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Title: Phase equilibria in stimuli-responsive gels : a PC-SAFT and molecular dynamics study of smart PNiPAAm gels
Language: English
Authors: Eckert, Kathrin 
Hu, Xihua 
Smirnova, Irina  
Chapman, Walter G. 
Other contributor: Rice University, United States, Houston, Texas 
Issue Date: 9-Sep-2022
Abstract (english): 
Smart materials undergo significant configurational changes 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 types range from physical (temperature) to chemical (solvent composition) or biochemical (enzymes). (1)

Despite being versatile materials of major interest, smart gels have few actual application examples, due to missing insights into the thermodynamics of the response behavior. Many model approaches were developed improving this understanding. The difficulty in modeling the swelling behavior lies in the superposition of the equilibrium with the swelling kinetics. (2,3)

In various previous studies, the swelling behavior of pNiPAAm (poly(N-iso-propyl acrylamide)) gels concerning temperature as external stimulus has already been investigated successfully (4). In our work, a self contained actuation of a flow-apparatus while operating an actual reaction (emulsion co polymerization) at preparative scale was developed (5). In these studies, a specially designed reactor, produced by additive manufacturing, allowed the exothermic reaction to heat the hydrogel above its LCST during flow through and reaction progression to regulate product and waste flows (5). The next step in our work is the extension from temperature to solvent composition as trigger for self-contained actuation.

Compared to the temperature, solvent composition is an otherwise complex parameter due to the occuring interactions between solvent molecules with each other and the polymer chain (4,6). For this reason, the prediction and understanding of swelling behavior in solvent mixtures is fundamental to the application of smart gels in chemical processes.

In this work, studies using the perturbed-chain statistical associating fluid theory (PC-SAFT) as well as molecular dynamic analysis are performed to model the swelling behavior of responsive gels in pure solvents and mixtures. For these studies, an esterification process was chosen as model reaction, which allows investigation of the influence of different solvent types: carboxylic acids, alcohols, water, esters, and alkanes as reaction medium.

It was proven, solvents containing hydrophilic groups such as carboxylic acids, alcohols, or water, exhibit positive swelling behavior. Strong interactions between the amide groups of the pNiPAAm lyogel can be observed in these mixtures. This behavior can be reproduced very well using PC SAFT.

In comparison, unpolar compounds such as hexane show low swelling degrees. The reason for this is the dominating interactions of the solvent with the isopropyl groups of the polymer. The difference of the interactions is also apparent in the conformations of the polymer in hydrophilic, compared to hydrophobic solvents, observable in molecular dynamic studies: In hydrophilic solvents, the polymer chain shows an open configuration, the amide groups of the polymer are easily accessible to the solvent molecules. In hydrophobic solvents, the polymer chain is folded, isopropyl groups are turned outwards, and the amide groups are located in the core of the structure away from the solvent molecules.

Starting from pure polymers, different solvent mixtures are analyzed in this work. MD simulations provide a useful tool for analyzing occurring interactions, while PC-SAFT is a tool for analyzing qualitatively the degree of occurring swelling of the lyogel.
Conference: International Conference on Materials Science, Engineering & Technology 2022 
DOI: 10.15480/882.4630
Institute: Thermische Verfahrenstechnik V-8 
Document Type: Presentation
Project: I³-Lab - Smart Reactors 
Functional and advanced insulating and energy harvesting/storage materials across climate adaptive building envelopes 
Peer Reviewed: No
License: CC BY 4.0 (Attribution) CC BY 4.0 (Attribution)
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