270846 Time Scale Effects On Rheology, Adhesion, and Hardness of Polymer Gels
Polymer gels are physically or chemically crosslinked networks of polymers which are swollen in a liquid. Polymer gels are used in diverse applications ranging from foods and drug delivery to adhesives and consumer products. Silicone based gels have found wide use in products ranging from medical products to cooking utensils. Due to the diverse nature of the application of polymer gels, the physical properties of the gel must be well understood in a wide variety of conditions. Polymer gels typically consist of two parts; the polymer chains bound into the gel network (gel) and unbound polymer chains (sol). Due to the binary nature of the gel, a very wide range of relaxation times ranging from very short (sol) to infinite (perfect gel network) are observed depending on the quality of the gel network formed. Gels in which every polymer chain is bound into the gel network have relaxation times that approach those of an elastic solid (i.e., infinite relaxation time) while gels containing large numbers of defects or imperfections have relaxations times that are much shorter and resemble those of viscous liquids.
Since there is a very large array of relaxation times for any given polymer gel, the measurement of the gel physical properties depends heavily on the time scale of the measurement. For measurements at low speeds or low frequencies, the gel response is dominated by the elastic gel network. The time scale of the measurement is slow enough that the polymer sol has time to relax and rearrange in response to the applied stress and so does not contribute significantly to the measurement. However, at high measurement speeds or frequencies, the polymer sol does not have time to rearrange and relax. The entangled chains store energy elastically and contribute to viscous dissipation of the applied energy resulting in a very different overall response of the polymer gel. The influence of time scale on the measurement of polymer gel rheology, sol polymer rheology, adhesion, and hardness are examined here for three fluorosilicone gels of varying equilibrium modulus ranging from very soft to hard, each containing about 50% polymer sol.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
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