A Review of Polyolefin-Insulation Materials in High Voltage Transmission; From Electronic Structures to Final Products.

This review focuses on the use of polyolefins in high-voltage direct-current (HVDC) cables and capacitors. A short description of the latest evolution and current use of HVDC cables and capacitors is first provided, followed by the basics of electric insulation and capacitor functions. Methods to determine dielectric properties are described, including charge transport, space charges, resistivity, dielectric loss, and breakdown strength. The semicrystalline structure of polyethylene and isotactic polypropylene is described, and the way it relates to the dielectric properties is discussed. A significant part of the review is devoted to describing the state of art of the modeling and prediction of electric or dielectric properties of polyolefins with consideration of both atomistic and continuum approaches. Furthermore, the effects of the purity of the materials and the presence of nanoparticles are presented, and the review ends with the sustainability aspects of these materials. In summary, the effective use of modeling in combination with experimental work is described as an important route toward understanding and designing the next generations of materials for electrical insulation in high-voltage transmission.

In-Depth Understanding of the Effect of the Distribution of Substituents on the Morphology and Physical Properties of Ethylcellulose: Molecular Dynamics Simulations Insights.

Ethylcellulose (EC) is a crucial cellulose derivative with widespread applications, particularly in the pharmaceutical industry, where precise property adjustments through chemical modification are imperative. The degree of substitution (DS) and the localization of substituents along the cellulose chains are pivotal factors in this process. However, the impact of the substituent location within the repeating unit of EC remains unexplored. To address this gap, we conducted molecular dynamics simulations on amorphous EC, comparing randomly and uniformly substituted ethyl groups in the repeating units. This comprehensive study of pairwise interactions revealed significant differences in intramolecular and intermolecular hydrogen-bonding capabilities, depending on whether the hydroxyl groups were substituted at C2, C3, or C6. While our simulations demonstrated that substituent localization in the repeating unit influenced the density, number of hydrogen bonds, and conformations, the DS emerged as the dominant determinant. This insight led us to propose and validate a hypothesis: a straightforward linear function using the properties of uniform models and molar fractions can predict the properties of randomly substituted EC with a given DS. This innovative approach is anticipated to contribute to the selection of cellulose derivatives with desirable properties for the pharmaceutical industry and new applications in other fields.

Effects of multi-functional additives during foam extrusion of wheat gluten materials.

To broaden the range in structures and properties, and therefore the applicability of sustainable foams based on wheat gluten expanded with ammonium-bicarbonate, we show here how three naturally ocurring multifunctional additives affect their properties. Citric acid yields foams with the lowest density (porosity of ~50%) with mainly closed cells. Gallic acid acts as a radical scavenger, yielding the least crosslinked/ aggregated foam. The use of a low amount of this acid yields foams with the highest uptake of the body-fluid model substance (saline, ~130% after 24 hours). However, foams with genipin show a large and rapid capillary uptake (50% in one second), due to their high content of open cells. The most dense and stiff foam is obtained with one weight percent genipin, which is also the most crosslinked. Overall, the foams show a high energy loss-rate under cyclic compression (84-92% at 50% strain), indicating promising cushioning behaviour. They also show a low compression set, indicating promising sealability. Overall, the work here provides a step towards using protein biofoams as a sustainable alternative to fossil-based plastic/rubber foams in applications where absorbent and/or mechanical properties play a key role.

Nanostructures of etherified arabinoxylans and the effect of arabinose content on material properties.

To further our understanding of a thermoplastic arabinoxylan (AX) material obtained through an oxidation-reduction-etherification pathway, the role of the initial arabinose:xylose ratio on the material properties was investigated. Compression molded films with one molar substitution of butyl glycidyl ether (BGE) showed markedly different tensile behaviors. Films made from low arabinose AX were less ductile, while those made from high arabinose AX exhibited elastomer-like behaviors. X-ray scattering confirmed the presence of nanostructure formation resulting in nano-domains rich in either AX or BGE, from side chain grafting. The scattering data showed variations in the presence of ordered structures, nano-domain sizes and their temperature response between AX with different arabinose contents. In dynamic mechanical testing, three transitions were observed at approximately -90 °C, -50 °C and 80 °C, with a correlation between samples with more structured nano-domains and those with higher onset transition temperatures and lower storage modulus decrease. The mechanical properties of the final thermoplastic AX material can therefore be tuned by controlling the composition of the starting material.

Centrifuge fractionation during purification of cellulose nanocrystals after acid hydrolysis and consequences on their chiral self-assembly.

The inherent colloidal dispersity (due to length, aspect ratio, surface charge heterogeneity) of CNCs, when produced using the typical traditional sulfuric acid hydrolysis route, presents a great challenge when interpreting colloidal properties and linking the CNC film nanostructure to the helicoidal self-assembly mechanism during drying. Indeed, further improvement of this CNC preparation route is required to yield films with better control over the CNC pitch and optical properties. Here we present a modified CNC-preparation protocol, by fractionating and harvesting CNCs with different average surface charges, rod lengths, aspect ratios, already during the centrifugation steps after hydrolysis. This enables faster CNC fractionation, because it is performed in a high ionic strength aqueous medium. By comparing dry films from the three CNC fractions, discrepancies in the CNC self-assembly and structural colors were clearly observed. Conclusively, we demonstrate a fast protocol to harvest different populations of CNCs, that enable tailored refinement of structural colors in CNC films.

Biodegradable Fiber-Reinforced Gluten Biocomposites for Replacement of Fossil-Based Plastics.

Biocomposites based on wheat gluten and reinforced with carbon fibers were produced in line with the strive to replace fossil-based plastics with microplastic-free alternatives with competing mechanical properties. The materials were first extruded/compounded and then successfully injection molded, making the setup adequate for the current industrial processing of composite plastics. Furthermore, the materials were manufactured at very low extrusion and injection temperatures (70 and 140 °C, respectively), saving energy compared to the compounding of commodity plastics. The sole addition of 10 vol % fibers increased yield strength and stiffness by a factor of 2-4 with good adhesion to the protein. The biocomposites were also shown to be biodegradable, lixiviating into innocuous molecules for nature, which is the next step in the development of sustainable bioplastics. The results show that an industrial protein coproduct reinforced with strong fibers can be processed using common plastic processing techniques. The enhanced mechanical performance of the reinforced protein-based matrix herein also contributes to research addressing the production of safe materials with properties matching those of traditional fossil-based plastics.

Anion-Specific Adsorption of Carboxymethyl Cellulose on Cellulose.

Integration of fiber modification step with a modern pulp mill is a resource efficient way to produce functional fibers. Motivated by the need to integrate polymer adsorption with the current pulping system, anion-specific effects in carboxymethylcellulose (CMC) adsorption have been studied. The QCM-D adsorption experiments revealed that CMC adsorption to the cellulose model surface is prone to anion-specific effects. A correlation was observed between the adsorbed CMC and the degree of hydration of the co-ions present in the magnesium salts. The presence of a chaotropic co-ion such as nitrate increased the adsorption of CMC on cellulose compared to the presence of the kosmotropic sulfate co-ion. However, anion-specificity was not significant in the case of salts containing zinc cations. The hydration of anions determines the distribution of the ions at the interface. Chaotropic ions, such as nitrates, are likely to be distributed near the chaotropic cellulose surface, causing changes in the ordering of water molecules and resulting in greater entropy gain once released from the surface, thus increasing CMC adsorption.

Coarse-Grained Model for Prediction of Hole Mobility in Polyethylene.

Electrical conductivity measurements of polyethylene indicate that the semicrystalline structure and morphology influence the conductivity. To include this effect in atomistic charge transport simulations, models that explicitly or implicitly take morphology into account are required. In the literature, charge transport simulations of amorphous polyethylene have been successfully performed using short oligomers to represent the polymer. However, a more realistic representation of the polymer structure is desired, requiring the development of fast and efficient charge transport algorithms that can handle large molecular systems through coarse-graining. Here, such a model for charge transport simulations in polyethylene is presented. Quantum chemistry calculations were used to define six segmentation rules on how to divide a polymer chain into shorter segments representing localized molecular orbitals. Applying the rules to amorphous systems yields distributions of segments with mode and median segment lengths relatively close to the persistence length of polyethylene. In an initial test, the segments of an amorphous polyethylene were used as hopping sites in kinetic Monte Carlo (KMC) simulations, which yielded simulated hole mobilities that were within the experimental range. The activation energy of the simulated system was lower compared to the experimental values reported in the literature. A conclusion may be that the experimental result can only be explained by a model containing chemical defects that generate deep traps.

Stretchable, Strong, Recyclable Helicide Elastomer Based on Dynamic Covalent Interactions.

Current methods for making and disposing synthetic polymers have been widely pursued and are largely unsustainable. As a part of the solution, the reversible nature of dynamic covalent bonds emerges as an extraordinarily diverse and valuable feature in the development of exotic molecules and extended structures. With these bonds, it should be possible to construct recyclable and mechanically interlocked molecular structures using relatively simple precursors with preorganized geometries. A new helicide-based elastomer network is developed here with self-healing, recycling, and degradation features using a similar concept. The best self-healing performance (100%) was noted over 10-20 min, with various H2O, HCl, and NaOH solutions that delivered mechanical properties in the 1-1.4 MPa range. For hydrolytic degradation, the parameters are defined based on the type of binding, the pH of the solutions, and the copolymer network, which endowed a degradation time of approximately 4-11 h for each prepared sample. However, due to the reversible nature of the dynamic bonds, the material showed good recyclable mechanical properties compared to the pristine samples after five consecutive cycles, which meet the requirements of recyclable materials and recyclable packaging.

Large-scale synthesis of 2D-silica (SiOx) nanosheets using graphene oxide (GO) as a template material.

Graphene oxide (GO) was used in this study as a template to successfully synthesize silicon oxide (SiOx) based 2D-nanomaterials, adapting the same morphological features as the GO sheets. By performing a controlled condensation reaction using low concentrations of GO (<0.5 wt%), the study shows how to obtain 2D-nanoflakes, consisting of GO-flakes coated with a silica precursor that were ca. 500 nm in lateral diameter and ca. 1.5 nm in thickness. XPS revealed that the silanes had linked covalently with the GO sheets at the expense of the oxygen groups present on the GO surface. The GO template was shown to be fully removable through thermal treatment without affecting the nanoflake morphology of the pure SiOx-material, providing a methodology for large-scale preparation of SiOx-based 2D nanosheets with nearly identical dimensions as the GO template. The formation of SiOx sheets using a GO template was investigated for two different silane precursors, (3-aminopropyl) triethoxysilane (APTES) and tetraethyl orthosilicate (TEOS), showing that both precursors were capable of accurately templating the graphene oxide template. Molecular modeling revealed that the choice of silane affected the number of layers coated on the GO sheets. Furthermore, rheological measurements showed that the relative viscosity was significantly affected by the specific surface area of the synthesized particles. The protocol used showed the ability to synthesize these types of nanoparticles using a common aqueous alcohol solvent, and yield larger amounts (∼1 g) of SiOx-sheets than what has been previously reported.

Green Chemistry to Modify Functional Properties of Crambe Protein Isolate-Based Thermally Formed Films.

Proteins are promising precursors to be used in production of sustainable materials with properties resembling plastics, although protein modification or functionalization is often required to obtain suitable product characteristics. Here, effects of protein modification were evaluated by crosslinking behavior using high-performance liquid chromatography (HPLC), secondary structure using infrared spectroscopy (IR), liquid imbibition and uptake, and tensile properties of six crambe protein isolates modified in solution before thermal pressing. The results showed that a basic pH (10), especially when combined with the commonly used, although moderately toxic, crosslinking agent glutaraldehyde (GA), resulted in a decrease in crosslinking in unpressed samples, as compared to acidic pH (4) samples. After pressing, a more crosslinked protein matrix with an increase in ß-sheets was obtained in basic samples compared to acidic samples, mainly due to the formation of disulfide bonds, which led to an increase in tensile strength, and liquid uptake with less material resolved. A treatment of pH 10 + GA, combined either with a heat or citric acid treatment, did not increase crosslinking or improve the properties in pressed samples, as compared to pH 4 samples. Fenton treatment at pH 7.5 resulted in a similar amount of crosslinking as the pH 10 + GA treatment, although with a higher degree of peptide/irreversible bonds. The strong bond formation resulted in lack of opportunities to disintegrate the protein network by all extraction solutions tested (even for 6 M urea + 1% sodium dodecyl sulfate + 1% dithiothreitol). Thus, the highest crosslinking and best properties of the material produced from crambe protein isolates were obtained by pH 10 + GA and pH 7.5 + Fenton, where Fenton is a greener and more sustainable solution than GA. Therefore, chemical modification of crambe protein isolates is effecting both sustainability and crosslinking behavior, which might have an effect on product suitability.

Molecular Dynamics Simulations of Cellulose and Dialcohol Cellulose under Dry and Moist Conditions.

The development of wood-based thermoplastic polymers that can replace synthetic plastics is of high environmental importance, and previous studies have indicated that cellulose-rich fiber containing dialcohol cellulose (ring-opened cellulose) is a very promising candidate material. In this study, molecular dynamics simulations, complemented with experiments, were used to investigate how and why the degree of ring opening influences the properties of dialcohol cellulose, and how temperature and presence of water affect the material properties. Mechanical tensile properties, diffusion/mobility-related properties, densities, glass-transition temperatures, potential energies, hydrogen bonds, and free volumes were simulated for amorphous cellulosic materials with 0-100% ring opening, at ambient and high (150 °C) temperatures, with and without water. The simulations showed that the impact of ring openings, with respect to providing molecular mobility, was higher at high temperatures. This was also observed experimentally. Hence, the ring opening had the strongest beneficial effect on "processability" (reduced stiffness and strength) above the glass-transition temperature and in wet conditions. It also had the effect of lowering the glass-transition temperature. The results here showed that molecular dynamics is a valuable tool in the development of wood-based materials with optimal thermoplastic properties.

Experimental and simulated distribution and interaction of water in cellulose esters with alkyl chain substitutions.

This study investigated the effect of the average length of substituted side chains in different cellulose esters on water sorption and the water association mechanism. For this purpose, a set of esters with a similar total degree of substitution was selected: cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate. Dynamic vapor sorption was used to determine the effect of the side chain length on sorption, desorption, and the occurrence of water clustering. Since water association in the structure was of interest, molecular dynamics simulations were performed on cellulose acetate and cellulose acetate propionate. This study showed that cellulose acetate appears to be water-sensitive and experiences hysteresis upon water sorption, which was attributed to structural changes. The simulations also showed that water is screened out by the side chains and forms intermolecular hydrogen bonds, primarily to the carbonyl oxygen rather than the residual hydroxyl groups.

Sustainable Wheat Protein Biofoams: Dry Upscalable Extrusion at Low Temperature.

Glycerol-plasticized wheat gluten was explored for producing soft high-density biofoams using dry upscalable extrusion (avoiding purposely added water). The largest pore size was obtained when using the food grade ammonium bicarbonate (ABC) as blowing agent, also resulting in the highest saline liquid uptake. Foams were, however, also obtained without adding a blowing agent, possibly due to a rapid moisture uptake by the dried protein powder when fed to the extruder. ABC's low decomposition temperature enabled extrusion of the material at a temperature as low as 70 °C, well below the protein aggregation temperature. Sodium bicarbonate (SBC), the most common food-grade blowing agent, did not yield the same high foam qualities. SBC's alkalinity, and the need to use a higher processing temperature (120 °C), resulted in high protein cross-linking and aggregation. The results show the potential of an energy-efficient and industrially upscalable low-temperature foam extrusion process for competitive production of sustainable biofoams using inexpensive and readily available protein obtained from industrial biomass (wheat gluten).

Robust Assembly of Cross-Linked Protein Nanofibrils into Hierarchically Structured Microfibers.

Natural, high-performance fibers generally have hierarchically organized nanosized building blocks. Inspired by this, whey protein nanofibrils (PNFs) are assembled into microfibers, using flow-focusing. By adding genipin as a nontoxic cross-linker to the PNF suspension before spinning, significantly improved mechanical properties of the final fiber are obtained. For curved PNFs, with a low content of cross-linker (2%) the fiber is almost 3 times stronger and 4 times stiffer than the fiber without a cross-linker. At higher content of genipin (10%), the elongation at break increases by a factor of 2 and the energy at break increases by a factor of 5. The cross-linking also enables the spinning of microfibers from long straight PNFs, which has not been achieved before. These microfibers have higher stiffness and strength but lower ductility and toughness than those made from curved PNFs. The fibers spun from the two classes of nanofibrils show clear morphological differences. The study demonstrates the production of protein-based microfibers with mechanical properties similar to natural protein-based fibers and provides insights about the role of the nanostructure in the assembly process.

Clustering and cross-linking of the wheat storage protein α-gliadin: A combined experimental and theoretical approach.

Our aim was to understand mechanisms for clustering and cross-linking of gliadins, a wheat seed storage protein type, monomeric in native state, but incorporated in network while processed. The mechanisms were studied utilizing spectroscopy and high-performance liquid chromatography on a gliadin-rich fraction, in vitro produced α-gliadins, and synthetic gliadin peptides, and by coarse-grained modelling, Monte Carlo simulations and prediction algorithms. In solution, gliadins with α-helix structures (dip at 205 nm in CD) were primarily present as monomeric molecules and clusters of gliadins (peaks at 650- and 700-s on SE-HPLC). At drying, large polymers (Rg 90.3 nm by DLS) were formed and ß-sheets increased (14% by FTIR). Trained algorithms predicted aggregation areas at amino acids 115-140, 150-179, and 250-268, and induction of liquid-liquid phase separation at P- and Poly-Q-sequences (Score = 1). Simulations showed that gliadins formed polymers by tail-to-tail or a hydrophobic core (Kratky plots and Ree = 35 and 60 for C- and N-terminal). Thus, the N-terminal formed clusters while the C-terminal formed aggregates by disulphide and lanthionine bonds, with favoured hydrophobic clustering of similar/exact peptide sections (synthetic peptide mixtures on SE-HPLC). Mechanisms of clustering and cross-linking of the gliadins presented here, contribute ability to tailor processing results, using these proteins.

Highly insulating thermoplastic nanocomposites based on a polyolefin ternary blend for high-voltage direct current power cables.

Octyl-silane-coated Al2O3 nanoparticles are found to be a promising conductivity-reducing additive for thermoplastic ternary blends comprising low-density polyethylene (LDPE), isotactic polypropylene and a styrenic copolymer. The ternary blend nanocomposites were prepared by compounding the blend components together with an LDPE-based masterbatch that contained the nanoparticles. The nanoparticles did not affect the superior stiffness of the ternary blends, compared to neat LDPE, between the melting temperatures of the two polyolefins. As a result, ternary blend nanocomposites comprising 38 wt% polypropylene displayed a storage modulus of more than 10 MPa up to at least 150 °C, independent of the chosen processing conditions. Moreover, the ternary blend nanocomposites featured a low direct-current electrical conductivity of about 3 × 10-15 S m-1 at 70 °C and an electric field of 30 kV mm-1, which could only be achieved through the presence of both polypropylene and Al2O3 nanoparticles. This synergistic conductivity-reducing effect may facilitate the design of more resistive thermoplastic insulation materials for high-voltage direct current (HVDC) power cables.