Abstract
Soft actuators are increasingly being required for a variety of application ranging from robotics to biomedicine. This work reports on the development of printable materials for soft actuator applications based on ionic liquids (ILs) and a fluorinated polymer, poly(vinylidene fluoride) (PVDF). ILs sharing the same cation 1‑butyl‑3-methylimidazolium, [Bmim]+ and different anions (tricyanomethanide, [C(CN3)]−, dicyanamide, [N(CN2)]− and thiocyanate, [SCN]−) were incorporated into the PVDF polymer matrix at 40% wt. and processed by direct writing printing technique.Rheological measurements of the IL/PVDF solutions allowed to stablish a correlation between shear stress and viscosity, being observed a shear thinning behavior. Independently of the IL anion, the inclusion ILs leads to variations in the sample morphology related to the formation of significantly smaller spherulites than in PVDF with well-defined borders and an increase of the electroactive β phase content and crystallinity degree of the polymer. The incorporation of the ILs into the PVDF matrix induces a mechanical plasticizing effect. A maximum ionic conductivity of 5.2 × 10−5 S/cm has been achieved for the [Bmim][N(CN2)]/PVDF composite. The potential of the developed printable materials as soft actuators has been experimentally demonstrated and theoretically evaluated, the highest displacement of 1.0 mm at an applied voltage of 4 Vpp being obtained for [Bmim][SCN]/PVDF. Finally, the implementation of an all printed micro gripper shows the potential of the materials for applications.
Highlights
Additive manufacturing technologies are being increasingly developed and applied in recent years, due to their strong potential in a wide variety of applications, improved integration of materials into devices, the ability to obtain rapid prototypes and solutions in nonconventional shapes, design versatility, compatibility with a wide range of materials, low costs and environmental friendliness [1]
Of the anion type and, even when the crystallization phase is mainly determined by the processing temperature [63], as discussed before, the presence of the ILs leads to an increase of the β phase content with respect to pristine PVDF from 67% to 82%
For all the printed materials, PVDF crystallizes into the β phase due to the low temperature processing and, independently of the anion type, the incorporation of ILs into the PVDF matrix leads to a further increase of the electroactive phase content as a result of the ion-dipole interactions between the IL anions and cations a the PVDF dipoles
Summary
Additive manufacturing technologies are being increasingly developed and applied in recent years, due to their strong potential in a wide variety of applications, improved integration of materials into devices, the ability to obtain rapid prototypes and solutions in nonconventional shapes, design versatility, compatibility with a wide range of materials, low costs and environmental friendliness [1]. Electroactive polymers (EAPs) are being developed for applications in robotics [26], environmental sensing and remediation [27] or tissue engineering [28], due to their high mechanical flexibility, low density, biocompatibility and processability in a variety of shapes [29] These materials are suited for sensor and actuator applications, due to their high response, versatility, pattern ability, and compatibility with low temperature fabrication processes [24]. The influence of the fluorinated matrix type was addressed upon the incorporation of the IL [Emim][TFSI] into PVDF and its co-polymers being observed a clear matrix type dependence [30] In this scope, a new interesting and needed approach relies on the development of printable actuators. The IL/PVDF printed films morphology, physical-chemical, thermal and mechanical properties were studied, together with their performance as soft actuators and their implementation into a micro gripper device
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