Structural behavior within the normal operating temperature range
Minimal molecular chain loosening: When the temperature is below the material's glass transition temperature or melting point, molecular chain segments do not undergo large-scale slippage; only slight thermal motion occurs, and the overall structure remains intact.
Stable chemical bonds: High-energy chemical bonds in the polymer backbone do not break, and there is no significant drop in molecular weight; irreversible structural damage-such as degradation or decomposition-does not occur.
No significant change in microstructure: The crystalline regions of semi-crystalline materials remain stable, and the three-dimensional network structure of cross-linked materials remains intact; the material's dimensions and mechanical properties essentially remain at their original levels.
For example, PPSU (below 220°C) and PEEK (below 260°C) maintain molecular structural stability during long-term operation; they do not become brittle or crack even after undergoing thousands of high-temperature sterilization cycles.
Structural changes beyond tolerance limits
Physical structural changes: Once the temperature exceeds the heat deflection temperature, molecular chain segments in amorphous regions begin to slide freely, causing the material to soften and deform; macroscopically, this manifests as dimensional warping or collapse.
Chemical structural changes: When the temperature approaches or exceeds the thermal decomposition temperature, chemical bonds in the backbone break, triggering thermal degradation reactions; molecular weight drops sharply, small-molecule substances are released, and material properties deteriorate.
For example, when PC plastic exceeds 300°C, molecular chains gradually break, releasing harmful monomers, and the material becomes brittle and cracks.
Slow changes due to long-term thermal aging: Even if the temperature does not reach the melting point, prolonged exposure to high temperatures near the upper limit causes slow oxidation and cross-linking reactions; the molecular chain structure gradually changes, eventually leading to a decline in performance.