Meaning
Polymer physics metrics describe the time it takes for a single chain to escape from the imaginary tube formed by its surrounding entanglements. Tube disengagement time is a fundamental concept in the reptation theory used to explain the movement of long molecules in a dense melt. This value is directly related to the longest relaxation time of the polymer and governs its slow flow and recovery properties.
Relaxation Rate
Internal structural changes happen as the chains wiggle and slide through the mesh of their neighbours. The tube disengagement time marks the point where the molecule has completely forgotten its original configuration and has moved into a new part of the melt. This process is highly sensitive to the molecular weight of the polymer, increasing with the cube of the chain length.
Chain Motion
Dynamics of the melt are restricted by the physical barriers created by the other molecules. Below the tube disengagement time, the polymer behaves more like a solid because the chains are trapped and can only move back and forth within their tubes. Only after this time has passed can the material flow like a liquid, which is an important consideration when designing the hold time and cooling phases of a moulding cycle.
Viscoelastic Effect
Performance of the resin during processing depends on the relationship between the machine speed and this internal clock of the polymer. If the moulding cycle is faster than the tube disengagement time, the material will build up notable internal stresses that can lead to part warping or shrinkage. A moulder must balance the need for speed with the time required for these molecular motions to occur, ensuring the part reaches a stable state before it is ejected from the tool.
This temporal property is a primary factor in determining the residual stress levels in high precision optical components.
