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New Insight into the Relationship Between Molecular Effects and the Rheological Behavior of Polymer-Thickened Lubricants Under High Pressure

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Abstract

The effects of temperature, pressure and shear stress on the viscosity of simplified automotive lubricants—polymer-thickened base oil solutions—were investigated. Various polymers—with different molecular weights and conformations (comb, linear and star)—were used at low concentration (1.2 % w/w) in a hydrocracked mineral base oil: a poly(alkylmethacrylate), an olefin copolymer and a poly(isoprene-styrene hydrogenated). Their rheological behavior was studied and modeled with a Vogel-Tamman and Fulcher equation, a modified Williams-Landel-Ferry-Yasutomi relationship and a Carreau-Yasuda-like formula. Then, the Einstein’s law was used to rapidly and simply determine the hydrodynamic radii of polymers as a function of temperature and pressure. Calculations from Flory equations, intrinsic viscosities and direct measurements confirmed the relevance of this methodology. Finally, molecular considerations allowed a good understanding of the rheological response of polymer solutions.

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Abbreviations

a, n :

Parameters for the Carreau-Yasuda model

C 1, C 2, A 1, A 2, B 1, B 2 :

WLF constants

c :

Concentration in weight of active polymer (g cm−3)

c*:

Critical concentration (g cm−3)

D F :

Fragility parameter

F :

Dimensionless relative thermal expansion of the free volume

G :

Shear modulus (kPa)

h :

Film thickness (nm)

k :

Liquid thermal conductivity (W m−1 K−1)

l :

Mean monomer size (cm)

M :

Mass of one molecule (g)

m0 :

Molecular mass of one monomer (g mol−1)

M n :

Number average molecular mass (g mol−1)

M w :

Weight average molecular mass (g mol−1)

N :

Number of monomers (N = M w/m0)

N a :

Avogadro number (mol−1)

Na :

Nahme-Griffith number

N experiment :

Number of experimental data

N parameter :

Number of parameters in the model

n :

Refractive index

PDI:

Polydispersity index

p :

Pressure (Pa)

R g :

Radius of gyration (nm)

R h :

Hydrodynamic radius (nm)

Relative root mean square error:

\(\sqrt {\frac{{\sum {\left( {\frac{{\eta_{\text{experiment}} - \eta_{\text{model}} }}{{\eta_{\text{experiment}} }}} \right)^{2} } }}{{N_{\text{experiment}} - N_{\text{parameters}} }}}\)

T :

Temperature (°C)

\(T_{\infty }\) :

Vogel temperature at which the viscosity diverges (°C)

T g(0):

Glass transition temperature at atmospheric pressure (°C)

T g(p):

Glass transition temperature depending on pressure (°C)

α :

Local pressure–viscosity coefficient (GPa−1)

α*:

Reciprocal asymptotic isoviscous pressure coefficient (GPa−1)

β :

Temperature–viscosity coefficient (K−1)

Φ :

Volume fraction

Φ 0 :

Universal Flory constant (mol−1)

[η]:

Intrinsic viscosity (L g−1)

η :

Viscosity (Pa s)

η experiment :

Experimental viscosity (Pa s)

η model :

Modeled viscosity (Pa s)

η red :

Reduced viscosity (L g−1)

η r :

Relative viscosity (cm3 g−1)

\(\eta_{0}\) :

Low shear viscosity (Pa s)

\(\eta_{\infty }\) :

Viscosity extrapolated to infinite temperature (Pa s)

\(\eta_{g}\) :

Viscosity at the glass transition (Pa s)

τ :

Shear stress (Pa)

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Acknowledgments

The authors are deeply grateful to Total for its financial and technical support. We would like to thank Dr. J. Eckelt and Prof. Dr. B.A. Wolf from WEE-Solve (Auf der Burg 6, D-55130 Mainz, Germany) for having performed LCST measurements. We also acknowledge Prof. C. Chassenieux, Prof. J.-F. Tassin and D. Chaveroux from the laboratory Polymères, Colloïdes, Interfaces (UMR Université du Maine CNRS-6120, 72085 Le Mans Cedex 9, France) for having kindly provided chromatography and light scattering results.

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Mary, C., Philippon, D., Lafarge, L. et al. New Insight into the Relationship Between Molecular Effects and the Rheological Behavior of Polymer-Thickened Lubricants Under High Pressure. Tribol Lett 52, 357–369 (2013). https://doi.org/10.1007/s11249-013-0214-y

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