Secondary Ion Mass Spectrometry of Poly(styrene-co-isoprene) Diblock Copolymers:  Improved Surface Sensitivity Using the High-Mass Region

• Published in: Macromolecules 2001-03, Vol.34 (6), p.1845-1854
• Main Authors: Mehl, John T, Hercules, David M
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Structure, morphology and analysis
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma000544l

Thin film preparations of symmetric poly(styrene-co-isoprene) (PS−PI) diblock copolymers on silver were examined by TOF-SIMS. This diblock system is known to segregate, forming a PI block layer at the surface. A direct comparison between the low- and high-mass regions of TOF-SIMS spectra is presented. 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 present at the surface. Thick polymer films were found not to have measurable quantities of PS at the surface. This indicates that surface segregation of 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 diblocks are composed entirely of PI. In the high-mass region, large PI fragment ions form a repeat spectral pattern, providing unambiguous identification of the PI chain. Absence of PS fragment ions in the high-mass region suggests that the escape depth for high-mass ions is less than for low-mass ions. The estimated escape depth of high-mass ions is ≤5 Å. This is a 2-fold improvement in surface sensitivity over low-mass ions, which have a reported information depth of approximately 10 Å. It is probable that high-mass ions are formed in less-energetic regions of the collision cascade further from the center of impact and closer to the surface. Various molecular weight (MW) PS−PI diblocks were examined, and it was observed that the relative intensity of the R n − Δ and R n clusters is sensitive to the molecular weight of the PI block. PI homopolymers, similar in MW to the diblock copolymers, yield high-mass spectra nearly identical to PS−PI diblocks, supporting the conclusion that the surface of the diblocks is composed only of PI.