Environmentally Sensitive Polymers and Hydrogels

Abstract

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.

Synthesis and property of temperature and pH sensitive xanthan-MA/PNIPAAm hydrogels

“…The data indicate that all XMNs have a higher LCST than the pure PNIPAAm hydrogel(XMN0) at 33 ℃ [10].…”

Hydrophilic–hydrophobic hydrogels for cartilage replacement

“…Cross-linking density can be increased to improve the properties of the hydrogel, but crosslinking generally makes materials stiffer or embrittled (Tienen et al, 2004). Research has shown that the type (ionic versus covalent), number, and length of cross-linking in hydrogels can dramatically affect the elastic moduli and toughness of the system (Hoffman, 1991). Another route to strengthening the material involves utilizing a reinforcement material such as fabric or foam (Liu et al, 2008), but this may introduce stress risers into the material (Mansmann, 2003).…”

Synthesis and characterization of partially biodegradable, temperature and pH sensitive Dex–MA/PNIPAAm hydrogels

“…Among the family of temperature intelligent hydrogels, poly(Nisopropylacrylamide) (PNIPAAm) hydrogel is one of the most widely studied. It exhibits a lower critical solution temperature (LCST) or transition temperature (T tr ) at B33 C in aqueous solution and shows an abrupt thermoreversible change in volume as external temperature cycles around this critical temperature [14,15].…”