Secondary ion mass spectrometry of poly(styrene-co-isoprene) diblock copolymers: Improved surface sensitivity using the high-mass region

Thin film preparations of symmetric poly(styrene-co-isoprene) (PS-PI) diblo ck copolymers on silver were examined by TOF-SIMS. This diblock system is k nown to segregate, forming a PI block layer at the surface. A direct compar ison between the low- and high-mass regions of TOF-SIMS spectra is presente d. In the low-mass region, characteristic PI ions indicate that the surface is primarily composed of PI; however, a detectable amount of PS is also pr esent at the surface. Thick polymer films were found not to have measurable quantities of PS at the surface. This indicates that surface segregation o f thin film preparations on silver is not as extensive as in thick films or that a matrix effect exists due to the silver foil. In contrast, the high- mass region of TOF-SIMS spectra indicates that the surfaces of PS-PI dibloc ks are composed entirely of PI. In the high-mass region, large PI fragment ions form a repeat spectral pattern, providing unambiguous identification o f the PI chain. Absence of PS fragment ions in the high-mass region suggest s that the escape depth for high-mass ions is less than for low-mass ions. The estimated escape depth of high-mass ions is less than or equal to5 Angs trom. This is a a-fold improvement in surface sensitivity over low-mass ion s, which have a reported information depth of approximately 10 Angstrom. It is probable that high-mass ions are formed in less-energetic regions of th e collision cascade further from the center of impact and closer to the sur face. Various molecular weight (MW) PS-PI diblocks were examined, and it wa s observed that the relative intensity of the R-n - Delta and R-n clusters is sensitive to the molecular weight of the PI block. PI homapolymers, simi lar in MW to the diblock copolymers, yield high-mass spectra nearly identic al to PS-PI diblocks, supporting the conclusion that the surface of the dib locks is composed only of PI.