Radiation synthesis of low swelling acrylamide based hydrogels and ...

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376 N. Mahmudi et al. / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 375–378 the sample, and k is the deformation ratio (deformed length/initial length). For a homogenous network of gaussian chains, the elastic modulus of gel swollen to equilibrium, G, is related to the network cross-link density by Eq. (3) [3]. G ¼ A q M c RT m 2=3 2r m1=3 2m ; where q is the polymer density. The prefactor A, equals 1 for an affine network and (1 2//) for a phantom network. The effective cross-link density, m e , of a cross-linked structure can be obtained from the results of compressive strain measurements using Eqs. (2)–(4): M c ¼ q m e : In our previous studies we have shown that simple compression analyses and equations derived from phantom network theory can be used for the determination of effective cross-link density of highly swollen hydrogels without needing some polymer-solvent based parameters as in the case of swelling [4]. In this study we compared swelling and mechanical analyses for the determination of cross-link density of hydrogels prepared by ionizing radiation with relatively low degree of swelling. 2. Experimental Four components were used in the preparation of acrylamide–methacrylamide–methylenebisacrylamide (AAm/ MAAm/MBA/water) and acrylamide–2-hydroxyethyl methacrylate–methylenebisacrylamide (AAm/HEMA/ MBA/water) hydrogels, namely acrylamide, methacrylamide, and 2-hydroxyethyl methacrylate as monomers and methylenebisacrylamide as the cross-linking agents and water as dispersing medium. The mass proportion of the monomers in the initial mixtures is summarized in Table 1. The AAm/MAAm/MBA/water and AAm/HEMA/ MBA/water solutions were placed in PVC straws of 3 mm diameter and irradiated at 15 kGy and 6.6 kGy doses, respectively. They have been determined to be minimum doses corresponding to complete conversion. Fresh ð3Þ ð4Þ hydrogels obtained in long cylindrical shapes were cut into pieces 3–4 mm in length. Unreacted monomer and uncrosslinked polymers were removed by washing the gels for two days in distilled water. They were dried in vacuum oven in 315 K. Percentage gelation i.e. percentage conversion of monomers and cross-linking agent into insoluble networks, was based on the total weight of the cross-linking agent and monomers in the initial mixture. Washed and dried hydrogels were left to swell in distilled water at room temperature to determine the parameters of swelling. Swollen gels removed from the water bath at regular intervals were dried superficially with filter paper, weighed and immediately placed in the same bath still in equilibrium swelling state. Elastic properties and shear modulus of hydrogels were determined by using a Zwick Z010 model Universal Testing Instrument and uniaxial compression module. The crosshead speed was 5 mm/min. 3. Results and discussion 3.1. Swelling behavior of hydrogels For the characterization of the network structure and determination of effective cross-link density of prepared hydrogels their swelling behavior at pH 7 was first investigated. The percentage swelling of hydrogels was calculated by the following equation; S%ðmÞ ¼½ðm t m o Þ=m o Š100; where m t and m o are the weights of the swollen and dry gels respectively. Representative swelling curves for AAm/MAAm/MBA systems are given in Fig. 1. Very similar curves were obtained for the other hydrogel systems. The % equilibrium swelling values of all prepared hydrogels were collected in Table 2. As can be seen from this table % swelling of hydrogels is lower than 550%. The equilibrium value of swelling was used in each case to calculate the volume fraction of polymer (m 2m ) by using Eq. (5) given below where q and q w are the densities of dry gel and water. W is the weight fraction of polymer in swollen gel. 1=m 2m ¼ 1 þ q=q w ðw 1 1Þ : ð5Þ Table 1 Mass composition of monomers and cross-linking agent in the feed solutions and corresponding abbreviations used for the hydrogels Gel code Mass of monomers, cross-linking agent and water AAm (g) MAAm (g) HEMA (ml) MBA (%) a Water (ml) 0.2AAm0.2MAAm2MBA 0.2 0.2 – 0.5 1 0.2AAm0.2MAAm4MBA 0.2 0.2 – 1.0 1 0.2AAm0.2MAAm6MBA 0.2 0.2 – 1.5 1 1AAm1HEMA0.05MBA 1.0 – 1.0 0.05 2 1AAm1HEMA0.1MBA 1.0 – 1.0 0.1 2 1AAm1HEMA0.2MBA 1.0 – 1.0 0.2 2 1AAm1HEMA0.4MBA 1.0 – 1.0 0.4 2 1AAm1HEMA0.8MBA 1.0 – 1.0 0.8 2 a MBA(%) equal to (mass of MBA/mass of AAm + mass of MAAm or HEMA) * 100.
N. Mahmudi et al. / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 375–378 377 % S 600 500 400 300 200 100 1: 0.2AAm0.2MAAm2MBA 2: 0.2AAm0.2MAAm4MBA 3: 0.2AAm0.2MAAm6MBA 0 0 500 1000 1500 2000 2500 Time (min) Fig. 1. Swelling degree versus time for AAm/MAAm/MBA hydrogels. The v parameter of hydrogels is generally calculated by using the following equation [5]. v ffi 1=2 þ m 2m =3; ð6Þ which is the result of an assumption as M c goes to infinity which makes the denominator in Eq. (1) equal to zero i.e. lnð1 m 2m Þþm 2m þ vm 2 2m ¼ 0: ð7Þ Serial expansion of the logarithmic term lnð1 m 2m Þ¼ ð m 2m þ m 2 2m =2 m3 2m =3 Þ for small m 2m values gives the following equation; m 2 2m =2 m3 2m =3 þ vm2 2m ¼ 0; ð8Þ upon rearrangement is converted into Eq. (6). This equation shows that v parameter approaches to 0.5 for small m 2m values. In this study, we have calculated M c ðM cðsÞ Þ values of hydrogels by using m 2m and v obtained from Eq. (6) and other relevant parameters from swelling experiments and collected the results in Table 2. To account for calculations of these parameters from swelling experiments the subscript ‘‘s’’ was used throughout in the abbreviations. 3.2. Mechanical properties of hydrogels and determination of M c For the investigation of the effect of MBA on the mechanical properties of AAm/MAAm and AAm/HEMA 1 2 3 hydrogels and for the determination of true M c ðM cðmÞ Þ and v parameter, uniaxial compression was applied by using the Universal Testing Instrument. The subscript ‘‘m’’ was used to indicate that these parameters are calculated from mechanical measurements. Typical stress–strain curves of hydrogels were given in Fig. 2. As can be seen from the figure, the magnitude of stress increased with increasing MBA content in the hydrogel for a given strain. Shear modulus values of hydrogels were calculated by using elastic deformation theory and Eq. (2). [1]. When the equation is applied to the initial stages of deformation, plots of f versus k k 2 yield straight lines, Fig. 3 where k is the deformation ratio and equal to L/L o . L o and L are the lengths of the undeformed and deformed hydrogels during compression, respectively. The G value was calculated from the slope of the lines and listed in Table 2. By using G values and other relevant experimental parameters, M c and m e were calculated from Eqs. (3) and (4) and collected in Table 2. As can be seen from Table 2. the magnitudes of M c calculated from mechanical properties are different from those obtained by using swelling experiments. Large difference was attributed to using incorrect v parameter in the modified Flory–Rehner equation. The actual v parameters were calculated by using M cðmÞ values and Eq. (1). Recalculated v parameters (v m ) and the differences between v s and v m are also given in Table 2. For the investigation of the effect of v parameter on the M c values the theoretical M c values were Stress, kPa 150 125 100 75 50 25 1: 0.2AAm0.2MAAm2MBA 2: 0.2AAm0.2MAAm4MBA 3: 0.2AAm0.2MAAm6MBA 0 0 5 10 15 20 25 30 Strain, % Fig. 2. Strain versus stress curves of AAm/MAAm/MBA hydrogels. 3 2 1 Table 2 Network properties and cross-link densities of hydrogels Sample %S v s M cðsÞ (g/mol) M cðmÞ (g/mol) v m m e(s) (mol/cm 3 ) m e(m) (mol/cm 3 ) v s v m G (kPa) 0.2AAm/0.2MAAm/2MBA 540 0.54 39,600 6790 0.52 3.29E 05 1.92E 04 0.02 29.0 0.2AAm/0.2MAAm/4MBA 360 0.55 9750 2970 0.54 1.32E 04 4.35E 04 0.01 77.1 0.2AAm/0.2MAAm/6MBA 300 0.56 6230 2880 0.55 2.09E 04 4.51E 04 0.01 87.3 1AAm/1HEMA/0.05MBA 410 0.55 14,385 7745 0.55 9.02E 05 1.68E 04 0.00 46.7 1AAm/1HEMA/0.1MBA 380 0.56 11,915 6780 0.55 1.08 E 04 1.91E 04 0.01 53.9 1AAm/1HEMA/0.2MBA 370 0.56 10,020 6030 0.55 1.29 E 04 2.15E 04 0.01 62.4 1AAm/1HEMA/0.4MBA 330 0.56 8805 4780 0.55 1.46 E 04 2.68E 04 0.01 79.4 1AAm/1HEMA/0.8MBA 280 0.57 5450 3660 0.56 0.000237 3.52E 04 0.01 108.0
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