Polyvinylpyrrolidone Molecular Weight Controls Silica Shell Thickness on Au Nanoparticles with Diglycerylsilane as Precursor

Several strategies have been described for the preparation of silica-encapsulated gold nanoparticles (SiO₂–AuNP), which typically suffer from an initial interface between gold and silica that is difficult to control, and layer thicknesses that are very sensitive to minor changes in silane concentration and incubation time. The silica shell thicknesses are normally equal to or larger than the gold particles themselves, which is disadvantageous when the particles are to be used for biodiagnostic applications. We present a facile and reproducible method to produce very thin silica shells (3–5 nm) on gold nanoparticles: the process is highly tolerant to changes in reaction conditions. The method utilized polyvinylpyrrolidone (PVP) of specific molecular weights to form the interface between gold and silica. The method further requires a nontraditional silica precursor, diglycerylsilane, which efficiently undergoes sol–gel processing at neutrality. Under these conditions, higher molecular weight PVP leads to thicker silica shells: PVP acts as the locus for silica growth into an interpenetrating organic–inorganic hybrid structure

Thermoplastic Silicone Elastomers through Self-Association of Pendant Coumarin Groups

Although there are many benefits associated with thermoplastic elastomeric silicones, very few examples exist: silicone elastomers are normally thermoset materials. We have discovered that the simple incorporation of coumarin groups on linear silicone polymer backbones creates physical silicone polymeric networks that exhibit thermoplastic elastomeric properties in the absence of covalent cross-links. A range of materials was prepared by incorporating four different concentrations of coumarin along the silicone backbone using thermal azide/alkyne cycloaddition reactions: higher coumarin concentrations lead to more tightly cross-linked, higher modulus materials. Intermediate properties could be obtained by mixing silicones with different coumarin loadings in the melt. Physical cross-links arise from 1:1 coumarin complexes. As a consequence, it is possible to reduce cross-link density by adding silicones bearing a single coumarin to an elastomer. The physical interactions between coumarin-triazoles on the silicone polymers could be temporarily overcome thermally as shown by tensile, rheometry and thermal remolding experiments. The simple expedient of grafting coumarin groups, which cross-link reversibly through head-to-tail π-stacking, to silicone chains allows one to tailor the mechanical properties of these thermoplastic elastomers, enhancing their utility

Reductive Degradation of Lignin and Model Compounds by Hydrosilanes

The exploitation of lignin, the second most abundant naturally occurring polymer on earth, has been hampered by its network structure, which makes it difficult to process. Hydrosilanes have previously been shown to convert aryl ethers to hydrolyzable silyl ethers in the presence of B­(C₆F₅)₃. We demonstrate that the process is general and can be used to convert model lignin compounds to both aryl silyl ethers and alkanes. The relative reactivity of functional groups on model lignin compounds was found to be phenol > primary alcohol > methoxybenzene > alkyl silyl ethers. The process thus leads to cleavage of β-O-4, α-O-4, and methoxybenzene groups with concomitant silylation of phenolic and secondary alcohol groups. At longer time points provided sufficient silane was present, the full reduction of primary and secondary alcohols to alkyl groups was observed. Softwood lignin itself could only be partially solubilized (∼30%) even using excess hydrosilane and high catalyst loadings; the products were not characterized in detail. The lack of further degradation was attributed to its highly branched network structure containing 5-5, β-5, 4-O-5, and other linkages derived from coniferyl alcohol monomers that are not susceptible to reductive silylation. By contrast, over 95% of hardwood lignin was efficiently reduced/degraded into organosoluble products by the monofunctional hydrosilane HMe₂SiOSiMe₃ over a few hours at 50 °C. The molecular weight of the silylated products was consistent with oligomeric structures comprised of 3–8 linked aryl groups. This process holds promise to increase the accessibility to value-added products using lignin as a starting material

Spontaneous Emergence of Nonlinear Light Waves and Self-Inscribed Waveguide Microstructure during the Cationic Polymerization of Epoxides

We report spontaneous pattern formation due to modulation instability (MI) of a broad, uniform, incandescent beam as it propagates through a fluid medium undergoing cationic ring-opening polymerization of epoxide moieties and show that the dynamics of the process can be controlled through polymerization kinetics. By strong contrast, MI in the half century-old field of nonlinear light propagation has until now been described predominantly in terms of optical parameters such as coherence, intensity and wavelength. The increase in refractive index (Δ<i>n</i>) originating from the cross-linking polymerization of biscycloaliphatic epoxy monomers pushes the system into a nonlinear regime, where normally negligible spatial noise becomes greatly amplified. The perturbed optical field stabilizes by spontaneously dividing into thousands of self-trapped filaments of light. Because each filament inscribes a permanent microscopic channel along its propagation path, the initially isotropic fluid medium solidifies into a densely packed array of self-induced waveguides. These experiments demonstrated the strong correlation between the kinetics of cationic polymerization and dynamics of MI; the beam becomes unstable only within a narrow parameter range where a critical balance is struck between the photoresponse speed (determined by polymerization rate) and the magnitude of Δ<i>n</i> (determined by extent of cross-linking). The former needs to be sufficiently fast to respond to noise while the latter must be large enough to generate high-refractive index seeds that trigger MI. Outside of this range, MI is entirely suppressed. This study reveals that nonlinear waveforms emerge in a familiar, widely employed epoxide photopolymer system, which for the first time highlights the possibility of tuning MI through the kinetics of a photochemical reaction

Silica Shell/Gold Core Nanoparticles: Correlating Shell Thickness with the Plasmonic Red Shift upon Aggregation

Differences in the wavelengths of the surface plasmon band of gold nanoparticles (AuNP) – before and after particle aggregation – are widely used in bioanalytical assays. However, the gold surfaces in such bioassays can suffer from exchange and desorption of noncovalently bound ligands and from nonspecific adsorption of biomolecules. Silica shells on the surfaces of the gold can extend the available surface chemistries for bioconjugation and potentially avoid these issues. Therefore, silica was grown on gold surfaces using either hydrolysis/condensation of tetraethyl orthosilicate <b>1</b> under basic conditions or diglyceroxysilane <b>2</b> at neutral pH. The former precursor permitted slow, controlled growth of shells from about 1.7 to 4.3 nm thickness. By contrast, 3–4 nm thick silica shells formed within an hour using diglyceroxysilane; thinner or thicker shells were not readily available. Within the range of shell thicknesses synthesized, the presence of a silica shell on the gold nanoparticle did not significantly affect the absorbance maximum (∼ 5 nm) of unaggregated particles. However, the change in absorbance wavelength upon aggregation of the particles was highly dependent on the thickness of the shell. With silica shells coating the AuNP, there was a significant decrease in the absorbance maximum of the aggregated particles, from ∼578 to ∼536 nm, as the shell thicknesses increased from ∼1.7 to ∼4.3 nm, because of increased distance between adjacent gold cores. These studies provide guidance for the development of colorimetric assays using silica-coated AuNP

Silicone Microemulsion Structures Are Maintained During Polymerization with Reactive Surfactants

Bicontinuous microemulsions exhibit domain structures on the nanoscale (<20 nm). Normally, such fine details are lost during the conversion from a fluid microemulsion to solid elastomeric materials, as a consequence of interfacial destabilization via polymerization of either the oil phase or monomers in the aqueous phase. Very little is known about the polymerization of silicone microemulsions and the morphological changes that occur upon transition from a nanostructured liquid to a solid matrix. Silicone microemulsions polymerized by free radical (aqueous phase) and condensation (silicone phase) processes, respectively, were characterized by small-angle X-ray scattering and transmission electron microscopy. It was found that cross-linking of the silicone phase alone led, over time, to large increase of the size of the microemulsion nanodomains. By contrast, photoinduced polymerization of a reactive surfactant and acrylic monomers in the aqueous phase was effective at retaining bicontinuous nanomorphology, irrespective of the degree of cross-linking of the silicone phase

Versatile Surface Modification of Cellulose Fibers and Cellulose Nanocrystals through Modular Triazinyl Chemistry

The ability to tune the interfacial and functional properties of cellulose nanomaterials has been identified as a critical step for the full utilization of nanocellulose in the development of new materials. Here, we use triazine chemistry in a modular approach to install various functionalities and chemistries onto cellulose fibers and cellulose nanocrystals (CNCs). The surface modification is demonstrated in aqueous and organic media. Octadecyl, monoallyl-PEG, benzyl, and propargyl triazinyl derivatives were grafted onto cellulose/CNCs via aromatic nucleophilic substitution in the presence of base as hydrochloric acid scavenger. The covalent nature and degree of substitution of grafted aliphatic, polymeric, alkyne chains, and aromatic rings were characterized through Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, elemental analysis, and thermogravimetric analysis. In addition, AFM and DLS analysis showed minimal change in the geometry and individualized character of CNCs after surface modification. X-ray diffraction analysis confirmed that the modification happened only at the CNC surface, while the bulk crystalline core remained unmodified. Modified cellulose/CNCs showed hydrophilic or hydrophobic properties depending on the grafted functionality, which resulted in stable colloidal suspensions of CNCs in polar and nonpolar organic solvents. Furthermore, the reactive nature of propargyl-modified cellulose was demonstrated by the successful grafting of an azido-fluorescein dye via copper-catalyzed Huisgen 1,3-dipolar cycloaddition. The triazinyl chemistry thus presents a versatile route for tuning the interfacial properties of nanocellulose, with the possibility of postmodification for applications that require the conjugation of molecules onto cellulose through bio-orthogonal chemistries

Phototunable Cross-Linked Polysiloxanes

Silicone elastomers are normally thermoset materials. While their inherent properties make them highly valuable, it would be of interest to develop stimuli-responsive silicones whose properties could be reversibly tuned at will. In the case of silicone polymers, a particularly interesting trigger is light, since silicone elastomers can readily be formulated to be transparent. We describe the utilization of coumarin-modified silicones for this purpose. On their own, the presence of coumarin groups converts silicone oils into thermoplastic elastomers through physical (noncovalent) cross-linking. UV-irradiation permits covalent cross-linking through [2 + 2] cycloadditions and is accompanied by loss of most physical cross-links. Higher energy photons permit, in part, photoinitiated retro-cycloaddition and a subsequent decrease in covalent cross-link density. It is thus possible to tailor the physical properties of the elastomer to increase and/or decrease the modulus of the elastomer using light and to convert thermoreversible thermoplastics, by degree, into thermosets

Surface Behavior of Boronic Acid-Terminated Silicones

Silicone polymers, with their high flexibility, lie in a monolayer at the air–water interface as they are compressed until a critical pressure is reached, at which point multilayers are formed. Surface pressure measurements demonstrate that, in contrast, silicones that are end-modified with polar groups take up lower surface areas under compression because the polar groups submerge into the water phase. Boronic acids have the ability to undergo coordination with Lewis bases. As part of a program to examine the surface properties of boronic acids, we have prepared boronic acid-modified silicones (SiBAs) and examined them at the air–water interface to better understand if they behave like other end-functional silicones. Monolayers of silicones, aminopropylsilicones, and SiBAs were characterized at the air–water interface as a function of end functionalization and silicone chain length. Brewster angle and atomic force microscopies confirm domain formation and similar film morphologies for both functionalized and non-functionalized silicone chains. There is a critical surface pressure (10 mN m^–1) independent of chain length that corresponds to a first-order phase transition. Below this transition, the film appears to be a homogeneous monolayer, whose thickness is independent of the chain length. Ellipsometry at the air–water interface indicates that the boronic acid functionality leads to a significant increase of film thickness at low molecular areas that is not seen for non-functionalized silicone chains. What differentiates the boronic acids from simple silicones or other end-functionalized silicones, in particular, is the larger area occupied by the headgroup when under compression compared to other or non-end-functionalized silicones, which suggests an in-plane rather than submerged orientation that may be driven by boronic acid self-complexation