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

Surfaces of several styrene (St)-methyl methacrylate (MMA) random copolymers have been analyzed by ToF-SIMS and XPS in order to detect any possible surface segregation of one of the two components and/or any specific matrix effect 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 the copolymer. The absolute intensity of characteristic peaks from styrene or 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, decreased with increasing styrene content, while the intensities of MMA fragments decreased. All fragments exhibited intensity lower than that expected from a simple linear combination calculated from intensities associated with the MMA. and St homopolymers. By contrast, some characteristic styrene peaks (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. Intensities for other MMA characteristic fragments such as C3H3O-, C4H5O2-, C8H13O2-, and C9H13O4- at mit = 55, 85, 141, and 185, respectively, strongly decreased with increasing styrene content. These experiments as well as previous work on polystyrenes show that specific interactions between adjacent species take place during secondary ion emission, especially for the C7H7+ fragment.