On a New Method to Determine the Yield Stress in Lubricating Grease

An experimental study using both a controlled stress and a controlled strain rheometer has been undertaken to characterize lubricating grease in shear, creep, stress relaxation, and oscillatory flow, with a main focus on determining the yield stress. The yield stress was examined using a cone–plate and parallel-plate system with smooth and rough surfaces. Clear discrepancies were observed in the yield stress values obtained using different techniques where oscillatory strain sweep measurements seem to be the best choice. This technique is less sensitive to wall slip, shows good reproducibility, and is relatively easy to perform. The method also shows that the yield stress is a function of the imposed frequency and therefore of the time domain. At lower values of shear—that is, in the linear viscoelastic regime—there is no structural breakdown and the rheology of the grease can be described by the Maxwell model where the stress and the strain are almost proportional to each other. Based on this observation, a novel method to determine the yield stress is proposed: “The yield stress can be determined from the point where this linearity no longer applies.” This method is compared to those that are commonly used. The yield stress was found to depend exponentially on temperature and linearly on frequency

Carbon-based two electron sigma-donor ligands beyond classical N-heterocyclic carbenes

Ghadwal R. Carbon-based two electron sigma-donor ligands beyond classical N-heterocyclic carbenes. DALTON TRANSACTIONS. 2016;45(41):16081-16095.N-Heterocyclic carbenes (NHCs) are an important class of compounds that are indeed regarded as most versatile carbon-donor ligands in transition metal and organometallic catalysis. In addition, NHCs are also capable of stabilizing a variety of highly reactive main group compounds with intriguing properties. The enormous success of NHCs prompted the investigation of other carbon-based neutral ligands with additional structural and electronic features. Reactivity studies of NHCs and their complexes unveiled routes to new carbon donor ligands. Such NHC-derived ligands can be grouped into three categories: (i) N-heterocyclic olefins (the donor site is extended by an intervening carbon atom), (ii) ditopic carbanionic-NHCs (an additional coordination site is generated by the deprotonation of an NHC), and (iii) abnormal-NHCs (the carbene center of an NHC is moved to different positions). This article summarizes the recent advances in NHC chemistry of compounds featuring the aforementioned ligands, which are exclusively derived by the functionalization of NHCs, 1,3-bis(2,6-diisopropylphenyl) imidazol-2-ylidene (IPr) in particular