Description and calibration of an elasto-osmometer

Abstract

An elasto-osmometer is described in which a stretched and swollen strip of crosslinked silicone rubber serves as indicator. The change in retractive force which occurs when the solvent around the strip is replaced by a solution containing a polymeric solute, which does not penetrate into the strip, is inversely proportional to the number average molecular weight of the solute. The instrument can be operated at elevated (∼90°C) as well as at room temperature and can yield two or three molecular weights in a working day. Although calibration by means of a sample of known molecular weight is the best operating procedure, it is also possible to put elasto-osmometry on an absolute basis. To this end the network parameters of the swollen silicone strip have to be determined. Examples of both procedures are given for a number of samples of polystyrene and polyethylene, ranging from 2 000 to 14 000 in molecular weight. The theory indicates that with isotropic silicone rubber strips molecular weights up to at least 40 000 should be attainable. The instrument yields erroneous results at molecular weights below 5 000 because of penetration of solute into the swollen strip.