Molecular Space
Theoretical description of the empty space between molecules in a polymer matrix explains how substances diffuse through plastics and elastomers. The free volume model relates the movement of penetrant molecules to the fluctuating gaps within the polymer chains. This framework allows researchers to predict how temperature and pressure affect gas permeation rates.
It assumes that diffusion occurs only when a gap of sufficient size opens next to a gas molecule.
Diffusion Mechanism
Thermal energy drives the continuous motion of polymer segments, which constantly rearranges the empty space inside the material. When the temperature rises above the glass transition point, the rate of these molecular movements increases dramatically. This change expands the available empty space, which allows larger molecules to migrate through the polymer matrix.
Penetrant molecules themselves can act as plasticizers, which increases the local mobility of the chains and further accelerates the diffusion process. The mathematical formulation of the free volume model uses these physical interactions to calculate the transport coefficients of various gases.
Model Constraint
Limitations emerge when applying this theory to highly crystalline polymers or composite materials. The free volume model assumes a homogeneous distribution of empty space, which does not represent the structured regions of semi-crystalline polymers. Glassy polymers below their transition temperature also deviate from the predicted behavior due to frozen non-equilibrium states.
These discrepancies force engineers to use empirical corrections for accurate sensor seal design.
Engineering Application
Sensor design utilizes these mathematical predictions to select protective coatings for sensitive electronics. Permeability estimates help determine the lifetime of hermetically packaged microelectromechanical devices. These calculations ensure that moisture does not reach the internal circuitry during the intended operating life.
