Chapter 3
Network Inhomogeneities in Polymer Gels Wilhelm Oppermann, Brigitte Lindemann, and Bettina Vögerl Institute o f Textile a n d F i b e r C h e m i s t r y , University o f Stuttgart, P f a f f e n w a l d r i n g 55,
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D - 7 0 5 5 0 Stuttgart, G e r m a n y
Spatial inhomogeneities present in ionic and nonionic hydrogels depend systematically on network density and on the degree of swelling. To obtain a quantitative measure, the mean -square fluctuation of the refractive index, , and the correlation length, ξ, were determined by static light scattering. Upon increase of the network density, rises markedly while ξ remains essentially unchanged. When ionic gels are swollen in aqueous solutions containing various salt concentrations, a series of different degrees of swelling is obtained. In such a series, ξ rises with increasing swelling while decreases, as expected due to three-dimensional expansion and dilution. The data determined in the state of formation of the network, however, deviate strongly from this tendency with being some decades lower than in the swollen state. 2
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Hydrogels made by crosslinking copolymerization are interesting materials for a variety of pharmaceutical and hygienic applications. The macroscopic properties such as swelling behavior and elasticity are quite well understood (7). However, little information is available on the internal structure and network topology.
© 2003 American Chemical Society In Polymer Gels; Bohidar, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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One feature of particular interest is the presence of network inhomogeneities. These can be visualized as strongly crosslinked regions in a less densely crosslinked environment. The degree of such spatial inhomogeneities depends strongly on the conditions at preparation of the network and on the chemistry of the crosslinker. It was shown previously that there is a clear correlation between the reactivity of the crosslinker, the efficiency of the cross-linking reaction, and the size and density of network inhomogeneities (2). In this paper we report on the influence of the degree of swelling on the structural parameters as deterrnined by static light scattering experiments.
Experimental Materials Hydrogels were prepared by copolymerizing acrylic acid or 50%neutralized acrylic acid with N,N'-methylenebisacrylamide in aqueous solution. The monomer concentration was 10 or 15 % (weight/volume). Great care was taken to avoid dust particles in the samples. Therefore, appropriate starting solutions were filtered through Nalgene cellulose acetate filters into the light scattering cuvettes in a glove box, where polymerization took place. The polymerization reaction was initiated by the redox system tetramethyl ethylendiamine (72 mg/100 ml) / Na S 0 (80 mg/100 ml). To study gels in the swollen state, the samples were removed from the cuvettes and immersed in NaCl solutions of different concentrations for several days. After attainment of swelling equilibrium, the weight gain was determined and the degree of swelling, Q, was calculated as mass of swollen gel per mass of dry polymer. Cylindrical specimens of appropriate diameter were punched out of the swollen gels and brought back into the cuvettes. The small space between sample and glass was filled with the corresponding salt solution to reduce light reflections. 2
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Light scattering experiments The measurements were carried out in a modified Sofica apparatus equipped with a HeNe-laser and a computerized data acquisition system. It was calibrated against a toluene standard. The cuvette was rotated to several positions and an ensemble average of scattering intensity was taken to account for the fixed structure of the gels. The scattering intensity expressed as the Raleigh ratio R(q) is assumed to be due to thermal concentration fluctuations (ergodic contribution) and to
In Polymer Gels; Bohidar, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
39 inhomogeneities resulting from the crosslinking of the chains by which a certain inhomogeneous structure is frozen in (non-ergodic contribution) (3,4). Rgd(q) = Rth.(q) + Rcr.(q) We are interested in the latter contribution. The experimental means to determine this quantity rests on the assumption that RthXq) of E l is practically identical to that of a solution of linear macromolecules under the same conditions, Rs i.(cO- This is not exactly the case, but applies within 20% error. In particular, the thermal scattering of a gel should never become appreciably larger than that of the corresponding solution. A detailed discussion of this point is given in (3), where the relevant literature is thoroughly reviewed. When the total scattering of the gel exceeds that of the solution to a significant extent, the error in Rth.(q) has no big effect, and Rc .(q) can be determined safely as the so-called excess scattering intensity: a
e
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Rc .(q) = R (q) r
E
= RgdCq) -
R«L(CJ)
by subtracting the scattering intensity measured on an uncrosslinked sample of the same polymer (and salt) concentration from the scattering intensity of the gel. The Debye-Bueche analysis (5,6) was employed to obtain the correlation length, and the mean-square refractive index fluctuation, , of the gels from the relationship: 2
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R (q) = 47iK^ /(l + q f )
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l
Here K = 8i( n
