Degradation strategies for structural characterization of insoluble synthetic polymers by mass spectrometry

With the advent of soft ionization techniques such as electrospray (ESI) and matrix-assisted laser desorption/ionization (MALDI) to produce intact gas-phase ions from nonvolatile macromolecules, mass spectrometry has become an essential technique in the field of polymeric materials. However, (co)polymers of very high molecular weight or with reticulated architectures still escape ESI or MALDI, mainly due to solubility issues. Strategies developed to tackle such an analytical challenge all rely on sample degradation to produce low-mass species amenable to existing ionization methods. Yet, chain degradation needs to be partial and controlled to generate sufficiently large species that still contain topological or architectural information. The present article reviews the different analytical degradation strategies implemented to perform mass spectrometry of these challenging synthetic polymers, covering thermal degradation approaches in sources developed in the 2000s, off-line sample pre-treatments for controlled chemical degradation of polymeric substrates, and most recent achievements employing reactive ionization modes to perform chemolysis on-line with MS.

Differentiation of Four Polyvinylidene Fluoride Polymers Based on Their End Groups by DART-FT-ICR MS and Kendrick Plots

Nowadays, synthetic polymers are produced and used in many materials for different applications. Matrix-assisted laser desorption/ionization or electrospray mass spectrometry are classically used to investigate them, but these techniques require sample preparation steps, which are not always suitable for the study of insoluble or formulated polymers. Alternatively, direct real-time (DART) ionization analysis may be conducted without sample preparation. Four polyvinylidene fluoride (PVDF) polymers involving the C2H2F2 repeating unit coming from different suppliers have been analyzed by DART Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in negative-ion mode. The obtained mass spectra systematically displayed an oligomeric distribution between m/z 400 and 1300 of [M - H]-, [M + O2]•-, and [M + NO2]- ions. Kendrick plots were used to ease the identification of PVDF end-groups and establish a difference between the samples. Both commercial PVDF polymers shared the same α+ω end groups formula, which confirmed a similar polymerization process for their synthesis. The two other PVDFs were clearly different from the commercial ones by the occurrence of specific end-groups. MS/MS and MS3 experiments were conducted to obtain structural information on these end-groups.

Thermal Stabilization of Nafion with Nanocarbon Materials

The stability of Nafion-carbon composites is important for the efficient functioning of fuel cells. The thermal decomposition of Nafion, nanostructured carbon materials, such as multi-walled carbon nanotubes, graphene-like materials, and their composites, have been studied using constant heating rate thermogravimetry in air. Materials were characterized by quantitative and qualitative analysis methods, such as thermogravimetry, X-ray photoelectron spectroscopy, scanning, and transmission electron microscopy with field emission. In Nafion-carbon composites, an increase in the thermal stability of the Nafion polymer is observed due to the formation of surface compounds at the Nafion-carbon interface. In this case, the degree of stabilization is affected by both the component composition of the composite and the structure of the nanocarbon material. The greatest effect was obtained in the case of using thermally expanded graphite (few-layer graphene). Nafion is distributed to a greater extent over the surface of the carbon material due to its high structural accessibility. The most thermally stable composite is Nafion-graphene in a mass ratio of components 1:4 with one stage Nafion degradation at 422 °C, whereas the degradation of pristine Nafion occurs in three stages at 341, 413, and 430 °C. The dependences of thermal stability and features of thermal degradation on the composition and structure of composites are discussed.

Qualitative Analysis of Acid Salts with Direct Analysis in Real Time Mass Spectrometry by Conversion to the Free Acid Form

Organic acid salts typically have very low volatility and are not well suited for analysis by Direct Analysis in Real Time mass spectrometry (DART-MS). However, qualitative analysis of organic acid salts by DART can be facilitated by the addition of a strong acid to convert the compounds to the free acid form. Examples are presented here for inorganic salts (sodium and potassium perchlorate) and several organic salts, including three disodium salts and a mixed sodium/potassium salt.