molecular weights were determined by multistandard calibration (41), a procedure found to greatly enhance reproducibility and accuracy of the results. Experiments were conducted immediately after sample preparation and three days later. 

Figure 2.4 Schematic diagram of a vapor phase osmometer. Reprinted with permission from J. E. Mark, Physical Chemistry of Polymers, ACS Audio Course C-89, American Chemical Society, Washington, DC, 1986. Copyright 1986, American Chemical Society.

In the vapor phase osmometer, two matched thermistors are located in a ther-mostatted chamber which is saturated with solvent vapor. A drop of solvent is placed on one thermistor and a drop of polymer solution of equal size on the other thermistor. The solution has a lower vapor pressure at the test temperature (Eq. 2-72), and so the solvent condenses on the solution thermistor until the latent heat of vaporization released by this process raises the temperature of the solution sufficiently to compensate for the lower solvent activity. At equilibrium, the solvent has the same vapor pressure on the two temperature sensors but is at different temperatures. 

In fact, thermal equilibrium is not attained in the vapor phase osmometer, and the foregoing equations do not apply as written since they are predicated on the existence of thermodynamic equilibrium. Perturbations are experienced from heat conduction from the drops to the vapor and along the electrical connections. Diffusion controlled processes may also occur within the drops, and the magnitude of these effects may depend on drop sizes, solute diffusivity, and the presence of volatile impurities in the solvent or solute. The vapor phase osmometer is not a closed system and equilibrium cannot therefore be reached. The system can be operated in the steady state, however, and under those circumstances an analog of expression (3-6) is... 

Vapor phase osmometers differ in design details. The most reliable instruments appear to be those which incorporate platinum gauzes on the thermistors in order to ensure reproducible solvent and solution drop sizes. In any case, the highest purity solvents should be used with this technique, to ensure a reasonably fast approach to steady state conditions. 

The upper limit of molecular weights to which the vapor phase osmometer can be applied is usually considered to be 20,000 g mol . Newer, more sensitive machines have extended this limit to 50,000 g niol" or higher. The measurements are convenient and relatively rapid and this is an attractive method to use, with the proper precautions. 

Figure 4.8 Schematic diagram of a vapor phase osmometer.

Figure 4.7 Schematic diagram of a vapor-phase osmometer. (After Rudin, 1982.)...

D. E. Burge, Calibration of vapor phase osmometers for molecular weight measurements, J. Appl. Polym. Sci. 24, 293 (1979). 

Number-average molecular weights ("Rn) of several of the polysulfone oligomers were measured using chloroform solutions at 40 C with a Wescan model 233 vapor phase osmometer. Sucrose octaacetate was used to calibrate the instrument. 

Polymer MW can be determined by gel permeation chromatography (GPC) calibrated with polystyrenes (Pepper, 1978). Pol)mers number average MW can be determined in acetonitrile by using vapor phase osmometer (Leonard et al, 1966). 

The molecular weight and its distribution of prepolymers were determined in THF by gel permeation chromatography (Waters Associates). Four different pore size columns (10, 10, 10 and 10 A i -Styrogels) and a refractive index detector were used. The instrument was calibrated with polyisoprene samples which were prepared similarly in this laboratory and whose number-average molecular weights were determined with a vapor phase osmometer (Wescan, Model 232 A) in toluene at 50 C. 

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