PNIPAm Thermo-Responsive Nanofibers Mats:Morphological Stability and Response Behavior under Cross-Linking
Since 2004,poly(N-isopropylacrylamide)(PNIPAm)cross-linked thermo-responsive 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 non-cross-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 cross-linking 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 cross-linking 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 cross-linking 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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