Meaning
Physical models of polymer dynamics describe the empty space between polymer chains that allows molecular movement. In free volume theory, the total volume of a polymer is divided into the occupied volume and the empty space. This framework explains how plasticizers affect the transition between glassy and rubbery states.
Molecular Space
Empty space within the polymer matrix increases as temperature rises because thermal expansion drives the chains further apart. This expansion provides the polymer chains with more room to rotate and slide past one another. The amount of this empty space directly influences the rate of molecular diffusion and the overall viscosity of the polymer melt.
Diffusion Behavior
Water molecules and other low-molecular-weight substances migrate through the polymer matrix by moving between these transient empty spaces. When a plasticizer enters the polymer, it increases the total empty space and lowers the activation energy for molecular motion. This increase in diffusion rates allows moisture to penetrate the polymer more rapidly at elevated temperatures or during humid storage.
Thermal Transition
Glass transition temperature marks the point where the empty space reaches a critical threshold that permits cooperative motion of the polymer backbones. Below this transition, the polymer remains hard and brittle because the chains are locked into position with minimal room for movement. Above this temperature, the dramatic increase in available space results in a soft, flexible material that can be processed easily and shaped into finished components.
