Acrylamide precipitation polymerization in a continuous flow reactor: an in situ FTIR study reveals kinetics
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INVITED ARTICLE
Acrylamide precipitation polymerization in a continuous flow reactor: an in situ FTIR study reveals kinetics Pascal Fandrich1 · Lars Wiehemeier1 · Maxim Dirksen1 · Oliver Wrede1 · Tilman Kottke1 · Thomas Hellweg1 Received: 25 August 2020 / Revised: 8 September 2020 / Accepted: 29 September 2020 © The Author(s) 2020
Abstract In this work, we present a combination of a continuous flow reactor with in situ monitoring of the monomer conversion in a precipitation polymerization. The flow reactor is equipped with a preheating area for the synthesis of thermoresponsive microgels, based on N-isopropylacrylamide (NIPAM). The reaction progress is monitored with in situ FTIR spectroscopy. The monomer conversion at defined residence times is determined from absorbance spectra of the reaction solutions by linear combination with reference spectra of the stock solution and the purified microgel. The reconstruction of the spectra appears to be in good agreement with experimental data in the range of 1710 to 1530 cm−1 , in which prominent absorption bands are used as probes for the monomer and the polymer. With increasing residence time, we observed a decrease in intensity of the ν(C=C) vibration, originating from the monomer, while the ν(C=O) vibration is shifted to higher frequencies by polymerization. Differences between the determined inline conversion kinetics and offline growth kinetics, determined by photon correlation spectroscopy (PCS), are discussed in terms of diffusion and point to a crucial role of mixing in precipitation polymerizations. Keywords Microgels · NIPAM · Continuous flow reactor · in situ FTIR · Precipitation polymerization · Growth kinetics
Introduction In the past few years, the continuous synthesis of smart microgels has attracted more and more attention, because of the convenient up-scaling of production. Smart microgels are polymer particles, typically swollen with a solvent, which can respond to an external stimulus by a reversible volume phase transition (VPT) [1–5]. The size of these colloidal gels ranges from a few tens of nm to approximately 1 µm. The ability to respond to an external stimulus, such as a change of temperature [6], pH value [7], pressure [8], solvent [9], or ionic strength [10], makes microgels interesting candidates for a broad range of applications. The response typically results in a change of the particles size, Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00396-020-04762-w) contains supplementary material, which is available to authorized users. Thomas Hellweg
[email protected] 1
Dep. of Chemistry, Physical and Biophysical Chemistry, Bielefeld University, Universit¨atsstr. 25, 33615Bielefeld, Germany
which is of great interest in various fields, like controlled catalysis [11, 12], biocatalysis [13, 14], sensor design [15–18], drug delivery [19, 20], microreactors [21], smart membrane fabrication [22, 23], surface coating [24, 25] and microfluidics [26–28]. The properties of microgels can be
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