Stiff and strong polyethylene with shish kebab morphology by continuous melt extrusion

Abstract

In a previous work, it was shown that highly oriented fibres with 10GPa modulus could be obtained by continuous single-stage melt extrusion of a medium molecular weight polyethylene to which 3% ultra-high molecular weight (/ldw ~ 3 to 5 x 106 ) material had been added by solution blending. It was demonstrated that a special interlocking shish kebab structure was responsible for the favourable mechanical properties. In the present work, we succeeded in achieving the same effect from an unblended polyethylene by choosing starting materials with an inherently suitable molecular weight distribution. Both the high and low molecular weight tails of the distribution are very influential: the high tail contributes to the formation of extended-chain fibrils (which constitute the backbones of the shish kebabs), while the low tail affects melt extrudability and strength. Melt strength is important because unusually high tensile stresses are required during wind-up. The wind-up stress was measured and found to be an order of magnitude greater than that encountered in conventional melt spinning -where no shish kebabs are formed. The implications of the above findings for polymer processing, crystal morphology and melt rheology are discussed.

Crystallization and melting of polyethylene copolymers: Differential scanning calorimetry and atomic force microscopy studies

“…44 Subsequent studies of stress-induced crystallization of polyethylene elucidated this mechanism and revealed the detailed structure of these morphological units, which were called shish kebabs. [45][46][47][48][49][50][51][52] The detailed structure of polyethylene shish kebabs were revealed in recent studies, showing that the shish were sub-lm to $10 lm in length, $2-3 lm in diameter and contained $10-50 extended chain segments. 51,52 Also, a blend study revealed that mainly ultra-high molecular weight polyethylene chains ($5-6 Â 10 6 weight-average molecular weight) were involved in shish formation by a coil-stretch mechanism.…”

Analysis of microporous membranes obtained from polypropylene films by stretching

“…For example, the stick-slip phenomenon [7] was observed for a high molecular weight PP studied in our preliminary tests. The presence of low molecular weight tail (short chains) is generally undesirable for the formation of a row nucleated structure since it affects adversely the chain relaxation, but it improves melt processability via a smoother extrusion [8,9]. Obtaining a very uniform film is a major concern since any non-uniformity and thickness variations cause irregularities in the stress distribution.…”

Quantum Beam Studies on Polymer Crystallization under Flow

“…On the other hand, as mentioned in Introduction, TEM observations showed that shish (extended chain crystal) had a diameter of $10 nm. [7][8][9][10] These observations suggest that shish-kebab has hierarchic structure in a wide spatial scale. In this work we studied the hierarchic structure using small-angle neutron scattering in a very wide Q range.…”

“…In the Q range above about 0.02 Å À1 , on the other hand, the observed data in the normal direction show excess scattering, suggesting some extra structure which is not included in the cylinder model. As mentioned in the Introduction, Keller and coworkers [7][8][9][10] reported shish structure with diameter of $10 nm, which must correspond to extended chain crystal (the shish). We have assigned the excess scattering intensity above about 0.02 Å À1 to the shish structure.…”

“…One of the famous issues in the research filed is structure and formation mechanism of the so-called shish-kebab, which consists of long central fiber core (shish) surrounded by lamellar crystalline structure (kebab) periodically attached along the shish, and it is believed that the shish-kebab structure is a structure origin of ultra-high strength and ultra-high modulus fibers. [4][5][6][7][8][9][10] This is one of the reasons why so many studies have been performed on the structure and the formation mechanism of shish-kebab. In spite of the great efforts there are still many unsolved problems in the shish-kebab.…”