Experimental Determination of Carbon Dioxide Diffusivity in Low-density Polyethylene

Diffusion in molten polymers far above the glass-transition temperature is characteristic of many industrial processes such as polymerization, monomer recylcling, stripping, drying, coating and foaming. Many of these systems of practical importance exhibit a strong dependence of diffusion coefficients on concentration and temperature. These operations involve concentrated solutions of polymers and solvents, which are far removed from the dilute region where theoretical advancements have been most significant. The design and optimization of these applications reuires concentraion dependent diffiusivity data, which are scarce at present.

In this work, the calculus of variation is used to establish the necessary conditions of the concentration-dependent diffusivity for a unidirectional distributed parameter model, such that the model-predicted mass of absorbed gas in polymer matches with its experimental counterpart. A computational algorithm is implemented to solve the model, and obrain the diffusivities of carbon dioxide gas in low-density polyethylene (LDPE) melt, in the range of 352 to 1232 kPa, at 120^oC and 130^oC. The optimal diffusivities versus concentration curves obtrained indicate diffusivity as a significant function of concentration in the polymer medium. The peak diffusivity of carbon dioxide in low-density polyethylene melts for the above temperature and pressure range varies between 3.04 x 10^-9 m² /s. The above results obtrained are evaluated for their sensitivity with respect to maximum expected experimental variation in saturation weight fraction of the gas. The sensitivity of diffusivity to change in above system parameters is maximum at its peak value and is less than 2% with respect to its base value.