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Effects of Extreme Hydrostatic Pressure on Elastomeric Material for Soft Robots

Introduction to Extreme Hydrostatic Pressure

• The Mariana Trench, the deepest trench on Earth (10,900 m), presents unique challenges for exploration due to extreme hydrostatic pressures.

• Soft robots can mimic the adaptability of deep-sea organisms to perform efficiently in such conditions.

• Extreme pressures significantly affect the molecular structure and properties of soft robot materials, particularly silicone rubber (SR).

Key Concepts and Findings

• Glass Transition Temperature and Pressure (Tg and Pg):

  • Application of 416.67 MPa pressure can induce the glass transition in SR at room temperature.

  • Traditional glass transition involves cooling to reach Tg; however, pressurizing offers a new realization mechanism.

  • By regulating both temperature and pressure, an equivalent extreme pressure environment can be established.

• Elasticity and Performance of Soft Robots:

  • Research indicates a decline in soft robot performance under extreme pressures; specifically, the voltage-induced area strain in actuators reduces significantly.

  • Understanding the effects of hydrostatic pressure on soft robot materials is crucial for future designs and applications.

Experimental Methodology

• Materials: Polydimethylsiloxane (PDMS) silicone rubber from Dow Chemical is commonly used. It includes a base agent and a curing agent mixed in a specific ratio.

• Sample Preparation: Samples are conditioned and solidified under controlled conditions to mitigate bubble formation and ensure uniformity.

• Characterization of Glass Transition Temperature:

  • Differential Scanning Calorimetry (DSC) is used to determine Tg, with measurements taken following a defined cooling and heating protocol.

  • The Tg for the SR samples was established at 148.38 K (approximately -124.77 °C).

Molecular Dynamics (MD) Simulations

• Molecular Dynamics simulations performed using Materials Studio and LAMMPS aimed to model the behavior of SR under pressure.

• The silicon-oxygen polymer chain was constructed to simulate SR accurately.

• Simulations corroborate the experimental findings regarding Tg and other molecular properties, establishing significant consistency.

Structural Changes Under Pressure

• Free Volume and Molecular Interaction:

  • The distribution of free volume (Vf) and its impact on molecular chain mobility was studied under various pressures, revealing marked changes in SR volume and density under extreme conditions (e.g., 110 MPa).

  • The fractional free volume (FFV) demonstrated an immediate reduction under pressure, highlighting increased molecular packing density.

• Bond Lengths, Angles, and Torsion Changes:

  • The analysis of bond lengths (BLs), bond angles (BAs), and torsion angles (TAs) indicated significant structural dynamics in SR under diverse ambient pressures, contributing to a thorough understanding of molecular interactions.

Glass Transition Mechanism

• Different Realization Mechanisms:

  • Glass transition achieved through cooling involves a compression of free volume, while pressure-induced transitions rely solely on forcing volume compression.

  • The study elucidates that changes in molecular distribution parameters (BLs, BAs, TAs) have negligible effects during pressures below 1 GPa.

  • Results emphasize that deep-sea pressures do not alter the distribution of SR's structural parameters when temperature is maintained constant.

Conclusions and Future Directions

• Understanding the interplay between temperature and pressure on the properties of SR is critical for designing effective soft robots for deep-sea explorations or extraterrestrial environments.

• The study provides essential theoretical foundations for choosing suitable elastomeric materials for soft robotics under challenging conditions:

  • Future designs should consider elastomers with lower Tg for enhanced performance under extreme conditions.

  • The findings also support the viability of achieving glass transition in SR under controlled pressure and temperature changes, presenting flexible design options for soft robots.