Unexpected Tribological Synergy in Polymer Blend Coatings: Leveraging Phase Separation to Isolate Domain Size Effects and Reduce Friction

Abstract

We employed a systematic processing approach to control phase separation in polymer blend thin films and significantly reduce dynamic friction coefficients (μ)s. We leveraged this modulation of phase separation to generate composite surfaces with dynamic friction coefficients that were substantially lower than expected on the basis of simple mixing rules, and in several cases, these friction coefficients were lower than those of both pure components. Using a model polyisoprene [PI]/polystyrene [PS] composite system, a minimum μ was found in films with PS mass fractions between 0.60 and 0.80 (μ = 0.11 ± 0.03); that value was significantly lower than the friction coefficient of PS (μ = 0.52 ± 0.01) or PI (μ = 1.3 ± 0.09) homopolymers and was comparable to the friction coefficient of poly(tetrafluoroethylene) [PTFE] (μ = 0.09 ± 0.01) measured under similar conditions. Additionally, through experiments in which the domain size was systematically varied at constant composition (through an annealing process), we demonstrated that μ decreased with decreasing characteristic domain size. Thus, the tribological synergy between PS and PI domains (discrete size, physical domain isolation, and overall film composition) was shown to play an integral role in the friction and wear of these PS/PI composites. Overall, our results suggest that even high friction polymers can be used to create low friction polymer blends by following appropriate design rules and demonstrate that engineering microstructure is critical for controlling the friction and adhesion properties of composite films for tribologically relevant coatings.

Exploiting Feedstock Diversity To Tune the Chemical and Tribological Properties of Lignin-Inspired Polymer Coatings

“…However, when measured with the HDPE probe, the friction coefficients increased with oxygen content, likely as a result of the change in adhesion between the probe and the film (see PVM and PSM data in Figure ). In general, the friction coefficients in Figure are similar to those for PS measured under similar conditions with glass (μ PS = 0.46) and HDPE (μ PS = 0.51) …”

“…Dynamic friction coefficients (μ i )s were measured using a custom linear reciprocating tribometer, which was operated in a sphere-on-flat geometry, as described in the literature. 52 The probe diameter was 6.35 mm, and the probe was made of either borosilicate glass or highdensity polyethylene (HDPE) depending on the experiment. Select time-dependent friction coefficient traces are shown in the Supporting Information, Figure S2.…”

“…In general, the friction coefficients in Figure 7 are similar to those for PS measured under similar conditions with (μ PS = 0.46) and HDPE (μ PS = 0.51). 52 Tribology Discussion. Although increasing oxygen content per repeat unit impacted the polymer−solvent interaction parameters with THF and CHCl 3 , leading to both increases in solvent compatibility (PGM → PSM) and decreases in solvent compatibility (PGM → PVM) [see Figure 4], depending on the position and type of the oxygen group, the polymers containing more oxygen groups generally were less soluble in the casting solvent (THF).…”

Arrested Elongated Interface with Small Curvature by the Simultaneous Reactive Compatibilization and Stereocomplexation

“…Due to the complex interplay of multiple factors during the blending process, a full comprehension on the formation and stability of co-continuous phase morphology nowadays is far from being elucidated. Most investigations on the formation of the co-continuous structure have been performed with an emphasis on the role of processing conditions, 17,18 component proportions, 19,20 viscosity ratios, 21−23 and interfacial tensions. 24−28 No attention has been paid to the interfacial modulus effects on the formation of co-continuous morphology, to our best knowledge.…”

Design of Friction, Morphology, Wetting, and Protein Affinity by Cellulose Blend Thin Film Composition

“…More recently, FFM was employed to correlate friction to viscoelastic relaxation (Hammerschmidt et al, 1996; Sondhauß et al, 2015) and to characterize photoreactive organic surface patterns of spin casted thin films (Hlawacek et al, 2009; Shen et al, 2014). Polyisoprene and polystyrene blend coatings were recently reported to show an unexpected tribological synergy (Emerson et al, 2017). By varying the composition of the blend films, it was possible to tune the tribological properties and achieve friction coefficients which are much lower than for the pure films.…”