Synthesis and Processing of PPTA in Ionic Liquids

Promovendus/a: Sven Dewilde

Promotor(en): Prof. dr. Koen Binnemans, Prof. dr. Wim Dehaen

Poly-p-phenylene terephthalamide (PPTA), commonly known under the brand names Kevlar® or Twaron® is a high performance material. Its high tensile strength is derived from a high crystallinity and crystal orientation resulting from its rigid and symmetric structure and the capability to form an extended hydrogen bond network. Currently, PPTA is synthesized in a NMP/CaCl2 solvent mixture. The chloride salt is essential for preventing premature precipitation of the polymer during synthesis. The aim of this PhD thesis was to investigate the potential of ionic liquids (ILs) as alternative solvents for the dissolution and synthesis of PPTA. Ionic liquids are solvents that consist entirely of ions. Additionally, new insights in the polymerization process of PPTA were gained.

In the first part of this thesis, various commercial and newly synthesized ionic liquids were tested towards the dissolution of PPTA. A set of short PPTA oligomers was used to make valuable comparisons between structurally different ionic liquids. Ionic liquids with coordinating anions are of prime importance. Anions with a high basicity are able to dissolve higher amounts of PPTA oligomer and the following trend in performance was observed: phosphate ≈ carboxylate > chloride. The cation plays a secondary role towards the solvent strength of the ionic liquid. Non-coordinating cations are best suited as they give the anion full potential to act as a hydrogen bond acceptor towards PPTA. Therefore, the following order in performance was observed for the cations: phosphonium ≈ ammonium > 2-methylpyridinium > imidazolium. Neither ionic liquids nor NMP/CaCl2 are capable of dissolving high molecular mass PPTA. However, the best performing ionic liquid, tributylethylphosphonium diethylphosphate ([P4442][Et2PO4]), is able to dissolve almost 10 times more of the pentamer model compound than the state-of-the-art industrial solvent.

In the second part, ionic liquids were used as solvents in low-temperature polycondensation reactions of PPTA. The reaction parameters such as the reaction temperature, monomer concentration, water content and viscosity of the solvent must be strictly controlled. Polymerization reactions with various ionic liquids diluted in a co-solvent were performed. Again, ionic liquids with coordinating anions are of prime importance for preventing premature precipitation of the polymer during synthesis. However, only the chloride anion was found to be suitable as too nucleophilic anions cause side reactions. Solvents containing imidazolium as cation were used to synthesize PPTA polymer with the highest molecular mass. A 25 wt%/75wt% of 3‑methyl-1-octylimidazolium chloride ([C8MIM][Cl]) and NMP was able to produce PPTA polymer with molecular mass close to the industrial benchmark. It was theorized that this time a coordinating cation is essential as polymerization does not rely on solubilization of PPTA but rather on gelation and network formation within the reaction mixture. Equal interactions between anion, cation, solvent and amide bonds create a network that extends to all components present in the reaction mixture.

Gel formation also did occur in pure [C8MIM][Cl] as solvent, indicating that this ionic liquid has the potential to be an alternative solvent for the synthesis of PPTA. However, EXAFS and solid-state NMR-spectroscopy indicated that a co-solvent that interacts with both the polymer and the ionic liquid might be needed for the formation of more stable gels and subsequently higher molecular masses of PPTA.