Disentangled UHMWPE/POSS nanocomposites prepared by ethylene in situ polymerization

Nickel-catalyzed in situ synthesis of UHMWPE/TiO2 composites with enhanced mechanical properties and adjustable photocatalytic degradabilities

Expanding the application field of polyolefin materials through functionalization has been a research hotspot in the past three decades. Here, a TiO2-supported anilinenaphthoquinone nickel catalyst was assembled and applied for in situ ethylene polymerization with high activity (>2000 kg mol-1h-1) to produce ultra-high molecular weight polyethylene (UHMWPE)/TiO2 composites with unique physicochemical performance. The UHMWPE/TiO2 composite films and fibers prepared by in-situ ethylene polymerization are superior to the samples from the blend system in issues such as TiO2 dispersibility, mechanical property, and photocatalytic degradability. The mechanical properties (strength up to 26.8 cN/dtex, modulus up to 1248.8 cN/dtex) of the obtained UHMWPE/TiO2 composite fibers are significantly improved with a very low dosage of TiO2 (as low as 1.4 wt‰). Moreover, UHMWPE/TiO2 composites obtained by coating Al2O3 and SiO2 on the surface of TiO2 not only retain the strong absorption of ultraviolet rays, but also effectively weaken the photocatalytic degradation effect.

Entanglements of Macromolecules and Their Influence on Rheological and Mechanical Properties of Polymers

Flexible macromolecules easily become entangled with neighboring macromolecules. The resulting network determines many polymer properties, including rheological and mechanical properties. Therefore, a number of experimental and modeling studies were performed to describe the relationship between the degree of entanglement of macromolecules and polymer properties. The introduction presents general information about the entanglements of macromolecule chains, collected on the basis of studies of equilibrium entangled polymers. It is also shown how the density of entanglements can be reduced. The second chapter presents experiments and models leading to the description of the movement of a single macromolecule. The next part of the text discusses how the rheological properties change after partial disentangling of the polymer. The results on the influence of the degree of chain entanglement on mechanical properties are presented.

Nanoscale effects of TiO2 nanoparticles on the rheological behaviors of ultra-high molecular weight polyethylene (UHMWPE)

Considering the molar mass between entanglements to be an intrinsic property of ultra-high molecular weight polyethylene (UHMWPE), the number of entanglements per chain increases with increasing molar mass, correspondingly making the UHMWPE intractable. Herein, we dispersed TiO₂ nanoparticles with different characteristics into UHMWPE solutions to disentangle the molecular chains. Compared with the UHMWPE pure solution, the viscosity of the mixture solution declines by 91.22%, and the critical overlap concentration increases from 1 wt% to 1.4 wt%. A rapid precipitation method was utilized to obtain UHMWPE and UHMWPE/TiO₂ composites from the solutions. The melting index of UHMWPE/TiO₂ is 68.85 mg, which is in sharp contrast to that of UHMWPE which is 0 mg. We characterized the microstructures of UHMWPE/TiO₂ nanocomposites using TEM, SAXS, DMA, and DSC. Accordingly, this significant improvement in processability contributed to the reduction of entanglements and a schematic model was proposed to explain the mechanism by which nanoparticles disentangle molecular chains. Simultaneously, the composite demonstrated better mechanical properties than UHMWPE. In summary, we provide a strategy to promote the processability of UHMWPE without sacrificing its outstanding mechanical properties.

Nanocomposites of Au/Disentangled UHMWPE: A Combined Optical and Structural Study

The term disentangled refers to polymers with fewer entanglements in the amorphous regions, a metastable condition that can significantly affect the material’s properties and processing behavior. The lower entanglement density in ultra-high molecular weight polyethylene (dis-UHMWPE) facilitates the solid-state processability into uniaxially-oriented specimens reaching very high draw ratios and crystallinities. In this study, Au/dis-UHMWPE nanocomposites were formulated and processed at variable draw ratios. Polarized light microscopy suggests gold nanoparticles are oriented in arrays following the drawing of polymer chains. The structural features, upon orientation, are studied by means of Raman spectroscopy, wide- and small-angle X-ray scattering, and near-infrared spectrophotometry. Crystallinity is found to increase by 15%, as calculated by wide-angle X-ray scattering. The change in optical absorbance in the visible spectrum indicates that, with orientation, the average size of gold aggregates increases, supported quantitatively by small-angle X-ray scattering. Since the gold nanoparticles are expected to be found within amorphous chain segments, the aforementioned findings are attributed to the increase of crystallinity and thus the decrease of available (amorphous) space.

In-Situ Bubble Stretching Assisted Melt Extrusion for the Preparation of HDPE/UHMWPE/CF Composites

In this work, a novel melt extrusion method under synergy of extensional deformation and in-situ bubble stretching (ISBS) and corresponding apparatus were reported. The structure and working principle were introduced in detail. Polymer composites composed of high density polyethylene (HDPE)/ultrahigh molecular weight polyethylene (UHMWPE)/carbon fiber (CF) were prepared by using this new method. Effects of CF and Azodicarbonamide (AC) contents on composites' morphology, rheological, thermal, and mechanical properties were experimentally investigated. SEM results showed that the CFs dispersed evenly in the matrix when the AC content was relatively high. DSC results showed that co-crystallization of HDPE and UHMWPE occurred in the composites, and the Xc of the composites decreased with the addition of AC or under high CF loadings. TGA results showed that the thermostability of the composites increased markedly with increasing CF loading. Mechanical properties showed that tensile strength increased by 30% with 9 wt % CF and 0.6 wt % AC added. The results aforementioned indicate that the novel melt extrusion method is a green and effective way to prepare HDPE/UHMWPE/CF composites.

Preparation and characterization of CNTs/UHMWPE nanocomposites via a novel mixer under synergy of ultrasonic wave and extensional deformation

In this work, design and development of a new melt mixing method and corresponding mixer for polymer materials were reported. Effects of ultrasonic power and sonication time on the carbon nanotubes (CNTs) filled ultra high molecular weight polyethylene (UHMWPE) nanocomposites were experimentally studied. Transmission Electron Microscopy images showed that homogeneous dispersion of CNTs in intractable UHMWPE matrix is successfully realized due to the synergetic effect of ultrasonic wave and extensional deformation without any aid of other additives or solvents. Differential scanning calorimetry results revealed an increase in crystallinity and crystallization rate due to the finer dispersion of the CNTs in the matrix which act as nucleating point. Composites' complex viscosity and storage modulus decreased sharply at first and then leveled off with the increase of sonication time or the ultrasonic power. The thermal stability and the tensile strength of the CNTs/UHMWPE nanocomposites improved by using this novel mixing method. This is the first method that combined the ultrasonic wave and the extensional deformation in which the elongation rate, sonication time and ultrasonic power can be adjusted simultaneously during mixing. The novel mixer offers several advantages such as environment-friendly, high mixing efficiency, self-cleaning and wide adaptability to materials.

Immobilization of isolated FI catalyst on polyhedral oligomeric silsesquioxane-functionalized silica for the synthesis of weakly entangled polyethylene

POSS-functionalized silica can isolate the anchored FI catalysts and hinder chain overlap in the polymerization of ethylene.