Synthesis and Self‐Assembly of Diborate Ester Polymers for Dielectric Elastomer Actuators

Inorganic nanoparticles have been widely used as fillers to modify dielectric elastomers to simultaneously improve mechanical and dielectric performances. Elegant interface design is crucial for achieving synergy between fillers and elastomers. Herein, we show that diborate ester polymers can both improve the interfacial compatibility between inorganic nanoparticles and elastomers and enhance the dielectric performances. The diborate ester polymers are derived from the condensation reaction between tetrakis(dimethylamino)diborane (or tetrahydroxydiborane) and a three‐armed catechol monomer. Choosing TiO2 nanoparticles as the inner core, a series of TiO2@diborate ester polymer core‐shell nanoparticles are synthesized. Density functional theory (DFT) calculations indicate that the diborate ester polymer shell tightly bonds to the surface of TiO2 through two dissociative bidentate bridging modes of the catechol monomer (absorption energy: −11.049 eV). These core‐shell nanoparticles can soften and tough polydimethylsiloxane (PDMS) and show enhanced interfacial binding affinity to PDMS than naked TiO2 nanoparticles. The fabricated composites have high dielectric constants (up to 6.8) and low dielectric losses (<0.03). The actuated strain and puncture voltage of the composites reach 25.9% and 80 V μm−1, respectively, which are much higher than those of PDMS and TiO2/PDMS. This work provides a promising route to fabricate core‐shell nanoparticles for high‐performance dielectric elastomers.

obvious advantages in the functionalization of dielectric elastomers due to the synergistic effect between polymeric and inorganic components.For example, coating BaTiO 3 nanowires with a high modulus polymethyl methacrylate shell not only improves the dispersion in the poly(vinylidene fluoride-co-trifluoro ethylene) matrix but also efficiently enhances stress transfer at the interface. [12]Insulating BaTiO 3 in a fluorinated aromatic polythiourea shell can evidently enhance the interfacial polarization of DEs.Through the core-shell structure design, it is easy to adjust the ratio of organic/inorganic components and the filler/matrix interface, which are important for the optimization of the thermal, mechanical, and dielectric properties of DEs.
Catechol derivatives (including catechol functionalized polymers) have been widely used to decorate inorganic materials for creating core-shell structures, due to the strong interface binding affinity of catechol groups. [13]For example, a strawberrylike barium titanate/tannic acid-iron ion/silver dielectric coreshell filler was designed to enhance the mechanical and electrical properties of natural rubber matrix composites. [14]Furthermore, hydroxylation of the filler surface can covalently crosslink natural rubber and provide an efficient energy dissipation mechanism to enhance the dielectric constant of the elastomer. [15]Diboranes containing B-B bonds have B-B (sp 2 -sp 2 ), B-B (sp 2 -sp 3 ), and B-B (sp 3 -sp 3 ) cores.This structural diversity allows a fascinating variety of chemical transformations. [16]For example, diboron compounds can modify the surface of inorganic semiconductor nanoparticles (such as TiO 2 , ZnO, and SnO 2 ), thus regulating the inorganic nanoparticle surface and properties. [16,17]Compared with the boric acid group, the diboron group has a potential advantage in modifying DEs due to its special interfacial regulation effect.Specifically, the Lewis acid diboron group can form a donor-acceptor complex through B ← N coordination, in which the transfer of charge directly affects the change of molecular conformation and the size of molecular polarity.However, the study of diboron compounds has generally focused on small molecules, only a few oligomers and covalent organic skeletons of diborate ester polymers have been researched. [18]erein, we show that the condensation reaction between diboron and catechol molecules adopts a nucleation polymerization mechanism, affording a new one-step route to diborate ester polymer nanospheres.By restricting this nucleation polymerization on the surface of inorganic nanoparticles based on the catechol-surface binding, we develop a general strategy to synthesize inorganic@diborate ester polymer core-shell nanoparticles with precisely controlled shell thicknesses.These core-shell nanoparticles can be used as fillers to enhance the dielectric properties of PDMS.The solution self-assembly of diborate ester polymers is driven by B ← N coordination, while their self-assembly on the surface of inorganic nanoparticles is promoted by the cooperation of catechol-surface binding and B ← N coordination.Density functional theory (DFT) calculations prove the strong interfacial absorption energy between the diborate ester polymer and the inorganic nanoparticles, and the improved interfacial interaction between core-shell nanoparticles and PDMS.These elegantly designed interfacial interactions not only reduce the modulus of the elastomers, but also significantly increase the tensile properties, dielectric constant, and electric actuated performances.

Solution Self-Assembly of Diborate Ester Polymers
We chose tetrakis(dimethylamino)diborane (TAD) to react with a three-arm catechol monomer (TBC, Scheme S1, Supporting Information) to afford diborate ester polymers (ABBP, Figure 1a).Optical photos shown in Figure S1, Supporting Information illustrate the formation of a brownish-yellow turbid suspension.Scanning electron microscopy (SEM) (Figure 1b) and transmission electron microscope (TEM) (Figure 1c) images show that ABBP has a spherical morphology.The size of ABBP nanospheres can be easily controlled from about 100 to 300 nm by tuning the monomer concentration (Figure 1d-i and S2, Table S1, Supporting Information).It is worth noting that the size of ABBP nanospheres increases with increasing monomer concentration within an appropriate concentration range (C TBC : 0.003-20 mmol L À1 , C TAD : 0.005-30 mmol L À1 ).Outside this concentration range, irregular nanoparticles were formed (Figure S3, Supporting Information).Energy dispersive X-ray (EDX) mappings (Figure 1j) indicate the presence of C, O, N, and B elements in the nanospheres.
The condensation polymerization between TAD and TBC was preliminarily confirmed by the Fourier transform infrared (FTIR) spectra (Figure 1k).For ABBP, peaks at 1610 and 1358 cm À1 are attributed to the phenyl borate stretching vibration and B─O vibration, respectively. [19]17b,17c] After polymerization, the tertiary ammonia B←N stretching vibration at 1327 cm À1 of the TAD molecule disappears, and the secondary amine N─H stretching vibration at 3030 cm À1 appears.Probably, the by-product NH(CH 3 ) 2 is attached to the polymer chains of ABBP through B ← N coordination. [20]V-vis absorption spectra of the reaction solution were acquired to monitor the condensation polymerization (Figure 1l).With the formation of ABBP, one weak absorption band centered at 492 nm appears, which can be attributed to the increased π conjugation in the polymer chains of ABBP.This is because the empty p orbital of the boron atom and the lone electron pair on the oxygen atom are easily conjugated with the benzene ring. [20]With the increasing reaction time, the characteristic absorption peak of TBC (350 nm) decreases gradually, providing another indication for the condensation polymerization between TBC and TAD (Figure 1m).The thermal stability of ABBP was characterized by thermogravimetric analysis (TGA).As shown in Figure S4a,b, Supporting Information, ABBP shows weight loss at temperatures ranging from 30 to 120 °C, which can be attributed to the evaporation of absorbed NH(CH 3 ) 2 and TAD.This is confirmed by the weakening of the peaks at 30-40 ppm (signals of methyl) in the solid-state 13 C NMR spectra of ABBP samples after high-temperature treatment (Figure S4b, Supporting Information).At a temperature higher than 120 °C, ABBP degrades gradually.The 11 B nuclear magnetic resonance (NMR) spectrum of TAD (inset of Figure 1n) and the solid-state 11 B NMR spectrum (Figure 1n) of ABBP were characterized.The B-B group in TAD has a typical sp 2 -sp 2 hybrid characteristic with an 11 B signal appearing at 36.8 ppm. [21]Compared with the chemical shift of model molecule bis(catecholate)diborane (BCDB) at 38.8 ppm (Figure S4c, Supporting Information), the boron signal peak in ABBP appears at 40.4 ppm.This is due to the formation of B ← N coordination between nitrogen atoms (from imine and secondary amine groups) and boron atoms in ABBP (Figure S4d-h, Supporting Information). [22]In general, the formation of B ← N coordination can transform the boron atom from sp 2 hybridization to sp 3 hybridization, [23] changing the geometry structure of the electron cloud around the boron atom. [24]The B ← N coordination can also be evidenced by the signal shift of the N element signal in the high-resolution X-ray spectroscopy (XPS) spectrum (Figure S5, Supporting Information).
UV-vis absorption spectra and SEM images were characterized to track the intermediate stages for the formation of polymer nanospheres.By gradually titrating a TAD (7 mmol L À1 ) ethanol solution into 1 mL of TBC (4.6 mmol L À1 ) ethanol solution, the UV-vis absorption spectrum evolution of the reaction solution was monitored (Figure 1o).Compared with TBC, the absorption peak of ABBP shows a weak persistent red shift in the wavelength range of 400-600 nm with the increasing addition of TAD (≤0.001 mL, as indicated by the red dotted line in Figure 1o), implying the growth of polymer chain length. [25]EM images (Figure S6, Supporting Information) reveal that the size of ABBP enlarges gradually with the continuous addition of TAD.These results provide evidence for the nucleation polymerization between catechol and diboron molecules.Notably, when using a two-armed catechol monomer with imine bonds (TPC, Scheme S2, Supporting Information) to react with TAD (APBP, Scheme S3, Supporting Information), no nanosphere can be formed and the resultant reaction solution was transparent (Figure S7 and S8, Supporting Information).Probably, only oligomers with low molecular weights can be formed from the reaction of TPC and TAD.These oligomers have a high solubility in the solvent because they can interact with the by-product NH(CH 3 ) 2 through B ← N coordination, thus preventing the formation of polymer aggregates.Crosslinked polymer networks can be formed by the reaction of a three-arm catechol molecule (HHTP) with TAD (AHBP, Scheme S4, Supporting Information), and nanospheres with tunable sizes can be obtained (Figure S9a-f, Supporting Information).This indicates that the formation of crosslinked structure and the B ← N coordination synergistically affect the formation of diborate polymer nanospheres.The formation process of diborate polymer nanospheres comprises a nucleation process and a nanospheres growth process (Figure S9g, Supporting Information).Diborate oligomers are first formed through condensation polymerization between catechol monomers and TAD.These oligomers self-assemble into nucleation sites under the driving force of the dative B ← N bond formed between imine and diborate groups in the polymer chains. [25]lso, free NH(CH 3 ) 2 molecules can interact with the diborate groups through dative B ← N bond, and incorporate into the polymer networks.According to this reaction mechanism, we also prepared another diborate ester polymer (denoted as HBBP) from the reaction of TBC and tetrahydroxydiborane (THD), which also forms monodispersed nanospheres.Detailed preparation and characterization procedures are given in supporting information (Scheme S5 and Figure S10-S14, Supporting Information).

Interface Self-Assembly of Diborate Ester Polymers
Diborate ester polymers can also self-assemble on the surface of inorganic nanoparticles (Figure 2a), because catechol groups can bond to the surface of inorganic materials, restricting the condensation polymerization on the nanoparticle surfaces.We used DFT calculations to verify the strong interfacial interaction between catechol monomer TBC and TiO 2 anatase (101) surface and optimized the geometry of TBC attached to the TiO 2 surface (Figure 2b and S15, S16, Supporting Information).The initial anchorage adsorption model is partly based on previous theoretical and experimental work. [26]During the geometric optimization, the molecular structure of TBC and the crystal plane structure of anatase (101) are allowed to relax in the overall geometry.We found that TBC has two dissociative adsorption modes on the surface of anatase (101), namely dissociative bidentate bridging mode and dissociative monodentate mode, which are similar to some reported catechol molecules. [27]In the former mode, two adsorbate O atoms (deprotonated -OH groups of TBC) bind to two surface fivefold coordinate Ti atoms with Ti─O distances of 2.310 and 2.267 Å.The dissociated two H atoms are adsorbed on two O atoms with H─O distances of 1.001 and 1.002 Å.These two H atoms also form hydrogen bonds with the adjacent O atoms, exhibiting H─O distances of 1.798 and 1.794 Å.In the latter mode, one adsorbate O atom of TBC is connected to the surface fivefold coordinate Ti atom with Ti─O distance of 2.132 Å.The dissociated H atom is adsorbed on the surface O atom of TiO 2 with H─O distance of 1.002 Å.This H atom also interacts with the adjacent oxygen atom (distance: 1.794 Å) through hydrogen bond.The dissociative H atom of the other OH group of TBC is bonded to the nearest surface O atom of TiO 2 with H─O distance of 0.992 Å and interacts with another O atom of TiO 2 to form an OH group (H─O distance: 1.929 Å).The adsorption energy of TBC on the surface of TiO 2 is calculated to be À11.049eV, which is even more negative than the sum of the adsorption energies of dissociative bidentate bridging mode and dissociative monodentate mode of catechol molecules. [28]Therefore, TBC can be stably adsorbed on TiO 2 to achieve a thermodynamically stable interface.
We controllably coated ABBP on the surface TiO 2 nanoparticles to generate core-shell structural TiO 2 @ABBPs.The TEM images of TiO 2 @ABBPs show that all nanoparticles have a darker core and a brighter shell.The shell thickness of TiO 2 @ABBPs can be finely tuned from 40 to 10 nm through the adjustment of monomer concentration (Figure 2c, Table S2, Supporting Information).Dark-field TEM image and EDX mappings (Figure 2d) indicate the composition difference between the core and the shell of the nanoparticles, as well as the presence of C, N, O, B, and Ti elements in TiO 2 @ABBPs.Using the same method, ZnO@ABBPs with shell thicknesses ranging from 10 to 40 nm were easily prepared (Figure S17, Supporting Information).TGA results (Figure S18, Supporting Information) further verified that a larger shell thickness means a higher content of ABBP in TiO 2 @ABBPs and ZnO@ABBPs.Furthermore, the X-ray diffraction patterns (XRD, Figure S19a,b, Supporting Information), Raman spectra (Figure S19c,d, Supporting Information), and XPS spectra (Figure S20, Supporting Information) of TiO 2 @ABBPs and ZnO@ABBPs were characterized, and the corresponding analyses were given in supporting information.In addition, there is sufficient evidence that the HBBP can also be controllably coated on TiO 2 and ZnO nanoparticles (Table S3, Figure S21 and S22, Supporting Information).

Modification of Dielectric Elastomers with Core-Shell Nanoparticles
By using TiO 2 @ABBPs as the filler to modify PDMS, we obtained a series of composites named TiO 2 @ABBP/PDMS (Figure 3a).The curing conditions of PDMS such as the solvent tetrahydrofuran (THF) and the thermal treatment have no evident effect on the ABBP shell, as verified by the dark-field TEM image and the EDX mappings (Figure S23, Supporting Information), implying that the core-shell structure of TiO 2 @ABBPs can be preserved in PDMS.SEM images show the fractured surface morphology of pure PDMS and TiO 2 @ABBP/PDMS composites.PDMS exhibits a relatively flat fracture surface (Figure 3b).Nanoparticle aggregates were formed in the control sample of TiO 2 /PDMS (Figure 3c).Compared with TiO 2 /PDMS, the interface between TiO 2 @ABBPs and PDMS matrix is blurrier, indicating that the ABBP shell improves the interfacial compatibility, leading to a better dispersion of TiO 2 @ABBPs in PDMS (Figure 3d-f ).The cross-sectional SEM images of the composites were acquired to verify the improved compatibility of TiO 2 @ABBP core-shell nanoparticles in PDMS (Figure S24a-l, Supporting Information).Note that naked TiO 2 nanoparticles aggregate obviously in PDMS, TiO 2 @ABBP coreshell nanoparticles show more homogeneous dispersion.Statistical analysis indicates that the average size of the aggregates decreases slightly with the increase of ABBP shell thickness (Figure S24d-h, Supporting Information).Moreover, the increase in aggregate size is not obvious when increasing the loading amount of TiO 2 @ABBPs from 2.5 to 10 wt%.The residual catechol and boric acid groups in ABBP can form hydrogen bonds with PDMS, thus improving the interfacial interaction.The contact angle of water (Table S4, Supporting Information) on  101) crystal plane of anatase TiO 2 in dissociative bidentate bridging mode and dissociative monodentate mode.c) TEM images of TiO 2 @ABBP ST=40 , TiO 2 @ABBP ST=36 , TiO 2 @ABBP ST=28 , TiO 2 @ABBP ST=20 , TiO 2 @ABBP ST=10 (the subscript "ST" represents the average shell thickness of the TiO 2 @ABBP nanoparticles).d) Dark-field TEM image and EDX mappings of a TiO 2 @ABBP ST=10 .
We also used DFT calculation to confirm the enhanced interfacial compatibility of the core-shell nanoparticles in PDMS.Since the inner core is enclosed by the outer shell, the core-shell nanoparticles can only interact with PDMS through the ABBP shell, making it reasonable to only consider the ABBP-PDMS interaction in DFT calculation (Figure S25a,b, Supporting Information).In the model simulating the ABBP-PDMS interface (Figure 3j, Figure S25c,d, Supporting Information), due to the limitation of the simulated PDMS molecular chain, siloxane with three repeating units participates in the calculation and only interacts with the catechol group at one end of ABBP (simplified model) in a dissociative monodentate mode.The H atom in the silicone chain forms hydrogen bond with the O atom (from one of the dissociated -OH of the catechol group) with a H─O distance of 1.752 Å.As a control experiment, we also simulated the TiO 2 /PDMS interface (Figure 3k, and S25e,f, Supporting Information).The oxygen atom in the silicone chain is bound to the surface Ti atom with the Ti─O distance of 2.195 Å.The two undissociated H atoms in the silicone chain are separately adsorbed to two O atoms on TiO 2 through hydrogen bonding with H─O distances of 1.001 and 1.002 Å.The adsorption energies of ABBP/PDMS and TiO 2 /PDMS systems are À2.263 and À2.191 eV, respectively, implying that the ABBP shell has a higher binding affinity to PDMS than naked TiO 2 .
99% and exhibits Young's modulus of 1.06 MPa.With 1.0 wt% loading amount of TiO 2 @ABBPs, the as-formed TiO 2 @ABBP ST=10 / PDMS, TiO 2 @ABBP ST=20 /PDMS, TiO 2 @ABBP ST=28 /PDMS, TiO 2 @ABBP ST=36 /PDMS, and TiO 2 @ABBP ST=40 /PDMS show elongations at break of 106%, 121%, 135%, 160%, 176%, and 196%, respectively.Correspondingly, their Young's moduli decrease from 0.99 to 0.44 MPa.The same evolution trend was observed when increasing the content of TiO 2 @ABBPs in PDMS.For example, increasing the loading amount of TiO 2 @ABBP ST=20 from 1.0 to 20.0 wt%, Young's moduli decrease from 0.79 to Optical images of 20.0 wt% TiO 2 @ABBP ST=20 /PDMS before stretching and after 400% stretching.Stress-strain curves of TiO 2 @ABBP/ PDMS composites with different b) ABBP shell thicknesses and c) filler loading amounts.Effects of d) ABBP shell thickness and e) filler loading amount on Young's modulus and elongation at break of TiO 2 @ABBP/PDMS composites.Dynamic mechanical analysis for E 0 (green), E 0 0 (purple) and tan δ (orange) of TiO 2 @ABBP/PDMS composites with f ) different ABBP shell thicknesses and g) different filler loading amounts.Plots of dielectric constant, dielectric loss, and conductivity against the frequency for TiO 2 @ABBP/PDMS composites with h, i, and j) different ABBP shell thicknesses and k, l, and m) different filler loading amounts.For comparison, the mechanical and dielectric performances of PDMS and TiO 2 /PDMS are given.0.17 MPa, respectively.Notably, the corresponding elongation at break is improved by 4-fold.The toughing effect of TiO 2 @ABBP for PDMS can be explained through the following aspects.First, crack deflection may be the primary mechanism, as evidenced by the SEM images of the fracture surfaces (Figure S26, Supporting Information).Second, the ABBP shell improves the interfacial compatibility and interaction between TiO 2 @ABBPs and PDMS matrix.
The dynamic mechanical analysis (DMA) frequency scan results of TiO 2 @ABBP/PDMS composites are shown in Figure 4f,g.Both TiO 2 /PDMS and TiO 2 @ABBP/PDMS composites exhibit storage moduli (E', green) evidently larger than loss moduli (E", purple), implying that the composites comprise wellestablished crosslinked polymer networks. [29]With the increase of frequency, the energy storage moduli (Figure 4f,g, green) and loss moduli (Figure 4f,g, purple) of TiO 2 @ABBP/PDMS composites have the same change trend as that of PDMS, which implies that the composites have good stability as PDMS.With the increase of the shell thickness (Figure 4f ) and loading amount (Figure 4g) of TiO 2 @ABBP core-shell nanoparticles, the E' (green) and E" (purple) of the composites decrease, illustrating an obvious softening trend.This may be due to the reduced crosslinking density of PDMS in the composites and the increased mobility of molecular chains.The ratio of the loss modulus to the storage modulus is defined as the loss factor (tan δ), which indicates the damping properties of the composite.The good elastic properties of the composites were further supported by small tan δ (<0.2) at all frequencies (Figure 4f,g, orange).With the increase of shell thickness and loading amount, the damping properties of all composites do not change obviously, further confirming the stable mechanical properties.
The dielectric constant, dielectric loss tangent, and conductivities of the composites with different ABBP shell thicknesses (Figure 4h-j) and different filler loading amounts (Figure 4k-m) were tested in the frequency range of 20-10 6 Hz.With 1.0 wt% loading amount of TiO 2 @ABBPs, both the dielectric constant (Figure 4h) and dielectric loss (Figure 4i) of the TiO 2 @ABBP/ PDMS composites are higher than those of PDMS, TiO 2 /PDMS and ABBP/PDMS.The dielectric constants and dielectric loss of the composites decrease slightly with increasing frequency.This is mainly due to the orientation polarization of the PDMS matrix and the interface polarization between the PDMS matrix and TiO 2 @ABBPs.A shell thickness of 20 nm results in the highest dielectric constant of TiO 2 @ABBP/PDMS composites (Figure S27a,b, Supporting Information), which is mainly related to the interface polarization.With the presence of an ABBP shell, a doubly charged interface between TiO 2 @ABBPs and PDMS will be formed, and the interface can capture more charge carriers, thereby improving the interface polarization of the composites.When the polymer shell thickness exceeds 20 nm, although the dipole polarizability of the elastomer increases, the interfacial polarizability in the elastomer of the same mass decreases, resulting in a gradual decrease in the dielectric constant of the elastomer. [30]In addition, with the increase in shell thickness, the aggregation of TiO 2 nanoparticles becomes evident during the formation of core-shell nanoparticles, which is also the reason for the decreased interfacial polarizability of the composites (Figure S27c,d, Supporting Information).With the increase of ABBP shell thickness, the dielectric constant of the composites increases first and then decreases.Notably, the dielectric loss values of the composites are all lower than 0.01, which may be attributed to the high interfacial interaction (including the hydrogen bonding interactions, etc.) and the good compatibility between TiO 2 @ABBPs and PDMS.The conductivities of the composites with different ABBP shell thicknesses are given in Figure 4j.At low frequencies (<10 4 Hz), the conductivities of all TiO 2 @ABBP/PDMS composites are similar to that of PDMS and independent of the ABBP shell thickness and the loading amount of TiO 2 @ABBPs.This phenomenon indicates that the electron transfer is limited due to the isolation effect of the ABBP shell at low frequencies.In the case of higher frequencies (>10 4 Hz), TiO 2 @ABBP/PDMS composites show a more rapid increase in conductivity than PDMS.This may be attributed to the excitation of electric charge, which causes charge transport in the interior of the composites. [31]It is the same as other composites filled with core-shell structures that have been reported, [9a] the dielectric constant (Figure 4k), dielectric loss (Figure 4l) and conductivity (Figure 4m) of the composites increase with the increasing loading amount of TiO 2 @ABBPs.In addition, we fabricated another composite named TiO 2 @HBBP/PDMS, which shows a similar development trend in dielectric constant, dielectric loss tangent, and conductivity with those of TiO 2 @ABBP/ PDMS composites (Figure S28, Supporting Information).
Circular membrane actuators were constructed (Figure 5a and Scheme S6, Supporting Information) to measure the actuated strains of pure PDMS, ABBP/PDMS, and TiO 2 @ABBP/PDMS composites.All samples show increased actuated strains with the enhancement of the electric field.With 1.0 wt% loading of fillers, TiO 2 @ABBP/PDMS composites with different ABBP shell thicknesses exhibit higher actuated strains (Figure 5b), illustrating the improved actuating performance.Also, the puncture voltages of TiO 2 @ABBP/PDMS composites are much higher than those of TiO 2 /PDMS and ABBP/PDMS (Figure 5c).With 1.0 wt % of loading of TiO 2 @ABBP ST=20 , the composite has the highest puncture voltage of 71 V μm À1 and the biggest actuated strain of 14.5%.Under the same stimulation voltage, a higher content of TiO 2 @ABBPs results in better actuating performance (Figure 5d).However, excessive loading of TiO 2 @ABBPs induces the reduction of puncture voltage (Figure 5e).Therefore, TiO 2 @ABBP ST=20 / PDMS with 5.0 wt% filler content has the optimized actuating performance, possessing actuated strain and puncture voltage of 25.9% and 80 V μm À1 , respectively (see Video S1, Supporting Information).Compared with the reported DEs (Table S5, Supporting Information), ABBP@TiO 2 /PDMS composites show advantages in comprehensive performance, such as excellent dielectric and mechanical properties, which can be attributed to the unique shell polarization characteristics of the core-shell fillers.In addition, the actuator shows high electrical durability and dynamic electromechanical reliability and maintains its actuation performance well over 500 (see Video S2, Supporting Information).The improved actuating performance of TiO 2 @ABBP/PDMS benefits from both the increased polarization effect (including dipole and interfacial polarization) and the softening of the composites.Also, improving the actuation performance in a nonprestretched configuration leads to the possibility of applying these composites for low-field-driven shapemorphing devices.

Conclusions
In conclusion, we have shown that B ← N coordination can drive the solution self-assembly of crosslinked diborate ester polymers to afford nanospheres with tunable sizes.By combining the driving forces of B ← N coordination and catechol-surface binding, crosslinked diborate ester polymers can also self-assemble on the surface of inorganic nanoparticles.We have successfully coated crosslinked diborate polymers on the surfaces of TiO 2 nanoparticles and ZnO nanoparticles and prepared core-shell nanoparticles with controllable shell thicknesses, including TiO 2 @ABBPs, ZnO@ABBPs, TiO 2 @HBBPs, and ZnO@HBBPs.By choosing TiO 2 @ABBPs as an example, we studied the potential application of diborate ester polymer-based core-shell nanoparticles in the modification of DEs.By introducing the TiO 2 @ABBP core-shell nanoparticles in the curing system of PDMS, TiO 2 @ABBP/PDMS composites were fabricated.Both DFT calculations and experimental results indicate that the diborate ester polymer shell is effective in improving the interface interaction and compatibility between TiO 2 nanoparticles and PDMS, thus evidently softening and toughing the composites.A thicker diborate polymer shell or a higher loading amount of the core-shell nanoparticles results in a lower modulus and a higher stretchability.A moderate shell thickness (≈20 nm) is optimal for achieving promising dielectric performances.The actuating performance of the TiO 2 @ABBP/PDMS composites is improved compared with pure PDMS and TiO 2 /PDMS composites.The actuated strain and puncture voltage of the  c) puncture voltage and maximum actuated area strain of 1.0 wt% TiO 2 @ABBP/PDMS with different ABBP shell thicknesses.For comparison, the corresponding performances of PDMS, 1.0 wt% ABBP/ PDMS, 1.0 wt% TiO 2 /PDMS are given.Effect of filler loading amount on the d) actuated area strain and e) puncture voltage and maximum actuated area strain of the TiO 2 @ABBP/PDMS composites.composites reaches 25.9% and 80 V μm À1 , respectively.This work provides a promising avenue to design novel high-performance dielectric elastomers.

Figure 1 .
Figure 1.a) Schematic diagram of the synthesis of ABBP.b) SEM and c) TEM images of ABBP nanospheres with an average size of 200 nm.d-f ) SEM and g-i) TEM images of ABBP nanospheres with average sizes of about 100, 210, and 280 nm, respectively.j) EDX element mappings of C, N, O, and B for ABBP nanospheres.k) FTIR spectra of TBC, ABBP, and TAD.l) UV-vis absorption spectra of TAD, TBC, and ABBP nanospheres.m) UV-vis absorption spectra tracing the formation of ABBP nanospheres.n) Solid-state 11 B NMR spectra of ABBP nanospheres and TAD (inset).o) UV-vis absorption spectra of the reaction mixture for the preparation of ABBP nanospheres.

Figure 4 .
Figure4.a) Optical images of 20.0 wt% TiO 2 @ABBP ST=20 /PDMS before stretching and after 400% stretching.Stress-strain curves of TiO 2 @ABBP/ PDMS composites with different b) ABBP shell thicknesses and c) filler loading amounts.Effects of d) ABBP shell thickness and e) filler loading amount on Young's modulus and elongation at break of TiO 2 @ABBP/PDMS composites.Dynamic mechanical analysis for E 0 (green), E 0 0 (purple) and tan δ (orange) of TiO 2 @ABBP/PDMS composites with f ) different ABBP shell thicknesses and g) different filler loading amounts.Plots of dielectric constant, dielectric loss, and conductivity against the frequency for TiO 2 @ABBP/PDMS composites with h, i, and j) different ABBP shell thicknesses and k, l, and m) different filler loading amounts.For comparison, the mechanical and dielectric performances of PDMS and TiO 2 /PDMS are given.

Figure 5 .
Figure 5. a) Manufacturing process of dielectric the elastomer actuator.b) Actuated area strain andc) puncture voltage and maximum actuated area strain of 1.0 wt% TiO 2 @ABBP/PDMS with different ABBP shell thicknesses.For comparison, the corresponding performances of PDMS, 1.0 wt% ABBP/ PDMS, 1.0 wt% TiO 2 /PDMS are given.Effect of filler loading amount on the d) actuated area strain and e) puncture voltage and maximum actuated area strain of the TiO 2 @ABBP/PDMS composites.