Molecular Interpretation of the Load-Elongation Curve of Crystalline Polymers

The load-elongation curve of a semicrystalline polymer with lamellar structure may have four distinct parts: the initial elastic part, the yield area, the necking area where the lamellar morphology is transformed into the fibrous structure, and the drawing area of the fibrous material up to the failure of the sample. The neck formation stops the elastic deformation of the lamellar structure and hence reduces the upper yield load to the load needed for the continuous necking. The drawing of the fibrous structure extends the interfibrillar tie molecules and shears the crystal blocks. The increase of the contribution of taut interfibrillar tie molecule to the axial elastic modulus of the drawn sample comes to an end as soon as the tie molecule gets so far unfolded that at least one end is completely pulled out of the crystal block it was originally anchored in. Swelling of the amorphous regions so much weakens these areas that they are not able to exert enough force on the lamellae for their complete transformation into microfibrils. But the easy separation of the lamellae perpendicular to the direction of the applied tensile force creates a great many channels which are responsible for the high gas permeability. The bending of the same lamellae yields the high rubber-like strain behavior in the draw direction. The fracture of the sample is caused by the growth and coalescence of microcracks up to a critical size crack. The process involves the rupture of most molecules which connect the opposite sides of any microcrack. This rupture increases the work per surface area of the microcrack but does not cause the failure of the sample.