Meaning
Specific polymer structures can transition between different crystalline arrangements under the influence of heat and stress. In certain semi-crystalline thermoplastics like polyamide or polybutadiene, the gamma phase transition marks the temperature or stress threshold where the metastable gamma crystal polymorph converts into the more stable alpha or beta form. This transition often occurs during post-mold annealing or under tensile loading.
It can cause sudden changes in the volume and tensile modulus of the part. If this occurs during use, it can lead to part failure or loss of tolerance.
Structural Rearrangement
Crystalline polymorphs rearrange their molecular packaging to achieve a lower free energy state when exposed to thermal energy. The gamma phase transition is typical in polymers that have been rapidly quenched from the melt, trapping them in a less dense structure. When heated above the glass transition temperature, these chains gain enough mobility to reorganize.
This reorganization increases the density of the material.
Mechanical Response
Toughness and yield strength are directly affected by the shifting crystalline phases during deformation. Since the gamma phase transition absorbs mechanical energy, it can sometimes improve the impact resistance of the polymer under specific loading rates. However, the resulting alpha phase is harder and less compliant.
This change can increase the brittle behavior of the molded part.
Process Stabilization
Annealing molded parts at a controlled temperature can trigger the gamma phase transition before the parts are put into service. This step ensures that the dimensional changes occur in the oven rather than during product operation.
