Spatiotemporal Mapping of Hole Nucleation and Growth during Block Copolymer Terracing with High-Speed Atomic Force Microscopy.

As a platform for investigating two-dimensional phase separation, we track the structural evolution of block copolymer thin films during thermal annealing with environmentally controlled atomic force microscopy (AFM). Upon thermal annealing, block copolymer films with incommensurate thickness separate into a terraced morphology decorated with holes. With in situ imaging at 200 °C, we follow the continuous progression of terrace formation in a single region of a cylinder-forming poly(styrene-block-methyl methacrylate) thin film, beginning with the disordered morphology on an unpatterned silicon substrate and continuing through nucleation and coarsening stages. Topographic AFM imaging with nanoscale resolution simultaneously captures ensemble hole growth statistics while locally tracking polymer diffusion through measurements of the film thickness. At early times, we observe homogeneous hole nucleation and isotropic growth, with kinetics following the predictions of classical nucleation theory. At later times, however, we find anomalous hole growth which arises due to the combination of Ostwald ripening and coalescence mechanisms. In each case, our real-space observations highlight the importance of hole interactions for determining coarsening kinetics, mediated either through the interconnected phase for Ostwald ripening or through binary collision events for coalescence.

Controlling the Morphology of Dynamic Thia-Michael Networks to Target Pressure-Sensitive and Hot Melt Adhesives.

A series of multistage (pressure-sensitive/hot melt) adhesives utilizing dynamic thia-Michael bonding motifs are reported. The benzalcyanoacetate Michael acceptors used in this work undergo bond exchange under ambient conditions without external catalysis, facilitating pressure-sensitive adhesion. A key feature of this system is the dynamic reaction-induced phase separation that lends reinforcement to the otherwise weakly bonded materials, enabling weak, repeatable pressure-sensitive adhesion under ambient conditions and strong adhesion when processed as a hot melt adhesive. By using different pairs of benzalcyanoacetate cross-linking units, the phase separation characteristics of the adhesives can be directly manipulated, allowing for a tailored adhesive response.

Reaction of NO2 with Groups IV and VI Transition Metal Oxide Clusters.

The addition of NO2 to Group IV (MO2)n and Group VI (MO3)n (n = 1-3) nanoclusters was studied using both density functional theory (DFT) and coupled cluster theory (CCSD(T)). The structures and overall binding energetics were predicted for Lewis acid-base addition without transfer of spin (a physisorption-type process) and the formation of either cluster-ONO (HONO-like or bidentate bonding) or NO3- formation where for both the spin is transferred to the metal oxide clusters (a chemisorption-type process). Only chemisorption of NO2 is predicted to be thermodynamically allowed at temperatures ≥298 K for Group IV (MO2)n clusters with the formation of surface chemisorbed NO2 being by far the most energetically favorable. The ligand binding energies (LBEs) for physisorption and chemisorption on the TiO2 nanoclusters are consistent with computational studies of the bulk solids. Chemisorption is only predicted to occur for (CrO3)n clusters in the form of a terminal nitrate containing species whereas the larger chemisorbed nitrate structures for (MoO3)n and (WO3)n were found to be metastable and unlikely to form in any appreciable amount at temperatures of 298 K and higher. NO2 is predicted to only be capable of physisorbing to (MoO3)n and (WO3)n at lower temperatures and therefore unlikely to bind NO2 at temperatures ≥298 K. Correlations between the (MO3)nNO2 ligand bond energies and the chemical properties of the parent (MO3)n clusters (Lewis acidity, ionization potentials, excitation energies, and M = O/M-O bond strengths) are described.

Direct Imaging of Interfacial Fluctuations in Confined Block Copolymer with in Situ Slow-Scan-Disabled Atomic Force Microscopy.

Using environmentally controlled, high-speed atomic force microscopy (AFM), we examine dynamic fluctuations of topographically confined poly(styrene-block-methyl methacrylate) (PS-b-PMMA) cylinders. During thermal annealing, fluctuations drive perturbations of the block copolymer (BCP) interface between polymer domains, leading to pattern roughness. Whereas previous investigations have examined roughness in room-temperature and kinetically quenched samples, we directly visualize the dynamics of PS/PMMA interfaces in real space and time at in situ temperatures above the glass transition temperature, Tg. Imaging under these experimentally challenging thermal annealing conditions is critical to understanding the inherent connection between thermal fluctuations and BCP pattern assembly. Through the use of slow-scan-disabled AFM, we dramatically improve the imaging time resolution for tracking polymer dynamics. Fluctuations increase in intensity with temperature and, at high temperatures, become spatially coherent across their confining potential. Additionally, we observe that topographic confinement suppresses fluctuations and correlations in the proximity of the guiding field. In situ imaging at annealing temperatures represents a significant step in capturing the dynamics of chain mobility at BCP interfaces.

Reaction of CO2 with UO3 Nanoclusters.

Adsorption of CO2 to uranium oxide, (UO3)n, clusters was modeled using density functional theory (DFT) and coupled cluster theory (CCSD(T)). Geometries and reaction energies were predicted for carbonate formation (chemisorption) and Lewis acid-base addition of CO2 (physisorption) to these (UO3)n clusters. Chemisorption of multiple CO2 moieties was also modeled for dimer and trimer clusters. Physisorption and chemisorption were both predicted to be thermodynamically allowed for (UO3)n clusters, with chemisorption being more thermodynamically favorable than physisorption. The most energetically favored (UO3)3(CO2)m clusters contain tridentate carbonates, which is consistent with solid-state and solution structures for uranyl carbonates. The calculations show that CO2 exposure is likely to convert (UO3)n to uranyl carbonates.

Reaction of CO2 with Groups 4 and 6 Transition Metal Oxide Clusters.

Density functional theory (DFT) and coupled cluster theory (CCSD(T)) were used to study the addition of CO2 to group 4 (MO2)n and group 6 (MO3)n (n = 1, 2, 3) nanoclusters. The structures and energetics arising from Lewis acid-base addition (physisorption) and formation of CO32- (chemisorption) of CO2 to these clusters were predicted. Physisorption and chemisorption of CO2 are predicted to be thermodynamically allowed for group 4 (MO2)n clusters, with chemisorption being more favored energetically. Correlations of the ligand binding energies (LBEs) for the group 4 clusters are made with the fluoride affinities and M-O and M═O bond strengths of the clusters. The LBEs for chemisorption on the Zr and Ti clusters are consistent with published experimental and computational studies of bulk solids. Physisorption LBEs for the Ti and Zr clusters are more exothermic than the bulk values, as the cluster models allow for better relaxation at the metal sites. Chemisorption is not predicted to occur with group 6 (MO3)n clusters, as the larger chemisorbed structures were all found to be metastable. CO2 is predicted to weakly physisorb to (WO3)n with physisorption correlating with the Lewis acidity of the metal site.