Photopolymerized PNIPAM/MWCNT-COOH Hydrogels

The PNIPAM/MWCNT–COOH hydrogels were prepared using a two-step photopolymerization method. First, NIPAM monomer (0.1 g/mL) and Irgacure 2959 photoinitiator (0.01 g/mL) were added to ultrapure water and mixed for 30 min in cold water bath using a magnetic stirrer (C-MAG HS7, IKA, Staufen, Germany). The solution was then refrigerated at 4 °C for 24 h in the dark. Next, 3-mL aliquots of the NIPAM/Irgacure solution were transferred to Petri dishes and photopolymerized under UV-A light (λ 365 nm, flux 2.3 mW/cm²) for 15 min to obtain a homogeneous gel of poly(N-isopropylacrylamide) (PNIPAM). In the second step, the carbon nanotubes were added to the PNIPAM gel at different concentrations (0.5%, 1%, 2% and 3% by weight with respect to the initial NIPAM content). The dispersions were sonicated for 15 min in ultrasonic bath (Elmasonic P30H, Elma, Singen, Germany) filled with cold water to prevent heating of the thermo-responsive gel, followed by magnetic stirring for further 15 min. Next, the crosslinking agent MBA and Irgacure 2959 were added to the dispersions using a ratio of NIPAM:MBA:Irgacure 2959 of 10:3:3 by weight. The mixtures were stirred in cold water bath for 30 min, stored at 4 °C for 24 h in the dark, and then immediately photopolymerized under UV-A light for 1 h. After the second step of photopolymerization, the hydrogels were washed by immersion in ultrapure water (5 cycles over 3 days) to remove unreacted monomers and reagents.
A Leica DMLP polarized microscope equipped with 5×, 10×, and 20× objective lenses was used to analyze the dispersion of the carbon nanotubes in the PNIPAM gel. The MWCNT-COOHs were dispersed in the gel by sonication at frequency 37 kHz for 15 min. For imaging, the mixtures were spread onto glass microscope slides (Menzel-Gläser) and covered with coverslips to prevent water evaporation.
The yield of the two-step reaction was determined for each type of the composite hydrogel using the following equation: $$\mathrm{Yield}(\%)=\frac{{\mathrm{W}}_{\mathrm{d}}}{{{\displaystyle \sum }}_{\mathrm{i}}{\mathrm{W}}_{\mathrm{i}}}\times 100$$
where W_d and W_i are the weight of the dried hydrogel and the weight of the initial monomer and crosslinker in solution, respectively. To determine W_d, the hydrogel samples (five replicates for each type of hydrogel) were dried at 50 °C for 24 h and immediately weighed to prevent them from absorbing moisture.