A series of densely branched comb polystyrenes with well-defined architectural details were prepared by living anionic polymerization via the “grafting-through” approach. A new, general method was developed for the synthesis of well-defined, comb polystyrenes with controlled, variable amounts and types of branch end functionalities (C8F17, OH or COOH), by combining living anionic polymerization and thiol-end “click” chemistry. Characterization by NMR, SEC, and MALDI-TOF mass spectrometry established their chemical structures and chain-end functionalities, which indicated precisely defined comb polystyrenes with controlled degrees of functionalization.
Surface segregation in blends of well-defined comb and linear polystyrenes was investigated by neutron reflectivity (NR) and Static Time-of-Flight Secondary Ion Mass Spectrometry (STOF-SIMS) measurements. The results indicate strong enrichment of combs at the blend surface. In contrast to previously reported examples of blends of long-branched and linear chains, a thermal annealing process is not necessary for achieving significant surface segregation in these comb/linear blends. It is found that architecture details of the comb component have a strong impact on the bulk miscibility with the linear component in a blend and therefore also on its surface segregation. A self-consistent field theory by Wu and Fredrickson is able to describe the surface segregation behavior in the comb/linear blends reasonably well after accounting for a surface attraction for the ends higher than anticipated by the theory. Measurements of the surface segregation, as well as macroscopic hydrophobicity in blends containing branch end-functionalized comb polystyrenes, show that comb molecules with 25% branch chain ends functionalized with hydrophobic or hydrophilic groups still enrich the surface, though the contact angle of a blend depends on whether the functionalities themselves reside right at the surface, or are buried beneath the outermost layer of chain segments.
The surface height fluctuation dynamics of melt films of densely branched unfunctionalized comb polystyrenes of thickness greater than 55nm and at temperatures 23 to 58 °C above the bulk Tg can be rationalized using the same hydrodynamic continuum theory (HCT) useful for describing the behavior of melt films of linear and cyclic chains. The film viscosities inferred from fits of the HCT to intensity-intensity time correlation functions measured by X-ray Photon Correlation Spectroscopy (XPCS) are the same as those measured in bulk with conventional rheometry for three combs, though for two of the comb architectures the films viscosities are distinctly higher than the bulk viscosities. These combs are the one most like a star polymer and the one closest to showing bulk entanglement behavior. These discrepancies between film and bulk viscosities are much smaller than those seen for less densely branched polystyrenes. The fact that surface dynamics data for various combs do not collapse onto a single curve in a plot that accounts for differences in Tg,bulk among the various chains indicates that besides the important value of Tg,bulk, the chain architecture also plays a key role in determining surface fluctuations.