Defense Date

2005

Document Type

Thesis

Degree Name

Master of Science

Department

Mechanical Engineering

First Advisor

Dr. Zoubeida Ounaies

Abstract

The main objectives of this thesis are 1) to evaluate the effect of cross-linking polar cyano phenyl (CN) groups on poly (dimethyl siloxane) (PDMS) and 2) to characterize the electromechanical properties of the resulting CN-PDMS blend as an electroactive actuator. Materials responding to an external stimulus are referred to as electroactive materials. There are several phenomena, which govern the mechanism in these materials, such as piezoelectricity, Maxwell's effect, ferroelectricity, electrostriction to name a few. These electroactive materials can be employed in several applications such as biomedical devices, robots, MEMs, aerospace vehicles, where the application is governed by the specific mechanism. However in order for the materials to be used effectively, they need to be thoroughly characterized to understand their behavior under factors like electric field, temperature, frequency and time.The present work focuses on developing an electroactive actuator, which has tailorable properties, allowing a wide operational temperature window from -100°C to 200°C and stability in harsh conditions. The characterization of the CN-PDMS polymer blend is done in two folds. First the physical properties of the polymer system are characterized by performing tests such as Dielectric Spectroscopy, Differential Scanning Calorimetery and Thermally Stimulated Current measurement. These techniques offer complete understanding of the structure-property relationship and effects of the functional groups on the dielectric and relaxation behavior of the polymer. The Dielectric Spectroscopy and the Thermally Stimulated Current analysis are used to elucidate the primary and the secondary relaxations, such as molecular mobility, interfacial polarization and dipolar relaxation. Dielectric Spectroscopy reveals that the molecular weight of PDMS does not affect the dielectric permittivity of the polymer blend. Also, Dielectric Spectroscopy clarifies the role of the CN polar group in the polarization of the CN-PDMS blend, inducing electromechanical strain in the polymer blend through electrostriction.The Differential Scanning Calorimetery is used to quantify the thermal behavior of the CN-PDMS polymer blend by quantifying properties such as melting temperature (Tm) and re-crystallization temperature of the PDMS polymer cross-linked with CN functional group. Results reveal that the thermal characteristics of the blend are not affected when PDMS is cross-linked with the functional CN moieties, meaning CN-PDMS maintains the advantages of PDMS in terms of stability towards harsh conditions, wide operating temperature and resistance to ultraviolet radiations.Following the physical characterization, electromechanical characterization of the CN-PDMS polymer blend is done to assess the electromechanical strain induced in the blend in response to electric field. The electromechanical strain is studied in two configurations; the electromechanical strain induced along the length of the polymer blend and induced through the thickness of the blend. These strain measurements are performed by applying both direct current as well as alternating current electric fields, and the induced electromechanical strain is studied as a function of amplitude and frequency of the electric field as well as the time of application of the electric field. The mechanism behind the development of the electromechanical strain and the nature of the strain under electric field is elucidated. The performance of the electroactive polymer is compared with several other polymeric actuators such as PVDF and PVDF-TrFE, polyurethane based actuators and ionomers. Comparison gives favorable results in terms of strains. In addition, CN-PDMS polymer system has the advantage of allowing control of processing of the blend, which is not present in all the other commercial electroactive polymers. The maximum electromechanical strain yielded along the length of the CN-PDMS polymer blend is 1.74 % when an electric field of 0.2MV/m is applied along the length of the polymer. Through the thickness, the maximum induced strain is 0.12 % for an electric field of 0.8 MV/m. Based on the nature of the strain yielded it is observed that the strain induced in the CN-PDMS blend is consistently proportional to the square of the electric field (E2). Moreover, the strain is driven by the concentration of the dipolar moieties (CN) present in the polymer blend.All the above-mentioned techniques used for thermal and electromechanical characterization of the CN-PDMS polymer blend illustrate the electrostrictive nature of the polymer under the study.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

June 2008

Included in

Engineering Commons

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