Molecular orientation evolution during low-density polyethylene blown film extrusion using real-ime Raman spectroscopy

Real-time polarized Raman spectroscopy was used in this study to measure the molecular orientation evolution during blown film extrusion of low-density polyethylene (LDPE). Spectra were obtained at different locations along the blown film line, starting from the molten state near the die and extending up to the solidified state near the nip rolls. The trans C-C symmetrical stretching vibration of polyethylene (PE) at 1132 cm(-1) was analyzed for films possessing uniaxial symmetry. For the given peak, the principal axis of the Raman tensor is coincident with the c-axis of the orthorhombic crystal, and was used to solve a set of intensity ratio equations to obtain second (< P-2(cos theta)>) and fourth (< P-4(cos theta)>) moments of the orientation distribution function. The orientation parameters (P-2, P-4) were found to increase along the axial distance in the film line even past the frost-line height (FLH). The P-2 values also showed an increasing trend with crystalline evolution during extrusion, consistent with past observations that molecular orientation takes place even after the blown film diameter is locked into place. it was also found that the integral ratio (I-1132/I-1064) obtained from a single, ZZ-back-scattered mode can provide a reasonable estimate of molecular orientation. These results indicate the potential of real-time Raman spectroscopy as a rapid microstructure monitoring tool for better process control during blown film extrusion. Copyright (C) 2008 John Wiley & Sons, Ltd.