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

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