Meaning
Thermodynamic spaces within solid polymer matrices represent the unoccupied volume available for segment mobility of macromolecular chains. Conceptually defined as total specific volume minus the occupied volume of atoms, free volume dictates glass transition behavior and small molecule diffusion in amorphous polymers. The parameter governs structural mobility in solid-state plastics across temperature ranges near and below the glass transition temperature.
It stops applying in molten polymer states where macro-conformations collapse into isotropic liquids dominated by continuous thermal fluctuation.
Thermal Contraction
Rapid cooling during injection moulding traps excess unoccupied space within frozen polymer chains. Quenching hot plastic inside cold mould cavities preserves elevated free volume levels, yielding lower part density and temporary dimensional instability. Over time, physical aging reduces free volume as polymer chains slowly relax into denser molecular packing configurations.
This structural relaxation causes post-moulding shrinkage and embrittlement in amorphous engineering resins such as polycarbonate and polymethyl methacrylate. Moulding datasheets report equilibrium density values, yet rapid tool cooling prevents parts from attaining these theoretical values immediately after ejection.
Diffusivity Control
Molecular packing density determines barrier performance against gas and solvent transport through polymer films. High packing density restricts spatial gaps, suppressing gas permeation through oriented packaging films. Increases in free volume expand transport channels, accelerating moisture absorption and gas migration rates in barrier layers.
Plasticizer incorporation expands molecular spacing, lowering glass transition temperatures and increasing flexibility in rigid vinyl formulations.
Volumetric Relaxation
Physical aging rates depend on storage ambient temperature relative to the glass transition boundary. Annealing moulded components below transition temperatures accelerates the removal of excess volume, stabilizing part dimensions before final assembly steps.
