Abstract:

This paper focused on the effects of temperature and shear rate on CO₂ diffusion behavior in the foaming process of critical CO_2-assisted injection molding. Taking poly（lactic acid） （PLA） and CO₂ as research objects， a PLA/CO₂ diffusion model was constructed by molecular dynamics simulation along with the CAMPASS force field on the basis of periodic boundary conditions and SA algorithm， minimizing the energy of the model. From two aspects of temperature and shear rate， the effects of PLA main chain activity， system energy response and radius of gyration on the diffusion behavior of CO₂ molecules in PLA during the foaming process of injection molding were investigated. Mean square displacement （MDS） analysis indicated that the CO₂ diffusion coefficient increased with an increase in temperature， but there was no influence from shear force. The CO₂ diffusion coefficient reached 0.219 8×10^-5 cm²/s a temperature of 388 K， and it also increased with an increase in shear rate. Moreover， a diffusion coefficient 17.743 6×10^-5 cm²/s was achieved at a temperature of 378 K and a shear rate of 1 ps^-1. According to the calculated results， the activity and system energy of the PLA molecular chain increased due to a rise in environmental temperature， resulting in a increase in the diffusion paths of CO₂ molecules together with an increase in the CO₂ diffusion rate. The energy and activity of the molecular chain increased with a continuous increase in the shearing force on the molecular chain. A single molecular chain was separated from the entangled network to form a flow orientation along the direction of the shear force， increasing the CO₂ diffusion rate accordingly.

Key words: poly（lactic acid）, supercritical carbon dioxide, molecular dynamics simulation, injection foam molding, diffusion coefficient