PhD Defence by Grammatiki Terzi

Popular Science Summary

Aromatic monomers are highly valuable compounds in polymer synthesis, offering unique properties that result in obtaining high-performance materials used in a wide range of applications. However, the production of these monomers is heavily dependent on fossil resources, which poses significant environmental hurdles. The constantly increasing demand for aromatic monomers exacerbates their environmental impact, highlighting the need for more sustainable alternatives in the plastic industry.   

This study focuses on incorporating 4-HPA, an aromatic monomer which can also be of bio-based origin by microbial processes. The direct self-polycondensation of 4-HPA was explored by applying conventional or microwave heating to the monomer. Results show that by conventional heating a liquid crystalline polyester (PHPA) was obtained. This characteristic of PHPA is particularly important, as it enables the polymer to be melt-processed at relatively low temperatures. Additionally microwave heating could also yield a polymer with the same properties, but additionally, it offers the advantage of significantly reducing reaction times, hence making the reaction more energy efficient. 

The liquid crystallinity of PHPA is highly advantageous for further valorization of the aromatic homopolymer in combination with other polyesters. Multiblock copolymers were synthesized by combining the resulting liquid crystalline homopolymer with polyesters like PLA, PCL, PETG, and PET through transesterification reactions. These copolyesters were further used as additives for blends.  By performing mechanical testing and hydrolytic degradation studies on the resulting polymer blends, it is shown that the blends maintain their mechanical properties, while their hydrolytic degradation rate is significantly accelerated.  Lastly, the study investigates the use of bio-based 4-HPA monomers which are obtained through biological processes.

These monomers contain impurities from both the biological process but also from purification steps. It was found that using the same reaction pathway as producing PHPA from a commercial 4-HPA, results in an impure polymer with characteristics very similar to one produced by pure commercial monomer. In addition it is shown that for this case, purity alone cannot determine if a monomer can produce polymers with the desired properties, but it is necessary to consider details regarding the overall processing and purification steps. By not requiring full purity for bio-based monomers, their overall extraction and purification costs can be significantly reduced, thereby lowering the carbon footprint and production costs of bio-based plastics.