No abstract

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 an18% and14% 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.

The molecular dimensions and melt rheology of a thermotropic all-aromatic liquid crystalline polyester (TLCP) composed of p-hydroxy benzoic acid, hydroquinone, terephthalic acid, and 2,4-naphthalenedicarboxylic acid is examined. The Mark-Houwink exponent (␣) of 0.95 is estimated for the TLCP. The persistence length estimated from molecular weight (M) and intrinsic viscosity ([]) data using the Bohdanecky-Bushin equation is about 95 Å, whereas that estimated from light scattering data is 117 Å. These persistence lengths and the observed ␣ value, both higher than those for flexible polymers, suggest that the present TLCP is a semirigid polymer. The zero shear melt viscosity ( 0 ) varies with approximately M 6 for molecular weight M Ͼ 3 ϫ 10 4 g/mol; below this molecular weight, 0 varies almost linearly with M. Widely different entanglement molecular weights (M e ) are predicted, depending on the method used; the plateau modulus estimates M e of about 8 ϫ 10 5 g/mol, whereas the ratio of mean square end-to-end distance and molecular weight (͗R 2 ͘ 0 /M) predicts M e 's either too small (0.33 g/mol) or too large (2.5 ϫ 10 6 g/mol), depending on the theory used. Although the change in the molecular weight dependency of melt viscosity appears to be associated with the onset of entanglement coupling of the semirigid molecules, its origin needs further investigation.