Acrylamide (AAm), the initiator, ammonium persulphate (APS), supplied by Merck, (Darmstad, Germany), the activator N,N,N’,N’-tetramethylethylenediamine (TEMED) were supplied by Merck (Schuchardt, Germany). Anionic co-monomer, 4-styrenesulfonic acid sodium salt (SSS) and poly(ethylene glycol) (PEG, M_w = 4000), were supplied by Fluka Chemical Co., (Steinheim, Germany). Poly(ethylene glycol)diacrylate (PEGDA, M_n = 700) as crosslinker were supplied from Aldrich, (Steinheim, Germany).

Cationic dye, Janus Green B, (JGB) used in sorption studies, were purchased from Fluka, Steinheim, Germany. Some properties of JGB were presented in Table 1. All chemicals were used as received.

To prepare AAm/SSS hydrogel systems, AAm weighing 1.0 g (14.07 mmol) was dissolved in 1.0 mL water. Then 0 mg, 10 mg/0.0485mmol, 20 mg/0.0970mmol, 30 mg/0.145mmol, 40 mg/0.194mmol, 50 mg/0.242mmol, 60 mg/0.291mmol, 70 mg/0.339mmol, 80 mg/0.388mmol of SSS were added to each AAm solutions, respectively. After these additions, for the synthesis, 0.040 mL/0.0640mmol of PEGDA and 0.2 mL/0.0438mmol aqueous solutions of APS (5.0 g APS/0.022 mol/100mL water) and 0.25 mL/0.0167mmol 1% concentration of TEMED were added these aqueous solutions. The solutions were placed in PVC straws of 3 mm diameter. After gelation, fresh hydrogels obtained in long cylindrical shapes were cut into pieces of 3 - 4 mm in length. They were washed 4 days in distilled water to remove unreacted materials, blotted dry with filter paper, dried in air and vacuum, and stored for swelling and sorption studies.

To prepare highly swollen AAm/SSS/PEG semi IPNs, same method was used as mentioned above with addition of 0.25, 0.50, 0.75 and 1.00 g PEG to aqueous monomer solution per 1.0 g of AAm.

For swelling studies, AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs were accurately weighed and transferred into water. Water uptake with respect to time was obtained by periodically removing the samples from water; quickly blot drying, and reweighing. The measurements were conducted at 25˚C ± 0.1˚C in a water bath.

For structural characterization, FT-IR analysis was made. Spectra were taken on KBr discs by using VARIAN FTS 800 FT-IR spectrophotometer.

The surface morphology of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs

was visualized by a scanning electronic microscope PHILLIPS XL-30S FEG, FEI QUANTA 250 FEG. The surface of the AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs was coated with gold under vacuum using EMITECH K550X.

Batch sorption studies were applied in all sorption experiments. Cationic dye, Janus Green B, (JGB) used in sorption studies and some properties of JGB were given in Table 1. Solutions of JGB concentration range 1.25 × 10⁻³ M - 2.00 × 10⁻³ M in distilled water were prepared. AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs containing 60 mg SSS was used in a known volume of dye solution until equilibrium was reached. For SSS effect on the dye sorption, 2.00 × 10⁻³ M aqueous solution of JGB was used. After sorption, dye solution was separated by decantation from the hydrogels. Spectrophotometric method was applied to dye solutions. Spectrophotometric measurements were carried out using a SHIMADZU UV 1601 model UV-VIS spectrophotometer at ambient temperature. The absorbance of these solutions was read at 660 nm for JGB [22]. Distilled water was chosen as the reference. The equilibrium concentrations of the cationic dye solutions were determined by means of precalibrated scales.

AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs were prepared by free radical solution polymerization. Dried AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs are glassy and very hard, but swollen gels are soft. SSS received attention in recent years due to its strongly ionizable charged group. The synthesis of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs via radical chain polymerization is a well-established procedure. Representative chemical structures of monomers and possible AAm/SSS copolymer have been presented at Figure 1.

To understand binding and crosslinking of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs during polymerization, FT-IR spectra of the hydrogel systems were evaluated and are presented in Figure 2 and Figure 3. In the FT-IR spectra of the hydrogels, the bands at about 1700 and 3100 - 3500 cm⁻¹ are important. The bands at 1600 - 1700 cm⁻¹ could be attributed to a shift in stretching vibration associated with hydrogen that is bonded directly to an overtone of the strong carbonyl absorption. The peak at 1650 - 1660 cm⁻¹ is the carbonyl group and related to amide groups and at 1500 - 1600 cm⁻¹ is the N-H bonding vibration. The much broader absorption peaks in the regions of 3100 cm⁻¹ and 3500 cm⁻¹ are N-H bands and related to “polymeric” bands. In Figure 3, there is a group of absorption peaks between 3400 - 3600 cm⁻¹, which is due to stretching bands of the OH groups, and the band at 1640 cm⁻¹ also corresponds to the OH

groups. The broad peak 3500 is characteristic peak of primary amine. On the other hand, it is thought that the peaks at 1200 cm⁻¹ are C-N bands, and the peaks at 2850 cm⁻¹ and 1400 cm⁻¹ show CH₂ groups on the polymeric chain. The characteristic absorption peak of SSS units is shown at 1040 cm⁻¹ due to S=O group [18] [23].

The characteristic absorption peak of PEG units is shown at 1100 - 1200 cm⁻¹ due to aliphatic ether bonds [23]. The peaks observed in the FT-IR spectra confirm the presence of AAm, SSS and PEG.

A fundamental relationship exists between the swelling of a polymer in a solvent and the nature of the polymer and the solvent. The percentage swelling (S%) of the hydrogels in distilled water was calculated from the following relation,

S % = m t − m o m o × 100 (1)

where m_t is the mass of the swollen gel at timet and m_o is the mass of the dry gel at time 0.

The water intake of initially dry hydrogels was followed for a period of time, gravimetrically. Swelling isotherms of the hydrogels were constructed and representative swelling curve is shown in Figure 4 and Figure 5.

Figure 4 and Figure 5 show that swelling increase with time up to certain level, and then levels off. This value of swelling may be called the “equilibrium percentage swelling” (S_eq%). The values of S_eq% of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs are used for the calculation of network characterization parameters. The values of S_eq% of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs are given Table 2. Table 2 shows that S_eq% of AAm hydrogels is 490%, but S_eq% of AAm/SSS hydrogels are 530% - 850% with the incorporation of SSS groups into AAm hydrogels, and S_eq% of AAm/SSS/PEG semi IPNs are 435% - 790% with the incorporation of PEG groups into AAm/SSS hydrogels, while S_eq% of AAm/PEG semi IPNs is 420%. In Table 2, S_eq% of the hydrogels increased with the SSS content in the copolymers. S_eq% of AAm/SSS hydrogels is

higher than S_eq% of AAm hydrogels. Table 2 also shows that S_eq% values of AAm/SSS/PEG semi IPNs lower than S_eq% values of AAm/SSS hydrogels. One of the reasons of these results is decreasing of hydrophilic character at crosslinked polymeric systems. Additionally, the PEG chains are located in the free space of crosslinked polymer networks; therefore, water diffusion is prevented by the PEG chains.

It is well known that the swelling of a hydrogel is induced by electrostatic repulsion of the ionic charges of its network. The ionic charge content is important. SSS contains many ionic groups (–SO₃Na) (Figure 1). The swelling increase is due to an increase in the anionic units. The salt group is almost completely ionized, and a large number of hydrophilic groups occur. The hydrophilic group numbers of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs are higher than those of AAm, and so the swelling values of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs are greater than that of AAm swelling values. Upon swelling the hydrogels were strong enough to retain their shape. There have been presented the photographs of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs at Figure 6 and Figure 7, as dry state and as swollen state. The capacity of swelling of the hydrogel systems can be seen from their photographs.

The water absorbed by AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs is quantitatively represented by the equilibrium water content (EWC), by using below equation [24] [25].

EWC = m s − m o m s (2)

Here, m_s is the mass of the swollen gel at time t (equilibrium), and m_o is the mass of the dry gel at time 0. The EWC values of all AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs were calculated. The values of EWC of the hydrogels are tabulated in Table 2. Generally, it is seen that an increasing of the values of EWC, if SSS have been added to composite hydrogel systems. Here, the main effect is the hydrophilic character of SSS groups.

In order to examine the controlling mechanism of the swelling processes, several kinetic models are used to test experimental data. The large number and array of different chemical groups on the polymer chains (e.g., amine, amide, carbonyl, carboxyl or hydroxyl) imply that there are many types of polymer-solvent interactions. It is probable that any kinetics is likely to be global. From a system design viewpoint, a lumped analysis of swelling rates is thus sufficient to the practical operation.

A simple kinetic analysis is a second-order equation in the form of

d S d t = k S ( S − S t ) 2 (3)

where k_s is the rate constant of swelling and S denotes the degree of swelling at equilibrium [26] [27]. After definite integration by applying the initial conditions S = 0 at t = 0 and S_t = S at t = t, Equation (3) becomes

t S = A + B t (4)

Here A is reciprocal of initial swelling rate r_o or 1/k_sS² and B is inverse of the degree of swelling at equilibrium.

To test the kinetics model, t/S vs. t graphs are plotted and representative graph is illustrated in Figure 8 and Figure 9. The calculated kinetic parameters are tabulated in Table 3.

As can be seen from Table 3, kinetics model is agreement with swelling experiments, since, as depicted in Table 2, Percentage swelling, S% (S_eq%) is changed with SSS and PEG content. Again, the initial swelling rate is changed with SSS content. This may be plausible since the ionic content of the network is enhanced with the extent of SSS groups in structure. It may be important to note that extent of SSS determines the swelling rate by increasing hydrophilicity.

When a glassy hydrogel is brought into contact with water, water diffuses into

the hydrogel and the network expand resulting in swelling of the hydrogel. Diffusion involves migration of water into pre-existing or dynamically formed spaces between hydrogel chains. Swelling of the hydrogel involves larger segmental motion resulting, ultimately, in increased separation between hydrogel chains.

Analysis of the mechanisms of water diffusion into swellable polymeric systems has received considerable attention in recent years, because of important applications of swellable polymers in biomedical, pharmaceutical, environmental, and agricultural engineering.

The following equation is used to determine the nature of diffusion of water into hydrogels [27] [28] [29].

F = M t M s = k t n (5)

Here, F is the fractional uptake at time t. Here, M_t and M_s are the mass uptake of the solvent at time t and the equilibrium, respectively. Equation (5) is valid for the first 60% of the fractional uptake. Fickian diffusion and Case II transport are defined by n values of 0.5 and 1.0, respectively. Anomalous transport behavior (non-Fickian diffusion) is intermediate between Fickian and Case II. That is reflected by n between 0.5 and 1.0 [27] [28] [29]. The values of (n) and (k) were calculated from the slope and the intercept of the plot of lnF against lnt, respectively.

For AAm/SSS hydrogel and AAm/SSS/PEG semi IPNs, lnF vs. lnt graphs are plotted and representative results are shown in Figure 10 and Figure 11. Diffusional exponents, (n) and diffusion constant, (k) are calculated and listed in Table 4.

Table 4 shows that the number determining the type of diffusion (n) is over

0.50. Hence the diffusion of water into the super water-retainer hydrogels is generally found to have a non-Fickian character. When the diffusion type is anomalous behavior, the relaxation and diffusion time are of the same order of magnitude. As solvent diffuses into the hydrogel, rearrangement of chains does not occur immediately.

The study of diffusion phenomena of water in hydrogels is of value in that it clarifies polymer behavior. For hydrogel characterization, the diffusion coefficients can be calculated by various methods. The diffusion coefficient, (D) of the water was calculated using the following equation [30] [31].

D = π r 2 ( k 4 ) 1 / n (6)

Here, D is in cm²·min⁻¹, r is the radius of a cylindrical polymer sample, (n) is the diffusional exponent and (k) is a constant incorporating characteristic of the macromolecular network system and the penetrant. The values of diffusion coefficient determined for AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs are listed in Table 4. Table 4 shows that the values of the diffusion coefficient of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs vary from 65.0 × 10⁻⁶ cm²·min⁻¹ to 294.0 × 10⁻⁶ cm²·min⁻¹. There is no good correlation between the values of the diffusion coefficient of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs.

For investigation of the effect of mass/content of PEG on the swelling properties of AAm/SSS/PEG semi IPNs, the related swelling isotherms of AAm/SSS/PEG hydrogels were constructed and representative swelling isotherms, swelling rate curves and the plot of lnF vs. lnt are plotted and representative results are shown in Figures 12-14. PEG effect on some swelling and diffusion parameters of AAm/SSS/PEG semi IPN systems containing 60 mg SSS are tabulated in Table 5.

It was shown that a decreasing of the equilibrium percentage swelling, equilibrium water contents, diffusion constant, diffusion exponent, diffusion coefficient, initial swelling rate, swelling rate constant and theoretical equilibrium percentage swelling values of AAm/SSS/PEG semi IPN systems when PEG has been added to the hydrogel systems. Incorporation of PEG into the copolymer network leads to lower degrees of swelling. On the other hand, also, it is seen that an increasing diffusion constant (k) of AAm/SSS/PEG hydrogels with increasing content of PEG in hydrogel systems from Table 5. Here, it was said that PEG chains were placed in the crosslinked polymeric systems, instead of crosslinked AAm and SSS molecules, it was seen that decreasing of the value of the

equilibrium swelling percent and related parameters, because of decreasing of hydrophilic character at crosslinked polymeric systems. In addition of this phenomenon, the PEG chains are located in the free space of crosslinked polymer networks; water diffusion is prevented by the PEG chains. This is also caused of decreasing of the equilibrium percentage swelling and related parameters.

SEM is the most used technique to investigate the shape, size, morphology, crosslink and porosity of hydrogels, or other related materials. Two SEM micrographs of the crosslinked copolymeric samples by PEGDA are presented in Figure 15 and Figure 16. If SSS is added to the hydrogel system, the observed crosslinked densities are very high (Figure 16). The micro-structural differentiations can be followed from Figure 15 and Figure 16. If Figure 16 is examined, it was seen that a lot of number of porous and crosslinked structure.

To observe the sorption of Janus Green B (JGB), AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs were placed in aqueous solutions of JGB and allowed to equilibrate for four days at 25˚C. At the end of this period AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs in the JGB solutions showed the dark coloration. But acrylamide hydrogel did not sorb any dye from solution.

For equilibrium sorption studies, the dye removal capacity (q) (mass amount as “mol” of sorption per unit mass (as gram) of the adsorbent, adsorption percentage (Ads%), and partition coefficient (K_d) can be investigated.

The dye removal capacity (q) of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs were evaluated by using the following equation:

q = ( C o − C ) v m (7)

where q is the dye removal capacity of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs (mol·g⁻¹), C_o and C are the concentration of the dye in the initial solution and the aqueous phase after treatment for a certain period time, respectively (mol·L⁻¹), v is the volume of the aqueous phase (L), m is the amount of dry AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs.

Adsorption percentage (Ads%) of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs was calculated by following equation.

Ads % = C o − C C o × 100 (8)

Here C_o and C were defined earlier.

The effect of SSS contents onto dye uptake capacity was measured. The dye removal capacity, the amount of dyes sorbed onto unit dry mass of the gel was calculated for uptake of dye within the hydrogel in 2.00 × 10⁻³ mol JGB in L of aqueous solutions, and presented in Table 6. Table 6 presents that the dye removal capacity of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs (1.35 × 10⁻⁴ - 4.12 × 10⁻⁴ mol·g⁻¹) and adsorption percentage of these (20% - 74%) both are increased with together. The dye removal capacity and adsorption percentage of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs gradually increased with increasing of SSS content in hydrogels and semi IPNs.

Equilibrium JGB sorption isotherms of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs are presented in Figure 17 & Figure 18. To Figure 17 & Figure 18, the sorption capacity of the hydrogel systems is increased with the increasing concentration of JGB. This is expected result.

Partitioning of dissolved constituents between an aqueous phase and adsorbents in waters and sediments has commonly been described by an empirical partition coefficient that simply relates the total concentration of a dissolved species to the total concentration of the adsorbed species. For this parameter, the given equation at below can be used [32] [33].

K d = C o − C C (9)

Here; K_d is empirical partition coefficient at equilibrium. C_o and C were defined earlier. Partition coefficients of JGB between dye solution and hydrogels were calculated, and are shown in Table 6. In Table 6, K_d values of AAm/SSS hydrogels is 0.32 - 2.85, but K_d values of AAm/SSS/PEG semi IPNs is 0.25 - 1.54 with the incorporation of PEG groups into the hydrogels.

Here, K_d values of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs (having 60 mg and more than this SSS content) are higher than 1.0. So, it can be said that synthesized crosslinked AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs (having

60 mg and more than this SSS content) could be used as potential adsorbent [32] [33].

For investigation of the effect of mass/content of PEG on the sorption properties of AAm/SSS/PEG semi IPNs, some sorption parameters such as dye removal capacity, adsorption percentage and partition coefficient of AAm/SSS/PEG semi IPN systems containing 60 mg SSS are tabulated in Table 7.

It was shown that a decreasing of adsorption percentage (55% - 44%) and partition coefficient (1.21 - 0.78) of AAm/SSS/PEG semi IPN systems when PEG has been added to the hydrogel systems. Incorporation of PEG into the copolymer network leads to lower values of adsorption percentage and partition coefficient of AAm/SSS/PEG semi IPN systems. Here, K_d values of AAm/SSS/PEG semi IPNs including 0 g, 0.25 g, 0.5 g and 0.75 g of PEG are higher than 1.0 (1.21 - 1.01). So, it can be said that synthesized crosslinked AAm/SSS/PEG semi IPNs (having 60 mg SSS and 0.0 g - 0.75 g PEG content) could be used as potential adsorbent. On the other hand, it can be said that there is no important changing of the dye removal capacity of AAm/SSS/PEG semi IPN systems when PEG has been added to the hydrogel systems (3.16 × 10⁻⁴ - 3.47 × 10⁻⁴ mol·g⁻¹).

Here, it was said that PEG chains were placed in the crosslinked polymeric systems, instead of crosslinked AAm and SSS molecules, it was seen that decreasing of the adsorption percentage, because of decreasing of hydrophilic character at crosslinked polymeric systems. In addition of this phenomenon, the PEG chains are located in the free space of crosslinked polymer networks; JGB sorption is prevented by the PEG chains. This is also caused by partition decreasing of the adsorption percentage.

The ionic charge content in the polymeric structure is important. SSS contains ionic units (–SO₃Na). The swelling degrees of the hydrogels increase due to increasing of the hydrophilic units on hydrogel structure (Figure 1). Therefore, AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs have many ionic groups that can increase interaction between the cationic dye molecules and anionic groups of hydrogels. The results of swelling studies are parallel character to the results of sorption studies. Both of them, it can be seen that swelling or sorption capability of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs are increased with increasing SSS content in copolymeric structure. The most important effect is hydrophilicity of copolymeric gels. Hydrophilicity of AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs becomes greater than that of AAm hydrogels, when addition of SSS to the crosslinked copolymeric structure.

For good binding analysis, Table 8 can be arranged. There can be many reasons for non-covalent interactions in the binding of JGB by AAm/SSS hydrogels and AAm/SSS/PEG semi IPNs. The main interactions between the hydrogel and

the monovalent cationic dye may be hydrophobic and hydrogen bonding. Specially, hydrogen bonding will be expected to occur between amine groups and nitrogen atoms on the dye molecules and the amine and carbonyl groups on the monomer unit of crosslinked polymer. Hydrophobic effects are especially aqueous solutions interactions which in the present case will involve those aromatic rings on the dye molecules and the methine and methyl groups on the gel. There can be some other interactions such as dipole-dipole and dipole-induced dipole interactions between the dye molecules and the hydrogel chains.