Swelling-Related Processes in Hydrogels

Abstract

The swelling equilibrium of gels can be explained by the Flory-Rehner theory. The FR theory calculates the difference of the chemical potential of a solvent inside a gel to the surrounding pure solvent as a sumof the mixing and the elastic part. The first is based on the Flory–Huggins theory, the second on the rubber elasticity theory (Sect. 1). An important role for gels with volume phase transition plays the Flory–Huggins interaction parameter and its dependencies on temperature and concentration. In a system with specific interactions between the solvent and the network, additional contributions to the chemical potential have to be taken into consideration. The application of hydrogels is based on changes of the gel properties, mostly the dramatic change of their swollen volume, in response to specific environmental stimuli. The velocity of changes of the degree of swelling is of outstanding importance. The kinetics of swelling and shrinking is determined by different diffusion processes (Sect. 2). The cooperative motion of the net chains is characterized by a cooperative diffusion coefficient Dcoop. The time dependence of the degree of swelling for gels of different geometries is followed and Dcoop calculated. Processes happening at the volume phase transition were discussed. Section 3 introduces then the fundamental concepts of the spectroscopic characterization of hydrogels. The focus of Sect. 3.1 is a practical hands-on advice that everyday practitioners of infrared and Raman spectroscopy will find useful. The reader will be introduced to spectroscopic methods and to sample preparation techniques. More advanced techniques like imaging spectroscopy and chemometric data analysis are discussed. Section 3.2 discusses the NMR imaging technique as an important method to visualize swelling-related processes in hydrogels.