Award Date

12-1-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Committee Member

Kwang Kim

Second Committee Member

Jeremy Cho

Third Committee Member

Woosoon Yim

Fourth Committee Member

Brendan O'Toole

Fifth Committee Member

Pushkin Kachroo

Abstract

Polyvinyl chloride (PVC) gels are a class of electroactive polymer that contain a PVC-based polymeric lattice and various amounts of plasticizer. PVC and plasticizer are industrially available and are commonly used in a wide range of applications including cable insulation, flexible hosing, and healthcare equipment. There are many plasticizer types available, some being extremely biocompatible with implementations in food-safe applications and even food additives for human consumption. Plasticizer weakly bonds to the long polymerized chains of the polymer lattice decreasing the modulus of the composite material. By adding excessive amounts of plasticizer, the bonding sites of these long carbon chains become saturated resulting in free interstitial plasticizer that has the ability to migrate under an applied electric field resulting in the deformation of the material and produces complex electroactive behaviors.The actuation characteristics of this material have been investigated in detail with a somewhat unified theory. The applied electric field causes migration of the interstitial plasticizer due to a change in space charge density at the anode. This results in a plasticizer-rich layer with extremely low material modulus, the compressive force observed in actuation is not completely accepted but can be the result of Maxwell and electrostatic pressures. This results in relatively high strain, near 20%, in a compliant biocompatible actuator. This naturally lends itself to biomechanical and biomimetic applications. The inherent transparency of these materials produces promising applications in varifocal lens technologies. While actuation underlying physics, performance, and applications have been studied in detail the sensing, or mechanoelectrical, properties have not been investigated in any detail. There has been a single study investigating the sensing behavior under impulse-type loading applications where this material is shown to exhibit mechanoelectrical transduction behavior but does not pursue an investigation of signal dependencies, underlying mechanisms, or possible applications of these soft polymer gel sensors. This qualitative study outlines the extent of the known mechanoelectrical transduction properties of these complex electroactive polymers. The properties, underlying mechanisms of mechanoelectrical transduction, and potential applications of these materials are widely unknown. This work aims to provide a fundamental foundation for these soft polymeric gel sensors. This is completed through four major branches including characterization of the mechanoelectrical transduction behavior through experimentation, presentation of a theoretical framework for underlying physics-based mechanisms, mathematical modeling of the mechanoelectrical behavior for prediction of sensor behaviors, and implementation of these unique sensors into in-lab demonstrations for potential applications to broaden the scope and use of these soft polymer gel sensors. The characterization is limited to compressive behavior due to complexity and possibly alteration of underlying mechanisms under varying mechanical deformations. A variety of plasticizers are investigated for sensing characteristics including environmentally friendly and biocompatible plasticizers for possible biomechanical or healthcare applications. This work explores other polymer lattice structures, such as thermoplastic polyurethane (TPU), to exhibit mechanoelectrical transduction in soft polymer gel sensors. This is foundational in that TPU has not been known to exhibit electroactive properties, and neither electromechanical nor mechanoelectrical transduction properties have been investigated. This work essentially shows the decoupling of mechanical and electrical properties of these soft polymer gel sensors to tune both independently for desirable material and sensor characteristics. The soft polymeric gel sensors used in experiments are approximately 12 mm in diameter with a 1 mm thickness and have sensitivity to strain inputs of less than 1%. This high sensitivity is exhibited across multiple plasticizers and polymer lattice structures and produces electric potentials in the mV range. The theoretical framework presented explains experimental phenomena and draws from electromechanical actuation theory. The theoretical framework is based on adsorptive theory in mobile interstitial plasticizer migrates upon material compression. The Langmuir isotherm is fit to steady-state experimental results for soft polymeric gel sensors and is also nondimensionalized to show the classical S-shape and ‘halfway point”. This is typically dependent on the bulk concentration of the adsorptive species, which may change as PVC and other polymer lattice structures are highly compressible. This theoretical framework may not provide all underlying physics but lays a foundation of underlying mechanisms and important material properties for mechanoelectrical transduction in soft polymer gel sensors. The mathematical model is based on the adsorptive theory developed during the experimentation of these gel sensors. COMSOL Multiphysics is used to couple solid mechanics, electrostatics, and transport properties of these materials during sensing of compressive deformation. This portion contains an extensive material property study including viscoelastic and hyperelastic properties. Viscoelastic properties are modeled through a standard linear solid model while hyperelasticity is modeled through a highly compressive Storakers material model. This model is fit to experimental data and shows very low residuals with a maximum of 0.646 kPa through the experimental region of 0 kPa to 50 kPa. This model may not incorporate all physics associated with mechanoelectrical transduction but does provide a predictive multiphysics model for mechanoelectrical transduction behavior of sensors with known material properties. Implementation of these soft polymer sensors presented includes both soft artificial skin and low-visibility flow sensing applications. A biocompatible soft polymer sensor has been applied to soft artificial skin applications and by the use of segmented electrodes to display locational sensing abilities. This soft robotic skin is highly compliant with tunable mechanical properties and can sense static and dynamic loading applications. The low-visibility flow sensing application presented shows soft polymer gel sensors in various orientations to measure flow rates from both time and frequency domain signals. Some initial work shows the potential of these sensors to detect vortex shedding frequencies and flow rate through the Strouhal-Reynolds relationship. This work aims to characterize the sensing abilities and provide a foundational theory for the underlying mechanisms of mechanoelectrical transduction in these soft polymer gel sensors. The mechanoelectrical properties of these soft sensors are widely unknown and this work provides a systematic approach to investigating the underlying mechanisms of mechanoelectrical transduction within these materials. The ability to tune both the mechanical and electrical properties of these sensors allows for possible use in a broad range of applications. This study acts as a fundamental framework for soft polymeric gel sensors.

Keywords

compliant sensor; electroactive polymers (EAP); polyvinyl chloride (PVC) gel; smart materials; soft polymer gel; soft sensing

Disciplines

Mechanical Engineering

File Format

pdf

File Size

9900 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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