From Gel to Crystal: Mechanism of HfO2 and ZrO2 Nanocrystal Synthesis in Benzyl Alcohol

Nonaqueous sol-gel syntheses have been used to make many types of metal oxide nanocrystals. According to the current paradigm, nonaqueous syntheses have slow kinetics, thus favoring the thermodynamic (crystalline) product. Here we investigate the synthesis of hafnium (and zirconium) oxide nanocrystals from the metal chloride in benzyl alcohol. We follow the transition from precursor to nanocrystal through a combination of rheology, EXAFS, NMR, TEM, and X-ray total scattering (PDF analysis). Upon dissolving the metal chloride precursor, the exchange of chloride ligands for benzylalkoxide liberates HCl. The latter catalyzes the etherification of benzyl alcohol, eliminating water. During the temperature ramp to the reaction temperature (220 °C), sufficient water is produced to turn the reaction mixture into a macroscopic gel. Rheological analysis shows a network consisting of strong interactions with temperature-dependent restructuring. After a few minutes at the reaction temperature, crystalline particles emerge from the gel, and nucleation and growth are complete after 30 min. In contrast, 4 h are required to obtain the highest isolated yield, which we attribute to the slow in situ formation of water (the extraction solvent). We used our mechanistic insights to optimize the synthesis, achieving high isolated yields with a reduced reaction time. Our results oppose the idea that nonaqueous sol-gel syntheses necessarily form crystalline products in one step, without a transient, amorphous gel state.

Morphological Study of Bio-Based Polymers in the Consolidation of Waterlogged Wooden Objects

The removal of water from archaeological wooden objects for display or storage is of great importance to their long-term conservation. Any mechanical instability caused during drying can induce warping or cracking of the wood cells, leading to irreparable damage of the object. Drying of an object is commonly carried out in one of three ways: (i) air-drying with controlled temperature and relative humidity, (ii) drying-out of a non-aqueous solvent or (iii) freeze-drying. Recently, there has been great interest in the replacement of the standard, but limited, polyethylene glycol with biopolymers for wood conservation; however, their behaviour and action within the wood is not completely understood. Three polysaccharides-low-molar-mass (Mw) chitosan (Mw ca. 60,000 g/mol), medium-molar-mass alginate (Mw ca. 100,000 g/mol) and cellulose nanocrystals (CNCs)-are investigated in relation to their drying behaviour. The method of drying reveals a significant difference in the morphology of these biopolymers both ex situ and within the wood cells. Here, the effect these differences in structuration have on the coating of the wood cells and the biological and thermal stability of the wood are examined, as well as the role of the environment in the formation of specific structures. The role these factors play in the selection of appropriate consolidants and drying methods for the conservation of waterlogged archaeological wooden objects is also investigated. The results show that both alginate and chitosan are promising wood consolidants from a structural perspective and both improve the thermal stability of the lignin component of archaeological wood. However, further modification would be necessary to improve the biocidal activity of alginate before it could be introduced into wooden objects. CNCs did not prove to be sufficiently suitable for wood conservation as a result of the analyses performed here.

Stress Dissipation in Cucurbit[8]uril Ternary Complex Small Molecule Adhesives

The ability to control supramolecular and macroscopic self-assembly and disassembly holds great potential for responsive, reversible adhesives that can efficiently broker stresses accumulated between two surfaces. Here, cucurbit[8]uril is used to directly adhere two functionalized mica substrates creating surface-surface interactions that are held together through photoreversible CB[8] heteroternary complexes. Comparison of single-molecule, bulk, and macroscopic adhesion behavior give insight into cooperativity and stress dissipation in dynamic adhesive systems.

Single-Molecule Force Spectroscopy Quantification of Adhesive Forces in Cucurbit[8]Uril Host-Guest Ternary Complexes

Cucurbit[8]uril (CB[8]) heteroternary complexes display certain characteristics making them well-suited for molecular level adhesives. In particular, careful choice of host-guest binding pairs enables specific, fully reversible adhesion. Understanding the effect of the environment is also critical when developing new molecular level adhesives. Here we explore the binding forces involved in the methyl viologen·CB[8]·naphthol heteroternary complex using single-molecule force spectroscopy (SMFS) under a variety of conditions. From SMFS, the interaction of a single ternary complex was found to be in the region of 140 pN. Additionally, a number of surface interactions could be readily differentiated using the SMFS technique allowing for a deeper understanding of the dynamic heteroternary CB[8] system on the single-molecule scale.

The Importance of Excess Poly(N-isopropylacrylamide) for the Aggregation of Poly(N-isopropylacrylamide)-Coated Gold Nanoparticles

Thermoresponsive materials are generating significant interest on account of the sharp and tunable temperature deswelling transition of the polymer chain. Such materials have shown promise in drug delivery devices, sensing systems, and self-assembly. Incorporation of nanoparticles (NPs), typically through covalent attachment of the polymer chains to the NP surface, can add additional functionality and tunability to such hybrid materials. The versatility of these thermoresponsive polymer/nanoparticle materials has been shown previously; however, significant and important differences exist in the published literature between virtually identical materials. Here we use poly(N-isopropylacrylamide) (PNIPAm)-AuNPs as a model system to understand the aggregation behavior of thermoresponsive polymer-coated nanoparticles in pure water, made by either grafting-to or grafting-from methods. We show that, contrary to popular belief, the aggregation of PNIPAm-coated AuNPs, and likely other such materials, relies on the size and concentration of unbound "free" PNIPAm in solution. It is this unbound polymer that also leads to an increase in solution turbidity, a characteristic that is typically used to prove nanoparticle aggregation. The size of PNIPAm used to coat the AuNPs, as well as the concentration of the resultant polymer-AuNP composites, is shown to have little effect on aggregation. Without free PNIPAm, contraction of the polymer corona in response to increasing temperature is observed, instead of nanoparticle aggregation, and is accompanied by no change in solution turbidity or color. We develop an alternative method for removing all traces of excess free polymer and develop an approach for analyzing the aggregation behavior of such materials, which truly allows for heat-triggered aggregation to be studied.