Ultrahigh Molecular Weight Poly Research Articles

Comparative analysis on the nanoindentation of polymers using atomic force microscopy

Using atomic force microscopy (AFM) nanoindentation, we have measured the mechanical properties of various polymers: low density polyethylene, polyvinyl alcohol, high density polyethylene, ultrahigh molecular weight polyethylene, polyvinyl chloride, polycarbonate, Nylon 6, poly(methyl methacrylate), polystyrene and polyacrylic acid. The hardness and Young's modulus of the polymers were obtained by AFM through both the force–indentation and area–depth curves. Our comparative analysis shows that the two methods give almost identical results with self-consistency.

Surface structure of nascent particles of ultrahigh molecular weight poly(ethylene) reactor powders

A comparative investigation of the surface structure of three ultrahigh molecular weight poly(eth-ylene) (UHMWPE) reactor powders that differ by their ability to be processed to high-performance fibers is carried out with a JEOL 6300 scanning electron microscope and a nanoluminograph, which makes it possible to study thermoluminescence of ultrathin near-surface layers of solids. The activation energies of relaxation processes in near-surface layers of nascent particles and the sizes of kinetic units of motion, for which the mobility is defrozen in the temperature range of the corresponding transitions, are calculated from the glow curves. The possible location of kinetic units in supermolecular formations resolved in micrographs and their influence on the dissolution of the reactor powder are discussed.

Fabrication and characterization of a novel TiO 2 nanoparticle self-assembly membrane with improved fouling resistance

A novel TiO 2 nanoparticle self-assembly membrane was prepared based on ultrahigh molecular weight poly(styrene-alt-maleic anhydride)/poly(vinylidene fluoride) (SMA/PVDF) blend membrane. TiO 2 nanoparticle solution was beforehand prepared via the controlled hydrolysis of titanium tetraisopropoxide. The diameter (10 nm or less) and anatase crystal structure were analyzed using transmission electron microscopy (TEM) and X-ray diffraction (XRD). The SMA/PVDF blend membranes prepared by the phase inversion method were immersed into the TiO 2 nanoparticle solution for a week to produce TiO 2 self-assembly membranes. The chemical compositions in membrane surface were analyzed by X-ray photoelectron spectroscopy (XPS). The membrane morphologies were measured by scanning electron microscopy (SEM). Finally, the membrane hydrophilicity, protein anti-fouling property and the molecular weight cutoff (MWCO) were characterized by water contact angle measurement, static protein absorption and filtration experiments, respectively. It is demonstrated that, in comparison to PVDF/SMA blend membrane, the permeability and anti-fouling ability of TiO 2 self-assembly membranes were significantly improved.

Electrospun biomimetic nanocomposite nanofibers of hydroxyapatite/chitosan for bone tissue engineering

The development of bioinspired or biomimetic materials is essential and has formed one of the most important paradigms in today's tissue engineering research. This paper reports a novel biomimetic nanocomposite nanofibers of hydroxyapatite/chitosan (HAp/CTS) prepared by combining an in situ co-precipitation synthesis approach with an electrospinning process. A model HAp/CTS nanocomposite with the HAp mass ratio of 30 wt% was synthesized through the co-precipitation method so as to attain homogenous dispersion of the spindle-shaped HAp nanoparticles ( ca. 100 × 30 nm) within the chitosan matrix. By using a small amount (10 wt%) of ultrahigh molecular weight poly(ethylene oxide) (UHMWPEO) as a fiber-forming facilitating additive, continuous HAp/CTS nanofibers with a diameters of 214 ± 25 nm had been produced successfully and the HAp nanoparticles with some aggregations were incorporated into the electrospun nanofibers. Further SAED and XRD analysis confirmed that the crystalline nature of HAp remains and had survived the acetic acid-dominant solvent system. Biological in vitro cell culture with human fetal osteoblast (hFOB) cells for up to 15 days demonstrated that the incorporation of HAp nanoparticles into chitosan nanofibrous scaffolds led to significant bone formation oriented outcomes compared to that of the pure electrospun CTS scaffolds. The electrospun nanocomposite nanofibers of HAp/CTS, with compositional and structural features close to the natural mineralized nanofibril counterparts, are of potential interest for bone tissue engineering applications.

Electrospinning Nanocomposite Nanofibers of Hydroxyapatite/Chitosan

This paper presents a two-step approach for electrospinning of hydroxyapatite/chitosan (HAp/CTS) nanocomposite, which was firstly prepared by an in situ co-precipitation method. Continuous HAp/CTS nanofibers with diameters of 214 ± 25 nm were produced successfully with the aid of an ultrahigh molecular weight poly(ethylene oxide) (UHMWPEO). The HAp nanoparticles with aggregations to some extent were incorporated along the electrospun CTS-based nanofibers. And their crystallite structure can be preserved to some extent although an acidic solvent system was used. Current nanocomposite nanofibers of HAp/CTS may be of potential interest for bone repair and regeneration application.

Amphiphilic graft copolymers based on ultrahigh molecular weight poly(styrene- alt-maleic anhydride) with poly(ethylene glycol) side chains for surface modification of polyethersulfone membranes

Amphiphilic graft copolymers having ultrahigh molecular weight poly(styrene- alt-maleic anhydride) (SMA) backbones and methoxyl poly(ethylene glycol) (MPEG) grafts were synthesized via the esterification between anhydride groups with hydroxyl groups. The synthesized graft copolymers, SMA- g-MPEGs, were used as additives in the preparation of polyethersulfone (PES) membranes via phase inversion process. X-ray photoelectron spectroscopy (XPS) analysis showed the comb-like graft copolymers spontaneously segregated to membrane surface during membrane formation. Water contact angle measurements and water absorbance experiments indicated the PES/SMA- g-MPEG blend membranes were much more hydrophilic than pure PES membrane. The blend membranes had stronger protein adsorption resistance than pure PES membrane did. After washed using de-ionized water for 25 days, the blend membranes exhibited higher hydrophilicity and stronger protein adsorption resistance. This phenomenon was attributed to the further accumulation of SMA- g-MPEG additives on membrane surface in aqueous conditions. SMA- g-MPEGs can be well preserved in membrane near-surface and not lost during membrane washing due to their high molecular weight and comb-like architecture.

Nanoscale Surface Modification of UltraHigh Molecular Weight Polyethylene (UHMWPE) Samples with the W + C Ion Implantation

AbstractIn this work, Ultra High Molecular Weight Poly Ethylene (UHMWPE) samples were implanted with W + C ion by using Metal-Vapour Vacuum Arc (MEVVA) ion implantation technique. Samples were implanted with W and C atoms with a fluence of 1017ion/cm2 and extraction voltage of 30 kV. Mechanism underlies this modification characterized with ATR-FTIR, UV-VIS-NIR Spectrum and Rutherford Backscattering Spectrometry (RBS). Surface morphology of implanted and unimplanted samples were examined in nanoscale with AFM.

Development of oriented morphology and tensile properties upon superdawing of solution-spun fibers of ultra-high molecular weight poly(acrylonitrile)

The uniaxial drawing of UHMW-PAN fibers spun from a dilute solution into methanol coagulation baths at different temperatures and the resultant structure and tensile properties of the drawn products were studied. Although the initial morphology of the fibers and the deformation mode in a lower draw ratio (DR t) range were significantly dependent on the temperatures of the coagulation bath, the tensile properties at a given DR t, as well as the maximum achieved ones, were comparable. Both the tensile modulus and strength increased steadily with the DR t and reached 35 and 1.8 GPa, respectively, at the highest DR t of ∼80. These tensile properties are among the highest ever reported for PAN fibers. The achievement of such high tensile properties for extremely drawn fibers is ascribed to the conformational changes of crystalline chains from the 3/1 helix to the planar-zigzag with increasing DR t, the improvement in the uniformity of the fiber diameter along the fiber axis, and the decrease in fiber diameter. Indeed, the tensile strength of fibers prepared from a dilute solution and having comparable moduli increased with a decrease in the fiber diameters. The reciprocal of the strength was proportional to the square root of the diameter as suggested by the Griffith theory. Extrapolation to a zero diameter yielded an ultimate tensile strength of 2.4±0.1 GPa for a fiber having a maximum achieved tensile modulus of 35±1 GPa.

Preparation of glycerol dimethacrylate-based polymer monolith with unusual porous properties achieved via viscoelastic phase separation induced by monodisperse ultra high molecular weight poly(styrene) as a porogen

The preparation of polymer-based monolith capillary was examined by the use of glycerol dimethacrylate (GDMA) as monomer and monodisperse standard polystyrene (PS) solution in chlorobenzene as porogen. Poly-GDMA monoliths were prepared in situ in test tubes with standard PS having the variety of molecular weight (defined as Mw hereafter) from 50,000 to 3,840,000, and their morphology was compared to that of poly-GDMA monolith prepared in situ with a poor porogenic solvent of GDMA. According to scanning electron micrograph (SEM) observation, the structure of poly-GDMA monolith prepared in situ with toluene as a poor porogenic solvent showed a typical agglomerated globular structure, whereas the morphology of poly-GDMA monolith prepared in situ with the polymer (PS) porogenic solution was transformed from the aggregated globule form to three dimensionally (3D) continuous skeletal structure with the increase of Mw of standard PS utilized. Along with this morphological transformation or change, in the case of poly-GDMA monolith prepared in situ with ultra high Mw standard PS porogenic solution, the pore size distribution showed a sharp bimodal distribution, with one peak being located around 4 nm in the mesopore range (2–50 nm) and the other peak located around 1–2 μm in the macropore range (>50 nm), respectively. The poly-GDMA capillaries were prepared in situ with toluene, low Mw (50,000, 600,000) PS solution in chlorobenzene and the above mentioned ultra high Mw PS solution in chlorobenzene as a porogen, respectively, and measured by μ-HPLC with benzene and n-alkyl phenyl ketone as solutes for the evaluation in aqueous methanol (MeOH/H 2O = 50/50–80/20, v/v). The permeability of capillaries prepared in situ with ultra high Mw standard PS polymer porogenic solution was much larger, compared to those of the capillaries prepared in situ with low Mw standard PS polymer porogenic solution or with toluene as porogen. On the other hand, the column efficiency was better in the case of the capillary prepared in situ with the ultra high Mw PS solution than in the latter capillaries. Those observations indicated that the ultra high Mw standard PS polymer porogenic solution should delay dynamically the phase separation of polymerizing mixture because of its visco-elasticity and should contribute to the creation of three dimensionally continuous skeletal monolith structure better to afford high separation efficiency.

Novel High and Ultrahigh Molecular Weight Poly(propylene) Plastomers by Asymmetric Hafnocene Catalysts

AbstractSummary: The novel asymmetric ansa‐complexes [1‐(9‐η5‐fluorenyl)‐2‐(2,5,7‐trimethyl‐indenyl)ethane]hafnium dichloride (7a) and [1‐(9‐η5‐fluorenyl)‐2‐(2,4,6‐trimethyl‐indenyl)ethane]hafnium dichloride (7b) were prepared and used as catalysts for propylene homopolymerization reactions after in situ activation. The synthetic route allows to separate the 4,6‐ and 5,7‐substituted ligand isomers before the complexation step. The orientation of the methyl groups to the “front” (4,6) or to the “back” (5,7) of the tetrahedral hafnocene dichloride species influences their performances in polymerization reactions. Whereas hafnocene (7b) which bears trimethyl substitution at 2,4,6‐positions of the indenyl moiety exhibits only moderate activity, the 2,5,7‐trimethyl substituted structure (7a) produces isotactic poly(propylene)s with high molecular weights (up to $\overline M _{\rm w}$ = 9.0 × 105 g · mol−1) and high activities [up to 3.2 × 105 kg of PP (mol Hf × h)−1]. A comparative analysis of polymerization data and mechanical behavior of 7a, and previously reported 6,7‐indenyl substituted complex 6b are reported.Typical stress‐strain curves of different types of poly(propylene)s.magnified imageTypical stress‐strain curves of different types of poly(propylene)s.

The Quantification of Physiologically Relevant Cross-Shear Wear Phenomena on Orthopaedic Bearing Materials Using the MAX-Shear Wear Testing System

Background: The occurrence of multi-directional sliding motion between total knee replacement bearing surfaces is theorized to be a primary wear and failure mechanism of ultra-high molecular weight poly(ethylene) (UHMWPE). To better quantify the tribologic mechanisms of this cross-shear wear, the MAX-Shear wear-testing system was developed to evaluate candidate biomaterials under controlled conditions of cross-shear wear. Method of approach: A computer controlled traveling x-y stage under a 3 degree-of-freedom statically loaded pin is used to implement the complex multi-directional motion pathways observed during TKR wear simulation. A MHz collection of dynamic x-y friction was available on all six environmentally controlled stations. The functionality of this testing platform was proven in a 100,000 cycle, 11.6 MPa, wear test using 15.0 mm diameter polished stainless steel spheres against flat GUR4150 UHMWPE. A five-pointed star wear pattern was used to incorporate the physiologically relevant cross-shear sliding conditions of stop/start, 50mm∕s entraining velocity and five crossing angles of 72°. Using normalized volumetric reconstruction of the resulting surface damage, a direct quantitative relationship between linear and cross-shear surface damage intensity was obtained. Results: Cross-shear surface damage volume loss was found to be 2.94 (±0.88) times that associated with linear sliding under identical tribologic conditions. SEM analysis of linear wear damage showed consistent fibril orientation along the direction of sliding while cross-shear wear damage showed multi-directional fibril orientations and increased surface roughness. Significant increases in discrete crossing-point friction coefficients were recorded throughout testing. Conclusions: This scientific approach to quantifying the tribologic effects of cross-shear provides fundamental wear mechanism data that are critical in evaluating potential biomaterials for use as in vivo bearings. Relevant multi-axis, cross-shear wear testing is necessary to provide quantifiable measures of complex biomaterials wear phenomena.

Morphology and enzymatic degradation of oriented thin film of ultrahigh molecular weight poly[(R)-3-hydroxybutyrate

Thin films of ultrahigh molecular weight poly[(R)-3-hydroxybutyrate] (P(3HB)) were sheared and isothermally crystallized at 100 degrees C. Transmission electron microscopy and atomic force microscopy (AFM) observations revealed that thick fibrous textures, on which lamellae are overgrown normal to the long axis of the fibril, run parallel to the shearing direction. A selected area electron diffraction pattern taken from the fibrils exhibits a fiber pattern of P(3HB) alpha-modification, and the crystallographic c-axis (chain axis) of P(3HB) is set parallel to the long axis of the fibril. In situ AFM observations of enzymatic degradation for the thin film were performed with an extracellular P(3HB) depolymerase from Ralstonia pickettii T1 in a buffer solution. The film surface and thickness became rougher and thinner, respectively, with time after adding the enzyme. During the degradation, fine shish-kebab structures appeared gradually. This fact supports that the amorphous region in the film is preferentially degraded rather than the crystalline one by the depolymerase. The in situ AFM observations also revealed that one thick fibril in the original film is composed of three different states, namely, finer fibril (shish), stacked lamellae (kebab) in edge-on state, and the surrounding amorphous phase.

Higher α-Olefin Polymerization Behavior of a Bis(Phenoxy-Imine)Titanium Complex/i-Bu3Al/Ph3CB(C6F5)4 Catalyst System

Abstract Catalytic behavior of a bis(phenoxy-imine)Ti complex/i-Bu3Al/Ph3CB(C6F5)4 catalyst system for higher α-olefin polymerization was investigated. The catalyst system exhibited higher activities towards α-olefins having larger molecular sizes and formed ultra-high molecular weight poly(higher α-olefin)s.

Low-temperature suspension polymerization of vinyl pivalate for the preparation of syndiotacticity-rich ultrahigh molecular weight poly(vinyl alcohol) microfibrils with high yield

To prepare ultrahigh molecular weight (UHMW) poly(vinyl pivalate) (PVPi) with high conversion and high linearity for a precursor of syndiotacticity-rich UHMW poly(vinyl alcohol) (PVA), vinyl pivalate (VPi) was suspension polymerized using a low-temperature initiator, 2,2'-azobis(2,4-dimethylvaleronitrile) (ADMVN), and the effects of polymerization conditions on the polymerization behavior and molecular structures of PVPi and PVA prepared by saponifying PVPi were investigated. Suspension polymerization was slightly inferior to bulk polymerization in increasing the molecular weight of PVA. In contrast, the former was superior in increasing the conversion of the polymer. Suspension polymerization of VPi at 25 °C by controlling various polymerization factors proved to be successful in obtaining PVA of UHMW (number-average degree of polymerization (Pn): 14,700–16,700), high syndiotactic diad content (62%), and of high yield (ultimate conversion of VPi into PVPi: 85–90%). In the case of bulk polymerization of VPi under the same conditions, maximum Pn, conversion of 15,800–17,000, and 25–35% were obtained, respectively. The degree of branching was lower and the Pn and syndiotacticity were higher with PVA prepared from PVPi polymerized at lower temperatures. All PVAs from PVPi suspension-polymerized at 25 °C were fibrous, with a high degree of crystallinity and orientation of the crystallites.

FI Catalysts: A New Family of High Performance Catalysts for Olefin Polymerization

AbstractFor Abstract see ChemInform Abstract in Full Text.

Measurement of Orientation Distributions Using Chemical Shift Amplification

A new three-dimensional magic angle spinning (MAS) experiment is proposed, based on a combination of the two-dimensional rotor-synchronized MAS experiment of Spiess and co-workers and a new chemical shift anisotropy amplification method. The new experiment is demonstrated on a macroscopically ordered sample of ultra-high molecular weight poly(ethylene).

FI Catalysts: A New Family of High Performance Catalysts for Olefin Polymerization

This paper reviews a new family of olefin polymerization catalysts. The catalysts, named FI catalysts, are based on non-symmetrical phenoxyimine chelate ligands combined with group 4 transition metals and were developed using “ligand-oriented catalyst design”. FI catalysts display very high ethylene polymerization activities under mild conditions. The highest activity exhibited by a zirconium FI catalyst reached an astonishing catalyst turnover frequency (TOF) of 64,900 s –1 atm –1, which is two orders of magnitude greater than that seen with Cp2ZrCl2 under the same conditions. In addition, titanium FI catalysts with fluorinated ligands promote exceptionally high-speed, living ethylene polymerization and can produce monodisperse high molecular weight polyethylenes (Mw/Mn<1.2, max. Mn>400,000) at 50 °C. The maximum TOF, 24,500 min –1 atm –1, is three orders of magnitude greater than those for known living ethylene polymerization catalysts. Moreover, the fluorinated FI catalysts promote stereospecific room-temperature living polymerization of propylene to provide highly syndiotactic monodisperse polypropylene (max. [rr] 98%). The versatility of the FI catalysts allows for the creation of new polymers which are difficult or impossible to prepare using group 4 metallocene catalysts. For example, it is possible to prepare low molecular weight (Mv∼103) polyethylene or poly(ethylene-co-propylene) with olefinic end groups, ultra-high molecular weight polyethylene or poly(ethylene-co-propylene), high molecular weight poly(1-hexene) with atactic structures including frequent regioerrors, monodisperse poly(ethylene-co-propylene) with various propylene contents, and a number of polyolefin block copolymers [e.g., polyethylene-b-poly(ethylene-co-propylene), syndiotactic polypropylene-b-poly(ethylene-co-propylene), polyethylene-b-poly(ethylene-co-propylene)-b-syndiotactic polypropylene]. These unique polymers are anticipated to possess novel material properties and uses.

Degradation of Ultrahigh Molecular Weight Poly(styrene-co-m-divinylbenzene)s as Network-Polymer Precursors in SEC Columns

Degradation of Ultrahigh Molecular Weight Poly(styrene-co-m-divinylbenzene)s as Network-Polymer Precursors in SEC Columns

Ultra-low-pressure water softening with pore-filled membranes

The design parameters for high-performance, ultra-low-pressure water softening membranes based on a construct involving pore-filling microporous substrates with polyelectrolyte gels are presented. The design parameters were then used to make a series of gel-filled membranes by in situ crosslinking of poly(vinylbenzyl chloride) with diamines (piperazine and 1,4-diazabicyclo[2.2.2]octane) or poly(4-vinylpyridine) with dihalides such as α,α'-dichloro- p-xylene in DMF solutions in the pores of either a TIPS poly(propylene) or non-woven ultra-high molecular weight poly(ethylene) support. The separation properties of these membranes were tested under ultra-low pressure (100 kPa) using untreated municipal tap water. These membranes exhibit excellent separation properties, comparable to those of a commercial thin-film nanofiltration membrane (Desal-51, Osmonics), and productivity (flux) markedly higher than that of the thin-film membrane. The results of this study clearly show the importance of the mechanical strength (rigidity) of the microporous supports in designing high-performance pore-filled membranes.

Gamma irradiation effect upon positron annihilation in ultra high molecular weight poly(ethylene oxide)

Positron annihilation technique was used to reveal the evolution of small pore structure of semi-crystalline ultra high molecular weight poly(ethylene oxide) under γ-irradiation. It has been established that the structure of poly(ethylene oxide) is improved under low dose irradiation (<20 kGy), while the concentration of free-volume holes in amorphous regions increases at higher doses. The results were compared with those from small angle X-ray scattering and wide angle X-ray scattering measurements of the same samples.