Extraction of stabilizers from polymers: Separation of oligomeric hindered amine light stabilizers and phenolic antioxidants from polyolefins using liquid chromatography and high-temperature solid-phase extraction.

The extraction of different stabilizers from a polymer matrix and the subsequent separation of said stabilizers is one of the most important as well as challenging undertakings in polymer chemistry. A multitude of stabilizers exists, each of which may be hard to extract, be difficult if not impossible to separate from other stabilizers or necessitate very selected and time-consuming intermediate stages for separation. Certain polymer matrices even pose additional challenges, such as polyolefins being only soluble at elevated temperatures. One of the most well-established approaches for the extraction of stabilizers is Soxhlet extraction. However, even this highly successful approach shows only limited success with regard to the extraction of the ever more relevant oligomeric stabilizers or the extraction of multiple stabilizers in a one-shot approach. Moreover, performing Soxhlet extractions often necessitates ≥24 h. For these reasons, alternative approaches for the extraction of stabilizers from polymers are highly sought after. An approach with enormous potential is solid-phase extraction, which allows the selective retention and enrichment of stabilizers. Herein, the very first application of high-temperature solid-phase extraction for the extraction of stabilizers from polyolefin matrices is described; as with other extraction techniques, the identification and quantification of the stabilizers is then allowed. At temperatures of 140-160°C, it was possible to adsorb common polyolefin stabilizers selectively on a silica solid phase from their polyolefin matrix. To predict high-temperature solid-phase extraction test conditions, first LC tests are necessary, offering an elegant approach for the separation of polyolefins from oligomeric stabilizers, which was not achievable until now.

Porous graphite as stationary phase for the chromatographic separation of polymer additives - determination of adsorption capability by Raman spectroscopy and physisorption.

Additives are added to polymers in small concentration to achieve desired application properties widely used to tailor the properties. The rapid diversification of their molecular structures, with often only minute differences, necessitates the development of adequate chromatographic techniques. While modified silica so far is the workhorse as stationary phase we have probed the potential of porous graphitic carbon (HypercarbTM) for this purpose. The results show that the multitude of physicochemical interactions between analyte molecules and the graphitic surface enables separations of polyolefin stabilizers with unprecedented selectivity. To support the chromatographic results the adsorption capability of HypercarbTM for selected antioxidants and UV absorbers has been determined by Raman spectroscopy and argon physisorption measurements. The shift of the Graphite-band in the Raman spectra of HypercarbTM upon infusion with additives correlates with the changes in the Adsorption Potential Distributions. The results of argon physisorption measurements go hand in hand with the chronology of desorption of the additives in liquid chromatography experiments. The elution sequence can be explained by van der Waals or London forces, π-π-interactions and electron lone pair donor-acceptor interactions between the graphite surface and analyte functional groups.

Porous graphite as platform for the separation and characterization of synthetic polymers - an overview.

Porous graphite as sorbent differs significantly from all other HPLC column packings. It stands out due to its chemically extremely homogeneous surface, which moreover is planar on an atomic level. This sorbent, according to its non-polar but polarizable surface, is able to adsorb polar as well as non-polar small molecules as well as macromolecules. Moreover, it enables their separation induced by minute differences in their molecular architecture, which includes the aspects of planarity, branching or tacticity of macromolecules. Although graphite had already been used many years for the separation of small molecules, the application of porous graphite for separations in the domain of synthetic polymers has been rare. In 2009 it was found that porous graphite enables the separation of polyethylene and polypropylene on the basis of their full adsorption and desorption, when suitable solvents are used. This approach has led to the fast elaboration of HPLC systems for separations of various polar modified as well as non-polar polyolefins. Due to pronounced adsorptive interactions, porous graphite is applicable even at temperatures as high as 160 °C. The results presented in this paper manifest that porous graphite enables to obtain important information about the composition distribution of various synthetic polymers, the architecture of macromolecules (i.e., branching) or their tacticity, and underlines its enormous application potential.