Meaning
Mathematical parameter that characterises the kinetics of phase transitions during polymer crystallisation. The avrami exponent provides insight into the dimensionality of crystalline growth and the nature of nucleation in a cooling melt. It relates the crystalline fraction of a polymer to time under isothermal conditions.
The value typically ranges between one and four.
Growth Dimension
Microscopic observation of spherulite development reveals that the growth pattern of a polymer relates directly to this value. An exponent of three indicates three-dimensional spherical growth from sporadic nucleation sites. Conversely, a value of two represents two-dimensional lamellar structures.
This differentiation helps moulders predict the mechanical properties of the finished part because the dimension of growth dictates how polymer chains align under stress. Spherulite growth affects impact resistance and ductility.
Solidification Boundary
The mathematical relationship holds true only during the primary phase of crystallisation. Secondary crystallisation causes a deviation from the model. Moulders must limit calculations to the initial conversion phase.
Cooling Consequence
Excessive cooling rates in thin-walled sections can suppress the crystalline transition entirely, making the exponent irrelevant for highly amorphous regions. In thick sections, the slower cooling allows the exponent to govern the final morphology. This difference in crystalline morphology leads to internal stress and uneven shrinkage.
Solidification speed determines part dimensions.
