(754a) Phase Equilibria in Responsive Gels: On the Way to Smart Reactors Beyond the Microscale

Smart materials represent a material class, which undergoes significant configurational changes in structure when exposed to certain stimuli. Therein, stimuli-responsive polymers and hydrogels are well-known materials, which show promising usability for the development of self-contained process control within the process unit geometries. Applicable stimuli range from temperature (e.g. poly(N-iso-propyl acrylamide), pH (e.g 2-hydroxy-3-methyl piperazinepropyl methacrylate), and electrical current to solvent concentrations or even specific molecules (e.g. polyethylene oxide-block-poly(N-aminidinododecyl acrylamide or DEAEMA/PEGDA by CO₂)_, or a double response by pH and CO₂. These stimuli are also of interest since they could enable actuation in chemical and biotechnological processes through process deviation-caused stimuli and system responses. Until now, such materials have been applied only at a small scale, e.g. for the microfluidics or as small (nano) particulates [1-3]. Their use as integrated parts of chemical or biotechnological reactors is more or less unknown, apart from a first proof-of-concept undertaken particularly in this work. Based on our findings, potential of these materials for smart and efficient technical applications at larger scale seems substantial. A successful development profits from a fundamental understanding of the thermodynamics and kinetics of these gel systems, which is only in an emerging state. The interplay of the fundamental thermodynamic understanding, the chemical and physical nature of the responsive polymers is discussed in this work. Special attention is given to the phenomena enabling to establish an approach to reactor systems that are capable of providing a self-contained, and thus smart process control. Particularly, the response time of the materials is crucial for engineering applications and must be correspondingly tailored to fit the typical residence time in the process.

In literature hydrogels as actuators were able to block/unblock flow-through channels by applying different temperatures below and above the LCST to a microreactor [1-3]. In this work we demonstrate for the first time that the self-contained actuation of a flow-apparatus is feasible while operating an actual reaction (emulsion co-polymerization) and at a preparative scale. By utilizing multi-material additive manufacturing of a specially designed reactor module, a facile incorporation method was developed for the stimuli-responsive hydrogel-based reactor. The coiled design of the reactor allowed the exothermic reaction to heat up the hydrogel above its LCST during flow through and reaction progression [4]. To understand this process deeper and to extend the whole concept, systematic experiments on gel/solvent system compatibility and corresponding LCST transitions were conducted. Further, apart from temperature another stimuli- solvent concentration- was investigated. The chosen solvent system consisted of compounds from a model esterification reaction of 1-butanol, acetic acid, n-butyl acetate, water, and n-hexane as additional solvent. The n-hexane amount, as well as the composition of the reactants, were varied. This, in turn, was translated into a form of motion inside an apparatus enabling the actuation of a process depending on the progression of the reaction.

Besides considerations on the equilibrium swelling state of the responsive gels in multi-component systems, the swelling kinetics of the polymer network was addressed. In our proposed smart reactor concept, we found that the residence time distribution of the components of a continuous reaction in conjunction with an appropriate designed reactor are sufficient to compensate for slower equilibrium response times, thereby enabling an adequate control of the process operation. In the theoretical part of the work first attempts to describe the behavior of hydrogels with PC-SAFT were undertaken. Although it is obvious, that the model should be further adopted to describe all the relevant equilibria in the multicomponent gel-containing system, qualitative agreement with LCST behavior was achieved.

Further, the production of stimuli-responsive hydrogels utilizing additive manufacturing technologies, also termed as 4D printing, was investigated. In this work we have, for the first time, successfully utilized commercially available SL printers to fabricate periodic open-celled structures for a multi-phasic flow apparatus, which enabled the control of fluiddynamic properties as well as mass transfer in a co-current bubbly flow in an apparatus. By using the phase transition behavior and the accompanying swelling and shrinkage of the responsive gel, the disperse gas phase break-up was controlled, thereby providing a larger or smaller total interfacial area between the liquid and gas flow. It was thus shown, that distinct photo-resist formulations with different phase transition triggers (LCST, UCST and pH) could be realized, thus enabling a more diverse application range of such structures [5]. In our current work, the utilization of these smart structures is further pursued in terms of self-contained process control of an actual (reactive) multi-phase system, which transcends the mere purpose of actuator valves. Thereby, again, thermodynamic and kinetics behavior of the system is assessed to understand and enable the broad application of such responsive hydrogel structures in industrial processes.

[1] A. Richter, S. Klatt, G. Paschew, C. Klenke, Lab Chip. 2009, 9 (4), 613–618. DOI: 10.1039/B810256B.

[2] A. Richter, G. Paschew, S. Klatt, J. Lienig, K.-F. Arndt, H.-J. Adler, Sensors. 2008, 8 (1), 561–581. DOI: 10.3390/s8010561.

[3] K.-F. Arndt, D. Kuckling, A. Richter, Polym. Adv. Technol. 2000, 11 (8–12), 496–505. DOI: 10.1002/1099-1581(200008/12)11:8/12<496::AID-PAT996>3.0.CO;2-7.

[4] Hu, X., Karnetzke, J., Fassbender, M., Drücker, S., Bettermann, S., Schroeter, B., Pauer, W., Moritz, H.-U., Fiedler, B., Luinstra, G. A., Smirnova, I., Smart reactors–Combining stimuli-responsive hydrogels and 3D printing. Chemical Engineering Journal, 2019, 123413 (online, doi: 10.1016/j.cej.2019.123413

[5] Hu, X., Spille, C., Schlüter, M., Smirnova, I., Ind. Eng. Chem. Res. 2020, 59, 43, 19458–19464

Acknowledgement: This work was done with financial support of I3 Lab "Smart reactors" of TUHH