Application of a Coupling Agent to Improve the Dielectric Properties of Polymer-Based Nanocomposites

Summary

Here, we demonstrate a simple and low-cost solution-casting process to improve the compatibility between the filler and the matrix of polymer-based nanocomposites using surface modified BaTiO₃ fillers, which can effectively enhance the energy density of the composites.

Abstract

In this work, an easy, low-cost, and widely applicable method was developed to improve the compatibility between the ceramic fillers and the polymer matrix by adding 3-aminopropyltriethoxysilane (KH550) as a coupling agent during the fabrication process of BaTiO₃-P(VDF-CTFE) nanocomposites through solution casting. Results show that the use of KH550 can modify the surface of ceramic nanofillers; therefore, good wettability on the ceramic-polymer interface was achieved, and the enhanced energy storage performances were obtained by a suitable amount of the coupling agent. This method can be used to prepare flexible composites, which is highly desirable for the production of high-performance film capacitors. If an excessive amount of coupling agent is used in the process, the non-attached coupling agent can participate in complex reactions, which leads to a decrease in dielectric constant and an increase in dielectric loss.

Introduction

The dielectrics applied in electrical energy storage devices are mainly characterized using two important parameters: the dielectric constant (ε_r) and the breakdown strength (E_b)^1,2,3. In general, organic materials such as polypropylene (PP) exhibit a high E_b (~10² MV/m) and a low ε_r (mostly <5)^4,5,6 while inorganic materials, especially ferroelectrics such as BaTiO₃, exhibit a high ε_r (10³-10⁴) and a low E_b (~10⁰ MV/m)^6,7,8. In some applications, flexibility and the ability to withstand high mechanical impacts are also important for fabricating dielectric capacitors⁴. Therefore, it is important to develop methods for preparing polymer-based dielectric composites, especially for the development of low-cost methods to create high-performance 0-3 nanocomposites with high ε_r and E_b^{9,10,11,12,13,14,15,16,17,18}. For this purpose, preparation methods based on ferroelectric polymer matrices such as the polar polymer PVDF and its correlated copolymers are widely accepted due to their higher ε_r (~10)^4,19,20. In these nanocomposites, particles with high e_r, especially ferroelectric ceramics, have been widely used as fillers^{6,20,21,22,23,24,25}.

When developing methods for manufacturing ceramic-polymer composites, there is a general concern that dielectric properties can be significantly influenced by the distribution of fillers²⁶. The homogeneity of dielectric composites is not only determined by the preparation methods, but also by the wettability between the matrix and fillers²⁷. It has been proven by many studies that the non-uniformity of ceramic-polymer composites can be eliminated by physical processes such as spin-coating^28,29 and hot-pressing^19,26. However, neither of these two processes change the surface connection between fillers and matrices; therefore, the composites prepared by these methods are still limited in improving ε_r and E_b^19,27. Additionally, from a manufacturing point of view, inconvenient processes are undesirable for many applications because they can lead to much more complex fabrication processes^28,29. In this regard, a simple and effective method is needed.

Currently, the most effective method to improve the compatibility of ceramic-polymer nanocomposites is based on the treatment of ceramic nanoparticles, which modifies the surface chemistry between fillers and matrices^30,31. Recent studies have shown that coupling agents can be easily coated on ceramic nanoparticles and effectively modify the wettability between fillers and matrices without affecting the casting process^{32,33,34,35,36}. For surface modification, it is widely accepted that for each composite system, there is a suitable amount of coupling agent, which corresponds to a maximal increase in energy storage density³⁷; excess coupling agent in composites may result in a decline in the performance of products^36,37,38. For dielectric composites using nano-sized ceramic fillers, it is speculated that the effectiveness of coupling agent mainly depends on the surface area of fillers. However, the critical amount to be used in each nano-sized system is yet to be determined. In short, further research is required to use coupling agents to develop simple processes for manufacturing ceramic-polymer nanocomposites.

In this work, BaTiO₃ (BT), the most widely studied ferroelectric material with high dielectric constant, was used as fillers, and the P(VDF-CTFE) 91/9 mol% copolymer (VC91) was used as the polymer matrix for the preparation of ceramic-polymer composites. To modify the surface of the BT nanofillers, the commercially available 3-aminopropyltriethoxysilane (KH550) was purchased and used as a coupling agent. The critical amount of the nanocomposite system was determined through a series of experiment. An easy, low-cost, and widely applicable method is demonstrated to improve the energy density of nano-sized composite systems.

Protocol

1. Surface modification of BT fillers

1. Prepare 20 mL of KH550 solution (1 wt% KH550 in 95 wt% ethanol-water solvent) and ultrasonicate for 15 min.
2. Weigh BT nanoparticles (i.e., the filler) and KH550, respectively, so that fillers can be coated with 1, 2, 3, 4, 5 wt% of the coupling agent. Treat 1 g of BT nanoparticles in 1.057, 2.114, 3.171, 4.228, and 5.285 mL of KH550 solution by 30 min ultrasonication.
3. Evaporate the water-ethanol solvent from the mixture at 80 °C for 5 h and then at 120 °C for 12 h in a vacuum oven.
4. Use the dry BT nanoparticles as the surface modified fillers to prepare BT-VC91 nanocomposites.

2. Preparation of BT-VC91 nanocomposites

1. Dissolve 0.3 g of VC91 powders in 10 mL of N,N-dimethylformamide (DMF) at room temperature by magnetic stirring for 8 h to obtain a homogeneous VC91-DMF solution.
2. Add 0.0542, 0.1145, 0.1819, 0.2578, 0.3437, and 0.4419 g of BT nanoparticles into 10 mL of VC91-DMF solution to obtain a final BT percentage of 5, 10, 15, 20, 25, and 30 vol% in the nanocomposites. Mix BT nanoparticles by magnetic stirring for 12 h and ultrasonication for 30 min to form a homogeneous BT-VC91-DMF suspension.
   NOTE: Both the unmodified BT and BT nanoparticles coated with the coupling agent are used.
3. Cast the suspension by evenly pouring the BT-VC91-DMF onto a preheated 75 mm x 25 mm glass substrate (3 mL per substrate). Keep the glass substrates with suspensions in the oven at 70 °C for 8 h to evaporate the DMF solvent to form composite films.
4. Release the composites from glass substrates using sharp tweezers to obtain free-standing BT-VC91 films. Anneal the films on a preheated dust-free paper at 160 °C in air for 12 h.

3. Characterization and measurement

1. Characterize the morphology and uniformity of nanocomposites using a scanning electron microscope (SEM). To do this, freeze BT-VC91 samples in liquid nitrogen and break to show fresh cross section with an approximate size of 5 mm x 30 µm (i.e., the ceramic-polymer interface). Then coat one side of the cross section with a gold layer with a thickness of 3−5 nm and characterize the composite structure using an SEM (Table of Materials).
2. Using a gold coater (Table of Materials), sputter gold layers with a positive circle shape, a diameter of 3 mm, and a thickness of ~50 nm on both sides of the nanocomposite prepared from step 2 to form the electrode for impedance testing.
3. Characterize the capacitance and dielectric loss of the nanocomposites over a frequency range from 100 Hz to 1 MHz using an impedance analyzer (Table of Materials) with the Cp-D function. In the testing, connect gold layers on both sides of the composite film with the two poles of fixture.
4. Calculate the dielectric constant (ε_r) of nanocomposites from the capacitance obtained by impedance analyzer using the parallel capacitor model:
   
   ε_r = dCp/ε₀A
   
   where ε₀ = 8.85 x 10^-12, A is the area of gold electrodes, d is the thickness of sample, and Cp is parallel capacitance obtained by connecting the gold electrodes with the fixture of impedance analyzer.
5. Characterize the breakdown strength of nanocomposites using a 10 kV high voltage supplier (Table of Materials). Increase the applied electric field evenly and continuously until the breakdown of each sample.
6. Characterize the polarization-electric (P-E) field hysteresis loop of nanocomposites using a ferroelectric tester. Record the P-E loops at each electric field while continuously increasing the electric field.

Results

The free-standing nanocomposite films with different contents of fillers were successfully fabricated as described in the protocol, and were labeled as xBT-VC91, where x is the volume percentage of BT in the composites. The effect of KH550 (coupling agent) on the morphology and microstructure of these BT-VC91 films was studied by SEM and shown in Figure 1. The SEM images of 30BT-VC91 nanocomposites with 1 and 5 wt% coupling agent are shown in Figure 1a and

Discussion

As discussed above, the method developed by this work could successfully improve the energy-storage performance of ceramic-polymer nanocomposites. To optimize the effect of such method, it is critical to control the amount of coupling agent used in ceramic-surface modification. For ceramic nanoparticles with a diameter of ~200 nm, it was experimentally determined that 2 wt% of KH550 could lead to a maximal energy density. For other composite systems, this conclusion may be used approximately when the fillers with the dia...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
3-Aminopropyltriethoxysilane (KH550)	Sigma-Aldrich	440140	Liquid, Assay: 99%
95 wt.% ethanol-water	Sigma-Aldrich	459836	Liquid, Assay: 99.5%
BaTiO3 nanoparticles	US Research Nanomaterials	US3830	In a diameter of about 200 nm
Ferroelectric tester	Radiant	Precision-LC100
Glass substrates	Citoglas	16397	75 x 25 mm
Gold coater	Pelco	SC-6
High voltage supplier	Trek	610D	10 kV
Impedance analyzer	Keysight	4294A
N, N dimethylformamide	Fisher Scientific	GEN002007	Liquid
P(VDF-CTFE) 91/9 mol.% copolymer
Scanning Electron Microscopy (SEM)	JEOL	JSM-7000F
Vacuum oven	Heefei Kejing Materials Technology Co, Ltd	DZF-6020