When temperatures plummet to minus dozens of degrees Celsius, most rubbers turn rigid like stone, and plastics may simply shatter. However, silicone rubber can remain soft, elastic, and deformable even at –60°C or as low as –100°C. This extraordinary low-temperature performance makes it a critical material for polar expedition equipment, high-altitude aircraft, cryogenic medical devices, and winter outdoor electronics. So why does silicone rubber maintain its characteristics in such harsh cold conditions? The answer lies once again in its unique molecular structure.

The Core Issue with Low Temperatures: Molecular Chain Motion Freezes

At low temperatures, materials become hard because the movement of molecular segments is "frozen." As temperature decreases, thermal motion weakens, preventing polymer chains from responding to external forces through rotation or sliding. Consequently, the material transitions from a highly elastic state to a glassy state, losing its elasticity. This transition temperature is known as the glass transition temperature (Tg). While natural rubber has a Tg around –70°C and styrene-butadiene rubber about –50°C, high-quality silicone rubber boasts a Tg that can be below –120°C. This means that under normal low-temperature environments, silicone rubber remains in a highly elastic state, with molecular chains capable of free movement.

Key Factors Leading to Such a Low Tg

The ability to have such a low Tg hinges on the low internal rotational barrier of the siloxane backbone (–Si–O–Si–). Due to the longer Si–O bond length (approximately 1.63 Å), flexible bond angles (130°–160°), and moderate electronegativity of oxygen atoms, the resistance to rotation between adjacent silicon atoms is minimal. In contrast, C–C bonds have higher rotational barriers, making the molecular chains more "rigid." Therefore, even at extremely low temperatures, the silicone rubber backbone can easily undergo conformational changes, maintaining material flexibility.

Impact of Side Groups on Low-Temperature Performance

The choice of side groups also significantly impacts low-temperature performance. Methyl groups, being small and symmetrical, do not overly hinder main chain movement. Introducing larger phenyl groups can improve radiation resistance but slightly raises the Tg; conversely, incorporating small amounts of vinyl or trifluoropropyl groups can further optimize low-temperature elasticity in specific systems. Engineers utilize this "molecular tailoring" strategy to customize optimal low-temperature performance for different applications.

Dynamic Mechanical Stability at Low Temperatures

Notably, silicone rubber's elasticity at low temperatures extends beyond static softness to dynamic mechanical stability. For example, after thousands of cycles of stretching and compressing at –55°C, its rebound rate can still exceed 80%, far surpassing most organic elastomers. This fatigue resistance makes it suitable for components requiring long-term dynamic operation in cold conditions, such as seals for low-temperature valves, freezer door gaskets, and joints in polar robots.

Practical Applications and Formulation Considerations

Silicone rubber's low-temperature advantages have been widely verified in practical applications. High-altitude drones flying in the stratosphere face external temperatures down to –70°C; using ordinary PVC would result in immediate cracking, whereas silicone rubber sheaths ensure uninterrupted signal transmission. Cryogenic storage tanks' seals must withstand short-term contact at –196°C, and specialized silicone rubber formulations meet these requirements. Even in Mars exploration missions, where day-night temperature differences exceed 100°C, silicone rubber-made instrument shock absorbers work reliably.

However, low-temperature performance is influenced by formulation factors such as filler type and quantity, the presence of plasticizers, and crosslink density. Excessive fillers like fumed silica can restrict segmental motion and raise the Tg, while using low molecular weight silicone oils as processing aids improves flowability but may exude at low temperatures, affecting performance. Thus, low-temperature grade silicone rubbers typically use high-purity raw rubber, optimized filler dispersion processes, and strict control over oligomer content.

Conclusion

In summary, the low-temperature elasticity of silicone rubber showcases its inherent structural elegance. The flexible siloxane backbone, woven from silicon and oxygen, acts like an unfrozen river flowing through the coldest realms, sustaining life's elasticity. It enables human technology to reach the peaks of Earth and the vastness of space, quietly safeguarding warmth and safety during the harshest winters.


Organic silicon buffer energy absorption material MY 3086