Thermo-responsive and Thermally Curable Poly(N-isopropylacrylamide) Copolymers and their Carbon Black Conductive Composites: Preparation, Electrospun Nanofibers, Characterization, and Morphology

In this study, the thermo-responsive and thermal crosslinkable poly(N-isopropylacrylamide-co-N-methylol acrylamide), poly(NIPAAm-co-NMA), copolymer and its conductive composites of poly(NIPAAm-co-NMA) with carbon black in the morphologies of films, hydrogels or nanofibers were prepared.
There are two parts in this research. The first one includes Chapter 2 and Chapter 3. In Chapter 2, it shows the preparation and characteristics of the thermo-responsive and thermal crosslinkable oly(NIPAAm-co-NMA) and the properties of hydrogels for the copolymers. Poly(NIPAAm-co-NMA) was copolymerized by thermally curable monomer N-(methylol acrylamide) (NMA) and N-isopropylacrylamide in water by initiators and then applied various curing time or temperature for thermal curing. The properties of gel fraction, swelling ratio, and lower critical solution temperature (LCST) were evaluated for the ratio of NMA and the curing conditions of the poly(NIPAAm-co-NMA) copolymers. The results showed the copolymer could be cured at low NMA ratio. The introduction of a crosslinking structure,NMA, into the temperature-responsive polyNIPAAm controlled the swelling capability and the properties of the crosslinked hydrogels.
In Chapter 3, thermo-responsive nanofibers were successfully prepared via electrospinning. Poly(NIPAAm-co-NMA) in methanol or water was used as the solution for preparing the electrospinning nanofibers. Thermal curing process was then applied on the copolymer nanofibers for thermal crosslinking and the crosslinked nanofibers could keep the fiber morphology and the copolymer characters while soaking in water. The properties of the copolymers in the morphologyof hydrogel or nanofibers were further investigated.
The second part includes Chapter 4 and Chapter 5, in which the properties of conductive composites with carbon black were studied. The acid-treatment carbon black was introduced into poly(NIPAAm-co-NMA) in Chapter 4 to prepare the temperature-dependent conductive films. It was found that the surface resistance of the conductive films not simply affected by the amount of water content, but also appeared significant drop when the temperature was higher than the LCST. It is noted that the poly(NIPAAm-co-NMA)/CB composites exhibited both temperature-dependent electric resistance and reproducible thermal-responsive characteristics.
In Chapter 5 the temperature-dependent conductive composite nanofibers were prepared by electrospinning. The morphologies of the nanofibers with different carbon black loading were evaluated and the crosslinked nanofibers were with good stability in water. The composites in nanofibers showed the lower percolation ratio and higher surface resistance response rate than the copolymer in films.