Comparison of different methods for the determination of the molar mass and molar mass distribution of poly(ethylmethylsiloxane)

Abstract

The application of a poly(dimethylsiloxane) (PDMS) calibration curve for size exclusion chromatography (SEC) to poly(ethylmethylsiloxane) (PEMS) leads to molar masses, which are 15–40% higher than expected compared to measurements of the absolute molar mass values. As for the analysis of physical properties of PEMS, the exact knowledge of the molar mass is fundamental, therefore PEMS calibration curves for both SEC and viscometry were established. Eleven narrow distributed linear PEMS samples in the range between 4 000 and 110 000 g/mol were synthesised by anionic ring‐opening polymerisation of a cyclic monomer. The molar masses of the PEMS samples were determined by matrix assisted laser desorption ionisation time‐of‐flight mass spectrometry (MALDI‐TOF‐MS), light scattering, membrane osmometry, vapour pressure osmometry, and by viscometry. The polydispersities calculated from MALDI‐TOF‐MS spectra were lower compared to the SEC results. With the help of universal calibration, a SEC calibration curve for PEMS was established. Small angle neutron scattering (SANS) experiments on a 50%/50% protonated/deuterated PEMS blend match exactly the molar mass obtained from the PEMS calibration curve for SEC.

Neutron Scattering Investigations on the Statics and Dynamics of Polydimethyl- and Polyethylmethylsiloxane Melts

“…So the desired molecular weights were achieved with an accuracy better than 10%. [12,13] The deuterated monomers were prepared in nine reaction steps starting from deuterated ethanol and methanol; the protonated monomers were synthesized from commercially available ethylmethyldichlorosilane. [14] Narrowly distributed protonated and deuterated PDMS were synthesized from the protonated respectively deuterated cyclic trimers in two reaction steps.…”

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“…Application of a PDMS calibration curve to these polymers overestimated the molar masses by between 15 and 40%. 15 Anionic ring opening polymerisation of 1 occurs by nucleophilic attack at both Q (route a) and D (routes b and c) silyl centres, followed by regioselective ring opening to give linear polysiloxanes with Q units as regular components of the polymer backbone. Multinuclear NMR measurements were used to assign the polymer structure and end group analysis was employed to determine the absolute molecular weight.…”