Spectroscopic polymer characterization without polymers: Benchmarking quantum chemistry in the gas phase

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The mechanical and thermal properties of real organic plastics depend not only on their chemical composition, but also on their pre-treatment, rendering an accurate theoretical modelling challenging. Therefore, it makes sense to create idealised bridgeheads for important polymer classes, where theory and experiment are more easily and accurately compared to each other. This is possible for short chain molecules which correspond to finite polymer segments. They permit to extrapolate via high sensitivity vibrational spectroscopy how much force is needed to stretch an infinite chain. They further allow to spectroscopically detect the critical chain length, beyond which inter-chain attraction overcomes chain stiffness and leads to hairpin arrangements. Both issues were addressed in preparatory work for the most abundant polymer polyethylene (PE). Specific modifications of polyethylene as well as the most important water soluble polymer polyethylene glycol (PEG) shall be characterized in an analogous way. For this purpose, a very high sensitivity gas phase Raman spectrometer was developed and shall be extended to the simultaneous recording of different spectral windows. The direct comparison with accurate and approximate quantum chemical calculations should allow for a reliable assessment of the latter in terms of mechanical and intermolecular properties.