Pyrolysis-Field Ionization Mass Spectrometry of Rubber Vulcanizates

Abstract

Abstract Time-/temperature-resolved Py-FIMS allows for the identification of rubber components in compounds containing normal organic additives. Signals due to polymer pyrolyzates, which may be masked by processing oil in nontemperature resolved spectra, are easily obtained. Py-FI spectra from cured BR and NR differ from the corresponding uncured samples in that signals from sulfur-containing oligomers are observed. For SBR, the signals from sulfur-containing pyrolyzates were not distinguishable in the complex mixture of hydrocarbon fragments that was produced. Therefore, unambiguous distinction between cured and uncured SBR was difficult. Since Py-FI mass spectra for rubber blends appear similar to the sum of the corresponding single component spectra, secondary reactions of chain fragments from the two blend components are minimal. These results are consistent with Curie-point Py-MS studies which also showed little interaction between components in blends. Since the Py-FI mass spectrum of the styrene-butadiene block copolymer is similar to the sum of single component spectra, it is obvious that styrene-butadiene sequences are not very abundant. In contrast, mixed oligomers containing both styrene and butadiene units are found for SBR copolymers. The absence of styrene dimer and trimer, as well as high-mass oligomers of butadiene, indicates that the amount of block styrene is very low. Furthermore, the large numbers of mixed oligomers indicates a random sequence distribution. In summary, Py-FIMS is a very effective technique for direct rubber compound analysis. The sample can be examined directly, without pretreatment, and both organic additives and the rubber components can be identified in the same experiment. With programmed heating of the rubber, one can obtain separate (time-/temperature-resolved) FI mass spectra for the organic additives and the rubber pyrolyzates. The results are interesting in that much higher mass oligomers can be observed by Py-FIMS than are detected by other methods of Py-MS. For example, while the low voltage Py-EIMS typically shows no higher oligomers than trimer or tetramer for diene rubbers, Py-FIMS shows sequences containing perhaps 15–20 monomer units. This improved performance is due mainly to (a) the close proximity of the pyrolysis chamber to the field emitter (which minimizes secondary reactions) and (b) the very soft ionization provided by the FI technique. In favorable cases, Py-FIMS can be used to study long sequences in homopolymers, copolymers, and blends. As we have noted, pendent groups (e.g., mercaptobenzothiazyl) and crosslinks may be detected among the pyrolysis products. While some rubbers (e.g., polyisoprene) thermally degrade to give high abundances of oligomers, others degrade in a more random fashion (e.g., SBR) to give very complex mixtures of pyrolyzates. Polystyrene is an interesting case in which thermal degradation by retropolymerization (unzipping) is so prevalent that the monomer is by far the dominant pyrolyzate, and long oligomeric sequences are precluded. Thus, while Py-FIMS can easily be used for qualitative identification of rubber components, more detailed information may or may not be discernible in analysis of a particular rubber sample.