Molecular Mobility
Physical concepts describe the unoccupied space between molecules in a liquid or solid that allows for structural rearrangement. Within polymer science, free volume theory explains how the mobility of polymer chains decreases as a material is cooled toward its glass transition. This internal space provides the room necessary for segments of the chain to rotate or slide past each other.
The theory links the macroscopic viscosity of a substance to its microscopic molecular packing.
Thermal Expansion
Temperature changes alter the amount of available space as the molecules vibrate more or less vigorously. According to free volume theory, the glass transition occurs when the unoccupied space drops below a critical value, effectively locking the chains in place. Below this temperature, the material behaves as a rigid solid because the segments can no longer move into new positions.
The thermal expansion coefficient changes at this point because the expansion of the free volume stops or slows significantly.
Diffusion Modeling
Small molecules like water or gas move through a polymer by jumping between these temporary gaps. By applying free volume theory, researchers can predict the permeability of a plastic to different substances based on the size of the gaps. The introduction of a plasticizer increases the total unoccupied space, which lowers the glass transition temperature and makes the material more flexible.
This model also accounts for physical aging, where the material slowly densifies over time as the extra space is squeezed out. Measuring the rate of this contraction helps in predicting the long term stability of optical fibers or precision sensors.
Theoretical Bound
Mathematical limits define the minimum volume a substance can occupy at absolute zero. Comparisons between this theoretical minimum and the measured volume at a given temperature provide the value of the free volume fraction.
