The orientation of the crystalline and amorphous regions in polyethylene film

Abstract

The orientation in the crystalline and amorphous regions of a series of polyethylene films prepared by the tubular films extrusion process has been studied by a combination of x‐ray diffraction and optical methods. All of these films show a relaxation orientation differing both from the fully relaxed type of orientation previously proposed by the authors, and from the more complicated type of orientation suggested by Keller. In all films there is cylindrical symmetry about the machine direction and the b‐axes of the crystallites are preferentially oriented in the plane perpendicular to this direction. The preferred directions of the a and c crystallite axes depend both on the extrusion conditions and on the nature of the polymer; at low blow‐up ratios and high haul of speeds the c axes tend to lie more nearly parallel to the machine direction, but as the blow‐up ratios is increased and the haul‐off speed decreased rotation of the crystallites about their b axes occurs, and eventually the a axes tend to lie more nearly parallel to the machine direction. Both the preferred crystallite orientation and its angular distribution have been measured for a large number of films made from various ethylene polymers, and a linear relation between a function of the orientation angle and the optical birefringence has been established. As the c axes tilt away from the machine direction towards the plane perpendicular to this direction the birefringence steadily decreases, and eventually becomes negative. An expression is given for the birefringence of such a cylindrically symmetrical array of crystallites tilted to various angles, and is used to predict the orientation at which the crystallite birefringence contribution will be zero and also the extreme values of the crystalline birefringence contribution in the fully drawn and fully relaxed conditions. By comparing the theoretical and experimental relations between crystallite orientation and birefringence, it has been deduced that in these films the amorphous material appears to provide a nearly constant small positive contribution to the birefringence irrespective of the birefringence contribution of the crystallite regions; this means that in the negatively birefringent films some of the molecules in the amorphous regions are crossed with respect to those in the crystalline regions. When allowance is made for this amorphous contribution, crystallinities calculated from the extrapolated observed and theoretical extreme values of the birefringence are in reasonable agreement with the x‐ray measured crystallinity of 45%. The mechanism resulting in the formation of this unusual orientation is considered in reference to the manufacturing process.

Experimental analysis of the thermoplastic film embossing process

“…33 The birefringence is related to the stress through the rheooptical law, which has been shown to be valid for both melt systems and amorphous materials.34 Although the rheooptical law is known not to be appropriate for semicrystalline materials, it still provides a qualitative means of illustrating the correlation between the a-axis orientation and the birefringence. The higher the stress level is, the greater the angle of inclination of the a-axis from the plane of the plastic film (MD-TD) and the greater the birefringence is.…”

Orientation and property correlations for LLDPE blown films

“…However, there are still controversies about the structure of these oriented polymers and they are not fully understood. The a ‐axis structure model was proposed first to explain PE film structure 1, 2. In the a ‐axis structure model, the a ‐axis of the crystalline unit cell for extruded PE films lies along the machine direction.…”

“…The structure, property, and processing relationships of oriented PE films have been studied for nearly 50 years. 1 However, there are still controversies about the structure of these oriented polymers and they are not fully understood. The a-axis structure model was proposed first to explain PE film structure.…”

Crystallite orientation in extruded polyethylene film

“…apparently prefer a axis orientation over row orientation. All these investigations have assumed cylindrical symmetry about the extrusion directions and both Holqes et al 6 and Aggarwal et al ' have attempted to verify that they indeed had such symmetry.…”