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A special issue of Polymers (ISSN 2073-4360). This special issue belongs to the section "Polymer Physics and Theory".

Deadline for manuscript submissions: closed (30 June 2020) | Viewed by 37622

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Special Issue Editors

Prof. Hsu-Wei Fang

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Guest Editor

Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan
Interests: bio-tribology; biomaterials; polymers; tissue engineering; cartilage repair; surface engineering; bone graft; biomolecular lubricants; artificial joint system; mesenchymal stem cell application

Prof. Chen-Ying Su

E-Mail Website
Guest Editor

Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, Taiwan
Interests: bio-tribology, biomolecular lubricants, bone graft, cartilage repair, adipose-derived stem cell application, genetics, molecular biology, developmental biology

Special Issue Information

Dear Colleagues,

This Special Issue focuses on the latest progress in the tribology of polymers. The properties of polymers change with time during the tribological process, resulting in problems. For example, the ultra-high molecular-weight polyester in the artificial joint system produces wear particles after extended use and induces oesteolysis. Wear in metals and ceramics can be reduced by external lubricants, but external lubricants often cause swelling in polymers. In the human body, not only the body fluid influences the lubrication of polymers but also the biological components. A good example is that the tribological movement between the contact lens and the eye causes conformational changes of tear proteins and subsequently results in the discomfort of the wearer. It is critical to understand the tribology of polymers in order to develop or modify polymers with high scratch resistance, low wear, or low friction. In addition, potential external lubricants might be identified after understanding the effects of the tribological process in polymers.

We aim to deliver recent the progress in the tribology of synthetic or natural polymers. We also aim to present potential mechanisms for reducing the friction, decreasing the wear, and increasing the lubrication of polymers. This Special Issue not only provides a better understanding of changes in the properties of polymers with time but also provides potential solutions for current problems of polymers during tribological processes.

Authors are welcome to submit their latest research in the form of original full articles, communications, or reviews on this topic.

Prof. Hsu-Wei Fang
Prof. Chen-Ying Su
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Polymers is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

Tribology of polymers
Friction
Wear
Lubrication
Scratch resistance
Synthetic polymers
Natural polymers
Bio-tribology
Biomedical polymers
Surface modification
Lubricant

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Published Papers (10 papers)

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Research

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13 pages, 4572 KiB

Open AccessArticle

Tribological and Mechanical Behavior of Graphite Composites of Polytetrafluoroethylene (PTFE) Irradiated by the Electron Beam

by Adrian Barylski, Andrzej S. Swinarew, Krzysztof Aniołek, Sławomir Kaptacz, Jadwiga Gabor, Arkadiusz Stanula, Zbigniew Waśkiewicz and Beat Knechtle

Polymers 2020, 12(8), 1676; https://doi.org/10.3390/polym12081676 - 28 Jul 2020

Cited by 15 | Viewed by 3228

Abstract

This research investigated the effect of irradiation with an electron beam energy of 10 MeV in doses of 26–156 kGy on polytetrafluoroethylene (PTFE) with a 15% and 20% graphite additive. The research has shown that mechanical (compression strength, hardness, and Young’s modulus) and sclerometric (coefficient of wear micromechanism and coefficient of resistance to wear) properties improve and tribological wear decreases as graphite content increases. Electron beam irradiation increases the degree of crystallinity of both materials to a similar extent. However significant differences in the improvement of all examined properties have been demonstrated for PTFE with higher (20%) graphite content subjected to the electron beam irradiation. This polymer is characterized by higher hardness and Young’s modulus, reduced susceptibility to permanent deformation, higher elasticity, compression strength, and above all, a nearly 30% reduction in tribological wear compared to PTFE with a 15% graphite additive. The most advantageous properties can be obtained for both of the examined composites after absorbing a dose of 104 kGy. The obtained results hold promise for the improvement of the operational life of friction couples which do not require lubrication, used for example in air compressors and engines, and for the possibility of application of these modified polymers. In particular PTFE with 20% graphite content, in the nuclear and space industry. Full article

(This article belongs to the Special Issue Tribology of Polymers)

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15 pages, 2186 KiB

Open AccessArticle

Quantify the Protein–Protein Interaction Effects on Adsorption Related Lubricating Behaviors of α-Amylase on a Glass Surface

by Nareshkumar Baskaran, You-Cheng Chang, Chia-Hua Chang, Shun-Kai Hung, Chuan-Tse Kao and Yang Wei

Polymers 2020, 12(8), 1658; https://doi.org/10.3390/polym12081658 - 25 Jul 2020

Cited by 2 | Viewed by 2529 | Correction

Abstract

Dental ceramic material is one of the widely preferred restorative materials to mimic the natural tooth enamel surface. However, it has continuously been degraded because of low wear resistance during mastication in the oral cavity. The friction involved was reduced by introducing the lubricant saliva protein layers to improve the wear resistance of the dental materials. However, little is understood regarding how the protein–protein interactions (PPI) influence the adsorbed-state structures and lubricating behaviors of saliva proteins on the ceramic material surface. The objective of this study is to quantify the influences of PPI effects on the structural changes and corresponding oral lubrications of adsorbed α-amylase, one of the abundant proteins in the saliva, on the dental ceramic material with glass as a model surface. α-Amylase was first adsorbed to glass surface under varying protein solution concentrations to saturate the surface to vary the PPI effects over a wide range. The areal density of the adsorbed protein was measured as an indicator of the level of PPI effects within the layer, and these values were then correlated with the measurements of the adsorbed protein’s secondary structure and corresponding friction coefficient. The decreased friction coefficient value was an indicator of the lubricated surfaces with higher wear resistance. Our results indicate that PPI effects help stabilize the structure of α-amylase adsorbed on glass, and the correlation observed between the friction coefficient and the conformational state of adsorbed α-amylase was apparent. This study thus provides new molecular-level insights into how PPI influences the structure and lubricating behaviors of adsorbed protein, which is critical for the innovations of dental ceramic material designs with improved wear resistance. Full article

(This article belongs to the Special Issue Tribology of Polymers)

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10 pages, 1344 KiB

Open AccessArticle

Correlation between Tribological Properties and the Quantified Structural Changes of Lysozyme on Poly (2-hydroxyethyl methacrylate) Contact Lens

by You-Cheng Chang, Chen-Ying Su, Chia-Hua Chang, Hsu-Wei Fang and Yang Wei

Polymers 2020, 12(8), 1639; https://doi.org/10.3390/polym12081639 - 23 Jul 2020

Cited by 5 | Viewed by 3627

Abstract

The ocular discomfort is the leading cause of contact lens wear discontinuation. Although the tear proteins as a lubricant might improve contact lens adaptation, some in vitro studies suggested that the amount of adsorbed proteins could not simply explain the lubricating performance of adsorbed proteins. The purpose of this study was to quantify the structural changes and corresponding ocular lubricating properties of adsorbed protein on a conventional contact lens material, poly (2-hydroxyethyl methacrylate) (pHEMA). The adsorption behaviors of lysozyme on pHEMA were determined by the combined effects of protein–surface and protein–protein interactions. Lysozyme, the most abundant protein in tear, was first adsorbed onto the pHEMA surface under widely varying protein solution concentrations to saturate the surface, with the areal density of the adsorbed protein presenting different protein–protein effects within the layer. These values were correlated with the measured secondary structures, and corresponding friction coefficient of the adsorbed and protein covered lens surface, respectively. The decreased friction coefficient value was an indicator of the lubricated surfaces with improved adaptation. Our results indicate that the protein–protein effects help stabilize the structure of adsorbed lysozyme on pHEMA with the raised friction coefficient measured critical for the innovation of contact lens material designs with improved adaptation. Full article

(This article belongs to the Special Issue Tribology of Polymers)

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15 pages, 7812 KiB

Open AccessArticle

Nano-Indentation Response of Ultrahigh Molecular Weight Polyethylene (UHMWPE): A Detailed Analysis

by Tanveer Iqbal, S. S. Camargo, Jr., Saima Yasin, Ujala Farooq and Ahmad Shakeel

Polymers 2020, 12(4), 795; https://doi.org/10.3390/polym12040795 - 2 Apr 2020

Cited by 8 | Viewed by 4386

Abstract

Nano-indentation, a depth sensing technique, is a useful and exciting tool to investigate the surface mechanical properties of a wide range of materials, particularly polymers. Knowledge of the influence of experimental conditions employed during nano-indentation on the resultant nano-mechanical response is very important for the successful design of engineering components with appropriate surface properties. In this work, nano-indentation experiments were carried out by selecting various values of frequency, amplitude, contact depth, strain rate, holding time, and peak load. The results showed a significant effect of amplitude, frequency, and strain rate on the hardness and modulus of the considered polymer, ultrahigh molecular weight polyethylene (UHMWPE). Load-displacement curves showed a shift towards the lower indentation depths along with an increase in peak load by increasing the indentation amplitude or strain rate. The results also revealed the strong dependence of hardness and modulus on the holding time. The experimental data of creep depth as a function of holding time was successfully fitted with a logarithmic creep model (R² ≥ 0.98). In order to remove the creeping effect and the nose problem, recommended holding times were proposed for the investigated polymer as a function of different applied loads. Full article

(This article belongs to the Special Issue Tribology of Polymers)

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8 pages, 1678 KiB

Open AccessArticle

The Bio-Tribological Effect of Poly-Gamma-Glutamic Acid in the Lysozyme-Ionic Contact Lens System

by Chen-Ying Su, Lung-Kun Yeh, Chi-Chun Lai, Mihaela Dubuisson, Yi-Fei Tsao, Ching-Li Tseng and Hsu-Wei Fang

Polymers 2020, 12(1), 156; https://doi.org/10.3390/polym12010156 - 7 Jan 2020

Cited by 3 | Viewed by 3510

Abstract

Feeling comfortable is an important issue for contact lens wearers as contact lenses are worn for an extensive period of time. It has been shown that the in vitro friction coefficient of contact lenses is correlated to the degree of in vivo comfort, thus many studies focus on establishing friction testing methods for investigating the friction coefficient of contact lenses or contact lens care solution. We have previously demonstrated the lubricating property of poly-gamma-glutamic acid (γ-PGA)-containing care solution, and it could reduce the high friction coefficient caused by lysozyme. However, the mechanism of how γ-PGA-containing care solution reduces the lysozyme-induced friction coefficient of contact lenses is unclear. We investigated the bio-tribological effect of γ-PGA on ionic contact lenses in the presence of lysozyme by testing load and velocity variations. The ability to remove lysozyme deposition by γ-PGA and viscosity analysis of γ-PGA-containing care solutions were also investigated to understand the potential mechanism. Our results showed that the friction coefficient of γ-PGA-containing care solution with lysozyme was the lowest in both load and velocity variations, and γ-PGA functions distinctly in the lysozyme-ionic contact lens system. We proposed a model of how γ-PGA could reduce the friction coefficient in these two conditions. Full article

(This article belongs to the Special Issue Tribology of Polymers)

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14 pages, 4039 KiB

Open AccessArticle

Tribological Behavior of Hydraulic Cylinder Coaxial Sealing Systems Made from PTFE and PTFE Compounds

by Andrea Deaconescu and Tudor Deaconescu

Polymers 2020, 12(1), 155; https://doi.org/10.3390/polym12010155 - 7 Jan 2020

Cited by 6 | Viewed by 5136

Abstract

Current trends concerning hydraulic cylinder sealing systems are aimed at decreasing energy consumption which can be materialized by minimizing leaks and reducing friction. The latest developments in the field of materials and sealing system geometries as well as modern simulation possibilities allow maximum performance levels of hydraulic cylinders. Reducing friction is possible by hydro-dynamic separation of the sliding and sealing points already at very low velocities and by using materials, such as plastomers, from polytetrafluoroethylene (PTFE) (virgin PTFE and filled PTFE). It is within this context that this paper discusses a theoretical and experimental study focused on the tribological behavior of coaxial sealing systems mounted on the pistons of hydraulic cylinders. It presents a methodology for the theoretical determination of the lubricant film thickness between the cylinder piston and the seal. The experimental installation used for measuring fluid film thickness is presented, and the results obtained under various working conditions are compared to the theoretical ones. For the analyzed working conditions related to pressure, speed, and temperature, the paper concludes with a set of criteria for the selection of the optimum seal material so as to maximize energy efficiency. Full article

(This article belongs to the Special Issue Tribology of Polymers)

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13 pages, 3848 KiB

Open AccessArticle

The Influence of Hydrothermal Aging on the Dynamic Friction Model of Rubber Seals

by Jian Wu, Hang Luo, Haohao Li, Benlong Su, Youshan Wang and Zhe Li

Polymers 2020, 12(1), 102; https://doi.org/10.3390/polym12010102 - 4 Jan 2020

Cited by 8 | Viewed by 3891

Abstract

Cylinder has become an indispensable and important pneumatic actuator in the development of green production technology. The sealing performance of the cylinder directly affects its safety and reliability. Under the service environment of the cylinder, hydrothermal aging of the rubber sealing ring directly affects the dynamic friction performance of the cylinder. So, the dynamic friction model of the cylinder has been developed based on the LuGre friction model, which considers the influence of hydrothermal aging. Here, the influences of the static friction coefficient and Coulomb friction coefficient on the friction model are analyzed. Then, the aging characteristic equation of rubber is embedded in the model for revealing the influence of aging on the friction coefficient of the model. Results show that the aging temperature, aging time, and compressive stress affects the friction coefficient; the variation of the static friction coefficient is larger than that of the Coulomb friction coefficient. The improved cylinder friction model can describe the influence of the aging process on the cylinder friction characteristics, which is of great significance in the design of the cylinder’s dynamic performance. Full article

(This article belongs to the Special Issue Tribology of Polymers)

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15 pages, 5923 KiB

Open AccessArticle

Evolution of Interfacial Friction Angle and Contact Area of Polymer Pellets during the Initial Stage of Ultrasonic Plasticization

by Bingyan Jiang, Yang Zou, Guomeng Wei and Wangqing Wu

Polymers 2019, 11(12), 2103; https://doi.org/10.3390/polym11122103 - 14 Dec 2019

Cited by 6 | Viewed by 3323

Abstract

Interfacial friction heating is one of the leading heat generation mechanisms during the initial stage of ultrasonic plasticization of polymer pellets, which has a significant influence on the subsequent viscoelastic heating according to our previous study. The interfacial friction angle and contact area of polymer pellets are critical boundary conditions for the analysis of interfacial frictional heating of polymer pellets. However, the duration of the interfacial friction heating is extremely short in ultrasonic plasticization, and the polymer pellets are randomly distributed in the cylindrical barrel, resulting in the characterization of the distribution of the interfacial friction angle and contact area to be a challenge. In this work, the interfacial friction angle of the polymer pellets in the partially plasticized samples of polymethyl methacrylate (PMMA), polypropylene (PP), and nylon66 (PA66) were characterized by a super-high magnification lens zoom 3D microscope. The influence of trigger pressure, plasticizing pressure, ultrasonic amplitude, and vibration time on the interfacial friction angle and the contact area of the polymer pellets were studied by a single factor experiment. The results show that the compaction degree of the plasticized samples could be enhanced by increasing the level of the process parameters. With the increasing parameter level, the proportion of interfacial friction angle in the range of 0–10° and 80–90° increased, while the proportion in the range of 30–60° decreased accordingly. The proportion of the contact area of the polymer pellets was increased up to 50% of the interfacial friction area which includes the upper, lower, and side area of the cylindrical plasticized sample. Full article

(This article belongs to the Special Issue Tribology of Polymers)

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Review

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30 pages, 8153 KiB

Open AccessReview

Scratch on Polymer Materials Using AFM Tip-Based Approach: A Review

by Yongda Yan, Shunyu Chang, Tong Wang and Yanquan Geng

Polymers 2019, 11(10), 1590; https://doi.org/10.3390/polym11101590 - 29 Sep 2019

Cited by 25 | Viewed by 5626

Abstract

As a brand new nanomachining method, the tip-based nanomachining/nanoscratching (TBN) method has exhibited a powerful ability at machining on polymer materials and various structures have been achieved using this approach, ranging from the nanodot, nanogroove/channel, bundle to 2D/3D (three-dimensional) nanostructures. The TBN method is widely used due to its high precision, ease of use and low environmental requirements. First, the theoretical models of machining on polymer materials with a given tip using the TBN method are presented. Second, advances of nanostructures achieved by this method are given, including nanodots/nanodot arrays, a nanogroove/channel, 2D/3D nanostructures and bundles. In particular, a useful approach called the ultrasonic vibration-assisted method introduced to integrate with TBN method to reduce the wear of the tip is also reviewed, respectively. Third, the typical applications of the TBN method and the nanostructures achieved by it are summarized in detail. Finally, the existing shortcomings and future prospects of the TBN method are given. It is confirmed that this review will be helpful in learning about this method and push the technology toward industrialization. Full article

(This article belongs to the Special Issue Tribology of Polymers)

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