ELECTROACTIVE POLYMER-BASED MATERIALS FOR RESPONSIVE ORIGAMI-INSPIRED STRUCTURES
Open Access
- Author:
- Ahmed, Saad
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 13, 2017
- Committee Members:
- Zoubeida Ounaies, Dissertation Advisor/Co-Advisor
Zoubeida Ounaies, Committee Chair/Co-Chair
Mary I Frecker, Committee Member
Paris R Vonlockette, Committee Member
Michael T Lanagan, Outside Member
Timothy William Simpson, Committee Member - Keywords:
- Electroactive polymer
Origami-inspired engineering
Self-clearing
Electrode
Dielectric breakdown
Actuators
P(VDF-TrFE-CTFE) - Abstract:
- With the ever-growing usage of technology in our day-to-day life, the demands on existing energy sources, sustainable technology, and space requirements are also increasing. As a result, increasing interests are focused on technologies with benefits such as reduced space requirements, reduced material consumption, and improved energy efficiency. Origami-inspired engineering has gained much attention among scientists, engineers and mathematicians as an emerging form of technology with potential advantages such as relatively simple assembly process, realization of a large number of structures/shapes from a single sheet and finally, capability of folding into a compact shape and then unfolding (deploy or launch) into a larger-complex shape. Self-folding mechanism coupled with origami-inspired engineering is of particular interest where the structure has the ability to fold and unfold in response to an external stimulus without manual assistance. Researchers have investigated several self-folding mechanisms to realize origami-inspired engineering. In this work, an electroactive polymer (EAP), more specifically poly (vinylidende fluoride-trifluoroehtylene-chlorotrifluoroethylene) P(VDF-TrFE-CTFE), is studied and implemented to achieve origami-inspired self-folding structures. P(VDF-TrFE-CTFE) is used to realize electric field driven origami-inspired smart structures because of high room temperature dielectric constant (~50), fast response and reversible actuation mechanism. Processing condition of P(VDF-TrFE-CTFE) is studied based on the microstructure and electromechanical properties. Then a detail electrical, electromechanical, analytical, thermal, mechanical characterization on P(VDF-TrFE-CTFE) is performed and presented. Through dielectric spectroscopy it is observed that the Curie transition of terpolymer shows a broad peak unlike normal ferroelectric polymer. Polarization study reveals very slim (less lossy) hysteresis loop. The current understanding of the origin of electrostrictive strain is attributed to phase transition from paraelectric to ferroelectric phases. In this work, electric field in-situ X-ray diffraction (XRD) and sum frequency generation (SFG) study are conducted to investigate and confirm the switching of α and γ phases to polar -phase. To enable electric field-driven on-demand bending and folding of P(VDF-TrFE-CTFE), various approaches are undertaken. First, electric field-driven large bending is achieved using a one layered unimorph actuator. Two approaches are pursued to convert the bending actuation into folding: notches and stiffener. While stiffener approach was not very successful, the notch approach showed more pronounced folding actuation. However, EAP actuators also possess drawbacks that impede their implementation in applications, namely they induce relatively low force and require high actuation voltages. To address these issues, a multilayered unimorph concept is proposed. Due to the complex fabrication process of multilayered unimorph actuator and the large number of experimental parameters, a universal analytical model is developed in this work to guide the experimental design and fabrication of these actuators. The model is based on a set of non-dimensional equations for electric field-induced curvature, tip displacement and blocked force, taking into account the wide range of design parameters (such as thickness, modulus, number of layers, electric field magnitude, etc.). First, analytical results are validated with experimental studies, then the model is used to predict the displacement, curvature, blocked force and maximum work output of multilayered unimorph actuators for various input parameters such as thickness, modulus, number of layers, electric field and stiffness contrast. An advantage of this combined modeling and experimental approach is the ability to maximize the performance of the designed actuator for the particular application of interest; for example, whether displacement or force is the goal would determine the number of layers and type of substrate used. Finally, origami-inspired smart structures are actuated using the insights from the developed electromechanical model. Although the multilayered actuator concept improves the electromechanical performance of P(VDF-TrFE-CTFE) actuator, with the increase of number of polymer layers the possibility of defect driven premature electrical breakdown also increases due to the requirement of high driving voltage. Polymer-based capacitors have the ability to clear defects with partial electrical breakdown and subsequent removal of a localized electrode section near the defect, which is known as self-clearing. A methodical approach to self-clear P(VDF-TrFE-CTFE) terpolymer to delay premature defect-driven electrical breakdown of the terpolymer actuators at high operating electric fields is proposed in this study. Breakdown results show that electrical breakdown strength is improved up to 18% in comparison to a control sample after self-clearing. Furthermore, the electromechanical performance in terms of blocked force and free displacement of terpolymer-based benders are examined after self-clearing and precleared samples show improved blocked force, free displacement and maximum sustainable electric field compared to control samples. The study demonstrates that controlled self-clearing of EAPs improves the breakdown limit and reliability of the EAP actuators for practical applications without impeding their electromechanical performance. In this study, the limitations of P(VDF-TrFE-CTFE) terpolymer are addressed and studied to achieve origami-inspired self-folding structures. First, processing conditions of terpolymer is studied and 9 hours annealing condition is selected based on the improved electromechanical performance. Electric field driven in-situ XRD and SFG is implemented which gives direct experimental evidence of the electric field induced reversible transition of and phases to phase. The feasibility of P(VDF-TrFE-CTFE) to achieve electric field driven on-demand bending and folding is experimentally demonstrated using various geometric approaches. Then, a universal analytical model for an EAP based on beam bending theorem is developed which can also be implemented to design multilayered actuators driven by other physical fields such as magnetic or thermal. The concept to couple active multilayered actuators with inactive origami-inspired structures is introduced and successfully actuated. Finally, a systematic method is introduced to induce controlled self-clearing of P(VDF-TrFE-CTFE), which improves electric field sustaining capability and reliably of P(VDF-TrFE-CTFE) devices.