Network Stretching, Slip Processes, and Fragmentation of Crystallites during Uniaxial Drawing of Polyethylene and Related Copolymers. A Comparative Study

Abstract

When polyethylene (PE) is deformed to large strains, the stress originates from both the viscous forces associated with the plastic deformation of the crystallites by slip and fragmentation processes and the entropic elastic forces arising from the stretching of the entangled amorphous regions. Relative weights of the different processes change with the crystallinity. The dependencies were analyzed in a comprehensive study on a series of samples encompassing a large range of crystallinities: PE, low-density PE (LDPE), and ethylene−vinylacetate copolymers. The comparison was based on measured true stress−strain curves for constant strain rates. For the samples with higher crystallinity, which show a necking, this was achieved by employing a video-controlled tensile testing machine. Recovery properties of the sample were studied in step-cycle runs, where the load was applied stepwise and interrupted after each step by an unloading−reloading loop. Simultaneous with the mechanical testing, the related texture changes were determined by a measurement of the WAXS patterns. In spite of the large changes in the gross mechanical properties from solid- to rubberlike, there exists a common general scheme for the deformation behavior. For all samples, one finds four characteristic points where the differential compliance changes. They may be associated with (1) the onset of isolated slip processes, (2) a change into a collective activity of the slips, (3) the beginning of crystallite fragmentation, and (4) chain disentanglement resulting in a finite truly irreversible deformation. When the crystallinity is increased, the stresses at the four transition points also increase. The related strains, however, remain essentially constant. Crystal textures are also a function of the imposed strain only, the dependencies being common for all samples. Experiments support the novel picture of a granular substructure of the crystalline lamellae as a basic structural feature. Block slips with the three surfaces as slip planes enable the system to easily react on each imposed strain in a well-defined way.

The degree of crystalline orientation as a function of draw ratio in semicrystalline polymers: a new model based on the geometry of the crystalline chain slip mechanism

“…It is clear that the crystal orientation data for all five materials tend to lie on a single curve. In other words, the observation reported by previous authors [7,8] of a single crystalline orientation-strain relation, irrespective of the polymer considered, is also verified in our case. Moreover, that single curve is represented well by the crystalline chain slip model, and this will be explored in more detail.…”

“…As long ago as 1978, Kilian and Pietralla [7] observed that the crystalline orientation data for drawn HDPE, LDPE and PVC, both stressed and relaxed, all lay on the same degree of orientation versus strain curve. More recently, this single crystalline orientation-strain relation was observed once again, this time in a series of semicrystalline polymers of crystallinities ranging from 18 to 76% [8].…”

“…An immediate corollary is that the crystal orientation does not depend directly on the deformation of the non-crystalline material, so that it is as if the crystals were embedded in a matrix whose deformation can accommodate the crystal slip processes. What makes this simple behaviour possible may be the block-like nature of the crystalline lamellae [19] as has been suggested by Hiss et al [8]. (The block-like nature follows from the fact that crystallization is a twostep process, with formation of blocks first, and then their fusion into lamellae).…”

“…The irreversibility of the crystalline deformation had already been implied in the work of Kilian and Pietralla [7] in which the crystalline orientation data for both stressed and relaxed HDPE and LDPE all lie on the same master orientation-strain curve. Hiss et al [8] made a similar observation and stated that "...there exists a one-to-one correspondence between the [crystalline] orientational order and the strain, independent of (...) whether the sample is stressed or unloaded." …”

Effect of Strain Rate on Microscopic Deformation Behavior of High-density Polyethylene under Uniaxial Stretching

“…Very similar results have been reported by in-situ WAXD [9,46] and Raman spectroscopy of HDPE [22,23] and iPP [24]. This oblique orientation in the yielding region is explained by the plastic deformation such as the intracrystalline slip [9,46] or the fragmentatioin process into the lamellar cluster units [22]. In the strain-hardening region, both orientation parameters drastically increased with the strain, and reached asymptotic values in the highly-strained region ( > 4).…”