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Dielectric Behaviour of Acrylic and Natural Rubber Dielectric Elastomers: Combined Effects of Frequency, Temperature and Mechanical Pre-stretch

This study investigates how frequency, temperature, and mechanical pre-stretch jointly influence the dielectric constants of acrylic (VHB 4910) and natural rubber (Oppoband 8001) elastomers, revealing that the acrylic variant exhibits significantly higher sensitivity to these factors due to its polar backbone, thereby providing critical data for optimizing material selection in soft robotic applications.

Original authors: Ajeet Kumar, Anjani Kumar, Abhishek Kumar Shukla, Md Shamim Shah

Published 2026-09-09

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Ajeet Kumar, Anjani Kumar, Abhishek Kumar Shukla, Md Shamim Shah

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where machines can move with the gentle, fluid grace of living tissue. This is the promise of soft robotics, a field that seeks to build artificial muscles and delicate grippers not from rigid metal, but from flexible, rubber-like materials. At the heart of this technology lies a special class of plastics known as electroactive polymers. These are smart materials that change their shape when an electric field is applied, much like a muscle contracting. For these materials to work effectively, they must be able to store and release electrical energy efficiently. This ability is measured by a property called the dielectric constant, which essentially tells us how well the material can hold an electric charge. However, this property is not fixed; it shifts depending on how hot or cold the material is, how fast the electricity is changing, and how much the material is being stretched. Understanding exactly how these factors interact is crucial for engineers who want to build reliable soft robots that can operate in the real world, where conditions are rarely perfect or static.

A team of researchers set out to map these shifting behaviors in two very different types of rubbery materials. One was an acrylic-based tape known as VHB 4910, a polar material with a chemical structure that makes it very responsive to electricity. The other was a natural rubber band called Oppoband 8001, a non-polar hydrocarbon material that is far less sensitive to electrical fields. The scientists wanted to see what happened when they subjected both materials to a wide range of conditions simultaneously. They tested the materials across a temperature spectrum from extremely cold at minus 80 degrees Celsius to a warm 60 degrees Celsius. They also varied the speed of the electrical signal from a slow one cycle per second to a rapid one million cycles per second. Finally, they physically stretched the materials in different directions, pulling them to various degrees of tension to see how the act of stretching itself changed their electrical nature.

The results revealed a clear pattern in how these materials behave. As the frequency of the electrical signal increased, the ability of both materials to hold a charge decreased. This happens because the tiny molecular dipoles inside the material, which act like miniature magnets trying to align with the electric field, cannot keep up when the field changes too quickly. The acrylic material showed a much sharper drop in performance as the speed increased compared to the natural rubber, a difference attributed to the fact that the acrylic has many more polar groups that struggle to reorient themselves rapidly. Temperature played an equally dramatic role. For the acrylic material, the ability to store charge peaked at 0 degrees Celsius. Below this point, the material became too stiff and glassy for the molecular dipoles to move freely, causing performance to plummet. Above this point, the molecules moved so wildly due to heat that they could no longer align neatly with the electric field, again reducing performance. The natural rubber behaved differently; it remained relatively stable across a much wider temperature range, only showing significant changes as it approached its own freezing point at minus 68 degrees Celsius.

Perhaps the most significant finding concerned the mechanical stretching of the materials. When the researchers pulled the rubber bands, the ability to store charge dropped in both cases, but the magnitude of the change was starkly different. For the acrylic material, stretching it from a relaxed state to a high tension caused its ability to store charge to fall by more than 32 percent. In contrast, the natural rubber saw a drop of only about 14 percent under similar stretching conditions. The researchers explained that stretching the acrylic material forces its long molecular chains to align in a specific direction, which physically restricts the polar groups from rotating and responding to the electric field. The natural rubber, lacking these strong polar groups, is less affected by this alignment. Additionally, the natural rubber undergoes a process where stretching causes it to become slightly more crystalline, which further dampens its electrical response, but this effect is far less pronounced than the restriction seen in the acrylic.

These findings provide a practical guide for choosing the right material for a specific job. If an application requires maximum electrical sensitivity and will operate in a narrow, comfortable temperature range around freezing, the acrylic material is the superior choice, despite its sensitivity to stretching. However, if a device needs to function reliably across a vast range of temperatures, from deep cold to warm heat, and must maintain consistent performance even when stretched, the natural rubber is the better candidate. The study confirms that there is no single "best" material for all soft robotics applications; instead, the choice depends entirely on the specific environmental and mechanical demands the device will face. By defining these operational limits, the research helps engineers design more robust and efficient soft machines that can thrive in the complex conditions of the real world.

Technical Summary: Dielectric Behaviour of Acrylic and Natural Rubber Dielectric Elastomers

Problem Statement
Dielectric elastomers (DEs) are critical components in electroactive polymer (EAP) actuators, sensors, and generators, where their performance is governed by the dielectric constant ( $\varepsilon_r$ ). While the dielectric constant is known to be sensitive to operational parameters such as frequency, temperature, and mechanical pre-stretch, existing literature often examines these factors in isolation or lacks comprehensive comparative data across a wide range of conditions. Specifically, there is a gap in understanding the combined effects of temperature, frequency, and mechanical pre-stretch on the dielectric properties of both polar acrylic elastomers and non-polar natural rubber. Furthermore, previous studies have reported inconsistent values for dielectric constants (e.g., for VHB 4910) depending on electrode types and testing conditions, necessitating a systematic investigation to define reliable operational limits for soft robotics and energy harvesting applications.

Methodology
The study conducted a comparative experimental analysis of two distinct dielectric elastomers:

VHB 4910: A polar acrylic-based elastomer (3M) with a thickness of 1 mm.
Oppoband 8001: A non-polar natural rubber (latex) fitness band with a thickness of 150 μm.

Experimental Setup:

Electrodes: Carbon conductive grease (MG Chemicals) was applied to both sides of the films, followed by gold-plated copper circular electrodes (20 mm diameter) to ensure consistent contact.
Deformation: Samples were subjected to biaxial pre-stretch states ranging from $\lambda_1 \times \lambda_2 = 1\times2$ to $3\times4$ using a custom-built stretching device. Uniaxial and pure-shear modes were also considered in the broader experimental design.
Conditions: Measurements were taken across a frequency spectrum of 1 Hz to 1 MHz. The temperature range for VHB 4910 was $-60^\circ\text{C}$ to $60^\circ\text{C}$ , while Oppoband 8001 was assessed in a broader spectrum of $-80^\circ\text{C}$ to $60^\circ\text{C}$ .
Instrumentation: A Novocontrol Spectroscopy Alpha Analyser was used with a 1 V bias voltage. Data was processed using WinDETA software.
Analysis: The glass transition temperatures ( $T_g$ ) were determined via Dynamic Mechanical Analysis (DMA) by identifying the peak of the damping factor ( $\tan \delta$ ). The experimental data was fitted to the Debye model and a combined pre-stretch–temperature expression (Equation 5) to derive phenomenological constants.

Key Results

Frequency Dependence: For both materials, the dielectric constant decreased as frequency increased due to dielectric relaxation. VHB 4910 exhibited a more significant drop compared to Oppoband 8001, attributed to the dipole relaxation of its polar acrylate side groups (specifically $\beta$ -relaxations). Oppoband 8001 showed negligible loss tangent increases with frequency, consistent with its non-polar hydrocarbon backbone.
Temperature Dependence:
- VHB 4910: The dielectric constant peaked at $0^\circ\text{C}$ . Below this temperature, it decreased as the material approached its glassy state ( $T_g = -42^\circ\text{C}$ ), restricting dipole mobility. Above $0^\circ\text{C}$ , increased thermal energy caused random dipole motion, reducing alignment and the dielectric constant at lower frequencies.
- Oppoband 8001: Showed a different trend, with the dielectric constant increasing as temperature rose above its $T_g$ ( $-68^\circ\text{C}$ ) before declining at higher temperatures. However, for higher frequencies above 1000 Hz, the dielectric constant decreased monotonically as temperature increased, because the thermal mobility of dipoles increases, hindering dipole polarization. The material demonstrated greater thermal stability in its dielectric constant compared to VHB.
Pre-stretch Dependence: Increasing the biaxial pre-stretch product ( $\lambda_1 \times \lambda_2$ $λ_{1} \times λ_{2}$ ) consistently reduced the dielectric constant for both materials.
- VHB 4910: At $0^\circ\text{C}$ , the dielectric constant dropped from 6.27 to 4.23 (a 32.53% reduction) as stretch increased from $1\times2$ to $3\times4$ . This sensitivity is attributed to the immobilization of polar dipoles as polymer chains align.
- Oppoband 8001: The reduction was significantly smaller, dropping from 2.60 to 2.23 (14.23% reduction) over the same stretch range. This is attributed to its non-polar nature and strain-induced crystallization, which lowers the effective dielectric constant.
Dielectric Loss: Dielectric loss decreased with increasing pre-stretch for both materials. VHB 4910 showed a steeper decline in loss compared to Oppoband 8001 due to its higher initial polarity.

Modeling and Quantitative Findings
The study fitted the data to a combined model: $\varepsilon(\lambda, T) = a(\lambda_1, \lambda_2) + \frac{b}{T} + c$ .

Electrostriction Constant ( $a$ ): VHB 4910 ( $a = -0.0410$ ) showed a steeper decline with pre-stretch than Oppoband 8001 ( $a = -0.0265$ ).
Temperature Coefficient ( $b$ ): VHB 4910 ( $b = 603$ ) exhibited stronger thermal dependence than Oppoband 8001 ( $b = 452$ ).
Baseline Permittivity ( $c$ ): VHB 4910 ( $c = 3.610$ ) was significantly higher than Oppoband 8001 ( $c = 1.529$ ), reflecting the higher polarity of the acrylic elastomer.
The model indicated that the contributions of pre-stretch and temperature are additive rather than coupled over the tested ranges.

Significance and Claims
The paper claims to provide a comprehensive repository of data defining the operational limits of these two elastomers under combined mechanical, thermal, and electrical loading.

Material Selection: The results suggest that VHB 4910 is optimal for applications requiring maximum permittivity within a narrow, near-ambient temperature band (specifically $-20^\circ\text{C}$ to $20^\circ\text{C}$ ), where high permittivity coincides with acceptable dissipation.
Thermal Stability: Oppoband 8001 is identified as the preferable choice for applications demanding thermal stability over a wide range ( $-80^\circ\text{C}$ to $40^\circ\text{C}$ ), offering a stable permittivity (varying less than 6%) and negligible loss tangent up to $60^\circ\text{C}$ .
Scientific Contribution: The study pioneers the examination of the simultaneous influence of pre-stretch, temperature, and frequency on dielectric constants, offering a phenomenological description (Equation 5) that aids in the design of resilient constitutive models for soft actuators, artificial muscles, and energy harvesters.

The authors conclude that while the combined effects of temperature and pre-stretch are linked to strain-induced crystallization and dipole alignment, the distinct molecular architectures of polar acrylics and non-polar natural rubbers result in significantly different dielectric behaviors, necessitating material-specific selection criteria for soft robotics and electromechanical systems.

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