Nacu Hernandez

Combining Metabolic Engineering and Electrocatalysis: Application to the Production of Polyamides from Sugar.

Biorefineries aim to convert biomass into a spectrum of products ranging from biofuels to specialty chemicals. To achieve economically sustainable conversion, it is crucial to streamline the catalytic and downstream processing steps. In this work, a route that combines bio- and electrocatalysis to convert glucose into bio-based unsaturated nylon-6,6 is reported. An engineered strain of Saccharomyces cerevisiae was used as the initial biocatalyst for the conversion of glucose into muconic acid, with the highest reported muconic acid titer of 559.5 mg L(-1) in yeast. Without any separation, muconic acid was further electrocatalytically hydrogenated to 3-hexenedioic acid in 94 % yield despite the presence of biogenic impurities. Bio-based unsaturated nylon-6,6 (unsaturated polyamide-6,6) was finally obtained by polymerization of 3-hexenedioic acid with hexamethylenediamine.

Gel Point Suppression in RAFT Polymerization of Pure Acrylic Cross-Linker Derived from Soybean Oil

Here we report the reversible addition-fragmentation chain transfer (RAFT) polymerization of acrylated epoxidized soybean oil (AESO), a cross-linker molecule, to high conversion (>50%) and molecular weight (>100 kDa) without macrogelation. Surprisingly, gelation is suppressed in this system far beyond the expectations predicated both on Flory-Stockmeyer theory and multiple other studies of RAFT polymerization featuring cross-linking moieties. By varying AESO and initiator concentrations, we show how intra- versus intermolecular cross-linking compete, yielding a trade-off between the degree of intramolecular linkages and conversion at gel point. We measured polymer chain characteristics, including molecular weight, chain dimensions, polydispersity, and intrinsic viscosity, using multidetector gel permeation chromatography and NMR to track polymerization kinetics. We show that not only the time and conversion at macrogelation, but also the chain architecture, is largely affected by these reaction conditions. At maximal AESO concentration, the gel point approaches that predicted by the Flory-Stockmeyer theory, and increases in an exponential fashion as the AESO concentration decreases. In the most dilute solutions, macrogelation cannot be detected throughout the entire reaction. Instead, cyclization/intramolecular cross-linking reactions dominate, leading to microgelation. This work is important, especially in that it demonstrates that thermoplastic rubbers could be produced based on multifunctional renewable feedstocks.

Pavement Friction Modeling using Texture Measurements and Pendulum Skid Tester

Pavement frictional behavior affects pavement performance in terms of vehicle safety, fuel consumption, and tire wear. Comprehending and interpreting pavement friction measurements is a challenging task, because of friction sensitivity to several uncontrollable factors. These factors include: pavement surface conditions, such as the type and thickness of contaminants and fluids on the surface and their interaction with friction forces; and the device operating conditions, such as sliding speed, material properties and geometry of the rubber slider used, and operating temperature. Despite the efforts to describe and quantify the impact of varying conditions on pavement friction, which ultimately will allow for a better harmonization of friction measurements, there is a need to better understand the link between the surface texture and physical friction measurements. In this paper, Persson’s friction model is used to analyze and understand the impact of surface texture on frictional behavior of dry pavement surfaces. The model was used to analyze 18 test locations, which were compared with the dry kinetic coefficients of friction (COF) estimated using a British pendulum tester (BPT). The results show that Persson’s friction model could predict the COF estimated from the BPT results with relatively high accuracy. In addition, the model could provide a profound explanation of the frictional forces mechanism. Finally, it was found that the mean profile depth (MPD) cannot provide a full picture of the frictional behavior. However, combining MPD with the Hurst exponent, texture measurements can potentially provide a full physical explanation of the frictional behavior for road surfaces.