PNIPAm Thermo-Responsive Nanofibers Mats: Morphological Stability and Response Behavior under Cross-Linking

Abstract

Since 2004, poly(N-isopropylacrylamide) (PNIPAm) cross-linked thermoresponsive nanofibers mats have emerged as a responsive material with a phase transition temperature that can be easily controlled. These mats overcome the limitations, such as a high production cost and slow response rate, of huge traditional PNIPAm hydrogels. They also overcome the poor water resistance of PNIPAm noncross-linked thermo-responsive nanofibers and, thus, have been widely studied. In 2017, continuous PNIPAm thermo-responsive nanofibers in pure aqueous solvents without beads were fabricated, which began the ecological and water-based era of uniform PNIPAm nanofiber production. In this review, we comprehensively analyzed the effects of physical and chemical cross-linking reaction types, cross-linking degree, cross-linking time, and cross-linking molecular weight on the morphological stability and response behavior of PNIPAm thermo-responsive nanofibers mats, providing theoretical support for their future crosslinking treatment. Because of their high specific surface area and porosity, PNIPAm thermo-responsive nanofibers mats are vulnerable to solvent erosion before cross-linking, which damage their morphology and reduce response rates and usage times. Increased water resistance and can be utilized repeatedly, by introducing cross-linking groups to these mats, such as in drug release, cell culture, drivers, and smart switches. Chemical cross-linking are more stable than physical cross-linking and can be divided into crosslinkers, chemical reactive cross-linking, and other cross-linking. The crosslinking networks produced by a cross-linking agent are more robust; however, the resulting nanofibers mats are not applicable to the human body owing to the small, non-degradable harmful molecules, such as formaldehyde and glutaraldehyde (GA). Random 3D networks generated by physical cross-linking are easier to break but relatively safe and pollution-free. The morphological stability and response behavior of PNIPAm thermo-responsive nanofibers mats are affected by the cross-linking. The cross-linking agent content and the cross-linking time are positively correlated with the morphological stability of PNIPAm thermo-responsive nanofibers mats. This is conducive to multiple recycling but has little effect on the response rate. Greener and more reliable cross-linking methods should be investigated to realize and expand the practical applications of PNIPAm thermo-responsive nanofibers mats, with increasing focus on the effect of crosslinking on the mechanical properties of the mats. We hope this review will result in ideas for improving the development and application of PNIPAm thermo-responsive nanofibers mats.

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