A Biomimetic Approach to Lubricate Engineering Materials

Troels Røn

Research output: Book/ReportPh.D. thesis

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Abstract

This PhD thesis consists of studies on biomimetic aqueous boundary lubrication by applying amphihilic copolymers and hydrophobin proteins as lubricant addtives. Studies on the temperature dependency of neat water and hydrogel lubrication were also conducted.
Amphiphilic diblock, triblock and graft copolymers were applied as synthetic boundary lubricant additives in water in relation to test the hypothesis that adsorbed polyelectrolyte brushes can displays the same superior lubricity over neutral brushes as has been observed for covalently anchored brushes. In the case of diblock copolymers, the diblocks consisted of a hydrophilic block of either neutral poly(ethylene glycol) (PEG) or negatively charged poly(acrylic acid) (PAA) and of a hydrophobic block of polystyrene (PS) or poly(2-methoxyethyl acrylate) (PMEA). Thus generating PEG-b-X or PAA-b-X, where X block was either PS or PMEA. Comparing the neutral PEG and charged PAA buoyant blocks, the neutral showed superior adsorption onto hydrophobic poly(dimethylsiloxane) (PDMS) surfaces from neutral aqueous conditions. Neutral PEG based copolymers showed substantial adsorption for both PS and PMEA as the anchoring block, whereas charged PAA-based copolymersshowed effective adsorption only for PMEA anchoring block. PAA-b-PS diblock copolymer’s poor lubricity for the PDMS-PDMS sliding contact was well correlated with poor adsorption. PAA-b-PMEA copolymers, despite their significant degree of adsorption, showed little lubricity. When adding NaClto the aqueous solution or by lowering the pH, both the adsorption and lubricity of the PAA-b-PMEA diblock copolymer solutions improved. The poor adsorption and inferior aqueous lubricating properties of the polyelectrolyte based (PAA) diblock copolymers compared to their PEG-based counterparts was mainly attributed the electrostatic repulsion between charged PAA blocks, hindering the facile formation of the lubricating layer under cyclic tribological stress.
It is well known that graft copolymers anchor more efficiently to surfaces than their diblock counterparts, thus the synthesis and study on lubricating capabilities of this amphiphilic structure were conducted. The graft copolymers consisting of a backbone poly(2-hydroxyethyl methacrylate) (PHEMA) and polyelectrolyte graft chains of either anionic poly(methacrylic acid) or cationic poly((2-dimethylamino) ethyl methacrylate) (PDMEAMA) was synthesized, and neutral MEA repeating units were incorporated in the graft chain to provide dilution of the charges. Cationic based P(MEA-co-DMAEMA)-graft-PHEMA did not show any adsorption or lubricating properties at neutral (pH 7.0, 1 mM ionic strength) or 150 mM NaCl conditions. Graft copolymer of anionic P(MEA-co-MAA)-graft-PHEMA copolymers did not show lubricating capabilities at neutral conditions either. However, the lubricity was significantly improved at saline conditions of 150 mM NaCl even at the slowest sliding speeds. Lubricity in this speed range was not observed for the PAA-b-PMEA diblocks in the diblock copolymer study. Graft copolymers were more successful than charged diblock copolymers in lubricating PDMS-PDMS sliding contact when charged moieties are present in the buoy chains.
The adsorption and aqueous lubricating properties of an amphiphilic triblock copolymers of PEG, PMEA and (PMAA) blocks, namely PEG-b-PMEA-b-PMAA was also studied. After adsorption onto a nonpolar hydrophobic surface from aqueous solution, an equal and homogeneous mixture of neutral PEG and charged PMAA chains is formed on the surface, with an adsorbed polymer mass comparable to its fully neutral counterpart, PEG-b-PMEA-b-PEG. The lubricity of PEG-b-PMEA-b-PMAA showed significant improvement compared to fully charged diblock of PAA-b-PMEA, which is attributed to dilution of charged moieties on the surface and subsequent improvement of the lubricating film stability in PDMS-PDMS sliding contact. However, no salt was needed to achieve the good lubricity in the sliding contact. The studies on amphiphilic block copolymers show that pure polyelectrolyte chains are not applicable as means of affording good lubricity as aqueous lubricant additives for boundary lubrication. Dilution or screening of the charges is necessary for obtaining good lubricity for amphiphilic block copolymers.
The influence of temperature on the lubricating properties of neat water for four tribopairs with different surface hydrophilicity and bulk elasticity moduli were investigated.The four contacts were soft and hard, and hydrophobic and hydrophilic. With increasing temperature, the coefficients of friction generally increased due to the decrease of water’s viscosity. This change was more clearly observed at the soft interfaces due to easier lubricating film formation of water at the lower contact pressure. Nevertheless, dominant lubrication mechanism appeared to be boundary and mixed lubrication even for soft interfaces at all mean speeds (10-1200 mm/s) and temperatures (1 to 90 °C) investigated.
The temperature dependency on the lubricating properties of thermoresponsive F127 hydrogel was also studied in the same contacts. The high viscosity of gelation at certain temperatures (ca. 20-60 °C) was conjectured to tune the lubricity of the F127 hydrogels. However, due to significant shear thinning in the hard contacts, there was no or very little lubricity of the F127 hydrogel. In the soft contact good lubricity was observed, albeit the lubricating performance was ascribed (depending on the sliding speed) to viscosities which were still much lower than the bulk gel, and to boundary lubrication of the F127 polymer. Shear thinning was, however, assessed as a compelling property in the soft contact causing lower friction. Amphiphilic proteins of hydrobins type II fungi hydrophobins, HFBI and FpHYD5, were studied as aqueous lubricant additives at PDMS tribopair interface. The two hydrophobins are featured as non-glycosylated and lighter (HFBI, m.w. ca. 7 kDa) vs glycosylated and heavier (FpHYD5, m.w. ca. 10 kDa) proteins. Their adsorption at PDMS-water interface were very similar. PDMS-PDMS sliding interface was effectively lubricated by the hydrophobin solutions, and showed a reduction in the coefficient of friction as much as by ca. two orders of magnitude. Better lubrication was provided at concentrations 1.0 mg/mL as compared to 0.1 mg/mL, espicially in low-speed regime, where boundary lubrication characteristic is dominant via ‘selfhealing’ mechanism. The glycosylated FpHYD5 revealed a better lubrication than HFBI. Two type II hydrophobins function more favorably compared to synthetic amphiphilic copolymer, PEO-PPO-PEO, with a similar molecular weight. This is ascribed to higher amount of adsorption of the hydrophobins to hydrophobic surfaces from aqueous solution.
Original languageEnglish
PublisherTechnical University of Denmark. Department of Mechanical Engineering
Number of pages184
Publication statusPublished - 2014

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