Molecular osmometer

Our primary objective in this section is the discussion of practical osmometry, particularly with the goal of determining the molecular weight of a polymeric solute. We shall be concerned, therefore, with the design and operation of osmometers, with the question of units, and with circumventing the problem of nonideality. The key to these points is contained in the last section, but the details deserve additional comment. 

Attempts were made to determine number average molecular weights (Mn) by osmometry (Mechrolab Model 502, high speed membrane osmometer, 1 to 10 g/1 toluene solution at 37 °C), however, in many instances irreproducible data were obtained, probably due to the diffusion of low molecular weight polymer through the membrane. This technique was abandoned in favor of gel permeation chromatography (GPC). 

If 0.6 N lithium bromide is added to the solution of the polyelectrolyte and also to the solvent on the opposite side of the osmometer membrane, the lowermost set of points in Fig. 145 (lower and left scales) is observed. The anion concentration inside and outside the coil is now so similar that there is little tendency for the bromide ions belonging to the polymer to migrate outside the coil. Hence the osmotic pressure behaves normally in the sense that each poly electrolyte molecule contributes essentially only one osmotic unit. The izjc intercept is lower than that for the parent poly-(vinylpyridine) owing to the increase in molecular weight through addition of a molecule of butyl bromide to each unit. 

Such an osmometer was first designed by Herzog and was further modified by Fuoss and Mead. This type of osmometers is widely used for the determination of Molecular weight. 

Vapour pressure osmometer is a variation of the isopiestic or of the isothermal distillation techniques by which a solvent and a solution in that solvent are placed side by side in a closed container. It measures the difference in temperature created by the condensation of solvent on a sensitive thermistor containing a solution of the solute whose Molecular weight is to be determined. 

The vapour pressure osmometer method is more acceptable of all the methods involving measurement of colligative properties because of the sensitivity of the detector. For ideal solvent-solvents with a low heat of vaporisation, the differential thermistors of the VPO can detect differences in temperature of the order of 0.001°C this sensitivity determines the Molecular weight of the samples upto 20,000. 

Owing to the very low degree of polymerisation of the polymers obtained, molecular weights (Mn) were determined with a MECROLAB Vapour Pressure Osmometer, model 301 A with benzene as solvent. For values of Mn ranging between 400 and 2000 the reproducibility was 30 units. 

The reaction products were precipitated in methanol containing 0.880 ammonia. The precipitated polymer was filtered off and dried at 50 °C in a vacuum oven. Evaporation of the filtrate yielded the oligomers. For molecular weight determinations, the two fractions were dissolved together in CC14. Molecular weights were determined with a vapour-pressure osmometer. 

Number-average molecular weight was measured in 1,2-di-chloroethane at 40°C using a Wescan vapor pressure osmometer. 

Number-average molar masses were determined using a vapor pressure osmometer (VPO) (Hitachi 117 Molecular Weight Apparatus) at 54.8 0.1°C in toluene (Fisher Scientific, certified A.C.S.) which was distilled from freshly crushed CaH2. The VPO apparatus was calibrated with pentaerythritol tetrastearate (Pressure Chemical). Gel permeation chromatographic (GPC) analyses were performed in tetrahydrofuran by HPLC (Perkin-Elmer 601 HPLC) using six y-Styragel columns (106, 105, 10l, 103, 500, and 100 A) after calibration with standard polystyrene samples. 

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. 

Vapor Pressure Osmometry. Number-average molecular weights were evaluated with a vapor pressure osmometer (Knauer) following a previously described method (18). 

Number average molecular weight was determined in benzene solution by Hitachi 117 vapor pressure osmometer(VPO) at 42°C. 

George R. Hill Molecular weight determinations are now being made with an osmometer. We assumed that molecular weight decreases with degree of extraction (or increase in temperature). 

The number-average molecular weights of poly(4-methyl-1 -pentene) s were measured in toluene at 37° C with a Hewlett-Packard membrane osmometer (model 503). 

Membrane Osmometry and Viscometry. Number-average molecular weights of PMMA were determined with a Mechrolab model 501 highspeed membrane osmometer in toluene at 60 °C except for samples 122-4 and 122-6 which were determined at 40°C. 

Two grams of HSPAN was added to 500 ml of water, the pH was adjusted from 9 to 6.5 with dilute hydrochloric acid, and 0.1 ml of Thermamyl 60L enzyme solution (Novo Enzyme Corp.) was added. The resulting mixture was heated at 95°C for 21 hr, and the clear yellow solution was exhaustively dialyzed against distilled water. Freeze drying yielded 1.235 g of polymer, which contained only about 5% residual carbohydrate (by infrared analysis). The number average molecular weight of this polymer was 44,000, as determined in 0.15N sodium chloride solution on a Melabs Model CSM-2 membrane osmometer equipped with a B-19 membrane (Schleicher and Schuell Co.). 

Number average molecular weights were determined in toluene using a Mechrolab high speed membrane osmometer with Schleicher and Schuell, type U.O. very dense cellophane membranes. Previous work has established that under these conditions diffusion of this type of polymer through the membrane is not detectable at molecular weights down to about 6000 (18). 

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