ANALYSIS OF MONOMETHYLSILOXANE 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 tha t of low mass size-exclusion chromatography (SEC). SFC gave far better results than SEC and appeared well adapted to the analysis of functio nalized or nonfunctionalized polydimethylsiloxanes (PDMSs) of high deg rees of polymerization (DP less than or equal to 70), the separation o f linear and cyclic oligomers of polysiloxanes and the partial separat ion of the different oligomer series of dimethyl- and monomethylsiloxa ne copolymers. Moreover, the study of repeatability using either Linea r or integral decompression restrictors showed that, in both cases, SF C analyses were repeatable for retention times (repeatibility of 0.5 t o 1%) as well as for quantitative measurements (repeatibility of 1.5 t o 2%), up to high degrees of polymerization (DP of about 50). Finally, the study of the retention of PDMSs and of polymonomethylsiloxane at constant CO2 density allowed their identification. We have demonstrate d that the logarithms of the retention factors are linear functions of the degree of polymerization. Therefore, we propose a strategy that a llows 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 In k'=f(DP), permits the characterization of the different polysiloxanes analysed. (C) 1998 Elsevier Science B.V. All rights reserved.