"Matrix" effects in ToF-SIMS analyses of styrene-methyl methacrylate random copolymers

Surfaces of several styrene (St)-methyl methacrylate (MMA) random copolymer s have been analyzed by ToF-SIMS and XPS in order to detect any possible su rface segregation of one of the two components and/or any specific matrix e ffect in the fragmentation processes. The observed O/(O + C) dependency on styrene content observed by XPS indicates that styrene-methyl methacrylate copolymers exhibit bulklike surfaces over the entire composition range of t he copolymer. The absolute intensity of characteristic peaks from styrene o r methyl methacrylate units was monitored by ToF-SIMS as a function of the styrene content. In positive mode, hydrocarbon fragments such as CH3+, C2H3 +, C2H5+, C5H5+, and C7H9+ at m/z = 15, 27, 29, 65, and 93, respectively, d ecreased with increasing styrene content, while the intensities of MMA frag ments decreased. All fragments exhibited intensity lower than that expected from a simple linear combination calculated from intensities associated wi th the MMA. and St homopolymers. By contrast, some characteristic styrene p eaks (such as C5H3+, C7H7+, C8H7+, and C8H9+ at m/z = 63, 91, 103, and 105, respectively) showed an absolute intensity higher than those observed for PSt and PMMA. In negative mode, fragments such as OH- and C2HO- at mit = 17 and 41 exhibited linear dependence with styrene content at the surface. In tensities for other MMA characteristic fragments such as C3H3O-, C4H5O2-, C 8H13O2-, and C9H13O4- at mit = 55, 85, 141, and 185, respectively, strongly decreased with increasing styrene content. These experiments as well as pr evious work on polystyrenes show that specific interactions between adjacen t species take place during secondary ion emission, especially for the C7H7 + fragment.