Measuring the specific volumes of polymers

Knowledge of the relationships between pressure, specific volume and temperature (PvT) or between pressure, density (the inverse of specific volume) and temperature (P, $\rho$ , T) of polymers is fundamental to the transformation of polymers into plastics, particularly in the most commonly used process: injection molding. During injection molding, a cavity is filled with molten thermoplastic polymer. The part contracts as it cools, and the volume occupied by the solidified part is less than that of the mold cavity (a phenomenon commonly referred to as shrinkage). This contraction corresponds to the organization of the material into a semi-crystalline structure that is denser than the amorphous phase of the molten state. In the crystalline phase, chain segments assemble in an orderly, compact fashion. In addition, during cooling, defects appear, some of which are appearance defects or shrinkage, others geometrical defects or warpage. PvT measurements make it possible to identify the critical parameters that control plastic injection molding, and to improve the parameters of software simulating these phenomena by making them more predictable.