Atomistic insights into anti-wear mechanisms and protective tribofilm formation in polytetrafluoroethylene composites

Polytetrafluoroethylene (PTFE) has a low friction coefficient but poor wear resistance (k ~ 10−3 mm3/Nm) against various surfaces. Mechanical modeling suggests that the enhanced anti-wear performance of PTFE composites (k ~ 10−5 mm3/Nm) relies on load support by filler in the matrix. Recent studies found that tribochemical polarization of PTFE polymers triggered the formation of highly protective transfer film, thus resulting in an exceptionally low wear (k ~ 10−7 mm3/Nm) in certain composites. However, atomistic interaction was believed to play an important role in the known anti-wear mechanisms, which has yet to be fully described. Here, environmental and computational experiments allowed detailed mechanistic studies for representative PTFE composites, including metal-, ceramic-, carbon-, and polymer-filled composites. Experimental results found that the protective and polarized transfer film formed only in environmental water/oxygen, which could also reduce the composite wear by 10-fold or more. Density-functional-theory (DFT) calculations revealed that the electrophilic atom at solid surface tends to defluorinate PTFE molecule, which enables the tribochemical products of polarized PTFE accumulated near the sliding surfaces. Molecular dynamics simulations suggested that the strengthening of nonbonding interactions resulted from polar polymers improved polymer composites' adhesion and cohesion strengths against steel counterface, which was responsible for the achievement of macro-scale ultralow wear in PTFE composites. The relation between the atomistic interactions and the macroscopic wear behavior of composites was systematically discussed.

Characterization of Low-Density Polyethylene and LDPE-Based/Ethylene-Vinyl Acetate with Medium Content of Vinyl Acetate

Low-density polyethylene (LDPE) and ethylene vinyl acetate copolymer (EVA), which are non-polar and polar polymers, are immiscible and form a polyphase system. In this study, LDPE was mixed with 2.5%, 5%, 7.5%, 10%, 12.5% Ethylene-vinyl acetate (EVA-28) with a medium content of vinyl acetate (28% VA), respectively by injection molding machine and LDPE. Tensile strength and flexural strength were tested according to ASTM D638-02 standard and ISO 178 standard. The results showed that adding EVA-28 increased the elongation at break of the LDPE/2.5% EVA, LDPE/5% EVA and LDPE/10% EVA blend samples. In addition, the tensile and flexural strength of the LDPE/EVA blend decreases gradually as the EVA-28 content in the blend increases. The hardness decreases with the increasing EVA-28 content. EVA-28 spherical particles appeared scattered on the surface of the LDPE matrix, in the highest EVA-28 percent sample (12.5% EVA-28), the number of particles appeared to be quite a lot, and was dispersed quite evenly on the surface. The LDPE/EVA-28 blend achieved a higher elongation at the break than LDPE, in which 10% EVA-28 gives the highest elongation at break.

Nanocellulose as a new sustainable material for various applications: a review

Purpose: This paper presents a comprehensive review of nanocellulose and its application in several applications, including composites, biomedical, and food packaging fields. Design/methodology/approach: General explanations about cellulose and nanocellulose have been described. Different types of nanocellulose (cellulose nanofibers, cellulose nanocrystals, bacterial nanocellulose) as well as their isolation processes (mechanical process, chemical process) have been reviewed. Several surface modifications have been explained to improve the dispersion of nanocellulose in non-polar polymers. The possible utilization of nanocellulose in composites, biomedical, and food packaging fields have also been analysed. Findings: This review presents three application fields at once, namely composites, biomedical, and food packaging fields. In the composite field, nanocellulose can be used as a reinforcing agent which increases the mehcnical properties such as tensile strength and toughness, and thermal stability of the final composites. In the biomedical field, nanocellulose is reinforced into hydrogel or composites which will be produced as tissue scaffolding, wound dressing, etc. It is found that the addition of nanocellulose can extend and control the drug release. While in the packaging field, nanocellulose is added into a biopolymer to improve the barrier properties and decrease the water and oxygen vapor transmission rates. Research limitations/implications: Nanocellulose has a hydrophilic nature, thus making it agglomerated and difficult to disperse in most non-polar polymers. Therefore, certain surface modification of nanocellulose are required prior to the preparation of composites or hydrogels.Practical implications: Further research regarding the toxicity of nanocellulose needs to be investigated, especially when applying it in the biomedical and food packaging fields. Originality/value: This review presents three application fields at once, namely composites, biomedical, and food packaging fields.

Suitability of serpentinized rock material as mineral filler in polymer matrices

<p>Mineral fillers can significantly affect the application properties of plastic materials. The structural and chemical properties of phyllosilicates provide the conditions to change the properties of polymeric material, e.g. flexural and tensile strength or thermal properties, according to the required application. Mineral fillers frequently used are clay minerals with a two-layer structure (serpentine-kaolin group) or three-layer structure (talc-pyrophyllite group, mica group, smectite group). The mineral fillers can be directly used or after surface modification, depending on the polar nature of the polymer. Polymers containing polar groups (hydrophilic polymers) are water-soluble, like polyvinyl alcohols and polysaccharides or can form hydrogen bonds, like polyamides, polyesters and polyvinyl fluorides. Hydrophobic (non-polar) polymers show an absence of polar groups (e.g. polyethylene, polypropylene) or mutual cancelling electrical dipole moments (e.g. polytetrafluorethylene). Minerals with a hydrophilic surface are directly applicable with polar polymers. For the application with non-polar polymers their surface require hydrophobization, whereas non-polar two-layer silicates are directly applicable with these polymers.</p><p>Serpentinized rock material is investigated with regard to its suitability as a polymer filler and its influence on the performance characteristics of various polymers. The samples origin from the Kraubath Ultramafic Massif, which represents part of an Early Paleozoic ophiolite, at the basement of the Austro-Alpine. The Kraubath complex is dominated by metamorphosed dunites and harzburgites, which origin from fractionation processes of the primary peridotite magma. Hydrothermal alteration led to a partly or entirely serpentinization of the ultramafic rocks. The serpentinization process of dunite, ortho-pyroxenite and harzburgite transformed Mg-containing silicates, like olivine and pyroxene to serpentine group minerals. Rock material with a high grade of serpentinization offers favourable conditions for the application as mineral filler.</p><p>The qualitative and quantitative XRD-analyses reveal a predominant occurrence of the antigorite. Further serpentine group minerals, like lizardite, occur in small amounts. Talc represents the second largest mineral phase. The rock material contains a few percentage of amphibole, chlorite, olivine (forsterite) and less than two percent of chromite and bronzite. In the two-layer structure of the main component antigorite, the charge of the tetrahedral layer is compensated by the charge of the octahedral layer. The three-layer structure of talc is electrostatically neutral, with no interlayer material. Therefore, serpentine minerals and talc are suitable for the application as mineral fillers in non-polar polymers, like polypropylene. Both influence the mechanical and tribological properties of polymers. Serpentine improves elasticity, tensile strength, stress at break, elongation at break, the mass wear rate and the coefficient of friction of the polymer but reduces the impact strength. Talc positively influences rigidity, shrinkage, creep properties, heat distortion under load and the coefficient of linear thermal expansion, however reduces toughness, long thermal ageing, impact strength and tensile strength. The further mineral phases are not considered to affect the application properties negatively. Regarding tensile strength and elasticity the ratio of serpentine to talc can influence the increase and decrease of these properties in non-polar polymers. The applicability of the practical implementation is investigated with nanoparticles of the serpentinized rock material in combination with polypropylene in varying proportions.</p>

Nematicidal Streptomyces against Meloidogyne incognita by hydrosoluble macromolecules

Background: Although actinomycetes, which produce antibiotics, have been reported to produce nematicidal metabolites, macromolecules with nematicidal activities have rarely been described. Results: The culture filtrates of Streptomyces albogriseolus strain DN41 and S. fimicarius strain D153 presented 82.13 % and 86.96 % nematicidal activity against Meloidogyne incognita, respectively, after 12 h. A pot experiment highlighted that these culture filtrates were characterised by significant efficacy. The biocontrol efficiency of strain DN41 reached 61.68 % and that of strain D153 68.33 % after 30 days at a dose of 100 mL pot-1. The active metabolites produced by the two strains were hydrosoluble and sensitive to heat, with molecular weights above 8000 Da for DN41 and of 1000–8000 Da for D153. However, these metabolites were negative to protein reaction. A chemical characteristic test indicated that these active metabolites were polar polymers that contained a large amount of sugar and phenolic hydroxyl compounds. Conclusions: These results revealed S. albogriseolus strain DN41 and S. fimicarius strain D153 as potential agents for the control of root-knot nematodes, as hydrosoluble macromolecules have good compatibility and are convenient for practical application.

Surface modification effects on nanocellulose – molecular dynamics simulations using umbrella sampling and computational alchemy

Topochemical modification of nanocellulose particles, in particular acetylation, is commonly used to reduce hygroscopicity and improve their dispersibility in non-polar polymers.

A theoretical framework for the biophysics of tubulins

Microtubules (MTs), the intracellular structures, are made-up of polar polymers that are composed of [Formula: see text] and [Formula: see text] tubulins. The functions of MTs are shape the way for vesicles movement and asexual mitosis division. However, one of the main functions of MTs is stability of cells. Fewer geometrical methods are available in the literature to explore the molecular dynamics (MDs) of a MT, which is a difficult task due to its microscopic size and complex structure. A structural mechanics model with rather similar properties to MT can demonstrate the dynamics of MT. The first and most important step for this process is to obtain the interaction force between tubulins, and a mechanical model can be used to simulate the mechanical and dynamical properties of MTs by using meso- and macro-scale simulations. This work reports the interaction properties of [Formula: see text]–[Formula: see text] tubulin in MT. During this research, with the aid of the MD simulations, the interaction energy in [Formula: see text]–[Formula: see text] dimer is evaluated. The alpha-beta force–distance diagram is sketched with the aid of force and energy formulae. Thus, the graphical analysis supported the findings of this study.