Thermoreversible gels were formed by dissolving a poly(methyl methacrylate)−poly(tert-butyl acrylate)−poly(methyl methacrylate) triblock copolymer in a variety of alcohols, including ethanol, 1-butanol, 2-ethylhexanol, and 1-octanol. The gels exhibit an ideal and reversible solid/liquid transition in each of these solvents, behaving as strong elastic solids at room temperature and as freely flowing liquids above the gel transition. The time-dependent elastic properties of the gels are governed by two transition temperatures. The first transition is the critical micelle temperature (cmt) near 60 °C, below which the PMMA blocks aggregate to form a physically cross-linked network. As the gels are cooled to room temperature, differential scanning calorimetry (DSC) reveals a second transition where the PMMA domains undergo a glass transition. The glass transition temperature of the PMMA domains increases when the gels are aged at room temperature. Time−temperature superposition can be applied below the cmt to give master curves describing the relaxation behavior of the gels in the vicinity of the glass transition of the PMMA domains. These relaxation times increase by 1 decade for every 8 K decrease in temperature, a result that is consistent with previous measurements of polymer relaxations in the vicinity of the glass transition temperature.

The use of instrumented indentation to characterize the mechanical response of polymeric materials was studied. A model based on contact between a rigid probe and a linear viscoelastic material was used to calculate values for the creep compliance and stress relaxation modulus for two glassy polymeric materials, epoxy and poly(methyl methacrylate), and two poly(dimethyl siloxane) (PDMS) elastomers. Results from bulk rheometry studies were used for comparison with the indentation stress relaxation results. For the two glassy polymers, the use of sharp pyramidal tips produced responses that were considerably more compliant (less stiff) than the rheometry values. Additional study of the deformation remaining in epoxy after indentation creep testing as a function of the creep hold time revealed that a large portion of the creep displacement measured was due to postyield flow. Indentation creep measurements of the epoxy with a rounded conical tip also produced nonlinear responses, but the creep compliance values appeared to approach linear viscoelastic values with decreasing creep force. Responses measured for the unfilled PDMS were mainly linear elastic, with the filled PDMS exhibiting some time-dependent and slight nonlinear responses in both rheometry and indentation measurements.

Dynamic nanoindentation was performed on a cured epoxy, a poly(methyl methacrylate) (PMMA), and two poly(dimethyl siloxane) (PDMS) samples of different crosslink densities. These samples were used to compare dynamic nanoindentation with classical rheological measurements on polymeric samples in the glassy and rubbery plateau regions. Excellent agreement between bulk rheological data and dynamic nanoindentation data was observed for the two glassy materials (epoxy and PMMA) and the less compliant PDMS sample. More divergent results were observed for the more compliant PDMS sample. The theoretical foundation and historical development of the working equations for these two types of instrumentation are presented and discussed. The major difference between nanoindentation and the more classical rheological results is in the treatment of the instrument-sample interface.

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