High-speed 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). 

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). 

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

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... 

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. 

Figure 4.4 Schematic diagram of essential components of a high-speed membrane osmometer. (Hewlett-Packard Corp., Avondale, Pa.)...

Characterization. Number average molecular weights of the block copolymers in toluene were determined at 25 °C by a Mechrolab high speed membrane osmometer fitted with an 0-8 membrane. Styrene mole fractions in the block copolymers were determined by H -NMR in CC14 at 60°C. To determine the effect of fractionation procedure, sedimentation patterns of tetra-hydrofuran solutions of fractionated samples were examined using a Spinco model E ultracentrifuge at 59,780 rpm. 

Laboratory designed instruments were developed in the 40 s and 50 s, e.g. by Zimm or by Flory. Later on, high speed membrane osmometers are commercially available, e.g., from Knauer, Hewlett-Packard or Wescan Instruments. External pressures may be applied to balance the osmotic pressure if necessary, e.g., Vink. The principle scheme of a membrane osmometer together with the corresponding... 

Number average molecular weights of the samples were measured by the osmotic pressure measurements. The Knauer high-speed membrane osmometer was used with Sartorius Ultra-cellafliter sd as a semipermeable membrane. 

Osmotic equilibrium is not reached quickly after the solvent and solution first contact the membrane. Periods of a few hours or more may be required for the pressure difference to stabilize, and this equilibration process must be repeated for each concentration of the polymer in the solvent. Various ingenious procedures have been suggested to shorten the experimental time. Much of the interest in this problem has waned, however, with the advent of high-speed automatic osmometers. 

High-speed, automated membrane osmometers monitor the fluid volume in one of the chambers and externally apply the osmotic pressure to the solution chamber to prevent flow. Since no flow through the membrane is necessary, they can reduce measurement time from days to hours or even minutes. 

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