The role of nanocellulose fibers, starch and chitosan on multipolysaccharide based films

Thin nanocomposite films of thermoplastic starch, chitosan and cellulose nanofibers (bacterial cellulose or nanofibrillated cellulose) were prepared for the first time by solvent casting of water based suspensions of the three polysaccharides. The role of the different bioploymers on the final properties (thermal stability, transparency, mechanical performance and antimicrobial activity) of the films was related with their intrinsic features, contents and synergic effects resulting from the establishment of interactions between them. Thermoplastic starch displays an important role on the thermal stability of the films because it is the most stable polysaccharide; however it has a negative impact on the mechanical performance and transparency of the films. The addition of chitosan improves considerably the transparency (up to 50 % transmittance for 50 % of chitosan, in respect to the amount of starch), mechanical performance and antimicrobial properties (at least 25 % of chitosan and no more than 10 % of cellulose nanofibers are required to observe bacteriostatic or bactericidal activity) but decrease their thermal stability. The incorporation of cellulose nanofibers had the strongest positive impact on the mechanical properties of the materials (increments of up to 15 and 30 MPa on the Young′s modulus and Tensile strength, respectively, for films with 20 % of BC or NFC). Nonetheless, the impact in thermal stability and mechanical performance of the films, promoted by the addition of chitosan and cellulose nanofibres, respectively, was higher than the expected considering their percentage contents certainly because of the establishment of strong and complex interactions between the three polysaccharides.     

Influence of the Epoxy/Acid Stoichiometry on the Cure Behavior and Mechanical Properties of Epoxy Vitrimers

Bisphenol A epoxy resin cured with a mixture of dimerized and trimerized fatty acids is the first epoxy vitrimer and has been extensively studied. However, the cure behavior and thermal and mechanical properties of this epoxy vitrimer depend on the epoxy/acid stoichiometry. To address these issues, epoxy vitrimers with three epoxy/acid stoichiometries (9:11, 1:1 and 11:9) were prepared and recycled four times. Differential scanning calorimetry (DSC) was used to study the cure behavior of the original epoxy vitrimers. The dynamic mechanical properties and mechanical performance of the original and recycled epoxy vitrimers were investigated by using dynamic mechanical analysis (DMA) and a universal testing machine. Furthermore, the reaction mechanism of epoxy vitrimer with different epoxy/acid stoichiometry was interpreted. With an increase in the epoxy/acid ratio, the reaction rate, swelling ratio, glass transition temperature and mechanical properties of the original epoxy vitrimers decreased, whereas the gel content increased. The recycling decreased the swelling ratio and elongation at break of the original epoxy vitrimers. Moreover, the elongation at break of the recycled epoxy vitrimers decreased with the epoxy/acid ratio at the same recycling time. However, the gel content, tensile strength and toughness of the original epoxy vitrimers increased after the recycling. The mechanical properties of epoxy vitrimers can be tuned with the variation in the epoxy/acid stoichiometry.     

Inverse Vulcanized Polymers with Shape Memory,Enhanced Mechanical Properties,and Vitrimer Behavior

The invention of inverse vulcanization provides great opportunities for generating functional polymers directly from elemental sulfur, an industrial by‐product. However, unsatisfactory mechanical properties have limited the scope for wider applications of these exciting materials. Here, we report an effective synthesis method that significantly improves mechanical properties of sulfur‐polymers and allows control of performance. A linear pre‐polymer containing hydroxyl functional group was produced, which could be stored at room temperature for long periods of time. This pre‐polymer was then further crosslinked by difunctional isocyanate secondary crosslinker. By adjusting the molar ratio of crosslinking functional groups, the tensile strength was controlled, ranging from 0.14±0.01 MPa to 20.17±2.18 MPa, and strain was varied from 11.85±0.88 % to 51.20±5.75 %. Control of hardness, flexibility, solubility and function of the material were also demonstrated. We were able to produce materials with suitable combination of flexibility and strength, with excellent shape memory function. Combined with the unique dynamic property of S?S bonds, these polymer networks have an attractive, vitrimer‐like ability for being reshaped and recycled, despite their crosslinked structures. This new synthesis method could open the door for wider applications of sustainable sulfur‐polymers.     

Enhancement of thermal,mechanical and barrier properties of EVA solar cell encapsulating films by reinforcing with esterified cellulose nanofibres

Solar cell encapsulating film based on ethylene vinyl acetate copolymer (EVA) was modified by using bacterial cellulose (BC) nanofibres. Bacterial cellulose was chemically modified with propionic anhydride prior to compounding with EVA in a twin screw extruder. The effects of fibre content on the mechanical, thermal, optical and barrier properties of the EVA composite films were investigated. Better mechanical and barrier properties of the EVA films were obtained when the modified BC nanofibres were used. The results were ascribed to the different chemical functional groups on the fibre surface, as verified by FTIR spectra. Deacetylation of the EVA was delayed and visible light transparency of the EVA films above 75% was retained. Overall, our study showed that it was possible to improve the barrier properties of EVA film without sacrificing much transparency by using a suitable type and content of cellulose nanofibres.     

Ultra-robust,self-healable and recyclable polyurethane elastomer via a combination of hydrogen bonds,dynamic chemistry,and microphase separation

Flexibility, robustness, transparency, and recyclability are critical to the application of self-healing polymer materials in the field of flexible electronics. However, integrating all the above properties remains a huge challenge to date. In this work, we put forward a facile strategy to prepare polyurethane (PU) elastomer with ultra-high strength and self-healing performance based on hydrogen bonds, disulfide dynamic chemistry, and microphase separation at the same time. Three different self-healing PUs were obtained by introducing disulfide bonds and different types of hydrogen bonds. A robust, transparent, and recyclable PU with amino-terminated chain extender (PUA) with fast and efficient self-healing performance was prepared. The mechanical and self-healing properties of the PUA were effectively balanced by the synergistic effect of reversible interaction of disulfide bonds and the formation of microphase separated structure. The results indicated that the PUA exhibited high transparency up to 90% and excellent mechanical property, e.g. the tensile strength and elongation at break can reach 37.10 MPa and 1080%, respectively. Meanwhile, it can achieve a high self-healing efficiency of 96.8% at 80 °C for 4 h and maintain 84% of the initial mechanical strength even after four times of recycling. Moreover, the colloid graphite/PUA flexible strain sensor was prepared by the combination of colloid graphite and PUA, which can accurately detect both large and tiny scale deformations.     

环氧树脂类玻璃高分子研究进展

类玻璃高分子(Vitrimer)是一类具有可逆共价交联网络的高分子,其能够在维持交联结构的同时实现交联网络的重构,兼具热固性高分子和热塑性高分子的双重优势.基于通用热固性树脂形成的Vitrimer材料不仅能具有良好的力学性能和耐溶剂性等,还能表现出类似热塑性树脂的流动性和重复加工性能,为从源头上实现交联树脂的回收和再利...     

Development and thermal behaviour of ternary PLA matrix composites

Biodegradable PLA composites were prepared using microcrystalline cellulose (MCC) and silver (Ag) nanoparticles. The main objective of the present study is to develop new biopolymer composites with good mechanical properties, thermal stability, maintaining the optical transparency and also providing antimicrobial properties through silver nanoparticle introduction. Composites were prepared with 1%wt of Ag nanoparticles and 5%wt of MCC using a twin-screw microextruder; film parameters were optimized in order to obtain a thickness range between 20 and 60 μm.PLA composites maintained optical transparency properties of the matrix, while MCC was able to reduce polymer permeability. Thermal analysis revealed that MCC increased PLA crystallinity and the mechanical properties of the composites demonstrated that tensile modulus was improved by microcrystalline cellulose.     

A Triple Crosslinking Design toward Epoxy Vitrimers and Carbon Fiber Composites of High Performance and Multi-shape Memory

It remains a challenge to use a simple approach to fabricate a multi-shape memory material with high mechanical performances. Here,we report a triple crosslinking design to construct a multi-shape memory epoxy vitrimer(MSMEV), which exhibits high mechanical properties,multi-shape memory property and malleability. The triple crosslinking network is formed by reacting diglycidyl ether of bisphenol F(DGEBF) with4-aminophenyl disulfide, γ-aminopropyltriethoxysilane(APTS) and poly(propylene glycol) bis(2-aminopropyl ether)(D2000). The triple crosslinking manifests triple functions: the disulfide bonds and the silyl ether linkages enable malleability of the epoxy network; the silyl ether linkages impart the network with high heterogeneity and broaden the glass transition region, leading to multi-shape memory property; a small amount of D2000 increases the modulus difference between the glassy and rubbery states, thereby improving the shape fixity ratio. Meanwhile,the high crosslinking density and rigid structure provide the MSMEV with high tensile strength and Young's modulus. Moreover, integrating carbon fibers and MSMEV results in shape memory composites. The superior mechanical properties of the composites and the recyclability of carbon fiber derived from the dissolvability of MSMEV make the composites hold great promise as structural materials in varied applications.     

Conversion of Cellulose and Lignin Residues into Transparent UV-Blocking Composite Films

The valorization of cellulose and lignin residues in an integrated biorefinery is of great significance to improve the overall economics but has been challenged by their structural recalcitrance, especially for lignin residue. In this work, a facile chemical conversion route to fabricating functional UV-blocking cellulose/lignin composite films through a facile dissolution–regeneration process using these biomass residues was proposed. Three representative lignin residues, i.e., aspen and poplar wood lignin, and corn stover (CS) lignin were assessed for their feasibility for the film fabrication. The UV-blocking performance of the composite films were comparatively investigated. Results showed that all these three lignin residues could enhance the UV-blocking property of the composite films, corresponding to the reduction in the optical energy band gap from 4.31 to 3.72 eV, while poplar lignin had a considerable content of chromophores and showed the best UV-blocking enhancement among these three assessing lignins. The enhancement of UV-blocking property was achieved without compromising the visible-light transparency, mechanical strength and thermal stability of the composite films even at 4% lignin loading. This work showed the high promise of integrating biomass residue conversion into lignocellulose biorefinery for a multi-production purpose.     

An Interfacial Dynamic Crosslinking Approach toward Catalyst-free and Mechanically Robust Elastomeric Vitrimer with a Segregated Structure

Elastomeric vitrimers with covalent adaptable networks are promising candidates to overcome the intrinsic drawbacks of conventional covalently-crosslinked elastomers; however, most elastomeric vitrimers show poor mechanical properties and require the addition of exogenous catalysts. Herein, we fabricate a catalyst-free and mechanically robust elastomeric vitrimer by constructing a segregated structure of sodium alginate (SA) in the continuous matrix of epoxidized natural rubber (ENR), and further crosslinking the composite by exchangeable hydroxyl ester bonds at the ENR-SA interfaces. The manufacturing process of the elastomeric vitrimer is facile and environmentally friendly without hazardous solvents or exogenous catalysts, as the abundant hydroxyl groups of the segregated SA phase can act as catalyst to activate the crosslinking reaction and promote the dynamic transesterification reaction. Interestingly, the segregated SA structure bears most of the load owing to its high modulus and small deformability, and thus ruptures preferentially upon deformation, leading to efficient energy dissipation.Moreover, the periodic stiffness fluctuation between rigid segregated SA phase and soft ENR matrix is beneficial to the crack-resisting. As a result,the elastomeric vitrimer manifests exceptional combination of catalyst-free, defect-tolerance, high tensile strength and toughness. In addition,the elastomeric vitrimer also exhibits multi-shape memory behavior which may further broaden its applications.     

Reprocessible Epoxy Networks with Tunable Physical Properties: Synthesis,Stress Relaxation and Recyclability

In order to extend the application of epoxy vitrimer,1,4-cyclohexanedicarboxylic acid (CHDA) was used as a co-curing agent and structure modifier for sebacic acid (SA) cured diglycidyl ether of bisphenol A (DGEBA) epoxy vitrimer to tailor the mechanical properties of epoxy vitrimers with 1,5,7-triazabicylo[4.4.0]dec-5-ene (TBD) as a transesterification catalyst.The glass transition temperature (Tg) ofvitrimer increased gradually with the increase in CHDA content.Vitrimers behaved from elastomer to tough and hard plastics were successfully achieved by varying the feed ratio of CHDA to SA.Both the Young's modulus and storage modulus increased apparently with the increase in CHDA content.Stress relaxation measurement indicated that more prominent stress relaxation occurred at elevated temperatures and the stress relaxation decreased with the increase of CHDA content due to the reduced mobility of the vitrimer backbone.The vitrimers showed excellent recyclability as evidenced by the unchanged gel fraction and mechanical properties after compression molded for several times.With tunable mechanical properties,the epoxy vitrimers may find extensive potential applications.     

Recombinant cellulose crosslinking protein: a novel paper-modification biomaterial

Cellulose-binding domains have been isolated from various cellulases, and proteins, which lack hydrolytic activity. The hypothesis that a cellulose-binding domain can be used to alter surface and mechanical properties of paper was tested. Two cellulose-binding domains from Clostriium cellulovorans were fused to form a cellulose crosslinking protein (CCP). The recombinant bifunctional cellulose-binding protein was expressed in E. coli, appliedby immersion onto Whatman cellulose filter paper, and its mechanical properties were tested. The purified protein improved the treated paper's mechanical properties (tensile strength, brittleness, Young's modulus and energy to break). In addition, cellulose crosslinking protein treatment was shown to transform filter paper into a more water-repellent paper. The binding of cellulose-binding domains to cellulose under a wide range of envi-ronmentalconditions, without the need for chemical reactions, and its biodegradability make them attractive moieties for the design of a new class of paper-modification materials.p>     

Improving the mechanical properties of CNF films by NMMO partial dissolution with hot calender activation

Reinforcing of cellulose nanofibril (CNF) films by partial dissolution with N-methylmorpholine-N-oxide (NMMO) was investigated. The method investigated is composed of impregnation of CNF film with liquid solution of NMMO followed by dry heat activation. The heat activation of the impregnated film was carried out using a heated calendering nip, which enabled simultaneous heating and compression. The partial dissolution of cellulose by NMMO caused a significant increase in the transparency of CNF film due to the decrease of film porosity and increased surface smoothness. The dry strength of the reinforced film was increased from 122 up to 195 MPa. Furthermore, the wet strength of the reinforced film was up to 70% greater than the dry strength of pure CNF film. The changes in the fibrillar structure were investigated with topographical imaging (SEM and AFM) and spectroscopically using NMR and FTIR. No significant changes in the fibril structure or cellulose morphology were observed. Moreover, the treated film resisted significant water pressure, highlighting CNF film’s permanent water resistance. The partial dissolution process with NMMO was also capable of reinforcing a CNF composite film with macro scale structural elements (lyocell short-cut fibres). The strategy investigated is a robust and fast method to improve the mechanical properties of fibrillary cellulose films, allowing them utilization in applications where improved water resistance and fully cellulosic character are required properties.     

Sugar beet cellulose nanofibril-reinforced composites

Cellulose was isolated from sugar beet chips, a by-product of sugar production, by wet chemistry. Further processing of the cellulose with a high-pressure homogeniser led to the disruption of cell walls into nanofibrils. Cellulose sheets obtained by casting and slow evaporation of water showed higher strength and stiffness when homogenised cellulose was used compared to unhomogenised cellulose. These cellulose sheets showed significantly better mechanical performance than Kraft paper tested for reference. The addition of cellulose nanofibrils to a polyvinyl alcohol and a phenol-formaldehyde matrix, respectively, demonstrated excellent reinforcement properties. The best mechanical performance was achieved for a composite with a phenol-formaldehyde resin content of 10%, which showed a tensile strength of 127 MPa, a modulus of elasticity of 9.5 GPa, and an elongation at break of 2.9%.     

Sequential Injection Analysis Hyphenated with Other Flow Techniques: A Review

In the last two decades, sequential injection analysis (SIA) became an important automation tool in several analytical fields as it provided advantages in terms of versatility, robustness, and consumption of samples and reagents.

The noteworthy versatility of an SIA selection valve allowed its association with several units, including devices typically associated with other flow techniques, resulting in generally better analytical performance.

This review discusses the hyphenization of SIA with other flow approaches outlining its advantages and motivations, which are related mainly with sample manipulation and solutions management. The future trends and perspectives in this field are also addressed.     

High-flux,porous and homogeneous PVDF/cellulose microfiltration membranes

Low hydrophilicity of membranes is probably the biggest concern in membrane filtration since it increases the costs for water treatment. Conversely, application of hydrophilic biopolymers (such as cellulose) is limited because of its complex and crystalline structure. Enabling the wide use of the most common biopolymer in nature is crucial to improve the performance of water treatment, especially in terms of membrane sustainability. Here, we study the effect of cellulose dissolution in the synthesis of homogeneous PVDF/cellulose membranes. Although only partial dissolution was achieved for studied samples, adding cellulose to the membranes greatly improved their water flux. Besides, the porous structure obtained after partial solvent removal indicates the water flux (and consequently the pore size) may be tailored according to the membrane production method. Therefore, the homogeneous cellulose microfiltration membranes studied here may have potential for water treatment considering their high-water flux and low complexity to produce.

     

Physical and mechanical testing of essential oil-embedded cellulose ester films

Polymer films made from cellulose esters are useful for embedding plant essential oils, either for food packaging or air freshener applications. Studies and testing were done on the physical and mechanical properties of cellulose ester-based films incorporating essential oils (EO) from lemongrass (Cybopogon citratus), rosemary pepper (Lippia sidoides) and basil (Ocimum gratissimum) at concentrations of 10 and 20% (v/w). Results obtained showed that, in all films, the addition of the essential oil caused a decrease in the water vapor permeability due to the hydrophobic nature of the oil. The use of 20% of EO caused lower transparency of the films, although the change was not observed visually. Mechanical testing was done on cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate. It was found that incorporation of lemongrass, basil and rosemary pepper EO significantly affected the Young's modulus, tensile strength and elongation at break of the cellulose ester films. The results suggested that the essential oils interacted with the polymers like plasticizers. The results were confirmed with thermal and microscopic studies.     

基于二维压电材料功能性器件的设计、构筑与性能研究

二维压电材料由于具有机械强度高、 性质多样、 柔性透明等特点, 吸引了广大科研人员的研究兴趣. 基于二维压电材料的柔性电子器件、 纳米传感器以及光电子器件等功能性器件也展现出了良好的性能和应用前景. 对此类器件的构筑和应用需要系统的设计和性能研究. 本综述围绕功能性的二维压电器件, 系统地论述了压电效应在(光)电子器件中的性能调控机制, 并总结其设计和制备流程以及如何实现多种功能性应用, 以期对此类器件的设计和研究提供参考.     

Electrically conductive transparent papers using multiwalled carbon nanotubes

We describe a novel class of electrically conductive transparent materials based on multiwalled carbon nanotubes (MWCNTs). Transparent nanocomposites were fabricated by incorporating an aqueous silk fibroin solution into bacterial cellulose membranes. The transparent nanocomposites had a high transmittance in the visible and infrared regions, regardless of the bacterial cellulose fiber content, due to the nanosize effect of the bacterial cellulose nanofibrils. This phenomenon allowed the preparation of a novel electrically conductive transparent paper. The high dispersity of the MWCNTs was realized by utilizing a bacterial cellulose membrane as a template to deposit them uniformly, thereby achieving electrically conductive transparent papers with outstanding optical transparency. The light transmittance and electrical conductivity varied according to the concentration of the MWCNT dispersion. Good optimal transparency and electrical properties were obtained with a light transmittance of 70.3% at 550 nm and electrical conductivity of 2.1 × 10^?3 S/cm when the electrically conductive transparent paper was fabricated from a 0.02 wt % aqueous MWCNT dispersion. In addition, the electrically conductive transparent papers showed remarkable flexibility without any loss of their initial properties. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1235–1242, 2008     

Layer-by-layer assembly of cationic guar gum,cellulose nanocrystals and hydroxypropyl methylcellulose based multilayered composite films

Eco-friendly sustainable materials provide an appealing template to replace contemporary synthetic-nonrenewable resource-based materials while maintaining the acceptable material properties to meet the performance requirements. Here, a layer-by-layer (LBL) self-assembly technique was used for fabricating multilayer composite films using all bio-based polymers/polysaccharides, i.e. cationic guar gum (CGg), carboxylated cellulose nanocrystals (cCNCs) and hydroxypropyl methylcellulose (HPMC). A five layered composite film was fabricated by depositing polymeric layers as follows: CGg→cCNCs→HPMC→cCNCs→CGg. The structural analysis of (CGg/cCNCs/HPMC)_5 L multilayered composite films indicated the existence of electrostatic interaction as well as H-bonding between polymeric layers that resulted in homogenous, dense and compact film surface with improved smoothness and strength properties. As compared to pure CGg film, the (CGg/cCNCs/HPMC)_5 L multilayered composite films showed improved tensile strength (84.8?% increment) and modulus (29.19?% improvement). Importantly, the deposition of HPMC layer contributed in achieving multilayer composite films with more flexible behavior (46.55?% improvement in elongation at break). Furthermore, owing to the high transparency (89.5?% transmittance), appreciable gas and oil barrier performance and resistance to various solvents (e.g. acetone, THF and DMAc), these multilayer films are promising candidates for various applications including renewable/sustainable packaging materials.