Ultraviolet (UV) light can trigger a plethora of useful photochemical reactions for diverse applications, including photocatalysis, photopolymerization, and drug delivery. These applications typically require penetration of high energy photons deep into materials, yet delivering these photons beyond the surface is extremely challenging due to absorption and scattering effects. Triplet-triplet annihilation upconversion (TTA-UC) shows great promise to circumvent this issue by generating high energy photons from incident lower energy photons. However, molecules that facilitate TTA-UC usually have poor water solubility, limiting their deployment in aqueous environments. To address this challenge, we leverage a nanoencapsulation method to fabricate water-compatible UC micelles, enabling on-demand UV photon generation deep into materials. We present two iridium-based complexes for use as TTA-UC sensitizers with increased solubilities that facilitate the formation of highly emissive UV-upconverting micelles. Furthermore, we show this encapsulation method is generalizable to nineteen UV-emitting UC systems, accessing a range of upconverted UV emission profiles with wavelengths as low as 350 nm. As a proof-of-principle demonstration of precision photochemistry at depth, we use UV-emitting UC micelles to photolyze a fluorophore at a focal point nearly a centimeter beyond the surface, revealing opportunities for spatially controlled manipulation deep into UV-responsive materials.

Shear banding in entangled polymer solutions is an elusive phenomenon in polymer rheology. One recently proposed mechanism for the existence of banded velocity profiles in entangled polymer solutions stems from a coupling of the flow to banded concentration profiles. Recent work [Burroughs et al., Phys. Rev. Lett. 126, 207801 (2021)] provided experimental evidence for the development of large gradients in concentration across the fluid. Here, a more systematic investigation is reported of the transient and steady-state banded velocity and concentration profiles of entangled polybutadiene in dioctyl phthalate solutions as a function of temperature (T), number of entanglements (Z), and applied shear rate (Wiapp), which control the susceptibility of the fluid to unstable flow-concentration coupling. The results are compared to a two-fluid model that accounts for coupling between elastic and osmotic polymer stresses, and a strong agreement is found between model predictions and measured concentration profiles. The interface locations and widths of the time-averaged, steady-state velocity profiles are quantified from high-order numerical derivatives of the data. At high levels of entanglement and large Wiapp, a significant wall slip is observed at both inner and outer surfaces of the flow geometry but is not a necessary criterion for a nonhomogeneous flow. Furthermore, the transient evolution of flow profiles for large Z indicate transitions from curved to “stair-stepped” and, ultimately, a banded steady state. These observed transitions provide detailed evidence for shear-induced demixing as a mechanism of shear banding in polymer solutions.

Embedding nanomaterials into polymer hydrogels enables the design of functional materials with tailored chemical, mechanical, and optical properties. Nanocapsules that protect interior cargo and disperse readily through a polymeric matrix have drawn particular interest for their ability to integrate chemically incompatible systems and to further expand the parameter space for polymer nanocomposite hydrogels. The properties of polymer nanocomposite hydrogels depend on the material composition and processing route, which were explored systematically in this work. The gelation kinetics of network-forming polymer solutions with and without silica-coated nanocapsules bearing polyethylene glycol (PEG) surface ligands were investigated using in situ dynamic rheology measurements. Network-forming polymers comprised either 4-arm or 8-arm star PEG with terminal anthracene groups, which dimerize upon irradiation with ultraviolet (UV) light. The PEG-anthracene solutions exhibited rapid gel formation upon UV exposure (365 nm); gel formation was observed as a crossover from liquid-like to solid-like behavior during in situ small-amplitude oscillatory shear rheology. This crossover time was non-monotonic with polymer concentration. Far below the overlap concentration (c/c* ≪ 1), spatially separated PEG-anthracene molecules were subject to forming intramolecular loops over intermolecular cross-links, thereby slowing the gelation process. Near the polymer overlap concentration (c/c* ∼ 1), rapid gelation was attributed to the ideal proximity of anthracene end groups from neighboring polymer molecules. Above the overlap concentration (c/c* > 1), increased solution viscosities hindered molecular diffusion, thereby reducing the frequency of dimerization reactions. Adding nanocapsules to PEG-anthracene solutions resulted in faster gelation than nanocapsule-free PEG-anthracene solutions with equivalent effective polymer concentrations. The final elastic modulus of nanocomposite hydrogels increased with nanocapsule volume fraction, signifying synergistic mechanical reinforcement by nanocapsules despite not being cross-linked into the polymer network. Overall, these findings quantify the impact of nanocapsule addition on the gelation kinetics and mechanical properties of polymer nanocomposite hydrogels, which are promising materials for applications in optoelectronics, biotechnology, and additive manufacturing.

Recent models have predicted entangled polymer solutions could shear band due to unstable flow-induced demixing. This work provides the first experimental probe of the in situ concentration profile of entangled polymer solutions under shear. At shear rates above a critical value, we show that the concentration and velocity profiles can develop bands, in quantitative agreement with steady-state model predictions. These findings highlight the critical importance of flow-concentration coupling in entangled polymer solutions.

The flow behavior of entangled polymer solutions in simple shear flow is often assumed to be uniform, both in terms of the velocity gradient and polymer concentration. However, there is growing evidence of nonuniform "banded" transient or steady state flows of entangled polymer solutions. The present work considers a distinct phenomenon, whereby transient banding is concomitant with the presence of local shear-enhanced concentration fluctuations. Experimental observations are made using combined rheological measurements with simultaneous particle tracking velocimetry and microscopy (rheomicroscopy) during startup shear in Taylor-Couette flow of entangled polymer solutions of polystyrene in a marginal solvent, dioctyl phthalate, at various entanglement numbers (Z). At high Z, the flow develops transient, nonhomogeneous "banded" velocity profiles over a wide range of imposed shear rates that are inverted from those expected for Taylor-Couette flow, which then relax to a nearly uniform shear rate at steady state. Rheomicroscopy reveals that these transient banded states are accompanied by strong shear-enhanced concentration fluctuations localized to the region of lowest local shear rate. We hypothesize that such finite-amplitude concentration fluctuations lead to increased dissipation that contributes a higher local "effective" fluid viscosity that, in turn, could produce the observed flow nonuniformity. In spite of the nonuniformity of the flow and the concentration fluctuations, the measured apparent rheology of the fluids is in qualitative agreement with both prior experimental studies and model predictions based on a Rolie-Poly model under the assumption that the fluid remains homogeneous, suggesting that this behavior is only observable through spatially resolved measurements of fluid flow and concentration.

Nanomaterials are regularly added to crosslinkable polymers to enhance mechanical properties; however, important effects related to gelation behavior and crosslinking kinetics are often overlooked. In this study, we combine cellulose nanocrystals (CNCs) with a photoactive poly(vinyl alcohol) derivative, PVA-SbQ, to form photocrosslinked nanocomposite hydrogels. We investigate the rheology of PVA-SbQ with and without CNCs to decipher the role of each component in final property development and identify a critical CNC concentration (1.5 wt%) above which several changes in rheological behavior are observed. Neat PVA-SbQ solutions exhibit Newtonian flow behavior across all concentrations, while CNC dispersions are shear-thinning <6 wt% and gel at high concentrations. Combining semi-dilute entangled PVA-SbQ (6 wt%) with >1.5 wt% CNCs forms a percolated microstructure. In situ photocrosslinking experiments reveal how CNCs affect both the gelation kinetics and storage modulus (G') of the resulting hydrogels. The modulus crossover time increases after addition of up to 1.5 wt% CNCs, while no modulus crossover is observed >1.5 wt% CNCs. A sharp increase in G' is observed >1.5 wt% CNCs for fully-crosslinked networks due to favorable PVA-SbQ/CNC interactions. A percolation model is fitted to the G' data to confirm that mechanical percolation is maintained after photocrosslinking. A ∼120% increase in G' for 2.5 wt% CNCs (relative to neat PVA-SbQ) confirms that CNCs provide a reinforcing effect through the percolated microstructure formed from PVA-SbQ/CNC interactions. The results are testament to the ability of CNCs to significantly alter the storage moduli of crosslinked polymer gels at low loading fractions through percolation-induced reinforcement.

Shear banding occurs in complex fluids that exhibit a non-monotonic constitutive instability, such as wormlike micelles, and potentially also in polymeric fluids with presumably monotonic constitutive behavior. However, velocity profiles for shear thinning fluids in geometries possessing a stress gradient, such as Taylor-Couette flow, could be misidentified as shear banding. To address this, we present a model-free experimental procedure to distinguish shear banding from strong shear thinning using high-resolution velocimetry. The approach is developed and validated using simulations using the d-Giesekus model and is based upon the behavior of the width of the apparent interface between the high and low shear rate regions. It is then tested using experimental data for model wormlike micellar solutions. The method allows shear banding to be distinguished from shear thinning in cases where this difference is otherwise indistinguishable. As a by-product, it also provides an estimate of the stress diffusivities for shear banding fluids.

Cyclodextrin complexation of chitosan presents a novel route to achieve nanofibers of chitosan and other difficult-to-electrospin biopolymers.

The Conversion of cooperatively assembled metal nitrate, citric acid, and an amphiphilic block copolymer, poly(methoxypoly[ethylene glycol methacrylate])-block-poly(butyl acrylate), films to their associated Carbonate is investigated using Fourier transform infrared spectroscopy (FTIR) and spectroscopic ellipsometry for both cobalt and copper. The processing conditions associated with the formation of the carbonate significantly impact the mesostructure generated. Ex situ FTIR measurements tracked the carbonate formation and consumption of citric acid to elucidate the kinetics of the reactions and were compared to the evolution in the film thickness and refractive index by in situ spectroscopic ellipsometry. From ellipsometry, the initial rate of thickness change appears to follow an Arrhenius temperature dependence with the apparent activation energy for Co (43 kJ/mol) approximately double that for Cu (23 kJ/mol). These data elucidating the reaction kinetics enable optimization of the temperature and reaction time for improved properties and decreased fabrication time. The temperature utilized to form the carbonate impacts the mesostructure that develops and the porosity in the resultant oxide film. The optimum temperature to maximize the porosity of the oxide films is an intermediate carbonate formation temperature where the rate of conversion is not too fast to disrupt the nanostructure, but the final conversion is sufficiently high to provide thermal resilience to the framework through calcination. This knowledge enables fabrication of ordered mesoporous oxides with porosities in excess of 60%.