Improved Dielectric Properties of Thermoplastic Polyurethane Elastomer Filled with Core–Shell Structured PDA@Tic Particles

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Article Improved Dielectric Properties of Thermoplastic Polyurethane Elastomer Filled with Core–Shell Structured PDA@TiC Particles

Xinfu He 1,2,*, Jun Zhou 1, Liuyan Jin 1, Xueying Long 1, Hongju Wu 1, Li Xu 1, Ying Gong 1 and Wenying Zhou 1,* 1 School of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, China; [email protected] (J.Z.); [email protected] (L.J.); [email protected] (X.L.); [email protected] (H.W.); [email protected] (L.X.); [email protected] (Y.G.) 2 Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, Ministry of Natural Resources, Xi’an 710021, China * Correspondence: [email protected] (X.H.); [email protected] (W.Z.)

Received: 10 June 2020; Accepted: 20 July 2020; Published: 27 July 2020

Abstract: Insulating interlayer between nanoparticles and polymer matrix is crucial for suppressing the dielectric loss of polymer composites. In this study, titanium carbide (TiC) particles were surface modified by polydopamine (PDA), and the obtained PDA@TiC powders were used to reinforce thermoplastic polyurethane (TPU). The results indicate that the PDA@TiC were homogenously dispersed in the matrix compared with the pristine TiC, and that the PDA@TiC/TPU composites show improved dielectric and mechanical properties, i.e., much lower dissipation factors and obviously enhanced dielectric breakdown strength, as well as higher tensile strength and elongation at break as compared to the raw TiC/TPU. The nanoscale PDA interlayer contributes to the dielectric and mechanical enhancements because it not only serves as an insulating shell that prevents TiC particles from direct contacting and suppresses the loss and leakage current to very low levels, but also enhances the interfacial interactions thereby leading to improved mechanical strength and toughness. The prepared flexible PDA@TiC/TPU with high permittivity but low loss will find potential applications in electronic and electrical applications.

Keywords: polymer matrix composites; electrical properties; mechanical properties; polyurethane; titanium carbide; polydopamine

1. Introduction Polymer-based dielectric composites have aroused great interest in researchers due to the materials being able to store electrical energy and their wide use in capacitors, voice control equipment, artificial muscles and other applications [1–4]. However, the dielectric constant (k) of most commercialized polymers are often very low (k < 10), which cannot meet the requirements of further miniaturization of electronic components in practical applications. Studies have shown that direct introduction of inorganic particles with high-k, such as barium titanate, calcium titanate and piezoelectric ceramics, can significantly increase the dielectric constant of polymer matrix composites [5–9]. Among all the electroactive polymers, thermoplastic polyurethane (TPU) has been regarded as a good flexible polymer host owing to its excellent electrical properties, high shape recoverability, and large deformation. Yuan et al. [10] prepared a novel TPU based composite and found that the dielectric constant of TPU increased from 8 to 41 with the incorporation of 1.0 wt % polyamide-1 (PA1), the loss tangent still kept at a low level of less than 0.02. Zheng et al. [11] blended TPU with poly (vinylidene fluoride) (PVDF) and found the soft chain of TPU was very helpful for

Materials 2020, 13, 3341; doi:10.3390/ma13153341 www.mdpi.com/journal/materials Materials 2020, 13, 3341 2 of 14 promoting the breakdown strength of composites, and a maximum value was obtained of up to 537.8 mV/m at 3 vol.% TPU with a hardness of 65. Xiao et al. [12] prepared a series of TPU/rGO (reduced graphene oxide) composites with high dielectric permittivity (151 at 1 kHz) with only 0.75 vol.% rGO. Meanwhile, its dissipation factors remain lower than 1 at 1 kHz. Chen et al. [13] synthesized poly (methyl methacrylate)-functionalized graphene/polyurethane (MG-PU) dielectric elastomer composites, and found the 1.5 wt % MG-PU film had high relative permittivity as up to 28.2. Titanium carbide (TiC) as a structural ceramic has attracted great interest in virtue of its certain conductivity (3 107 S/cm), resistance to corrosion, chemical stability and high permittivity (ε). × These unprecedented properties make it a promising candidate filler to prepare polymer-based high k nanocomposites [14–16]. However, like other ceramic fillers, only when the amount of TiC is high enough can a noticeably high dielectric constant of the composites be obtained, followed by a large dissipation factor and increased conductivity. Furthermore, the high filler loading also brings about other severe problems, such as the phase interfacial defects and poor mechanical and processing properties [17]. Many researchers used hydroxyl groups, coupling agents, surfactants and phosphoric acid to modify the surfaces of nanoparticles to combat this problem [18,19], and these surface modifiers can improve the dispersion of nanoparticles and form a more stable interface layer to suppress dielectric losses of the composites. Dopamine has recently been appreciated because of its strong adhesion and its easy interaction with various substrates via covalent and noncovalent interactions [2,20]. In addition, aromatic groups and hydrogen bonds are highly polarizable, which helps improve dielectric constants [21,22]. Zhang et al. [23] added Ag nanoparticles coated with polydopamine (PDA) into PVDF, and confirmed the dielectric constant of the obtained composites reached up to a value of 53 at 100 Hz. Thakur et al. [24] presented an environmentally friendly way to form polymethyl methacrylate (PMMA) with dopamine, and the functionalized PMMA films exhibited enhanced dielectric properties compared to pristine PMMA films. In this study, to reduce the dielectric loss of pristine TiC/TPU, the TiC nanoparticles was surface modified with polydopamine (PDA) to form a core–shell structured nanoparticle PDA@TiC, and the PDA@TiC/TPU composites were prepared by embedding PDA@TiC into a TPU matrix through the solution method. The microstructures of PDA@TiC and dielectric properties of the TiC/TPU and PDA@TiC/TPU composites were investigated in terms of PDA concentration and filler loading. So, the present work aims to provide a deep insight into the effect of PDA and fillers content, and frequency on dielectric and mechanical properties of TiC/TPU composites.

2. Materials and Methods

2.1. Materials TPU (1180A, BASF), dopamine hydrochloride (C H NO HCl) and TiC (40–100 nm) were 8 11 2· provided by Zhengfa Plastics Co. Ltd., Alfa, and Guangzhou Hongwu Material Technology Co. Ltd., respectively. N,N-Dimethylformamide (DMF, AR), Tris (hydroxymethyl)-methyl aminomethane (Tris, AR), HCl (38 wt % aqueous solution), ethanol (AR) and NaOH (AR) were all purchased from the Sinopharm Group. For all experiments, deionized water was used.

2.2. Preparation of PDA@TiC and PDA@TiC/TPU Composites Of the HCl solution 8.5 mL (0.1 mol/L) and 50 mL of the Tris solution (0.1 mol/L) were mixed and diluted with deionized water to obtain 100 mL of a Tris-buffer solution. Dopamine hydrochloride, which is equivalent to 0.1 g dopamine (DA), was added to 50 mL of the Tris-buffer solution, and precisely measured amount of TiC powder (molar ratio of TiC to DA, nTiC/nDA, were 20:1, 40:1 and 60:1, respectively) was then dispersed into the solution. The mixture was stirred for 48 h at room temperature, and then was vacuum filtered. The filter residue, after being washed with deionized water and ethanol Materials 2020, 13, 3341 3 of 14

several times, was dried at 40 ◦C for 12 h, to obtain the modified TiC powder. The prepared PDA@TiC powders were denoted as PDA@TiC 20, PDA@TiC 40 and PDA@TiC 60, respectively. − − − Of TPU 7 g was added into 10 mL DMF, and the mixture was stirred at 60 ◦C for 2 h to form a TPU solution. TiC or PDA@TiC x (where the value of x is determined by the ratio of n /n ) − TiC DA powder, which accounts for 1%, 4%, 7%, 10% and 13% of the total weight of the solid materials (TiC and TPU or PDA@TiC x and TPU), was then added to the TPU solution. The obtained mixture was − stirred for 8 h, then heated to 140 ◦C to evaporate most of the DMF. The resulting composite was further dried at 65 ◦C in a vacuum oven to remove the residual solvent. The prepared composites with different doping content of TiC or PDA@TiC were denoted as TiC/TPU 1, TiC/TPU 4, TiC/TPU 7, − − − TiC/TPU 10 and TiC/TPU 13 and PDA@TiC x/TPU 1, PDA@TiC x/TPU 4, PDA@TiC x/TPU 7, − − − − − − − − PDA@TiC x/TPU 10 and PDA@TiC x/TPU 13, respectively. − − − − 2.3. Characterizations Fourier transform infrared (FT-IR) analyses were performed with a spectrometer (iN10&iZ10, 1 Thermofisher) using KBr pellets in the wave number range of 400–4000 cm− . The Raman spectra were obtained on a Micro Raman Spectrometer (inVia, Renishaw) with an excitation wavelength at 532 nm. Thermal analysis was performed using a thermogravimetric (TG) analyzer (TGA/DSC1, Mettler Toledo Instruments) to observe the weight changes of the samples as temperature increased. The morphologies of TiC, PDA@TiC, TiC/TPU and PDA@TiC/TPU were studied via scanning electron microscopy (SEM, S-4800, Hitachi). Before testing, TiC/TPU and PDA@TiC/TPU composites need to be peeled off the copper foils and then brittle broken in liquid nitrogen rapidly. Core/shell-structured PDA@TiC particles were ultrasonic dispersed in ethanol for 10 min, and then dropped on copper mesh for observing their microstructures by using a transmission electron microscope (TEM, H-7650, Hitachi) operating at a voltage of 200 kV. The dielectric properties were examined in the frequency range of 40–107 Hz at room temperature using an Agilent 4294A impedance instrument (USA). Prior to measurement, the composite was well distributed between two circular copper foils (10 cm in diameter and 10 µm in thickness), and then was loaded into a cylindrical mold (10 cm in diameter) and was hot formed at 200 ◦C for 15 min with a pressure of approximately 15 MPa. The disk-shaped sample covered with copper foil was then cut into a disk with a diameter of 2 cm for testing. The breakdown voltage of the sample was tested using a HF5013 EHV voltage tester. The breakdown field strength of the sample was calculated using the following formula: E = Ub/d (1) where E is the breakdown field strength, kV/mm; Ub is the breakdown voltage, kV and d is sample thickness, mm. Tensile tests of the samples were conducted with a ZMGI 250 tensile tester (News SANS China), adopting ISO 527-3:1995 as a standard. Dumbbell samples were stretched at a speed of 100 mm/min.

3. Results and Discussions

3.1. FT-IR Analysis FT-IR spectra of TiC and PDA@TiC 60 are shown in Figure1. The absorption bands at 3472 1 − and 1636 cm− were caused by residual water absorbed in TiC. Compared with the curve of TiC, the characteristic absorption peaks of PDA appeared inside the curve of PDA@TiC 60. The enhanced 1 − peak of 3435 cm− was caused by stretching vibration of phenolic hydroxyl groups and Ar-NH. 1 1 The absorption bands at 2985 cm− and 2914 cm− were caused by a C-H stretching vibration. 1 The intense absorption features at 1632 cm− indicate an aromatic ring skeleton (C=C) stretching 1 vibrations and a N-H in-plane deformation vibration. A peak shown at 1384 cm− was caused by the in-plane vibration absorption of an unsaturated hydrocarbon (=C-H) in the structure of PDA. The new 1 1 peak at 1044 cm− was assigned to a C-O stretching vibration, and the last peak at 874 cm− was Materials 2020, 13, 3341 4 of 14

Materials 2020, 13, x FOR PEER REVIEW 4 of 14 Materials 2020, 13, x FOR PEER REVIEW 4 of 14 attributed to an out-of-plane deformation vibration of N-H. The results demonstrated that a layer of to an out-of-plane deformation vibration of N-H. The results demonstrated that a layer of PDA was toPDA an out was-of coated-plane successfully deformation onto vibration surface of of N TiC.-H. The results demonstrated that a layer of PDA was coated successfully onto surface of TiC. coated successfully onto surface of TiC.

FigureFigure 1. 1. FTIRFTIR spectra spectra of of TiC TiC and and PDA@Ti PDA@TiCC−6060.. Figure 1. FTIR spectra of TiC and PDA@TiC−−60. 3.2.3.2. Raman Raman A Analysisnalysis 3.2. Raman Analysis RamanRaman spectra spectra of of TiC TiC and and PDA@TiC PDA@TiC−6060 are are displayed displayed in in Figure Figure 22.. In contrast to pristine TiC the Raman spectra of TiC and PDA@TiC−60 are displayed in Figure 2. In contrast to pristine TiC the peakpeak intensityintensity was was obviously obviously weakened weakened at 200–700 at 200 cm–7001 incm the−1 PDA@TiCin the PDA@TiC60 spectrum.−60 spectrum. The attenuated The peak intensity was obviously weakened at 200–700− cm−1 in the PDA@TiC− −60 spectrum. The attenuatedsignals were signals caused were by PDAcaused coating. by PDA The coating. new strong The peaksnew atstrong 1371 peaks and 1584 at 1371 cm 1 andappeared 1584 cm after−1 attenuated signals were caused by PDA coating. The new strong peaks at 1371− and 1584 cm−1 appearmodiﬁcation,ed after whichmodification, were ascribed which were to the ascribed deformation to the ofdeformation the catechol of moiety the catechol in PDA. moiety An apparentin PDA. appeared after modification, which were ascribed to the deformation of the catechol moiety in PDA. Anbroad apparent absorption broad peak absorption was also peak observed was also at 2865 observed cm 1, attributableat 2865 cm−1 to, attributable the unsaturated to the C-H unsaturated stretching An apparent broad absorption peak was also observed− at 2865 cm−1, attributable to the unsaturated Cvibration-H stretching in the vibration aromatic in ring the of aromatic PDA. ring of PDA. C-H stretching vibration in the aromatic ring of PDA.

Figure 2. Raman spectra of TiC and PDA@TiC−60. FigureFigure 2. 2. RRamanaman spectra spectra of of TiC TiC and and PDA@TiC PDA@TiC−6060.. − 3.3. Microstructure Analysis 3.3.3.3. Microstructure Microstructure A Analysisnalysis As seen in Figure 3a, the pure TiC particle size was approximately 100 nm, and serious AsAs seenseen inin Figure Figure3a, the3a, purethe pure TiC particle TiC particle size was size approximately was approximately 100 nm, and 100 serious nm, aggregationsand serious aggregations appeared that result from an electrostatic interaction. Figure 3b shows that the particle aggregationsappeared that appear resulted from that result an electrostatic from an electrostatic interaction. interaction. Figure3b Figure shows 3b that shows the particlethat the particle size of size of PDA@TiC−60 was approximately µ0.2–1 µm. A smooth surface and relatively regular shape sizePDA@TiC of PDA@TiC60 was−60 approximately was approximately 0.2–1 0.2m.–1 A µ smoothm. A smooth surface surface and relatively and relatively regular regular shape shape were were observed− in PDA@TiC−60. The local agglomeration of the nanoparticles can be seen clearly to wereobserved observed in PDA@TiC in PDA@TiC60.− The60. The local local agglomeration agglomeration of theof the nanoparticles nanoparticles can can be be seen seen clearly clearly to to be be effectively blocked by− the surface coating. The energy dispersive spectrometry (EDS) analysis beeﬀ ectivelyeffectively blocked blocked by theby surfacethe surface coating. coating. The energyThe energy dispersive dispersive spectrometry spectrometry (EDS) (EDS) analysis analysis results results describing TiC before and after treatment are shown in Table 1. There are increasing contents resultsdescribing describing TiC before TiC andbefore after and treatment after treatment are shown are inshown Table 1in. ThereTable are1. There increasing are increasing contents ofcontents N and of N and O elements in PDA@TiC−60, indicating PDA has been deposited on TiC particles. Figure ofO N elements and O inelements PDA@TiC in PDA@TiC60, indicating−60, indicating PDA has been PDA deposited has been on deposited TiC particles. on TiC Figure particles.3c,d revealed Figure 3c,d revealed the dispers−ion and compatibility between particles and the matrix. It is visible that 3c,dthe dispersionrevealed the and dispers compatibilityion and compatibility between particles betwe anden theparticles matrix. and It the is visible matrix. that It is PDA@TiC visible that60 PDA@TiC−60 particles were more homogeneously dispersed in the composites. No obvious defect− PDA@TiC−60 particles were more homogeneously dispersed in the composites. No obvious defect was observed at the junction of the two phases that exist in PDA@TiC−60/TPU−13. One reason is was observed at the junction of the two phases that exist in PDA@TiC−60/TPU−13. One reason is ascribed to the strong phase interface adhesion resulting from the PDA coating. The other reason is ascribed to the strong phase interface adhesion resulting from the PDA coating. The other reason is Materials 2020, 13, 3341 5 of 14 particles were more homogeneously dispersed in the composites. No obvious defect was observed at the junction of the two phases that exist in PDA@TiC 60/TPU 13. One reason is ascribed to the − − strongMaterials phase 20 interface20, 13, x FOR adhesion PEER REVIEW resulting from the PDA coating. The other reason is there are5 hydroxylof 14 groups in the PDA structure that could form hydrogen-bonding interactions with C-N bonds [25]. there are hydroxyl groups in the PDA structure that could form hydrogen-bonding interactions with Therefore,Materials PDA 2020, 1 coating3, x FOR PEER enhances REVIEW interfacial interactions between ﬁllers and the matrix and5 improves of 14 C-N bonds [25]. Therefore, PDA coating enhances interfacial interactions between fillers and the uniformity of the internal structure of the materials. matrixthere are and hydroxyl improves groups uniformity in the PDAof the structure internal structurethat could of form the materials.hydrogen -bonding interactions with C-N bonds [25]. Therefore, PDA coating enhances interfacial interactions between fillers and the matrix and improves uniformity of the internal structure of the materials.

Figure 3. SEM images of TiC (a), PDA@TiC−60 (b), TiC/TPU−13 (c) and PDA@TiC−60/TPU−13 (d). Figure 3. SEM images of TiC (a), PDA@TiC 60 (b), TiC/TPU 13 (c) and PDA@TiC 60/TPU 13 (d). − − − − TableTable 1. 1.EDS EDS results results ofof TiC and PDA@TiC PDA@TiC−6060.. Figure 3. SEM images of TiC (a), PDA@TiC−60 (b), TiC/TPU−13 (c) and− PDA@TiC−60/TPU−13 (d). Element Content/wt % Atomic Ratio/% Ample Element Content/wt % Atomic Ratio/% Ample C TableO 1. EDSN results ofT TiCi and PDA@TiCC −60O. N Ti C O N Ti C O N Ti TiC 36.53 ± 3.42 6.93 ± 0.30 0 56.54 ± 3.72 65.22 ± 2.99 9.31 ± 0.05 0 25.47 ± 2.94 TiC 36.53 3.42 6.93 Element0.30 Content 0/wt % 56.54 3.72 65.22 2.99Atomic 9.31 Ratio/%0.05 0 25.47 2.94 PDA@TiCAmple− 60± 26.37 ± 0.87± 8.87 ± 1.02 1.50 ± 0.02 63.26± ± 0.14 50.73 ± 3.37± 16.30 ± 4.6±0 2.47 ± 0.12 30.50 ± 1.10 ± PDA@TiC 60 26.37 0.87C 8.87 1.02O 1.50 0.02N 63.26Ti 0.14 50.73C 3.37O 16.30 4.60N 2.47 0.12Ti 30.50 1.10 − ± ± ± ± ± ± ± ± TiC 36.53 ± 3.42 6.93 ± 0.30 0 56.54 ± 3.72 65.22 ± 2.99 9.31 ± 0.05 0 25.47 ± 2.94 ThePDA@TiC TEM image−60 26.37s of ± 0.87 PDA@TiC 8.87 ± 1.02 particles 1.50 ± 0.02 (Fig 63.26ure ± 0.14 4) 50.73reveal ± 3.37ed that16.30 TiC± 4.6 0particles 2.47 ± 0.12 were30.50 ±completely 1.10 Theencapsulated TEM images inside of a uniform PDA@TiC PDA particles shell, and (Figure the thickness4) revealedes of PDA that layers TiC particles were about were 20 nm completely and encapsulated40 nmThe for TEM insidePDA@TiC image a uniform−s60 of and PDA@TiC (b) PDA PDA@TiC shell, particles and−20 ,(Fig respectively. theure thicknesses 4) reveal ed of that PDA TiC layers particles were were about completely 20 nm and 40 nmencapsulated for PDA@TiC inside60 a anduniform (b) PDA@TiC PDA shell, and20, respectively.the thicknesses of PDA layers were about 20 nm and 40 nm for PDA@TiC− −60 and (b) PDA@TiC−−20, respectively.

Figure 4. TEM images of PDA@TiC−60 (a) and PDA@TiC−20 (b).

FigureFigure 4. TEM4. TEM images images of of PDA@TiC PDA@TiC−6060 (a (a) )and and PDA@TiC PDA@TiC−20 (20b). (b). − −

Materials 2020, 13, 3341 6 of 14

Materials 2020, 13, x FOR PEER REVIEW 6 of 14 3.4. TG Analysis 3.4. TG Analysis Figure5 depicts the results of the TG analysis of TiC particles, PDA@TiC 60 and dopamine, Figure 5 depicts the results of the TG analysis of TiC particles, PDA@TiC−60 and dopamine, respectively. TiC is a transitional metal carbide with similar properties to metals that have a high respectively. TiC is a transitional metal carbide with similar properties to metals that have a high melting point and high hardness so its weight was not affected by temperature, while, dopamine melting point and high hardness so its weight was not affected by temperature, while, dopamine is is an organic substance that decomposed readily at high temperatures. Dopamine’s weight began an organic substance that decomposed readily at high temperatures. Dopamine’s weight began to to decline dramatically at 250 C, which is consistent with the loss of features of PDA as reported decline dramatically at 250 °C, which◦ is consistent with the loss of features of PDA as reported in [1]. in [1]. Compared to TiC, PDA@TiC 60 particles also began to lose a small amount of weight at 250 C. Compared to TiC, PDA@TiC−60 particles− also began to lose a small amount of weight at 250 °C. This◦ This result further confirms the surface modification TiC with PDA, and corresponded to the results result further confirms the surface modification TiC with PDA, and corresponded to the results obtained using TEM. The weight loss of PDA@TiC 60 can be measured at approximately 2.8% of its obtained using TEM. The weight loss of PDA@TiC−60 can be measured at approximately 2.8% of its total original weight. total original weight.

Figure 5. TGATGA of of pristine pristine TiC, PDA@TiC −6060 and and dopamine. dopamine. − 3.5. Dielectr Dielectricic P Propertiesroperties The dependence of the dielectric properties of TiC/TPU and PDA@TiC 60/TPU composites The dependence of the dielectric properties of TiC/TPU and PDA@TiC−60−/TPU composites are shownare shown in Figure in Figure 6 The6 The dielectric dielectric constant constant of ofpure pure TPU TPU kep keptt at at a a low low value value and shows aa weakweak 3 frequencyfrequency-dependent-dependent behavior. behavior. Compared Compared to to pure pure TPU TPU (ε ( ε= =3.63.6 at at10 103 Hz),Hz), the the dielectric dielectric constants constants of TPUof TPU composites composites with with pristine pristine and and surface surface modified modified TiC TiC were were notably notably increased, increased, and and approach approacheded to ato monotonous a monotonous increase increase with with the the doping doping content content of of the the fillers fillers in in Figure Figure 6aa–c.–c. There may be threethree reasonsreasons contributedcontributed to to this thi result.s result. Strong Strong interfacial interfacial polarization polarization effects wouldeffects occurwould at lowoccur frequencies at low according to the Maxwell–Wagner effect; when ε σ ε σ exist (where ε and ε are frequencies according to the Maxwell–Wagner effect;TPU TiCwhen, TiCTPUTPUTiC TiC TPU existTPU (whereTiC TPU dielectric constants of TPU and TiC, σ and σ are conductivities of TPU and TiC), the effect of and TiC are dielectric constants of TPUTPU and TiC,TiC TPU and TiC are conductivities of TPU and 𝜀𝜀 𝜎𝜎 ≠ 𝜀𝜀 𝜎𝜎 𝜀𝜀 TiCinterfacial), the effect polarization of interfacial appears polarization in the interface appears between in the the interface PDA shell between and the TiCthe corePDA [26 shell–29], and and the the 𝜀𝜀 𝜎𝜎 𝜎𝜎 TiCtime core for the[26 polarization–29], and the is time abundant. for the The polarization inorganic is particles abundant. may The disrupt inorganic the phase particles separation may disrupt of TPU, theresulting phase in separation the disruption of TPU, of the resulting hydrogen in bonding the disruption between of TPU the chains hydrogen and thus bonding increasing between the dipole TPU chainspolarization and thus of the increasing TPU chains the underdipole low polarization frequencies of [the30,31 TPU]. Finally, chains the under introduction low frequencies of TiC provides [30,31]. Finally,additional the free introduction charges. The of TiC dielectric provides constant additional gradually free decreases charges. withThe dielectric increasing constant frequencies gradually due to decreasespolarization with failing increasing to keep frequencies pace with thedue changes to polarization of the applied failing electric to keep field, pace and with a strong the changes dielectric of therelaxation applied appears electric [32 field,], where and thea strong composites dielectric finally relaxation exhibit characteristics appears [32] related, where to the insulation. composites finally exhibit characteristics related to insulation. Figure 6e shows that with the increase of PDA@TiC−60 content, the dissipation factors of PDA@TiC−60/TPU exhibited a very slight increase (tan δ = 0.05–0.1 at 103 Hz). The change of PDA@TiC−60 doping content had little influence on the dissipation factors. However, the overall value in loss was much smaller than that of the TiC/TPU system (tan δ = 0.1–0.23 at 103 Hz), shown in Figure 6d,f. Materials 2020, 13, 3341 7 of 14

Materials 2020, 13, x FOR PEER REVIEW 7 of 14

FigureFigure 6. Frequency6. Frequency dependence dependence onon the dielectric constant constant of ofTiC/TPU TiC/TPU (a), (PDA@TiCa), PDA@TiC−60/TPU60 /(TPUb); (b); 3 − dielectricdielectric constant constant of of TiC TiC/TPU/TPU andand PDA@TiCPDA@TiC−60/TPU60/TPU with with different different filler filler content content at 10 at Hz 10 3(c)Hz; (c); frequency dependence on dissipation factor of TiC/TPU− (d), PDA@TiC−60/TPU (e) and dissipation frequency dependence on dissipation factor of TiC/TPU (d), PDA@TiC 60/TPU (e) and dissipation factor of TiC/TPU and PDA@TiC−60/TPU with different filler content at 103 −Hz (f). factor of TiC/TPU and PDA@TiC 60/TPU with different filler content at 103 Hz (f). − The dielectric dissipation factor (tan δ) measurement formula [33] is: Figure6e shows that with the increase of PDA@TiC 60 content, the dissipation factors of − PDA@TiC 60/TPU exhibited a very slightTan δ increase= tan δp + (tantan δGδ = 0.05–0.1 at 103 Hz). The change(2) of − PDA@TiCin which60 tan doping δp and tan content δG are hadthe relaxation little influence polarization on the loss dissipation and leakage factors. loss, respectively. However, The the tan overall − 3 valueδp inand loss tan was δG can much be calculated smaller than as: that of the TiC/TPU system (tan δ = 0.1–0.23 at 10 Hz), shown in Figure6d,f. ( ∞) tan δp = (3) The dielectric dissipation factor (tan δ) measurement𝜀𝜀J−∞𝜀𝜀 𝜔𝜔𝜔𝜔 formula [33] is: 2 2 𝜀𝜀J+𝜀𝜀 𝜔𝜔 𝜏𝜏 tan δG = × (4) 𝜎𝜎 1 Tan δ = tan δp + tan𝜀𝜀J−𝜀𝜀∞δG (2) 𝜔𝜔𝜀𝜀0 𝜀𝜀∞+1+𝜔𝜔2𝜏𝜏2 in which tan δp and tan δG are the relaxation polarization loss and leakage loss, respectively. The tan δp and tan δ can be calculated as: G εJ ε ωτ tan δ = − ∞ (3) p 2 2 εJ + ε ω τ ∞ Materials 2020, 13, x FOR PEER REVIEW 8 of 14 where is static dielectric constant of the medium, ∞ is dielectric optical permittivity, ω is angular frequency, τ is the time constant, σ is the conductivity and is the vacuum dielectric 𝜀𝜀J 𝜀𝜀 constant. 𝜀𝜀0 It can be determined from Equation (4) that when the medium’s conductivity (σ) is relatively Materials 2020, 13, 3341 8 of 14 high a proportionate increase in leakage loss will appear. Loss of polarization is relatively difficult to highlight. The conductivity and agglomeration of TiC make it easy to generate leakage current after σ 1 energizing, and it results in huge leakagetan δ =loss. In contrast, the covalent bonds between PDA and(4) G εJ ε ωε0 × ∞ TPU tie it down via the -OH, which is free to move. εThe+ number− 2 2 of moving charges reduces on the ∞ 1+ω τ surface of TiC. As a result, the loss caused by leakage current obviously decreases, and the overall where εJ is static dielectric constant of the medium, ε is dielectric optical permittivity, ω is angular dissipation factors value of the composite is effectively∞ cut down. Meanwhile, Equation (4) shows frequency, τ is the time constant, σ is the conductivity and ε is the vacuum dielectric constant. the dielectric loss is proportional to electrical conductivity, which0 is given expression to the tan δ of It can be determined from Equation (4) that when the medium’s conductivity (σ) is relatively TPU composites keeping consistent with the content of fillers. In the low-frequency region (103 Hz), high a proportionate increase in leakage loss will appear. Loss of polarization is relatively difficult to tan δp arises from the friction of polarization orientation. In the high-frequency region (105–107 Hz), highlight. The conductivity and agglomeration of TiC make it easy to generate leakage current after the tan δp mainly arises from the polymer itself, such as the C-N key polarization. Additionally, the energizing, and it results in huge leakage loss. In contrast, the covalent bonds between PDA and TPU insulating PDA shell on the surface of TiC particles prevents them from direct contact in the matrix, tie it down via the -OH, which is free to move. The number of moving charges reduces on the surface thereby suppressing the loss and decreasing the conductivity to low levels, and the shell thickness of of TiC. As a result, the loss caused by leakage current obviously decreases, and the overall dissipation PDA has an effect on the dielectric properties of the composites. So, the dielectric properties of factors value of the composite is effectively cut down. Meanwhile, Equation (4) shows the dielectric TiC/TPU can be effectively tuned by adjusting the thickness of the PDA interlayer. loss is proportional to electrical conductivity, which is given expression to the tan δ of TPU composites Figure 7 presents the effect of the mole ratio of TiC and DA on the dielectric constant and keeping consistent with the content of fillers. In the low-frequency region (103 Hz), tan δ arises from dissipation factor of the TPU composites when the filling quality is set as 13 wt %. Asp seen from the friction of polarization orientation. In the high-frequency region (105–107 Hz), the tan δ mainly Figure 7a, for the dielectric constant the TPU composites had a similar trend. The dielectric constantp arises from the polymer itself, such as the C-N key polarization. Additionally, the insulating PDA shell of the sample with a higher mole ration of DA (20:1) was 13 at 103 Hz and it remained constant until on the surface of TiC particles prevents them from direct contact in the matrix, thereby suppressing the it was at the frequency of 1 MHz. As shown in Figure 7c with decreasing the mole ratio of the DA loss and decreasing the conductivity to low levels, and the shell thickness of PDA has an effect on the mole proportion, the dielectric constant of composites increased from 13 to 31 at 103 Hz. Therefore, it dielectric properties of the composites. So, the dielectric properties of TiC/TPU can be effectively tuned is important to control the shell thickness. According to the Maxwell-Wagner-Sillar (MWS) effect, it by adjusting the thickness of the PDA interlayer. can be assumed that the charge carriers were accumulated at the interface of PDA and the TiC near Figure7 presents the e ffect of the mole ratio of TiC and DA on the dielectric constant and the percolation threshold. The adjacent TiC are considered as parallel electrodes whereas the layer of dissipation factor of the TPU composites when the filling quality is set as 13 wt %. As seen from PDA and PVDF as a dielectric material placed in between electrodes giving rise to several Figure7a, for the dielectric constant the TPU composites had a similar trend. The dielectric constant microcapacitors. With an increase in the molar amount of PDA, no changes had taken place in the of the sample with a higher mole ration of DA (20:1) was 13 at 103 Hz and it remained constant number of microcapacitors, but the thickness increased (Figure 4) of the dielectric layer, effectively until it was at the frequency of 1 MHz. As shown in Figure7c with decreasing the mole ratio of suppressing the electron accumulations at the interface of PDA@TiC and TPU, leading to a low the DA mole proportion, the dielectric constant of composites increased from 13 to 31 at 103 Hz. dielectric constant [30]. Therefore, it is important to control the shell thickness. According to the Maxwell-Wagner-Sillar Figure 7b represents in the low-frequency region (102–103 Hz), with the increase of the PDA (MWS) effect, it can be assumed that the charge carriers were accumulated at the interface of PDA coating thickness the loss factor obviously decreased and was kept at a relatively low level (<0.1). and the TiC near the percolation threshold. The adjacent TiC are considered as parallel electrodes There are two possible reasons for this phenomenon. The direct contact and agglomeration between whereas the layer of PDA and PVDF as a dielectric material placed in between electrodes giving rise particles among the matrix were effectively prevented by the PDA insulation coating on the surface to several microcapacitors. With an increase in the molar amount of PDA, no changes had taken of TiC, and the leakage current was then suppressed and stopped. The DC (direct current) leakage place in the number of microcapacitors, but the thickness increased (Figure4) of the dielectric layer, suppression was more obvious with the increased coating thickness (Figure 7c). Additionally, the effectively suppressing the electron accumulations at the interface of PDA@TiC and TPU, leading to a interfacial polarization becomes weaker at the same time [34–36], which leads to superior low dielectric constant [30]. performances for the composites.

Figure 7. Cont. Materials 2020, 13, 3341 9 of 14

Materials 2020, 13, x FOR PEER REVIEW 9 of 14

Figure 7. Frequency dependence on the dielectric constant (a), dissipation factors (b) of TPU and Figure 7. Frequency dependence on the dielectric constant (a), dissipation factors (b) of TPU and TPU composites with different mole ratio of titanium carbide to dopamine. Dielectric constant and TPU composites with diﬀerent mole ratio of titanium carbide to dopamine. Dielectric constant and dissipation factors of TPU composites with a different mole ratio of titanium carbide to dopamine at dissipation factors of TPU composites with a diﬀerent mole ratio of titanium carbide to dopamine at 103 Hz and room temperature (c). 103 Hz and room temperature (c).

DielectricFigure7b representsbreakdown in strength the low-frequency indicates regionthat a material (10 2–103 canHz), withstand with the increase the highest of the electric PDA coating field strengththickness under the loss the factor action obviously of an electric decreased field and to wasavoid kept being at a relativelydestroyed. low It is level calculated (<0.1). There by Weibull are two plotspossible according reasons to for the this following phenomenon. formula The [37] direct: contact and agglomeration between particles among the matrix were effectively prevented byP(E the) = PDA1 − exp( insulationE/E0)β coating on the surface of TiC, and(5 the) leakage current was then suppressed and stopped. The DC (direct current) leakage suppression was wmorehere obviousE is the withmeasured the increased breakdown coating strength thickness; P(E) (Figure is the7 cumulativec). Additionally, probability the interfacial as a function polarization of E; Ebecomes0 is the breakdown weaker at the strength same timewhen [34 the–36 cumulative], which leads failure to superior probability performances is 63.2% and for theis also composites. used to representDielectric Weibull breakdown breakdown strength strength indicates (Weibull that E aB material) and β is can a withstandshape parameter the highest to evaluate electric fieldthe scatterstrength of underthe measured the action breakdown of an electric strength. field to avoid According being destroyed.to the IEEE It is(I calculatednstitute of byElectrical Weibull plotsand Electronicaccording Engineers) to the following standard formula 930–2004, [37]: the probability ( ) of breakdown is given as: = (I − 0.44)/(n + 0.25) × 100%i (6) 𝑝𝑝 β P(E) = 1 exp(E/E0) (5) where i is the serial number when𝑝𝑝i the breakdown− data of E are sorted in ascending order and n is the numberwhere E ofis specimens the measured for each breakdown distribution strength; and nP(E) wasis 5 thein this cumulative research. probability as a function of E; E0 isE theB of breakdown PDA@TiC strength−60/TPU whenand theTiC/TPU cumulative nanocomposites failure probability were isrevealed 63.2% andin isFig alsoure used 8a,b to, respectively.represent Weibull Figure breakdown 8c presents strength the order (Weibull of EB isE PDA@TiCB) and β is−60 a shape/TPU− parameter1 > PDA@TiC to evaluate−60/TPU the−4 > scatter TPU >of PDA@TiC the measured−60/TPU breakdown−7 > PDA@TiC strength.−60/TPU According−10 > PDA@TiC to the IEEE−60/TPU (Institute−13 of> TiC/TPU Electrical−x and. It Electronicindicates thatEngineers) the addition standard of PDA@TiC 930–2004,− the60 particles probability greatly (pi) of improve breakdownd the is breakdown given as: field strength of the composites at low filler loading. The particles provided a large number of interfaces in the composite material, and enhanced the interfacep = coupling(I 0.44 )effect./(n + 0.25PDA@TiC) 100%−60/TPU was markedly superior to (6) i − × TiC/TPU because of PDA’s weak polar molecular structure and high electrical insulation. The enhwhereancedi is interfac the serialial number interactions when between the breakdown PDA@TiC data−60 of Eandare TPU sorted impart in ascendinged the composites order and n withis the highernumber dielectric of specimens strength for eachbecause distribution more electrical and n was energy 5 in this was research. needed to destroy the samples as E of PDA@TiC 60/TPU and TiC/TPU nanocomposites were revealed in Figure8a,b, respectively. comparedB with the raw− TiC/TPU. EB decreased with further increasing of two kinds of fillers, which Figure8c presents the order of E is PDA@TiC 60/TPU 1 > PDA@TiC 60/TPU 4 > TPU > was due to an excess of nanoparticlesB in the composites− producing− more empty− areas− and pores, PDA@TiC 60/TPU 7 > PDA@TiC 60/TPU 10 > PDA@TiC 60/TPU 13 > TiC/TPU x. It indicates especially in− the interface− region of inorganic− − particles and the− polymer− matrix [38,39].− that the addition of PDA@TiC 60 particles greatly improved the breakdown field strength of the − composites at low filler loading. The particles provided a large number of interfaces in the composite material, and enhanced the interface coupling effect. PDA@TiC 60/TPU was markedly superior to − TiC/TPU because of PDA’s weak polar molecular structure and high electrical insulation. The enhanced interfacial interactions between PDA@TiC 60 and TPU imparted the composites with higher dielectric − strength because more electrical energy was needed to destroy the samples as compared with the raw TiC/TPU. EB decreased with further increasing of two kinds of fillers, which was due to an excess of nanoparticles in the composites producing more empty areas and pores, especially in the interface region of inorganic particles and the polymer matrix [38,39]. Materials 2020, 13, 3341 10 of 14 Materials 2020, 13, x FOR PEER REVIEW 10 of 14

FigFigureure 8. WeibullWeibull plots plots of of PDA@TiC PDA@TiC−6060/TPU/TPU (a (a) )and and TiC/TPU TiC/TPU ( (bb)) and and the the breakdown breakdown strength strength of of − PDA@TiCPDA@TiC−6060/TPU/TPU and and TiC/TPU TiC/TPU with with different different weight loadings of fillers fillers ( c). − FFigureigure 99aa showsshows thatthat thethe tensiletensile strengthstrength ofof PDA@TiCPDA@TiC−60/TPU60/TPU r roseose with with an an increase increase in in the the − contentscontents ofof PDA@TiCPDA@TiC−60,60, whichwhich was was much much higher higher than than that that observed observed in pure in TPU.pure ThisTPU may. This account may − accountfor the PDA for the coating, PDA coating, which provides which provides a strong a interface strong interface bonding bonding force between force between TiC particles TiC particles and the andTPU the matrix, TPU and matrix, improved and improved dispersion dispersion of TiC [40, 41of]. TiC It seems [40,41] that. It tensile seems strength that tensile is dependent strength onis dependeinterfacialnt adhesion,on interfacial where adhesion, it almost where linearly it almost increases linearly with theincreases filler content. with the The filler tensile content. strength The tensileof PDA@TiC strength60 of/ TPUPDA@TiC13 shows−60/TPU a 169%−13 shows performance a 169% performance higher than higher that observed than that withobserved only with TiC. − − onlyThe mechanicalTiC. The mechanical properties properties were unsatisfactory were unsatisfactory with more with added more raw added TiC, r perhapsaw TiC, dueperhaps to the due poor to thecompatibility poor compatibility of the raw of TiC the with raw the TiC resin with matrix the resin and the matrix agglomeration and the agglomeration of TiC particles of [42 TiC]. Moreover,particles [42]the. phase Moreover, separation the phase introduces separation many introduces interfacial many defects interfacial and plays defects a negative and roleplays in a the negative improvement role in theof theimprovement mechanical of strength the mechanica [23,43].l Instrength Figure [239b, however,43]. In Figure increasing 9b however, TiC contents increasing would TiC causecontents an wouldopposite cause effect inan the opposite toughness effect for two in composites. the toughness Although for PDA@TiC two composites.60/TPU 13 showsAlthough the − − PDA@TiCelongation−60 at/TPU break−13 (EAB) shows was lowerthe elongation than that ofat pure break polyurethane (EAB) was it still lower shows than the lowestthat of value pure of polyurethanetoughness that it wasstill upshows to 740%, the lowest which value was obviously of toughness higher that than was that up observed to 740%, inwhich TiC/TPU was obviously13 (510%). higher than that observed in TiC/TPU−13 (510%). − Materials 2020, 13, 3341 11 of 14 Materials 2020, 13, x FOR PEER REVIEW 11 of 14

FigureFigure 9. 9. EffectEﬀect ofof TiCTiC and and PDA@TiC PDA@TiC60−60 loading loading amount amount in TiC in /TPUTiC/TPU and PDA@TiCand PDA@TiC60/TPU−60/TP on tensileU on − − tensilestrength strength (a) and (a strain) and (strainb), respectively. (b), respectively.

4.4. Conclusion Conclusionss CoreCore–shell–shell structured PDA@TiCPDA@TiC compositecomposite particles particles with with shell shell thicknesses thicknesses about about 20–40 20 nm–40 werenm weresuccessfully successfully synthesized. synthesized. The The dissipation dissipation factors factors of of the the PDA@TiC PDA@TiC−6060/TPU/TPU remainedremained at a a much much − lowerlower level level (tan (tan δδ == 0.050.05–0.1–0.1 at at 10 103 3Hz)Hz) when when compared compared with with that that of of pristine pristine TiC TiC (tan (tan δδ == 0.10.1–0.23–0.23 at at 101033 Hz)Hz) due due to to the the suppressing suppressing effect eﬀect of of the the PDA PDA shell shell surrounding surrounding TiC. TiC. The The dielectric dielectric constant constant of of the the compositescomposites with with 13 13 wt wt % % PDA@TiC PDA@TiC−6060 at at 10 103 3HzHz could could reach reach up up to to 31, 31, which which was was nearly nearly 9 9 times times that that − ofof the the pure pure TPU TPU ( (εε == 3.63.6 at at 10 1033 Hz).Hz). Increasing Increasing the the thickness thickness of of PDA PDA result resulteded in in better better dispersion dispersion of of PDA@TiCPDA@TiC particles particles in in TPU, TPU, and and slightly slightly decrease decreasedd both both the the dielectric dielectric constant constant and and the the dissipation dissipation factor.factor. PDA@TiC PDA@TiC−6060/TPU/TPU composites composites exhibit exhibiteded enhanced enhanced breakdown breakdown strength, strength, i.e., i.e., 24 2444 kV/mm kV/mm for for − TPUTPU with with 1 1 wt wt % % PDA@TiC PDA@TiC−6060 compared compared with with 190 190 kV/mm kV/mm for for TPU TPU with with 1 1 wt wt % % TiC, TiC, which which seem seemeded − attributableattributable to to the the PDA PDA s shell.hell. The The PDA PDA layer layer also also improve improvedd the the tensile tensile strength strength and and elongation elongation at at breakbreak of of PDA@TiC/TPU. PDA@TiC/TPU.

AuthorAuthor Contributions: Contributions: Conceptualization,Conceptualization, X. X.H.,H., J.Z J.Z.. and and W. W.Z.;Z.; methodology, methodology, X. X.H.,H., J.Z J.Z,, H. H.W.W. and and W. W.Z.;Z.; investigation, X.H., J.Z., L.J., X.L., H.W., L.X., Y.G. and W.Z.; writing—original draft preparation, X.H., J.Z. investigation, all the authors contributed; writing—original draft preparation, X.H., J.Z. and H.W; and H.W.; writing—review and editing, X.H., H.W. and W.Z.; visualization, X.H. and H.W.; supervision, X.H. and writingW.Z.; funding—review acquisition, and editing, W.Z. X.H All., H. authorsW. and have W.Z read.; visualization, and agreed toX. theH. and published H.W.; supervision, version of the X. manuscript.H. and W.Z.; funding acquisition, W.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 51577154. Funding: This research was funded by the National Natural Science Foundation of China, grant number Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the 51577154.study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publishReferences the results. 1. Li, Y.; Fan, M.; Wu, K.; Yu, F.; Chai, S.; Chen, F.; Fu, Q. Polydopamine coating layer on graphene for Referencessuppressing loss tangent and enhancing dielectric constant of poly(vinylidene fluoride)/graphene composites. 1. Li,Compos. Y.; Fan, Part M.; A Appl.Wu, K.; S. 2015Yu, ,F.;73 ,Chai, 859–862. S.; Chen, [CrossRef F.; Fu,] Q. Polydopamine coating layer on graphene for 2. suppressingJiang, L.; Betts, loss A.; tangent Kennedy, and D.; enhancing Jerrams, S.dielectric Improving constant the electromechanical of poly(vinylidene performance fluoride)/graphene of dielectric composites.elastomers Compos. using silicone Part A rubber—Appl. andS. 2015 dopamine, 73, 859– coated862. barium titanate. Mater. Des. 2015, 85, 7337–7342. 2. Jiang,[CrossRef L.; Betts,] A.; Kennedy, D.; Jerrams, S. Improving the electromechanical performance of dielectric 3. elastomersGómez, J.; using Recio, silicone I.; Navas, rubber A.; and Villaro, dopamine E.; Galindo, coated B.; barium Ortega-Murguialday, titanate. Mater. Des. A. Processing2015, 85, 7337 influence–7342. on 3. Gómez,dielectric, J.; Recio, mechanical, I.; Navas, and electricalA.; Villaro, properties E.; Galindo, of reduced B.; Ortega graphene-Murguialday, oxide-TPU A. Processing nanocomposites. influenceJ. Appl. on dielectric,Polym. Sci. mechanical,2019, 136, 47220.and electrical [CrossRef properties] of reduced graphene oxide-TPU nanocomposites. J. Appl. 4. Polym.Yang, J.;Sci. Zhang, 2019, 136 T.; Zhang,, 47220. Y.; Zhang, N.; Huang, T.; Wang, Y. Constructing the “core-shell” structured island 4. Yang,domain J.; Zhang, in polymer T.; Zhang, blends Y.; to achieveZhang, highN.; Huang, dielectric T.; constantWang, Y. and Constructing low loss. Polym. the “core Int.-shell”2020, 69structured, 228–238. island[CrossRef domain] in polymer blends to achieve high dielectric constant and low loss. Polym. Int. 2020, 69, 228–238. Materials 2020, 13, 3341 12 of 14

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