Elastic properties of thin poly(vinyl alcohol)-cellulose nanocrystal membranes

In spite of extensive studies on the preparation and characterization of nanocomposite materials, the correlation of their properties at the nanoscale with those in bulk is a relatively unexplored area. This is of great importance, especially for materials with potential biomedical applications, where surface properties are as important in determining their applicability as bulk characteristics. In this study, the nanomechanical characteristics of thin poly(vinyl alcohol) (PVOH)-poly(acrylic acid) (PAA)-cellulose nanocrystal (CNC) membranes were studied using the nanoindentation module in an atomic force microscope (AFM) and the properties were compared with the macro-scale properties obtained by tensile tests. In general, the elastic properties measured by nanoindentation followed the same trend as macro-scale tensile tests except for the PVOH 85-PAA 0-CNC 15 sample. In comparison to the macro-scale elastic properties, the measured elastic moduli with AFM were higher. Macro-scale tensile test results indicated that, in the presence of PAA, incorporation of CNCs up to 20?wt% improved the elastic modulus of PVOH, but when no PAA was added, increasing the CNC content above 10?wt% resulted in their agglomeration and degradation in mechanical properties of PVOH. The discrepancy between macro-scale tensile tests and nanoindentation in the PVOH 85-PAA 0-CNC 15 sample was correlated to the high degree of inhomogeneity of CNC dispersion in the matrix. It was found that the composites reinforced with cellulose nanocrystals had smaller indentation imprints and the pile-up effect increased with the increase of cellulose nanocrystal content.     

Spatially resolved characterization of cellulose nanocrystal-polypropylene composite by confocal Raman microscopy

Raman spectroscopy was used to analyze cellulose nanocrystal (CNC) -polypropylene (PP) composites and to investigate the spatial distribution of CNCs in extruded composite filaments. Three composites were made from two forms of nanocellulose (CNCs from wood pulp and the nano-scale fraction of microcrystalline cellulose) and two of the three composites investigated used maleated PP as a coupling agent. Raman maps, based on cellulose and PP bands at 1098 and 1460 cm(-1), respectively, obtained at 1 μm spatial resolution showed that the CNCs were aggregated to various degrees in the PP matrix. Of the three composites analyzed, two showed clear existence of phase-separated regions: Raman images with strong PP and absent/weak cellulose or vice versa. For the third composite, the situation was slightly improved but a clear transition interface between the PP-abundant and CNC-abundant regions was observed, indicating that the CNC remained poorly dispersed. The spectroscopic approach to investigating spatial distribution of the composite components was helpful in evaluating CNC dispersion in the composite at the microscopic level, which helped explain the relatively modest reinforcement of PP by the CNCs.     

Review of the mechanical properties of carbon nanofiber/polymer composites

In this paper, the mechanical properties of vapor grown carbon nanofiber (VGCNF)/polymer composites are reviewed. The paper starts with the structural and intrinsic mechanical properties of VGCNFs. Then the major factors (filler dispersion and distribution, filler aspect ratio, adhesion and interface between filler and polymer matrix) affecting the mechanical properties of VGCNF/polymer composites are presented. After that, VGCNF/polymer composite mechanical properties are discussed in terms of nanofibers dispersion and alignment, adhesion between the nanofiber and polymer matrix, and other factors. The influence of processing methods and processing conditions on the properties of VGCNF/polymer composite is also considered. At the end, the possible future challenges for VGCNF and VGCNF/polymer composites are highlighted.     

Polyoxymethylene composites with natural and cellulose fibres: Toughness and heat deflection temperature

Cellulose and abaca fibre reinforced polyoxymethylene (POM) composites were fabricated using an extrusion coating (double screw) compounding followed by injection moulding. The long cellulose or abaca fibres were dried online with an infrared dryer and impregnated fibre in matrix material by using a special extrusion die. The fibre loading in composites was 30 wt.%. The tensile properties, flexural properties, Charpy impact strength, falling weight impact strength, heat deflection temperature and dynamic mechanical properties were investigated for those composites. The fibre pull-outs, fibre matrix adhesion and cracks in composites were investigated by using scanning electron microscopy. It was observed that the tensile strength of composites was found to reduce by 18% for abaca fibre and increase by 90% for cellulose fibre in comparison to control POM. The flexural strength of composites was found to increase by 39% for abaca fibre and by 144% for cellulose fibre. Due to addition of abaca or cellulose fibre both modulus properties were found to increase 2-fold. The notched Charpy impact strength of cellulose fibre composites was 6-fold higher than that of control POM. The maximum impact resistance force was shorted out for cellulose fibre composites. The heat deflection temperature of abaca and cellulose fibre composites was observed to be 50 °C and 63 °C higher than for control POM respectively.     

Cross-linked nanocomposite hydrogels based on cellulose nanocrystals and PVA: Mechanical properties and creep recovery

Cellulose nanocrystal (CNC) reinforced poly(vinyl alcohol) (PVA) hydrogels with a water content of ∼92% were successfully prepared with glutaraldehyde (GA) as a cross-linker. The effects of the CNC content on the thermal stability, swelling ratio and mechanical and viscoelastic properties of the cross-linked hydrogels were investigated. The compressive strength at 60% strain for the hydrogels with 1 wt% CNCs increased by 303%, from 17.5 kPa to 53 kPa. The creep results showed that the addition of CNCs decreased the creep elasticity due to molecular chain restriction. The almost complete strain recovery (∼97%) after fixed load removal for 15 min was observed from the hydrogels with CNCs, compared with 92% strain recovery of the neat cross-linked PVA hydrogels. The incorporation of CNCs did not affect the swelling ratio and thermal stability of the hydrogels. These results suggest the cross-linked CNC-PVA hydrogels have potential for use in biomedical and tissue engineering applications.     

Effect of surface etching on interphase and elastic properties of a biocomposite reinforced using glass–silica particles

Biopolymer based composites are designed using glass–silica reinforcement. Surface etching of spherical glass–silica particles is performed using chemical and physical treatments. In particular, treatment with hydrofluoric acid proved to be efficient to achieve acceptable anchoring effect. Experimental testing of thermomoulded composites confirms that samples with chemically modified microbeads have improved mechanical properties, irrespective of phase content. A quantitative evaluation of the improvement of the starch/glass–silica interphase properties is achieved using a finite element model. Generation of typical microstructures is used to simulate phase arrangement and interphase properties. Microstructures are meshed taking into account the interphase region. Finite element results indicate that for all samples, interphase Young’s modulus is lower than those of the intrinsic materials. The thickness weighted interface modulus increases for composites where the mechanical adhesion is improved using HF chemical treatment.     

Adhesive surface interactions of cellulose nanocrystals from different sources

Adhesion plays an important role in the final properties of nanocomposites. This study explored the surface interaction of cellulose nanocrystals (CNCs) and the effect of CNC sources on adhesion between individual CNCs and the Si tip of an AFM cantilever using a force mapping technique called FMap. The adhesion between CNCs and a Si tip from five different sources has been studied: cotton, Whatman filter paper, hemp, softwood chemical kraft pulp, and softwood-dissolving pulp (alistaple). Mica was used as the background substrate to act as an internal standard. This study’s findings suggest that adhesion is not the same for all CNCs. Transmission electron microscopy and atomic force microscopy were used to determine the size and shape of each CNC. The experimental quantitative data showed that adhesion between CNCs and the Si tip has a close correlation with the diameter of the CNCs. X-ray photoelectron spectroscopy confirmed the presence of sulfate groups on the surface of the CNCs and a correlation between adhesion and surface chemistry of the CNCs was observed.     

聚乳酸/酯化纤维素纳米晶体共混膜性能研究

以聚乳酸(PLA)为基体,酯化纤维素纳米晶体(ECNC)为添加剂,制备了PLA/ ECNC共混膜。探讨了原始纤维素纳米晶体(CNC)与ECNC对PLA膜的透光率、表面形貌、热稳定性、亲疏水性及力学性能的影响。结果表明,与CNC相比,ECNC与PLA的相容性提高,透光率、热稳定性及力学性能也显著增强;经酯化的纤维素纳米晶体能降低CNC的亲水性,从而增强与PLA的界面黏合力,使CNC在PLA共混膜中的质量分数由小于1%提高到5%。该PLA/ECNC共混膜在包装塑料领域具有潜力,为制备出性能更加优良的可降解包装用塑料提供了一种简单可行的方法。     

Effect of fiber loading on the properties of treated cellulose fiber-reinforced phenolic composites

The effect of fiber loading on the properties of treated cellulose fiber-reinforced phenolic composites was evaluated. Alkali treatment of the fibers and reaction with organosilanes as coupling agents were applied to improve fiber–matrix adhesion. Fiber loadings of 1, 3, 5, and 7 wt% were incorporated to the phenolic matrix and tensile, flexural, morphological and thermal properties of the resulting composites were studied. In general, mechanical properties of the composites showed a maximum at 3% of fiber loading and a uniform distribution of the fibers in such composites was observed. Silane treatment of the fibers provided derived composites with the best thermal and mechanical properties. Meanwhile, NaOH treatment improved thermal and flexural properties, but reduced tensile properties of the materials. Therefore, the phenolic composite containing 3% of silane treated cellulose fiber was selected as the material with optimal properties.     

纤维素纳米晶/环氧树脂复合材料的制备及性能研究

以过硫酸铵氧化微晶纤维素得到纤维素纳米晶(CNC),与二乙烯三胺在N,N-二甲基甲酰胺(DFM)中发生缩合接枝反应,制备胺化纤维素纳米晶(ACNC)。采用溶液共混法,分别将CNC和ACNC与环氧树脂复合得到纤维素纳米晶/环氧树脂复合膜,纤维素纳米晶不仅起到增强剂的作用,还起到固化交联剂的作用,进而改善环氧树脂的性能。利用万能力学试验机、动态热机械性能、环境扫描电子显微镜、热重分析等对复合材料的性能加以表征分析。结果证明,当CNC和ACNC的添加量均为0.1%时,环氧树脂复合膜的机械强度最大;纤维素纳米晶的加入不仅能够提高环氧树脂的力学性能,还能显著改善其柔韧性,ACNC对环氧树脂复合膜的增强作用高于CNC,CNC的增韧作用强于ACNC。     

Reinforcing potential of micro- and nano-sized fibers in the starch-based biocomposites

Starch-based biocomposites reinforced with jute (micro-sized fiber) and bacterial cellulose (BC) (nano-sized fiber) were prepared by film casting. Reinforcement in the composites is essentially influenced by fiber nature, and amount of loading. The optimum amount of fiber loading for jute and bacterial cellulose in each composite system are 60 wt% and 50 wt% (of starch weight), respectively. Mechanical properties are largely improved due to the strong hydrogen interaction between the starch matrix and cellulose fiber together with good fiber dispersion and impregnation in these composites revealed by SEM. The composites reinforced with 40 wt% or higher bacterial cellulose contents have markedly superior mechanical properties than those reinforced with jute. Young’s modulus and tensile strength of the optimum 50 wt% bacterial cellulose reinforced composite averaged 2.6 GPa and 58 MPa, respectively. These values are 106-fold and 20-fold more than the pure starch/glycerol film. DMTA revealed that the presence of bacterial cellulose (with optimum loading) significantly enhanced the storage modulus and glass transition temperature of the composite, with a 35 °C increment. Thermal degradation of the bacterial cellulose component occurred at higher temperatures implying improved thermal stability. The composites reinforced with bacterial cellulose also had much better water resistance than those associated with jute. In addition, even at high fiber loading, the composites reinforced by bacterial cellulose clearly retain an exceptional level of optical transparency owing to the effect of the nano-sized fibers and also good interfacial bonding between the matrix and bacterial cellulose.     

Mechanical properties of cellulose nanofiber (CNF) reinforced polylactic acid (PLA) prepared by twin screw extrusion

The aim of this study was to develop cellulose nanofiber (CNF) reinforced polylactic acid (PLA) by twin screw extrusion. Nanocomposites were prepared by premixing a master batch with high concentration of CNFs in PLA and diluting to final concentrations (1, 3, 5 wt.%) during the extrusion. Morphology, mechanical and dynamic mechanical properties (DMA) were studied theoretically and experimentally to see how different CNF concentrations affected the composites’ properties. The tensile modulus and strength increased from 2.9 GPa to 3.6 GPa and from 58 MPa to 71 MPa, respectively, for nanocomposites with 5 wt.% CNF. The DMA results were also positive; the storage modulus increased for all nanocomposites compared to PLA; being more significant in the high temperature region (70 °C). The addition of nanofibers shifted the tan delta peak towards higher temperatures. The tan delta peak of the PLA shifted from 70 °C to 76 °C for composites with 5 wt.% CNF.     

Ductile unidirectional continuous rayon fibre-reinforced hierarchical composites

Endless rayon fibres (Cordenka®) were used to reinforce polyhydroxybutyrate (PHB) nanocomposites containing 2.5 wt.% nanofibrillated cellulose (NFC) to create truly green hierarchical composites. Unidirectional (UD) composites with 50–55% fibre volume fraction were produced using a solvent-free continuous wet powder impregnation method. The composites exhibit ductile failure behaviour with a strain-to-failure of more than 10% albeit using a very brittle matrix. Improvements at a model composite level were translated into higher mechanical properties of UD hierarchical composites. The Young’s moduli of rayon fibre-reinforced (NFC-reinforced) PHB composites were about 15 GPa. The tensile and flexural strength of hierarchical PHB composites increased by 15% and 33% as compared to the rayon fibre-reinforced neat PHB composites. This suggests that incorporation of NFC into the PHB matrix binds the rayon fibres, which does affect the load transfer between the constituents resulting in composites with better mechanical properties.     

Biomicrofibrilar composites of high density polyethylene reinforced with curauá fibers: Mechanical,interfacial and morphological properties

Vegetal fibers are used in polymer composites to improve mechanical properties, substituting inorganic reinforcing agents produced by non renewable resources, like fiberglass. The highest performance formulation in high density polyethylene, HDPE, composites reinforced with curauá fibers were studied, aiming to improve the interphase interaction and optimize the mechanical properties. The fiber content, the type and the concentration of coupling agent were tested. The composites and the pure materials were characterized by Fourier transform infrared spectroscopy and the fiber/matrix phase adhesion was evaluated by scanning electron microscopy. The mechanical properties and the micrographs showed that the best formulation is: 20 wt.% of milled curauá fibers and 2 wt.% poly(ethylene-g-maleic anhydride). The coupled composites are also less hygroscopic than the uncoupled composites. We conclude that the composites reinforced with curauá fibers have mechanical properties comparable to commercially produced composites of HDPE reinforced with fiberglass.     

Introducing cellulose nanocrystals in sheet molding compounds (SMC)

The mechanical properties of short glass fiber/epoxy composites containing cellulose nanocrystals (CNC) made using sheet molding compound (SMC) manufacturing method as well as the rheological and thermomechanical properties of the CNC-epoxy composites were investigated as a function of the CNC content. CNC up to 1.4 wt% were dispersed in the epoxy to produce the resin for SMC production. The addition of CNC in the resin increased its viscosity and slightly reduced the heat of reaction during the polymerization without altering the curing time and temperature and the effective pot life of the resin. The incorporation of 0.9 wt% CNC in the SMC composite resulted in increases in elastic modulus and tensile strength by ∼25% and ∼30% and in flexural modulus and strength by ∼44% and ∼33% respectively. Concentrations of CNC up to 0.9 wt% in the SMC composite did not alter the impact energy.     

碳纤维/聚芳基乙炔复合材料界面特性的原子力显微镜研究

为深入研究复合材料界面特性,分析复合材料界面改性机理,采用原子力显微镜以力调制模式研究了单向碳纤维/聚芳基乙炔 (CF/PAA) 复合材料横截面的表面形貌和硬度,得到了材料中各相的形态、分布和相对硬度等信息。通过对硬度图像进行统计学分析得到材料表面硬度分布直方图,对不同方法改性后的复合材料界面相特性进行了分析和比较。研究表明,CF未经表面改性处理时,复合材料中无明显的界面相出现,氧化处理后界面上出现硬度变化区域,界面相结构变得复杂,氧化结合高碳酚醛涂层处理后可以获得更为完善的界面相,在纤维与基体间形成一个模量适中的过渡区域,使材料的界面力学性能显著提高。      

Microscopic failure mechanisms of fiber-reinforced polymer composites under transverse tension and compression

The mechanical behavior of unidirectional fiber-reinforced polymer composites subjected to tension and compression perpendicular to the fibers is studied using computational micromechanics. The representative volume element of the composite microstructure with random fiber distribution is generated, and the two dominant damage mechanisms experimentally observed – matrix plastic deformation and interfacial debonding – are included in the simulation by the extended Drucker–Prager model and cohesive zone model respectively. Progressive failure procedure for both the matrix and interface is incorporated in the simulation, and ductile criterion is used to predict the damage initiation of the matrix taking into account its sensitivity to triaxial stress state. The simulation results clearly reveal the damage process of the composites and the interactions of different damage mechanisms. It can be concluded that the tension fracture initiates as interfacial debonding and evolves as a result of interactions between interfacial debonding and matrix plastic deformation, while the compression failure is dominated by matrix plastic damage. And then the effects of interfacial properties on the damage behavior of the composites are assessed. It is found that the interfacial stiffness and fracture energy have relatively smaller influence on the mechanical behavior of composites, while the influence of interfacial strength is significant.     

Cellulose diacetate reinforced with electrospun cellulose fiber: A new route to prepare an all cellulose-based composite

An all cellulose-based composite was produced by applying cellulose diacetate (CDA) and electrospun cellulose fibers (ECFs) as a matrix and reinforcing fibers, respectively. The ECFs were prepared by electrospinning CDA followed by alkali treatment. The alkali treatment of the electrospun CDA fibers removes acetyl groups and creates new crystalline domains similar to cellulose fibers, which may result in increased mechanical properties. The physical properties of ECFs are highly correlated with the acetyl content. The mechanical properties of the CDA/ECF composites were evaluated by measuring the tensile strength and Young’s modulus as a function of the hydrolysis time. The results showed that the integration of the CDA matrix with 15% weight/weight (w/w) ECF7, which was alkali-treated for 7 h, was optimal, resulting in an improved tensile strength and Young’s modulus by factors of 1.7 and 2.2, respectively. This study demonstrates that the increased mechanical properties are attributed to the reinforcement of ECFs as well as good interfacial adhesion.     

Analysis of effective elastic modulus for multiphased hybrid composites

Short fiber reinforced composites inherently have fiber length distribution (FLD) and fiber orientation distribution (FOD), which are important factors in determining mechanical properties of the composites. Since the internal structure has a direct effect on the mechanical properties of the composites, a Micro-CT was used to observe the three dimensional structure of fibers in the composites and to acquire FLD and FOD. It was successful to investigate FLD, FOD, and fiber orientation states and to predict the elastic modulus of the hybrid system. Since hybrid composites used in this study consist of three phases of particles, glass fibers, and matrix, theoretical hybrid modeling is required to consider reinforcing effects of both particles and glass fibers. Interaction between the particles and matrix was considered by using a perturbed stress–strain theory, the Tandon–Weng model. In addition, the laminating analogy approach (LAA) was used to predict the overall elastic modulus of the composite. Theoretical prediction of hybrid moduli indicated that there was a possibility of poor adhesion between glass fibers and matrix. The poor interfacial adhesion was confirmed by morphological experiments. This theoretical and experimental platform is expected to provide more insightful understanding on any kinds of multiphased hybrid composites.     

Effect of colloidal silica on the strength and energy absorption of glass fiber/epoxy interphases

Prior research has demonstrated that fiber-sizings can be designed to yield composite materials that simultaneously possess high energy absorption and structural properties. The improved mechanical properties resulted from control of the fiber surface chemistry and nano-scale topological features within the fiber–matrix interphase. The present study further explains the role of sizing chemistry and surface roughness on composite material performance. Model and commercial glass fiber epoxy specimens were fabricated using these fiber sizing systems resulting in interphase regions with varied surface topology and chemical functionality. Micromechanical measurements were performed using the microdroplet adhesion test method to quantify the fiber–matrix interfacial properties. Improvement in energy absorption and interfacial shear strength due to the presence of the nano-scale silica were quantified. Inspection of the failure modes revealed that the existence of colloidal silica promotes crack propagation along a more tortuous path within the interphase that results in progressive failure and contributes to increased energy dissipation.