Environmentally Sensitive Vinylpyrrolidone/methacrylic Acid Inter-complex Amphoteric Hydrogel: Preparation, Characterization, and Use in the Binding of Copper Ions

Environmentally sensitive hydrogels (ESH) with inter-complex and amphoteric properties were prepared by thermal free radical polymerization of vinylpyrrolidone (VP), methacrylic acid (MA), and N,N'-methylene bisacrylamide. Spectroscopic and thermal characterizations of ESHs were performed using FTIR and TGA. To determine the effects of ESHs on swelling properties, swelling and diffusion studies were performed at different pHs, temperatures, and salt solutions. While the inter-complex formation between VP and MA was monitored with UV, the amphoteric property and environmental sensitivity of VP/MA-H were determined by swelling studies. In the binding of Cu(II) ion binding onto VP/MA-H experiments, a Langmuir type (L) adsorption was observed concerning the Giles classification system. Binding parameters such as equilibrium constant (KL), monolayer coverage (Qm), and maximum fractional occupancy (FO%) were calculated as 0.16 L gESH, 30 MgCu(II) gESH, and 81%, respectively. In the light of these findings, it can be said that strong electrostatic interactions between the anionic groups in the VP/MA-H and Cu (II) cations are more effective in the interaction of the heavy metal ions with the hydrogel. The outputs of this study are important in the preparation of effective vinylpyrrolidone/methacrylic acid for hydrogel applications in electrostatic interactions, adsorption, and removal of organic toxic wastes.


Introduction
While hydrophilic cross-linked polymeric structures are defined as hydrogels [1,2], hydrogels that respond with volume changes to external stimuli such as pH, temperature, ionic strength, solvent, electric field, light, magnetic field, physiological fluids, are environmentally sensitive is named [3].
Homo-polymeric and co-polymeric polymers and gels prepared from vinylpyrrolidone, a water-loving monomer, are widely used in the scientific and technological field [4][5][6][7][8][9][10][11]. The unique physical and chemical properties (such as biocompatibility, non-toxicity, chemical stability, good solubility in water and many organic solvents, the tendency to complex with both hydrophobic and hydrophilic substances) of VP polymers and hydrogels have made it suitable as a biomaterial in several important medical and non-medical applications (pharmaceutical industry and medicine, optics and electrical applications, membranes, adhesives, ceramics, paper, coatings and inks, household, industrial and institutional, lithography and photography, fibers and textiles, environmental applications) [4][5][6][7][8][9][10][11].
While the number of publications on VP polymers since 1970 is around 5400, 230 of these publications are VP hydrogels in SCI. These publication numbers show that the research and development of VP hydrogels are still in their infancy compared to the continued and significantly increased efforts to develop hydrogels [12]. However, although methacrylic acid is mentioned in about 30 of these publications (such as crosslinking with gamma rays [13][14][15][16][17][18], photopolymerization [19], polymerization by ATR method [9], grafting polymerization [20], and IPN formation [21]), there are no studies on the preparation of copolymeric hydrogels by free radical polymerization using azobisisobutyronitrile (AIBN) initiator.
We aimed to prepare, characterize and investigate the Cu(II) ion binding properties of methacrylic acid copolymeric hydrogels of vinylpyrrolidone by free radical polymerization using an AIBN initiator. To improve the swelling properties of vinylpyrrolidone/methacrylic acid gels and to impart pH sensitivity it has been copolymerized for a monoprotic methacrylic acid (MA). The resulting gel is responsive to both pH, temperature, and salt solutions changes.
By copolymerization with a suitable monomer, the hydration degree and swelling kinetics of the final hydrogel are easily tuned.

Chemicals
The monomers such as vinylpyrrolidone (VP), methacrylic acid (MA), and a crosslinker such as N,N'methylenebisacrylamide (N-Bis), and an initiator such as azobisisobutyronitrie (AIBN) were purchased from Aldrich, Milwaukee Company. The chemical names, synonyms, abbreviations, and chemical structures of the monomers, crosslinker, and initiator are given in Table 1. The copper (II) sulfate used in the adsorption process was obtained from Merck KGaA, Darmstadt, Germany. The chemicals used were analytical purity (99%).

Preparation of hydrogels
Homo-polymeric hydrogels The vinylpyrrolidone hydrogel was synthesized by free radical homopolymerization using 0.81 mL of VP in 0.81 mL of distilled water. The initiator was (AIBN) (2.0 wt.%) and the crosslinker was N-Bis (2, 4, 6, or 8 wt.%) for the total mass of the monomers. We used deionized double distilled water as a solvent.
The methacrylic acid hydrogel was made in the same way as the vinyl pyrrolidone hydrogel preparation and 0.81 mL of MA was used instead of VP. However, since AIBN is insoluble in water, the initiator was first dissolved in MA.
The homopolymeric hydrogel prepared from vinylpyrrolidone monomer was named VP-H, and that prepared from methacrylic acid was designated MA-H.

Copolymeric hydrogels
In the preparation of vinylpyrrolidone/ methacrylic acid hydrogel, 0.81 mL of VP, 0.81 mL of MA, and 1.62 mL of distilled water were used and made with the same technique as homo-polymers. The co-polymeric hydrogel prepared from the co-monomer of vinylpyrrolidone monomer and methacrylic acid was named VP/MA-H.
Both the homo-polymer and co-polymer mixtures were transferred to flexible PVC straws with a heat-sealed bottom and placed in the oven at 65°C for 5 hours. After synthesis, the gels were removed from the tubes and cut to be approximately equal to each other. The hydrogels, which were washed several times with distilled water, were first dried at room temperature for 24 hours and then dried in an oven at 50 °C for 24 hours. These dry gels were used in characterization, swelling, and adsorption tests. TG thermal analyzer (Perkin Elmer Pyris 1 TGA, Shelton, USA)) was used for the thermogravimetric analysis of the hydrogels. ~10 mg of hydrogel (ESH) was heated from 10 °C to 600 °C at a heating rate of 10 °C min -1 and a flow rate of 20 mL min -1 nitrogen gas.
The swelling values of homo-or co-polymeric hydrogels (S, g gESH -1 ) were determined from the following equation [26] by measuring the masses of hydrogels in different aqueous media (such as different temperature, pH, or ionic solutions).
where mi and mt are the initial mass and the mass of swollen hydrogel at any time, respectively.

Binding
The cupric ion binding capacities of the VP/MA hydrogel were determined using an aqueous Cu(II) solution. The procedure was started by adding VP/MA hydrogel (~ 0.1 g) to the stock solution (100 mL) in an Erlenmeyer.
The initial metal ion concentration was 125 ppm for kinetic studies, and increasing cupric ion concentrations were also taken to evaluate the binding isotherms.
where Co and Ce are the concentrations of the Cu(II) metal ion in the aqueous solution (g Cu(II) L -1 ) before and after bindings, respectively. m is the hydrogel mass (gESH) and V is the total solution volume (L).
Unless otherwise stated, swelling and binding experiments were carried out at pH=7 and 25°C.

Intercomplex formation
To investigate the complex formation [22] between vinyl pyrrolidone and methacrylic acid, the UV spectra of the aqueous solutions of the monomer and co-monomer, and their 1:1 mixture were taken and presented in Figure 1. While the λmax values for N-vinyl pyrrolidone, methacrylic acid, and the mixture were found to be 288 nm, 290 nm, and 286 nm, respectively, the A values of these solutions were found to be 1.69, 2.06, and 1.45 in the same order. As seen in the UV-spectra (Figure 1), the mixture of monomer and co-monomer showed a small hypsochromic shift (blueshift) with hypochromic effect relative to VP and MA. Thus, a hydrogen bond is formed between the free electron pair in -C=O's and the hydroxyl group proton. Here, the energy of the n orbital decreases by the energy of the hydrogen bond, and a blueshift occurs. With these evaluations, it can be said that an inter complex is formed between VP and MA with the help of H-bonds.
Possible interactions and complex formation between N-vinylpyrrolidone and methacrylic acid in VP/MA-H can be illustrated in Figure 2.  Figure 3. Vinylpyrrolidone, methacrylic acid homopolymers, or vinylpyrrolidone/methacrylic acid co-polymer can be polymerized and crosslinked using a chemical initiator such as azobisisobutyronitrile (AIBN).
AIBN decomposes above 60 °C to form free radicals due to the possibility of radical formation (free radical formation) during the reaction. The first possible step for polymerization is the transfer of an unpaired electron to make the monomeric units reactive, and the reaction between the VP (or MA) molecules and the radical. The free radical then reacts with the vinylpyrrolidone monomer or methacrylic acid comonomer, breaking the double bond of the monomer or comonomer and retaining an unpaired electron at the base of the formed chain, forming a new free radical (propagation step; homopolymerization or copolymerization). This species then interacts with the crosslinker (N-Bis), forming species with two radical sites that combine in the molecule and form a chain of crosslinked vinylpyrrolidone/methacrylic acid copolymers (propagation step; crosslinking). The reactivity ratios of monomer VP and co-monomer MA are given as r1 (=44.66) and r2 (=3.38) [13]. Due to the high r1 and low r2 values, the polymerization ability of VP with itself and other monomers is quite strong. For this reason, the mixing ratio of the feed was chosen as 1:1. Since r1 > 1 and r2 > 1, not every radical has a preference and the copolymer has a completely random sequence of VP and MA monomers [13]. However, VP is more reactive than MA. As a result, the copolymer will consist of a more substantial portion of the more reactive sample in the array of random repeat units. Thus, it can be said that VP/MA hydrogels are randomly copolymerized.
radical formation copolymerization crosslinking Figure 3. Plausible copolymerization and crosslinking mechanism of VP/MA hydrogel. The bands observed around 3700 -3500 cm -1 in the FT-IR spectra of the hydrogels shown in Figure 3 show O-H stretching. The band at 1750 cm -1 in the spectrum of MA and the bands at 1750 cm -1 in the other spectra depend on C=O stretching. In particular, the shift of the band at 1750 cm -1 in the spectrum of MA to 1700 cm -1 in that of the mixture can be interpreted as the formation of intermolecular H-bonds between the carbonyl group in VP and the carboxyl group in MA [15]. The inability to observe the N-C=O bending band at 568 cm -1 observed in VP in the mixture may also be due to the formation of intermolecular H-bonds. While C-H bending band at 1400 cm -1 and the CH2 band at 1020 cm -1 were observed in all spectra, CH2

FT-IR analysis
wagging band was observed at 1300 cm -1 in the spectrum of the mixture. These bands may be an indication that the crosslinker has entered the structure. In the spectrum of the hydrogel obtained from a monomer, a comonomer, and a crosslinker at a 1:1 feed ratio, the observation of all functional groups in VP, MA, and crosslinker may be an indication of the formation of a crosslinked vinyl pyrrolidone/methacrylic acid hydrogel.

TG analysis
The thermogram for VP/MA-H is presented in Figure 5. At 600 o C, approximately 8% of residue remains at the end of VP/MA-H thermal decomposition. [6,15].

Swelling
The swelling of the VP-H, MA-H, and VP/MA-H occurs because of the osmotic pressure difference caused by the presence of the water-loving repeat units in the threedimensional cross-linked network. Penetrant intake of initially dry VP-H, MA-H, and VP/MA-H was followed for a while, gravimetrically [24,25]. All the hydrogels absorbed the fluids and swelled at a higher rate in the beginning. After a certain period, the fluid uptake became constant, and the VP-H, MA-H, and VP/MA-H hydrogels achieved their equilibrium swelling capacity (Seq).

Influences of cross linker concentration on swelling
The equilibrium swelling values of hydrogels prepared with different concentrations of N-Bis between 2% and 8% were plotted against the crosslinker concentration and shown in Figure 6. As can be seen in Figure 6, the equilibrium swelling values of the hydrogels decreased as the crosslinker concentration increased. By increasing the crosslinker concentration up to a certain concentration, a more intense bonding between the main chains, an increase in crosslinking points, and consequently pore shrinkage occur. This makes it harder for the penetrant to penetrate the hydrogel and reduces swelling.
In this study, it was preferred to use the crosslinker concentration giving the highest swelling value.

Influence of hydrogels type on the hydrogels swelling
Swelling plots were constructed and represented in Figure 7 for the homo-polymers VP-H, MA-H, and co-polymer VP/MA-H hydrogels.  The time (t1/2, min) at which the swelling is one-half the equilibrium value (S1/2=Seq/2) was found in Figure 7. Considering that swelling shows a first-order kinetic behavior, swelling rate constants of the prepared hydrogels were found from the equation k = 0.693 / t1/2.
Experimental Seq, t1/2, and kexp values of the hydrogels are given in Table 2. In addition, in Figure 7, which represents the dynamic swelling behavior of hydrogels, it is seen that the rate of penetrant uptake increases rapidly in the early times and begins to flatten in the following times. In this case, the swelling behavior may be compatible with the exponential rise to the maximum equation. By adapting the exponential rise to maximum relation to determine swelling parameters of the hydrogels [24], the following equation can be written; where S (g gESH −1 ) is swelling at time t, power parameter Smax is equilibrium swelling (g gESH −1 ), t is time (min) for swelling, and ks (min -1 ) stand for the swelling rate constant. The value of the rate constant is a measure of the ease with which the fluid penetrates the hydrogel. The inverse of the rate constant value (1/ks) gives the rate parameter (τ, min). A high value of the power parameter indicates that the swelling is very high, while a small rate parameter indicates that the swelling is fast. Also, the ratio of power parameter to the rate parameter gives the swelling rate (SR, g gESH −1 min −1 ) at time τ, and τ value is a measure of the SR (i.e. the lower the τ value, the higher the rate of swelling) [24].
Nonlinear regression was applied to equation (1) to calculate the parameters. The correlation coefficients (r 2 ) of the graphs drawn according to equation (1) (in Figure 7) are 0.991 and above, indicating that 1 equation can be used to find the swelling parameters of the prepared hydrogels.
The Smax and ks values found in equation (1), along with the standard error (SE) and correlation coefficients (r 2 ), are given in Table 3. Calculated τ and SR values are also added to Table 3.

Diffusion
The following equation is used to determine the nature of diffusion of a penetrant into hydrogels.
where F is the fractional uptake at time t, kD is a constant incorporating characteristic of the network system and the penetrate, and n is the diffusion exponent, which is indicative of the transport mechanism. Equation 2 is valid for up to rate parameter of swelling of the polymer.
For a cylindrical gel, n = 0.45-0.50 corresponds to Fickian-type diffusion process, while 0.50 < n < 1.0 indicates non-Fickian or anomalous transport and n = 1 implies case II (relaxation controlled) transport [23]. For the prepared hydrogels, F versus t graphs are plotted and are shown in Figure 8. Diffusion exponents (n) and diffusion constants (k) were calculated from the nonlinear regression of the F and t plots from the experimental data shown in Figure 8 and are summarized in Table 4 along with the standard error (SE) and correlation coefficients (r 2 ).  [23,24].

Influence of pH on the swelling
The polymeric networks containing ionizable functional groups exhibit pH responsivity [25][26][27][28][29]. The responsibility of surrounding media pH on the swelling values of the prepared hydrogels at 25 °C between pH = 1-13 is shown in Figure 9.

Influence of temperature on the swelling
To examine the effect of temperature on swelling [36][37][38][39], the hydrogels were inflated to equilibrium at 25 and 50 °C. The variation of swelling with temperature is shown in Figure   11 with a bar graph. The swelling of the hydrogels decreased with increasing temperature. The reason why the swelling of the hydrogels decreases with increasing temperature may be that the hydrophobicity of the -CH3 group on the MA unit and the pyrrolidone ring on the VP unit prevents hydrogen bond formation [40][41][42].
As a result, it can be said that the prepared VP-H, MA-H, and VP/MA-H are sensitive to temperature, and these temperature sensitivities can be calculated from the maximum and minimum swelling values difference as 22.58, 3.34, and 17.89 g gESH -1 , respectively.

Influence of ions on the swelling
The ions and counter ions play an important role in the swelling behavior of hydrogels [3,43]. Ion-sensitive swelling behavior was investigated in aqueous solutions of NaCl, KCl, and Na2CO3 salts at 0.1 mol L -1 concentrations at 25 °C, and ion-sensitive swelling of hydrogels is shown in Figure 12. Hydrogels swell less in salt solutions than in water. The swelling values decreased according to the following sequence water, NaCl, KCl, Na2CO3. As the hydrogels were swelled in saline solutions, the acid and pyrrolidone ring groups were neutralized by the cations in the external solution, and the swelling were decreased. When the fixed charges on polymeric side chains were fully neutralized, the hydrogel showed nonionic behavior. In various saline solutions, hydrogels showed the Donnan effect when the charges on the polymeric side chain were neutralized and then showed a salting-out effect with the gels going to a nonionic state [3].
These hydrogels, which are prepared by using the pH-, temperature-and ion-sensitivity of the vinylpyrrolidine monomer and the pH-and ion-sensitivity of the methacrylic acid comonomer, with are a durable, homogeneous appearance, can be defined as environmentally sensitive, stimuli-responsive, smart or intelligent hydrogels.

Cu(II) Binding
To observe uptake of heavy metal ions [44]

Binding kinetics
The binding of solute molecules to the surface of an adsorbent is the adsorption process.
Adsorption kinetics, on the other hand, is a curve (or line) that describes the rate at which a solute is retained or released from an aqueous medium to the solid-phase interface at a given adsorbent dose, temperature, and pH [47].
The differential and integral representations of the pseudo-first and second-order binding kinetics equations can be written as follows [47].
The binding kinetics of Cu(II) to the VP/MA-H was studied and is given in Figure 14.
The curves of the integral equals of pseudo-order applied to these data are also shown on the same graph. On the other hand, the parameters found as a result of the pseudo-first-and secondorder non-linear kinetic models applied to the experimental data are presented in Table 5 together with the standard error and correlation coefficients. The fact that the r 2 values of the first-order binding kinetic curve at all pHs are smaller than the r 2 values of the second-order curve and the second-order curve better overlaps with the experimental points, indicating that the binding of Cu(II) to VP/MA follows a second-order kinetic. In this model, the rate of binding of the solute is assumed to be proportional to the available sites on the hydrogel, and the reaction rate depends on the amount of solute on the surface of the adsorbent, that is, the driving force (Qe-Qt) is proportional to the number of active sites present in the hydrogel [48].

Binding isotherm
Binding isotherms indicate the distribution of adsorbate molecules between a liquid phase and a solid phase when the adsorption process reaches an equilibrium state [24]. To determine the binding of the Cu(II) onto VP/MA-H, a plot of the amount of adsorption (Q) against the free concentration (C) of ion solution is shown in Figure 15. The binding curve of Cu(II) ions on VP/MA-H in Figure 13 resembles a hyperbola.
Although curves resembling hyperbola generally show L (Langmurian) type in Giles adsorption classification, binding types such as S, C, L in this classification are determined by the exponential value (nF) of the Freundlich equation. n F is a measure of the deviation of isotherm from the linear form, i.e. heterogeneity factor. The Freundlich equation is given as: where kF is the Freundlich constant, equal to adsorption capacity at C = 1. The nF values are related to the Giles classification, S, L, and C type isotherm. nF < 1 correspond to S shape, nF = 1 to C type, and nF > 1 to L type [24]. On the other hand, higher values of kF represent an easy uptake of adsorbate from the solution.
Freundlich parameters are calculated from the nonlinear regression of the plots in Figure 15. The correlation coefficient value was found to be r 2 =0.993. The calculated Freundlich exponent nF to take a value of 2.22 indicates that the binding of Cu(II) ions to the VP/MA hydrogel is the L-type isotherm.
L-type (Langmuir type) binding isotherms in the Giles classification system for adsorption of a solute from its solution [46]. In this type of bonding isotherm, the initial curvature indicates that as more sites in the substrate are filled, it becomes increasingly difficult for a bombarding solute molecule to find a suitable free space. This means that the solute molecule adsorbed is not in strong competition with the solvent.
The binding equation for Langmuirian isotherms is as follows; Where KL is the binding constant, i.e., the equilibrium constant for the attachment of a cupric ion onto a site by a specific combination of noncovalent forces. Here Qm is the site density (the limiting value of Q for monolayer coverage) which is therefore of the density of the sites along the polymer chain.
Langmuir parameters of binding are calculated from the nonlinear regression of the plots in Figure 15 and are summarized in Table 7 along with the standard error (SE) and correlation coefficients (r 2 ). Also, using the experimentally found maximum Q value (Qexp) and the calculated monolayer capacity values (Qm), the fractional occupancy percentage (FO%) can be calculated from the given equation [46]; FO% = Q exp Q m × 100 9 The notations in this equation are already defined. The calculated FO% value is added to the last column of Table 6.
The monolayer coating value of Cu(II) binding to the VP/MA-H was found to be approximately 30 MgCu(II) gESH -1 and it was observed that approximately 20% of the binding surface could still be filled with cupric ions.

Thermal degradation of Cu(II) loaded VP/MA hydrogel
TG thermogram of Cu(II) loaded VP/MA-H was taken and presented in Figure 16. In the thermograms given in Figure 16 In conclusion, it can be said that the VP/MA hydrogel is a model hydrogel for removing heavy metal ions and cationic pollutants from the environment, as well as in environmentally sensitive swelling areas.