Author: Nirmal Sigamani

Publisher:

Published: 2015

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Both ionic and electronic electroactive polymers (EAPs) have displayed great potential as actuators. Current ionic EAPs have limited practical application due to their slow response time and their low blocked force; furthermore, their ion transport-based mechanism necessitates the presence of an electrolyte, which complicates issues of packaging and device lifetime. On the other hand, despite the advantages of electronic EAPs such as their efficient electromechanical coupling and relatively rapid response time, there are major obstacles blocking their transition to application as well; most notably, they require high actuation voltages (threshold voltage needed to generate electroactive strain) and they have low blocked stress (the stress at which the actuator stops moving). Hence, the main objective of this study was to develop a new kind of polymer nanocomposite for actuator applications that would exhibit simultaneous improvement in both electromechanical response and strain energy density. To fulfill this objective, existing PVDF-based electroactive polymers were modified using different types of carbon nanofillers. An effort was made to observe the impact of these nanofillers on the microstructure of the polymer that would then lead to a better understanding of the maximum possible improvement of the electromechanical response. As a first step, we investigated the impact of the 2-dimensional GO and reduced GO on the electromechanical response of PVDF, a polar polymer. The 1 wt % reduced-GO-PVDF nanocomposites showed a tremendous improvement in dielectric permittivity and electrical conductivity. The dielectric permittivity at 1 KHz increased almost eight fold, while the electrical conductivity showed an increase of four orders of magnitude in comparison to the corresponding values for the unmodified PVDF. The reduced GO-PVDF polymer films showed a bending actuation response with a DC electric field, thus demonstrating its potential as EAP. The mechanism responsible for this bending actuation response is determined to be electrostriction, because the strain (S11) exhibited a quadratic response with the applied electric field while Joule heating and Maxwell stress effects were shown to be negligible. The coefficient of electrostriction value (M1133) for the 1 wt % reduced GO-PVDF was found to be 1.7 x 10-16 (m2/V2), which is higher than that for most of the existing electrostrictive polymers like polyurethane and PVDF TrFE CTFE terpolymer, whose values lie in the range of 14 x 10-18 (m2/V2) to 8 x 10-18 (m2/V2). Although coefficient of electrostriction of reduced GO-PVDF is higher than most of the existing electroactive polymers, the relatively high electrical conductivity and low breakdown limits their use for practical applications. So next step was to exploit the advantages of a conductive carbon nanostructure while controlling its network to better impact its electrical properties which could also lead to higher breakdown strength. To achieve this, the impact of the hybridization between SWNT and GO on the microstructure and the electrical properties of PVDF was studied. Increasing the content of insulative GO helped to disrupt the percolated network of the SWNT, lowering the electrical conductivity and dielectric loss. The synergistic effect of the hybrid nanofillers on the microstructure of PVDF was then analyzed. The hybrid nanofillers had a favorable influence on crystallization, leading to a higher degree of crystallinity. Enhancement in the ferroelectric strain for the stretched nanocomposite was observed. Due to the hybridization of SWNT and GO and subsequent stretching, a high dielectric breakdown strength of 140 MV/m was found for a nanocomposite with a 0.25 wt % SWNT and 0.25 wt % GO compared to 0.6 MV/m for 0.25wt% SWNT-PVDF.Based on the promising impact of hybrid nanofillers on the ferroelectric polymer PVDF, a similar polymer with a relaxor ferroelectric character is considered owing to its higher inherent electroactive response and higher breakdown strength. Given that it is not broadly studied, there was a need to understand structure-property relationship of the PVDF TrFE CTFE terpolymer. Hence, the effect of processing conditions (such as annealing times and isothermal crystallization temperatures) on the microstructure and the subsequent electromechanical properties were analyzed. This structure-property analysis helped to understand the relation between the different types of crystalline phases and the degrees of crystallinity as well as to observe crystal sizes as they relate to the electric field induced strain. It was found that a higher degree of crystallinity was required to achieve a higher strain; at the same time, if the average crystal sizes are high, then the crystals act as a hindrance to random dipole movement and thus detrimentally affect the electroactive strain. Next, with the better understanding about the structure-property of terpolymers, the effect of adding SWNTs was investigated. The dispersion of the SWNT terpolymer nanocomposites using both physical dispersion and chemical dispersion (APS modification) was studied. The APS modification helped to achieve a good dispersion in the nanocomposites, which delayed the percolation of the SWNT network, leading to a low dielectric loss and low electrical conductivity. The inclusion of SWNT-APS also changed the microstructure of terpolymers; these films showed a lower % crystallinity compared to that for pure terpolymer and low proportion of [alpha]-phase, especially at lower weight fractions of SWNT-APS. As a final step, the effect of the hybrid SWNT/GO on both microstructure and electromechanical properties of the terpolymer were studied. The hybrid nanofillers were chemically modified to form a covalent bond between them to improve their interaction. The morphology of the hybrid nanofillers after the chemical modification was studied for two different chemical modification routes: one using thionyl chloride, other using NHS and EDAC as catalysts. Of the two methods, the NHS and EDAC catalyst method showed a strong uniform interaction, confirmed by SEM images and FTIR results, with a shift in the peak to 1630 cm-1. Finally, the effect of hybrid SWNT and GO on the electromechanical properties were studied and, interestingly, the hybrid terpolymer nanocomposite film showed a lower electroactive strain compared to pure terpolymer at the same applied electric field. WAXS and DSC results suggest that this reduction is partly due to the change in the crystallinity and to the SWNT hindrance effect on the crystalline phase transformation which is responsible for the electroactive strain. In this dissertation, it was successfully shown that using hybrid SWNT-GO both high coefficient of electrostriction (increase by 60 %) and high breakdown strength (140 MV/m) can be achieved by exploiting the actuation capabilities of SWNT in PVDF while GO acted as insulative filler. Also, the type of the fillers in the nanocomposites route had a strong influence on the actuation mechanism of relaxor ferroelectric polymers. The microstructure-property study highlights the importance of choosing the right type of nanofillers for further advancement in the field of EAPs.