Pyrolytic Depolymerization of Polyolefins Catalyzed by Zirconium-based UiO-66 Metal-Organic Frameworks.

Polyolefins such as polyethylenes and polypropylenes are the most-produced plastic waste globally, yet are difficult to convert into useful products due to their unreactivity. Pyrolysis is a practical method for large-scale treatment of mixed, contaminated plastic, allowing for their conversion into industrially-relevant petrochemicals. Metal-organic frameworks (MOFs), despite their tremendous utility in heterogenous catalysis, have been overlooked for polyolefin depolymerization due to their perceived thermal instabilities and inability of polyethylenes and polypropylenes to penetrate their pores. Herein, we demonstrate the viability of UiO-66 MOFs containing coordinatively-unsaturated zirconia nodes, as effective catalysts for pyrolysis that significantly enhances the yields of valuable liquid and gas hydrocarbons, whilst halving the amounts of residual solids produced. Reactions occur on the Lewis-acidic UiO-66 zirconia nodes, without the need for noble metals, and yields aliphatic product distributions distinctly different from the aromatic-rich hydrocarbons from zeolite catalysis. We also demonstrate the first unambiguous characterization of polyolefin penetration into UiO-66 pores at pyrolytic temperatures, allowing access to the abundant Zr-oxo nodes within the MOF interior for efficient C-C cleavage. Our work highlights the potential of MOFs as highly-designable heterogeneous catalysts for depolymerization of plastics which can complement conventional catalysts in reactivity.

Polyurea-urethane Temperature-responsive Hydrogels for Sustained Delivery of Anti-VEGF Therapeutics.

Temperature-responsive hydrogels, or thermogels, have emerged as a leading platform for sustained delivery of both small molecule drugs and macromolecular biologic therapeutics. Although thermogel properties can be modulated by varying the polymer's hydrophilic-hydrophobic balance, molecular weight and degree of branching, varying the supramolecular donor-acceptor interactions on the polymer remains surprisingly overlooked. Herein, to study the influence of enhanced hydrogen bonding on thermogelation, we synthesized a family of amphiphilic polymers containing urea and urethane linkages using quinuclidine as an organocatalyst. Our findings showed that the presence of strongly hydrogen bonding urea linkages significantly enhanced polymer hydration in water, in turn affecting hierarchical polymer self-assembly and macroscopic gel properties such as sol-gel phase transition temperature and gel stiffness. Additionally, analysis of the sustained release profiles of Aflibercept, an FDA-approved protein biologic for anti-angiogenic treatment, showed that urea bonds on the thermogel were able to significantly alter the drug release mechanism and kinetics compared to usage of polyurethane gels of similar composition and molecular weight. Our findings demonstrate the unrealized possibility of modulating gel properties and outcomes of sustained drug delivery through judicious variation of hydrogen bonding motifs on the polymer structure.

Enhancing the photocatalytic upcycling of polystyrene to benzoic acid: a combined computational-experimental approach for acridinium catalyst design.

Converting polystyrene into value-added oxygenated aromatic compounds is an attractive end-of-life upcycling strategy. However, identification of appropriate catalysts often involves laborious and time-consuming empirical screening. Herein, after demonstrating the feasibility of using acridinium salts for upcycling polystyrene into benzoic acid by photoredox catalysis for the first time, we applied low-cost descriptor-based combinatorial in silico screening to predict the photocatalytic performance of a family of potential candidates. Through this approach, we identified a non-intuitive fluorinated acridinium catalyst that outperforms other candidates for converting polystyrene to benzoic acid in useful yields at low catalyst loadings (≤5 mol%). In addition, this catalyst also proved effective with real-life polystyrene waste containing dyes and additives. Our study underscores the potential of computer-aided catalyst design for valorizing polymeric waste into essential chemical feedstock for a more sustainable future.

A Polyanionic Tartrate-containing Temperature-responsive Hydrogel.

Thermogels, a class of hydrogels which show spontaneous sol-gel phase transition when warmed, are an important class of soft biomaterials. To date, however, most amphiphilic polymers that are able to form thermogels in aqueous solution are uncharged, and the influence of ionisable groups on thermogelation are largely unknown. Herein, we report the first example of a polyanionic amphiphilic multi-block copolymer, containing multiple pendant carboxylate groups, that can form transparent thermogels spontaneously when warmed up to physiological temperature. We demonstrate that introducing negative charges onto thermogelling polymers could significantly alter the properties of the micelles and thermogels formed. Furthermore, the polymer's polyanionic character provides new options for modulating the gel rheological properties, such as stiffness and gelation temperatures, through electrostatic interactions with different cations. We also demonstrated that the polyanionic thermogel allowed slower sustained release of a cationic model drug compound compared to an anionic one over 2 weeks. The findings from our study demonstrate exciting new possibilities for advanced biomedical applications using charged polyelectrolyte thermogel materials.

Optical properties of ultrafine line and space polymeric nanogratings coated with metal and metal-dielectric-metal thin films.

Noble metal and metal-dielectric-metal ultrathin films were deposited on the surfaces of ultrafine polymeric nanogratings, which were fabricated using nanoimprint lithography. Experimental results showed dramatic differences of the surface morphologies for single metal and triple metal-dielectric-metal films deposited on flat and corrugated polymeric surfaces. The effect of the surface morphology on the optical properties was hence investigated and analyzed under linearly polarized light. The surface plasmon resonances of single metal and triple metal-dielectric-metal films deposited on polymeric nanograting surfaces were also characterized based on the Kretschmann prism-coupling method. The single metal and triple metal-dielectric-metal films deposited on polymeric nanograting surfaces are important for the study of photon-plasmon interactions (i.e. couplings and conversions) at the interfaces between a nanograting and metal films.

Reversible recovery of nanoimprinted polymer structures.

A shape memory polymer, Nafion, has its shape memory simultaneously programmed and patterned with micro- and nanometer-scale surface textures using a nanoimprint process. Highly ordered and well-defined micro- and nanometer surface textures, for example, high aspect ratio (~5) micropillars, form the permanent shape memory of the Nafion films. When damaged, these permanently shaped micro- and nanostructures possess repair ability through a heat treatment. Reversible recoveries of the damages caused by mechanical and irradiation exposure have been demonstrated. The recovery retains above 90% of the structural fidelity, which is comparable to the shape recovery in bulk film.

New approach for multilayered microstructures fabrication based on a water-soluble backing substrate.

We demonstrate a new approach for woodpile microstructure fabrication. The method involves the use of polyvinyl alcohol (PVA) as a sacrificial substrate for the transfer of SU-8 films to prepatterned structures. The surface activated PVA substrate allows good wettability of SU-8 film and its solubility in water eliminates the need of delaminating SU-8 structures from the substrate. This makes the fabrication process much simplified and we successfully demonstrate eight-layer stacking of gratings. Fourier transform infrared spectra of single-layer and four-layer grating structures show a broader transmission dip spectrum compared to their film counterparts, indicating their potential use as broadband terahertz (THz) absorbers.

Nanoimprinted ultrafine line and space nanogratings for liquid crystal alignment.

Ultrafine 50 nm line and space nanogratings were fabricated using nanoimprint lithography, and were further used as an alignment layer for liquid crystals. The surface morphologies of the nanogratings were characterized and their surface energies were estimated through the measurement of the contact angles for two different liquids. Experimental results show that the surface energies of the nanogratings are anisotropic: the surface free energy towards the direction parallel to the grating lines is higher than that in the direction perpendicular to the grating lines. Electro-optical characteristics were tested from a twisted nematic liquid crystal cell, which was assembled using two identical nanogratings. Experimental results show that such a kind of nanograting is promising as an alternative to the conventional rubbing process for liquid crystal alignment.

The effect of micro and nanotopography on endocytosis in drug and gene delivery systems.

Endocytosis is a fundamental biological process and is also the key mechanism for drug and non-viral gene delivery. The importance of topographical cues in modulating cell behaviors has become increasingly evident, but the influence of topography on endocytosis has however only been sparsely studied. We hypothesize that topography can enhance cellular endocytosis, and in turn the non-viral transfection efficiency. Nano- to microtopographical patterns were fabricated using nano-imprinting lithography (NIL). We first investigated if the substrate topographies could modulate endocytosis and in turn the cellular transfectability. Our results showed increased internalization of fluorescently labeled dextran by human mesenchymal stem cells (hMSCs) and monkey kidney fibroblasts (COS7) when they were cultured on micro- and nanopillars. When the hMSCs were introduced to green-fluorescent protein (GFP) encoding plasmid with Lipofectamine, highest transfection efficiency was observed in cells on nanopillars. Tunable detachable substrate topographies were also fabricated using NIL to promote endocytosis in different cell types, and our results show hMSCs phagocytosis of these polymeric structures. Besides being important in understanding the fundamental process of endocytosis, the current research results may also lead to applications utilizing nanotopography to enhance drug and gene delivery.

Direct imprinting of high resolution TiO(2) nanostructures.

We demonstrate a different approach to direct nanoimprint lithography of oxides, in particular TiO(2), using the metal methacrylate route which not only gives very high resolution ( approximately 20 nm) but also provides yields of approximately 100% over areas > 1 cm x 1 cm. TiO(2) was imprinted using a polymerizable liquid 'TiO(2) resin' consisting of a mixture of titanium methacrylate, ethylene glycol dimethacrylate, and azobis-(isobutyronitrile). The resin underwent free radical polymerization when imprinted using a silicon mold at 110 degrees C with pressures as low as 10 bar. Polymerization strengthens the imprinted structures, thereby giving approximately 100% yield after demolding. Heat-treatment of the imprinted structures at 400 degrees C resulted in the loss of organics and their subsequent shrinkage ( approximately 75%) without the loss of integrity or aspect ratio, and converted them to TiO(2) nanostructures as small as approximately 20 nm wide. Furthermore, our method demonstrates that large imprinted areas of sub-100-nm features can be achieved by sub-micron molds which translate into huge cost savings with the added flexibility of direct patterning of urinary as well as multi-component oxides.

Wafer-scale near-perfect ordered porous alumina on substrates by step and flash imprint lithography.

Nanoporous anodic aluminum oxide (AAO) has been widely used for the development of various functional nanostructures. So far, highly ordered AAO on substrates could only be prepared using a nanoindentation method via hard stamping and lithographic techniques that are not scalable to a wafer-scale. Here we report on a step and flash imprint lithography (SFIL)-based method to fabricate a near-perfect ordered AAO with square and hexagonal lattice configuration on silicon substrate over 4 in. wafer areas. SFIL was used to prepattern a polymer mask layer, and wet-etching process was employed to transfer the nanopatterns to aluminum (Al) films, thus creating ordered nanoindentation on the Al surface. The ordered nanoindentation guides the growth of nanochannels in the anodization step to create the ordered nanoporous structures. The proposed wafer-scale process is compatible with standard semiconductor fabrication and offers substantial advantages over conventional Al patterning methods in terms of patterning areas, throughput, process simplicity, and process robustness, allowing up to 10 000 imprints or pattern transfer to the Al films.