Optimization of crosslinking and water absorption measurement in citric acid-based polyesters

This thesis explores the synthesis and optimization of partially bio-based superabsorbent polymers (SAPs) derived from citric acid (CA) and dimethylolpropionic acid (DMPA), with a focus on crosslinking behavior and water absorption capacity. While superabsorbent polyesters based on renewable monomers represent a promising and environmentally friendly alternative to conventional acrylic-based SAPs, such materials are still at an early research stage. In contrast to commercially dominant acrylic SAPs, which suffer from poor degradability and a non-biobased origin, the polyester systems investigated here represent an emerging and rather underexplored class of SAP materials. This work therefore investigates the formation of crosslinked, hydrophilic polyester networks through different crosslinking strategies, comparing the reactivity and performance of carboxylic, hydroxyl and epoxy functional crosslinking agents. The thesis further aims to identify optimal synthesis conditions and analytical methods that allow reliable quantification of crosslinking efficiency and water absorption capacity. Functionalized polyesters based on citric acid and dimethylolpropionic acid were synthesized and investigated, where functionalization was achieved either by ionic neutralization or by sulfonation. The influence of the degree of neutralization (0-75%) on crosslinking efficiency and swelling behavior was systematically investigated, as it determines the balance between hydrophilicity and network stability. Crosslinking was carried out with DMPA, aliphatic diols (1,4-butanediol, 1,6-hexanediol and 1,10-decanediol) and an epoxide (1,4-butanediol diglycidyl ether, BDDGE) using various setups including a vacuum drying cabinet, GC oven with vacuum setup and an oil-bath aperture. In this context, diol crosslinking was found to exhibit lower reproducibility and stronger dependence on reaction conditions due to formation of reaction water, whereas epoxide-based crosslinking provided more robust and reproducible network formation. Analytical characterization of soluble fractions was performed via gel permeation chromatography (GPC) and nuclear magnetic resonance spectroscopy (NMR). Further, samples were analyzed using attenuated total reflectance Fourier transformed infrared spectroscopy (ATR-FTIR) and thermogravimetric analysis (TGA). Concerning water-absorption capacity measurements, a comparison between the traditional tea-bag method and an advanced time-resolved tensiometer approach provided valuable insight into the reliability and reproducibility of the different measurement approaches. Among the tested systems, crosslinking with 1,6-hexanediol at low neutralization degrees of 25% yielded water-insoluble networks with pronounced swelling capacity. At 25% neutralization, a gel content of approximately 74% and a water absorption capacity of about 690% were obtained. In contrast, the non-neutralized system exhibited a high gel content of 99%, but only limited water uptake of 60%, confirming the strong dependence of water absorption on ionization level. Notably, MALDI analysis revealed that, in addition to esterification, transesterification reactions occurred during hexanediol crosslinking, resulting in longer reaction times to obtain significantly crosslinked species. Epoxide-based crosslinking using BDDGE proved to be the most efficient route towards stable and highly crosslinked products, even at moderate temperatures of 120 °C. For sulfonated polyester systems, water absorption capacities of up to 1400% were obtained with BDDGE as crosslinker. In contrast, neutralized polyesters reached maximum absorption values of only 380% at a neutralization degree of 25%. Overall, the findings demonstrate that citric acid-based polyesters represent a promising platform for bio-based SAPs. The results highlight how a controlled degree of neutralization and an appropriate choice of crosslinking agent and reaction parameters determine the resulting material performance.