SAXS experiments on voids in gel-spun polyethylene fibres

Abstract

The morphology and properties of extracted gel-spun polyethylene fibres depend on the spinning conditions. The main structures in the extracted fibre are shish-kebabs and lamellae. Equatorial small-angle X-ray scattering (SAXS) experiments show that the former structure is very porous due to the presence of lamellar overgrowth preventing a close package of the backbone fibrils, whereas the latter structure is relatively dense. After hot-drawing, due to melting/recrystallization, both structures are transformed on a 100 nm scale into a dense structure consisting of shish-kebabs or fibrils containing a void volume fraction of about 1%, as revealed by the scattering power of equatorial SAXS experiments. Moreover, a slight decrease of the equatorial intensity especially at the smallest angles after treating the hot-drawn fibres with paraffin oil, points to a small contribution of multiple scattering to the equatorial scattering. This implies the presence of a superstructure of not too closely packed macrofibrils. SAXS measurements of strained ultra-high-strength polyethylene fibres show that no or very little void formation is involved in the fracture mechanism. Most probably this is due to the (partly) fibrillar structure.

Two‐stage drawing process to prepare high‐strength and porous ultrahigh‐molecular‐weight polyethylene fibers: Cold drawing and hot drawing

“…Here, Q is only correlated with the volume ( V ) of the scattering subject and the difference of the electronic density (ρ). In PE, (ρ c − ρ a ) 2 ≈ 6.7 × 10 −3 mol 2 electron 2 /cm 6 , (ρ c − ρ v ) 2 ≈ 3.26 × 10 −1 mol 2 electron 2 /cm 6 , and (ρ a − ρ v ) 2 ≈ 2.39 × 10 −1 mol 2 electron 2 /cm 6 ; ρ a , ρ c and ρ v are the electronic density of the amorphous phase, crystalline phase and the voids, respectively . The ρ difference between the voids and amorphous phase or crystalline phase was approximately 30–50 times larger than that between the amorphous phase and the crystalline phase; this implied that the scattering intensity was much more dependent on the formation of voids in fibers.…”

“…Usually, an ultradrawing process is used to improve the mechanical strength of the fibers; these processes include cold drawing, hot drawing, local drawing, or multistage drawing. However, the porosity of the ultimate fibers in previous works appeared rather low, although a high strength and modulus were achieved . Until now, related studies have placed almost all of their emphasis on the ultimate mechanical performance and mechanism of structural development of the UHMWPE fibers .…”

“…[7][8][9][10][11] Nevertheless, the fibers prepared in these ways always lack favorable mechanical properties. 7,12 In addition, Pennings and coworkers [13][14][15][16] prepared porous UHMWPE fibers, which were produced by the gel spinning of a semidilute solution of UHMWPE in paraffin oil followed by the extraction of paraffin oil in n-hexane. The obtained fibers were comprised of plentiful voids and lamellar stacks (or shish kebabs) but had a very low mechanical strength.…”

Structural Evolution of UHMWPE Fibers during Prestretching Far and Near Melting Temperature: An In Situ Synchrotron Radiation Small‐ and Wide‐Angle X‐Ray Scattering Study

“…Both SAXS and WAXS results indicate that the kebab crystals (lamellae) are transformed into extended‐chain crystals, which contribute to the great strength of fibers. However, few studies have focused on the structural evolution during prestretching of the gel‐spun fibers …”

“…However, few studies have focused on the structural evolution during prestretching of the gel-spun fibers. [25][26][27][28][29] Since the structural evolution during prestretching is strongly affected by the dynamics of the chains and crystal, the stretching temperature and plasticizer enhanced chain mobility are significant factors for adjusting the precursor structure during prestretching. [30][31][32][33] When stretching near the melting temperature, chain mobility is greatly enhanced, and stretchinduced melting may occur, especially with the presence of a plasticizer.…”