On the Development of Orientation in LDPE Blown Films

The orientational order in a number of low density polyethylene (LDPE) films blown at varying extrusion temperatures is studied with four dif ferent experimental techniques: wide-angle X-ray scattering (WAXS), Fourier- transform infrared (FTIR) dichroism, polarized Raman spectroscopy and bire fringence. From these experimental studies a number of so-called order parameters is obtained that make up the orientational distribution function of the lamellar crystals in the films. The findings support the morphological model that consists of arrays of stacked lamellae which are twisted. These lamellae grow radially outward from a nucleation center along the crystalline b-axis, whereas the a- and c-axes rotate around this direction. With increasing extrusion temperature an increasing amount of twisting takes place, due to a reduced melt stress at the time of crystallization and a decreased number of aligned nuclei being formed. Furthermore, with increasing extrusion tempera ture the small amount of amorphous order decreases to practically zero above an extrusion temperature of around 150°C. From a comparison of the four experimental techniques it can be concluded that WAXS is the most comprehensive choice, yielding pole figures which directly show a qualitative picture of the crystalline orientational distribution. FTIR and Raman give a limited number of order parameters, but are less elabo rate in use than WAXS. Birefringence measurements are easily performed, but only give average values of both the crystalline and amorphous order. Using crystalline values from other measurements, these birefringence measure ments can then be used to obtain the amorphous order.