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Investigation of mechanical and thermodynamic properties of pH-sensitive poly(N,N-dimethylaminoethyl methacrylate) hydrogels prepared with different crosslinking agents.

INTRODUCTION

In recent years, intelligent stimuli-responsive hydrogels have generated considerable research interest due to their ability to change their volume and properties in response to environmental stimuli such as temperature, pH, salt, and certain chemicals (1-4). These materials can exhibit pronounced changes in their swelling behavior, network structure, permeability, or mechanical strength in response to such stimuli. Based on their dramatic swelling and deswelling behavior, stimuli-responsive hvdrogels are being utilized for new potential applications in numerous fields including chemical transducer, separation, drug delivery, and artificial organ (5-7). One particular type of the responsive polymers that has received much attention is poly(N,N-dimethy laminoethyl methacrylate) (PDMAEMA) in various modifications. The intelligent materials prepared from PDMAEMA and its copolymers have attracted much interest for their [hernial and pH-stimuli response (8), (9).

Several studies showed that the mechanical properties and the swelling behavior of hydrogels depend upon the nature of the polymer, the polymer-solvent interaction parameter and the degree of crosslinking. The type and the amount of the crosslinking agent used in the preparation are also important, since the physical states of hydrogels can be altered with the changing of degree of crosslinking in the structure. Using multifunction monomer as a cross-linker during preparation by free-radical polymerization is an essential step towards the improvement in their chemical stability. N,N'-methylenebis(acrylamide) (BAAm) and ethylene glycol dimethacrylate (EGDMA) are often used as crosslinking agents to prepare hydrogels containing amide and acrylate groups which has been the subject of numerous papers related to their synthesis, swelling, selectivity, sensitivity, and water-uptake studies (10-12).

Ilaysky and Hrouz (13) systematically investigated the effect of the amount of BAAm as crosslinking agent on the properties of poly(acrylamide)-based hydrogels. Kabul et al. (14) studied the fast swelling of highly porous superabsorbent hydrogels prepared using BAAm as a water-soluble crosslinker and 1,4-butanediol diacrylate as an oil-soluble crosslinker. They reported that the higher crosslinker concentration, especially in the case of BAAm, causes decreased gelation times. Hence, the swelling rate of hydrogels increases by increasing BAAm concentration which is related to the decreased gelation time. Moreover, a series of poly(N-isopropyl acrylamide) (PNIPA) copolymeric hydrogels with different hydrophobic comonomers was prepared by Xue and Hamley (15) in the presence of BAAm and glyoxal bis(diallylacetal) (GLY) as crosslinking agents. The investigation of the effect of different type of crosslinkers and their concentration on the swelling ratio and on the LCST behavior showed that the swelling capacity of hydrogels can be enhanced by using GLY to replace BAAm as a crosslinker. It was also found that the values of LCST are not affected by the crosslinker concentration. Moreover, Lee and Lin (16) prepared thermosensitive copolymeric hydrogels from NIPA and poly(ethylene glycol) methylether acrylate with different crosslinkers such as BAAm, tetraethyleneglycol cliacrylate (TEGDA), and EGDMA. They showed that the hydrogels crosslinked with TEGDA exhibit the largest swelling ratio and the swelling ratios of copolymeric hydrogels increase with the hydrophilicity of the crosslinker used in the preparation. Recently, Cavus and Gurdag (17) reported the effect of the amount of crosslinker TEGDA on the swelling ratio of poly(N,N-dirnethy I am inoethyl methacrylate-co-2-aerylamido-2-methylpropane-l-sulfonic acid) copolymeric hydrogels.

However, the influence of the type of the crosslinking agent on the mechanical and thermodynamic properties as well as pH-sensitive phase transition behavior of homopolymeric poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) hydrogels has not been reported before. Therefore, the main aim of this work is to report the findings of a comprehensive and systematic study of the effect of the type of the crosslinker on the elasticity and swelling properties of PDMAEMA hydrogels prepared at various gel preparation concentrations. For the preparation, tetraethyleneglycol dimethacrylate (TEGMA) and NH-methylenebis(acrylamide) (BAAm) were selected as different crosslinking agents. PDMAEMA hydrogels were prepared by free-radical crosslinking polymerization of DMAEMA in the presence of the crosslinkers. The swelling capacity in water and pH-dependent phase transition behavior as well as the elasticity and mechanical properties of PDMAEMA hydrogels were also investigated as a function of the gel preparation concentration. The experimental conditions that are required to obtain hydrogels with good swelling and improved mechanical properties were analyzed using the available theories.

EXPERIMENTAL

Materials

N,N-dimethylaminoethyl methacrylate (DMA.EMA) as main monomer and tetraethylene glycol dimethacrylate (TEGMA) as crosslinker were purchased from Fluka Chemical. N,N'-methylenebis(acrylamide) (BAAm) as crosslinker, ammonium persulfate (APS) and N,N,N',N'-tetramethylethylenediamine (TEMED) as redox-initiator system were obtained from Merck Chemical. Distilled water was used for the preparation of hydrogels as well as for the swelling experiments. Hydrochloric acid (Merck), potassium dihydrogen phosphate (Riedel-de Haen), potassium phosphate (J. T. Baker), disodium hydrogen phosphate (Merck) and sodium chloride (Merck) were used for the swelling experiments in buffer solutions. All solvents and starting reagents were of the highest available purity.

Preparation of Crosslinked PDMAEMA

Poly(N,N-dimethylaminoethyl methaciylate) (PDMAEMA) hydrogels were prepared by free-radical crosslinking polymerization. The redox initiator system consisting of 3.51 mM APS and 24.9 mM TEMED was used to initiate the gelation reaction. For the preparation of hydrogels, freshly prepared APS stock solution (1.0 ml), DMAEMA and the crosslinker were mixed in a graduated flask. After bubbling nitrogen for 20 min to eliminate oxygen from the pre-gel solution, TEMED stock solution (1.0 ml) was added and then, the reaction solution was completed with distilled water. After shaking the flask, the solution was poured into several polypropylene syringes with the inner diameters of 5.0 mm and length of 15.0 cm and then, the syringes were sealed and the polymerization reaction was conducted at -18[degrees]C for 24 h. The initial molar concentration of the monomers in terms of the polymer network concentration just after the gel preparation [[phi].sub.2.sup.0] was varied while the crosslinker ratio X (the mole ratio of the crosslinker to the monomer DMAEMA) was fixed at 1/80.

In the free-radical crosslinking copolymerization of DMAEMA monomer and BAAm or TEGMA as crosslinking agent using APS--TEMED as a redox-initiator system, the first step occurs between APS and TEMED in which the TEMED molecule is left with an unpaired valance electron. (18). The crosslinking mechanism of DMAEMA monomer with BAAm and TEGMA were illustrated in Fig. 1.

Characterization

Upon completion of the polymerization reaction, the hydrogel samples were carefully removed from the syringes without destroying their cylindrical shapes. The resulting hydrogels were cut into samples of about 10 mm in length and then the hydrogel samples were immersed in an excess of distilled water for 1 week to remove the residual unreacted monomers and the linear polymers. Then, the hydrogel samples were dried according to the following procedure: the swollen hydrogel samples were successively washed with solutions whose compositions were changed gradually from water to pure acetone. This solvent exchange process facilitates the final drying of the hydrogel samples. The collapsed hydrogel samples after the treatment with acetone were dried in vacuum oven at room temperature and the weight of dry hydrogels [m.sub.dry] was determined and then stored in a vacuum desicator. The volume fraction of the crosslinked polymer network after the gel preparation denoted by [[phi].sub.2.sup.0] was calculated as:

[[phi].sub.2.sup.0] = [[1 + ([q.sub.F] - 1)p/[d.sub.1]].sup.-1] (1) where [q.sub.F] is the dilution degree after the gel preparation (mass of gel after preparation/mass of dried gel), p and [d.sub.1] are the densities of PDMAEMA polymer and water, respectively. The values p and [d.sub.1] used in this study were 1.20 and 1.00 g/ml, respectively. By assuming the monomer conversion is complete after the crosslinking, the theoretical value of [[phi].sub.2.sup.0] was calculated from the initial molar concentration of the monomers [C.sub.o], by using the equation, [[phi].sub.2.sup.0] = [10.sup.-3] [C.sub.o][[bar.V].sub.r], where [[bar.V].sub.r] is the molar volume of PDMAEMA repeat units (in ml [mol.sup.-1]). For PDMAEMA hydrogel prepared with theoretical value of [[phi].sub.2.sup.0] = 0.085, the experimentally determined [[phi].sub.2.sup.0] value calculated from Eq. 1 was found as 0.107. Because the experimental [[phi].sub.2.sup.0] values are very close to the theoretical ones, the overall average copolymer composition can be approximated to the initial feed composition. Furthermore, the slight variation in the values indicates the presence of the residual water in the dried samples.

Swelling Measurements

The swelling behavior of PDMAEMA hydrogels were investigated in water, in buffer solutions at different pH-values as well as in aqueous salt (NaCl) solutions. First, the equilibrium swelling measurements of hydrogels in the form of rods of 4 mm in diameter were carried out in water at 24[degrees]C [+ or -] 0.5[degrees]C. To reach the swelling equilibrium, the hydrogels were immersed in water for at least 2 weeks replacing the water every other day. After equilibrium swelling was reached, the hydrogels were removed and the excess surface water was lightly dried with filter paper. The swelling equilibrium was tested by measuring the diameter of the hydrogel samples. To achieve good precision, four measurements were carried out on samples of different length taken from the same hydrogel, We results indicated that the hydrogel is stable and the response is repeatable. The linear swelling ratio with respect to the state of preparation [alpha] was determined by measuring the diameter of the hydrogel samples after equilibrium swelling in water D and after preparation [D.sub.0] by a calibrated digital compass (Mitutoyo Digimatic Caliper, Series 500, resolution: 0.01 mm). Then, the linear swelling ratio [alpha] was calculated according to the following equation:

[alpha] = D/[D.sun.0] (2)

Once equilibrium swelling was attained the normalized volume of the equilibrium swollen hydrogel [V.sub.eq] (volume of equilibrium swollen hydrogel/volume of the hydrogel just after preparation) was calculated as:

[V.sub.eq] = (D/[D.sub.0] (3)

Then, the volume fraction of the crosslinked polymer in the equilibrium swollen hydrogel [[phi].sub.2,eq] was calculated by using the normalized volume of the equilibrium swollen hydrogel and the volume fraction of the crosslinked polymer network after the gel preparation by:

[[phi].sub.2,eq] = [[phi].sub.2.sup.0]/[V.sub.eq] (4)

Similar procedures were performed for the swelling studies of PDMAEMA hydrogels in aqueous salt (NaCI) solutions. The ionic strength of the salt solutions ranged from [10.sup.-5] to 1.0 M. For this reason, water-equilibrated hydrogel samples were placed in aqueous NaCl solutions and allowed to swell until the equilibrium swelling: ~ 2 weeks were needed to reach the equilibrium. The swelling measurements in aqueous NaCl were carried out in direction of increasing salt concentration from water up to 1.0 M NaCl. The approach to equilibrium swelling of PDMAEMA hydrogels in aqueous salt solutions was monitored gravimetrically and the swelling ratio of PDMAEMA hydrogels in the salt solutions was calculated as:

[q.sub.w]([NaCl.sub.aq]) = mass gel in salt solution/[m.sub.dry] (5)

Then, the volume swelling ratio of hydrogels [q.sub.v] was calcukited according to the following equation:

[q.sub.v] = 1 + ([q.sub.w] - 1)[rho]/[d.sub.1] (6)

Each swelling ratio reported in this study is an average of four separate swelling measurements performed in parallel.

Uniaxial Compression Measurements

The gel strength of the samples was measured by using a home-made compression measurement apparatus of ITU (Istanbul Technical University). The compressive stressstrain experiments were conducted on PDMAEMA samples after their preparation and at their equilibrium swelling in water. For the measurement, a cylindrical PDMAEMA hydrogel sample of 5 mm in diameter and 7-8 mm in length was placed on a digital electronic balance. A load was transmitted vertically to the hydrogel through a rod fitted with a PTFE (Teflon) end-plate. The force acting on the gel F was calculated from the reading of the balance m as F = mg, where g is gravitational acceleration (g = 9.8030 m [s.sup.-2]). The resulting deformation [DELTA]l = l-[l.sub.0] where [l.sub.0] and l are the initial undeformed and deformed lengths, respectively, was measured using a digital comparator (IDC type Digimatic Indicator 543-262, Mitutoyo) which was sensitive to the displacements of [10.sup.-3] mm. The deformation ratio [lambda] (deformed length/initial length) was calculated using the equation, [lambda] = 1-[DELTA]/[ll.sub.o] The force and the resulting deformation were recorded after 20 s of relaxation. The measurements were conducted up to about 20% compression of the sample. The corresponding stress f was calculated as f = F/A, where A is the cross-sectional area of the sample, A = [pi] [([D.sub.0]/2).sup.2], in which [D.sub.0] is its initial diameter. Eq. 7 was used to calculate the elastic modulus G of resulting hydrogels:

f = G([lambda] - [[lambda].sup.-2]) (7)

At low strains, the plot of the stress versus -([lambda] - [[lambda].sup.-2]) would yield a straightline whose slope is G. According to the theory of rubber elasticity, for a network of Gaussian chains, the elastic modulus of swollen gels G is given by the following equation (19), (20):

G = [[Av.sub.e]RT([[phi].sub.2.sup.0]).sup.2/3][[phi].sub.2,eq.sup.1/3] (8)

where lie is the effective crosslink density, [[phi].sub.2,eq] is the volume fraction of the crosslinked polymer in the equilibrium swollen hydrogel, the front factor A equals to I for an affine network and 1-2/[phi] for a phantom network, in which [phi] is the functionality of the crosslinks, R and T are in their usual meanings. For PDMAEMA hydrogels, using the G and [[phi].sub.2.sup.0] values together with Eq. 8, one may calculate the effective crosslink densities [v.sub.e] of the resulting hydrogels given by the relation as:

[v.sub.e] = [rho]/[[bar.M].sub.c] (9)

where [[bar.M].sub.c] is the average molecular weight of the network chains and the network chain length N is related to the effective crosslink density [v.sub.c] according to the following equation:

N = [([v.sub.e][V.sub.1]).sup.-1] (10)

where [V.sub.1] is the molar volume of segment, which is taken as the molar volume of water (18 ml/mol). Because the hydrogels prepared in this study were highly swollen, the phantom network model ([phi] = 4) was used to calculate the network chain length of resulting PDMAEMA hydrogels.

RESULTS AND DISCUSSION

PDMAEMA hydrogels with at a fixed chemical crosslink density (X = 1/80) were prepared by using different crosslinking agents at various polymer concentrations ranging from dilute to concentrated solutions. The fundamental properties such as the equilibrium swelling ratio, the gel strength, the crosslinking density and the average molecular weight between crosslink points for PDMAEMA hydrogels crosslinked with different crosslinking agents were investigated as a function of the gel preparation concentration.

Effect of Crosslinker Type on the Swelling Ratio

Variation of the type and the concentration of the crosslinking agent cause considerable difference in swelling behavior of the resulting hydrogels. A fundamental relationship exists between the swelling of a crosslinked network, the nature of the polymer and the crosslinker. In Fig. 2, the filled symbols show the linear swelling ratio [alpha] of PDMAEMA hydrogels crosslinked with TEGMA in water plotted as a function of the crosslinked polymer concentration after the gel preparation [[phi].sub.2.sup.0]. For comparison, the swelling data for PDMAEMA hydrogels crosslinked with BAAm are also shown in figure by the open symbols. Data for PDMAEMA hydrogels crosslinked with BAAm were taken from our previous work (21). In Fig. 2, it is seen that a increases with increasing [[phi].sub.2.sup.0] while is not a monotonic function of the polymer concentration [[phi].sub.2.sup.0]. The main difference between two systems lies in the lower polymer concentration [[phi].sub.2.sup.0]. For [[phi].sub.2.sup.0] < 0.10, as it is seen that [alpha] increases sharply with 4 for PDMAEMA hydrogels crosslinked with BAAm. However, in this regime, [alpha] is a decreasing function of [[phi].sub.2.sup.0] for PDMAEMA hydrogels crosslinked with TEGMA due to the fact that increasing the initial monomer concentration results in decreased expansion ratios of hydrogels with respect to after-preparation state. When the chemical structures of the crosslinkers BAAm and TEGMA are compared, the higher amount of swelling is expected in case of hydrogels crosslinked with TEGMA. However, the mechanical measurements showed that the average network chain length N of PDMAEMA hydrogels prepared with TEGMA is a strong decreasing function of the crosslinked polymer concentration after the gel preparation in the range of [[phi].sub.2.sup.0] < 0.10 (Fig. 6). Because N is used to determine the distance between two successive crosslinks, for [[phi].sub.2.sup.0] < 0.10, the smaller value of the hydrogels with TEGMA indicates higher crosslinking density networks. The results in Fig. 2 also indicated that the swelling ratio of PDMAEMA hydrogels crosslinked with BAAm is larger than those for hydrogels crosslinked with TEGMA over the entire range of the polymer network concentration. Although the TEGMA bears four oxyethylene repeating units, which allow the water molecules hydrogen bonding on its chains, it seems that the amido group in BAAm has larger affinity for water molecules. The increased hydrophilicity leads to increased water uptake and hence, the hydrogels crosslinked with BAAm exhibit higher swelling ratio over the entire range of the gel preparation concentration.

From Fig. 2, it is also seen that for [[phi].sub.2.sup.0] > 0.10, the linear swelling ratios of PDMAEMA hydrogels increase with increase in the initial monomer concentration. The obtained results for PDMAEMA hydrogels in Fig. 2 confirm to the previous results. Shibayama et al. (22) studied a series of PNIPA hydrogels formed at a fixed crosslinker ratio but at varying [[phi].sub.2.sup.0] between 0.03. and 0.08. Then, they reported that the linear swelling ratio of hydrogels is independent on the initial monomer concentration. This result was explained with the chain entanglements acting as additional crosslink points, whose number increases with increasing concentration. Recently, Okay and coworkers investigated the linear swelling ratio and the effective network chain length of poly(N,N-dimethylacrylamide) (PDMAAm) hydrogels as a function of the gel preparation concentration. It was found that the linear swelling ratio of hydrogels first decreases but then increases as the gel preparation concentration is increased. The [[phi].sub.2.sup.0]-dependence of swelling ratio is due to the variation of the network chain length N depending on the gel preparation concentration (11), (23).

Effect of Crosslinker Type on the Gel Strength

The desired features of hydrogels include high swelling capacity, high swelling rate, and good strength under stress. The typical stress-strain dependencies of PDMAEMA hydrogels just after their equilibrium swelling in water are shown in Fig. 3. Although the chemical erosslink density of PDMAEMA hydrogels is the same, the slope of the stress-strain isoterms varies depending on the type of the crosslinker and the gel preparation concentrations.

The gel strength of PDMAEMA hydrogels was evaluated from the elastic modulus obtained from Eq. 7. The results are collected in Fig. 4, in which, the elastic modulus G of PDMAEMA hydrogels crosslinked with TEGMA is shown by the solid symbols and that of hydrogels crosslinked with BAAm is shown by the open symbols as a function of the volume fraction of the crosslinked polymer after the gel preparation [[phi].sub.2.sup.0]. For PDMAEMA hydrogels, the influence of different crosslinkers on the elastic modulus after equilibrium swelling in water indicates that the elastic modulus of hydrogels crosslinked with TEGMA is higher than that of hydrogels crosslinked with BAAm over the entire range of the gel preparation concentration. Additionally, this result can be reflected by the effective crosslink density ve and the average network chain length N between the crosslink points. By using the G, [[phi].sub.2,eqn], and [[phi].sub.2.sup.0] values of hydrogels together with Eqs. 8-10, the effective crosslink densities ve of PDMAEMA hydrogels were calculated. The results for the phantom network model ([phi] = 4) are shown in Fig. 5 as solid symbols for PDMAEMA hydrogels crosslinked with TEGMA plotted against [[phi].sub.2.sup.0] and that of hydrogels crosslinked with BAAm by open symbols. The curves in Fig. 5 represent the crosslinking efficiency [[epsilon].sub.xl], calculated using the equation given by:

[[epsilon].sub.xl] = [V.sub.e]/[V.sub.chem] (11)

where [V.sub.chem] is the chemical crosslink density of PDMAEMA hydrogels, which would result if all the crosslinker molecules formed effective crosslinks in the resulting hydrogel. Because X = 1/80, [v.sub.chem] = 191 mol [m.sup.3] and should be constant over all [[phi].sub.2.sup.0] range. Thus, it can be concluded that ex! gives the fraction of the crosslinker molecules consumed in the formation of elastically effective crosslinks.

From the comparison of Figs. 4 and 5, it is seen that both the swollen elastic moduli G and the effective crosslink density [v.sub.e] of PDMAEMA hydrogels first increase with [[phi].sub.2.sup.0] and then decrease with further increasing [[phi].sub.2.sup.0]. This is because, the crosslinker TEGMA contains four oxyethylene repeating units, it bears long chain spacer between two vinyl groups and also during the swelling process, the network chains are forced to attain more elongated and less probable configurations. The absorption of water by the hydrogel also causes the network to expand and its chains to stretch. As a result, the chains making up the network structure is assumed in a stretched conformation as the polymer network swells. Because the network chains in these swollen hydrogels are in the expanded configuration with respect to their dry state, the increase of the elastic modulus is connected with high stretching of the network chains. In this region, the higher crosslink density (Fig. 5) may cause stronger thermodynamic force which makes water to diffuse faster. As a result, it may be expected that a highly crosslinked network offer a higher rate of swelling. However, as it is seen that the elastic moduli of hydrogels decrease as the polymer network concentration further increased. In this region, the swelling of PDMAEMA hydrogels also increases with increasing [[phi].sub.2.sup.0] as can be seen from Fig. 2. Because the crosslink density of hydrogels is related to the swelling ratio, the crosslinker content, the structure of crosslinker and the gel preparation concentration, it can be concluded that the gel strength can be improved by the crosslink density and the type of the crosslinker used in the preparation.

In Fig. 6, the solid symbols show the average network chain length N of PDMAEMA hydrogels crosslinked with TEGMA and the open symbols show that of the hydrogels crosslinked with BAAm. The gel preparation concentration dependence of the swelling of corresponding hydrogels is due to the variation of the network chain length N depending on the concentration. FOr the present PDMAEMA hydrogel system, N decreases continuously with increasing initial monomer concentration for the hydrogels prepared with BAAm. In the case of PDMAEMA hydrogels crosslinked with TEGMA, as [phi].sub.2.sup.0] is increased, N decreases rapidly up to about [phi].sub.2.sup.0] = 0.10. At low polymer concentrations, for [phi].sub.2.sup.0] < 0.10, N is very sensitive to the gel preparation concentration due to decreasing probability of cyclization as the monomer concentration is increased and hence, linear swelling ratio [alpha] decreases (Fig. 2). Then, N increases slightly with further increasing [phi].sub.2.sup.0]. The slight variation of N in this high concentration regime is clue to the reducing reactivity of pendant vinyl groups during crosslinking as well as due to the increasing extent of the chain entanglements (23). Further, it can be said that the crosslink density of a gel is reciprocal to [[bar.M].sub.c], but proportional to its elastic modulus. The elasticity results shown in Table 1 also indicated that the values of [[bar.M].sub.c] obtained from Eq. 9 for PDMAEMA hydrogels crosslinked with BAAm are larger than those for hydrogels crosslinked with TEGMA.

TABLE 1. Parameters of PDMAEMA hydrogels.

                        TEGMA                               BAAm

[[phi].sub.2.sup.0]   [v.sub.e]   [[epsilon].sub.xl] x   [v.sub.e]
                         (mol            [10.sup.3]           (mol
                     [m.sup.-3])                        [m.sup.-3])

0.076                       2.22                  11.7         1.46

0.085                       3.32                  17.5         1.68

0.095                       81.6                   431         1.85

0.142                      120.1                   635         4.71

0.190                      112.2                   593         4.13

0.285                      125.9                   665         17.5

0.332                      103.5                   547         44.3

0.408                      60.78                   320         30.9

[[phi].sub.2.sup.0]  [[epsilon].sub.xl] x
                          [10.sup.3]

0.076                                 7.7

0.085                                 8.9

0.095                                 9.7

0.142                                24.9

0.190                                21.8

0.285                                93.0

0.332                                 234

0.408                                 163

[[phi].sub.2.sup.0] is the crosslinked polymer concentration after
the gel preparation. [v.sub.e] is the effective crosslink density.
[[epsilon].sub.xl] is the crosslinking efficiency of the resulting
hydrogels.


As it is well known that one of the characteristic parameters of crosslinked polymer network structure is the polymer-solvent interaction parameter [chi], which depends on the temperature and the composition of the structure. According to Flory-Rehner theory of swelling equilibrium (24), the polymer-solvent interaction parameter [chi] of PDMAEMA with water was calculated at T = 298 K from the following equation:

[chi] = -ln(1 - [[phi].sub.2,eq]) + [[phi].sub.2,eq] + 0.5([rho]/[[bar.M].sub.c])[V.sub.1][([[phi].sub.2,eq]).sup.1/3][([[phi].sub.2.sup.0]).sup.2/3]/[([[phi].sub.2,eq]).sup.2] (12)

By using the theoretical values of [[phi].sub.2.sup.0], the experimentally determined values of [[phi].sub.2,eq] and [[bar.M].sub.c] values found from the elasticity measurements and the other related parameters, the interaction parameter [chi] of PDMAEMA-water system was calculated as [chi] = 0.474 [+ or -] 0.03 for the hydrogels crosslinked with TEGMA and that of [chi] = 0.472 [+ or -] 0:02 for the hydrogels crosslinked with BAAm in the range of [[phi].sub.2,eq] between 0.0098 and 0.064. However, it is not possible to make meaningful comparison of the present network parameters with literature values because such values are usually scant for hydrophobiezilly modified PDMAEMA hydrogels. However, Emileh et al. reported [chi] parameter for PDMAEMA hydrogels crosslinked using EGDMA as the crosslinking agent as 0.57 at 25[degrees]C (12). In this respect, the calculations in this work reveal that the [chi] parameter of PDMAEMA--water system which describes the total interaction between the PDMAEMA network and water is nearly independent on [[phi].sub.2,eq] in the range of interest.

Water Content at Various Conditions

As it is well known that the swelling capacity of hydrogels depends on the nature of the polymer, the composition of the hydrogel and the characteristics of the external solution. In this work, the composition of PDMAEMA hydrogels [[phi].sub.2.sup.0] was varied widely between 0.076 and 0.469. The pH-sensitive swelling behavior of resulting hydrogels was investigated in buffer solutions ranging from pH 2.1 to 10.7 at room temperature. Figure 7 shows the effect of pH on the water content of PDMAEMA hydrogels crosslinked with TEGMA and with BAAm at different gel preparation concentrations.

It can be observed that the hydrogels crosslinked with TEGMA are in the swollen-state in acidic condition and in the collapsed-state in alkaline condition, demonstrating We same characteristics of hydrogels crosslinked with BAAm. In a narrow range of pH between 7.7 and 8.0, it was observed that the hydrogels exhibit a sharp pH-sensitive phase transition. The experimental results presented here indicate that the hydrogels expand in acidic conditions, because the tertiary amine side groups of DMAEMA chains become protonated in this region and hence, the charge density on the network increases by increasing degree of the protonation. Because the internal osmotic pressure increases the attractive interactions between the network chains and the water molecules, the equilibrium swelling capacity of hydrogels also increase in the acidic pH region. On the other hand, with increase in pH., the amino nitrogen are deprotonated and the excess swelling is reduced. It was found that the hydrogels shrank in basic condition owing to the coiled conformation due to ionic affinity. Another point shown from the Fig. 7 is that the extent of the transition is also affected by the type of the crosslinkers used in the preparation. In the range of pH between 7.7 and 8.0, swelling is a strong decreasing function of pH; the hydrogel crosslinked with TEGMA at [[phi].sub.2.sup.0] = 408 obtained a 6.5-fold larger swelling ratio in buffer solution with pH = 7.7, whereas the hydrogel crosslinked with BAAm at the same gel preparation concentration obtained a 4.7-fold larger swelling ratio. For PDMAEMA hydrogels crosslinked with BAAm, the relatively higher degree of protonation in the acidic pH region provides higher swelling capacity of the hydrogels and resulted in the larger transition.

To predict the effect of salt concentration on the water absorbency of PDMAEMA hydrogels, the swelling measurements were carried out in aqueous salt solutions ranging concentrations from [10.sup.-5] to 1.0 M. The volume swelling ratio of PDMAEMA hydrogels [q.sub.v] was calculated using the Eq. 6. The results for PDMAEMA hydrogels crosslinked with TEGMA and with BAAm collected in Fig. 8 showed that the water absorbency gradually decreased with an increase in the salt concentration. Additionally, it is seen that the water absorbency of PDMAEMA hydrogels crosslinked with BAAm is higher than that of the hydrogels series crosslinked with TEGMA. This is mainly because of the decrease in the difference of osmotic pressure between the network of the resulting gel and the external solution, that is, the water molecule is hard to infiltrate into the gel. Thus, the volume swelling ratio of hydrogels decreases with increasing salt concentration. Another point shown in Fig. 8 is that swelling is a strong decreasing function of rising NaCI concentrations; for [[phi].sub.2.sup.0] = 0.408, PDMAEMA hydrogels crosslinked with BAAm obtained a 12-fold larger swelling ratio in [10.sup.-5] M NaCI solution whereas the hydrogels crosslinked with TEGMA exhibit 9-fold larger swelling ratio. It can be concluded that PDMAEMA hydrogels crosslinked with TEGMA and BAAm exhibit strong salt sensitive swelling behavior over the entire range of the NaCI concentration which promotes their use in drug delivery devices. Also the hydrogels containing PDMAEMA are suitable to be used in pH-sensitive drug delivery systems because of their pH-sensitive swelling behavior.

CONCLUSIONS

In the present work, the investigation of the effect of different type of crosslinkers on the swelling behavior and the physical properties of PDMAEMA hydrogels is the main purpose. The process and the hydrogel swelling behavior were investigated in terms of the type and concentration of the crosslinkers. It was observed that the type of the crosslinker used in the gel preparation significantly affects the swelling behavior and the mechanical properties of the resulting hydrogels. The results showed that the swelling behavior of these hydrogels is related to their chemical structures, their compositions, and the type of external solutions. In addition, it was also found that the swelling ratio of PDMAEMA hydrogels crosslinked with BAAm is larger than those for hydrogels crosslinked with TEGMA over the entire range of the polymer network concentration. From the comparison of the chemical structures of the crosslinkers BAAm and TEGMA, it can be concluded that TEGMA-crosslinked hydrogels is more flexible since the distance between two vinyl groups is higher than that of BAAm. In case of TEGMA, the effective crosslink density was found to be higher than that of BAAm, and it can be resulting from the difference in the reactivity ratios of the crosslinkers. Thus, the higher reactivity ratio of TEGMA than BAAm is confirmed by the higher effective crosslink density. Because the increasing crosslink density decrease the swelling capacity of hydrogels, the larger swelling ratio for the present hydrogels prepared with BAAm is obtained due to their lower effective crosslink density. The degree of crosslink which strongly determines the swelling capacity of hydrogels can be controlled easily by varying the type and amount of the crosslinker used in the preparation. It was observed that increasing swelling capacity by increasing the gel preparation concentration is related to crosslink density. The pH-responsive swelling of the hydrogels increased in acidic pH region due to the protonation of tertiary amino nitrogen of the network chains. The hydrogels exhibit very sharp pH-sensitive phase transition in a very narrow range of pH between 7.7 and 8.0. The pH-sensitive phase-transition phenomenon is associated with the protonation of tertiary amine side groups of DMAEMA chains. This swelling property as a function of pH could make them as potential candidates in biomedical applications.

Correspondence to: Nermin Orakdogen; e-mail: orakdogen@itu.edu.tr

Contract gram sponsor: FARED.

DOI 10.1002/pen.23308

Published online in Wiley Online Library (wileyonlinelibrary.com).

[c] 2012 Society of Plastics Engineers

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Nermin Orakdogen

Department of Chemistry, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey
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Date:Apr 1, 2013
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