Chris Toprakcioglu

Photocontrol over Cucurbit[8]uril Complexes: Stoichiometry and Supramolecular Polymers

Herein we report the photocontrol of cucurbit[8]uril (CB[8])-mediated supramolecular polymerization of azobenzene-containing monomers. The CB[8] polymers were characterized both in solution and in the solid state. These host-guest complexes can be reversibly switched between highly thermostable photostationary states. Moreover, a remarkable stabilization of Z-azobenzene was achieved by CB[8] complexation, allowing for structural characterization in the solid state.

Activation Energies Control the Macroscopic Properties of Physically Cross-Linked Materials†

Here we show the preparation of a series of water-based physically cross-linked polymeric materials utilizing cucurbit[8]uril (CB[8]) ternary complexes displaying a range of binding, and therefore cross-linking, dynamics. We determined that the mechanical strength of these materials is correlated directly with a high energetic barrier for the dissociation of the CB[8] ternary complex cross-links, whereas facile and rapid self-healing requires a low energetic barrier to ternary complex association. The versatile CB[8] ternary complex has, therefore, proven to be a powerful asset for improving our understanding of challenging property-structure relationships in supramolecular systems and their associated influence on the bulk behavior of dynamically cross-linked materials.

Multifunctional supramolecular polymer networks as next-generation consolidants for archaeological wood conservation

Significance The preservation of cultural heritage is of widespread importance all over the world. Yet the lack of development in the field of conservation treatments means the fate of some of the most culturally important artifacts in the world remain in jeopardy. In the preservation of waterlogged wooden artifacts, conservators rely almost exclusively on poly(ethylene glycol) doped with a broad-spectrum biocide. The concept of a chemotactic consolidant, one that can adapt to the artifact it is treating, as described here has never before been described for an archaeological/conservation treatment. Additionally, the cross-links holding the consolidant together are entirely reversible, resulting in a material that is a greener, safer, and sustainable alternative to current conservation strategies. The preservation of our cultural heritage is of great importance to future generations. Despite this, significant problems have arisen with the conservation of waterlogged wooden artifacts. Three major issues facing conservators are structural instability on drying, biological degradation, and chemical degradation on account of Fe3+-catalyzed production of sulfuric and oxalic acid in the waterlogged timbers. Currently, no conservation treatment exists that effectively addresses all three issues simultaneously. A new conservation treatment is reported here based on a supramolecular polymer network constructed from natural polymers with dynamic cross-linking formed by a combination of both host-guest complexation and a strong siderophore pendant from a polymer backbone. Consequently, the proposed consolidant has the ability to chelate and trap iron while enhancing structural stability. The incorporation of antibacterial moieties through a dynamic covalent linkage into the network provides the material with improved biological resistance. Exploiting an environmentally compatible natural material with completely reversible chemistries is a safer, greener alternative to current strategies and may extend the lifetime of many culturally relevant waterlogged artifacts around the world.

Complexation of lysozyme with adsorbed PtBS-b-SCPI block polyelectrolyte micelles on silver surface.

We present a study of the interaction of the positively charged model protein lysozyme with the negatively charged amphiphilic diblock polyelectrolyte micelles of poly(tert-butylstyrene-b-sodium (sulfamate/carboxylate)isoprene) (PtBS-b-SCPI) on the silver/water interface. The adsorption kinetics are monitored by surface plasmon resonance, and the surface morphology is probed by atomic force microscopy. The micellar adsorption is described by stretched-exponential kinetics, and the micellar layer morphology shows that the micelles do not lose their integrity upon adsorption. The complexation of lysozyme with the adsorbed micellar layers depends on the micelles arrangement and density in the underlying layer, and lysozyme follows the local morphology of the underlying roughness. When the micellar adsorbed amount is small, the layers show low capacity in protein complexation and low resistance in loading. When the micellar adsorbed amount is high, the situation is reversed. The adsorbed layers both with or without added protein are found to be irreversibly adsorbed on the Ag surface.

Neutron reflectivity study of adsorbed diblock copolymers

SummaryIn this paper, we summarize our cumulative work on neutron reflectivity studies of polystyrene-poly(vinyl-2-pyridine) (PS-PVP) and polystyrene-polyethylene oxide (PS-PEO) adsorbed at a quartz-solvent interface. Deuterated toluene was chosen as the solvent since it is a good solvent for PS and a poor one for either of the other two blocks. In this case, the polystyrene dangles into the solvent while the other block acts as an anchor. The neutron reflectivity studies reveal that the form of the polymer density profile normal to the substrate may be varied from an extended «brush» to a condensed «mushroom» conformation by manipulating the ratio of the molecular weights of the two blocks. In addition, we present new data on the PS-PEO system in a poor solvent, deuterated cyclohexane, under conditions of shear flow in Poiseuille geometry. We find that when the PS-PEO diblock is absorbed from cyclohexane and is allowed to relax, the PS chain takes on a «mushroom» conformation. However, when the shear is applied, the layer shear thickens due to the PS chains extending to nearly twice their original lengths.

Shear effects on the geometry of polystyrene-polyethylene oxide copolymers at the solid-solvent interface

Abstract The geometry of polystyrene (PS)-polyethylene oxide (PEO) diblock copolymers at the quartz-solvent interface has been examined as a function of solvent quality and shear by neutron reflectometry. Under conditions where a brush geometry is observed for the static PS free block, extremely large shear rates (∼ 10 000/s) produce little or no effect on the structure, in agreement with the recent theoretical considerations. The geometry of the adsorbed polymer was also examined as a function of solvent quality. Under conditions where the polymer can phase separate, the effect of shear increases. Sparser PS coverage, which tends toward the mushroom geometry in good solvents, increases solvent penetration between the molecules. In poor solvents, the PS film may have lateral inhomogeneity and areas of little coverage. Again, this increased penetration of solvent is suggested to account for the increased sensitivity of the molecules to deformation under conditions of shear.

Reflectivity studies on adsorbed block copolymers under shear

The authors report neutron reflectivity data on (poly)styrene-(poly)ethylene oxide (PS-PEO) diblock copolymers adsorbed onto quartz from the selective solvent cyclohexane (a non-solvent for PEO and a poor solvent for PS). The PEO ``anchor block`` adsorbs strongly to form a thin layer on the quartz substrate, while the deuterated PS chains dangle into the solvent. They find that under static conditions the density profile of the PS block in a poor solvent can be well described by a Schultz function which is indicative of a polymer ``mushroom.`` Furthermore, they have studied the same system under shear at shear rates from 0--400s{sup {minus}1}. They find that there is a dramatic increase in the thickness of the PS layer under shear in cyclohexane and that the relaxation time from the shear-on profile back to the static profile is on the order of several days.

Metallic inclusions in a non-uniform lattice

This paper is part of a larger project which aims to investigate the fabricational and electromagnetic advantages of creating integrated antenna systems using emerging nano-manufacturing technology. It has been known for several decades that the effective permittivity of a mixture consisting of dielectric inclusions in a host dielectric can be controlled by varying the permittivity, size and spacing of the inclusions. Various authors have developed theoretical equations to analyse these structures but are typically limited to spherical inclusions in a uniform cubic lattice. This paper extends this work from spheres in a uniform mesh to investigate thin metallic inclusions in a dielectric host using a non-uniform lattice (spacing in x, y and z, not the same). Electromagnetic simulations of these structures have been compared to canonical equations of spheres in a cubic uniform lattice with the same volume ratio.