4. Thermo-Responsive Nanofiber Mats Fabricated by Electrospinning

Copolymers of N-isopropylacrylamide and stearyl acrylate (PNIPA-SAX) with various SA feed ratios (X = 1–10 mol%) were synthesized and electrospun into nanofiber mats. It was found that average diameter of nanofibers electrospun at concentration of 25 % and voltage of 30 kV linearly increased from 165 nm (PNIPA) to 497 nm (PNIPA-SA10) with increasing the SA content. The PNIPA-SAX (X = 3–10 mol%) nanofiber mats were insoluble in water at 25 °C, in which inter-polymer and inter-fiber physical cross-links were formed through hydrophobic interaction of stearyl side-chains. With increasing the temperature from 25 to 40 °C the PNIPA-SA3 nanofiber mat exhibited significant volume contraction of 66 %, while that of a single nanofiber estimated by AFM measurements was found to be 37 %. The results allowed us to conclude that not only swelling-deswelling but also dissociation-association of the nanofibers via hydrophobic interactions were crucially important for the macroscopic volume changes of the nanofiber mats.