Dependence on tacticity for phase transition of poly (N-isopropylacrylamide) in ionic liquid

Significance 

In the field of polymers, the development of poly(N-isopropylacrylamide) has paved the way for potential applications in sensing and biomedical materials owing to their unique properties. Unfortunately, poly(N-isopropylacrylamide) material in an aqueous medium cannot be used at high temperatures or stored for a long time in the open air. To this end, ionic liquid solvents have been chosen as a perfect replacement for the aqueous medium due to their suitable properties including ionic conductivity, thermal and chemical stability, and non-volatility. Several studies have been conducted to improve the applications of poly(N-isopropylacrylamide) materials by investigating the phase transition properties. However, the aforementioned studies including many others have not yet resolved the tacticity problem in the ionic liquid solvents.

Presently, several methods including phase transition, gelation, glass transition among others have been developed to investigate and understand the effects of tacticity on the physical and chemical properties of the poly(N-isopropylacrylamide) materials. Even though remarkable achievements have been realized, systematical effects of the poly(N-isopropylacrylamide) in ionic liquids is still missing. Therefore, researchers have been looking for suitable solvents and have identified imidazolium ionic liquids as a promising solution for systematic investigation of the effect of tacticity on the poly(N-isopropylacrylamide) materials.

To this note, Shenzhen University researchers: Dr. Chandra Sekhar Biswas, Professor Florian J. Stadler, and Dr. Zhi-Chao Yan (corresponding author) from College of Materials Science and Engineering investigated the phase transition properties of the poly(N-isopropylacrylamide) in ionic liquids. Their fascinating work is currently published in the research journal, Polymer.

Briefly, in their previous research work, the research team successfully synthesized different poly(N-isopropylacrylamide) with meso-diad contents ranging from 47%-82%. Therefore, they first dissolved samples of the initially obtained products in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide. Specifically, the influence of molecular weight, tacticity as well as concentration on the upper critical solution temperature-based phase transition was determined.

The authors observed that the increase of the transition temperature results from a corresponding increase of the isotacticity due to the formation of a strong hydrogen bond by the amide groups aligned on side of the backbone. In addition, it was noted that at low concentrations, the phase diagram of isostatic poly(N-isopropylacrylamide) exhibited upper critical solution temperature peaks similar to the phase diagram observed in a typical polymer solution. This was attributed to the fact that the dissolution of the isostatic poly(N-isopropylacrylamide) in the hydrophobic ionic liquid was major as a result of the entropy-driven effect. Also, it was worth noting that the increase of the transition temperature with molecular weight was as a result of entropic effects too.

The study by Shenzhen University researchers is the first to successfully achieve phase diagrams of the poly(N-isopropylacrylamide) with a high meso-diad content ranging from 72%-82%. For all the investigated samples, the phase transition exhibited hysteresis effect that was not sensitive to other significant factors like the concentration, tacticity and molecular weight even though it was influenced by the heating and cooling effects. Altogether, it will pave way for understating and enhancing the transition properties of poly(N-isopropylacrylamide) materials.

About the author

Dr. Zhi-Chao Yan received Bachelor Degree of Material Chemistry from Sun Yat-sen University, China (2009), and PhD Degree of Polymer Chemistry and Physics from Institute of Chemistry, Chinese Academy of Sciences, China (2015). Then, he moved to the Foundation for Research and Technology – Hellas (FORTH) for postdoc research (2015-2016). Now, he is an assistant professor at College of Materials Science and Engineering, Shenzhen University, Shenzhen, Guangdong China.

His current research interest includes rheology of complex fluid, cyclic polymers, polymer solutions and gels based on ionic liquids.

Email: [email protected]

Reference

Biswas, C. S., Stadler, F. J., & Yan, Z. C.* (2018). Tacticity effect on the upper critical solution temperature behavior of Poly(N-isopropylacrylamide) in an imidazolium ionic liquid. Polymer, 155, 101-108.

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