Unraveling the structure of high-performance fibres
Aramid fibres have extreme tensile strength and are widely applied as high performance materials. The most important is the Poly(p-phenylene terephthalamide) (PPTA) polymer. It is best known as Twaron or Kevlar.
The fibres are not prefect single crystals, which makes their characterisation difficult. In particular one wants to know how single polymer chains pack together at the atomic length scale and how this correlates with fibre properties and performance.
How to determine how the chains pack together?
X-ray diffraction yields much detail about the possible packing-motifs, but, as the material is not a perfect crystal, it cannot resolve all details. It provides constraints, and predicts a set of possible packings that the chains can have. With first-principles quantum-mechanical calculations in combination with NMR experiments we can further narrow down the possible packings.
How does it work?
In an NMR (nuclear magnetic resonance) experiment the fibre is brought into a very strong magnetic field. The field induces quantum-mechanical currents in the material. These currents give rise to induced magnetic fields, that the NMR experiment can probe at the nuclear positions of 1H and 13C. In particular the induced ring currents in the terephthaloyl and phenylenediamine rings give rise to relatively strong magnetic fields that are sensed (as the so-called NMR chemical shift) by the 13C and 1H nuclei in neighbouring polymer chains. We can calculate these chemical shifts with quantum mechanical calculations. In fact, we mimic the physical process that occurs in the real material in silico. We can do this for specific crystal packings, and compare to actual measured shifts, to assess the suitability of the packing-motifs. Thus we further narrow down the possible motifs and found out that fibres prefer to have the rings on neighbouring chains next to each other, i.e. at the same “height”.
The results were published in Macromolecules 49, 5548-5560 (2016) .
This work is an illustration of a important trend in materials characterization where the combination of accurate experimental NMR measurements with high-quality predictive computational modelling gives it a strong boost. This is particularly important for materials that are not prefect crystals.
This work was a collaboration of Teijin Aramid and the solid state NMR and theoretical chemistry groups of the Institute for Molecules and Materials of Radboud University. It was funded by TA-COAST project 053.21.101. The computational study was part of the research programma of the Stichting voor Fundamenteel Onderzoek der Materie (FOM) with financial support from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO).