Mechanical characterization of active poly(vinyl alcohol)–poly(acrylic acid) gel

Abstract

Active polymer gels expand and contract in response to certain environmental stimuli, such as the application of an electric field or a change in the pH level of the surroundings. This ability to achieve large, reversible deformations with no external mechanical loading has generated much interest in the use of these gels as actuators and “artificial muscles.” This work focuses on developing a means of characterizing the mechanical properties of active polymer gels and describing how these properties evolve as the gel actuates. Poly(vinyl alcohol)–poly(acrylic acid) (PVA–PAA) gel was chosen as the model material for this work because it is relatively simple and safe to both fabricate and actuate. PVA–PAA gels are fabricated on-site using a solvent-casting technique. These gels expand when moved from acidic to basic solutions, and contract when moved from basic to acidic solutions. The mechanical properties of the gel were characterized by conducting uniaxial tests on thin PVA–PAA gel films. A testing system has been developed which can measure stress and deformations of these films in a variety of liquid environments. The experimental results on PVA–PAA gels show these materials to be relatively compliant, and slightly viscoelastic and compressible. These gels are also capable of large recoverable deformations in both acidic and basic environments.