Preparation of Adhesive Hydrogels Based on Phase Separation

As the focus on smart materials continues to grow, the development of self-healing hydrogels has advanced significantly in recent years. Traditional hydrogels suffer from poor mechanical properties and limited or no adhesion, hindering their usage in popular fields such as sensors. To expand the applications of hydrogels, it is crucial to enhance their adhesion properties. In this study, a non-adhesive PVA hydrogel was produced using the freeze-thaw method. The PVA hydrogel was then soaked in a 70% acetone solution, which caused the phase separation of the hydrogel. This phase separation technique transforms the original non-adhesive hydrogel into a strong adhesive with high adhesion, addressing the shortcomings of PVA hydrogels in practical engineering applications and providing a novel approach for the preparation of new adhesives.


Introduction
In the past decades, hydrogels have received a lot of attention for their good biological properties.Hydrogels can be divided into covalently bonded cross-linked hydrogels and noncovalently bonded cross-linked hydrogels, or a combination of both methods.Covalently crosslinked hydrogels are those whose network structure consists of covalent linkages formed between gel molecules that are cross-linked.In contrast, non-covalently cross-linked hydrogels (supramolecular hydrogels) form cross-linked networks through interactions such as electrostatic forces.Supramolecular gels are solid-phase materials formed based on noncovalent bonding and self-assembly.The driving forces in the formation of supramolecular materials are hydrogen bonds, van der Waals forces, conjugate π-π stacking interactions, metal coordination interactions, host-guest interactions, and ion-ion interactions [1] .In recent years, hydrogels with adhesion properties have gradually become a hot research topic in the field of hydrogels, and many dynamic covalent bonds, such as imine bonds, disulfide bonds, etc., have been employed to form self-healing hydrogels.However, this method is more complicated.Using the phase separation method, which does not require special treatment of the gel surface or the introduction of specific groups.All that needs to be done is to immerse the hydrogel in a mixture of good and bad solvents for phase separation.There are two main effects of the phase separation method on the adhesion of the gel.On the one hand, due to the decrease in the water content of gels, the volume contracts, and the density of the surface polymer surface of the gel rises rapidly after the separation is separated, which is conducive to the generation of dense keys between the gel and the solid interface.On the other hand, during the separation of the gel phase, due to the large number of mechanical dissipations during the interface separation, the dense polymer can act as a break-up key to consume energy.However, the dilute polymer phase may retain the overall integrity and ductility of the gel, giving hydrogels high toughness [2] .Additionally, because phase separation is a reversible process, the phase separation of the gel can be reversed by adding drip water to the solid-gel interface, such that the surface density and overall energy dissipation of the gel will be significantly reduced and the gel can be easily separated from the solid surface.Likewise, when the removed gel is re-immersed in the solvent mix, it again undergoes phase separation and regains its high strength and adhesion.
PVA material has become an important medical material as a result of its excellent degradation and biocompatibility [3] .This article uses the conventional frozen-thawing method to obtain the required non-sticky PVA gel.By separating PVA gel in 70% of the acetone solution for phase separation to obtain an adhesive PVA hydraulic gel [4] .Stretching and peeling of the PVA gel before and after phase separation show that the mechanical properties and adhesion of the PVA gel after phase separation are improved over before phase separation.Using this method of separation, there is no need to specially treat the gel surface, and there is no need to introduce a specific group.What you need to do is to separate the hydraulic solvent and the poor solvents of the solvents and the bad solvents.Can be converted into a tough adhesive with adhesive gels into adhesive.This greatly expands the scope of application of hydrogels in the engineering field [5] , enhances the practical value of hydrogels, and provides new ideas for the preparation of new types of adhesives.

Preparation of phase separated PVA hydrogels
Weigh 10 g of PVA powder with an electronic balance, dissolved the weighed PVA powder in the solvent mixture with 90 g of DMSO and water in a 3:1 ratio , and stir continuously at 90 °C until the PVA powder was completely dissolved.The mixture was then placed in glass molds and chilled at -40 °C for 24 hours.Subsequently, the mixture was thawed at room temperature, and organogels were obtained.PVA hydrogels were created by immersing the organogels in deionized water for 72 hours to allow for solvent exchange.The obtained PVA hydrogel was divided into two parts and configured with 70 wt% acetone solution, and one of the PVA hydrogels was immersed in 70 wt% acetone solution for 12 h, during which the solvent mixture was changed periodically to ensure a stable solvent concentration.

Raman Spectroscopy
The PVA gels before and after phase separation treatment were analyzed using Raman spectroscopy with an excitation wavelength of 532 nm and all other parameters set to default.

Tensile test
The PVA hydrogel before and after phase separation was cut into long strips of 2.5 mm width, 12 mm length, and 2.5 mm thickness and placed on the stretching machine.The tensile machine was controlled to perform pull-off and cyclic tensile tests on the sample to obtain the tensile curve of the sample.By processing and analyzing the tensile curves, the deformation process and modulus changes of the hydrogels before and after phase separation during the tensile process were analyzed, and the effects of phase separation treatment on the tensile properties of PVA hydrogels were investigated according to the stress-strain curves.

Peel test
The PVA hydrogel before and after phase separation was cut into rectangles with a width of 8 mm, a length of 20 mm and a thickness of 2-4 mm.Before the start of the test, a portion of the gel was applied to the solid surface and pressed with 2.5 kg of weight for 5 s.Subsequent peel tests were performed at a constant peel speed of 100 mm min -1 .

SEM
The PVA hydrogels were dried at 50 °C before and after phase separation.The dried samples were then gold-plated, and the surface morphology was observed using a scanning electron microscope.

Thermal Analysis
The PVA hydrogels before and after phase separation were dried at 50 °C.The dried samples were placed in a crucible, capped, and tested.The atmosphere was nitrogen.The temperature range is set to 35-300 °C, and the temperature rise rate is set to 15 °C/min.Prepare PVA hydrogel by the freezing-thawing method divided the prepared hydrogel into two copies, one without any treatment and the other processed by phase separation.As shown in Figure 1, the peak strength of the PVA hydrogel sample at 2800-3000 cm -1 [6] shows that the peak strength of 2800-3000 cm -1 [6] increased, which indicates that the number of gel surface polymer chains after phase separation increases.The generation of dense keys between glue and solid interface improves the adhesion ability of gel to solid interface.

COMSE-2023
Journal of Physics: Conference Series 2671 (2024) 012009 When touching the water gel with your hands, you can obviously feel that the gel after separation is more rough than the gel surface before the phase was separated, and the gel overall was more tough and difficult to deform.Figure 2 shows the appearance of the hydrogel before and after the phase separation.It can be clearly seen that after the separation processing, the PVA hydrogel becomes opaque and the volume is significantly reduced.A non-uniform phase separation (PS) network structure of local dense polymer phase and diluted polymer phase is formed.The moisture content of PVA hydrogel is reduced, and the density of the polymer is significantly increased.6 SEM shows the appearance characteristics of PVA hydrogel before and after separation.As shown in Figure 3, the surface of the gel without phase separation is relatively flat and uniform, and the gelatin surface that is separated by the phase has a local dense polymer phase and a thinner polymer, so it is uneven.This also confirms the separation of PVA hydrogel in the mixed solvent.After the separation of PVA hydrogel polymer chains, the surface aggregation density of the gel rises, which is conducive to the dense key between the gel and the solid interface.The generation has improved the adhesion of gel.The stretch performance tests were performed for PVA hydrogel (NPS) and PVA hydrogel (PS) separated by phase-separated PVA hydrogel (NPS).As shown in Figure 4, the PVA hydrogels that have not been separated by phase separation are crispy and weak, showing lower fracture stress, fracture response, and Yang's modulus.In contrast, the fracture stress, fracture strain, and Young's modulus of PVA hydrogels after phase separation treatment were significantly higher than those of NPS gels.Due to the formation of a phase-separated (PS) network structure consisting of dense and dilute polymer phases, the dense polymer phase can act as a sacrificial bond that breaks to consume energy; however, the dilute polymer phase maintains the overall integrity and ductility of the gel, allowing the gel to withstand higher loads and elongation.

COMSE-2023
Journal of Physics: Conference Series 2671 (2024) 012009 The phase separation of PVA hydrogel (NPS) and PVA hydrogel (PS) was used to perform the peeling test.The corresponding peel force per unit width versus displacement curves are shown in Figure 5.The PVA hydrogel without phase separation showed essentially no viscosity, so no data were recorded for the peel test.It can be seen from the figure that the hydrogel after phase separation possesses a certain viscosity.This is due to the fact that the phase separation process decreases the water content of the gel, the gel volume shrinks, and the gel surface polymer density increases rapidly after the phase separation, which facilitates the generation of dense bonds between the gel and the solid interface and enhances the adhesion of the gel to the solid.However, the adhesion exhibited by the gels after phase separation in this experiment was far from the strong adhesion described in the literature, which may be due to the insufficient time for phase separation of the gels and the insufficient degree of phase separation of the gels, resulting in the low adhesion of the gels.In order to prove that the thermal stability of the hydrogel has been significantly improved after the separation of the phase, the melting point of the hydrogel before and after the phase separation was tested.It can be seen from Figure 6 that the melting point of the PVA hydrogel before and after the separation is within the temperature range of 220-230 °C.The PVA hydrogel before phase separation has only one peak, which is the melt peak of the PVA hydrogel.And the PVA hydrogel after phase separation treatment showed two peaks in the temperature range of 200-230 °C.After phase separation occurs, the PVA hydrogel internally separates into a partially dense polymeric phase and a partially loose polymeric phase.The peak at 202.7 °C is caused by the breakage of covalent bonds in the dense polymer phase, and the peak at 227.2 °C is the melting peak of the PVA hydrogel after phase separation.After the phase separation process, the melt peak in the DSC curve shifted to the high temperature direction, and the peak shape was sharper.This indicates that the thermal stability of the PVA hydrogels was also improved after phase separation.

Conclusion
In this study, we have prepared a tough adhesive using a phase separation method.Phase separation was induced by immersing the non-adhesive PVA hydrogels in a mixture of acetone and water.Equilibrium homogeneous hydrogels in a mixture of good and bad solvents form a non-uniform phase separation (PS) network structure of locally dense polymer phases and dilute polymer phases.Phase-separated hydrogels can adhere effectively to solid substrates,

Figure 1 .
Figure 1.Raman spectra of PVA hydrogels before and after phase separation.PVA hydrogel samples without phase separation treatment (NPS) and PVA hydrogel samples after phase separation treatment (PS)

Figure 3 .
Figure 3. SEM images of PVA hydrogels before and after phase separation: (a) PVA hydrogel after phase separation; 1000 times (b) PVA hydrogel before phase separation; 1000 times (c) PVA hydrogel after phase separation 2000 times (d) PVA hydrogel before phase separation; 2000 times.

Figure 4 .
Figure 4. Tensile stress-strain curves of PVA hydrogels before and after phase separation.PVA hydrogel samples without phase separation treatment (NPS) and PVA hydrogel samples after phase separation treatment (PS)

Figure 5 .
Figure 5. Peel force per unit width versus displacement curves of PVA hydrogels before and after phase separation

Figure 6 .
Figure 6.DSC curves of PVA hydrogels before and after phase separation