Enhanced Flexibility and β-Phase Crystallization in PVDF/BaTiO₃ Composites via Ionic Liquid Integration for Multifunctional Applications

Abstract

Piezoelectric polymer composites, particularly polyvinylidene fluoride (PVDF) blended with barium titanate (BT), show promise for wearable technologies as both energy harvesters and haptic actuators. However, these composites typically exhibit limited electromechanical coupling and insufficient β-phase formation. This study presents a novel approach using ionic liquids (ILs) to enhance PVDF-based piezoelectric composite performance. Through solution-casting methods, we examined the effect of IL concentration on the structural, mechanical, and piezoelectric properties of PVDF/BT composites. Results demonstrate that the use of IL significantly improves β-phase crystallization in PVDF while enhancing electrical properties and mechanical flexibility, which are key requirements for effective energy harvesting and haptic feedback applications. The optimized composites show a 25% increase in β-phase content, enhanced flexibility, and a 100% improvement in piezoelectric voltage output compared to other more conventional PVDF/BT systems. The IL-modified composite exhibits superior piezoelectric response, generating an output voltage of 9 V and an output power of 40.1 µW under mechanical excitation and a displacement of 138 nm when subjected to 13 V peak-to-peak voltage, making it particularly suitable for haptic interfaces. These findings establish a pathway toward high-performance, flexible piezoelectric materials for multifunctional wearable applications in human–machine interfaces.

1. Introduction

The intersection of wearable technology and human–machine interfaces has created an urgent demand for materials that can simultaneously harvest energy from human motion and provide tactile feedback through haptic actuation [1,2]. This dual functionality is particularly crucial for creating self-powered, interactive wearable systems that enhance user experience while minimizing power requirements. Piezoelectric materials, which convert mechanical energy to electrical signals and vice versa, represent an ideal material for addressing these complementary needs.

Among all piezoelectric materials that meet the requirements needed in wearable technologies and haptic feedback applications, flexible polymer-based piezoelectric materials have attracted significant attention due to their ability to be shaped and structured in various forms using different polymers, such as polyvinylidene fluoride (PVDF) and its copolymers [3,4], polyamide [5], or Nylon [6]. Polyvinylidene fluoride (PVDF) has emerged as a leading candidate among piezoelectric polymers due to its flexibility, biocompatibility, and processability. However, the piezoelectric properties of PVDF are strongly influenced by its crystalline phases. PVDF exhibits five polymorphic crystalline forms—α, β, γ, δ, and ε—each determined by specific processing conditions. Notably, the β-phase possesses the most pronounced electroactive characteristics among them [6]. Therefore, several approaches have been followed in the literature to boost β-phase transformation including nucleating with ceramic particles or through the application of high electrical field. When PVDF is combined with ceramic particles, these composites offer enhanced piezoelectric performance while maintaining the mechanical advantages of polymers.

Recent research by Abdolmaleki and Agarwala [7] demonstrated that incorporating hydrothermally synthesized BaTiO₃ nanoparticles at 5 wt.% in PVDF matrices significantly enhanced β-phase formation, resulting in increased piezoelectric output voltage compared to pure PVDF and showed that BaTiO₃ is not only acting as a nucleating agent for boosting β-phase crystallinity, but it also ameliorates the electric properties of PVDF by a synergistic ferroelectric polarization effect.

Similarly, Ben Ayed et al. [8] reported the use of Zn -BCZT ceramics for the realization of nanocomposites based on PVDF-HFP. The composite exhibits an enhancement of the crystal phase transformation leading to enhanced piezoelectric performances, a maximum peak-to-peak output voltage of 3.3 V, and output power of 2.13 μW for 1 MΩ load resistance and under excitation of a mechanical shaker at a frequency of (40 Hz). In addition, Wankhade et al. [9] prepared a nanocomposite formed with poly(vinylidene fluoride) and lead zirconia titanate (PZT) that was synthesized via a solution route for energy harvesting applications. They illustrated that pure PVDF consists of mainly α–phase, which possesses spherulitic morphology. With the addition of PZT, the reduction of spherulite dimension is observed, indicating the nucleating effect of PZT as well as the enhancement of β-phase transformation.

Among ceramic particles, lead-free materials are gaining attention due to environmental and health concerns associated with lead-based piezoelectric. BaTiO₃ (BT) particles have emerged as particularly promising fillers due to their excellent ferroelectric properties, environmental friendliness, and ability to nucleate β-phase crystallization in PVDF [10,11].

However, traditional PVDF/BT composites face persistent challenges that limit their practical application in wearable technologies and haptic feedback applications. These include insufficient β-phase content, the crystalline phase responsible for piezoelectric properties in PVDF, poor electromechanical coupling, and inadequate flexibility for conformable wearable applications.

A critical limitation for haptic feedback applications is the high driving voltage typically required for actuating these composites. Conventional PVDF and PVDF/BT actuators often require operational voltages exceeding 50 V to produce tactile sensations detectable by human skin [12,13].

Previous approaches to enhance PVDF/BT composites have explored various strategies including mechanical stretching, electrical poling, and the incorporation of nanofillers. While these methods have shown moderate success, they often require complex processing techniques or compromise the material’s flexibility. A more effective and straightforward approach is needed to simultaneously enhance the piezoelectric response and mechanical properties of these composites while enabling low-voltage operation suitable for wearable applications.

Consequently, a new generation of actuators capable of inducing large deformation at low frequencies and operating voltages is now emerging, with the most promising advancements centering on the strategic incorporation of ionic liquids (ILs) within polymer matrices [14].

Ionic liquids (ILs) are essentially molten salts, typically comprising an organic cation paired with an inorganic anion [15]. These unique materials exhibit distinctive properties including superior ionic conductivity, negligible vapor pressure, exceptional thermal and electrochemical stability, and a wide electrochemical operating window [16]. Such characteristics make them particularly valuable as functional additives in advanced composite systems. In [17], printed soft actuators have been realized based on both ILs and poly(vinylidene fluoride) (PVDF) to induce the highest displacement of 1.0 mm at an applied voltage of 4 V peak-to-peak voltage. In addition, the addition of ionic liquid in PVDF polymer illustrates as well their effectiveness in energy harvesting. Liu et al. [18] reported about the development of an energy harvester using an ionic liquid (IL)-assisted fused deposition modeling (FDM) approach that delivers a maximum voltage output of up to 4.7 V and an area current density of 17.5 nA cm⁻². In this study, they demonstrated that the incorporation of IL promotes and stabilizes the formation of β-phase crystals during both melt extrusion and the FDM printing process, achieving a β-phase content of up to 98.3% in the printed PVDF. As a result, the IL not only acts as a modifier to induce the formation of β-crystals in PVDF but also serves as a plasticizer, enhancing the material’s processability during melt extrusion and printing. Furthermore, Ye et al. [19] enhanced the PVDF piezoelectricity by introducing oriented nanobound state ions using an ionic liquid (IL). The IL increases PVDF crystallinity and dipole orientation through ion–dipole interactions, boosting polarization strength to 140.0 mC/m² and remnant polarization to 121.0 mC/m². The modified film exhibits excellent linear sensing behavior. Its inverse piezoelectric coefficient reaches 51.8 pm/V, representing a 116% improvement over commercial PVDF.

In this study, we present a novel strategy utilizing ionic liquids (ILs) as functional additives to significantly improve the performance of PVDF/BT composites. Ionic liquids salts that remain liquid at room temperature offer unique advantages as modifying agents due to their ionic conductivity, thermal stability, and ability to interact with polymer chains. We hypothesize that the incorporation of ILs can facilitate β-phase formation in PVDF while simultaneously enhancing charge mobility and mechanical flexibility, addressing the key limitations of conventional PVDF/BT composites.

Through systematic variation of IL concentration in solution-cast PVDF/BT composites, we investigate the structural, mechanical, electrical, and piezoelectric properties of these novel materials. Our findings demonstrate that IL integration represents a promising pathway toward high-performance, flexible piezoelectric composites capable of both efficient energy harvesting and effective haptic feedback in wearable technologies.

2. Materials and Methods

2.1. Materials

In this study, high-performance piezoelectric composites were engineered using carefully selected materials. Polyvinylidene fluoride (PVDF, Mw ~530,000) from Sigma Aldrich (Burlington, MA, USA) served as the primary polymer matrix due to its exceptional piezoelectric properties and ability to form electroactive phases. High-purity barium titanate (BaTiO₃) powder (99.5% purity, <2 μm particle size) was incorporated as the ceramic filler for its superior ferroelectric properties and optimal dimensions for matrix integration. N, N-Dimethylformamide (DMF) was utilized as the solvent medium to ensure uniform dispersion of BalTiO₃ nanoparticles throughout the PVDF matrix, preventing agglomeration and enhancing interfacial compatibility. The ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (EMIM[BF₄]) was added as a multifunctional additive, serving as a conductive agent, β-phase nucleation promoter, and plasticizer, enabling homogeneous solutions with enhanced processability for composite film fabrication.

2.2. Preparation of Polymer Composite Films

The realization of PVDF/BaTiO₃/IL composites involves a refined methodology as detailed in Figure 1. The process begins with the dissolution of PVDF in DMF through controlled heating at 70 °C with continuous agitation for 2 h, yielding a transparent polymeric solution. Subsequently, BaTiO₃ powder (15 wt.%) undergoes dispersion in DMF at 55 °C, followed by brief sonication to prevent agglomeration and ensure better particle distribution. Then, the ionic liquid was integrated into the ceramic suspension. Herein, the IL functions as both a dispersing agent and interfacial modifier, significantly enhancing compatibility between the polymer matrix and ceramic filler. In this study, the aim is to investigate the impact of IL on the final properties of the composite. Therefore, composites containing different content of ILs were made ranging from 5 wt.% to 20 wt.%. The resultant composite solution is cast onto glass substrates and subjected to a two-stage thermal treatment protocol: initial low-temperature drying at 30 °C for 30 min followed by extended curing at 60 °C for 12 h, facilitating optimal β-phase formation and structural integrity in the final composite films.

2.3. Characterization of the Developed Polymer Composite Films

The objective of this morphological and structural characterization was to evaluate the influence of ionic liquid concentration on the PVDF/BaTiO₃ composite microstructure and crystalline phases. Surface morphology was analyzed using scanning electron microscopy (Nova200 NanoLab SEM by Thermo Fischer Scientific, Waltham, MA, USA) operating at 9 kV, which generates high-resolution surface images by scanning a focused electron beam across the sample and detecting the resulting electron emissions. To mitigate charging effects during imaging, samples were prepared with a dual-layer conductive coating: a manual silver layer application, followed by gold sputtering specifically for samples containing lower ionic liquid concentrations.

The structural analysis was conducted for BT/PVDF-based polymer bio-composites containing different amounts of ionic liquids ranging from 0 wt.% to 20 wt.% using Fourier-transform infrared spectroscopy (FTIR) with an INVENIO spectrometer from Bruker, Germany, operating in attenuated total reflectance (ATR) mode. This spectroscopic investigation was performed to identify and quantify the crystal phase transformation within the polymer matrix, particularly focusing on the enhancement of electroactive β-phase content as a function of ionic liquid incorporation. The impedance measurements were carried out using a laboratory impedance analyzer (Agilent 4294A from Keysight Technologies company, Santa Rosa, CA, USA), which was connected to a computer running a LABVIEW program for generating Nyquist plots. Based on the obtained impedance spectra, the dielectric properties were determined over a frequency range of 40 Hz to 110 MHz.

During the measurements, the polymer composites were placed between two parallel silver electrodes to ensure consistent contact. The tensile test was used in this study to determine the influence of ILs on the mechanical properties. This study was performed according to ISO 527-3 standards [20] to evaluate the mechanical behavior of the samples based on 15 wt.% BT/PVDF composites containing different IL concentrations. The tensile test was carried out using a universal testing machine ElectroPuls^® E10000 Instron (Norwood, MA, USA) 10 kN load cell. The crosshead speed was 2 mm/min. Five samples were tested to determine the average properties. These measurements provide comprehensive insights into the composite’s mechanical performance and how it varies with the addition of IL.

2.4. Characterization of the Developed Polymer Composite Films as Generator and Actuator

To evaluate the functionality of the developed materials for energy harvesting and actuators two different setups were used. The direct piezoelectric effect of the composites was examined using a tapping machine and the output voltage was used using a digital oscilloscope as shown in Figure 2a. The tapping frequency used during the direct piezoelectric tests was 12 Hz, which represents the maximum frequency achievable with the tapping machine used in this experiment. In general, it was observed in many previous works that increasing the frequency leads to a higher output voltage, which in turn enhances the power output [11,21]. This is due to more frequent mechanical deformation of the piezoelectric material, resulting in greater charge generation over time.

To assess the performance of the optimized composite as an actuator, a commercial piezoelectric patch was used to evaluate the displacement of the composite film as demonstrated in Figure 2b. The prepared composite film was mounted on top of the commercial patch and connected to a voltage generator capable of producing a ±15 V square wave signal. The generator supplied the input voltage to the composite film, inducing bending in the commercial actuator. This bending, in turn, acted on the commercial patch, generating an output voltage, which was observed on a digital oscilloscope. In this study, a square wave voltage of 13 Vpp was applied at 100 Hz frequency corresponding to the tactile range of human haptic perception.

3. Results and Discussions

The SEM analysis in Figure 3a reveals a homogeneous distribution of BT particles within the PVDF polymer matrix, resulting in a smooth, unstructured surface. With the addition of IL, structured domains begin to emerge, indicating enhanced crystal phase transformation in the polymer. Figure 3b illustrates the impact of a low concentration of IL. The sample containing 5 wt.% IL exhibits spherulitic morphology associated with the α–phase. With the addition of IL, the reduction of spherulite dimension is observed, indicating the nucleating effect of IL as well as the enhancement of β-phase transformation as shown in Figure 3d. However, it was remarked that due to the addition of IL, micro-voids form. These voids promote particle agglomeration, which may impact both the mechanical and electrical properties of the composite. Therefore, it can be concluded that the addition of more IL over 15 wt.% will inhibit the material properties.

These voids were similarly observed in the work of Zhou et al. [22] where it was found that the surface of the PVDF film without IL was relatively flat and void-free. By adding IL, voids began to form on PVDF’s surface between spherical crystals, resulting in a significant change in the surface morphology.

The FTIR spectra presented in Figure 4 provides crucial insights into the crystalline phase composition of PVDF/BaTiO₃ composites with varying ionic liquid (IL) concentrations. The spectra demonstrate the significant influence of ionic liquid addition on the polymorphic structure of PVDF. The β-phase content is critical for maximizing the piezoelectric effect in haptic actuators. The proportion of the crystalline β-phase within the films is determined using the following Equation (1):

$$F\left(\beta \right)={\displaystyle \frac{A_{\beta }}{1.26A_{\alpha }+A_{\beta }}}\times 100$$

(1)

The α-phase is identified by absorption peaks at approximately 760, 975, and 1210 cm⁻¹, while the electroactive β-phase is evidenced by peaks at around 840 and 1430 cm⁻¹.

As ionic liquid concentration increases progressively from 5 wt.% to 10 wt.% and 15 wt.%, a clear enhancement of the β-phase characteristic peaks is observed, particularly at 840 cm⁻¹ and 1430 cm⁻¹. This indicates that the ionic liquid effectively promotes the formation of the electroactive β-phase crystalline structure in PVDF.

However, at the highest IL concentration (20 wt.%), there appears to be saturation, with less pronounced β-phase peaks compared to the 15 wt.% IL sample. This spectroscopic observation is precisely quantified in the bar graph Figure 3b, which demonstrates that β-phase content increases systematically from approximately 70% (15 wt.% BaTiO₃ without IL) to a maximum of approximately 88–90% at the optimal IL concentration of 15 wt.%. This analysis confirms the existence of a non-linear relationship, where IL addition beyond this optimal point (15 wt.%) results in diminished β-phase formation, with content decreasing to approximately 80–82% as shown for the sample with 20 wt.% IL. The enhancement of β-phase formation by ionic liquids in PVDF occurs through specific ion–dipole interactions between IL components and polymer chain segments. EMIM⁺ cations preferentially interact with electronegative CF₂ groups, while BF₄⁻ anions coordinate with electropositive CH₂ groups, creating complementary electrostatic interactions that stabilize the all-trans (TTTT) conformation characteristic of the β-phase [23]. Simultaneously, EMIM[BF₄] achieves directional arrangement of PVDF molecular chains, creating localized electric fields that orient the CF₂ and CH₂ dipoles perpendicular to the polymer chain axis [24]. This dual mechanism of electrostatic stabilization and field-induced alignment provides both thermodynamic driving force and kinetic facilitation for β-phase formation, explaining the superior effectiveness of ionic liquids compared to conventional additives.

Therefore, FTIR measurement demonstrates that ionic liquid addition serves as an effective chemical approach for promoting β-phase transformation in PVDF/BaTiO₃ composites.

The stress–strain curves in Figure 4a reveal that increasing IL concentration significantly enhances flexibility and stretchability while sacrificing some rigidity and initial strength. The sample without IL exhibits the highest tensile strength around 30 MPa but fails at a relatively low strain around 45%. Therefore, this sample exhibits the highest stiffness, Young’s modulus of ~1250 MPa, while all IL-containing samples demonstrate significantly reduced moduli. Tensile strength decreases dramatically with initial IL addition from ~27 MPa to ~8 MPa, then gradually increases with higher IL concentrations. This suggests that while small amounts of IL disrupt the polymer matrix strength, higher concentrations may contribute to alternative strengthening mechanisms, possibly through better interfacial interactions or more uniform BT distribution. As a result, the 15 wt.% IL sample appears to offer a balanced compromise between mechanical properties, showing moderate strength ~12 MPa, good elasticity ~82%, and reasonable modulus ~550 MPa, making it particularly suitable for flexible and wearable applications where durability is required. Furthermore, the results strongly support those ionic liquid functions as an effective plasticizer in these composites, significantly improving their deformation capacity while maintaining adequate mechanical integrity. The complex impedance spectroscopy data presented in Figure 5 provides critical insights into the electrical behavior of the PVDF/BaTiO₃ composite films with varying ionic liquid (IL) concentrations. The Nyquist plots (Z″ vs Z′) in Figure 5a,b reveal characteristic semicircular arcs. These semicircular features signify the presence of both inter-particle charge transfer mechanisms and polarization resistance phenomena at the material interfaces. Figure 5a shows the impedance response of the sample containing only BaTiO₃ (15 wt.%) without ionic liquid, displaying a steep impedance curve with high resistance values reaching approximately 2.0 × 10⁸ Ω. In contrast, Figure 5b demonstrates a systematic decrease in the semicircle diameter with increasing IL concentration indicating the reduced bulk resistance. The sample containing 15 wt.% BT and 15 wt.% IL demonstrates an optimal impedance profile, indicating enhanced ionic mobility and charge transport capabilities. This impedance analysis confirms that ionic liquid serves as an effective conductivity enhancer in these composite systems.

The frequency-dependent dielectric properties of the composites are illustrated in Figure 5c,d, which display the real part of the permittivity (ε′) and dielectric loss tangent (tanδ), respectively. The real (ε′) and imaginary (ε″) components of the complex dielectric permittivity (ε*) were calculated from the impedance data using the relation:

$$\epsilon *={\epsilon }^{\prime }-{j\epsilon }^{″}={\displaystyle \frac{1}{j\omega c_0{z}^{\ast }}}$$

(2)

where $\omega =2\pi f$ is the angular frequency, and $C_0$ is the geometric capacitance of the sample, calculated as:

$$C_0={\epsilon }_0.{\displaystyle \frac{A}{d\prime }}$$

(3)

Here, ε₀ is the vacuum permittivity ($8.854\times {10}^{-12}\mathrm{F}/\mathrm{m}$), A is the electrode area, and d is the sample thickness. The dielectric loss tangent (tanδ) was then obtained using the relation:

$$tan\delta =\epsilon ″/{\epsilon }^{\prime }$$

(4)

In Figure 5c, the ε′ values of all samples exhibit a typical dispersion behavior, decreasing with increasing frequency due to the inability of dipolar and interfacial polarization mechanisms to follow the alternating electric field at high frequencies. The pristine 15 wt.% BT/PVDF sample shows the lowest ε′ values throughout the frequency range, indicating limited polarizability due to the absence of mobile ionic species. The incorporation of IL has been shown to result in a significant increase in ε′, particularly at low frequencies. This observation underscores the enhanced space charge polarization and Maxwell–Wagner–Sillars (MWS) interfacial effects induced by the IL. It is noteworthy that the sample with 20 wt.% IL exhibited the highest ε′ value, surpassing 100 at 140 Hz. This finding substantiates the substantial impact of IL in enhancing dipole density and interfacial charge accumulation. Figure 5d provides further evidence for this trend through the tanδ profiles, which reflect the energy dissipation within the system. A notable peak is evident in all IL-containing samples at low frequencies, which is attributed to interfacial polarization relaxation processes. The highest tanδ value, which exceeds 3.0 at 40 Hz, is observed for the 10 wt.% IL sample. This observation indicates an optimal balance between charge storage and conduction losses at this composition. As the IL content increases beyond this point, a gradual decrease in tanδ is observed, indicating a transition from dielectric relaxation dominance to enhanced ion conduction. This finding aligns closely with the observed impedance behavior, thereby validating the hypothesis that the addition of ionic liquid does not exclusively result in a reduction of resistance; rather, it also leads to an enhancement of dielectric responsiveness through the combined action of interfacial and bulk mechanisms. Figure 6 presents comprehensive characterization data for the piezoelectric generators fabricated with varying ionic liquid (IL) concentrations, illustrating their electrical output performance under mechanical stimulation. Figure 6a displays the time-dependent output voltage generated by different piezoelectric composite formulations when subjected to repetitive mechanical loading via a standardized tapping machine. The voltage signals exhibit characteristic alternating positive and negative peaks, corresponding to the compression and release cycles of the piezoelectric material. During compression, piezoelectric charges migrate between the electrodes, generating a positive voltage signal. Conversely, when mechanical pressure is released, these charges return to their equilibrium state, producing a negative voltage response. Notably, the composite containing 15 wt.% ionic liquid demonstrates superior performance, generating the highest output voltage among all tested compositions around 9 V. Furthermore, the power output (P_out) of the piezoelectric generator is determined by measuring the voltage across a variable resistance placed parallel to the nanogenerator. The power output was calculated based on the following Equation (5):

$$P_{out}={\displaystyle \frac{V_{out}^2}{R}}$$

(5)

where V_out is the amplitude of the output voltage and R is the external resistance.

The power curves reveal a characteristic pattern where output power initially increases with load resistance, reaches a maximum value at an optimal resistance, and then decreases at higher resistance. Consistent with the voltage output data, the 15 wt.% IL composite exhibits superior power generation capabilities across the entire resistance range, achieving a maximum power output of approximately 4.01 × 10⁻⁵ W. The comparative analysis in

Table 1. A comparative table of the developed PENG to other reported PVDF-based PENGs.

Type of Sample	Output Voltage (V)	Output Power	Applied Force	Poling Condition	Ref
PVDF/1 wt.% ZnO/	1.81	0.21 μW/cm2	2.5 N	-	[25]
PVDF/2 wt.% Fe–ZnO	2.4	1.17 μW/cm2	2.5 N	-	[25]
BCZT/P(VDF-HFP)	2.5	-	-	Without poling	[26]
BaTiO3/P(VDF-TrFE)	~100 mV	0.02 μW	Bending tests with a maximum displacement ± 7 mm	In situ poling	[27]
(PVDF)-[0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7 Ca0.3 )TiO3]-graphene oxide (GO	4	6.4 μW	Simple hand tapping	Without poling just a hot pressing method	[28]
Rb4Ag2BiBr9-PVDF	60	1.8 μW	100 kPa pressure	Without poling	[29]
BaTiO3/PVDF-HFP NG	2.21	0.22		-	[30]
PVDF/4%KNN NRs	3.7	-	-	15 kV at 80 °C	[31]
15 wt.% BaTiO3/PVDF + 15 wt.% IL	~9	4.01 × 10−5 W/cm2	-	Without poling	This work

demonstrates that the developed PVDF-based PENG in this work exhibits competitive performance among reported piezoelectric nanogenerators. The 15 wt.% BaTiO₃/PVDF + 15 wt.% IL composite achieved an output voltage of ~9 V and a power density of 4.01 × 10⁻⁵ W/cm², representing a significant advancement in self-powered energy harvesting devices. Notably, this performance was achieved without requiring electrical poling, which is a considerable advantage over several other systems that necessitate high-voltage poling conditions (e.g., PVDF/4%KNN requiring 15 kV at 80 °C).

The direct piezoelectric performance revealed that the composite with 15 wt.% BaTiO₃ and 15 wt.% IL demonstrated markedly superior performance compared to all other sample formulations. Therefore, in this part, the actuation test was conducted for the sample containing 15 wt.% IL.

When subjected to a square wave 13 V_PP excitation signal, the piezoelectric patch coupled to our composite actuator produced induced voltage measurements reaching 150 mV, directly corresponding to the actuator’s mechanical displacement. The correlation between the generated voltage (V) and mechanical strain (ε) follows the relationship expressed in Equation (6):

$$V={t\times \sigma \times g}_{31}$$

(6)

In Equation (6), t represents the piezoelectric patch thickness, g₃₁ denotes the piezoelectric voltage coefficient for PZT-5A (approximately 11 × 10⁻³ Vm/N), and σ signifies the applied mechanical stress. Furthermore, the correlation between strain (ε) and stress is characterized by Equation (7):

$$\epsilon ={\displaystyle \frac{\sigma }{E}}$$

(7)

where E corresponds to Young’s modulus of PZT-5A (63 GPa or 63 × 10⁹ Pa). The displacement d can be calculated using Equation (8):

$$d=\epsilon .L$$

(8)

where L corresponds to the commercial piezoelectric patch length.

Quantitative analysis reveals an approximate piezoelectric patch displacement of 138 nm as depicted in Figure 6c. This measured displacement validates the exceptional performance capabilities of the BT+IL hybrid composite system for practical actuation applications. Furthermore, this result demonstrates the effectiveness of incorporating ionic liquids with polymer materials to enhance their properties, enabling the development of robust, flexible, low-voltage actuators particularly well-suited for haptic feedback applications.

4. Conclusions

This work has successfully developed a novel composite material based on PVDF/BaTiO₃ using a solution-casting method, with significant enhancements achieved through the strategic incorporation of ionic liquid (IL). The addition of IL has proven to be a remarkably effective approach for promoting β-phase transformation in PVDF without requiring complex mechanical stretching or electrical poling processes typically associated with piezoelectric polymer processing.

The addition of the IL was found to have a significant impact on the overall material properties of the composite. Through the addition of IL, the mechanical properties were changed enormously to have enhanced flexibility and stretchability. The 15 wt.% IL sample appears to offer a balanced compromise between mechanical properties, showing moderate strength, good elasticity, and reasonable modulus, making it particularly suitable for flexible and wearable applications.

The introduction of ionic liquid into the PVDF/BT composite system has resulted in extraordinary enhancement of space charge polarization and Maxwell–Wagner–Sillars (MWS) interfacial effects. This phenomenon has been conclusively confirmed through dielectric characterization, with the modified composites exhibiting substantial increases in both dielectric permittivity and loss tangent (tan δ) values. These enhanced dielectric properties play a crucial role in improving the electromechanical coupling efficiency of the material system, directly contributing to its superior performance in energy harvesting and haptic actuation applications.

This enhancement of piezoelectric performance was also quantified through FTIR measurement that confirmed that ionic liquid incorporation effectively promotes the transformation from non-polar α-phase to piezoelectrically active β-phase in PVDF, with a well-defined optimization point at 15 wt.% IL. The quantitative analysis demonstrated an increase from approximately 70% β-phase content in the baseline composite to 88–90% in the optimized formulation, representing a substantial improvement in electroactive phase formation.

As a direct result of these structural and property enhancements, the sample with 15 wt.% IL exhibits the highest output voltage and power among all realized composites when subjected to repetitive load at 12 Hz using a tapping machine, generating approximately 9 V and 4.01 × 10⁻⁵ W. Furthermore, this optimized sample demonstrates a displacement of 138 nm when subjected to a low driving voltage of only 13 Vpp, making it particularly promising for low-power haptic feedback applications in wearable devices.

Therefore, incorporation of IL into PVDF/BaTiO₃ overcomes the need for post-polarization treatments, simplifies the fabrication process, and achieves a high β-phase content in mold-casted PVDF composites.

The findings of this work establish a new pathway for developing high-performance, flexible piezoelectric materials that simultaneously address the persistent challenges of insufficient piezoelectric response and high operating voltages that have limited previous PVDF-based systems for haptic applications.