Synthesis and characterisation of polyamides based on 2,5-furandicarboxylic acid as a sustainable building block for engineering plastics

Polyphthalamides (PPAs) are promising engineering thermoplastics employed in several demanding applications. At present, most of the commercially available PPAs are based on non-renewable petroleum derived resources. Herein, we investigated the synthesis of a semi-aromatic polyamide, poly(hexamethylene furanamide) (PA6F), based on a bio-based monomer, 2,5-furandicarboxylic acid (FDCA), using melt polycondesation in the presence of two catalysts: Ti-isopropoxide (TIPT) and Ti-citrate (TIC). Initial experimentation and optimisation conducted in a thin-film reactor led to PA6F having number-average molecular weight (Mn) of 14 000 g mol−1 at very low catalyst loading (400 ppm TIPT). The reaction was scaled-up to further assess the catalytic activity of these catalysts. Both catalysts displayed strong selectivity for the ester amidation reaction compared to the uncatalysed system, resulting in PA6Fs with improved molecular weights and glass transition temperatures. Incorporating a slight excess (4.5 mol%) of hexamethylenediamine (HMDA) in the feed, resulted in further enhancement in the glass transition temperature; an increase of up to 10 °C was observed. The polymer structure and properties were extensively investigated with the help of a range of analytical techniques. The resultant polymer showed high glass transition temperature (130 °C) and elastic modulus (3.5 GPa), and comparable thermal stability to its structural analogue poly(hexamethylene terephthalamide) (PA6T). For the uncatalysed system, MALDI-ToF mass spectrometry revealed a series of methylated chain ends at both oligomerisation and polycondensation steps, inhibiting molecular weight growth.