I. Basic Characteristics of Materials
Phenyl Silicone Rubber
Phenyl silicone rubber is a high-performance special rubber material. By introducing phenyl groups on the side chain of polysiloxane, its temperature resistance is significantly improved. Depending on the content of phenyl groups, it can be classified into low phenyl (5-10%), medium phenyl, and high phenyl types. Among them, low phenyl silicone rubber has the best low-temperature resistance. Its curing temperature can be as low as -115℃, and it can still maintain flexible elasticity at -100℃. 
Fluorine rubber
Fluorine rubber refers to synthetic polymer elastomers whose main chain or side chain carbon atoms contain fluorine atoms. The minimum operating temperature of standard fluorine rubber is approximately -20℃ to -25℃. Special modified fluorine rubber can withstand low temperatures down to -40℃. Its glass transition temperature (Tg) is -30℃ to -40℃, and the brittle point is approximately -40℃. 

II. Comparison of Low Temperature Resistance Parameters
Performance indicators Phenolic silicone rubber Fluorine rubber
Minimum operating temperature -100℃ (low phenolic type) -40℃ (special modified type)
Hardening temperature -115℃ (low phenolic type) -
Glass transition temperature -140℃ -30℃ to -40℃
Crystallization temperature - -40℃
Temperature resistance range -100℃ to 250℃ -40℃ to 250℃
As can be seen from the table, phenolic silicone rubber significantly outperforms fluorine rubber in terms of low temperature resistance, especially in extremely low temperature environments. 

III. Analysis of Performance Differences
‌Molecular Structure Differences‌: The introduction of the phenyl group in the phenyl silicone rubber disrupts the regularity of the dimethylsiloxane structure, significantly reducing the crystallization temperature and glass transition temperature of the polymer, thereby achieving excellent low-temperature resistance. However, the carbon-fluorine bonds in the fluorine rubber are stable, but the molecular chains are relatively rigid, and they tend to become brittle at low temperatures. 
Different modification methods: Phenyl silicone can achieve the best low-temperature performance by adjusting the phenyl content (5-10%). However, fluorine rubber requires the introduction of a 20%-40% molar ratio of PAVE monomer to reduce the glass transition temperature to -30°C to -40°C. 
Elastic retention ability: Phenyl silicone rubber can still retain its elasticity at -100℃, while the elasticity of fluorine rubber significantly decreases below -40℃. 

IV. Differences in Application Scenarios at Low Temperatures
Main Applications of Phenyl Silicone Rubber
Aerospace sealing materials (such as aircraft fuel systems)
Sealing for cryogenic equipment (such as liquid nitrogen, liquid helium equipment)
Polar scientific research equipment
Nuclear power radiation protection coatings
Electronics and electrical component potting materials 
Main applications of fluorosilicone rubber
Automobile engine seals (in environments above -20℃)
Petrochemical equipment (such as seals for liquefied natural gas pumps and valves)
Chemical pipeline seals
Fuel system components
General industrial seals 

V. Selection Suggestions
‌Extremely low temperature environment (below -100℃)‌: Phenyl silicone rubber must be selected, especially the low phenyl type (with a phenyl content of 5-10%). 
Medium low-temperature environment (-40℃ to -20℃): Special modified fluororubber can be considered, but it should be noted that its elasticity will decrease as the temperature drops. 
Overall performance requirements: If both chemical corrosion resistance and certain low-temperature performance are required, fluororubber might be a better choice; if low-temperature performance is the primary consideration, phenolic silicone rubber is superior. 
Cost factor: Phenyl silicone rubber is generally more expensive than fluorine rubber. In non-extremely low-temperature environments, fluorine rubber may be a more economical choice. 

VI. Summary
Phenyl silicone rubber exhibits significantly superior low-temperature resistance compared to fluorine rubber. Low phenyl silicone rubber can maintain elasticity at -100°C, while fluorine rubber, even after modification, can only reach the low-temperature resistance limit of -40°C. This difference mainly stems from the different molecular structure designs of the two materials. In practical applications, the appropriate material should be selected based on the specific working temperature and environmental conditions. For extreme low-temperature environments, phenyl silicone rubber is the only feasible option.