Thermal response of double network hydrogels with varied composition

2.1 Hydrogel synthesis

The single network (SN) hydrogel was synthesized using thermally activated redox polymerization. An aqueous solution was prepared containing monomer, N,N′-diethylacrylamide (DEAAm; Sigma-Aldrich, St. Louis, MO, USA), crosslinking agent, N,N′-methylenebisacrylamide (MBAAm; Sigma-Aldrich, St. Louis, MO, USA), initiator, ammonium persulfate (APS; Sigma-Aldrich, St. Louis, MO, USA), and catalyst, N,N,N′,N′-tetramethylenediamine (TEMED; Sigma-Aldrich, St. Louis, MO, USA). DEAAm, MBAAm, and APS were dissolved in deionized water and flushed with nitrogen for approximately 10 min. Then, TEMED was added to the reaction mixture and shaken well for proper mixing. The reaction solution with a volume of approximately 3 mL was injected between 7 cm × 5 cm glass plates and placed in the refrigerator at 5°C. After 24 h of polymerization process, samples were washed for 3 days with distilled water, which was replaced every 24 h. Table 1 shows the chemical compositions of reaction mixtures used in the preparation of the SN hydrogels, which were the base for the subsequent preparation of double hydrogels.

The DN hydrogels were prepared using the following procedure: specimens of approximately 3 cm × 3 cm × 0.1 cm were cut out from the SN hydrogel samples at 20°C and immersed for 24 h in a large volume of aqueous solutions containing second monomer (acrylamide [Aam], DEAAm, or 2-acrylamido-2-methyl-1-propanesulfonic acid [AMPS]); a crosslinking agent, MBAAm; and a photoinitiator, 2-oxoglutaric acid (OGA) at the same temperature. The chemical compositions of the swelling solutions used in the preparation of DN hydrogels are shown in Table 2. Chemical structures of PDEAAm, PAAm, and PAMPS are shown in Figure A1 (in Appendix).

Before the swelling procedure, the prepared solutions were flushed with nitrogen for approximately 30 min. DN hydrogels were prepared at room temperature by UV irradiation (365 nm, 3 h) of the swollen specimens of the SN hydrogel fixed between two glassy plates separated by a spacer from silicone rubber with a rectangular void of dimension 4 cm × 4 cm. After synthesis, residual unreacted reagents contained in the hydrogel samples were washed out three times with a large amount of distilled water.

The coding of DN samples refers to that of the second component with molar concentration used in their preparation, e.g., the second component in DN-DA0.5 hydrogel was prepared from 0.5 M concentration of AAm monomer solution; similarly, DN-DD1 was prepared from 1 M concentration of DEAAm solution.

2.2 Swelling behavior

The swelling capacity of the prepared hydrogels was evaluated by gravimetric measurement. Samples with dimensions of approximately 1.5 cm × 1.5 cm × 0.1 cm were swollen to equilibrium in 50 mL of distilled deionized water at room temperature and then thermally equilibrated for 2 h at the starting temperature for swelling experiments (13°C). The samples were then heated to the next temperature and weighed after reaching equilibrium (2 h). The time period was determined using kinetic experiments in which a temperature step change from 15°C to 55°C was applied. It was observed that the swelling ratio remained practically unchanged after 2 h (Figure A2).

To determine the mass of the dry samples m _dry, the hydrogel samples were first air-dried at room temperature for 1 day and then vacuum-dried at 80°C for another (one) day. The swelling ratio s(T) was calculated as follows:

where m(T) is the mass of the hydrogel sample swollen at temperature T.

2.3 ¹H NMR spectroscopy

The ¹H NMR spectra were recorded with a Bruker Avance 500 liquid-state spectrometer (Bruker, Karlsruhe, Germany) (11.7 T, 16 scans, recycle delay of 20 s). The integrated intensities were determined using spectrometer software with an accuracy of ±1%. During the variable temperature experiments, the temperature was set with an accuracy of ±0.2°C using a BVT3000 temperature unit. Before data acquisition, the samples were tempered for 15 min.

The quantitative characterization of changes occurring during the heating process in hydrogels was obtained from the p-fraction analysis (degree of collapsing) using the relation

where I(T) is the integrated intensity of a given polymer resonance in the spectrum at temperature T > T ₀. I ₀(T ₀) is the integrated intensity of this resonance when no phase transition or other reason for the reduced mobility of polymer segments occur. Since integrated intensities decrease with absolute temperature as 1/T, Eq. 2 contains a T/T ₀ correction term. For T ₀, we took the temperature where the integrated intensity of the given signal was the highest and, therefore, p(T ₀) = 0 (22,23,24,25).

2.4 Differential scanning calorimetry (DSC)

DSC was used to analyze the heat associated with the phase-change of the hydrogel materials and the temperature transition of the hydrogels on a DSC-8500 instrument (Perkin-Elmer). The flow rate of the purge gas (nitrogen) through the DSC cell was 20 mL·min⁻¹. Temperature calibration was performed using mercury, distilled water, and indium. The heat flow was calibrated using the melting heat of indium.

The mass of the measured sample was within the range ca. 20–40 mg. The measurements were performed under a nitrogen gas atmosphere at a heating or cooling rate of 10°C·min⁻¹. The first heating scan was run from 10°C to 70°C, then held at 70°C for 1 min, followed by a first cooling scan to 10°C and subsequently held at 10°C for 5 min. After that, the second heating scan from 10°C to 70°C and the second cooling scan from 70°C to 10°C were carried out.

2.5 Elastic properties

The uniaxial compression measurements were conducted with a rectangular-shaped sample in Perkin-Elmer DMA7e apparatus at two temperatures (15°C and 61°C). The rectangular-shaped samples were prepared with a cross-section of approximately 8 mm × 8 mm and a height of 1–3 mm at room temperature. During the experiment, the sample was placed between two circular metal discs with a diameter of 20 mm and, at the same time, immersed in distilled deionized water maintained at a selected temperature. Prior to measurement, the samples and metal discs were equilibrated for 2 h in a thermostatic vessel. The compression was performed uniaxially with increasing force at a constant rate (50 or 100 mN·min⁻¹), and then the stress–strain curve of each sample was recorded. The Young’s modulus, E, was determined as a slope of the initial region of the stress–strain curves following Eq. 3 for the stress τ.

where F is the applied force, A ₀ is the initial cross-section area of the sample, and λ = (l/l ₀) is the ratio of the deformed height (l) to the initial height (l ₀) of the specimen cut out from the hydrogel.

2.6 Thermodynamic two-state models

To determine thermodynamic parameters of the phase transition in hydrogels from NMR and gravimetric data, we used a model based on two exchangeable states, i.e., swollen (hydrated) state and collapsed state. At temperatures lower than the transition point, the hydrogel exists in a swollen state and contains bound water molecules. As the temperature rises, the network shrinks and releases the water, resulting in a collapsed network that retains a certain amount of permanently bound water. A schematic representation of the model can be seen in Figure 1a.

Figure 1

Schematic two-state (swollen-collapsed) model of the temperature-dependent deswelling process of DN hydrogels (a). Theoretical dependence of collapsed fraction p(T) (b) and swelling ratio s(T) (c) as defined by Eqs. 4 and 5, respectively. Fitted parameters are highlighted in red. Adapted from ref. [21], originally published under a CC BY 4.0 license.

Eq. 4 was used for the description of the phase-separated fraction of hydrogel p(T) as obtained from NMR experiments (for a schematic plot, see Figure 1b). Eq. 4 was previously derived and used for solutions of temperature-sensitive polymers [26,27]. However, it remains valid for hydrogel systems since it is based on a similar two-state model (Figure 1a).

where ΔH _NMR and ΔS _NMR are values of endothermic enthalpy and entropy changes associated with the phase transition process in hydrogels, p _max is the maximum fraction of polymer units that participate in the phase transition, R is the gas constant (8.314 J·mol⁻¹·K⁻¹), and T is the absolute temperature. The onset temperature T NMR on (Figure 1b) is obtained as the cross-point of the tangent at the p(T) curve’s inflexion point and the x-axis. Similarly, the offset temperature T NMR off (Figure 1b) is obtained at the cross-point of the tangent with y = p _max line. The width of the phase transition can be determined as ΔT _NMR = T NMR off – T NMR on (Figure 1b).

The thermodynamic model used to describe the temperature-dependent swelling ratio in gravimetric experiments was derived in a similar manner as the case of p(T) curves from variable-temperature NMR spectra (21) as depicted schematically in Figure 1a. At low temperatures (T ≤ T ₀), water molecules are bound in swollen hydrogel network. When the temperature rises above T ₀ value, the hydrogel collapses, and at the same time, water molecules are released from the hydrogel interior. At the end of this process, although most water molecules are released outside the hydrogel structures (free water), a certain amount remains permanently bound inside the collapsed hydrogel network (permanently bound water).

Based on the model of gravimetric deswelling experiments, the equation for swelling ratio s(T) (Figure 1c) was derived (21) in the following form:

where ΔH _grav and ΔS _grav are the standard changes of enthalpy and entropy, respectively, determined from gravimetric measurement. The parameters α – 1 and β have the meaning as low- and high-temperature limit values of s(T) curve, respectively, and using these parameters, the extent of deswelling could be determined as their difference Δ s = α − 1 − β (Figure 1c). At the same time, parameters α and β could be expressed as follows:

where m(T ₀) is the mass of the hydrogel sample swollen at the lowest measured temperature T ₀ and m _pbw is the mass of permanently bound water molecules, which stay in the collapsed network above the phase transition (Figure 1a). The onset and offset temperature T grav on and T grav off , respectively, and the width of the phase transition ΔT _grav were determined from s(T) curves (Figure 1c) analogous to those parameters obtained from NMR experiments.

3.1 Deswelling gravimetric experiments

The temperature dependences of the swelling ratio for all prepared hydrogels are illustrated in Figure 2. The measured curves clearly indicate that all DN hydrogels consistently exhibit higher swelling ratios compared to the SN-D hydrogel from which they were prepared. Additionally, it is observed that the swelling ratio values measured at lower temperatures for DN hydrogels are inversely dependent on the molar concentration of the second component, e.g., the hydrogel DN-DA0.5 swells more than the hydrogel DN-DA3, and this trend holds regardless of the chemical structure of the second component.

Figure 2

Temperature dependence of swelling ratio for the hydrogels composed of PDEAAm/PAAm (a), PDEAAm/PDEAAm (b), and PDEAAm/PAMPS (c). Experimental points for PDEAAm/PAAm and PDEAAm/PDEAAm hydrogels are fitted using Eq. 5.

The PDEAAm/PAMPS hydrogels demonstrate a swelling ratio approximately one order of magnitude higher swelling ratio than the DN hydrogels composed of PDEAm/PAAm and PDEAAm/PDEAAm, which is due to the high hydrophilicity of PAMPS. PAMPS, as a hydrogel containing sulfonic acid, belongs to the class of strong polyelectrolyte hydrogels, exhibiting a high degree of ionization and increased swelling capacity (28). The high amount of water absorbed in the PDEAAm/PAMPS hydrogels apparently prevents these hydrogels from deswelling with increasing temperature, and this effect is more obvious for the DN hydrogels with 1 and 2 M concentrations of PAMPS. For the sample DN-DS0.5, a change in swelling ratio was detected at around 60°C. The DN hydrogels PDEAAm/PAAm and PDEAAm/PDEAAm exhibit thermo-responsive behavior within the measured temperature range and show deswelling with increasing temperature depending on the feed molar concentration of AAm and DEAAm monomers, respectively.

The deswelling curves of SN-D, PDEAAm/PAAm, and PDEAAm/PDEAAm hydrogels were fitted using Eq. 5 and relevant thermodynamic parameters (ΔH _grav, α − 1, Δs, γ, T grav on , and ΔT _grav) are plotted against the molar monomer concentration of the second component in DN hydrogels (Figure 3).

Figure 3

Parameters characterizing deswelling of PDEAAm/PAAm and PDEAAm/PDEAAm hydrogels in dependence on molar concentration of AAm and DEAAm in the reaction mixture, respectively: enthalpy ΔH _grav (a), low-temperature limit of the swelling ratio α − 1 (b), extent of deswelling Δs (c), the ratio of the mass of permanently bound water to the total mass of water contained in a fully swollen network γ (d), onset transition temperature T grav on (e), and the width of transition ΔT _grav (f).

In comparison to the SN-D hydrogel, the DN hydrogels composed of PDEAAm/PAAm show higher transition enthalpy ΔH _grav (Figure 3a) and broader transition region ΔT _grav (Figure 3f). As the amount of PAAm component increases, the onset temperature T grav on shifts to higher values (Figure 3e). As previously mentioned, the swelling of PDEAAm/PAAm hydrogels at low temperatures depends on the monomer concentration of AAm. The parameter α − 1, which corresponds to the low-temperature limit of the swelling ratio, thus decreases from 24.9 to 14.3 for hydrogels in the order DN-DA0.5 > DN-DA1 > DN-DA2 > DN-DA3 (Figure 3b). Due to higher swelling ratio at lower temperatures α − 1, DN-DA0.5, and DN-DA1 hydrogels show high extent of deswelling Δs in comparison with SN-D hydrogel. The extent of deswelling Δs decreases with the concentration of AAm and for the hydrogel DN-DA3, the temperature-dependent change in the swelling ratio is insignificant. The incorporation of the PAAm component in the PDEAAm/PAAm hydrogels, known for its strong affinity to water molecules, increases the hydrophilicity of the hydrogels, leading to a greater degree of swelling. This increased water content creates a higher resistance to temperature-induced changes in the hydrogel structure. As temperature increases, hydrogels tend to dehydrate and undergo deswelling. However, in DN hydrogels with a higher concentration of PAAm, the strong affinity between PAAm and water molecules helps retain a larger amount of water within the hydrogel network, even at elevated temperatures. This retention of water prevents significant structural changes and reduces the extent of deswelling. In addition, the interpenetration of polymer chains in PDEAAm/PAAm hydrogels, along with the hydrophilicity of the PAAm component, enables a greater water retention within the hydrogel network. This interpenetration restricts the movement and conformational changes in the polymer chains, resulting in increased resistance to temperature-induced structural transitions.

DN PDEAAm/PDEAAm hydrogels comprise two thermo-responsive PDEAAm networks of different polymer concentrations. At lower temperatures, PDEAAm/PDEAAm hydrogels absorb slightly more water than SN hydrogel, as indicated by an increase in the α − 1 parameter from 12.9 for SN-D hydrogel to 16.1 for DN-DD2 hydrogel. Figure 3e demonstrates that the onset temperature, which signifies the initiation of deswelling, slightly increases with the concentration of DEAAm units in the second component. Compared to SN hydrogel, the enthalpy value ΔH _grav for PDEAAm/PDEAAm hydrogels is somewhat larger, the temperature range of deswelling is narrower and the transition is therefore sharper. This is likely a result of the higher concentration of temperature-sensitive DEAAm units in the DN hydrogels and the interpenetration of polymer chains within DN network, which promotes the formation of collapsed structures in PDEAAm/PDEAAm hydrogels.

It is also of interest to observe parameter γ = β α − 1 , which represents the ratio of the mass of permanently bound water to the total mass of water contained in a fully swollen network (Figure 3d). PDEAAm/PDEAAm hydrogels exhibit values of γ ranging from 0.01 to 0.04, indicating that these hydrogels contain virtually no permanently bound water within their collapsed structures. On the other hand, PDEAAm/PAAm hydrogels show a significant increase in the γ parameter, reaching a value of 0.5 for DN-DA3 hydrogel. This signifies that water–polymer interactions are favored in the hydrophilic structures of PDEAAm/PAAm hydrogels, with approximately 50% of water molecules remaining in the bound state throughout the measured temperature range.

3.2 ¹H NMR spectra

The high-resolution ¹H NMR spectra of hydrogels DN-DA1 and DN-DD1 recorded under the same instrumental conditions at 28°C and 50°C are presented in Figure 4. The assignment of resonances to various types of protons and to residual water (HDO) is seen in the spectra measured at 28°C.

Figure 4

¹H NMR spectra of DN-DA1 (a) and DN-DD1 hydrogels (b) in D₂O recorded at 28°C and 50°C.

From the NMR spectra in Figure 4, it is evident that all the polymer signals are clearly distinguishable at 28°C. This is because the polymer chains of the swollen hydrogels are flexible, resulting in a complete signal in the high-resolution NMR spectra. However, as the temperature increases, the signals of the PDEAAm component become significantly broadened. This is due to the collapsing PDEAAm units becoming immobile, rendering the corresponding NMR lines undetectable in high-resolution NMR spectra (22,23). Contrarily, the signals of the main chain group CH of PAAm and the main chain group CH₂ of both components PAAm and PDEAAm marked as CH(A) and CH₂(D+A), respectively, in the spectra of DN PDEAAm/PAAm hydrogels (Figure 4a) do not change with the increase in the temperature. Obviously, the PAAm units remain mobile even at higher temperatures as they are not involved in compact globular structures.

For illustration, Figure A3 displays the ¹H NMR spectra of the DN-DS hydrogel measured at two distinct temperatures. The polymer bands remain unaffected by temperature variations, and the hydrogels containing PAMPS demonstrate no observable modifications at the microscopic level, similar to the results observed in swelling experiments.

The fraction of immobile polymer units, referred to as the p-fraction, was calculated by analyzing the temperature-dependent integral intensities of polymer signals using Eq. 2. All PDEAAm signals in the studied hydrogels showed the same dependence of p-fraction on temperature. Figure 5 illustrates the p-fractions p(T) of hydrogels composed of PDEAAm/PAAm and PDEAAm/PDEAAm. The data were fitted using Eq. 4 and the corresponding achieved parameters (ΔH _NMR, p _max, T NMR on , and ΔT _NMR) are plotted vs the molar monomer concentration of the second component in DN hydrogels in Figure 6.

Figure 5

Temperature dependences of p-fraction determined from PDEAAm CH₃ signal in hydrogels composed of PDEAAm/PAAm (a) and PDEAAm/PDEAAm (b). Experimental points are fitted according to Eq. 4.

Figure 6

Characteristic parameters as obtained from NMR spectra analyses of PDEAAm/PAAm and PDEAAm/PDEAAm hydrogels in dependence on molar concentration of AAm and DEAAm in the reaction mixture, respectively: enthalpy ΔH _NMR (a), maximum of p-fraction p _max (b), onset transition temperature T NMR on (c), and the width of transition ΔT _NMR (d).

For PDEAAm/PAAm hydrogels, the parameter p _max, indicating the maximum value of p-fraction, varies significantly with the concentration of PAAm units. When the SN-D hydrogel reaches a p _max value of 0.85, the DN-DA3 hydrogel exhibits a decreased p _max value of 0.2. This suggests that 80% of PDEAAm units are not included in the collapsed structures but remain in interaction with water and they are hydrated together with PAAm units. Consequently, this hydrogel retains a relatively high swelling ratio even at higher temperatures. Similar to the results from swelling experiments, the value of ΔH _NMR decreases, the extent of deswelling ΔT _NMR increases, and the transition becomes broader with the increase in the AAm concentration in PDEAAm/PAAm hydrogels. A similar effect of the hydrophilic component on the temperature dependence of p-fraction was observed for IPN hydrogels of PNIPAm and PAAm (24).

From Figure 6b, it is evident that p _max for SN-D hydrogel reaches the value 0.85, but for DN PDEAAm/PDEAAm hydrogels, an increase in p _max to 1 can be observed and PDEAAm signals in the NMR spectrum completely disappear at higher temperatures (Figure 4b). The introduction of the second component, PDEAAm, during the formation of PDEAAm/PDEAAm hydrogels apparently enhances the formation of compact globular structures in which all polymer units of DN hydrogels are involved. The high enthalpy values ΔH _NMR achieved by the SN-D (500 kJ·mol⁻¹) and PDEAAm/PDEAAm hydrogels (around 400 kJ·mol⁻¹) are attributed to the high cooperativity of the polymer chains during the temperature change as discussed in Section 3.4.

The temperature width of transition ΔT _NMR does not vary significantly for PDEAAm/PDEAAm hydrogels and ranges from 6°C to 8°C. The onset temperature T NMR on decreases noticeably with an increasing concentration of DEAAm in the second component, e.g., DN-DD2 hydrogel shows T NMR on 6°C lower compared to SN-D hydrogel. Obviously for hydrogels with a higher concentration of PDEAAm units, the conditions for the transition of polymer units from swollen to collapsed structures occur at lower temperatures. Interestingly, the onset temperature T grav on detected in the swelling experiments slightly increases with the PDEAAm concentration in the PDEAAm/PDEAAm hydrogels (Figure 3e), and at the same time T grav on values are about 10–15°C lower than T NMR on . The mentioned differences result from the fact that using NMR and swelling experiments, various processes are detected during the temperature-induced changes in hydrogels. First, the process of water release from the interior hydrogels begins, which is detected using swelling experiments. Then, the hydrogel units are packed into globular structures, depending on the density (concentration) of the PDEAAm units and this effect is observed in NMR spectra as decrease in polymer signals.

3.3 DSC

The DSC calorigrams of all studied hydrogels detected during the first and second heating are shown in Figure A4. In all the studied samples, a broad endothermic transition was observed, which is associated with the demixing of water molecules from the hydrogel. A similar broad DSC peak has been observed in PDEAAm solutions (29) and IPNs of PDEAAm and PAAm (25). The onset temperature of demixing was determined as the point of intersection of the leading edge of exotherm with the extrapolated baseline. The specific enthalpy of demixing was obtained by integration of the endotherms from the second heating and recalculated to the weight of dry sample (J·g⁻¹ of dry polymer).

Figure 7 illustrates the specific enthalpy of demixing per unit mass of polymer ΔH _DSC and the values of onset temperature of demixing temperature T DSC on in SN and DN hydrogels as a function of the molar monomer concentration of the second DN component. As it follows from Figure 7b for PDEAAm/PDEAAm hydrogels, the temperature T DSC on does not change with PDEAAm concentration, while for PDEAAm/PAAm hydrogels, this value is about 1–3°C higher than in corresponding SN hydrogel. The specific enthalpy of demixing in the SN-D hydrogel (ΔH _DSC = 29.1 J·g⁻¹ = 3.70 kJ·mol⁻¹) is consistent with the reported enthalpy value for aqueous PDEAAm solutions (30). For DN PDEAAm/PDEAAm hydrogels, the enthalpy of demixing is increasing with DEAAm concentrations, obtaining value ΔH _DSC = 49.9 J·g⁻¹ for DN-DD2 hydrogel. These high ΔH _DSC values are evidently a result of the highly concentrated DEAAm units in PDEAAm/PDEAAm hydrogels. The endotherms registered for PDEAAm/PAAm hydrogels are less distinct (Figure A4a), and the corresponding specific enthalpy of demixing are low (ΔH _DSC ≅ 8–12 J·g⁻¹) due to the presence of hydrophilic PAAm in DN hydrogels. These findings are consistent with the results obtained from deswelling experiments (Figure 3a).

Figure 7

Transition characteristics of PDEAAm/PAAm and PDEAAm/PDEAAm hydrogels determined during heating by DSC in dependence on molar concentration of AAm and DEAAm in the reaction mixture, respectively: specific enthalpy of demixing per unit mass of polymer network ΔH _DSC (a) and onset transition temperature T DSC on (b).

3.4 Cooperative domains

The enthalpy changes ΔH _NMR and ΔH _grav obtained by fitting of Eqs. 4 and 5, respectively, are referred to as the “van’t Hoff transition enthalpy” and are associated with the concept of cooperative domains (21,31). A cooperative domain is considered to consist of cooperating molecules that undergo a state change simultaneously (31). The ratio of the van’t Hoff transition enthalpy ΔH _NMR (in J·mol⁻¹ of cooperative monomer units) acquired from NMR (Figure 6a) and the calorimetric enthalpy ΔH _DSC obtained from DSC (Figure 7a), recalculated per mol of collapsing PDEAAm monomer units, provides the number of PDEAAm monomer units present in one cooperative domain. Analogously, the gravimetric deswelling measurements detect the water released from the hydrogel, and thus the corresponding unit of the van’t Hoff transition enthalpy ΔH _grav (Figure 4a) is J·mol⁻¹ of cooperative water units. The number of water molecules in one cooperative water unit can be calculated from the ratio of the van’t Hoff transition enthalpy ΔH _grav obtained from gravimetric experiments, and the calorimetric enthalpy ΔH _DSC, which needs to be correlated to the number of moles of water molecules released during the collapse of one gram of the network sample (21). The data and equations used to calculate the number of PDEAAm monomer units and water molecules in a cooperative domain are clearly presented in Tables A1 and A2 (in Appendix), respectively.

The number of PDEAAm monomer units and water molecules contained in one cooperative domain are summarized in Table 3. The sizes of cooperative water units for SN-D and DN PDEAAm/PDEAAm hydrogels are practically the same. The cooperative monomer units are the largest for SN-D hydrogel, and increasing concentration of DEAAm units in PDEAAm/PDEAAm hydrogels leads to a lower number of monomer units per cooperative domain. If we consider that the molar ratio of crosslinker MBAAm and monomer DEAAm was 1:125 during the preparation of the SN-D hydrogel (Table 1), then the number of PDEAAm units in a cooperative domain (110 monomer units), in a rough approximation, corresponds to the average number of monomer units between two network junctions. In the interpenetrating structure of PDEAAm/PDEAAm hydrogels, the formation of additional physical junctions effectively shortens the chain length between two junctions, resulting in a reduced size of cooperative monomer units.

PDEAAm/PAAm hydrogels show a significant swelling ratio and thus their cooperative water units contain 10,000–20,000 water molecules. However, cooperative monomer units are relatively small. For example, the DN-DA3 hydrogel, with the highest AAm content, contains only 20 monomer units in a cooperative domain due to the relatively low number of collapsing PDEAAm units.

3.5 Elastic properties

During the handling of the samples, we observed that SN-D hydrogel is fragile, whereas the DN hydrogels, formed by incorporating a second network, exhibit much higher strength. The Young’s modulus E of hydrogels at 15°C is plotted against the swelling ratio in Figure 8a. Despite DN hydrogels having a higher swelling ratio and thus containing more water, their Young’s modulus values are 2–3 times higher than those of the SN-D hydrogel. This can be attributed to the reinforcing effect of the second network in DN hydrogels. As expected, higher swelling ratios for DN hydrogels lead to a decrease in Young’s modulus.

Figure 8

Relationship of swelling ratio and Young’s modulus E for hydrogels at 15°C (a), dependence of Young’s modulus E for PDEAAm/PAAm and PDEAAm/PDEAAm hydrogels at 15°C and 60°C on molar concentration of AAm and DEAAm, respectively (b).

Figure 8b illustrates the dependence of Young’s modulus on the molar concentration of the second DN component in swollen and deswollen hydrogels at 15°C and 60°C, respectively. A significant increase in modulus, nearly one order of magnitude, is noticeable with the increase in the temperature. This change corresponds to the transition between a liquid-like swollen hydrogel containing a significant amount of water and flexible polymer chains, and a solid-like collapsed hydrogel from which most of the water has been excluded. Deswollen PDEAAm/PAAm hydrogels exhibit slightly lower modulus values compared to PDEAAm/PDEAAm hydrogels because even at higher temperatures, they still retain water within their structures, as indicated by the swelling experiments (Figure 2a).