Molecular Space
Polymeric structures consist of both occupied molecular chains and empty gaps between them. This unoccupied space is known as free volume, which determines the mobility of molecules within the polymer matrix. The magnitude of this space dictates how easily gases and vapor can migrate through a membrane.
Diffusion Coefficient
Water vapor transport through a sensor’s protective layer is highly dependent on the density of the barrier material. Because free volume represents the path of least resistance, it directly governs the diffusion coefficient of penetrants. Larger voids allow water molecules to leap between cavities with minimal energy resistance.
Glass Transition
Amorphous materials change from a rigid, glassy state to a flexible, rubbery state when heated. This transition occurs because thermal expansion increases the free volume to a point where molecular chains can rotate freely. Sensing elements embedded in polymers must operate well below this transition region to maintain stable calibration.
Permeation Rate
Sensor life is often limited by how quickly corrosive gases can penetrate protective packaging. Measuring the free volume through techniques like positron annihilation lifetime spectroscopy helps engineers select polymers with low permeability. This selection ensures that internal electronics remain dry and protected from corrosive atmospheres.
