Lisa Mauck Weiland

Prediction of the ionic polymer transducer sensing of shear loading

While ionic polymer transducers (IPTs) have often been studied as actuators, they also show considerable promise as sensors. In addition to responding to bending, IPT sensors can also detect compression, tension and shear. Existing IPT models focus primarily on actuation, while the few available sensing models are limited to bending. This is due in part to the lack of a viable hypothesis for a physical mechanism that can explain the existence of a sensing signal in all deformation modes. Identification of the fundamental sensing mechanism is desirable as it could ultimately be exploited to design IPT sensors with an optimized response. Here it is hypothesized that the dominant physical mechanism for sensing is streaming potential, as it holds promise for explaining the existence of IPT sensing in any mode of deformation. We present a model for predicting the current developed when an IPT is subject to shear deformation, the creation of which has previously been elusive. The presented study employs a finite element approach to predict diluent flow with respect to the surrounding hydrophobic amorphous region under shear loading. Assessment of multiple flow path orientations with respect to load is used to impose volume averaging, and subsequently to project transducer current. Both peak and transient response are considered. Model predictions compare well with previously reported experimental results.

Light Activated Shape Memory Polymer Characterization

Since their development, shape memory polymers (SMPs) have been of increasing interest in active materials and structures design. In particular, there has been a growing interest in SMPs for use in adaptive structures because of their ability to switch between low and high stiffness moduli in a relatively short temperature range. However, because a thermal stimulus is inappropriate for many morphing applications, a new light activated shape memory polymer (LASMP) is under development. Among the challenges associated with the development of a new class of material is establishing viable characterization methods. For the case of LASMP both the sample response to light stimulus and the stimulus itself vary in both space and time. Typical laser light is both periodic and Gaussian in nature. Furthermore, LASMP response to the light stimulus is dependent on the intensity of the incident light and the time varying through the thickness penetration of the light as the transition progresses. Therefore both in-plane and through-thickness stimulation of the LASMP are nonuniform and time dependent. Thus, the development of a standardized method that accommodates spatial and temporal variations associated with mechanical property transition under a light stimulus is required. First generation thick film formulations are found to have a transition time on the order of 60 min. The characterization method proposed addresses optical stimulus irregularities. A chemical kinetic model is also presented capable of predicting the through-thickness evolution of Youngs modulus of the polymer. This work discusses in situ characterization strategies currently being implemented as well as the current and projected performance of LASMPs.

Modeling the proton sponge hypothesis: examining proton sponge effectiveness for enhancing intracellular gene delivery through multiscale modeling

Dendrimers have been proposed as therapeutic gene delivery platforms. Their superior transfection efficiency is attributed to their ability to buffer the acidification of the endosome and attach to the nucleic acids. For effective transfection, the strategy is to synthesize novel dendrimers that optimize both of these traits, but the prediction of the buffering behavior in the endosome remains elusive. It is suggested that buffering dendrimers induce an osmotic pressure sufficient to rupture the endosome and release nucleic acids, which forms to sequestrate most internalized exogenous materials. Presented here are the results of a computational study modeling osmotically driven endosome burst or the ‘proton sponge effect.’ The approach builds on previous cellular simulation efforts by linking the previous model with a sponge protonation model, then observing the impact on endosomal swelling and acidification. Calibrated and validated using reported experimental data, the simulations offer insights into defining the properties of suitable dendrimers for enhancing gene delivery as a function of polymer structure.

High Energy Density Nastic Materials: Parameters for Tailoring Active Response

The engineered active material considered mimics bulk deformation similar to nastic movements in the plant kingdom. Controlled transport of charge and fluid across a selectively permeable membrane employing biological processes is employed to achieve bulk deformation. The membrane may contain biological ion pumps, ion channels, and ion exchangers surrounding a spherical inclusion in a polymer matrix; in this study only ion pumps and exchangers are considered. This work examines parameters of significance to designs employing active materials, including free strain, blocked stress, impedance matched work, and rate of response. The effects of varying material design parameters, such as system geometry and membrane permeability are considered to aid in the custom design of an active material appropriate to a given application. Predictions suggest that the rate of initial response may be on the order of msec with impedance matched work in excess of 200 kJ/m3.

Strain induced anisotropic properties of shape memory polymer

Heat activated shape memory polymers (SMPs) are increasingly being utilized in ambitious, large deformation designs. These designs may display unexpected or even undesirable performance if the evolution of the SMPs mechanical properties as a function of deformation is neglected. Yet, despite the broadening use of SMPs in complex load bearing structures, there has been little research completed to characterize how the material properties change upon application of large strain. The following is an experimental investigation into the strain induced anisotropic properties of the SMP Veriflex®. It is found that under large uniaxial strain the SMPs stiffness in the transverse direction can be reduced as much as 86%, while the toughness in the axial direction may increase by an order of magnitude in some cases. A generalized analysis suggests that this trend should be expected for any SMP.

Monte Carlo simulation of a solvated ionic polymer with cluster morphology

A multiscale modeling approach for the prediction of material stiffness of the ionic polymer Nafion is presented. Traditional rotational isomeric state theory is applied in combination with a Monte Carlo methodology to develop a simulation model of the conformation of Nafion polymer chains on a nanoscopic level from which a large number of end-to-end chain lengths are generated. The probability density function of end-to-end distances is then estimated and used as an input parameter to enhance existing energetics-based macroscale models of ionic polymer behavior. Several methods for estimating the probability density function are compared, including estimation using Johnson distributions, Bezier distributions, and cubic splines.

Ionic polymer transducers in sensing: Implications of the streaming potential hypothesis for varied electrode architecture and loading rate

Explored is the mechanism of streaming potential in the sensing response of ionic polymer transducers, sometimes referred to as ionic polymer metal composites. It is argued that, unlike previous hypotheses, the streaming potential hypothesis is insensitive to the assumed ionomer morphology, and accommodates experimental observation of sensing under shear loading. As demonstration a simplified model for sensing under bending deformation is presented for a material system displaying a cylindrical channel morphology. Specific demonstration scenarios considered include predicting the evolution of sensing with electrode particulate volume fraction and type. In addition, evolution of sensing for both step and oscillatory loading is considered.

Mechanical and Curing Properties of a Styrene-based Shape Memory Polymer

Presented is an experimental investigation into the characteristics of a particular styrene-based shape memory polymer, Veriflex®. Tensile, 3 point bend, and creep tests are conducted yielding the Young’s modulus, yield strength, flexural modulus, flexure strength, and creep modulus of the polymer both above and below the glass transition temperature. The results of the characterization may be used to populate a linear constitutive model appropriate for design. To populate the constitutive model parameters for viscosity, retardation time, and coefficient of thermal expansion are also developed. Initial experiments in this effort revealed the tendency for residual monomer to remain after curing. While the residual monomer ultimately evaporates from the sample, when present it acts as a plasticizer thereby increasing the glassy state compliance. Thus, also presented are strategies employed for eliminating residual monomer during sample preparation.

The streaming potential method for modeling the electromechanical responses of ionic polymer transducers

A streaming potential method for modeling the electromechanical responses due to imposed deformation of ionic polymer transducers (IPTs) is presented. It has been argued that imperfect ion pairing results in the availability of free counterions within the hydrophilic regions, thereby resulting in the presence of an electrolyte within these regions in the hydrophobic polymer matrix. When there is a net relative motion of this electrolyte with respect to the electrode, a streaming potential should result. It is hypothesized that a streaming potential mechanism within the electrode regions should be able to predict sensing responses for all modes of deformation. Based on a recently introduced parallel waterchannel morphology in Nafion® membrane, this model successfully addresses the physics of sensing in IPT bending. A linear relationship between the tip deflection of an IPT cantilever beam and the current generated in the IPT is achieved. The result trends show a good agreement with the experimental measurements. While this work studies the bending mode, it is able to be adapted for the other three sensing modes.

Experimental investigation of the streaming potential hypothesis for ionic polymer transducers in sensing

Ionic polymer transducers (IPTs) are ionomers that are plated with conductive media such as metals, leading to capacitive behavior. IPTs exhibit bending deformation when a voltage difference is applied across the surfaces of the transducer, thus displaying actuation. A current is generated when they are deformed, thus exhibiting sensing. However, the mechanisms responsible for actuation and sensing differ; research to date has focused predominantly on actuation, while identification of the dominant mechanism responsible for IPT sensing remains an open topic. The goal of this work is to initiate experimental investigations of the streaming potential hypothesis for IPT sensing. This hypothesis argues that the presence of unbound counter-ions within the hydrophilic phase of an ionic polymer behaves as an electrolyte in the presence of the electrode. Thus, as per classic streaming potential analyses, relative motion of the electrolyte with respect to the electrode will result in the evolution of a streaming potential. According to this hypothesis, the extent of communication between the electrode and electrolyte becomes important in the evolution of an electrical signal. This study experimentally explores the effect of electrode architecture on the sensing response where the IPTs are prepared via the direct assembly process (DAP). The DAP is selected because it enables control over the fabrication of the electrode structure. In this study, cantilevered IPT samples having different electrode composition are tested under several step input tip displacements. The experimental outcomes are consistent with predicted trends via streaming potential theory. (Some figures may appear in colour only in the online journal)