Low Viscosity Precursors of Polyether Ketone Ketone, Their in situ Polymerisation and Application to Composite Formation
Creators: name: Szmalko, David
Publisher: name: RMIT University
Year: 2025
Subjects: classificationCode: 401609
subject: Polymers and plastics
classificationCode: 401602
subject: Composite and hybrid materials
classificationCode: 401408
subject: Manufacturing processes and technologies (excl. textiles)

description: The aerospace industry is in a period of transition, as new materials and methods of manufacturing are sought to address the global challenges of climate change and carbon emissions. As a part of this transition, traditional metal structures are increasingly being replaced by carbon fibre reinforced composite materials. Currently composite materials in aerospace are largely comprised of thermosets with low viscosity precursors allowing for high throughput manufacturing. These are limited by being brittle, irreparable without replacement and unable to be recycled; an increasing concern as their adoption grows.

A polymer that can address these concerns is PolyEther Ketone Ketone (PEKK), an ultra-high-performance thermoplastic. Commonly used as a matrix with carbon fibre, it is lightweight with exceptional material properties and chemical resistance. In addition to this PEKK is inherently recyclable and sees use in various aerospace parts. The properties of the PEKK polymer can also be fine-tuned by altering the backbone with different structural isomers resulting from para-substituted terephthaloyl chloride and meta-substituted isophthaloyl chloride starting materials, with this ratio of para to meta links being known as the T/I ratio. Adversely, PEKK’s usefulness as a composite matrix is limited by its high viscosity in the melt. As PEKK’s current predominant synthesis requires that the polymer must be melted before use, a method by which PEKK composites could be produced from a low viscosity resin would be ideal.

One method that could produce PEKK from low viscosity precursors is entropy driven ring-opening polymerisation (ED-ROP). ED-ROP occurs through the ring-opening of large, strainless, low viscosity ring structures known as macrocyclic oligomers (MCOs). Driven by the increased conformational entropy of the linear polymer, polymerisation occurs without the release of heat, volatiles or by-products. ED-ROP is therefore well suited to the production of composites by an in situ polymerisation, where low viscosity PEKK MCOs could be injected into a mould, wet-out the fibres, and polymerise without contamination or voids in the finished part. Such a process could provide high-throughput manufacturing options. Accordingly, the research proposed for this PhD project was to synthesise MCOs of PEKK, to polymerise them via an in situ ED-ROP with methods analogous to resin infusion, and investigate their interaction with carbon fibre. 

A review of the literature found no complete examples of a PEKK MCO, and hence no examples of their ED-ROP nor composite formation with reinforcing fibres such as carbon fibre (CF). Examples from the broader family of polyaryl ether ketones (PAEKs) have been explored however and were informative for MCO synthesis conditions, as well as the mechanics, conditions and initiators used for this type of polymerisation. In addition, the properties of the PEKK polymer, such as crystallinity and thermal characteristics, the use of sizings for PEKK/CF composites and the effects of para and meta structural isomers on the polymer’s properties, were also examined. 

From this review experimentation began with the pseudo-high dilution synthesis of polyether ketone (PEK) to assess reaction set-up and conditions. This approach was then applied to produce novel MCOs of the homopolymers m PEKK and p PEKK in good yields, with optimisation experiments finding that caesium carbonate and enhanced removal of water improved the yields of cyclic species. Characterisation by Nuclear Magnetic Resonance (NMR) spectroscopy, Matrix-Assisted Laser Desorption/Ionisation Time of Flight (MALDI-ToF) mass spectrometry and Gel Permeation Chromatography (GPC) was required to distinguish between the MCOs and linear polymer, due to their structural similarities. Dimer species of two homopolymer MCOs were also successfully isolated, crystallised and analysed by X-ray crystallography to prove the cyclic structure. Attempts to further optimise the reaction conditions by a high-throughput assessment utilising Rapid Automated Materials Processing (RAMP) proved unsuccessful. Despite this, the synthesis was also expanded to produce novel MCOs of PEKK with three T/I ratios related to those of commercial PEKK products. NMR spectroscopy of both these MCOs and a commercial 60/40 T/I PEKK showed good agreement.

These novel MCOs were then polymerised by ED ROP. Polymerisations were carried out in a (Differential Scanning Calorimetry) DSC instrument with conditions analogous to an in situ polymerisation, and the successful formation of polymers determined by the appearance of a glass transition as well as changes to the morphology and solubility of the material. Glass transitions for both PEKK homopolymers and the three T/I ratio grades were identified and showed good agreement with a commercial sample, and further polymerisation aspects investigated. ThermoGravimetric Analysis (TGA) was used to show no weight loss during polymerisation, as should be expected of an ED-ROP. TGA was also employed to investigate the properties of the purified PEKK MCO dimers, as PAEK dimers had been noted in literature to impair ED-ROP.
Due to the insolubility of the PEKK polymer and its structural similarity to the MCOs comprehensive examination of the polymerisation proved difficult. Further analysis by GPC was therefore sought to determine the degree of polymerisation, and a thioketalisation strategy from literature was needed to ensure the solubility of the PEKK. This strategy was tested against the commercial sample and the MCOs, and the full derivatisation confirmed by NMR spectroscopy. Polymerisation experiments were conducted to identify optimal ED-ROP conditions with regards to temperature, time, type and amount of initiator, and type of PEKK MCO. Some indication of ring-opening polymerisation had occurred, but high molecular weight polymerisation could not be confirmed. The deleterious effects from the presence of oxygen and water were identified as a potential cause for this, along with possible impairment from the presence of the MCO dimer. Polymerisation was then with a neat sample of m-PEKK MCOs, with the dimer removed and 2 mol% Caesium Fluoride (CsF) initiator, under high vacuum conditions. The resultant polymer was thioketalised, analysed by GPC and, once corrected for the derivatisation, displayed a successful high molecular weight polymerisation, with a Mn of 22,000 g/mol and a Mw of 38,000 g/mol, the first example of a high molecular weight PEKK polymer formed by the ED-ROP of MCOs. Subsequent experiments under low vacuum also showed some evidence of ED-ROP on GPC but did not achieve the same molecular weights.

Formation of test composites of PEKK/CF by in situ polymerisation was also examined. Analysis was tailored to properties highly important to composite structures that could be assessed on a small scale. The viscosity of the MCOs was examined, and the melt complex viscosity found to be significantly lower than that of PEKK polymer. Test composites were then formed by in situ ED-ROP under low vacuum, being the first example of a PEKK/CF composite formed by ED-ROP. The samples were assessed by Scanning Electron Microscopy (SEM), with it being observed that the polymer had thoroughly penetrated the carbon fibre tow and wetted out the fibres. Cryofracturing of the composite samples revealed a close interphase between the fibre and matrix with no visible voids. Recovery and analysis of the PEKK showed a glass transition similar to those previously observed. GPC analysis, using the thioketalisation methods discussed, revealed that polymerisation had taken place but did not display a high molecular weight polymer.