Rheology of 3D printable ceramic suspensions: effects of non-adsorbing polymer on discontinuous shear thickening.

Concentrated suspensions of particles at volume fractions (ϕ) ≥ 0.5 often exhibit complex rheological behavior, transitioning from shear thinning to shear thickening as the shear stress or shear rate is increased. These suspensions can be extruded to form 3D structures, with non-adsorbing polymers often added as rheology modifiers to improve printability. Understanding how non-adsorbing polymers affect the suspension rheology, particularly the onset of shear thickening, is critical to the design of particle inks that will extrude uniformly. In this work, we examine the rheology of concentrated aqueous suspensions of colloidal alumina particles and the effects of adding non-adsorbing polyvinylpyrrolidone (PVP). First, we show that suspensions with ϕalumina = 0.560-0.575 exhibited discontinuous shear thickening (DST), where the viscosity increased by up to two orders of magnitude above an onset stress (τmin). Increasing ϕalumina from 0.550 to 0.575 increased the viscosity and yield stress in the shear thinning regime and decreased τmin. Next, PVP was added at concentrations within the dilute and semi-dilute non-entangled regimes of polymer conformation (ϕPVP = 0.005-0.050) to suspensions with constant ϕalumina = 0.550. DST was observed in all cases and increasing ϕPVP increased the viscosity and yield stress. Interestingly, increasing ϕPVP also increased τmin. We posit that the free PVP chains act as lubricants between alumina particles, increasing the stress needed to induce thickening. Finally, we demonstrate through direct comparisons of suspensions with and without PVP how non-adsorbing polymer addition can extend the extrusion processing window due to the increase in τmin.

Harmonized Life-Cycle Inventories of Nanocellulose and Its Application in Composites.

Cellulose nanocrystals (CNC) and nanofibers (CNF) have been broadly studied as renewable nanomaterials for various applications, including additives in cement and plastics composites. Herein, life cycle inventories for 18 previously examined processes are harmonized, and the impacts of CNC and CNF production are compared with a particular focus on GHG emissions. Findings show wide variations in GHG emissions between process designs, from 1.8-1100 kg CO2-eq/kg nanocellulose. Mechanical and enzymatic processes are identified as the lowest GHG emission methods to produce CNCs and CNFs. For most processes, energy consumption and chemical use are the primary sources of emissions. However, on a mass basis, for all examined production methods and impact categories (except CO emissions), CNC and CNF production emissions are higher than Portland cement and, in most cases, are higher than polylactic acid. This work highlights the need to carefully consider process design to prevent potential high emissions from CNCs and CNF production despite their renewable feedstock, and results show the magnitude of conventional material that must be offset through improved performance for these materials to be environmentally favorable.

Sustained Dye Release Using Poly(urea-urethane)/Cellulose Nanocrystal Composite Microcapsules.

The aim of this study is to develop methods to reinforce polymeric microspheres with cellulose nanocrystals (CNCs) to make eco-friendly microcapsules for a variety of applications such as medicines, perfumes, nutrients, pesticides, and phase change materials. Surface hydrophobization treatments for CNCs were performed by grafting poly(lactic acid) oligomers and fatty acids (FAs) to enhance the dispersion of nanoparticles in the polymeric shell. Then, a straightforward process is demonstrated to design sustained release microcapsules by the incorporation of the modified CNCs (mCNCs) in the shell structure. The combination of the mCNC dispersion with subsequent interfacial polyurea (PU) to form composite capsules as well as their morphology, composition, mechanical properties, and release rates were examined in this study. The PU microcapsules embedded with the mCNC were characterized by Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). The morphologies of the microcapsules were characterized by optical microscopy (OM) and scanning electron microscope (SEM). The rupture stress and failure behavior of the microcapsules were determined through single-capsule compression tests. Oil-soluble Sudan II dye solution in mineral oil was utilized as a model hydrophobic fill, representing other latent fills with low partition coefficients, and their encapsulation efficiency was measured spectroscopically. The release rates of the encapsulated dye from the microcapsules were examined spectroscopically by both ethanol and 2-ethyl-1-hexanol medium at room temperature. The concentration of released dye was determined by using UV-vis absorption spectrometry (UV-vis). The mCNC embedded poly(urea-urethane) capsules have strong and dense walls, which function as excellent barriers against leakage due to their extended diffusion path length and ensure enough mechanical strength from rupture for handling or postprocessing.

Hygroscopic Swelling Determination of Cellulose Nanocrystal (CNC) Films by Polarized Light Microscopy Digital Image Correlation.

The coefficient of hygroscopic swelling (CHS) of self-organized and shear-oriented cellulose nanocrystal (CNC) films was determined by capturing hygroscopic strains produced as result of isothermal water vapor intake in equilibrium. Contrast enhanced microscopy digital image correlation enabled the characterization of dimensional changes induced by the hygroscopic swelling of the films. The distinct microstructure and birefringence of CNC films served in exploring the in-plane hygroscopic swelling at relative humidity values ranging from 0% to 97%. Water vapor intake in CNC films was measured using dynamic vapor sorption (DVS) at constant temperature. The obtained experimental moisture sorption and kinetic profiles were analyzed by fitting with Guggenheim, Anderson, and deBoer (GAB) and Parallel Exponential Kinetics (PEK) models, respectively. Self-organized CNC films showed isotropic swelling, CHS ∼0.040 %strain/%C. By contrast, shear-oriented CNC films exhibited an anisotropic swelling, resulting in CHS ∼0.02 and ∼0.30 %strain/%C, parallel and perpendicular to CNC alignment, respectively. Finite element analysis (FEA) further predicted moisture diffusion as the predominant mechanism for swelling of CNC films.

Thermal conductivity in nanostructured films: from single cellulose nanocrystals to bulk films.

We achieved a multiscale description of the thermal conductivity of cellulose nanocrystals (CNCs) from single CNCs (∼0.72-5.7 W m(-1) K(-1)) to their organized nanostructured films (∼0.22-0.53 W m(-1) K(-1)) using experimental evidence and molecular dynamics (MD) simulation. The ratio of the approximate phonon mean free path (∼1.7-5.3 nm) to the lateral dimension of a single CNC (∼5-20 nm) suggested a contribution of crystal-crystal interfaces to polydisperse CNC film's heat transport. Based on this, we modeled the thermal conductivity of CNC films using MD-predicted single crystal and interface properties along with the degree of CNC alignment in the bulk films using Hermans order parameter. Film thermal conductivities were strongly correlated to the degree of CNC alignment and the direction of heat flow relative to the CNC chain axis. The low interfacial barrier to heat transport found for CNCs (∼9.4 to 12.6 m(2) K GW(-1)), and their versatile alignment capabilities offer unique opportunities in thermal conductivity control.

Thermal expansion of self-organized and shear-oriented cellulose nanocrystal films.

The coefficient of thermal expansion (CTE) of cellulose nanocrystal (CNC) films was characterized using novel experimental techniques complemented by molecular simulations. The characteristic birefringence exhibited by CNC films was utilized to calculate the in-plane CTE of self-organized and shear-oriented self-standing CNC films from room temperature to 100 °C using polarized light image correlation. CNC alignment was estimated via Hermans order parameter (S) from 2D X-ray diffraction measurements. We found that films with no preferential CNC orientation through the thickness (S: ∼ 0.0) exhibited an isotropic CTE (∼25 ppm/K). In contrast, films with aligned CNC orientations (S: ∼0.4 to 0.8) had an anisotropic CTE response: For the highest CNC alignment (S: 0.8), the CTE parallel to CNC alignment was ∼9 ppm/K, while that perpendicular to CNC alignment was ∼158 ppm/K. CNC film thermal expansion was proposed to be due primarily to single crystal expansion and CNC-CNC interfacial motion. The relative contributions of inter- and intracrystal responses to heating were explored using molecular dynamics simulations.

Performance of Advanced Waterborne Wood Coatings Reinforced with Cellulose Nanocrystals.

The main objective of this study was to examine the impact of cellulose nanocrystals (CNCs) in advanced waterborne wood coatings such as polycarbonate urethane (PCU) and hybrid alkyd varnish (HAV) in terms of coating performance, mechanical properties, optical properties, and water permeation and uptake properties. The influence of CNCs on the overall quality of the various waterborne wood coatings was investigated by incorporating different percentages of CNCs. Varying CNC content in coating formulations showed that CNCs are effective for waterborne wood coatings; CNCs offer both higher scratch and impact resistance as compared to neat coatings and have a significant reduction in water vapor permeation through a film with little increase in water vapor uptake at high concentrations. It was observed that the CNC darkened and reduced gloss in the coatings and viscosified the dispersion. These research findings suggest that CNCs are well-dispersed at lower concentrations, but at high concentrations, agglomeration occurred. Thus, while CNCs can give better mechanical and permeation performances at contents of up to 5 wt %, at 1 wt % CNCs can still provide modest scratch and chip resistance improvement without loss of optical properties (gloss and color) while retaining a similar water uptake. Overall, it can be concluded that CNCs have the potential to be used as a reinforcement filler in high-performance waterborne wood coatings.

Phone Camera Nano-Biosensor Using Mighty Sensitive Transparent Reusable Upconversion Paper.

Lycopene, a natural colorant and antioxidant with a huge growing market, is highly susceptible to photo/thermal degradation, which demands real-time sensors. Hence, here a transparent upconversion nanoparticles (UCNPs) strip having 30 mol % Yb, 0.1 mol % Tm, and ß-NaYF4 UCNPs, which shows an intense emission at 475 nm, has been developed. This strip has been found to be sensitive to lycopene with a detection limit as low as 10 nM using a smartphone camera, which is due to static quenching that is confirmed by the lifetime study. In comparison to previous paper strips, here the transparent strip has minimal scattering with maximum sensitivity in spite of not using any metal quenchers. An increase in strip hydrophobicity during the fabrication process complements the strip to selectively permeate and present an extraction-free substitute analysis for chromatography. Hydrophobicity endows the strip with the capability to reuse the strip with ∼100% luminescence recovery.

Effect of steric hindrance on the properties of antibacterial and biocompatible copolymers.

The development of polymers that are both bactericidal and biocompatible would have many applications and are currently of substantial research interest. It is well known that polymers of alkyl-quaternized poly(4-vinylpyridine) are known to be effective against a wide range of microbes: when copolymerized with monomers that form biocompatible materials, they has also been shown to possess biocompatible properties. However, the relationship of the various physical and chemical properties of these polymers and copolymers with the antibacterial and biocompatible properties remains poorly understood: maximizing the selectivity and performance of these materials is absolutely needed before they have the potential for commercial applications. Maximizing the performance will require a complete understanding of the effect of physical and chemical adjustments on these quaternized polymer bactericides. This article seeks to explore and characterize the effect of one specific property, steric hindrance, on the copolymers' antibacterial and biocompatible properties. We have thus synthesized and characterized a new class of copolymers from 2-vinylpyridine and poly(ethylene glycol) methyl ether methacrylate, measured its bactericidal and biocompatible properties, and compared its performance to chemically similar but sterically different polymer bactericides. This work thereby enables both a greater understanding of the properties of the 2-vinylpyridine copolymers and an improved understanding of the material properties that are vital for the development of antibacterial polymers.

Excellent dispersion of MWCNTs in PEO polymer achieved through a simple and potentially cost-effective evaporation casting.

A simple, reliable and potentially cost-effective composite film casting procedure is presented using the evaporation of solvent (water) from a dilute mixture of multiwalled carbon nanotubes (MWCNTs) and polyethylene oxide (PEO) polymer. It is found that the fabrication method develops excellent dispersion of MWCNTs in PEO confirmed by morphology observations, final crystallinity of polymer (amorphous) and a lower percolation threshold (closer to theoretical value) as well as higher electrical conductivity. A film thickness prediction model is derived based upon the fact that final film thickness is mainly dependent upon the dimensions of the casting mold and the loading of the MWCNTs and polymer. This simple model provides important insight that the material loss and the actual density of the base polymer are critical factors making the current casting method truly cost effective and controlling final thickness.

Influence of alignment and microstructure features on the mechanical properties and failure mechanisms of cellulose nanocrystals (CNC) films.

The mechanical properties of cellulose nanocrystal (CNC) films critically depend on many microstructural parameters such as fiber length distribution (FLD), fiber orientation distribution (FOD), and the strength of the interactions between the fibers. In this paper, we use our coarse-grained molecular model of CNC to study the effect of length and orientation distribution and attractions between CNCs on the mechanical properties of neat CNCs. The effect of misalignment of a 2D staggered structure of CNC with respect to the loading direction was studied with simulations and analytical solutions and then verified with experiments. To understand the effect of FLD and FOD on the mechanical performance, various 3D microstructures representing different case studies such as highly aligned, randomly distributed, short length CNCs and long length CNCs were generated and simulated. According to the misalignment study, three different failure modes: sliding mode, mixed mode, and normal mode were defined. Also, comparing the effects of FOD, FLD, and CNC interaction strength, shows that the adhesion strength is the only parameter that can significantly improve the mechanical properties, regardless of loading direction or FOD of CNCs.

Structure-Property Relationship of Cellulose Nanocrystal-Polyvinyl Alcohol Thin Films for High Barrier Coating Applications.

CO2 and O2 gas permeability are paramount concerns in food packaging. Here, the permeability of cellulose nanocrystals (CNCs) and polyvinyl alcohol (PVA) coatings was explored as it relates to varied CNC content. Specifically, this work focuses on the role of PVA in rheology and barrier performance of the CNC films. Results show that shear-casted CNC films are transparent and have a high-order parameter, which is attributed to the shear-thinning behavior of the CNCs. The barrier performance of the CNC films improved because of the synergistic effect of having both alignment of CNCs and a lower free volume. The CNC-PVA films exhibited excellent barrier performance as compared to traditional engineered polymers, even much higher than high barrier ethylene-vinyl alcohol copolymer films. Furthermore, the moisture sensitivity of the films was greatly diminished with the addition of PVA. Overall, the results show applicability of CNC-PVA coating formulations for high barrier packaging applications.

Recent Developments in Cellulose Nanomaterial Composites.

Cellulose nanomaterials (CNMs) are a class of materials that have recently garnered attention in fields as varied as structural materials, biomaterials, rheology modifiers, construction, paper enhancement, and others. As the principal structural reinforcement of biomass giving wood its mechanical properties, CNM is strong and stiff, but also nontoxic, biodegradable, and sustainable with a very large (Gton yr-1 ) source. Unfortunately, due to the relatively young nature of the field and inherent incompatibility of CNM with most man-made materials in use today, research has tended to be more basic-science oriented rather than commercially applicable, so there are few CNM-enabled products on the market today. Herein, efforts are presented for preparing and forming cellulose nanomaterial nanocomposites. The focus is on recent efforts attempting to mitigate common impediments to practical commercialization but is also placed in context with traditional efforts. The work is presented in terms of the progress made, and still to be made, on solving the most pressing challenges-getting properties that are competitive with currently used materials, removing organic solvent, solving the inherent incompatibility between CNM and polymers of interest, and incorporation into commonly used industrial processing techniques.

Alignment of Cellulose Nanofibers: Harnessing Nanoscale Properties to Macroscale Benefits.

In nature, cellulose nanofibers form hierarchical structures across multiple length scales to achieve high-performance properties and different functionalities. Cellulose nanofibers, which are separated from plants or synthesized biologically, are being extensively investigated and processed into different materials owing to their good properties. The alignment of cellulose nanofibers is reported to significantly influence the performance of cellulose nanofiber-based materials. The alignment of cellulose nanofibers can bridge the nanoscale and macroscale, bringing enhanced nanoscale properties to high-performance macroscale materials. However, compared with extensive reviews on the alignment of cellulose nanocrystals, reviews focusing on cellulose nanofibers are seldom reported, possibly because of the challenge of aligning cellulose nanofibers. In this review, the alignment of cellulose nanofibers, including cellulose nanofibrils and bacterial cellulose, is extensively discussed from different aspects of the driving force, evaluation, strategies, properties, and applications. Future perspectives on challenges and opportunities in cellulose nanofiber alignment are also briefly highlighted.

Structure-activity relationships of antibacterial and biocompatible copolymers.

The development of polymers that are both bactericidal and biocompatible would have many applications and are currently of research interest. Following the development of strongly bactericidal copolymers of 4-vinylpyridine and poly(ethylene glycol) methyl ether methacrylate, biocompatibility assays have been completed on these materials to measure their potential biocompatibility. In this article, a new methodology for measuring protein interaction was developed for water-soluble polymers by coupling proteins to surfaces and then measuring the adsorption of copolymers onto these surfaces. Ellipsometry was then used to measure the thickness of adsorbed polymers as a measurement of biocompatibility. These results were then compared and correlated with the results of other biocompatibility assays previously conducted on these polymers, affording a greater understanding of the biocompatibility of the copolymers as well as improving the understanding of the effect of hydrophilic and hydrophobic groups that is vital for the development of these materials.

Influence of Free Volume Determined by Positron Annihilation Lifetime Spectroscopy (PALS) on Gas Permeability of Cellulose Nanocrystal Films.

Cellulose nanocrystals (CNCs) are of increasing interest for packaging applications because of their biodegradability, low cost, high crystallinity, and high aspect ratio. The objective of this study was to use positron annihilation lifetime spectroscopy (PALS) to investigate the free volume of CNC films with different structural arrangements (chiral nematic vs shear-oriented CNC films) and relate this information to gas barrier performance. It was found that sheared CNC films with higher CNC alignment have lower free volume and hence have more tortuosity than chiral nematic self-assembled films, which lowers gas diffusion throughout the films. The overall barrier performance of the aligned CNC film obtained in this study has a higher barrier performance than high barrier polymer films like PVOH and EVOH. Furthermore, a modified model was developed for single-component CNC films to predict the gas permeability with variation of CNC alignment with validation by the data taken.

An emerging mobile air pollution source: outdoor plastic liner manufacturing sites discharge VOCs into urban and rural areas.

The in situ manufacture of cured-in-place-pipe (CIPP) plastic liners in damaged sewer pipes is an emerging mobile source of anthropogenic air pollution. Evidence indicates volatile organic compounds (VOCs) can be released before, during, and after manufacture. The chemical composition of a popular uncured styrene-based CIPP resin was examined, along with the VOCs that remained in the new cured composite. The roles of curing temperature and heating time in waste discharged into the air were examined. Uncured resin contained approximately 39 wt% VOCs. Multiple hazardous air pollutants were present, however, 61 wt% of the uncured resin was not chemically identified. A substantial mass of VOCs (8.87 wt%) was emitted into the air during manufacture, and all cured composites contained about 3 wt% VOCs. Some VOCs were created during manufacture. Curing temperature (65.5-93.3 °C) and heating time (25-100 min) did not cause different composite VOC loadings. High styrene air concentrations inhibited the detection of other VOCs in air. It is estimated that tens of tons of VOCs may be emitted at a single CIPP manufacturing site. Regulators should consider monitoring, and potentially regulating, these growing mobile air pollution and volatile chemical product sources as they are operating in urban and rural areas often in close proximity to residential and commercial buildings.

Multifunctional Bio-Nanocomposite Coatings for Perishable Fruits.

Hunger and chronic undernourishment impact over 800 million people, which translates to ≈10.7% of the world's population. While countries are increasingly making efforts to reduce poverty and hunger by pursuing sustainable energy and agricultural practices, a third of the food produced around the globe still is wasted and never consumed. Reducing food shortages is vital in this effort and is often addressed by the development of genetically modified produce or chemical additives and inedible coatings, which create additional health and environmental concerns. Herein, a multifunctional bio-nanocomposite comprised largely of egg-derived polymers and cellulose nanomaterials as a conformal coating onto fresh produce that slows down food decay by retarding ripening, dehydration, and microbial invasion is reported. The coating is edible, washable, and made from readily available inexpensive or waste materials, which makes it a promising economic alternative to commercially available fruit coatings and a solution to combat food wastage that is rampant in the world.

In vitro biocompatibility studies of antibacterial quaternary polymers.

Quaternized copolymers of 4-vinylpyridine and poly(ethylene glycol) methyl ether methacrylate are known to have antibacterial properties and have displayed biocompatibility in red blood cell hemolysis assays. The results from hemolysis assays have shown substantial promise, but the technique is rudimentary and only a first step toward the determination of biocompatibility. The present paper further explores the biocompatibility of these copolymers through comprehensive cell viability assays performed on Caco-2 human epithelial cells cultivated in vitro. We have shown that these copolymers are biocompatible at concentrations above their minimum bactericidal concentrations, leading to selectivity values that compare well with other microbicidal products.

Application of a high throughput bioluminescence-based method and mathematical model for the quantitative comparison of polymer microbicide efficiency.

Quaternized poly(4-vinyl pyridine)-based copolymers are known to be effective against a wide range of bacteria and possess biocompatible properties. Extensive testing of a wide range of copolymers is necessary to further explore and enhance the biocidal properties. However, testing is hampered by labor-intensive bacteria testing techniques. The present paper presents a new testing method, based on bioluminescent reporter strains to enable fast evaluation of bactericidal properties. The reported method enables us to create real-time characterization of bacteria behavior with far less labor than required through traditional testing methods. A mathematical model was also developed to characterize the change in bacteria populations exposed to biocides and to enable the quantitative comparison of minimum bactericidal concentrations.