Binary Systems
In this study the synthesis of XLG/AMPS hydrogels involved the combination of XLG nanoparticles with AMPS monomers and subsequent crosslinking to form a stable gel network. XLG nanoparticles act as a reinforcing agent, while AMPS monomers contribute to the hydrophilicity and reactivity of the hydrogel. Various methods can be employed for the synthesis of XLG/AMPS hydrogels, including free radical polymerization, in situ gelation, and self-assembly approaches. We particularly used the free radical method of polymerization to control the rheological and mechanical properties of the gels, such as gelation time, mechanical strength, and swelling behavior, by adjusting the concentrations of XLG and AMPS monomer.
XLG systems without AMPS monomers were investigated in the first series to examine how ionic interactions affect rheology. The viscoelastic properties of the system were studied by changing the ratio of XLG gels. Increased concentrations of XLG in the system result in a rise in ionic interactions, causing the system to be more viscous (Fig. 1). The results of the study showed that the viscosity increased with the same way when the ratio of XLG gels increased. XLG had a limit of 0.1 g∙mL− 1 in the system due to precipitation, which causes it to become viscous, while 0.01–0.05 is sufficient. DN gels, particularly XLG, have a swellable polymeric structure. Our current work focuses on its ionic structure and interactions. We aim to understand how the polymer behaves in response to changes in ionic strength. In addition, we are looking into the role of the ionic structure in the mechanical and rheological properties of gels.
The rheological properties of XLG/AMPS hydrogels were explored by focusing on their viscoelastic behavior, shear-thinning characteristics. XLG/AMPS hydrogels exhibit unique viscoelastic behavior, meaning they possess both viscous (liquid-like) and elastic (solid-like) characteristics. This behavior is a result of the interactions between the XLG nanoparticles, AMPS monomers in a polymer network.
During the incorporation of AMPS into the XLG system, the hydrogels exhibited predominantly elastic behavior, characterized by the ability to store, and recover energy. This elastic response arises from the physical entanglements between the polymer chains and the reinforcement provided by the Laponite nanoparticles. As a result, these hydrogels display excellent shape recovery and resistance to deformation. The viscoelastic behavior of the polymers was not affected by increased concentrations of XLG in the AMPS-XLG system, which is an excellent result (Fig. 2). This indicates that XLG does not interact with the polymer molecules in a significant manner. Furthermore, it suggests that the viscoelastic properties of the polymer remain unchanged even at high XLG concentrations (Fig. 2b). This behavior has important implications for the application of this material in various industries.
Current study shows that XLG/AMPS hydrogels exhibit shear-thinning behavior, which means their viscosity decreases as the shear rate or applied stress increases. This property is advantageous for various applications as it allows for easy processing, injection, and spreading of hydrogel. The shear-thinning behavior arises from the disruption of the physical entanglements within the hydrogel network under shear stress. As the shear rate increases, XLG nanoparticles and polymer chains align and slide past each other, leading to a decrease in resistance to flow. Upon the removal of the shear stress, the hydrogel recovers its original viscosity, exhibiting a reversible shear-thinning behavior. The thixotropic behavior arises from the reversible breakdown and reformation of physical interactions within the hydrogel network. Continuous agitation disrupts the physical entanglements between the XLG nanoparticles and polymer chains, resulting in a decrease in viscosity. Once the agitation ceases, the hydrogel gradually rebuilds its network structure, leading to the recovery of viscosity over time.
Figure S1a and b demonstrates that the samples have a gel state of up to 0.1 g∙mL− 1 without AMPS, but with AMPS, they become a highly distributed solution of up to 0.2 g∙mL− 1. This increase in solubility allows for higher concentrations of the sample and greater efficiency in the extraction process. The addition of AMPS also improves the viscosity of the sample, creating a more uniform solution. This in turn allows for more precise and accurate measurements of the sample. Additionally, the increased viscosity also improves the stability of the sample, allowing for better preservation of the sample over time.
The increasement in XLG concentration doesn’t change the frequency dependence of the dynamic system (Fig. 3). This frequency dependence is due to the ability of XLG to absorb energy and dissipate it as heat. This property enables XLG to change the dynamic behavior of the system, resulting in higher efficiency in the energy transfer process. This significantly reduces the energy loss in the system, resulting in improved performance and efficiency. It makes XLG an ideal material for energy transfer applications, as its properties can be tailored to accommodate specific processes.
In Fig. 3, the loss factor δ decreased as XLG concentration increased, indicating increased crosslinking but no stacking or precipitation. The results suggest that XLG can be used to control the rheological properties of the system without compromising product stability. Furthermore, the addition of XLG had no significant impact on the viscosity of the system. It is extensively diffused and acts as a macro crosslinker. XLG is a suitable stabilizer for these systems, providing beneficial rheology modifications without increasing viscosity and improved stability and rheological properties. Additionally, XLG/AMPS compounds reduce swelling effects as well. This results in a significant improvement in the physical and chemical properties of the formulation. XLG also enables the formulation of products with improved shelf life and longer product stability.
Overall XLG nanoparticles interact with the polymer chains through electrostatic interactions, hydrogen bonds, and van der Waals forces. These interactions promote stronger interfacial adhesion and improved stress transfer between the nanoparticles and the polymer matrix, resulting in enhanced mechanical properties. By incorporating XLG nanoparticles into the hydrogel, a tortuous path is formed for water or solute diffusion.This diffusion increases the resistance of the gels to swelling and helps maintain its structural integrity under mechanical stress.The layered structure of XLG provides a reinforcing effect, improving the gels’ stiffness and structural integrity. The mechanical properties of the resulting hydrogel were increased by varying upward the concentration of XLG nanoparticles. Increasing the nanoparticle concentration generally leads to improved mechanical strength and stiffness, up to a certain threshold where excessive nanoparticle loading may compromise the integrity of gels. Surface modifications of XLG nanoparticles can be employed to enhance their compatibility with the AMPS system. By functionalizing the nanoparticle surface and introducing AMPS polymer chains with affinity for the nanoparticle surface, stronger interactions have been achieved, resulting in improved mechanical properties.XLG/AMPS hydrogels possess a high-water absorption capacity due to the hydrophilic nature of AMPS monomers. The presence of XLG nanoparticles within the hydrogel network can modulate the swelling behavior, allowing for the control of gel properties and release kinetics. The results for a 0.5 w/v AMPS solution show that the addition of a large amount of AMPS property is prominent, and it’s brittle in the swelling stage (Figure S2).
Table 1
Mechanical parameters of samples at after preparation and swollen states. Young modulus = E, fracture stress = σf, fracture strain = εf % and toughness = W. Standart deviations are given in parenthesis and they are smaller than 5% for εf %.
Sample | XLG / g | E / kPa | σf / kPa | εf% | W / kJ∙m− 3 | State |
XLG-PA-1 | 1 | 219 (14) | 5458 (35) | 83 | 665 (39) | After prep. |
XLG-PA-2 | 2 | 1238 (76) | 1295 (89) | 48 | 222 (17) |
XLG-PAM-1 | 1 | 282 (4) | 1222 (53) | 76 | 192 (8) |
XLG-PAM-2 | 2 | 728 (37) | 775 (17) | 56 | 138 (10) |
XLG-PA-1 | 1 | 1000 (88) | 620 (4) | 47 | 49 (2) | Swollen |
XLG-PA-2 | 2 | 478 (12) | 340 (22) | 45 | 424 (9) |
XLG-PAM-1 | 1 | 163 (4) | 705 (15) | 72 | 122 (5) |
XLG-PAM-2 | 2 | 450 (4) | 520 (9) | 52 | 105 (4) |
Although the mechanical properties at after-synthesis conditions are promising for the AMPS/XLG system, the values of the swollen samples are limited due to the high swelling capacity of AMPS. Although the first method that comes to mind to correct this is to reduce the AMPS content or increase the chemical crosslink ratio, these applications will affect the dispersion of Laponite (XLG) or cause the hydrogel to turn into conventional brittle gels. In these conditions, MAPTAC, a cationic monomer, was included in the structure to limit the swelling of AMPS and increase the effect of reversible bonds in the structure. The ionic bonds between anionic and cationic monomers were strengthened by entanglement, and the targeted mechanical properties were achieved.
In the second set of experiments in which the ternary system was examined, the swelling degree of XLG-PAM hydrogels prepared at equimolar concentrations by mass and volume after synthesis was monitored for 10 days. The hydrogels in all XLG contents reached swelling equilibrium in distilled water within 48 h (Figure S3). At equilibrium swelling ratio by mass mrel,eq and volume Vrel,eq are plotted against the XLG content, indicating the magnitude of the interaction with increasing XLG amount and decreasing degree of swelling (Figure S3b).
The stress-strain curves of XLG-PAM ternary systems obtained under compression tests and the mechanical parameters such as toughness W, modulus E, fracture stress σf and strain εf% calculated as a result of these measurements are summarized in Figs. 4 and 5 for samples that swollen and after preparation states.
To examine the ternary system in detail, cylindrical samples were subjected to compressive deformation at different strain rates \(\dot{\epsilon }\). XLG-PAM hydrogels show strain sensitive mechanical properties owing to their dominant physical cross-linked nature. Figures 6a and b show the stress-strain curves measured in the range of 10− 3 – 10− 1 s− 1 strain rate \(\dot{\epsilon }\) for samples in the after preparation and swollen states, respectively. Figure 6c shows the ftacture stress varying depending on the strain rate \(\dot{\epsilon }\). For both after preparation (filled symbols) and swollen samples (open symbols), the fracture stress is directly proportional to the logarithmic strain rate ln\(\dot{\epsilon }\). Young’s modulus E and ftacture stress σf values are shown in Fig. 6d-g with horizontal columns. Filled columns belong to after preparation samples, and open columns belong to swollen samples.