Morphology-Property Relationships in Semicrystalline Aerogels of Poly(ether ether ketone)
Talley, Samantha Jo
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The phase diagrams for the thermoreversible gelation of poly(ether ether ketone) (PEEK) in dichloroacetic acid (DCA) and 4-chlorophenol (4CP) were constructed over broad temperature and concentration ranges, revealing that PEEK is capable of dissolving and forming gels in DCA and 4CP up to a weight fraction of 25 wt.%. Highly porous aerogels of PEEK were prepared through simple solvent exchange and solvent removal of the PEEK/DCA or PEEK/4CP gels. Solvent removal utilized freeze-drying (sublimation) methods or supercritical CO2 drying methods. Varying the weight fraction of PEEK dissolved in solution determined PEEK aerogel density. Mechanical properties (in compression) were shown to improve with increasing density, resulting in equivalent compressive moduli at comparable density regardless of preparation method (concentration variation, gelation solvent, solvent removal method, or annealing parameters). Additionally, density-matched aerogels from various MW PEEK showed a correlation between increasing MW and increasing compressive modulus. Contact angle and contact angle hysteresis revealed that PEEK aerogels have a high contact angle, exceeding the conditions necessary to be classified as superhydrophobic materials. PEEK aerogel contact angle decreases with increasing density and a very low contact angle hysteresis that increases with increasing density, regardless of gelation solvent or drying method. Small angle neutron scattering (SANS) contrast-matching experiments were used to elucidate the morphological origin of scattering features, wherein it was determined that the origin of the scattering feature present in the small angle scattering region was stacked crystalline lamella. Ultra-small angle X-ray scattering (USAXS)/SAXS/Wide angle X-ray scattering (WAXS) was then used to probe the hierarchical nanostructure of PEEK aerogels across a broad range of length scales. The Unified Fit Model was used to extract structural information, which was then used to determine the specific surface areas of PEEK aerogels. Regardless of gelation solvent, gel concentration, or solvent removal method, all PEEK aerogels display high surface areas as determined by SAXS and high surface areas as determined by nitrogen adsorption methods. Surface area values determined from SAXS data were consistently higher than that measured directly using nitrogen adsorption, suggesting that pore densification diminishes the accessible aerogel surface area.
- Doctoral Dissertations