Experimental evidence for inhomogeneous swelling and deformation in statistical gels

Abstract

Polymer concentration correlations in a network swollen by a solvent are studied by small-angle neutron scattering. The network is made by statistical cross-linking of long chains in solution. At small scattering vectors, the intensity strongly increases upon swelling. Upon stretching, unusually oriented isointensity lines ("butterflies") are observed. These effects are well described by a model of fractal heterogeneities of cross-linking giving rise to nonhomogeneous swelling or deformation.PACS numbers: 64.60.Cn, 64.70.Pf, 82.70.Gg According to most current theories the properties of polymer networks are determined by their structural units alone, namely, the elementary meshes that connect two nearest-neighbor cross-links. No correlations are thought to exist on distance scales larger than the average mesh size. 1 "" 3 These approaches have not been entirely satisfactory, as their description of the elastic properties of rubbers and gels is essentially limited to semiquantitative estimates of behavior. On a microscopic scale, they cannot explain a striking anomaly which has been found recently in studies of the structure of rubbers by small-angle neutron scattering (SANS). A network containing small free labeled (deuterated) chains was subjected to a uniaxial deformation and the isointensity lines were mapped using a bidimensional neutron detector. The pattern observed was anisotropic, as expected, but unexpectedly was oriented in the wrong direction. 4,5 The observed double-winged figures, called "butterfly patterns," had their long axes aligned in the stretching direction. 4 This phenomenon has attracted attention because its understanding might bring new elements to the theoretical debate of the modeling of microstructural properties of networks. 6 " 8 In this Letter, we present the results of two experiments designed to shed some light on this situation. We look for the existence of butterfly patterns in a somewhat different situation, namely, the uniaxial elongation of statistically cross-linked gels. The appearence of such figures was predicted recently for such a case on the basis of establishment of correlations between substructures larger than the average mesh size. 9 This new model establishes a connection between the butterfly phenomenon and a strong increase of scattering intensity predicted to appear in a swelling experiment. Thus, we investigate by SANS the swelling and the stretching of networks synthesized under conditions as close as possible to the model assumptions.Semidilute solutions are good reference systems for discussing the structure of gels. They can be viewed as homogeneous networks of finite lifetime and the way they scatter light or neutrons is well described by existing theories. 3 When a contrast is established between the polymer and the solvent, their neutron-scattering intensity I(q) has an Ornstein-Zernicke form I(q)~I(0)/ (1 +q 2 £ 2 ) for qZ, < 1. £ is the correlation length scale beyond which the elementary segments of the chains can be considered as randomly distri...

Small-angle neutron scattering on polymer gels: phase behavior, inhomogeneities and deformation mechanisms

“…The scattered intensity patterns at 42 1C were similar to the so-called butterfly pattern (or, more exactly, the abnormal butterfly pattern). 48 The scattering intensity contour had a lobe in the stretching direction. At 46 1C, a distinct peak appeared in both undeformed and deformed gels.…”

“…35 Deformation mechanisms of polymeric systems have been studied by SANS. [47][48][49][50] Although SANS is a suitable means to study the deformation mechanisms of polymeric systems, chucking a polymer gel onto a stretching device is difficult in most cases because of its fragility. Geissler et al 51 used a mold with an elliptical hole and allowed a shrunken gel to swell anisotropically.…”

Les polymères comme matériaux désordonnés : l'apport de la diffusion de neutrons

“…Elles reflètent le plus souvent l'existence de fluctuations gelées (piégées par la réaction) de la répartition spatiale des points de réticulation [13]. On pourra aussi marquer (généralement avant réticulation) certaines des chaînes à partir desquelles le réseau est construit.…”

Response of Disordered Polymer Networks to External Fields: Regular Lattices Built from Complex Subunits

“…Therefore, under contact with an excess of solvent, the solvent is mainly absorbed by the regions between the clusters, and the gel swells heterogeneously. 1,2 Moreover, dynamic and static light scattering experiments on hydrogels have also revealed highly heterogeneous structures. [3][4][5] As demonstrated in the recent work of Oppermann and co-workers on polyacrylamide hydrogels, 5 the gel structure can be visualized as being built from densely cross-linked clusters (cross-link agglomerations) distributed in surroundings whose cross-link density is lower.…”

“…That one is indeed confronted with quite complex systems is well-documented experimentally. Thus, small-angle neutron scattering experiments 1,2 have shown that in statistical gels the local density of cross-links varies widely: The random arrangement of cross-links in the gel leads to local regions (clusters), whose cross-link density is much higher than on the average. Therefore, under contact with an excess of solvent, the solvent is mainly absorbed by the regions between the clusters, and the gel swells heterogeneously.…”