Ultrahigh-molecular-weight Polyethylene Gel Research Articles

Constructing hierarchically elongated porous structure in ultrathin ultrahigh molecular weight polyethylene membrane: Achieving high selectivity with remained permeability

Anisotropic pores with a high aspect ratio exhibit higher permeability as well as selectivity than circular pores. Herein, highly selective porous membranes exhibiting elongated slit-like pores and disk-like interlayer large pores are fabricated successfully by biaxial stretching and further constrained uniaxial stretching of ultrahigh molecular weight polyethylene gel films. The thickness and the surface roughness of the porous membrane decrease, and the pore size distribution become narrow during biaxial stretching, while the porous structure become elongated during constrained uniaxial stretching. The thickness decreases from 173.23 μm to 2.33 μm, which reduces the resistance to water passage and then increases the water permeability from approximately 540 L/ (m2 h bar) to 780 L/ (m2 h bar). Due to the presence of mineral oil, the porosity is maintained and even high ratios of constrained uniaxial stretching do not close the pores. The elongation of the porous structure resulted in a significant reduction of the membrane pore size. At a total stretching ratio of 144 times, the pore size of the porous membrane with only biaxial stretching is 0.098 μm, while that of the porous membrane with biaxial stretching and further constrained uniaxial stretching is 0.048 μm. The separation accuracy and stability of the porous membrane improve distinctly, ascribing that the small pore size can reject small particles and prevent the particles from entering the membrane. This study provides one novel strategy to regulate the porous structure of porous polymer membranes for improving the separation efficiency.

Structural evolution of ultra-high molecular weight polyethylene gel film stretching at different temperatures with reservation of shish crystals

Shish crystal is the key structure to achieve low-dimensional ultra-high molecular weight polyethylene (UHMWPE) products with high performance. UHMWPE gel film containing shish crystals was prepared by precise temperature control in the gel molding, and its structural evolution in the stretching flow field at different temperatures was investigated by in-situ wide angle X-ray diffraction (WAXD)/small angle X-ray scattering (SAXS), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). It was found that shish crystals accelerated the structural evolution in both room and high temperature stretching, promoting more highly oriented crystals along the stretching direction. In room temperature stretching, the lamellae were deflected vertically to the stretching direction and gradually accumulated leading to a stabilization of the crystallinity index and the degree of orientation of the crystals along the stretching direction. However, in high temperature stretching, this intermediate orientation was broken and the transition to shish-kebab crystals continued, leading to a sharp increase in the crystallinity index and the degree of orientation of the crystals along the stretching direction.

Rheological behavior of ultrahigh molecular weight polyethylene/low‐density polyethylene blending gels with high solid content

For developing polyethylene (PE) fibers with relatively high mechanical properties but low cost, the rheological behaviors of ultrahigh molecular weight polyethylene (UPE) and low density polyethylene (LDPE) blending gels (UL blending gels) were investigated in terms of the shear‐induced chain interactions and the sol–gel transitions. UL blending gels with a fixed blend ratio 1/1 of UPE and LDPE but different solid contents (SCs) ranging from 2 wt% (UL‐2) to 10 wt% (UL‐10) were prepared using paraffin oil as solvent. The UL‐10 showed a more significant shear thinning behavior than others, and exhibited a little bit lower apparent viscosity than UPE gel with 5% SC (UPE‐5) at elevated temperature even though the SC of UL‐10 is double of that of UPE‐5. UL blending gel with low apparent viscosity and high SC could ensure smoothly and high‐efficient spinning. Rheological measurements confirmed no significant solid‐liquid phase separation of the system of UPE and LDPE in paraffin oil. At the same time, the macromolecular orientation under shear and structural viscosity also assured the blending gel UL‐10 an excellent spinning performance. UL blending fibers were prepared. The tensile strength of UL‐10 fiber reached 1.2 GPa which would satisfy industrial applications demanding relative high mechanical properties. POLYM. ENG. SCI., 58:22–27, 2018. © 2017 Society of Plastics Engineers

Determination of molecular weight distribution and composition dependence of monomeric friction factors from the stress relaxation of ultrahigh molecular weight polyethylene gels

The stress relaxation of ultrahigh molecular weight polyethylene (UHMWPE) gels in a broad composition range ( 0.025≤ϕP≤1.0) was studied within the framework of the reptation model. The molecular weight distribution (MWD) was determined at an optimum gel concentration ( ϕP=0.365) by regularization analysis of the stress relaxation data. The calculated relaxation moduli [ G(t)] by using MWD evaluated at a certain composition (UHMWPE gels, ϕP=0.365) were consistent with the experimental G(t) of other compositions in a broad range ( 0.218≤ϕP≤1.0). That is, the MWD (regularization method) was self-consistent within the framework of the reptation model. The composition dependence of the monomeric friction factor ( ζeff) of the UHMWPE gels was examined by use of the double reptation (DR) model with the determined MWD. The Ferry's free volume model of the molecular weight dependence of ζeff can be extended to the composition dependence of ζeff. In the low-concentration gels ( 0.025≤ϕP≤0.17), the experimental G(t)...

The Thermal and Mechanical Properties of Ultra-High Molecular Polyethylene/Montmorillonite Clay (UHMWPE/MMT) Nanocomposites Using Gel and Pressure-Induced Flow Process (PIF)

This paper examines hybrid organic-inorganic polymer nanocomposites incorporating organically modified montmorillonite (MMT) and ultra-high molecular weight polyethylene (UHMWPE). UHMWPE/MMT hybrid nanocomposites were prepared using gel and pressure-induced flow (PIF) processing methods at a gel concentration of 8 wt.% UHMWPE with various organoclay contents (0, 5, 10, 15, and 20 pphr). The interlayer properties of the nanocomposites were studied by X-ray diffraction (XRD) transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The thermal and mechanical interfacial properties of the nanocomposites were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and the use of a universal test machine (UTM). XRD and TWM indicated that the nanocomposites were formed upon dispersion of MMT in the polymer matrix. From the DSC, TGA and DMA results, we found that the thermal stability of the UHMWPE nanocomposites increased as the MMT content increased. The nanocomposites showed higher tensile strengths than those of pure UHMWPE gel sheet. These findings indicate that the interfacial and mechanical properties were improved by the addition of MMT and PIF processing.

Study on The Modification of Ultrahigh Molecular Weight Polyethylene Fiber with Polar Polymers

Due to the non-polar nature of ultrahigh molecular weight polyethylene (UHMWPE), surface modification was needed to improve the adhesion of UHMWPE fibers. In this paper, UHMWPE gel fibers were treated with a solution of polar polymer during the extracting process, then the modified UHMWPE fibers were made after multi-step hot drawing. The physical and chemical changes in the fiber structure were examined with scanning electron microscopy and Fourier transform infrared absorption spectroscopy. The mechanical properties and interfacial adhesion properties of modified UHMWPE fibers were investigated with tensile testing. The results showed that polar groups were introduced into the surface of UHMWPE fiber during the treatment process; this could improve the interfacial shear strength of UHMWPE fibers without damaging the excellent mechanical properties of fibers. The interfacial shear strength of UHMWPE fiber with epoxy resin increased with increasing draw ratio. It was shown that ethylene/vinyl acetate copolymer (EVA) is an effective modifier. After modification, the shear strength of the interface between the modified UHMWPE fiber and epoxy increased by 100%, and the integrated mechanical properties of the modified UHMWPE fibers were also optimum.

A zone drawing method to determine optimum polymer concentration and gelation temperature of high molecular weight polymer film

The zone drawing method has been used to determine optimum initial concentration and gelation/crystallization temperature for suitable macromolecular entanglements. Ultra-high molecular weight (UHMW) polyethylene (PE) gel films were prepared from decalin solution with different polymer concentrations and different gelation temperatures and then drawn under various zone drawing conditions. It was found that the initial concentration and gelation temperature of PE solution caused significant changes in draw ratio of UHMW PE gel film. That is, at initial concentration of 0.5 g/dl and at gelation temperature of 25°C, the one-step zone draw ratio revealed a maximum and gradually decreased at higher or lower concentrations. In addition, the lower the initial concentration, the lower the gelation/crystallization temperature. This tendency was similar to the maximum draw ratio data. Conclusively, it was identified that the optimum polymer concentration and gelation temperature of UHMW PE solution could be obtained simply by the zone draw ratio.

Effect of zone drawing accompanied with crosslinking on the structure and properties of ultrahigh molecular weight polyethylene gel film

To enhance the thermal properties of ultrahigh molecular weight (UHMW) (viscosity-average molecular weight of 6 × 106) polyethylene (PE) gel film, this was crosslinked by dicumyl peroxide (DCP) during a high-temperature zone drawing, which is effective to orient film. Through a series of experiments, it turned out that crosslinking actualized by an optimum amount of DCP and high-temperature zone drawing technique caused significant changes in the structure and properties of UHMW PE gel film. That is, crosslinking increased storage modulus of UHMW PE gel film at 25°C, resulting in improving thermal properties of the film. On the contrary, the crosslinking effect played a hindering role in raising the draw ratio of UHMW PE gel film. Maximum storage modulus of 165 GPa at 25°C could be obtained at the draw ratio of 324 of uncrosslinked homo-PE gel film. In the case of crosslinked PE gel film, the highest storage modulus at 25°C reached 65 GPa at maximum draw ratio of 150. Crosslinked film exhibited high modulus, even at 190°C, to some extent, while uncrosslinked homo-PE gel film was molten completely at 150°C. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1583–1590, 1997

Determination of the optimum polymer concentration for ultrahigh molecular weight polyethylene gel films by zone drawing method

A new method using a simple zone drawing technique was suggested for determining optimum initial concentration of polymer solution which has suitable macromolecular entanglements. Ultra high molecular weight (UHMW) polyethylene (PE) gel films were prepared from decalin solutions with concentrations of 0.3, 0.5, 0.7, and 1.0 g/dL, and different gelation/crystallization temperatures. In order to investigate drawing behaviors of UHMW PE gel films with different solution concentrations, those prepared were drawn under various zone drawing conditions. Through a series of experiments, it turned out that the initial concentration of PE solution in decalin caused significant changes in the draw ratio of UHMW PE gel film. That is, the one-step zone draw ratio of the gel film for initial concentration of 0.5 g/dL was at its maximum and gradually decreased at higher or lower concentrations. Moreover, this tendency was similar to the maximum draw ratio data. Thus, it was shown that the initial concentration of 0.5 g/dL is the optimum polymer concentration to produce maximum draw ratio in this work. Based on these results, it may be concluded that the optimum concentration of the initial UHMW PE solution can be determined directly by the zone draw ratio.

On the structure of ultra-high molecular weight polyethylene gels

Gel-crystallized ultra-high molecular weight polyethylene samples have been studied by transmission electron microscopy (TEM) and differential scanning calorimetry. The microstructure of the gels was investigated by substitution of the solvent by methacrylate monomers which were subsequently cured by ultraviolet radiation at low temperature. TEM images of RuO 4-stained thin sections showed a network of lamella crystals in which the links were formed by local contacts of the crystal surfaces. Electron spectroscopic imaging yielded carbon concentration profiles across the width of isolated crystals. Annealing of the dried gels above T α illustrated a high mobility of the polyethylene chains within the crystal above the α-transition and well below the melting point. Long lamellae became decorated by shorter and thinner crystallites which appear to be grown perpendicular on the surface of the long ones.

Structure and orientation development in biaxial stretching of ultrahigh molecular weight polyethylene gel films

AbstractUltrahigh molecular weight polyethylene (UHMWPE) gel films were prepared by gelation crystallization from decalin solutions. According to wide‐ and small‐angle X‐ray scattering (WAXD and SAXS) studies, single crystal mat texture with crystal c‐axis oriented normal to film surface generally develops. However, randomly oriented structure can also develop if an external force is applied to gel films during gelation crystallization. Both textures invariably yield high drawability in uniaxial mode with outstanding modulus and strength. Biaxial films, typically 5 × 5, 6 × 6, 8 × 8, and 10 × 10 times the original dimensions, were prepared at 130°C–135°C in a biaxial stretcher. Optical microscopic Investigations reveal the development of interwoven fibrillar structure in all specimens. WAXD and SAXS studies show that lamellar structure transforms to fibrillar texture during biaxial stretching. Crystal orientation is characterized by WAXD pole figure and infrared dichroic methods. Mechanical studies suggest that tensile modulus and strength appear uniform. In comparison with uniaxial deformation, significant improvement in the lateral strength is seen in the biaxially stretched films.