Effect of cross-links on the structure of polymer gels

This paper describes measurements of the macroscopic thermodynamic properties of swollen networks (osmotic swelling pressure, elastic modulus) in relation to the equivalent uncross-linked solutions. The presence of cross-links in a gel introduces a second characteristic distance scale, which is visible in small angle neutron and X-ray scattering (SANS and SAXS), in addition to that of the thermal fluctuations. The two length scales vary differently as the degree of swelling of the gel is modified: the correlation length of the osmotic fluctuations increases as the gel swells, while that of the elastic constraints is practically independent of swelling. Several observations are also described on gels containing free polymer chains of the same chemical nature as the network. These measurements show that the free chains in the gel have thermodynamic and scattering properties that closely resemble those of the network. Isotopic labelling of the free chains distinguishes them from the surrounding network and allows their individual properties to be examined. It is found that their radius of gyration shrinks, owing to their interaction with the cross-links in the network. Copyright (C) 1996 Published by Elsevier Science Ltd.