Influence of End Group and Molecular Weight on Polybutadiene Fingerprint Secondary Ion Mass Spectra X. Vanden Eynde* and P. Bertrand Unite ´ de Physico-Chimie et de Physique des Mate ´ riaux (PCPM), Universite ´ Catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium P. Dubois † and R. Je ´ ro ˆ me Center for Education and Research on Macromolecules (CERM), Universite ´ de Lie ` ge, Ba ˆ t. B6, Sart-Tilman, B-4000 Lie ` ge, Belgium Received May 4, 1998; Revised Manuscript Received June 26, 1998 ABSTRACT: Polybutadiene samples of different molecular weight have been synthesized by anionic polymerization as initiated by sec-butyllithium with low polydispersity and a major content of 1,2-vinyl units. They have been analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) in order to investigate the sensitivity of this method toward the sec-butyl end group and toward the molecular weight. The SIMS spectra show the characteristic fragment of the end group, C 4H9 + at m/z ) 57, whose the peak intensity is strongly dependent on the polymer molecular weight, as is the case for almost all the fragment intensities. A model consistent with the peak intensity variations is used to give some new insights into the fragmentation mechanism at the end groups and within the main chain. Moreover, the analysis of the end group fragment allows M n to be readily determined up to Mn ) 4 × 10 4 from, for example, the Y(53)/Y(57) intensity ratio where Y(53) is the intensity of the deprotonated repeat unit ([M - H] + ). Other Mn calibration methods have also been used and are discussed in terms of their accuracy and physical meaning. Introduction Secondary ion mass spectrometry is now widely used to characterize the molecular structure and functionality of the surface of different polymer materials such as polymer blends, surface-treated polymers (plasma, chemi- cal functionalization). 1 The chemical information is derived from the secondary molecular ions which are the fingerprints of the surface molecular structure. Many homopolymer spectra are now available in librar- ies of static secondary ion mass spectroscopy (SIMS) spectra. 2 The static SIMS has proven to be very useful in discriminating between many types of hydrocarbon polymers, such as polyethylene, polypropylene, poly- isobutylene, and polystyrene, 3-5 in contrast to X-ray photoelectron spectroscopy (XPS), which requires a careful study of the valence band. This work is focused on the study of 1,2-polybutadiene (PB). Its SIMS fingerprint spectrum was previously studied for both secondary ion polarities and for both types of monomer units: 1,4-cis/trans and 1,2-vinyl. 3-6 The assignment of ion molecular structures to the main peaks is such that there is no peak characteristic of the 1,4- and 1,2- units, but rather, the relative peak intensity changes with the PB molecular structure. 6 As observed for other polymers, these relative intensities are expected to change with the molecular weight. The molecular weight dependent fragmentation has been already studied for several polymers, such as polystyrene, 7-9 deuterated polystyrene, 10 poly(methyl methacrylate), 11 perfluoropolyether, 12-14 poly(ethylene glycol), 15 polycarbonate, 12,14 and many polyolefins. 16 In the polyolefin series, Galuska analyzed monodisperse 1,4-polybutadiene and proposed a nonlinear calibration curve for the M n calculation from an intensity ratio accurate up to M n ≈ 1 × 10 4 . 16 Since the molecular structure of the end group was unknown, this author was unable to identify the end group characteristic fragments. Nevertheless, he proved that knowledge of M n at the surface of polymeric materials was of great practical interest for applications. Indeed, low molec- ular weight polymers can be localized by imaging the surface. A first application concerns the composite materials, where short chain segregation has been observed to occur at the matrix-fiber interface, with detrimental effect on the adhesion. 17,16 Another ex- ample of this new capability of the time-of-flight (ToF) SIMS technique was the detection of defects in a polycarbonate CD-ROM surfaces due to a localized segregation of short polymer chains. 13 Recently, we proposed a model able to account for the influence of the end groups on the absolute intensity of the characteristic polymer fragments and for the effect of molecular weight, as well. The molecular character- istic features actually influence the fragmentation. For instance, if a hydrogen end group is substituted by a sec-butyl one, the fragmentation is modified. 9 Minor modifications of the end group structure, such as isomerization, are also important, as proved by the analysis of polystyrene (PS) 8 and poly(butyl methacry- late) 18,19 containing different isomers of the butyl group. Indeed, for PS, aromatic species are formed when the secondary butyl isomer is the end group, in contrast to what happens when a tertiary butyl end group is used. 8 Lub et al. have also observed that the butyl isomer influences the formation of enolate fragments. 19 Indeed, the dehydrogenation cannot occur in the case of the tert- butyl enolates. * Corresponding author. E-mail: vandeneynde@pcpm.ucl.ac.be. † Current address: Services des Mate ´riaux Polyme `res et Com- posites, Universite ´ de Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium. 6409 Macromolecules 1998, 31, 6409-6416 S0024-9297(98)00706-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/03/1998