to a soft gel for certain pH values. Synthesis pH sensitive polymers can be synthesized using several common polymerization methods. Functional groups may need to be protected so that they do not react depending on the type of polymerization. The masking can be removed after polymerization so that they regain their pH-sensitive functionality. Living polymerization is often used for making pH sensitive polymers because molecular weight distribution of the final polymers can be controlled. Examples include group transfer polymerization (GTP), atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer (RAFT). Graft copolymers are a popular type to synthesize because their structure is a backbone with branches. The composition of the branches can be changed to achieve different properties. Hydrogels can be produced using emulsion polymerization. Characterization Contact angle Several methods can be used to measure the contact angle of a water drop on the surface of a polymer. The contact angle value is used to quantify wettability or hydrophobicity of the polymer. Degree of swelling Equal to (swollen weight-deswelled weight)/deswelled weight *100% and determined by massing polymers before and after swelling. This indicates how much the polymer swelled upon a change in pH. pH critical point The pH at which a significant structural change in how the molecules are arranged is observed. This structural change does not involve breaking bonds, but rather a change in conformation. For example, a swelling/deswelling transition would constitute a reversible conformational change. The value of the pH critical point can be determined by examining swelling percentage as a function of pH. Researchers aim to design molecules that transition at a pH that matters for the given application. Surface changes Confocal microscopy, scanning electron microscopy, Raman spectroscopy, and atomic force microscopy are all used to determine how the surface of a polymer changes in response to pH. Applications Purification and separation pH sensitive polymers have been considered for use in membranes. A change in pH could change the ability of the polymer to let ions through, allowing it to act as a filter. Surface modification pH sensitive polymers have been used to modify the surfaces of materials. For example, they can be used to change the wettability of a surface. Biomedical Use pH sensitive polymers have been used for drug delivery. For example, they can be used to release insulin in specific quantities. References Smart materials