Shear field induced diffusion and molecular weight fractionation during polymer processing

It has been hypothesized that a high shear gradient induced during polymer processing could cause an increased migration of oligomer to the surface, thereby leading to “plateout” on mold and roll tooling. To test this hypothesis, poly(ethylene terephthalate) (PET) and poly(ethylene‐co‐cyclohexylenedimethylene terephthalate) (PETG) were extruded into film, and the molecular weight probed as a function of the depth from the surface by incremental milling. GPC analysis showed an 18% and 14% higher concentration of cyclic oligomer on the surface of these films as compared with the center for PET and PETG respectively. These results compare favorably with predictions from a theoretical shear field model based on a Convected Maxwell fluid. Results from this study should lead to a better understanding of how oligomer buildup at the die wall may relate to roll plateout, die drooling, die lubrication, and possibly even melt fracture. Polym. Eng. Sci. 44:1283–1294, 2004. © 2004 Society of Plastics Engineers.