Membrane osmometer

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). 

Osmosis is the passage of a pure solvent into a solution separated from it by a semipermeable membrane, which is permeable to the solvent but not to the polymeric solute. The osmotic pressure n is the pressure that must be applied to the solution in order to stop the flow. Equilibrium is reached when the chemical potential of the solvent is identical on either side of the membrane. The principle of a membrane osmometer is sketched in Figure 2. 

Average of cryoscopy, ebulliometry, 11,000 15,300 18,500 and vapour-phase osmometry Rapid membrane osmometer, first 19,900 22,100 27,900 observable values (5-7 min after sample introduction)... 

Rapid membrane osmometer, 36 min 29,400 25,000 32,800 after sample introduction... 

The membrane osmometer is a device for measuring the osmotic pressure, n, of a biopolymer solution separated from the pure solvent by a semi-permeable membrane (Tanford, 1961 Edmond and Ogston, 1968 Tombs and Peacocke, 1974 Edsman and Sundelof, 1987 Amur et al.,... 

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). 

Osmometry. Measurements were made in tetrahydronaphthalene at 130 °C by using a membrane osmometer (Mechrolab model 502). The plots of osmotic pressures at different concentrations are shown in Figure 5. Comparing the osmotic pressure results with those of GPC (interpretation 1) it appears that molecules larger than A = 50 may have permeated the membrane. If this is the case, the MN by osmometry is too high. 

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. 

Figure 2.2 Schematic diagram of a membrane 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.

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.). 

Molar mass determination of the acetylated true graft copolymer, the separated branch after acetylation and the acetylated mother cellulose, was carried out with a high speed membrane osmometer using chloroform, benzene, and chloroform, resp., as solvents. 

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). 

The number average molecular weight was measured in benzene solution by Mechrolab membrane osmometer 502 and also by Hitachi vapor pressure osmometer 115. 

Obtained with a Hewlett-Packard high speed membrane osmometer using a super-dense membrane at 29.5°C. in toluene. 

Finally, several attempts have been made to develop an absolute molar mass detector based on osmotic pressure measurements. Commercially available membrane osmometers are designed for static measurements, and the cell design with a flat membrane is not suited for continuous flow operation. Different from the conventional design, Yau developed a detector which measures the flow resistance of a column caused by osmotic swelling and deswelling of soft gel particles used for the packing (see Fig. 12) [65,78]. With a microbore gel column, a... 

Measured with membrane osmometer (at 37 °C in toluene) Measured with v Kir pressure osmometer (at 57 °C in DMF)... 

Analysis of the C z and C e olefin mixtures by gas chromatography and/or NMR indicated that in both cases the internal isomers were the predominant product. With all the alkylaluminum coinitiators the ratio of internal to terminal olefin was 3 1. Product characterization data are shown in Table 1. Number average molecular weights of polyisobutylenes were determined using a Hewlett-Packard 503 high speed membrane osmometer at 37° C and toluene solvent. Intrinsic viscosities were obtained with a Ubbelohde capillary viscometer at 30° C in cyclohexane solvent. The viscosity average molecular weights were calculated... 

Measured in toluene with Hewlett-Packard 502 Membrane Osmometer irradiation with RUL 3500 8 lamps under high vacuum. 

The osmolarity of the cell suspension increases during a fermentation process as a result of metabolic events (0yaas, 1989). The influence on the cells and the product is still unclear. Therefore, with each development of a new cell culture process, a study on the importance of this parameter must be performed. Unfortunately, there is no on-line measurement system commercially available, although sterile installation of a membrane osmometer should be possible. 

Schematic representation of a membrane osmometer. The osmotic pressure is determined from the height...

Number-average molecular weights were determined in toluene solutions using a membrane osmometer (Mechrolab, Hewlett-Packard 503 with S S-08 membranes). Size exclusion chromatographic analyses in chloroform were performed by HPLC (Perkln-Elmer 601 HPLC) using two y-Styragel columns (lO", 10 A) after calibration with standard polystyrene samples. 

Dynamic osmometers reach equilibrium pressures in 10 to 30 minutes and indicate osmotic pressure automatically. Several types are available. Some commonly used models employ sensors to measure solvent flow through the membrane and adjust a counteracting pressure to maintain zero net flow. A commercially available automatic osmometer operates on the null-point principle. In this high-speed membrane osmometer schematically represented in Fig. 4.4, the movement of an air bubble inside the capillary immediately below the solvent cell indicates the solvent flow to the solution cell. Such movement is immediately detected by a photocell, which in turn is coupled to a servomechanism. If any movement of the air bubble is detected by a photocell, the servomechanism is stimulated to move the solvent reservoir upward or downward in order to adjust the hydrostatic pressure such that the solvent flow is completely arrested. The pressure head of the reservoir gives the osmotic head. Some osmometers also use strain gauges on flexible diaphragms to measure the osmotic pressure directly. 

Figure 4.4 Schematic diagram of essential components of a high-speed membrane osmometer.

Products were characterized by a Varian T-60 H1 NMR spectrometer, a Waters Associates Ana-Prep Gel Permeation Chromatograph, and a Mecrolab 503 High Speed Membrane Osmometer. 

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