Tailoring the mass distribution and functional group density of dimethylsiloxane-based films by thermal evaporation

The tailoring of molecular weight distribution and the functional group density of vinyl-terminated polydimethylsiloxane (PDMS) by molecular beam deposition is demonstrated herein. Thermally evaporated PDMS and its residue are characterized using gel permeation chromatography and nuclear magnetic resonance. Thermal fragmentation of vinyl groups occurs for evaporation temperatures above 487 K (214 degrees C). At a background pressure of 10(-6) mbar, the maximum molecular weight distribution is adjusted from (700 +/- 100) g/mol to (6100 +/- 100) g/mol with a polydispersity index of 1.06 +/- 0.02. The content of vinyl-termination per repeating unit of PDMS is tailored from (2.8 +/- 0.2)% to (5.6 +/- 0.1)%. Molecular weights of vinyl-terminated PDMS evaporated at temperatures above 388 K (115 degrees C) correspond to those attributed to trimethyl-terminated PDMS. Side groups of linear PDMS dominate intermolecular interactions and vapor pressure.