3.1 Measurement of frontal fronts
The frontal polymerization reaction creates a frontal front, which is an interface between the polymer produced by the reaction and the unreacted monomer. As illustrated in Fig. 3(a), the frontal position versus time curve shows that the frontal front moves toward the monomer region at a constant rate and completes the polymerization reaction rapidly in less than 6 min. As can be observed in Fig.(b), the frontal temperature change curve has a nearly horizontal section at the beginning, indicating that spontaneous polymerization has not occurred in FP28. As the β-CD content in the hydrogel increases, the movement of the frontal front decreases gradually, and Vf decreases from 2.82 to 1.41 cm/min when the β-CD content increases from 0 to 1wt%. The FP maximum temperature decreased from 168.7°C to 142.3°C. The increase of β-CD content decreased the Vf value because β-CD as an inert substance in the polymerization reaction would lead to thermal dispersion, which decreased the polymerization reaction temperature, thus slowing down the reaction rate, and the Tmax value also decreased29.
Table 2
Parameters of frontal polymerization
Sample | β-CD (wt%) | Tmax (℃) | Vf (cm/min) |
FP0 | 0 | 168.7 | 2.82 |
FP1 | 0.25 | 160.6 | 2.18 |
FP2 | 0.50 | 150.3 | 1.92 |
FP3 | 1.00 | 142.3 | 1.41 |
3.2 Fourier Infrared Spectroscopy (FTIR)
To further analyze the hydrogel condition, the hydrogel was analyzed by infrared spectroscopy, and the results are shown in Fig. 4. Figure 4(a) shows the FTIR spectral profile of AM, the absorption peak at 3338 cm− 1 is the stretching vibration peak of -NH2 on the amide group (-CONH2), and the absorption peak at 1664 cm− 1 is the stretching vibration peak of C = O and the C = C vibration peak30, 31. From Fig. 4(b), it can be seen that there is a strong absorption peak of P(AA-AM) at 3438 cm− 1, which corresponds to the stretching vibration peak of -NH2 stretching vibration peak, and the absorption peak at 1647 cm− 1 corresponds to the stretching vibration peak of C = O and C = C vibration peak. And the absorption peak at 2927 cm− 1 is an asymmetric vibration of the C-H band, which is caused by the cleavage of C = = C in acrylamide32. The absorption peak present at 1449 cm− 1 is a symmetric stretching peak formed by the dissociation of the hydroxyl group of AM into COO− during the polymerization process33. The IR spectral curve of β-CD shows that the absorption peak at 3388 cm− 1 is the O-H stretching vibration peak, and the absorption peak at 1368 cm− 1 is the bending vibration peak of O-H34. From the FTIR spectral profile of P(AA-AM)/β-CD, it can be seen that there are a large number of absorption peaks identical to those of P(AA-AM), but a stronger absorption peak appears at 1374 cm− 1, which corresponds to the bending vibration peak generated by O-H in β-CD. The above results indicate that β-CD enters in the polymer network of P(AA-AM) hydrogel.
3.3 Microscopic morphological of composite hydrogels (SEM)
To study the effect of β-CD on the internal structure of P(AA-AM) hydrogels, SEM observations were performed on the hydrolyzed composite hydrogels. Before performing SEM on the hydrogels, the hydrogels were pre-frozen after one week of immersion and subsequently freeze-dried at -60°C for 48 h. After the freeze-drying treatment, the SEM scans of the four sets of hydrogels, the morphology is shown in Fig. 5. The cross section of FP0 appears to have folds, which may be caused by the collapse of the polymer network and the contraction of the structure during freezing. When the ice sublimates from the hydrogel, the flexible polymer chains in the hydrogel come into contact with each other, resulting in a collapsed hydrogel network35, 36. The addition of β-CD makes the cross section of the hydrogel dense and smooth, which is due to the increased cross-link density of β-CD during polymerization, which contributes to the formation of a denser polymer network in the hydrogel, resulting in the contact between polymer chains becoming more frequent and the structure of the hydrogel becoming more dense.
3.4 Mechanical properties
In order to test the mechanical properties of the hydrogels, tensile and compression experiments were performed on the hydrogels, and the experimental results are shown in Fig. 6. From Fig. 6(a), we can see that the tensile strength of the hydrogel gradually increases with the increase of β-CD in the hydrogel, and the highest tensile strength of FP3 hydrogel can reach 2.0 MPa, which is about two times of that of FP0 hydrogel. Figure 6(b) shows the compression curve of the hydrogel, and the compressive strength of the hydrogel increases with the increase of β-CD. The above results show that the introduction of β-CD enhances the mechanical properties of the composite hydrogels, which is due to the role of β-CD as a chemical cross-linker in the polymerization process. The increase of β-CD content in the hydrogel network increases the cross-link density of the hydrogel. The higher the crosslinking density of the hydrogel, the denser the gel network formed, the more hydrogen bonds formed within and between molecules, and the stresses were dispersed at the fractures, resulting in the enhancement of the mechanical strength of the hydrogel37, 38. The mechanical properties of hydrogels can usually be enhanced by increasing the concentration of hydroxyl groups of the constituent groups of polymeric materials, so the mechanical properties of hydrogels were also improved with the addition of β-CD39.
3.4 PH responsive
The effect of pH buffer solution on the dissolution equilibrium behavior of hydrogels is presented in Fig. 7. As can be observed in Fig. 7, the trend of hydrogel changes remained basically the same in solutions with different pH values. At pH 2.4, the carboxylic acid groups exist in the form of -COOH, which reduces the electrostatic repulsive force of the polymer chains and causes the polymer chains of the whole hydrogel network to be intertwined and contracted, so the swelling capacity of the hydrogel is low at low this environment40. When the pH is at 4.8 environment, the functional groups start to dissociate, the hydrogen bonding between -COOH groups is weakened, and the osmotic pressure inside the hydrogel gradually increases, leading to an increase in ESR33. The reason for the reduced swelling behavior of the hydrogel near pH = 7 may be due to the hydrogen bonding present within the molecules of -CONH2 groups, which interacts with the hydrogen bonding between the molecules of carboxylic acid groups leading to an increase in cross-link density, resulting in the contraction of the polymer network31. When the pH is greater than 7, the carboxylic acid groups ionize, -COOH dissociates into -COO−, and the ions between generates strong electrostatic repulsion, which expands the polymer network and thus leads to an increase in osmotic pressure inside the hydrogel, resulting in an increase in the swelling rate of the hydrogel. As can be show in Fig. 7, the swelling rate of the hydrogel gradually decreases with the increase of β-CD content, which is because the increase of β-CD content increases the cross-link density of the hydrogel, while the hydrophobic cavity of β-CD can prevent water molecules from penetrating into the hydrogel, thus limiting the swelling performance of the hydrogel41.
3.5 Drug loading of hydrogels
We chose tetracycline hydrochloride as the model drug and loaded it into the hydrogel. Figure 8 shows the drug loading data of the hydrogel, the drug loadings of FP0, FP1, FP2 and FP3 were 63.51, 75.61, 115.63 and 154.67 mg/g, with the increase of β-CD content, the loading of the hydrogel to the drug increased. Figure 9 shows a schematic diagram of the uptake of drug molecules by the hydrogel. β-CD/P(AA-co-AM) composite hydrogel is loaded with drug mainly by two ways. One is, the hydrogel swelling leads to the drug-containing solution into the hydrogel network; the other is, the hydrophobic cavity of β-CD can bind to the hydrophobic end of tetracycline hydrochloride, so that β-CD can form host-guest inclusion complexes with drug molecules, increasing the affinity of the polymer network for drug molecules and increasing the number of drug molecules captured by the hydrogel, thereby enhancing the drug adsorption capacity of the hydrogel42, 43. The increase in β-CD content in the hydrogels allowed more drugs to be loaded into the polymer network, improving the drug loading of the hydrogels.
3.6 Drug release of hydrogels
The cumulative drug release rate of the hydrogel is shown in Fig. 10 by performing drug release tests on the hydrogel, thereby investigating the effect produced by β-CD on drug release. In drug release studies, when deionized water penetrates into the polymer matrix, the trapped drug molecules are released from the hydrogel due to diffusion that occurs due to osmotic pressure. As can be shown in Fig. 10, the drug release efficiency of the hydrogels showed a significant difference with the increase of β-CD content in the hydrogels, and the drug release rate gradually decreased. At the beginning, the greater osmotic pressure between the drug molecules in the hydrogel network and the external environment resulted in a greater drug release velocity from the hydrogel within 600 min, while after 600 min, the drug release velocity from the hydrogel started to decrease. β-CD content increases the cross-link density of the hydrogel and makes the polymeric network of the polymer more compact, resulting in a restricted rate of water entry into the hydrogel, thus making the drug molecule's slow release time becomes longer44–46.