Active Polymers

Active polymers is a diverse class that can actuate and change shape in response to an environmental stimuli. These can be amorphous, elastomeric or glassy, semicrystalline, and liquid crystalline, and can activate in response to heat, light, solvent absorption, pH, and electrical field.  Our work seeks to develop experimental methods and physics-based models to identify key mechanisms of the active response and understand how the physical properties of the polymers, actuation conditions, processing conditions, and storage conditions affect the active response.

Collaborators

  • David Safranski, Ph.D., Research Scientist, MedShape, Inc., Atlanta Georgia
  • Chris Yakacki, Ph.D. Assistant Professor, Department of Mechanical Engineering, University of Colorado Denver
  • Carl Frick, Ph.D. Assistant Professor, Department of Mechanical Engineering, University of Wyoming
  • David Gracias, Ph.D. Professor, Department of Chemical and Biomolecular Engineering, Johns Hopkins University
  • Harold Park, Ph.D., Associate Professor, Department of Mechanical Engineering, Boston University

Shape Memory Polymers

 

Shape-memory behavior in polymers describes the ability to store a programmed shape indefinitely and fully recover to an original shape in response to an environmental trigger.  The materials are attractive for medical applications because they can be designed for biocompatibility, biodegradability, and controlled drug delivery. Moreover, they can store and recover large deformation, which is desirable for deployable and morphing structures (Perkins et al. 2004).  For amorphous networks, shape-memory behavior is realized by programming around the glass transition temperature, Tg. In a typical shape-memory cycle, the material is heated above Tg, deformed to the desired shape, then cooled rapidly below the Tg to fix the programed shape for indefinite storage.   The permanent shape can be recovered by heating the device to above the Tg or by immersing in solvent to depress the Tg.

The aims of this project is to investigate the effect of thermomechanical programming (deformation and temperature history), thermomechanical properties, and physical and environmental aging on the shape memory behavior under a variety of mechanical constraints.  Our approach is to develop constitutive models that describe the key time-dependent  mechanisms of the glass transition: (1) viscoelasticity, (2) structural relaxation, and (3) stress-activated viscous flow below Tg.

smp1smp2smp3

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Thermoresponsive Hydrogels

Hydrogels describe a broad class of heavily swollen crosslinked polymer networks.   The swelling of a hydrogels equilibrated in solution can be affected by different environmental stimuli, including the pH and temperature of the solution.  Differential swelling of multi-layered structures can further be exploited to design controllable self-folding structures.  The aim of this project is to develop constitutive models for the nonequilibrium swelling and mechanical behavior of hydrogels. We then apply the models to investigate the design of active hydrogel structures and devices, such as grippers.

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Temperature (C)

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Dielectric Elastomers

Before applying voltageDielectric elastomers is a type of electroactive polymer that can undergo large deformation in response to an applied electric field.  Unlike shape memory polymers and hydrogels, the electroactive is fast in dielectric elastomers. Moreover, the material is light weight, inexpensive to manufacture, and has goo electromechanical conversion efficiency. When prestretched and sandwiched between two compliant electrodes, a  dielectric elastomer membrane can stretch by as much as 160% \cite{pelrine2000high}. Dielectric elastomers ha applications for  robotic actuators, artificial limbs as well as energy harvesters.  However, the material is highly viscoelastic, which can significantly limit the performance of dielectric elastomers.  For example, undesired creep of an electro-actuated membrane can induce electric breakdown and development of the pull-in instability and wrinkling instability. We have been developing methods to characterize the temperature-dependent viscoelastic behavior of dielectric elastomers, as well as numerical methods to study the development of viscoelasticity induced structural instabilities.

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courtesy of Dr. H. Park

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Liquid Crystal Elastomers

lce1Liquid crystal elastomers (LCE) are composed of stiff, rod-like segments (mesogens) connected by a network of flexible polymer chains. The cross linked flexible network is mobile, which allows the initially randomly oriented mesogens to rotate and align to form anisotropic crystalline domains in response to temperature (thermotropic), light (lyotropic), and mechanical loading. A transition from an isotropic disordered state to an oriented state with a single preferred orientation is called an isotropic-nematic transition.  The nematic-isotropic temperature transition allows thermotropic LCE to exhibit a 2 way, reversible shape change, which makes the polymers ideal for soft actuators that require large deformation.  The deformation induced mesogen orientation produce a soft-elastic (super-elastic, pseudo-elastic) response, where small increases in stress results in large increases in strains.  This phenomenon is highly rate-dependent and can produce a  ductile and highly dissipative material.  The goal of our research is to understand the effect of viscoelastic on the temperature-induced 2-way shape change and soft-elasticity behavior.

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