The Effect of Microsphere Inclusion on the Mechanical Properties of Cross-linked Polyacrylamide Gels (2013)

Undergraduates: Samuel Kirby, Will Daniel

Faculty Advisor: Sergei Sheiko
Department: Chemistry

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The goal of this project is to design elastomeric or gel networks whose mechanical and acoustic properties can be varied by external stimuli. This can be accomplished by inserting expandable microbubbles into these networks. Controlled mechanical properties of the microbubbles along with their programmed expansion will affect both the structure and properties of the surrounding network. These microbubbles serve as modifiable fillers that reversibly transition between hard reinforcing particles and soft voids by varying the external temperature.
For this purpose we have prepared polyacrylamide gels with hollow, gas-filled polymeric microspheres (microbubbles) embedded as filler particles in the gels and studied their mechanical and acoustic properties as a function of temperature, cross-linking density of the gel and microbubble concentration. At temperatures below glass transition of the polymer shell of the microbubbles, we observed typical viscoelastic behavior in the rubber plateau of the gels. Gels containing loads of pre-expanded polymer microbubbles between 30 and 50 percent by volume displayed a retardation of the storage modulus, and accompanying increase in loss modulus upon being heated to 373 K, the glass transition temperature of the microsphere shell. This decrease in storage modulus is attributed to the transition of the polymer microbubbles from hard glassy spheres to soft, compressible elastomeric bubbles.