Polymer Communication Lattice heterogeneities at various crosslinker contents—a gel swelling study O ¨ . Pekcan a, * , S. Kara b a Department of Physics, Istanbul Technical University, Maslak 80626, Istanbul, Turkey b Department of Physics, Trakya University, 22030 Edirne, Turkey Received 1 February 2000; received in revised form 16 March 2000; accepted 31 March 2000 Abstract Disk shaped poly acrylamide (PAAm) gels were prepared from acrylamide (AAm) with various N,N 0 -methylenebis(acrylamide) (Bis) contents by free radical crosslinking copolymerization (FCC) in water and dried before use in swelling experiments. Photon transmission experiments for these gels were performed using an UV–visible (UVV) spectrometer. Transmitted light intensity, I tr from the gel increased at very early times when the PAAm gels are immersed in water, then decreased exponentially as the swelling time increased. Decrease in I tr was attributed to the lattice heterogeneities which might be originated between “frozen blob clusters” and holes in the swelling gel. Decrease in I tr was modeled using the Li–Tanaka equation from which cooperative diffusion coefficients, D c were determined for various Bis content gels. It is observed that the D c values increased as the Bis content increased.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Li–Tanaka equation; Poly acrylamide (PAAm) gels; N,N 0 -Methylenebis(acrylamide) (Bis) 1. Introduction Swelling is directly related to the viscoelastic properties of a gel. The gel elasticity and the friction between the network and solvent play an important role on the kinetics of the gel swelling [1–3]. It has been known that the relaxa- tion time of swelling is proportional to the square of a linear size of the gel [1] which has been confirmed experimentally [3]. One of the most important features of the gel swelling process is that it is isotropic, i.e. when the radius increases 10%, the axial length increases 10% in a long cylindrical gel. The elastic and swelling properties of permanent networks can be understood by considering two opposing effects, the osmotic pressure and the restraining force. Usually the total free energy of a chemically crosslinked network can be separated into two terms: the bulk and the shear energies. In a swollen network the characteristic quan- tity of the bulk free energy is the osmotic bulk modulus, K. The other important energy, the shear energy, keeps the gel in shape by minimizing the non-isotropic deformation. The characteristic coefficient of these forces is the shear modu- lus, m , which can be most directly evaluated by stress– strain measurements [4,5]. Li and Tanaka [6] have devel- oped a model where the shear modulus plays an important role that keeps the gel in shape due to coupling of any change in different directions. This model predicts that the geometry of the gel is an important factor, and swelling is not a pure diffusion process. It has been known that the swelling and elastic properties of gels are strongly influ- enced by large-scale heterogeneities in the network struc- ture [4,7]. In the swollen state these imperfections manifest themselves in a non-uniformity of polymer concentration. These large-scale concentration heterogeneities do not appear during the gelation but only in the gel swollen at equilibrium [8]. Light scattering experiments by Bastide et al. seem to confirm this picture [9]. The swelling processes of polyacrylamide gels in solvent have been extensively studied [10–12]. It has been reported that PAAm gels undergo continuous or discontinuous volume phase transitions with temperature, solvent compo- sition, pH and ionic composition [10]. When an ionized PAAm gel is allowed to swell in water, an extremely inter- esting pattern appears on the surface of the gel and the volume expansion increases by adding some amount of sodium acrylate [12]. If PAAm gels are swollen in an acet- one–water mixture, gel aging time plays an important role during the collapse of the network [12]. The kinetics of swelling of PAAm gels was studied by light scattering and the cooperative diffusion coefficient of the network was measured [1,13]. Small angle X-ray and dynamic light scattering were used to study the swelling properties Polymer 41 (2000) 8735–8739 0032-3861/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0032-3861(00)00264-0 * Corresponding author. Tel.: + 90-0212-285-3213; fax: + 90-0212- 285-6366. www.elsevier.nl/locate/polymer