Diffusivity of gases and main‐chain cooperative motions in plasticized poly(vinyl chloride)

Permeability and time‐lag measurements for H 2 and CO in poly(vinyl chloride) (PVC) plasticized with tricresyl phosphate show that the apparent diffusion coefficients at first decrease as the plas‐ticizer concentration is increased. The diffusion coefficients then increase as the additive concentration is raised above 15 wt %. These changes in the apparent diffusion coefficients can be related to the behavior of a variety of mechanical properties and are attributed to antiplasticization and plasticization effects of low and high concentrations of tricresyl phosphate, respectively. The antiplasticization‐plasticization effects reflect altered molecular motions of the polymer. Carbon‐13 NMR rotating‐frame relaxation rate measurements show directly that the cooperative main‐chain molecular motions of PVC are reduced when the additive acts as an antiplasticizer and are increased when the polymer is plasticized. Both the apparent diffusion coefficient and the rotating‐frame relaxation rate have a similar dependence on additive concentration. An application of the molecular theory of diffusion of Pace and Datyner accounts qualitatively for the way in which additives alter the average chain interaction energy, cooperative polymer main‐chain motions, and the diffusion coefficients of gaseous penetrants.