TITLE: TUNABLE FLEXOELECTRICITY IN ADDITIVELY MANUFACTURED POLYMERS

ABSTRACT: Flexoelectricity is the electrical response that originates when insulating materials are subjected to a strain gradient. This effect is generally considered to be small but known to depend sensitively on material microstructure. This research first explores the hypothesis that the microstructure produced by additive manufacturing (AM) can strongly influence flexoelectric response. It is found that minor changes to this microstructure influenced by fused filament fabrication, a mainstream approach for additively manufacturing thermoplastics, can lead to enormous changes in the magnitude and polarity of the flexoelectric response of polylactic acid (PLA). To explain these changes, a layer dipole model (LDM) is proposed that connects the in-plane shear in each layer to the electrical polarization that it produces. This model explains three independent mechanisms that were identified and that collectively allow one to drastically increase the flexoelectric effect by 173 fold: (1) choosing printing settings to optimize the geometry of pores between extruded lines, (2) choosing the infill of each layer such that bending-induced strain produces productive in-plane shear stresses, and (3) post-deposition annealing of the printed material to increase its crystallinity. Further implementation of this model seeks an understanding of this effect in a wider array of material compositions for improved integration into strain sensors. A shear piezoelectric effect has been reported in PLA, and the impact on its flexoelectric response will be studied through comparison with other polymers prepared with identically optimized manufacturing conditions. Annealed PLA coupons differing seemingly only in color have also been observed with flexoelectric coefficients of varying magnitudes and opposite polarities. Future research will focus on the dependence of the flexoelectric effect on the profile of additives present in a 3D-printing filament. Material characterization methods such as X-ray Diffraction and Differential Scanning Calorimetry will be employed on commercially available polymer filaments, while custom extrusion will be exploited to identify flexoelectric alterations in selectively chosen material compositions. Finally, these fundamental studies will be integrated in a strain-sensing application in which 3D-printed structures are displaced by vibrations along its surface. Solidworks simulations will pair with these experiments to propose expected natural frequencies and modal shapes, allowing for an informed placement of electrodes for amplified polarization measurements at vibrations of chosen frequencies. The LDM will act alongside simulation results to support this unique tuning of strain sensing properties in 3D printed polymers. This understanding will enable future sensors in which the structural material is also responsible for electromechanical functionality.

COMMITTEE: ADVISOR/CHAIR Professor Keith Brown, ME/MSE/Physics; Professor Joerg Werner, ME/MSE; Professor Scott Bunch, ME/MSE