Enhanced Ion Conductivity through Hydrated, Polyelectrolyte-Grafted Cellulose Nanocrystal Films

Grafting of nonpolar polyisoprene as midblock and polar polylactide as outer block onto cellulose to produce bio-based thermoplastic elastomers

Sustainable cellulose-graft-diblock copolymers (cellulose-graft-polyisoprene-block-polylactide, MCC-g-PI-b-PLLA, MCC was chemically-modified microcrystalline cellulose) were designed and delicately synthesized for the first time by a mild and facile procedure via combining ring-opening polymerization and reversible addition-fragmentation chain transfer polymerization techniques, which conquered the difficulties in synthesis due to the distinct chemical properties among the MCC backbone, PI and PLLA blocks. Two separate glass transition temperatures were detected for MCC-g-PI-b-PLLA copolymers by differential scanning calorimetry, indicative of microphase separation, consistent with small-angle X-ray scattering and atomic force microscopy measurements. The α-form crystals of PLLA outer blocks were revealed by wide-angle X-ray diffraction measurement. The monotonic and step-cyclic tensile tests revealed that the mechanical property could be tuned by changing the molecular mass of PI midblock. The two ends of the soft PI midblock were anchored by rigid cellulose backbone and hard PLLA outer block, respectively, which served as physical crosslinking points. The rigid cellulose backbones brought the PI rubbery domains and PLLA hard domains together to form a whole hierarchical microstructure, endowing the copolymers with distinguished mechanical property. This work provided a feasible approach to producing environmental-friendly cellulose-based copolymers with fully biomass-derived feedstocks, having a profound impact on the development of sustainable polymer-based materials.

Anisotropic membrane with high proton conductivity sustaining upon dehydration

In fuel cells and electrolyzers, suboptimal proton conductivity and its dramatic drop at low humidity remain major drawbacks in proton exchange membranes (PEMs), including current benchmark Nafion. Sustained through-plane (TP) alignment of nanochannels was proposed as a remedy but proved challenging. We report an anisotropic composite PEM, mimicking the water-conductive composite structure of bamboo that meets this challenge. Micro- and nanoscale alignment of conductive pathways is achieved by in-plane thermal compression of a mat composed of co-electrospun Nafion and poly(vinylidene fluoride) (PVDF) nanofibers stabilizing the alignment. This translates to pronounced TP-enhanced proton conductivity, twice that of pure Nafion at high humidity, 13 times larger at low humidity, and 10 times larger water diffusivity. This remarkable improvement is elucidated by molecular dynamics simulations, which indicate that stronger nanochannels alignment upon dehydration compensates for reduced water content. The presented approach paves the way to overcoming the major drawbacks of ionomers and advancing the development of next-generation membranes for energy applications.

Polyelectrolyte Cylindrical Brushes in Hairy Gels

We considered dispersions of cylindrical polyelectrolyte (PE) brushes with stiff backbones, and polymer-decorated nanorods with tunable solubility of the brush-forming PE chains that affected thermodynamic stability of the dispersions. We focused on thermo-induced and deionization-induced conformational transition that provokes loss of aggregative dispersion stability of nanorods decorated with weakly ionized polyions. A comparison between theoretical predictions and experiments enabled rationalization and semi-quantitative interpretation of the experimental results.

Polymer-Grafted Nanoparticles with Variable Grafting Densities for High Energy Density Polymeric Nanocomposite Dielectric Capacitors

Designing high energy density dielectric capacitors for advanced energy storage systems needs nanocomposite-based dielectric materials, which can utilize the properties of both inorganic and polymeric materials. Polymer-grafted nanoparticle (PGNP)-based nanocomposites alleviate the problems of poor nanocomposite properties by providing synergistic control over nanoparticle and polymer properties. Here, we synthesize "core-shell" barium titanate-poly(methyl methacrylate) (BaTiO₃-PMMA) grafted PGNPs using surface-initiated atom transfer polymerization (SI-ATRP) with variable grafting densities of (0.303 to 0.929) chains/nm² and high molecular masses (97700 g/mL to 130000 g/mol) and observe that low grafted density and high molecular mass based PGNP show high permittivity, high dielectric strength, and hence higher energy densities (≈ 5.2 J/cm³) as compared to the higher grafted density PGNPs, presumably due to their "star-polymer"-like conformations with higher chain-end densities that are known to enhance breakdown. Nonetheless, these energy densities are an order of magnitude higher than their nanocomposite blend counterparts. We expect that these PGNPs can be readily used as commercial dielectric capacitors, and these findings can serve as guiding principles for developing tunable high energy density energy storage devices using PGNP systems.

Ordered Nanostructures in Thin Films of Precise Ion-Containing Multiblock Copolymers

We demonstrate that ionic functionality in a multiblock architecture produces highly ordered and sub-3 nm nanostructures in thin films, including bicontinuous double gyroids. At 40 °C, precise ion-containing multiblock copolymers of poly(ethylene-b-lithium sulfosuccinate ester) n (PESxLi, x = 12 or 18) exhibit layered ionic assemblies parallel to the substrate. These ionic layers are separated by crystalline polyethylene blocks with the polymer backbones perpendicular to the substrate. Notably, above the melting temperature (T m) of the polyethylene blocks, layered PES18Li thin films transform into a highly oriented double-gyroid morphology with the (211) plane (d 211 = 2.5 nm) aligned parallel to the substrate. The cubic lattice parameter (a gyr) of the double gyroid is 6.1 nm. Upon heating further above T m, the double-gyroid morphology in PES18Li transitions into hexagonally packed cylinders with cylinders parallel to the substrate. These layered, double-gyroid, and cylinder nanostructures form epitaxially and spontaneously without secondary treatment, such as interfacial layers and solvent vapor annealing. When the film thickness is less than ∼3a gyr, double gyroids and cylinders coexist due to the increased confinement. For PES12Li above T m, the layered ionic assemblies simply transform into disordered morphology. Given the chemical tunability of ion-functionalized multiblock copolymers, this study reveals a versatile pathway to fabricating ordered nanostructures in thin films.

Nanocomposites Assembled via Electrostatic Interactions between Cellulose Nanocrystals and a Cationic Polymer

On account of their high strength and stiffness and their renewable nature, cellulose nanocrystals (CNCs) are widely used as a reinforcing component in polymer nanocomposites. However, CNCs are prone to aggregation and this limits the attainable reinforcement. Here, we show that nanocomposites with a very high CNC content can be prepared by combining the cationic polymer poly[(2-(methacryloyloxy)ethyl) trimethylammonium chloride] (PMETAC) and negatively charged, carboxylated CNCs that are provided as a sodium salt (CNC-COONa). Free-standing films of the composites can be prepared by simple solvent casting from water. The appearance and polarized optical microscopy and electron microscopy images of these films suggest that CNC aggregation is absent, and this is supported by the very pronounced reinforcement observed. The incorporation of 33 wt % CNC-COONa into PMETAC allowed increasing the storage modulus of this already rather stiff, glassy amorphous matrix polymer from 1.5 ± 0.3 to 6.6 ± 0.1 GPa, while the maximum strength increased from 11 to 32 MPa. At this high CNC content, the reinforcement achieved in the PMETAC/CNC-COONa nanocomposite is much more pronounced than that observed for a reference nanocomposite made with unmodified CNCs (CNC-OH).