Liquid-State NMR Analysis of Nanocelluloses.

Recent developments in ionic liquid electrolytes for cellulose or biomass dissolution has also allowed for high-resolution 1H and 13C NMR on very high molecular weight cellulose. This permits the development of advanced liquid-state quantitative NMR methods for characterization of unsubstituted and low degree of substitution celluloses, for example, surface-modified nanocelluloses, which are insoluble in all molecular solvents. As such, we present the use of the tetrabutylphosphonium acetate ([P4444][OAc]):DMSO- d6 electrolyte in the 1D and 2D NMR characterization of poly(methyl methacrylate) (PMMA)-grafted cellulose nanocrystals (CNCs). PMMA- g-CNCs was chosen as a difficult model to study, to illustrate the potential of the technique. The chemical shift range of [P4444][OAc] is completely upfield of the cellulose backbone signals, avoiding signal overlap. In addition, application of diffusion-editing for 1H and HSQC was shown to be effective in the discrimination between PMMA polymer graft resonances and those from low molecular weight components arising from the solvent system. The bulk ratio of methyl methacrylate monomer to anhydroglucose unit was determined using a combination of HSQC and quantitative 13C NMR. After detachment and recovery of the PMMA grafts, through methanolysis, DOSY NMR was used to determine the average self-diffusion coefficient and, hence, molecular weight of the grafts compared to self-diffusion coefficients for PMMA GPC standards. This finally led to a calculation of both graft length and graft density using liquid-state NMR techniques. In addition, it was possible to discriminate between triads and tetrads, associated with PMMA tacticity, of the PMMA still attached to the CNCs (before methanolysis). CNC reducing end and sulfate half ester resonances, from sulfuric acid hydrolysis, were also assignable. Furthermore, other biopolymers, such as hemicelluloses and proteins (silk and wool), were found to be soluble in the electrolyte media, allowing for wider application of this method beyond just cellulose analytics.

Effect of Ionic Strength and Surface Charge Density on the Kinetics of Cellulose Nanocrystal Thin Film Swelling.

This work explores cellulose nanocrystal (CNC) thin films (<50 nm) and particle-particle interactions by investigating film swelling in aqueous solutions with varying ionic strength (1-100 mM). CNC film hydration was monitored in situ via surface plasmon resonance, and the kinetics of liquid uptake were quantified. The contribution of electrostatic double-layer forces to film swelling was elucidated by using CNCs with different surface charges (anionic sulfate half ester groups, high and low surface charge density, and cationic trimethylammonium groups). Total water uptake in the thin films was found to be independent of ionic strength and surface chemistry, suggesting that in the aggregated state van der Waals forces dominate over double-layer forces to hold the films together. However, the rate of swelling varied significantly. The water uptake followed Fickian behavior, and the measured diffusion constants decreased with the ionic strength gradient between the film and the solution. This work highlights that nanoparticle interactions and dispersion are highly dependent on the state of particle aggregation and that the rate of water uptake in aggregates and thin films can be tailored based on surface chemistry and solution ionic strength.

Precise quantification of nanoparticle surface free energy via colloidal probe atomic force microscopy.

Interfacial interactions of nanoparticles (NPs) in colloids are greatly influenced by the NP surface free energy (SFE). Due to the intrinsic physical and chemical heterogeneity of the NP surface, measuring SFE is nontrivial. The use of direct force measurement methods, such as colloidal probe atomic force microscopy (CP-AFM), have been proven to be effective for the determination of SFE on relatively smooth surfaces, but fail to provide reliable measurements for rough surfaces generated by NPs. Here, we developed a reliable approach to determine the SFE of NPs by adopting Persson's contact theory to include the effect of surface roughness on the measurements in CP-AFM experiments. We obtain the SFE for a range of materials varying in surface roughness and surface chemistry. The reliability of the proposed method is verified by the SFE determination of polystyrene. Subsequently, the SFE of bare and functionalized silica, graphene oxide, and reduced graphene oxide were quantified and validity of the results was demonstrated. The presented method unlocks the potential of CP-AFM as a robust and reliable method of the SFE determination of nanoparticles with a heterogeneous surface, which is challenging to obtain with conventionally implemented experimental techniques.

Coupling Particle Ordering and Spherulitic Growth for Long-Term Performance of Nanocellulose/Poly(ethylene oxide) Electrolytes.

Development of lithium-ion batteries with composite solid polymer electrolytes (CPSEs) has attracted attention due to their higher energy density and improved safety compared to systems utilizing liquid electrolytes. While it is well known that the microstructure of CPSEs affects the ionic conductivity, thermal stability, and mechanical integrity/long-term stability, the bridge between the microscopic and macroscopic scales is still unclear. Herein, we present a systematic investigation of the distribution of TEMPO-oxidized cellulose nanofibrils (t-CNFs) in two different molecular weights of poly(ethylene oxide) (PEO) and its effect on Li+ ion mobility, bulk conductivity, and long-term stability. For the first time, we link local Li-ion mobility at the nanoscale level to the morphology of CPSEs defined by PEO spherulitic growth in the presence of t-CNF. In a low-MW PEO system, spherulites occupy a whole volume of the derived CPSE with t-CNF being incorporated in between lamellas, while their nuclei remain particle-free. In a high-MW PEO system, spherulites are scarce and their growth is arrested in a non-equilibrium cubic shape due to the strong t-CNF network surrounding them. Electrochemical strain microscopy and solid-state 7Li nuclear magnetic resonance spectroscopy confirm that t-CNF does not partake in Li+ ion transport regardless of its distribution within the polymer matrix. Free-standing CSPE films with low-MW PEO have higher conductivity but lack long-term stability due to the existence of uniformly distributed, particle-free, spherulite nuclei, which have very little resistance to Li dendrite growth. On the other hand, high-MW PEO has lower conductivity but demonstrates a highly stable Li cycling response for more than 1000 h at 0.2 mA/cm2 and 65 °C and more than 100 h at 85 °C. The study provides a direct link between the microscopic dynamic, Li-ion transport, bulk mechanical properties and long-term stability of the derived CPSE and, and as such, offers a pathway towards design of robust all-solid-state Li-metal batteries.

Nanocellulose in Emulsions and Heterogeneous Water-Based Polymer Systems: A Review.

Nanocelluloses (i.e., bacterial nanocellulose, cellulose nanocrystals, and cellulose nanofibrils) are cellulose-based materials with at least one dimension in the nanoscale. These materials have unique and useful properties and have been shown to assemble at oil-water interfaces and impart new functionality to emulsion and latex systems. Herein, the use of nanocellulose in both emulsions and heterogeneous water-based polymers is reviewed, including dispersion, suspension, and emulsion polymerization. Comprehensive tables describe past work employing nanocellulose as stabilizers or additives and the properties that can be tailored through the use of nanocellulose are highlighted. Even at low loadings, nanocellulose offers an unprecedented level of control as a property modifier for a range of emulsion and polymer applications, influencing, for example, emulsion type, stability, and stimuli-responsive behavior. Nanocellulose can tune polymer particle properties such as size, surface charge, and morphology, or be used to produce capsules and polymer nanocomposites with enhanced mechanical, thermal, and adhesive properties. The role of nanocellulose is discussed, and a perspective for future direction is presented.

Elucidating the effect of enzymatic polymerized polysaccharide particle morphology on emulsion properties.

Exploiting the shape of Pickering stabilizers offers the ability to unlock the full potential of nanoparticle-stabilized emulsions for applications in enhanced oil recovery, pharmaceuticals, cosmetics, and coatings. In this work, we utilize engineered polysaccharide particles derived from the enzymatic polymerization of glucose from sucrose with controlled shape for the stabilization of dodecane-in-water emulsions. Altering the particle shape (spherical aggregates, fibrids, or platelets), while maintaining a neutral surface charge allows for a systematic examination of the role of particle shape in the stabilization of emulsions. We find that platelet-shaped particles reduce the interfacial tension and result in the smallest droplet size, while emulsions stabilized by aggregates and fibrids are governed by a network of particles in the continuous phase. Exploiting the synergy between these particles allowed for the tuning of their microstructure and rheological signature which allows us to map and tailor these emulsions for a wider variety of applications.

In situ monitoring of the morphology evolution of interfacially-formed conductive nanocomposite films and their use as strain sensors.

HYPOTHESIS: Understanding and monitoring the film formation of interfacially formed layered films allows for the design of conductive nanocomposite films suitable for strain sensing. EXPERIMENTS: To understand the mechanism of interfacial film formation, the hexane/water interface was monitored during the evaporation process via confocal laser scanning microscopy. Scanning electron microscopy and atomic force microscopy were utilized to investigate final film morphology. Tensile testing was used to determine their mechanical properties under uniaxial strain. FINDINGS: Conductive nanocomposite films were formed at the hexane/water interface. Due to their low colloidal stability in hexane, the Vulcan carbon (VC) nanoparticles settled to the hexane/water interface prior to the onset of paraffin wax precipitation. Consequently, after the evaporation of hexane a two-layer structured film was formed. The bottom (water-facing, VC-rich) layer was conductive due to the existence of a percolated network of nanoparticle aggregates, while the top (hexane facing, paraffin-rich) layer was not conductive. The films showed high sensitivity for strains between 1% and 10%. We propose that the mechanism of strain sensing is similar to that of layer-structured sensors fabricated through embedding conductive nanofillers onto flexible polymeric substrates. The advantage of the films derived by the method proposed here is their ease of fabrication as well as their low cost.

Role of interparticle interactions on microstructural and rheological properties of cellulose nanocrystal stabilized emulsions.

HYPOTHESIS: Microstructural and rheological properties of particle-stabilized emulsions are highly influenced by the nanoparticle properties such as size and surface charge. Surface charge of colloidal particles not only influences the interfacial adsorption but also the interparticle network formed by the non-adsorbed particles in the continuous phase. EXPERIMENTS: We have studied oil-in-water emulsions stabilized by cellulose nanocrystals (CNCs) with two different degrees of surface charge. Surface charge was varied by means of acidic or basic desulfation. Confocal microscopy coupled with rheology as well as cryogenic scanning electron microscopy were employed to establish a precise link between the microstructure and rheological behavior of the emulsions. FINDINGS: CNCs desulfated with hydrochloric acid (a-CNCs) were highly aggregated in water and shown to adsorb faster to the oil-water interface, yielding emulsions with smaller droplet sizes and a thicker CNC interfacial layer. CNCs desulfated using sodium hydroxide (b-CNCs) stabilized larger emulsion droplets and had a higher amount of non-adsorbed CNCs in the water phase. Rheological measurements showed that emulsions stabilized by a-CNCs formed a stronger network than for b-CNC stabilized emulsions due to increased van der Waals and H-bonding interactions that were not impeded by electrostatic repulsion.

Incorporating Cellulose Nanocrystals into the Core of Polymer Latex Particles via Polymer Grafting.

Surface-initiated atom transfer radical polymerization was used to graft hydrophobic poly(butyl acrylate) from cellulose nanocrystals (CNCs) resulting in compatibilized CNCs that were successfully incorporated inside the core of polymer latex particles. CNCs are anisotropic nanoparticles derived from renewable resources and have potential as reinforcing agents in nanocomposites. However, challenges due to the incompatibility between cellulose and hydrophobic polymers and processing difficulties, such as aggregation, have limited the performance of CNC nanocomposites produced to date. Here, CNCs were incorporated into the miniemulsion polymerization of methyl methacrylate by adding polymer-grafted CNCs to the monomer phase. A poly(methyl methacrylate)-CNC nanocomposite latex was subsequently produced in situ, whereby polymer-grafted CNCs (with optimized graft length) were located inside the latex particles, as shown by transmission electron microscopy. This work provides a method for controlling the location of CNCs in latex-based nanocomposites and may extend the use of CNCs in commercial adhesives and coatings.

Grafting-from cellulose nanocrystals via photoinduced Cu-mediated reversible-deactivation radical polymerization.

In this work we describe the grafting of cellulose nanocrystals (CNCs) by surface-initiated photoinduced Cu-mediated reversible-deactivation radical polymerization (RDRP). Initially, CNCs obtained through sulfuric acid hydrolysis were functionalized with a tertiary bromo-ester moiety as an initiating group for the subsequent RDRP of methyl acrylate, targeting three different degrees of polymerization for the polymer grafts: 50, 300 and 600. The polymerizations proceeded in DMSO in the presence of CuBr2 and Me6TREN as the catalytic system utilizing a UV source (λmax≈360nm). The technique proved highly versatile for the modification of CNCs with poly(methyl acrylate), where considerably high grafting was achieved in short reaction times (90min), with simple purification steps. CNC morphology was maintained and polymer grafts were evident through FT-IR spectroscopy, thermal analysis, contact angle measurements, X-ray photoelectron microscopy and x-ray diffraction.