Rheological behavior of highly loaded cellulose nanocrystal/poly(vinyl alcohol) composite suspensions

Recent emphasis on the pilot scale production of cellulosic nanomaterials has increased interest in the effective use of these materials as reinforcements for polymer composites. An important, enabling step to realizing the potential of cellulosic nanomaterials in their applications is the materials processing of CNC/polymer composites through multiple routes, i.e. melt, solution, and aqueous processing methods. Therefore, the objective of this research is to characterize the viscoelastic behavior of aqueous nanocomposite suspensions containing cellulose nanocrystals (CNCs) and a water-soluble polymer, poly(vinyl alcohol) (PVA). Specifically, small amplitude oscillatory shear measurements were performed on neat PVA solutions and CNC-loaded PVA suspensions. The experimental results indicated that the methods used in this study were able to produce high-quality nanocomposite suspensions at high CNC loadings, up to 67 wt% with respect to PVA. Additionally, the structure achieved in the nanocomposite suspensions was understood through component attributes and interactions. At CNC loadings near and less than the percolation threshold, a polymer mediated CNC network was present. At loadings well above the percolation threshold, a CNC network was present, indicated by limited molecular weight dependence of the storage modulus. Overall, these results provide increased fundamental understanding of CNC/PVA suspensions that can be leveraged to develop advanced aqueous processing methods for these materials.     

Waterborne polyurethane-coated polyamide/fullerene composite films: Mechanical,thermal, and flammability properties

Novel waterborne polyurethane (WPU) was prepared and coated on nylon 12 (Ny12) and fullerene-C60 (Full-C60, 1–10 wt%) composite films using simple dip coating technique. In Ny12/Full-C60 composite, fullerene nanoparticles were dispersed in a wavy layered pattern, whereas coated WPU/Ny12/Full-C60 films depicted uniform pattern of non-overlapping scales. WPU/Ny12/Full-C60 1–10 showed higher values of tensile strength and modulus, 91.4–98.1 MPa and 52.2–57.9 GPa, respectively. In WPU/Ny12/Full-C60 1–10, maximum degradation temperature was increased to 598°C and char yield to 35%. Increasing fullerene content from 1 to 10 wt% decreased maximum peak heat release rate from 209 to 132 kW m^?2, i.e., 53% reduction in flammability compared to WPU.     

Preparation and property assessment of neat lignocellulose nanofibrils (LCNF) and their composite films

Lignocellulose nanofibrils (LCNF) were produced from thermo-mechanical pulp (TMP) using a micro-grinder and were characterized with respect to fiber diameter and thermal stability. The initial water content in the TMP affected the defibrillation process and longer grinding time was necessary for the air-dried TMP, resulting in LCNF with higher fibril diameter. As compared to the reference cellulose nanofibrils (CNF) produced through a refining process, LCNF was less thermally stable and started to degrade at a temperature that was 30 °C lower than that of CNF. LCNF obtained from the never-dried TMP was combined with various additives (10 wt%) to produce composite films. The neat LCNF and composite films did not reach the mechanical properties of the neat CNF film that was evaluated as reference. However, the addition of poly(vinyl alcohol) (PVA) at 10 wt% on a dry basis did cause a 46 and 25% increase in tensile strength and elastic modulus, respectively. Other additives including cellulose nanocrystals, bentonite and CNF were also found to increase to some extent the Young’s modulus and ductility of the LCNF composite films whereas the addition of talc did not improve the film performance. Water absorption of neat LCNF films was lower than the reference CNF and was negatively affected by the addition of PVA.     

Properties of poly(acrylamide)/TEMPO-oxidized cellulose nanofibril composite films

Self-standing composite films consisting of 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized cellulose nanofibril (TOCN) and anionic poly(acrylamide) (PAM) in various weight ratios were prepared by casting and drying of homogeneous mixtures of aqueous TOCN dispersion and PAM solution. PAM/TOCN composite films consisting of 25 % PAM and 75 % TOCN had clearly higher Young’s modulus (13.9 GPa) and tensile strength (266 MPa) than 100 % TOCN film (10.8 GPa and 223 MPa, respectively) or 100 % PAM film (4.9 GPa and 78 MPa, respectively), showing that PAM molecules have mechanical reinforcement ability in TOCN matrix. Some attractive interactions are likely formed between TOCN element surfaces and PAM molecules. In contrast, no such mechanical improvements were observed for poly(vinyl alcohol)/TOCN or oxidized starch/TOCN composite films prepared as references. Moreover, the mechanical properties of the PAM/TOCN composite films were further improved by controlling molecular mass and branching degree of the PAM. The high optical transparency and low coefficient of thermal expansion of the 100 % TOCN film were mostly maintained in the TOCN composite film containing 25 % PAM.     

Influence of drying restraint on physical and mechanical properties of nanofibrillated cellulose films

Nanofibrillated cellulose (NFC) is a renewable and biodegradable fibril that possesses high strength and stiffness resulting from high level hydrogen bonding. Films made from NFC shrink and distort as they transition from a wet state (20 wt% solids) to a state of moisture equilibrium (90 wt% solids at 50 % RH, 23 °C). Material distortions are driven by development of moisture gradients within the fibril network and effectively reduce mechanical performance. For this study, NFC was extracted from softwood holocellulose by first employing a chemical pretreatment [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl catalyzed oxidation] followed by mechanical fibrillation using ultrasound energy. To assess the problem of film distortion, neat NFC films were dried at 50 % RH, 23 °C under one of the following three restraint conditions: fully restrained, partially restrained, and uniaxially drawn. The influence of restraint condition on the resulting physical and mechanical properties was evaluated. Raman and X-ray results showed that fibrils in the uniaxially drawn specimens tended to align with the drawing axis, whereas no in-plane orientation effects were observed for the fully or partially restrained specimens. Fully restrained specimens had a respective strength and stiffness of 222 MPa and 14 GPa in every (in-plane) direction. However, samples that were wet-drawn to a 30 % strain level had a respective strength and stiffness of 474 MPa and 46 GPa in the direction of draw. Mechanical properties for axially drawn specimens had both fibril alignment and fibril straightening contributions.     

Development of continuous process enabling nanofibrillation of pulp and melt compounding

Microfibrillated cellulose (MFC), a mechanically fibrillated pulp mostly consisting of nanofibrils, is a very attractive material because of its high elastic modulus and strength. Although much research has been done on composites of MFC and polypropylene (PP), it has been difficult to produce such composites at an industrial level because of the difficulties in using MFC in such composites are not only connected to the polarity (that can be improved with compatibilizers), but also with the challenge to make a homogeneous blend of the components, and also the low temperature stability of cellulose that could cause problems during processing. We developed a new processing method which enables continuous microfibrillation of pulp and its melt compounding with PP. Never-dried kraft pulp and powdered PP were used as raw materials to obtain MFC by kneading via a twin-screw extruder. Scanning electron microscopy showed nano to submicron wide fibers entangled in the powdered PP. MFC did not aggregate during the melt compounding process, during which the water content was evaporated. Maleic anhydride polypropylene (MAPP) was used as a compatibilizer to reinforce interfacial adhesion between the polar hydroxyl groups of MFC and non-polar PP. We investigated the effect of MAPP content on the mechanical properties of the composite, which were drastically improved by MAPP addition. Needle-leaf unbleached kraft pulp (NUKP)-derived MFC composites had better mechanical properties than needle-leaf bleached kraft pulp (NBKP)-derived MFC composites. Injection molded NUKP-derived MFC composites had good mechanical and thermal properties. The tensile modulus of 50 wt% MFC composite was two times, and the tensile strength 1.5 times higher than that of neat PP. The heat distortion temperature of 50 wt% MFC content composite under 1.82 MPa flexural load was increased by 53 °C, from 69 to 122 °C. This newly developed continuous process using powder resin has the potential for application at an industrial level.     

Resin impregnation of cellulose nanofibril films facilitated by water swelling

Flexible composite films were produced by impregnating aqueous phenol formaldehyde (PF) resin into water-swollen cellulose nanofibril (CNF) films. CNF films were prepared using a pressurized filtration method in combination with freeze drying. The freeze-dried films were swollen with water then impregnated with PF resin by soaking in aqueous resin solutions of varying concentrations. Small amounts of PF slightly enhanced the tensile properties of CNF films. The formulation with the best mechanical properties was CNF/PF films with 8 wt % resin exhibiting tensile stress and toughness of 248 MPa and 26 MJ/m³, respectively. Resin concentrations higher than about 8 % resulted in composites with decreased tensile properties as compared to neat CNF films. The wet strength of the composite films was significantly higher than that of the neat CNF films. The resulting composites showed greater resistance to moisture absorption accompanied by reduced thickness swelling when soaked in water as compared to neat CNF films. The composites also showed decreased oxygen permeability at low humidity compared to neat films, but the composites did not show improved barrier properties at high humidity.     

Preparation and characterization of chemically bonded aramid-titania hybrids using isocyanatopropyltrimethoxysilane

Transparent aramid based titania hybrid films have been prepared by the sol–gel process. A mixture of m- and p-phenylenediamines was reacted with terephthaloyl chloride forming aromatic polyamide chains in dimethylacetamide solvent. The titania network was generated insitu in this matrix by the hydrolysis and condensation of the various amounts of tetraethylorthotitanate. Hybrid films with concentrations of titania varying from 2.5 to 12.5 wt% were prepared; the higher percentages of titania in the organic matrix showed a tendency towards phase separation. These films were tested for their thermo-mechanical properties. To achieve a further improvement in properties of the matrix, the aramid chain was functionalized and the inorganic network was chemically bonded using isocyanatopropyltrimethoxysilane. The bonded hybrids showed a narrower distribution of titania particles and these were distributed as a co-continuous phase. The glass transition temperature (Tg) of the hybrid films measured through dynamic mechanical analysis showed a relatively higher increase with inclusion of titania in the covalently bonded hybrids. The maximum value of Tg noted in the chemically bonded composites with 12.5 wt% titania was 361 °C and the storage modulus value was 5.214 GPa at 100 °C, showing an increase of 62 % over the pure polymer. The hybrid films with titania showed an improved UV-stability as compared to the pure polymer.     

Shear rheology of micro-fibrillar cellulose aqueous suspensions

Micro-fibrillar cellulose aqueous suspensions with different fiber lengths were prepared by mechanical refining of softwood pulp fiber suspensions at different specific refining energies. Effects of refining energy level, micro-fiber concentration and temperature on the rheological properties of these aqueous suspensions were studied. These microfibers form a three-dimensional network, which displays typical shear-thinning behavior with little thixotropic tendency, at concentrations as low as 0.5 wt%. A viscoelastic analysis showed that these micro-fibrillar cellulose suspensions at different concentrations (from 0.5 to 2 wt%) exhibit a viscoelastic gel-like behavior [G′ > G″ over an extended range of frequencies (ω) and a weak dependency of G′ on ω] at 25 °C. The storage modulus, G′, at 1 rad/s increased strongly upon increasing concentration from 0.5 to 2 wt% following a power law with an exponent of 3.2. However, increasing the temperature decreases the storage modulus, G′, due to weakening or disruption of intermolecular interactions at elevated temperatures. The viscoelastic behavior changes to liquid-like, with G″ > G′ at the investigated frequency range, for the suspensions at 85 °C.     

Magnetic soft silicone elastomers with tunable mechanical properties for magnetically actuated devices

Polydimethylsiloxane (PDMS)/iron oxide magnetic nanoparticle (NP) composites with tailored mechanical properties are prepared for use in magnetically actuated soft devices based on their controlled deformation by the application of an external magnetic field. This investigation reports the synthesis and functionalization of iron oxide NPs, the preparation of the PDMS/NP composites, the evaluation of NP dispersion using scanning electron microscopy (SEM) and optical microscopy, and the mechanical characterization of the composite films. Characterization includes rheological measurements as well as stress‐strain curves to obtain the Young modulus and elongation at break. SEM is used to probe individual NP dispersion, whereas optical microscopy provides rapid access to quantitative information about the size and distribution of particle aggregates. Results for nonfunctionalized (nf), oleic acid (OA)‐coated, and stearic acid (SA)‐coated iron oxide NPs and their blends are presented. PDMS elastomers containing both OA‐ and SA‐coated iron oxide NPs are found to have very low Young moduli with substantially higher resistance to failure than neat PDMS. For example, a formulation containing 2.5 wt% OA‐coated NPs and 2.5 wt% SA‐coated iron oxide NPs has a modulus of 0.15 MPa (compared with 0.24 MPa for neat PDMS), while it can withstand an elongation of about 1.5 times its initial length compared with only 0.3 times for neat PDMS. As a comparison, the modulus of the most commonly used commercial PDMS elastomer (Sylgard 184) is an order of magnitude higher than that of the composites prepared here, whereas maximum elongation is similar for the two. The formulations developed in this work could be used in applications where high deformability is required with limited magnetic field strength and/or NP loading.     

Preparation and Characterization of Polypropylene Carbonate Bio-Filler (Eggshell Powder) Composite Films

In recent years, biodegradable polymer composites have attracted considerable attention due to inadequate and depleting petroleum resources and to replace nonbiodegradable synthetic polymers posing environment problems. In the present work, biodegradable composites based on polypropylene carbonate (PPC)/eggshell powder (ESP) were prepared by the solution-casting method using chloroform as the solvent. Polypropylene carbonate was loaded with 1 to 5 wt% of eggshell powder (particle size < 40 µm). Characterization of the composites was accomplished by Fourier transform-infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), and X-ray diffraction (XRD) analysis, and morphological studies were carried out by optical microscopy (OM) and scanning electronic microcopy (SEM). The tensile properties of the composite films were found to be higher than those of neat PPC matrix and increased with ESP content up to 4 wt.% and then decreased. This work offers an easy path to manufacture ecofriendly PPC/eggshell powder composite films with improved properties, and reducing, in some cases, the demand for petroleum-based plastics such as polyolefins.     

Mechanical and optical properties of SiO<Subscript>2</Subscript> thin films deposited on glass

The optical and mechanical properties of amorphous SiO₂ films deposited on soda-lime silicate float glass by reactive RF magnetron sputtering at room temperature were investigated in dependence of the process pressure. The densities of the films are strongly influenced by the process pressure and vary between 2.38 and 1.91 g cm^?3 as the pressure changes from 0.27 to 1.33 Pa. The refractive indices of the films shift between 1.52 and 1.37, while the residual compressive stresses in the deposited films vary in the range from 440 to 1 MPa. Hardness and reduced elastic modulus values follow the same trend and decline with the increase of process pressure from 8.5 to 2.2 GPa and from 73.7 to 30.9 GPa, respectively. The abrasive wear resistance decreases with the density of the films.     

Liquid crystalline thermosets based on anisotropic phases of cellulose nanocrystals

A new class of liquid crystalline thermosets (LCTs) was successfully produced containing lyotropic cellulose nanocrystals (CNCs) as the primary mesogenic component (up to 72 wt%) by the addition of non-mesogenic epoxy monomers. Cellulose-based LCTs were produced by totally aqueous processing methods and ultimately cured at elevated temperatures to produce ordered networks of ‘frozen’ liquid crystalline (LC) phases. Various degrees of birefringence were obtained via self-assembly of CNCs into oriented phases as observed by polarized optical microscopy and transmission electron microscopy. X-ray diffraction measurements highlighted the effects of texture of CNCs within LCT films compared to lyophilized CNCs. Cellulose-based LCT films uniquely exhibited thermo-mechanical properties of both traditional LCTs and LC elastomers, such as high elastic modulus (~1 GPa) under ambient conditions and low glass transition temperature (~?25 °C), respectively. The development of LCTs based on CNCs and aqueous processing methods provides a renewable pathway for designing high performance composites with ordered network structures and unique optical properties.     

Interplay of colloidal stability of cellulose nanocrystals and their dispersibility in cellulose acetate butyrate matrix

Cellulose nanocrystals (CNC) prepared from eucalyptus cellulose CNCs were modified by the reaction with methyl adipoyl chloride, CNCm, or with a mixture of acetic and sulfuric acid, CNCa. The CNC were either dispersed at 0.1 wt% in the pure solvents ethyl acetate (EA), tetrahydrofuran (THF) and dimethylformamide (DMF) or in cellulose acetate butyrate (CAB) solutions prepared in these solvents at 0.9 wt%. The colloidal behavior of these dispersions was systematically investigated using a phase separation analyzer LUMiReader^®. The mechanical properties and morphological features of the films resulting from the mixtures of CAB and CNC were determined by dynamic mechanical analysis, optical microscopy and atomic force microscopy, respectively. Regardless the functional group attached to the surface of CNC, the best colloidal stability was observed for dispersions prepared in CAB/DMF solution. Higher degree of substitution of modified CNCs favored the colloidal stability in EA and THF. Composite films prepared from CAB/DMF solutions were more homogeneous and presented better mechanical performance than those prepared in CAB/EA or CAB/THF. The mechanical performance of composites and neat CAB prepared from DMF was CAB/CNCs > CAB/CNCm > CAB/CNCa > CAB, indicating that the modification weakens the percolation process, which is mediated by H bonding.     

Electrical and mechanical properties of the zone-drawn polypyrrole films

The zone-drawing method (ZD) was applied to electrochemically synthesized polypyrrole films containing tosylate (PPy/TsO) and the mechanical and electrical properties of the resulting films were investigated. It was found that the electrical conductivity of the zone-drawn film reached 365 S cm⁻¹ in the drawing direction, which was 4.7 times that of the original film. The tensile properties of the zone-drawn film were improved and Young's modulus and strength at break increased to 4.32 GPa and 90.1 MPa from 0.53 GPa and 40.4 MPa of the as-synthesized film, respectively. The dynamic storage modulus (E) increased by the zone-drawing over a whole experimental temperature range and attained 7.0 GPa at room temperature and 4.0 GPa even at 200°C. © 1996 John Wiley & Sons, Inc.     

Effect of preparation process of microfibrillated cellulose-reinforced polypropylene upon dispersion and mechanical properties

Microfibrillated cellulose (MFC)-reinforced polypropylene (PP) was prepared via two engineering approaches: disintegration of the pulp by a bead mill followed by a melt-compounding process with PP (B-MFC-reinforced PP); and disintegration of the pulp mixed with PP by a twin screw extruder followed by a melt-compounding process (T-MFC-reinforced PP). The effects that the engineering process and the microfibrillation of the pulp had upon the dispersion and mechanical properties were investigated through tensile tests, rheological analysis and X-ray computed tomography. The bead-milling method enabled a uniform microfibrillation of the pulp to under 100 nm, which corresponded to a surface area of 133–146 m²/g for the pulp, found by the Brunauer–Emmett–Teller (BET) analysis. The T-MFC-reinforced PP with 30 wt% MFC content exhibited a tensile modulus of 5.3 GPa and a strength of 85 MPa, whereas the B-MFC-reinforced PP composites with the same content of MFC exhibited values of 4.1 GPa and 59.6 MPa, respectively. Rheological analysis revealed that the complex viscosity and storage modulus at 170 °C of T-MFC-reinforced PP with 30 wt% MFC content are 5–7 and 5–8 times higher than that of B-MFC-reinforced PP, respectively. This indicated that T-MFC was more dispersed in the PP than B-MFC. Therefore, T-MFC produced a more rigid interconnected network in the matrix during the melting state than B-MFC.     

Enhanced toughness and strength of conductive cellulose-poly(butylene succinate) films filled with multiwalled carbon nanotubes

Cellulose (CE) composite films with high tensile strength, modulus, remarkable elongation as well as excellent electrical conductivity were successfully prepared by dispersing poly(butylene succinate) (PBS) and multiwalled carbon nanotubes (MWCNTs) in CE matrix via the help of ionic liquid 1-allyl-3-methylimidazolium chloride. Fourier transform infrared spectroscopy and differential scanning calorimetry results verified that a physical interaction junction existed between PBS and CE. Scanning electron micrograph (SEM) showed that the low content PBS was uniformly dispersed in CE matrix, leading to a tough and ductile fractured surface. The elongation at break of CE composite film with 1 wt% PBS was increased to 25.9 %, which showed an increase of 325 % compared to that of neat CE film (6.07 %). But high-content PBS acted as the structural defect in the CE matrix. MWCNTs were further added to improve the mechanical and conductive properties of the composite film. The tensile strength and Young’s modulus of MWCNT/CE-PBS composite film with 4 wt% MWCNTs were respectively increased by 33.6 and 140 % compared to CE-PBS film. The electrical conductivity of MWCNT/CE-PBS film was also improved by 8–9 orders of magnitude from 2.5 × 10^?14 to 1.3 × 10^?5 S/m.     

Preparation and properties of 3-aminopropyltriethoxysilane functionalized graphene/polyurethane nanocomposite coatings

Silicone-modified graphene was successfully synthesized by treating graphene oxide with 3-aminopropyltriethoxysilane (AMEO) and then reduced by hydrazine hydrate. Subsequently, the AMEO-functionalized graphene was incorporated into polyurethane (PU) matrix to prepare AMEO-functionalized graphene/PU nanocomposite coatings. The functionalized graphene could disperse homogenously by means of a covalent connection with PU. AMEO-functionalized graphene (AFG)-reinforced PU nanocomposite coatings showed more excellent mechanical and thermal properties than those of pure PU. A 227 % increase in tensile strength and a 71.7 % improvement of elongation at break were obtained by addition 0.2 wt% of AFG. Meanwhile, thermogravimetric analysis reveals that thermal degradation temperature was enhanced almost 50 °C higher than that of neat PU, and differential scanning calorimetry analysis demonstrates that glass transition temperature decreased by around 9 °C. The thermal conductivity of AFG/PU nanocomposite coatings also increased by 40 % at low AFG loadings of 0.2 wt%.     

Novel high-strength,micro fibrillated cellulose-reinforced polypropylene composites using a cationic polymer as compatibilizer

Novel high-strength, micro-fibrillated cellulose (MFC)-reinforced polypropylene (PP) composites were prepared using maleic anhydride polypropylene (MAPP) and a cationic polymer having a primary amino group (CPPA) as coupling agents. Un-dried kraft pulp was micro-fibrillated into nano- to submicron-wide fibrils by kneading with powdered PP and the coupling agents via a twin-screw extruder. The composites were prepared by injection molding. The MFC-reinforced PP composites containing both coupling agents MAPP and CPPA (combination system) showed extremely high mechanical strength compared with the MFC-reinforced composite containing only MAPP. The tensile strength of a 30 wt% MFC-PP composite containing the combination system was 27 % higher than that of the composite containing only MAPP, and more than 60 % higher than that of neat PP. In addition, the heat distortion temperature, under a 1.82-MPa flexural load, of the composite with the combination system was 17 °C higher than that of the composite with MAPP only, and 34 °C higher than that of neat PP. The anisotropy of the modulus and strength in the injection-molded MFC composites was lower than that of glass fiber-reinforced PP.     

Cationic wood cellulose films with high strength and bacterial anti-adhesive properties

In this work, periodate oxidized birch wood pulp and microfibrillated cellulose (MFC) were cationized using Girard’s reagent T or aminoguanidine. Cationic celluloses were used to obtain films via solvent-casting method, and the effects of the cationization route and the cellulose fiber source on the properties of the films were studied. Thermal and optical properties of the films were measured using differential scanning calorimetry and UV–Vis spectrometry, and the morphology of the films was examined using an optical microscope and a field emission scanning electron microscope. Bacterial anti-adhesive properties of the films were also studied using a modified leaf print method and against Staphylococcus aureus and Escherichia coli. Both cationizing agents exhibited similar reactivity with periodate oxidized celluloses, however, MFC had significantly higher reactivity compared to birch pulp. The films with high tensile strength (39.1–45.3 MPa) and modulus (3.5–7.3 GPa) were obtained from cationized birch pulp, aminoguanidine modification producing a film with slightly better mechanical properties. Modulus of the films was significantly increased (up to 14.0 GPa) when MFC was used as a cellulose fiber source. Compared to the unmodified MFC films, the cationic MFC films were less porous and significantly more transparent; however, they had slightly lower tensile strength values. It was found that aminoguanidine modified celluloses had no culturable bacteria on its surface and also exhibited resistance to microbial degradation, whereas there were culturable bacteria on the surface of Girard’s reagent modified films and they were partially degraded by the bacteria.