Meaning
Crystal growth at the boundary of a polymer melt and a solid surface differs from the bulk solidification of the material. Interfacial crystallization occurs when a filler, a fiber or the mould wall itself acts as a site for the nucleation of polymer chains. This process creates a distinct layer with different mechanical properties than the rest of the part.
The quality of this interface determines the efficiency of stress transfer between the polymer matrix and reinforcements.
Nucleation Site
High-energy surfaces provide a lower barrier for the initiation of crystal growth compared to the surrounding melt. During interfacial crystallization, the polymer chains align along the surface of a glass fiber or a carbon nanotube. This alignment creates a highly ordered transcrystalline layer that improves the stiffness of the composite.
The density of these sites is a function of the surface treatment applied to the reinforcement.
Bond Strength
Adhesion between the polymer and a secondary phase relies on the structural integrity of the boundary layer. Effective interfacial crystallization bridges the gap between the soft matrix and the rigid filler, preventing delamination under load. If the interface is weak, the part will fail at much lower stress levels than predicted by the material datasheet.
Thermal management during the moulding cycle is used to optimize the thickness and structure of this crystalline region.
Cooling Rate
Rapid heat removal from the mould wall can suppress the formation of large crystals at the surface. Slower cooling allows more time for interfacial crystallization to occur, which can increase the brittleness of the surface layer. Moulders adjust the tool temperature to control this effect, aiming for a balance between cycle time and part performance.
Measuring the crystallinity at the interface requires specialized analytical techniques like polarized light microscopy or X-ray diffraction. This boundary layer determines the durability of reinforced plastic components.
