Environmentally Sensitive Polymers and Hydrogels
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ous membrane, then one observes sharp changes in wettability or permeability, respectively, when the polymer is environmentally stimulated. These changes are usually reversible, although the recUssolution 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 EL 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, specific ion, solvent, or electrically responsive sys-
Allan S. Hoffman Table I: Examples of Environmental Stimuli. Introduction 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 precipita-
tion 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 por-
ApH A Temperature A Chemical or biochemical agents A Solvent or salts A Electrical field A Electromagnetic radiation A Mechanical stress A Sound input
Table II: Examples of Polymer Responses.
ApH or AT
Solmita©© H,0 H20 ApH or AT
H,0
H,0
H,0 H,0
Phase Separation precipitation Optical clears up or opacifies; becomes colored Shape shrinks or swells Mechanical hardens or softens Surface becomes nonwetting Chemical/biochemical stimulates or inhibits reactions or recognition processes Electrical generates signal; electrochemical reaction occurs Permeability bulk or porous membranes open and shut
Figure 1. Phase change behavior ofpH and temperature-sensitive polymers and hydrogels.
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MRS BULLETIN/SEPTEMBER 1991
Environmentally Sensitive Polymers and Hydrogels
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Hydrogen-bound w a t e r ;
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LCST p o l y m e r chaln
Figure 2. Schematic interactions of water molécules with an LCST polymer below and above its LCST.
tems, 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 hâve 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
Table III: Some Polymers and Surfactants that Show LCST Behavior In Aqueous Solutions.
Table IV: Applications of "Smart" Polymeric Biomaterials.
Ether Groups Polyethylene oxide (PEO) Poly(EO/PO) copolymers PEO-PPO-PEO triblock surfactants Alkyl-PEO block surfactants Poly(vinyl methyl ether) Alcohol Groups Hydroxypropyl acrylate Hydroxypropyl methylcellulose Hydroxypropyl cellulose Methylcellulose Polyvinyl alcohol (and derivatives) Substituted Amlde Groups
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