Analysis of monomethyl- and dimethylsiloxane polymers and copolymers (functionalized or nonfunctionalized) by supercritical-fluid chromatography

The performance of capillary supercritical-fluid chromatography (SFC) applied to the analysis of various polysiloxanes was compared with that of low mass size-exclusion chromatography (SEC). SFC gave far better results than SEC and appeared well adapted to the analysis of functionalized or nonfunctionalized polydimethylsiloxanes (PDMSs) of high degrees of polymerization (DP⩽70), the separation of linear and cyclic oligomers of polysiloxanes and the partial separation of the different oligomer series of dimethyl- and monomethylsiloxane copolymers. Moreover, the study of repeatability using either linear or integral decompression restrictors showed that, in both cases, SFC analyses were repeatable for retention times (repeatibility of 0.5 to 1%) as well as for quantitative measurements (repeatibility of 1.5 to 2%), up to high degrees of polymerization (DP of about 50). Finally, the study of the retention of PDMSs and of polymonomethylsiloxane at constant CO 2 density allowed their identification. We have demonstrated that the logarithms of the retention factors are linear functions of the degree of polymerization. Therefore, we propose a strategy that allows these complex mixtures to be identified, when spiking with pure standards is not possible. This method, based on the identification of the first oligomer of a polysiloxane using the relation ln k′= f( DP), permits the characterization of the different polysiloxanes analysed.