Biaxial Extensional Deformation Dynamics of Semi Crystalline Polymer Sheets
Biaxial extensional strain hardening controls sheet thinning, requiring precise melt strength, tool friction, and temperature tuning to set uniform container wall thickness.
Hermans Orientation Function
Meaning
Mathematical expressions describe the average alignment of polymer chain segments relative to a specific reference axis in a solid material. Often applied to films and fibres, the hermans orientation function ranges from a value of zero for random distribution to one for perfect alignment parallel to the draw direction. Negative values indicate that the polymer chains lie perpendicular to the reference axis.
Molecular Alignment
Determining this value involves analyzing wide-angle X-ray diffraction or infrared dichroism data to evaluate the average angle of polymer crystals. The hermans orientation function provides a direct calculation of the structural anisotropy resulting from shear forces during extrusion. When the melt flows through a narrow die, the shear forces stretch the molecules along the flow direction, and rapid cooling freezes this orientation.
Physical Property
Highly oriented films show high tensile strength in the machine direction but tear easily in the transverse direction. When the hermans orientation function deviates from the target value during extrusion, the film may fail thickness and tear specifications. While virgin resins with uniform molecular weight distributions orient predictably, regrind blends present inconsistent chain lengths that lead to non-uniform molecular alignment across the film width.
Process Control
Controlling the cooling rate after the die exit represents the primary method for managing this alignment. If cooling is too slow, thermal relaxation occurs, which reduces the orientation and decreases the final tensile strength of the extruded film.