Elastic properties of poly(N-isopropylacrylamide) and poly(acrylamide) hydrogels studied by scanning force microscopy

Poly(N-isopropylacrylamide) (PNIPAAm) and poly(acrylamide) (PAAm) hydrogels of varying cross-linker concentration were characterized for their swelling behavior in water. Local elastic properties of these hydrogels were studied using atomic force microscopy (AFM) in a temperature-controlled liquid cell. Elastic moduli (Young's moduli) of both PNIMAAm and PAAm hydrogels were determined at different temperatures and varying cross-linker concentrations. Spherical probes (micrometer sized) as well as sharp conical probes were used in force vs displacement data acquisition. Young's moduli were determined by evaluating the indentation of the gel surface as a function of the probing load. On PNIPAAm. hydrogel surfaces a dramatic increase of the stiffness (up to 10 times) was observed when crossing the phase transition at similar to33 degreesC. The cross-linker concentration has a strong influence on Young's modulus for temperatures above the critical temperature while only small variations were seen for temperatures below 33 degreesC. PAAm. hydrogels show a different behavior: Young's moduli were found to be just slightly dependent on the temperature though the cross-linker concentration again exerts a strong influence.