Enhancing Mesopore Volume and Thermal Insulation of Silica Aerogel via Ambient Pressure Drying-Assisted Foaming Method.

Ambient pressure drying (APD) of silica aerogels has emerged as an attractive method adapting to large-scale production. Spring-back is a unique phenomenon during APD of silica aerogels with volume expansion after its shrinkage under capillary force. We attribute the intense spring-back at elevated drying temperatures to a dense structure formed on the surface and the formation of positive internal pressure. Furthermore, an APD-assisted foaming method with an in situ introduction of NH4HCO3 was proposed. NH4HCO3 decomposing at drying temperatures hastened the emergence of positive pressure, thereby increasing the expansion volume. Compared to the previous method, the porosity of silica aerogel increased from 82.2% to 92.6%, and mesopore volume from 1.79 cm3 g-1 to 4.54 cm3 g-1. By adjusting the amount of the silicon source, silica aerogels prepared by the APD-assisted foaming method generated higher volume expansion and lower thermal conductivity. After calcination to remove undecomposed ammonium salts, the hydrophobic silica aerogel with a density of 0.112 g cm-3 reached a mesopore volume of 5.07 cm3 g-1 and a thermal conductivity of 18.9 mW m-1·K-1. This strategy not only improves the thermal insulation properties, but also offers a significant advancement in tailoring silica aerogels with specific porosity and mesopore volume for various applications.

Effect of caffeic acid esters on antioxidant activity and oxidative stability of sunflower oil: Molecular simulation and experiments.

Polyphenol, though used as antioxidants in food industry, suffers from poor solubility issues in vegetable oil. Usually, its solubility would be enhanced through esterification. This work investigated the antioxidant activity and oxidative stability of caffeic acid (CA) and its derivative modified esters by molecular simulation and experiments. Density functional theory (DFT) and molecular dynamic analysis revealed the antioxidant mechanism of CA esters attributing to the comprehensive effects. The lower hydrogen dissociation energy (ΔG) of CA esters with catechol moiety caused the transformation of antioxidant into quinone via the double hydrogen atom transfer reaction. Particularly, the second reduced hydrogen dissociation energy was the keypoint. The strong non-bond energy and hydrogen bond allowed CA esters and oil molecules to interact more efficiently. Hence, the ester moieties enhanced the antioxidant activity with 4.5-6.5 % ΔG reduction compared to CA. Rancimat and DSC assays validated the theoretical predictions. This result shows that the antioxidant activity of CA and its esters could be predicted by this molecular simulation way, which may aid in designing of new polyphenol antioxidant structure.

Effects of Nonhydroxyl Oxygen Heteroatoms in Diethylene Glycols on the Properties of 2,5-Furandicarboxylic Acid-Based Polyesters.

With regard to polyesters based on biobased 2,5-furandicarboxylic acid (FDCA), our work presents a new strategy, heteroatom substitution, to adjust the thermal and gas barrier properties. The effects of nonhydroxyl oxygen heteroatoms in the diols on the properties of FDCA-based polyesters were first investigated by a combination of an experiment and molecular simulation. The results demonstrated that the introduction of oxygen heteroatoms significantly influenced the thermal and gas barrier properties. As for the two model polymers with a very similar skeleton structure, poly(pentylene 2,5-furandicarboxylate) (PPeF) and poly(diethylene glycol 2,5-furandicarboxylate) (PDEF), their Tg exhibited an obviously increasing order. Moreover, they showed similar thermal stability and thermal oxidative stability. Dynamic mechanical analysis, positron annihilation lifetime spectroscopy, and molecular dynamics simulation indicated that the gas barrier properties followed the sequence of PDEF > PPeF mainly due to the decreased chain mobility and smaller fractional free volume. In-depth analysis of the effects of heteroatom substitution has an important directive significance for the design and preparation of new high glass transition temperature or novel excellent gas barrier materials. Through the manipulation of different heteroatoms in the diols, the polyesters with varied properties can be expected.

Insight into the anti-aging mechanisms of natural phenolic antioxidants in natural rubber composites using a screening strategy based on molecular simulation.

The failure of materials upon aging has led to the accumulation of waste and environmental pollution. Adding antioxidants (AOs) to the composites is one of the most effective ways to retard aging. However, traditional synthetic AOs are always detrimental to the environment and human health. The selection of antioxidants from streams by experiments will also definitely cost a lot of time and money. In addition, the complexity of thermo-oxidative aging factors along with the lack of quantitative tools significantly hampers its applications. So, building a screening strategy to quickly and easily find an appropriate and eco-friendly AO is imperative. In this study, we chose natural rubber (NR) as a matrix and provided a screening strategy based on diverse natural phenolic antioxidants to evaluate their ability in protecting NR composites. Thymol, α-tocopherol, and lipid-soluble epigallocatechin gallate (lsEGCG) were chosen from 18 natural phenolic antioxidants as potential alternative candidates. They were proved, indeed, to enhance the oxidative time in NR from experiments. Our results emphasized that thymol, α-tocopherol, and lsEGCG were promising alternatives for AOs in NR, and the in vitro toxicity test suggested that they are biocompatible. This study may develop a new strategy preference for screening the antioxidants by combining molecular simulation with the validation of experimental approaches, and therefore guide the AO molecular design with a more accurate theoretical prediction.