A new Membrane Osmometry Detector for GPC

K-D. Bures,

Membrane Osmometry is a well known method for molecular weight determination of polymers. This method has been used for decades but lost meaning in the past 15 years due to upcoming methods, like static light scattering or viscometry, coupled to separation technique. A new development opens now new possibilities for the fast determination of absolute Mn values.

The core of a membrane osmometer is build by a chamber separated into two compartments by a semi-permeable membrane. One of the compartments, called reference compartment, is sealed and connected to a sensitive pressure transducer device capable of resolving pressure changes within a few fractions of a Pascal. The second compartments, sample compartment, is accessible for sample loading. When both compartments are filled with pure solvent, no pressure is detected. When sample enters the sample compartment, osmosis takes place and solvent molecules diffuse through the membrane toward the sample compartment. This creates a reduction of the pressure in the reference compartment which is then measured by the pressure transducer. For very small concentrations, according to the van Hoff equation,

Eq. 1: Equation 1

the measured osmotic pressure is function of molecular weight of the sample, of the concentration and of the temperature.

The traditional method of operation implies measurement of a set of concentrations of the polymer under investigation and graphical determination of the limiting value of osmotic pressure. Application of a traditional membrane osmometer in a continuous flow environment like GPC, was impossible due to the slow response time of the membranes as well as because of the large sample volume usually necessary to complete a measurement. Also, the poor availability of membranes with low molecular weight cutoff as well as pressure transducers with high sensitivity, narrowed in the past the available measurement range from 10 000 g/mol to aprox. 100 000 g/mol, limiting therefore the field of application.

The need for a truly absolute Mn values within a modern Gel Permeation Chromatography (GPC) or continuous flow environment, led to the development of a new type of membrane osmometer capable to cover a wide range of molecular weights with high accuracy and precision.

Fig. 1Measured Molar Masses

The osmotic chamber of the Mn-2010 is designed to operate in a continuous flow environment, as it can be found in semi-online or in GPC applications thus allowing very fast molecular weight determination. Typical measurement times are as low as few minutes for semi-online measurements, in GPC measurement time is dictated by the required elution volume. Also, unlike older instruments, a single, highly diluted concentration of the polymer under investigation is necessary to obtain the absolute Mn value. This technique can be easily automated via an automatic sample injector, avoiding therefore the need of constant human assistance.

Fig. 2Elugram

In the semi-online configuration, a small sample volume of rd. 100 µl is injected and transported through the osmotic cell by a constant flow of pure solvent, delivered by a pulseless pump. In this case, calculation of Mn only requires knowledge of the sample concentration, injected sample volume, flow rate and solvent density. No further parameter or calibration is necessary. The obtained Mn is then called “Bulk Mn” and corresponds to the value obtained in former times by measuring a set of different concentrations to obtain the limiting value of the osmotic pressure.

In case molecular weight distribution is of interest, then connection of the Mn-2010 to a GPC system is necessary. Due to the nature of GPC, it is also necessary to measure concentration along with the osmotic pressure in order to calculate the molecular weight at every single point of elution volume. This is done with a refractive index detector connected along with the osmometer. This set-up delivers then two parameters, the osmotic pressure and the concentration of the sample. This method, called “Slice” method, calculates local concentration as well as osmotic pressure at every points of the chromatogram. The combination then allows calculation of local molecular weight which then turn into molecular weight averages Mn, Mw and Mz.

Fig. 3: Results

A number of different polystyrene standards in THF was measured at a flow rate of 1 ml/min with the osmometer in order to determine precision and accuracy of the method. As shown in Fig 1, the available measurement range expands from 5 000 g/mol to 500 000 g/mol with excellent match to the expected values. Below 5000 g/mol, which is the molecular weight cutoff of the membrane, permeation occurs and a correct determination is not possible. Unlike other mass-sensitive detectors like Multi Angle Laser Light Scattering (MALLS) or viscometers, the osmometer requires no calibration whatsoever and no sample specific parameter is needed for calculation of molecular weight. This fact makes this techniques very attractive in all those applications where other techniques fail like in case of co-polymers. As a matter of fact, Osmometry is currently the only method which delivers absolute molecular weight of co-polymers, independently of composition or structure. Table 1 shows a comparison of results obtained with different techniques for the same compounds. Clearly, results obtained from conventional GPC, depends upon the Standards used for calibration (Polystyrene) while MALLS results depend upon the dn/dc value used for calculation (Polystyrene).

Table 1:
Comparison of results obtained with different techniques for PS-PB and PS-PIP

Sample Mw, g/mol
Mn, g/mol
Mn, g/mol
(GPC Cal)
Mn, g/mol
Mn, g/mol
Poly(styrene -b-butadiene)
[PS 4000 PB 16600]
20600 18559 16500 10120 12100
Poly(styrene -b-isoprene)
[PS 6400 PIP 14900]
21300 20882 22700 15700 20800

Due to its ability to deliver accurate and fast molecular weights from a single measurement, the Mn-2010 Membrane Osmometer represents a breakthrough in polymer characterization, opening new possibilities for absolute characterization of homo- and co-polymers.