posted on 2023-11-28, 04:30authored byJim H. C. Lee, Sendhil K. Poornachary, Xin Yi Tee, Liangfeng Guo, Connie K. Liu, Liling Zhang, Tiedong Sun, Qiubo Chen, Jianwei Zheng, Pui Shan Chow
Viscosity
improvement property of a lubricant additive
is commonly
attributed to polymer coil expansion with increasing temperature,
although only some polymer chemistries show conformance to this conceptual
mechanism. Herein, we show that the polarity of base oil governs whether
this mechanism underlies the action of a viscosity modifier (VM) by
combining experimental and computational studies. Poly(butyl methacrylate)
(PBMA) dissolved in diethylene glycol diethyl ether (DGDE) or a mixture
of DGDE (polar solvent) and squalane (SQ, nonpolar solvent) was used
as a model lubricant oil system. Specific viscosity of the polymer
solutions measured over a wide range of additive concentrations and
temperatures revealed that thickening efficiency of the VM decreased
with decreasing base oil polarity. While the VM counteracted temperature-induced
thinning of the low polarity base oil, in the polar solvent, the polymer
did not enhance the solution viscosity at higher temperatures. Aiming
to unravel the molecular mechanism underlying viscosity improvement
at elevated temperatures in the different solvent systems, the polymer
conformation and size in the dispersing oil were determined by combining
solution viscosity, small-angle X-ray scattering measurements, and
coarse-grained molecular dynamics simulations. Collectively, the experimental
and simulation results show that the coil-swelling model underpins
viscosity improvement of the polymer solution in DGDE, and the viscosity
of PBMA in DGDE/SQ solution increased with temperature due to polymer
association in solution. However, the thermoresponsive behavior of
the polymer is more pronounced in the mixed solvent system due to
their higher propensity to aggregate at elevated temperatures.