Synthesis of physically crosslinked PAM/CNT flakes nanocomposite hydrogel films via a destructive approach

Carbon nanotube (CNT)-based hydrogels have recently found a wide variety of applications due to the unique physical and chemical properties of CNTs. CNTs can be used as a nanofiller and/or crosslinker to produce nanocomposite hydrogels with good mechanical and structural properties. In this research, a novel method was reported for producing polyacrylamide (PAM)/oxidized-multiwalled carbon nanotube (O-MWCNT) flakes nanocomposite hydrogel films without using any organic cross-linker or surfactant. Through a mechanism dependent on the reactive oxygen species (ROS), some O-MWCNTs were broken down in situ into small flakes in the aqueous solutions containing acrylamide (AM) and sodium persulfate (NaPS) at the temperature range of 85–90 °C. Simultaneously, in situ polymerization of the AM monomers occurred using free radicals, which resulted in the formation of PAM chains. The flakes acted as crosslinkers by forming hydrogen bonds with PAM chains and formed a hydrogel network after 48 h at room temperature. The hydrogels were characterized by different techniques (FT-IR, Raman, FE-SEM, TEM, TGA, tensile test). The porous structure of the hydrogel films as well as micro-network structures with unique morphologies were observed by SEM. The O-MWCNT flakes and some undegraded O-MWCNTs in the hydrogel network were also observed by TEM. The results showed that PC2I2H hydrogel film, as an evolved hydrogel, has excellent swelling performance in aqueous solutions at different pH and temperatures. In addition, this hydrogel showed a tensile strength of 103 MPa in the dry state and an elongation of 703% in the swollen state.

Part 3. Preparation, analysis and characterization of the hydrogel films S10 Fig. S6. The photographs of the preparation process of as-synthesized hydrogel films S10 Table S3. Assignments of IR bands for the hydrogel films S10 Fig. S7. The SEM image of the PC 3 I 3 film S11 Fig. S8. The micro-network structures of different hydrogel films S12 Fig. S9. The SEM images of PC 2 I 2 H hydrogel film after 6 months from synthesis S13 Table S4. The mechanical properties of as-synthesized hydrogel films S14 Table S5. The mechanical properties of swollen hydrogel films S14 Table S6. Summary of the TGA results for the hydrogel films S14 Part 4. Stability studies of the hydrogels at different pH and temperatures S15 Fig. S10. Swelling curves of the PC 0.5 I 1 H hydrogel film in distilled water at different pH values S16 Fig. S11. Swelling curves of the PC 1 I 1 H hydrogel film in distilled water at different pH values S19 Fig. S12. Swelling curves of the PC 2 I 2 H hydrogel film in distilled water at different pH values S19 Therefore, FTIR analysis indicated that oxidation by the acid mixture treatment has been able to produce many oxygen-containing functional groups, especially carboxyl, on walls of the nanotubes, which significantly increased the hydrophilicity of the nanotubes (Fig. S3).  The decrease in the swelling ratio of the hydrogels in the range of 0.5-52 h (76 h) was calculated according to the following formula: Where and are the swelling ratios after 0.5 and 52 h (76 h).
0.5ℎ 52ℎ(76ℎ) According to Table S1, compared with other hydrogels, PC 1 I 1 H hydrogel showed the highest stability along with the lowest decrease in swelling ratio and the highest swelling ratio after S8 52 h. Therefore, since the amount of NaPS used in PC 1 I 1 H was equal to O-MWCNT, the optimal amount of NaPS equivalent to the amount of O-MWCNT was selected for the synthesis of subsequent hydrogel films.       Fig. S7 The SEM image of the PC 3 I 3 film with a 500x magnification.

Part 4. Stability studies of the hydrogels at different pH and temperatures
Swelling behavior of the as-synthesized hydrogel films at different pH. 1 M HCl and 1 M NaOH solutions were used to prepare solutions with pH 0 and 14, respectively. The solutions were also diluted with distilled water to obtain solutions with pH 2 (HCl solution) and 12 (NaOH solution). Other solutions with different pH values were prepared by diluting solutions with pH 2 and 12. Fig. S10-S12 show the swelling behavior of different hydrogel films at the pH range of 0-14 during 76 h. As shown in Fig. S10, the maximum swelling ratio of the PC 0.5 I 1 hydrogel at different pH is in a neutral range (pH 6 to 8) during 2 and 3 h. This may be because the hydrogels can swell due to osmotic pressure caused by lower ionic strength of the external solution 9 . The decrease in swelling of PC 0.5 I 1 H hydrogel in an acidic medium may be justified by the interaction of hydrogen bonds among carboxylic acid groups (resulting from the hydrolysis of amide groups from PAM, as well as carboxylic groups on degraded O-MWCNTs), which produces additional physical crosslinks 10 . The formation of hydrogen bonds between carboxyl groups is more likely to occur at very acidic pH values (pH<3) 9 . In addition, the interaction of H + ions with OH groups can also induce a decrease in concentration of ions into the network and reduce swelling 11 . At pH 4 to neutral, a large number of carboxyl groups are ionized to carboxylate groups, which increases the repulsion between negative charges and leads to the swelling of hydrogel network. The decrease of hydrogel swelling in the basic range (pH 8-14) may be due to the osmotic pressure resulting from the increase in the concentration of OHions in the external solution. In addition, the charge screening effect of counter ions (Na + ) in salt solutions with pH> 9 can reduce the repulsion between negative charges of carboxylate groups. This prevents water from entering S17 the hydrogel and thus reduces swelling. After 76 h from immersing the PC 0.5 I 1 hydrogel into distilled water, the swelling ratio decreased significantly at all pH values (except pH 14, Fig.   S10). This may be attributed to the dissolution of the PC 0.5 I 1 H hydrogel film in aqueous solution, which is associated with a reduction in mass (Fig. S10) 12 . In addition, the high swelling at pH 14 is due to repulsive forces between the carboxylate groups, resulting from the hydrolysis of a large number of amide groups(from PAM) and the carboxyl groups( from On the other hand, the swelling curves of PC 1 I 1 H and PC 2 I 2 H hydrogels ( Fig. S11 and S12, respectively) in the pH range of 0-14 are different from those of PC 0.5 I 1 H hydrogel, especially S18 at 2 and 3 h. This may be due to increased stability of the hydrogel films against dissolution by increasing the amount of the O-MWCNT flakes (as crosslinkers) in the hydrogel. For PC 1 I 1 H and PC 2 I 2 H hydrogel films, the highest swelling ratio is observed at pH 14( Fig. S11 and S12). This may be due to repulsive interactions between negatively charged carboxylate groups resulting from the hydrolyzed amide groups in the hydrogel and carboxyl groups on the flakes (as well as free O-MWCNTs). In addition, a time-dependent increasing trend in the swelling ratio is observed at pH14 (Fig. S11 and S12). Under severe alkaline conditions, large amounts of amide groups in the PAM network are hydrolyzed to carboxylate groups over time. Therefore, as the density of carboxylate groups increases, the repulsion between their negative charges increases, leading to a significant increase in swelling. In addition, in alkaline hydrolysis of PAM, there is nearly alternation between amide and carboxylate groups. Under these conditions, the amide groups are partially converted to ionized carboxyl groups. The resulting negative charges repulse hydroxyl ions and do not allow hydrolysis of both adjacent amide groups. Thus, the next groups are possible to hydrolyze, which is an alternating tacticity 13 . Swelling in the pH range of 8-12 is influenced by two factors, osmotic pressure and repulsion between carboxylate groups, which act in the opposite directions ( Fig.   S10-S12). As pH increases from 8 to 10, the number of hydroxide ions in the solution increases, and the swelling decreases due to the osmotic pressure created by these ions. By increasing pH to 12, the number of hydroxide ions in the solution increases significantly, which can easily hydrolyze amide groups to carboxylate groups in the hydrogel network. The repulsion between negative charges of the carboxylate groups leads to swelling in the hydrogel. Swelling of different hydrogel films in the pH range of 2-6 may be further affected by osmotic pressure. As the concentration of H + ions in the aqueous solution increases, a S19 concentration difference occurs between the hydrogel and the external solution, reducing the flow of water into the hydrogel and thus reduces swelling. the swelling curves of the hydrogels at pH 0 show a time-dependent decreasing trend in swelling during 2,3 and 76 h ( Fig. S10-S12). This may be due to the hydrolysis of weak crosslinks created between the flakes and the PAM chains, as well as the weight loss of the hydrogel films by releasing unreacted monomers into the solution. Decreased concentration of ions in the network under acidic conditions is another factor in reducing swelling 11 .
As shown in Fig. S11 and S12, the swelling ratios in PC 1 I 1 H and PC 2 I 2 H hydrogels during 2 and 3 h at pH 0 is higher than those of other pH values (Except pH 14). This may be because some weak crosslinks in the hydrogel network are broken under severely acidic conditions, which reduces the density of the crosslinking and increases the volume of some pores in the hydrogel, and ultimately leads to a temporary increase in swelling. Whereas, the PC 0.5 I 1 H hydrogel network with lower crosslinks density collapses under severe hydrolysis at pH 0, resulting in a decrease in swelling (Fig. S10). The decrease in swelling after 76 h may be attributed to the dissolution of the hydrogel films in aqueous solution.

S21
The swelling ratios of different hydrogel films in aqueous solutions with different pH values after 76 h are given in Table S7. Compared with other hydrogel films, the PC 2 I 2 H hydrogel shows more changes in the swelling ratio at different pH values (Table S7).