Dresden 2011 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 34: Poster: Elastomers and Gels

CPP 34.6: Poster

Wednesday, March 16, 2011, 17:00–19:00, P2

Mechanical properties of the thermo-responsive hydrogel PNIPAM investigated by Brillouin spectroscopy — •Johannes Wiedersich, Andreas Meier-Koll, Peter Müller-Buschbaum, and Winfried Petry — TU München, Physik Department, LS Funkt. Mat., James-Franck-Str. 1, 85748 Garching, Germany

Poly(N-isopropyl acrylamide) (PNIPAM) is a thermo-responsive polymer that shows a volume phase transition at the lower critical solution temperature (LCST) around 33 ^∘C. Below the LCST the polymer incorporates water molecules into its network. At higher temperatures the polymer network collapses and precipitates.

We investigate high frequency sound propagation and attenuation of this hydrogel system by means of Brillouin spectroscopy, covering the range from well below to well above the phase transition. We probe the structure and dynamics of the system on length scales close to the size of a solvated PNIPAM molecule and on timescales on the order of those of fluctuations of hydrogen bonds.

Both the sound velocity and acoustic attenuation increase with increasing concentration. This is attributed to a larger elastic modulus at higher concentrations and to a stronger damping of sound waves for more concentrated solutions, respectively. Deviations from purely exponential damping of sound waves are observed.

Above the LCST two phases coexist: One is water-like with respect to sound propagation, the other one displays a higher sound velocity than the homogeneous solution.