MOLECULAR-WEIGHT DEPENDENCE AND END-GROUP EFFECTS ON THE SURFACE-TENSION OF POLY(DIMETHYLSILOXANE)

The surface tensions of amine-, hydroxyl-, and methyl-terminated poly(dimethylsiloxane) (PDMS) oligomers with molecular weights ranging from 1000 to 75 000 are measured by pendant drop tensiometry. The surface tension increases with molecular weight for the methyl-terminated polymers, decreases with molecular weight for the amine-terminated polymers, and is nearly independent of molecular weight for the hydroxyl-terminated polymers. This behavior is attributed to differences in surface tension of the end groups and the repeat unit of the chain (sigma(infinity) - sigma(e)); the methyl end group has a lower surface energy than the PDMS backbone whereas the surface energy of the amine group exceeds that of PDMS. The surface tension of the hydroxyl end group more closely matches that of the repeat unit of the chain. The surface tensions of the homologous series of materials extrapolate to the same value at infinite molecular weight when surface tension is assumed to follow an inverse dependence on molecular weight, indicating that this is the correct correlation for high molecular weight polymeric materials. Group contribution methods are used to predict the surface tensions for each group of materials. When compared to experimental results, these predictions suggest that the methyl end groups are preferentially attracted to the surface whereas amine end groups are depleted from the surface.