Research On Electro-mechanical Performance Of Polyurethane Dielectric Elastomer Composites

Dielectric elastomer electro-active polymer is a kind of novel intelligent material.Dielectric elastomers have attracted considerable interest,especially following the publication of elevated electric field-induced strain.They offer good performance such as large electric induced deformation,fast response speed,high flexibility,light quality and good flexibility in an unusually broad range of applications covering aeronautics and astronautics,mechanical engineering,biomedical fields,and so on.As one of the most important dielectric elastomers,polyurethane has a rather low modulus,a fast speed of response,high efficiency,excellent biocompatibility,high electromechanical coupling efficiency,high reliability,and high durability,thus it becomes one of the most promising classes of ideal flexible micro-actuator materials.However,it has low dielectric constant just as the other dielectric elastomers.A large number of attempts have been made to increase the dielectric constant and electro-mechanical performance of the polyurethane elastomers to meet the needs in electromechanical devices.In this paper,we adopt two major strategies to improve the electro-mechanical performance of the polyurethane elastomers.On the one hand,in order to achive the high dielectric constant we add graphene and functionalized graphene conductive nanofillers into the polyurethane matrix to improve the dielectric constant and generate large-strain dielectric composites based on the percolation theory.On the other hand,in order to achive the low modulus small molecule polyol fillers are utilized to weaken the hydrogen bond interactions of polyurethane chains.We finally designed and prepared a series of novel polyurethane dielectric elastomer composites.The fabrication process of composites,micro-structure of functional fillers,and interfacial interaction between filler-matrix has been discussed.The relationship between the material micro-structure and electric induced strain has been focused.Furthermore the influence mechanism of interfacial effect on electric field induced strain and electro-mechanical properties of the flexible micro-actuator unit has been revealed.The main work in this dissertation is summarized as follows:1.Graphene can induce a considerable improvement in the mechanical,thermal,and electrical properties of the resulting graphene-polymer composites at very low loading contents.However,graphene is an atomically thin sheet of sp2 bonded carbon atoms.Good dispersion of such hydrophobic fillers in polymer is a processing challenge.We observed that there are lots of obvious holes and flaws in graphene-polyurethane composite.The apparent phase separation is attributed to the poor dispersion and serious graphene aggregation.So we here demonstrate that polyurethane dielectric elastomer filled with titanium dioxide functionalized graphene,fabricated as a flexible micro-actuator,displays evident electric stimulus response and electric field induced strain.The hydrophilicity and wettability of graphene can be greatly enhanced by decorating nano-TiO₂ onto graphene,which causes the improved interface interactions between fillers and polymers.Nano-decoration has no effect on relaxation components and crystallization structures of dielectric elastomer.Higher dielectric constant,less loss modulus,elevated breakdown strength,and better electric stimulation responses are achieved due to enhanced filler-matrix interface.The electric induced strain of functionalized graphene-polyurethane composite is 1.8 times of that of graphene-polyurethane.2.In order to increase the organic functional groups on the surface of the graphene to strengthen the interface interacts of graphene and the polyurethane matrix,the surface of the graphene is covalently functionalized by flexible organic molecules to improve the dispersion of graphene in polyurethane matrix.Poly?methyl methacrylate?-functionalized graphene and hyperbranched aromatic polyamide functionalized graphene nanocomposites are prepared by atom transfer radical polymerization and hyperbranched polymerization.The functionalization of graphene nano-fillers can greatly improve the dispersibility of graphene in polyurethane.This allows for construction of more micro-capacitors in composites,leading to higher dielectric and electric induced strain performance of functionalized graphene-polyurethane dielectric elastomer.The largest electric induced strain value of poly?methyl methacrylate?functionalized graphenepolyurethane is 1.2 times of that of graphene-polyurethane under 1000 V direct current field.Meanwhile the largest electric induced strain value of hyperbranched aromatic polyamide functionalized graphene-polyurethane composites is 2.2 times of that of graphene-polyurethane under 38 MV/m alternating electric field.3.Generally covalent functionalization of graphene tends to disrupt the sp2-hybridized network required for good electron/hole conduction,thus compromising the electrical conductivity.Noncovalent functionalization of graphene sheets through CH-? and/or ?-? interactions are the preferred choice for tuning the interfacial properties without compromising conductivity.In order to fully harness the exceptional properties of ultra-high dielectric constant copper phthalocyanine oligomer fillers and graphene nanofillers,the surface of graphene is noncovalent functionalized with copper phthalocyanine to form the novel copper phthalocyanine-graphene hybrid structures.The electromechanical property of a diaphragm type actuator comprising a copper phthalocyanine oligomer noncovalent functionalized graphene-polyurethane dielectric elastomer composite is described.The composite film had a dielectric constant of 102 with a dielectric loss of 0.13 at 3.0 wt% hybrid filler under 1 k Hz.At 37 MV/m,the electric field induced strain of copper phthalocyanine noncovalent functionalized graphene-polyurethane is more than 1.8 and 1.6 times higher than that of copper phthalocyanine-polyurethane and graphene-polyurethane.Furthermore,a noncovalently interface modified layer was designed and built up by using conducting polymers that bridged the polyurethane dielectric elastomer matrix molecules with graphene platelets.The dielectric constant and the electromechanical actuation strain of conducting polymers functionalized graphene-polyurethane dielectric elastomer composites were improved dramatically in comparison with the pure polyurethane and graphene-polyurethane composites.A large increase in electric field induced thickness strain was obtained from 27% for pure polyurethane to 68% and 98% with the addition of 3.0 wt% polyaniline and polypyrrole functionalized graphene under a 38.5 V/?m electric field.Excitingly ultra high permittivity and significantly enhanced electric field induced strain has been found in polyurethane composites modified by poly?3,4-ethylenedioxythiophene?-poly?styrenesulfonate?conducting polymer noncovalent functionalized graphene.The dielectric composite with the unique “sandwich” structure composite exhibits ultra high permittivity?350 at 1 k Hz?,low dielectric losss?⁰.2 at 1 k Hz?,low loss modulus?200 MPa?,and low loss tangent??0.4?.The maximum thickness strain of 164% is significantly higher than reported values for polyurethane elastomers and nanocomposites.4.Because a large number of hydrogen bonds exist in polyurethane,these hydrogen bonds largely limit the mobility of the polarized groups of polyurethane molecules,and thus the polarization ability of polyurethane chains.Thus,based on this point,small molecule polyol fillers are utilized to weaken the hydrogen bond interactions of polyurethane chains in the final study of this paper.We expect that the dielectric constant of polyurethane/polyol blends can be increased by the increase in the dipole polarization ability of polyurethane chain segments by the weakening of hydrogen bonds of polyurethane chains.Meanwhile,the modulus of polyurethane/polyol blends might decrease by the weakening of hydrogen bonds in the hard segment of polyurethane and the plasticizing effect of polyols in soft segment of polyurethane matrix via van der Waals effects.The simultaneous increase in dielectric constant and decrease in modulus can result in obvious increase in electromechanical sensitivity and actuated strain.The polyol-polyurethane all organic composites have better electromechanical response than the pristine polyurethane.The results show that the polyurethane/glycerol sample presents higher strain levels than the pure polyurethane,polyurethane/ethylene glycol,and polyurethane/triethylene glycol counterparts.The highest actuation strain is obtained for the polyurethane/glycerol blend with a mass ratio of 50%.It has the dielectric constant of 32.71 at 40 Hz,tensile strength of 16.37 MPa,compliance of 9.71×10-5 1/Pa.The maximum thickness actuation strain reaches 49.7% at an electric field?37.4 MV/m?,which is nearly a 2-fold increase over that of pure polyurethane?30.7%?.