Environmentally sensitive polymers and hydrogels exhibit sharp changes in behavior in response to relatively small changes in conditions such as temperature or pH. Typical changes include precipitation of the polymer in water or collapse of a hydrogel with expulsion of a large fraction of the gel pore water. These changes are depicted in Figure 1. If such a polymer is grafted or adsorbed on another polymer surface or within the pores of a porous membrane, then one observes sharp changes in wettability or permeability, respectively, when the polymer is environmentally stimulated. These changes are usually reversible, although the recdissolution or reswelling processes are often slower than the precipitation or deswelling steps. Other examples of environmental stimuli and polymer system responses are listed in Tables I and II.
A large number of polymers display sharp responses to such stimuli. Many are based on vinyl monomers, such as anionic and cationic monomers for pH, specificion, solvent, or electrically responsive systems, and N-alkyl substituted acrylamides (e.g., N-isopropyl acrylamide, or NIPAAm) for temperature-sensitive systems. Other copolymer compositions may contain the responsive moiety as a pendant group on one of the monomers. Photoresponsive groups have been incorporated in such a way.
The ratio in the responsive polymer of the responsive monomeric component to the “nonresponsive” comonomer (e.g., the pH-sensitive monomer, acrylic acid, versus acrylamide or the temperature-sensitive monomer, NIPAAm versus acrylamide) will control both the sharpness and intensity of the response as well as the specific environmental conditions where it begins.