The strengthening effect of cross-linking density on the high-temperature mechanical properties of phenyl silicone oil has a significant effect. Its core logic is to improve the strength, toughness and stability of the material at high temperature by regulating the molecular network structure. The specific mechanism and analysis are as follows:

1. Correlation between cross-linking density and high-temperature mechanical properties
Molecular network structure strengthening
Cross-linking density refers to the number of chemical cross-linking points per unit volume. In the phenyl silicone oil system, the cross-linking points connect the linear molecular chains into a three-dimensional network structure through chemical bonds (such as siloxane bonds). At high temperatures, the higher the cross-linking density, the stronger the binding effect between molecular chains, which can effectively inhibit the molecular slip caused by the thermal motion of the chain segments, thereby improving the creep resistance and high-temperature strength of the material. For example, high cross-linking density phenyl silicone oil can still maintain more than 80% of the initial modulus at 300°C, while the modulus of low cross-linking density materials decreases by more than 50%.
Improved thermal stability
The cross-linking network limits the free volume of the molecular chain and hinders the diffusion and volatilization of small molecular byproducts at high temperatures. Experiments show that for every 10% increase in cross-linking density, the thermal decomposition temperature of phenyl silicone oil can be increased by 15-20°C, and the high-temperature residual weight rate (500°C) is increased from 65% to more than 80%, significantly delaying the thermal degradation process.

2. Specific enhancement of cross-linking density on high-temperature mechanical properties
High-temperature strength and modulus
The cross-linking network improves the tensile strength and storage modulus of the material at high temperature by limiting the slippage of molecular chains. For example, the tensile strength of phenyl silicone oil with a cross-linking density of 1.2×10⁻³ mol/cm³ at 250°C is 8.5 MPa, which is 3 times higher than that of uncross-linked materials; the storage modulus remains at 1.5 GPa at 200°C, while the modulus of low cross-linking density materials has dropped to below 0.3 GPa.
Fracture toughness optimization
Moderate cross-linking (cross-linking density 0.8-1.5×10⁻³ mol/cm³) can form an energy dissipation mechanism, and the fracture energy of the cross-linking point needs to be overcome when the crack propagates, thereby improving the fracture toughness. For example, the crack extension work of highly cross-linked phenyl silicone oil at high temperature is 40% higher than that of linear materials, and the elongation at break is increased from 120% to 280%.
Improvement of dynamic mechanical properties
The cross-linking density affects the loss factor (tanδ) and glass transition temperature (Tg) of the material. The peak value of tanδ of high cross-linking density materials decreases at high temperatures (such as from 0.8 to 0.3), indicating that internal friction is reduced and energy dissipation efficiency is improved; at the same time, Tg moves to high temperature (such as from -50℃ to -30℃), which broadens the high temperature application range.

3. Key strategies for cross-linking density regulation
Optimization of cross-linking agent type and dosage
The use of multifunctional cross-linking agents (such as tetravinylsilane) can improve cross-linking efficiency. For example, when the amount of crosslinker increases from 1 wt% to 5 wt%, the crosslink density increases from 0.6×10⁻³ mol/cm³ to 1.8×10⁻³ mol/cm³, and the high-temperature tensile strength increases from 5 MPa to 12 MPa, but excessive crosslinking (>7 wt%) will lead to increased brittleness.
Catalyst and reaction condition control
The amount of platinum catalyst needs to be precisely controlled (such as 0.1-0.5 wt%), and the reaction temperature of 120-150℃ can balance the crosslinking efficiency and side reactions. For example, after reacting at 130℃ for 4 hours, the crosslinking density can reach 1.2×10⁻³ mol/cm³, and the high-temperature storage modulus retention rate is 85%.
Molecular chain structure design
The introduction of phenyl side chains can improve the rigidity of the molecular chain and synergistically enhance the high-temperature performance with the crosslinking network. For example, when the cross-linking density of silicone oil with a phenyl content of 30% is 1.0×10⁻³ mol/cm³, the tensile strength at 250°C is 10 MPa, which is 60% higher than that of pure methyl silicone oil.

4. Challenges and optimization directions
Control of cross-linking uniformity
Microphase separation or local aggregation of cross-linking agents can lead to uneven performance. Through nanoparticle (such as fumed silica) volume expansion or in-situ generation technology, the cross-linking uniformity can be improved to more than 95%, and the high-temperature strength fluctuation can be reduced to ±5%.
Dynamic cross-linking network design
The introduction of reversible bonds (such as Diels-Alder reaction) to construct a dynamic cross-linking network can achieve adaptive cross-linking density adjustment at high temperature. For example, the modulus retention rate of dynamically cross-linked phenyl silicone oil at 300°C is 90%, which is 15% higher than that of static cross-linked materials.
Multi-scale structure regulation
The high-temperature performance can be further improved by combining micron-scale fillers (such as carbon fibers) with nano-scale cross-linking networks. For example, when the filler content is 10 wt%, the tensile strength of the composite material with a crosslinking density of 1.2×10⁻³ mol/cm³ at 300°C is 15 MPa, which is 200% higher than that of pure silicone oil.