3.1 Chemical characterization
In all the investigated samples, there is a small band at ~ 1640 cm-1 with a small intensity which may be attributed to O-H bending due to the absorption of water. Water also has a broad band at ~ 3400 cm-1 (3), so it is expected that the band at ~ 3270 cm-1 might also a have contribution from water. However, since the peak at ~ 1640 cm-1 has a constant intensity in the samples, we assume that the changes observable in the band at ~ 3270 cm-1, which is present in the collected spectra, are not linked to the contribution of water (see Fig. 2-a). In the PVA spectrum, the asymmetrical and symmetrical CH2 stretching vibrations appear at 2938 and 2910 cm-1, respectively. The same spectrum shows the peaks at 1708 and 1560 cm-1 which represent stretching carbonyl group (C=O) of the acetate group. This originates from remains of the manufacturing of PVA through hydrolysis of polyvinyl acetate. The peak at 1647 cm-1 is assigned to O-H bending due to the absorption of water. The C-H bending peak of CH2 group appears at 1414 cm-1. The peaks at 1376 and 1239 cm-1 correspond to the rocking of methyl groups (-CH3) or -CH2 wagging vibrations, respectively while the peak at 1327 cm-1 is attributed to H-C-H and C-O-H bending vibrations. The significant peak at 1141 cm-1, corresponding to C-C and C-O-C stretching vibrations, is indicative of the crystallinity of PVA. The C-O stretching vibration occurs at 1085 cm-1 and the peak at 916 cm-1 is attributed to the -CH2 stretching vibration. The vibration at 832 cm-1 is assigned to C-C stretching and C-H out-of-plane vibrations (38-40).
As reported, a chemical interaction between two materials after mixing them can lead to changes in the resulting ATR-FTIR spectrum. These changes may consist of shifts in the characteristic peaks or the appearance of new ones (41). In our case, when borax is added to PVA solution, the resulting film shows significant changes to the O-H broad band of pure PVA; the band becomes broader with a lower intensity and shifts to higher wavenumbers which is associated with the interaction between PVA and borax (see Fig. 2-a) (42). In addition, there is a slight shift occurring in the characteristic peaks of pure PVA. The ATR-FTIR spectrum of PVA-B also confirms the presence of the asymmetric stretching relaxation of B-O-C at 1420 and 1334 cm-1, indicating the crosslinking of PVA with borate ions through the formation of tetrahedral complexes. The peaks at 830 and 661 cm-1 are attributed to B-O stretching from the residual of B(OH)4- and bending of B-O-B linkages within borate networks, respectively (43).
The ATR-FTIR spectrum of PVA-B/AG blend shows the characteristic bands of agarose such as the C-O-C and glycosidic linkage at 1069 cm-1. The absorption bands at 931 and 893 cm-1 are associated with 3,6-anhydrogalactose and the C–H bending vibrations of anomeric carbon in ß-galactose residues, respectively (44-46). Upon mixing PVA-B with AG, the hydroxyl broad band changes in intensity and position. In addition, other slight changes in the intensity and peak position of characteristic PVA-B and AG peaks are observed. For instance, the two peaks of AG at 1065 and 1038 cm-1 shift to higher frequencies at 1069 and 1044 cm-1; these changes reflect the formation of intermolecular hydrogen bonds between PVA-B and AG (see Fig. 2-b) (47, 48).
3.2 Liquid phase retention
Fig. 3 summarizes the mass loss of different hydrogel formulations after 20 minutes in the thermogravimetric balance. When no solvents other than water are present, 3% PVA-B loses liquid faster than 4% PVA-B and 3%/1% PVA-B/AG. On the other hand, 4% PVA-B has a slight advantage in retaining liquid over 3%/1% PVA-B/AG. These results can be confirmed by the those obtained from the SEM micrographs of the same hydrogel formulations (see section 3.4.). The samples with smaller size of the pores the better they would retain the liquids. For example, the 4% PVA-B hydrogel retained the liquids better than the 3% PVA-B and 3%/1% PVA-B/AG hydrogels because its small pores' size. The same behavior is observed for the hydrogel formulations loaded with solvents. However, due to the tendency of xylene of becoming expelled, the corresponding formulations does not follow the same trend. The retained concentrations of 20% xylene by the 3% PVA-B, 4% PVA-B, and 3%/1% PVA-B/AG hydrogel formulations are as follows: 13.5%, 17.5%, and 19% respectively. The weight loss of all the hydrogel formulations follows an inverse trend relative to the boiling points of the three investigated organic solvents (water: 100 °C, EtOH: 78 °C, AC: 56 °C, and Xyl: 139-142 °C) (49).
3.3 Liquid phase diffusion into porous surfaces
The results obtained by means of this test are similar to those of the liquid phase retention test (see Figs. S3 & S4). The formulations characterized by a low liquid retention cause more rhodamine B dye to diffuse than those with a high liquid retention. In this case, 3% PVA-B shows the most important diffusion of the dye; then 3%/1% PVA-B/AG and 4% PVA-B. There is a minor difference between the hydrogel formulations 4% PVA-B and 3%/1% PVA-B/AG. As expected, the diffusion into the plaster tiles is considerably higher than that into the limestone ones due to the high porosity of the plaster tiles, even at different application times.
It is worth noting that during peeling off of the hydrogels from the surface of the plaster tiles, both PVA-B formulations (3% & 4%) strongly adhered to the tiles and left behind a layer of the hydrogel (see Fig. 4). Only the 3%/1% PVA-B/AG DN hydrogel did not show that problem; the hydrogel slab could be peeled off almost intact. The process was repeated with the formulations loaded with the three solvents (AC, EtOH, & Xyl) and again the hydrogels without the agarose adhered to the plaster tiles while the agarose-containing ones did not. This can be related to difference in the mechanical properties of the PVA-B formulations and the PVA-B/AG double network. In order to test the possible differences in mechanical properties between pure PVA-B and the PVA-B/AG double network, it was decided to perform tensile tests.
3.4 Morphological structure
Fig. 5-a illustrates the large interconnected pores of the 3D network of 3% PVA-B hydrogel. When agarose is added to the formulation, the thickness of the pore walls is strengthened, leading to a reduction in the size of the pores (see Fig. 5-b). In contrast, the 4% PVA-B hydrogel shows a large number of pores with a smaller size than that of 3% PVA-B and 3%/1% PVA-B/AG hydrogel (see Fig. 5-c).
3.5 Mechanical strength
The four replicates of 4% PVA-B all broke at different elongation points whereas the 3%/1% PVA-B/AG hydrogel did not break in any of the four measurements, allowing the machine to reach its maximum gauge length (see Figs. 6 & S5). These results indicate that the 4% PVA-B hydrogel is less flexible at these large elongations than the 3%/1% PVA-B/AG DN hydrogel. Recently, in combination with other materials, this DN gel was used by Kim et al. (30) to create a high-performance self-healing flexible planar supercapacitor. The superior mechanical strength exhibited by the PVA-B/AG DN hydrogel may be ascribed to: a) the hydrogen bonds formed between the PVA-B system and the AG polymers inside this hydrogel (30), b) the strengthened walls of the pores of the DN hydrogel. These results confirm the difference in behavior noticed in section (3.3) where the 3% PVA-B and 4% PVA-B hydrogels were damaged during peeling off from the plaster tiles while the 3%/1% PVA-B/AG DN hydrogel could be completely peeled off in one piece.
3.6 Rheological properties
Oscillatory shear measurements were carried out to study the effect of AG on PVA-B systems when they are blended together. The elastic storage modulus (G’, a measure of the elastic response of a material) and loss modulus (G”, a measure of the viscous response of a material) were measured. When G’ is larger than G”, the material exhibits elastic behavior while when G’<G”, the material shows viscous behavior (26). The frequency sweep curves for all measured hydrogel formulations are plotted in Figs. S6 and S7. From the frequency sweep curves, the intrinsic elastic modulus (G’0) and apparent relaxation time (τc) can be obtained. G’0 is calculated based on the average of the last three highest frequency points of G’. τc is obtained from the crossover frequency point (ωc) of the two moduli (G’ and G”), according to: (τc = 2π/ωc). G’0 is referred to as the ‘stiffness’ of the system and it should be higher than 400 Pa to provide enough elasticity of the hydrogel to be peeled off (35). On the other hand, when the crossover point shifts to lower frequencies τc increases and the hydrogel shows a better shape stability (50) (see Fig. S8).
PVA-B systems are viscoelastic; at high frequencies (or short loading times) the behavior is elastic and the storage modulus (G’) is dominating the behavior. At lower frequencies (or longer loading times), the loss modulus (G”) or the viscous behavior is dominating. In accordance with the literature (26), upon increasing the concentration of PVA, the hydrogels become stiffer at high frequencies and show a higher viscosity in the low frequency domain (see Fig. 7-a).
In Figs. 7-b and S8-a, data recorded for the hydrogel formulations containing a constant amount of PVA-B and increasing amounts of AG, are plotted. The data show that increasing AG also leads to a stiffer behavior, a higher storage and loss modulus at all frequencies investigated. The same trend can be observed on the τc data (see Fig. S8-b). In Figs. 7-c and d hydrogel formulations with various amounts of both components are plotted; in Fig. 7-c, the comparison between using 4% of PVA-B versus replacing 1% with AG is obvious. Both hydrogel formulations (4% PVA-B and 3%/1% PVA-B/AG) behave similar in this test. However, the 3%/1% PVA-B/AG hydrogel formulation has a slightly higher τc value than the 4% PVA-B hydrogel (see Fig. S8-b). In Fig. 7-d, a similar comparison is shown, again the effect of 1% change in the concentrations of PVA-B and AG is studied, this time at a lower concentration level; 3%/0.5% PVA-B/AG is compared to 2%/1.5% PVA-B/AG. In this case, it is clear that these samples are not similar. The high frequency behavior is again rather similar, but from around 0.2 Hz and lower differences are obvious. The hydrogel formulation 3%/0.5% PVA-B/AG behaves as expected, there is a crossover of storage and loss moduli, indicating that the sample is moving into a flow region. This is not the case for sample 2%/1.5% PVA-B/AG. In this sample, the storage modulus curve remains above the loss modulus curve; therefore, there is no crossover. As the frequency reduces, the storage modulus moves into a stable region. This is typical for the existence of crosslinks reducing further flow. Although the 2%/1.5% PVA-B/AG hydrogel formulation resembles the behavior of a true gel, when considering the measured G’0 values, it is a flexible hydrogel.
Upon adding cleaning agents to the 3%/1% PVA-B/AG DN, the frequency sweep curves exhibit differences in G’0 and τc data (see Figs. S7 & S9). All the cleaning agents added to the formulation induced increases in G’0 and τc values except the formulation loaded with 5% C25. This formulation only shows minor differences in response compared to 3%/1% PVA-B/AG with no cleaning agents included. The aforementioned increase agrees with the results presented in (24, 35, 51). The inclusion of organic liquids in PVA-B systems strengthens the hydrogel network, as the organic solvents stimulate the boron ions to bind to the PVA chains. The noticeable increase in the intrinsic storage modulus when EDTA is loaded into the hydrogel may be ascribed to the formation of hydrogen bonds between some of the EDTA ions and PVA chains in the PVA-B system (26). The G’0 measurements seem to suggest that the loading of PVA-B gels with increasingly low polarity solvents, produces stiffer hydrogels which can be due to the syneresis effect. More investigations are needed to confirm this observation. There is no clear correlation between τc and the polarity of the loaded solvents. Moreover, the G’ curve of formulation, loaded with 0.5% EDTA, is higher than G” all over the frequency range, indicating more gel-like behavior.
It was decided to perform creep tests to measure the behavior at long timescales, as opposed to frequency sweeps which are very suited to measure at short time scales. Some results are plotted in Figs. 8-a-d, where the strain is plotted versus the creep time. In Fig. 8-a, increasing the concentration of the PVA-B increases the stiffness or reduces the deformation seen in the creep test. It is also observed that the slope of the creep curve becomes constant at longer timescales, which is typical for viscoelastic materials (52). If the strain versus creep time reaches a constant slope, this indicates that a steady state condition has been reached and the inverse of this slope corresponds to a zero-shear viscosity level. In Fig. 8-b, creep curves are shown for samples containing a fixed amount of PVA-B and increasing concentrations of AG. In this case the effect of adding AG is very pronounced and it changes the shape of the creep curves. After adding AG, the creep curves resemble those of viscoelastic solids. The slope levels off with time, indicating that flow has almost stopped. 3%/0.5% PVA-B/AG hydrogel formulation shows this behavior and adding more AG strengthens this effect. This can be attributed to the additional network formed by AG and the strengthened walls of the pores.
In Figs. 8-c and d, various concentrations of both components (PVA-B and AG) are compared; (4%, 3%/1%, and 4%/1%,) and (3%/0.5%, 3%/1.5%, and 2%/1.5%) respectively. For example, when comparing the 4% PVA-B sample to the 3%/1% PVA-B/AG sample, the creep data clearly show that replacing 1% of PVA-B with AG is not equivalent; the creep curves clearly differ. A similar effect can be observed when comparing the 3%/0.5% to the 2%/1.5% PVA-B/AG hydrogel formulations. In addition, it is interesting to note that decreasing the concentration of PVA-B while using a fixed concentration of AG, appears to have a stronger effect. For example, when comparing 3%/1% PVA-B/AG to 4%/1% PVA-B/AG, the 3%/1% PVA-B/AG hydrogel reaches a lower end strain; so it is stiffer as compared to 4%/1% PVA-B/AG hydrogel (see Fig. 8-c). And similarly, when comparing 3%/1.5% PVA-B/AG and 2%/1.5% PVA-B/AG hydrogel formulations, the 2%/1.5% PVA-B/AG hydrogel has undergone less deformation as the 3%/1.5% PVA-B/AG hydrogel (see Fig. 8-d). This could indicate that the crosslinks formed by AG are more effective or more evenly distributed if the concentration of PVA-B is lower. We note that the reproducibility for all oscillation and creep tests decreases when adding more AG to the hydrogel formulations.
Finally, an empirical relation between creep and frequency sweeps was established, expressing the time range of the creep test as a frequency range. The strain can be transformed into a modulus using the constant stress/creep strain. In Fig. 9, where the creep data is overlaid on the frequency sweep data, it can be seen that the empirical relation is valid for these materials. When comparing samples with the same total concentration of gellant (i.e. the sum of PVA-B and AG), it is clear that the samples show the same behavior at high frequency but deviate at low frequencies. In practice, when hydrogels of this type are applied on the surface for typically 10 to 30 minutes in the frequency range from 0.00027 to 0.00050 Hz, differences in the rheological behavior of the hydrogels are observed.
3.7 Vertical flow test
The stability of a hydrogel in the vertical position is an important aspect especially when it is applied on a wall for long time. Flowing of the hydrogel negates the feature of selective application, in this case, the hydrogel reaches undesired areas of the surface to be treated. In this test, the 3%/1% PVA-B/AG hydrogel shows a better shape stability in the vertical position than the 4% PVA-B hydrogel (see Fig. S10). The 3% PVA-B formulation shows the least shape stability and flows significantly. As a general observation, the addition of solvents hinders the flow behavior and this coincides with the results collected from oscillation and creep measurements. However, the results of the formulations loaded with Xyl exhibit different results than those collected from the frequency sweep tests. In the flow test, the hydrogels loaded with Xyl have better shape stability than those loaded with AC and EtOH which is not the case in the data of τc (see Fig. S9). These differences can be ascribed to the expelled Xyl during the preparation of the hydrogels (see Table S3). It should be taken into account that the degree of flow is also dependent on the size and/or the thickness of the hydrogel slab to be applied.
3.8 Self-healing behavior
This self-healing character can be beneficial in some cleaning treatments such as the cleaning of 3D objects. It is possible to cut the hydrogel into pieces and assemble them on the surface of the object to allow a good contact to the whole surface including all the curves. In a few minutes, the pieces will heal and form one single piece to cover the entire area to be treated. Later, the hydrogel can be cut and peeled off piece by piece. The exposed area can be cleared with a cotton swab to remove swollen deposits. Moreover, if the hydrogel pieces are still usable after the first application, they can be put together to heal as a big single piece again that can be recut for another second application.
PVA-B hydrogels have certain self-healing characteristics due to the dynamic nature of the didiol complexation in the network. The labile bonds formed between tetrahedral borate ions and OH-groups of PVA chains can be easily broken and reformed under ambient conditions (see Fig. 10-a). Furthermore, the mobility of PVA strands and free borate ions facilitates the bonds formation across the contact interface and, consequently, the self-healing process takes place (53-55).
Based on the macroscopic tests, it is obvious that the PVA-B/AG hydrogel has similar healing properties as PVA-B hydrogels. For both 3% PVA-B and 3%/1% PVA-B/AG hydrogels, when the dyed hydrogel pieces were brought together, they healed into one piece under ambient conditions within ten minutes and without external stimuli. This was confirmed by lifting the hydrogels vertically (see Figs. 10-b & c).
Microscopically, the speed of the self-healing character in the PVA-B/AG DN hydrogel (c. 4 minutes) is slower than that of PVA-B hydrogel (c. 2 minutes), which may be ascribed to the fact that the agarose hinders the mobility of the PVA chains and borate ions in the network, as illustrated in Fig. S11. In general, this decrement in self-healing speed occurs when the gel fraction (PVA and/or AG) is increased. For instance, increasing the concentration of PVA polymer in PVA-B system to 5% decreases the self-healing speed to approximately 4 minutes compared to 2 minutes for the 3% PVA-B hydrogel.