Effect of processing techniques and residual solvent on the thermal/mechanical properties of epoxy-cellulose nanocrystal nanocomposites

Epoxy nanocomposites reinforced with 0–5 wt% cellulose nanocrystal (CNC) were fabricated by solution casting method. The CNCs were first dispersed in the hardener together with a co-solvent and subsequently mixed with epoxy monomer using two different protocols: bulk mixing and dropwise mixing. The dropwise mixing resulted in more homogeneously-dispersed CNCs and provided better mechanical properties, particularly higher tensile strength, tensile modulus, and work of fracture with increasing CNC content. Investigation of various CNC/hardener mixtures with different solvent content showed that the residual solvent in the composite mixture had an impact on the curing behavior of the nanocomposite epoxy and may have lowered its crosslinking density, leading to T_g depression. However, the presence of the solvent was shown to be essential for the preparation of a well-dispersed CNC phase in the hardener solution, and to form homogenous composite mixtures. Our results show that the reinforcement effect due to higher CNC concentration is more significant than the solvent effect in the hardener system and shows greater improvement in mechanical properties. The CNC component reversed the solvent plasticizing effect through its superior mechanical reinforcing effects, and the dropwise mixing process led to better dispersion compared with the bulk mixing process.     

Crosslinked poly(vinyl alcohol) composite films with cellulose nanocrystals: Mechanical and thermal properties

In this work, poly(vinyl alcohol) (PVA) and cellulose nanocrystals (CNCs) were crosslinked using sodium tetraborate decahydrate (borax) to improve the mechanical and thermal properties of the neat PVA. The results showed that the CNCs affected the crystallization behavior of the crosslinked PVA. The crystallization temperature of the crosslinked PVA with CNCs increased considerably from ～152 to ～187 °C. The continuous improvement of the thermal stability was observed with the increasing content of CNCs in the crosslinked PVA films. Additionally, the strong interaction between the CNCs and PVA was theoretically estimated from the Young's modulus values of the composites. Thermodynamic mechanical testing revealed that the crosslinked PVA composite films with CNCs could bear higher loads at high temperature compared to the films without the CNCs. At 60 °C, 2.7 GPa was reported for the storage modulus of the crosslinked composites with 3 wt % of CNCs, twice as high as that for the crosslinked films without CNCs. Moreover, creep results were improved when CNCs were added in the crosslinked nanocomposites. The materials prepared in this work could broaden the opportunities for applications in a wide range of temperatures. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45710.     

Thermally activated shape memory behavior of melt‐mixed polyurethane/cellulose nanocrystal composites

Building blocks made from renewable sources attract increasing attention for the design of new polymer systems. Recently, in this particular context, cellulose nanocrystals (CNCs) have gained great interest in both academic research and industry, mainly on account of their ability to reinforce range of polymer matrices and afford nanocomposites with attractive mechanical properties. The limited thermal stability of conventionally produced cellulose nanocrystals (CNCs) has, however, so far limited the range of polymers that could be used as basis for melt‐processed CNC nanocomposites. We herein show that a commercially accessible nanocrystal source, a particular grade of microcrystalline cellulose (MCC), can easily be converted into thermally stable CNCs by ultrasonication in phosphoric acid. A scalable melt‐mixing process was used to produce nanocomposites of these CNCs with a thermoplastic polyurethane (TPU) elastomer. A significant improvement of the room temperature storage modulus from 40 MPa (neat polymer) to 120 MPa (10% w/w CNC) was observed. The introduction of CNCs not only increased the stiffness of the polymer matrix, but also improved the shape memory properties of the nanocomposite. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45033.     

Starch‐based biocomposite films reinforced with cellulose nanocrystals from garlic stalks

The study was conducted to reinforce starch‐based biocomposite films using cellulose nanocrystals (CNCs) from garlic stalks. An average yield of 4.6% by mass based from air‐dried garlic stalks was obtained through alkali delignification, acid hydrolysis and sonication. The isolated CNCs are spherical and have an average diameter of 35 nm and crystallinity of 62%. Fourier transform infrared spectra correspond to the structure of cellulose, but some absorption bands corresponding to hemicelluloses were also noticed. Starch‐based biocomposite films with varying amount of the isolated CNCs as reinforcing filler were prepared by solution casting and evaporation method. Scanning electron micrographs of the films showed homogeneous dispersion of CNC in the starch matrix. Improvement in tensile strength and modulus was at maximum when the starch to CNC ratio is 100:5. The thermal stability of the films, on the other hand, decreased with the addition of CNC. Finally, CNC‐reinforced films had lower moisture uptake than nonreinforced films. POLYM. COMPOS. 34:1325–1332, 2013. © 2013 Society of Plastics Engineers     

Melt processing of polyamide 12 and cellulose nanocrystals nanocomposites

The fabrication of nanocomposites of polyamide 12 (PA12) and cellulose nanocrystals (CNCs) isolated from cotton and tunicates is reported. Through a comparative study that involved solution‐cast (SC) and melt‐processed materials, it was shown that PA12/CNC nanocomposites can be prepared in a process that appears to be readily scalable to an industrial level. The results demonstrate that CNCs isolated from the biomass by phosphoric acid hydrolysis display both a sufficiently high thermal stability to permit melt processing with PA12, and a high compatibility with this polymer to allow the formation of nanocomposites in which the CNCs are well dispersed. Thus, PA12/CNC nanocomposites prepared by melt‐mixing the two components in a co‐rotating roller blade mixer and subsequent compression molding display mechanical properties that are comparable to those of SC reference materials. Young's modulus and maximum stress could be doubled in comparison to the neat PA12 by introduction of 10% (CNCs from tunicates) or 15% w/w (CNCs from cotton) CNCs. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42752.     

Characterization and mechanical properties of ultraviolet stimuli‐responsive functionalized cellulose nanocrystal alginate composites

The poor mechanical properties of alginate when exposed to aqueous solution have been a problem plaguing researchers within the biomedical field. In order to be able to improve the mechanical properties in a systematic manner functionalized cellulose nanocrystals (CNCs) were added to alginate and UV‐induced crosslinked following an azo‐initiated free radical polymerization. CNCs were functionalized with 4‐pentenoic acid (PA‐g‐CNCs) using a simple, environmentally friendly solvent‐free esterification. The dimensional and crystallinity properties of PA‐g‐CNCs remained unchanged following esterification. Thermogravimetric analysis, Fourier transform infrared spectroscopy, and ¹³C nuclear magnetic resonance indicated that 4‐pentenoic acid was present on the surface of CNCs through bulk analysis. These PA‐g‐CNCs were then used in the creation of composites with an azo‐initiator to induce UV‐dependent crosslinking for the improvement of the mechanical properties of alginate. It was shown that the properties of alginate can be enhanced with the addition of functionalized CNCs to nanocomposites in mechanical testing in wet and dry conditions. These results suggest that the addition of PA‐g‐CNCs and crosslinking by UV‐dependent free radical polymerization improves the performance of alginate when tested in dry conditions, but without any apparent dependence to azo‐initiated crosslinking when exposed to water in regards to mechanical properties. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45857.     

Morphology and properties tuning of PLA/cellulose nanocrystals bio-nanocomposites by means of reactive functionalization and blending with PVAc

A novel method for the preparation of PLA bio-nanocomposites containing cellulose nanocrystals (CNCs) is reported. In order to enhance interfacial adhesion and dispersion of nanocrystals into PLA matrix, functionalization of PLA and CNCs by radical grafting of glycidyl methacrylate (GMA) and pre-dispersion of CNCs in poly (vinyl acetate) (PVAc) emulsion were applied. Morphologies, thermal and mechanical properties of nanocomposites for CNCs content of 1–6 wt.% were examined. Addition of functionalized components (PLA-GMA, CNC-GMA) and/or PVAc dispersed CNCs both improved the phase distribution of nanofiller and tensile properties, compared to the binary PLA/CNC nanocomposites. Thermal analyses demonstrated that glass transition, melting temperature and crystallinity of PLA were affected by the PVAc amount. Nanocomposites with PVAc dispersed CNCs exhibited higher thermal resistance than other composites. The filler effectiveness (CFE) was evaluated for all samples on the basis of storage modulus values: CNC-GMA and PVAc dispersed CNCs (3 wt. %) resulted the most effective fillers.     

Green esterification: A new approach to improve thermal and mechanical properties of poly(lactic acid) composites reinforced by cellulose nanocrystals

Cellulose nanocrystal (CNCs)‐reinforced poly(lactic acid) (PLA) nanocomposites were prepared using twin screw extrusion followed by injection molding. Masterbatch approach was used to achieve more efficient dispersion of CNCs in PLA matrix. Modified CNCs (b‐CNCs) were prepared using benzoic acid as a nontoxic material through a green esterification method in a solvent‐free technique. Transmission electron microscopy images did not exhibit significant differences in the structure of b‐CNCs as compared with unmodified CNCs. However, a reduction of 6.6–15.5% in the aspect ratio of b‐CNCs was observed. The fracture surface of PLA‐b‐CNCs nanocomposites exhibited rough and irregular pattern which confirmed the need of more energy for fracture. Pristine CNCs showed a decrease in the thermal stability of nanocomposites, however, b‐CNCs nanocomposites exhibited higher thermal stability than pure PLA. The average storage modulus was improved by 38 and 48% by addition of CNCs and b‐CNCs in PLA, respectively. The incorporation of b‐CNCs increased Young's modulus, ultimate tensile stress, elongation at break, and impact strength by 27.02, 10.90, 4.20, and 32.77%, respectively, however, CNCs nanocomposites exhibited a slight decrease in ultimate strength and elongation at break. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46468.     

Cellulose nanocrystal‐based poly(butylene adipate‐co‐terephthalate) nanocomposites covered with antimicrobial silver thin films

In this study, we reported the preparation and prospective application of the nanocomposites of poly(butylene adipate‐co‐terephthalate) (PBAT) reinforced with cellulose nanocrystals (CNCs). CNCs were isolated from bleached sugarcane bagasse by acid hydrolysis and functionalized with adipic acid. Nanocomposites were prepared with different concentration of CNCs (0.8, 1.5, and 2.3 wt% CNC) by solution‐casting method and then were covered with silver thin film by magnetron sputtering. The results showed that the surface modification increased the degree of crystallinity of nanocrystals from 51% to 56%, decreasing their length and diameter. Moreover, AFM‐IR spectroscopy revealed that the modified CNCs were covered by adipic acid molecules, improving the dispersion of nanocrystals in PBAT. Well‐dispersed modified CNCs acted as heterogeneous nuclei for crystallization of PBAT, and increased the storage modulus of the polymer by more than 200%. These improvements in thermal and mechanical properties of CNC‐based PBAT associated with the decrease of 56% in the Escherichia coli biofilm formation on nanocomposites (antibacterial properties) qualify the CNC/PBAT nanocomposites covered with silver thin films to be used as food packaging. POLYM. ENG. SCI., 59:E356–E365, 2019. © 2019 Society of Plastics Engineers     

Development and characterization of bionanocomposites based on poly(3‐hydroxybutyrate) and cellulose nanocrystals for packaging applications

Poly(3‐hydroxybutyrate) (PHB)‐based bionanocomposites were prepared using various percentages of cellulose nanocrystals (CNCs) by a solution casting method. CNCs were prepared from microcrystalline cellulose using sulfuric acid hydrolysis. The influence of CNCs on PHB properties was evaluated using differential scanning calorimetry, Fourier transform infrared spectroscopy, X‐ray diffraction, thermogravimetry and tensile testing. Vapor permeation and light transmission of the materials were also measured. Differential scanning calorimetric tests demonstrated that CNCs were effective PHB nucleation agents. Tensile strength and Young's modulus of PHB increased with increasing CNC concentration. Moreover, the PHB/CNC bionanocomposites exhibited reduced water vapor permeation compared to neat PHB and had better UV barrier properties than commodity polymers such as polypropylene. It was found that nanocomposites with 6 wt% of CNCs had the optimum balance among thermal, mechanical and barrier properties. © 2016 Society of Chemical Industry     

Reinterpretation of the mechanical reinforcement of polymer nanocomposites reinforced with cellulose nanorods

The mechanical reinforcement of nanocomposites containing nanorods‐like fillers such as cellulose nanocrystals (CNCs) is often interpreted by adapting the classical parallel–series model, assuming a simple hyperbolic dependence between the percolation threshold and aspect ratio. However, such assumptions are valid only for nanorods with high aspect ratio and often are misinterpreting the reinforcement obtained at low volume fraction of filler loading. To elucidate this intriguing scenario, we proposed a new approach and validated it by compiling and reinterpreting some of available literature that represent the experimental reinforcement with CNCs. Our approach showed better accuracy, specifically for the cases of CNC nanorods with lower aspect ratio. We conclude that this route permits a more realistic evaluation of the mechanical reinforcement, where a physical parameter accounting the polymer filler association is introduced. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45254.     

Poly(methyl methacrylate)‐grafted cellulose nanocrystals: One‐step synthesis,nanocomposite preparation,and characterization

Cellulose nanocrystals (CNCs) are ideal reinforcing agents for polymer nanocomposites because they are lightweight and nano‐sized with a large aspect ratio and high elastic modulus. To overcome the poor compatibility of hydrophilic CNCs in non‐polar composite matrices, we grafted poly(methyl methacrylate) (PMMA) from the surface of CNCs using an aqueous, one‐pot, free radical polymerization method with ceric ammonium nitrate as the initiator. The hybrid nanoparticles were characterized by CP/MAS NMR, X‐ray photoelectron spectroscopy, infrared spectroscopy, contact angle, thermogravimetric analysis, X‐ray diffraction, and atomic force microscopy. Spectroscopy demonstrates that 0.11 g/g (11 wt %) PMMA is grafted from the CNC surface, giving PMMA‐g‐CNCs, which are similar in size and crystallinity to unmodified CNCs but have an onset of thermal degradation 45 °C lower. Nanocomposites were prepared by compounding unmodified CNCs and PMMA‐g‐CNCs (0.0025–0.02 g/g (0.25–2 wt %) loading) with PMMA using melt mixing and wet ball milling. CNCs improved the performance of melt‐mixed nanocomposites at 0.02 g/g (2 wt %) loading compared to the PMMA control, while lower loadings of CNCs and all loadings of PMMA‐g‐CNCs did not. The difference in Young's modulus between unmodified CNC and polymer‐grafted CNC composites was generally insignificant. Overall, ball‐milled composites had inferior mechanical and rheological properties compared to melt‐mixed composites. Scanning electron microscopy showed aggregation in the samples with CNCs, but more pronounced aggregation with PMMA‐g‐CNCs. Despite improving interfacial compatibility between the nanoparticles and the matrix, the effect of PMMA‐g‐CNC aggregation and decreased thermal stability dominated the composite performance.     

Modified cellulose nanocrystals enhancement to mechanical properties and water resistance of vegetable oil-based waterborne polyurethane

Environmentally friendly and lightweight silylated cellulose nanocrystal (SCNCs)/waterborne polyurethane (WPU) composite films that exhibit excellent mechanical properties and water resistance were prepared. The cellulose nanocrystals (CNCs) of the filamentous structure were surface-modified by γ-aminopropyltriethoxysilane (APTES) and then introduced into a castor oil-based aqueous polyurethane (WPU) matrix by in situ polymerization. The morphology and silylation degree of CNCs were characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and Fourier infrared transform spectroscopy at different APTES concentrations. The results showed that the surface of the nanocellulose crystal has the best silylation morphology and thermal stability with incorporation of 6 wt % APTES. The thermal stability, mechanical properties, surface morphology, and water resistance of the nanocomposites were investigated by TGA, tensile test, SEM and optical contact angle, water absorption test, and mechanical property test after immersed in water. It was found that the effective introduction of modified CNCs resulted in a significant increase in tensile strength at high levels, and the thermal stability and hydrophobicity of the material were improved simultaneously, reaching the percolation threshold at a 0.50 wt % SCNCs as determined theoretically. This study provided an approach to the design and development of surface-modified CNCs/vegetable oil-based polymer composites by using an appropriate concentration of silane coupling agent to modify CNCs and improve the compatibility between nanocellulose and vegetable oil-based polymer matrices. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48228.     

Functionally Graded Polyurethane/Cellulose Nanocrystal Composites

The preparation and investigation of functionally graded polymer nanocomposites, which have a concentration gradient of cellulose nanocrystals (CNCs) along one direction, is reported here. As a test bed, a series of nanocomposites consisting of a thermoplastic polyurethane (PU) and 0–15% w/w CNCs is prepared via solvent casting and the mechanical properties of films of these materials are characterized by dynamic mechanical analyses and tensile tests. The formation of graded materials is accomplished by lamination of films with varying CNC content. The processing conditions are optimized to achieve intimate fusion of the individual layers. The elimination of internal interfaces is evidenced by an elongation at break of up to 500%. In order to explore potential applications of graded PU/CNC nanocomposites, structure‐dependent actuation in response to water is demonstrated in a bioinspired architecture. In addition, the damping behavior of cylindrical shaped composites is investigated by way of compression tests. The results show that functionally graded PU/CNC composites show good damping behavior over a much larger range of forces than the neat PU or the homogeneous nanocomposites.     

Influence of the Botanic Origin of Starch Nanocrystals on the Morphological and Mechanical Properties of Natural Rubber Nanocomposites

The structural and mechanical properties of natural rubber (NR) nanocomposites filled with starch nanocrystals (SNC) extracted from four different starch sources are investigated. The aim of this work is to explore the influence of botanic sources on final properties of nanocomposites and SNC reinforcing capability. A general trend seems to be that the higher the amylose content of native starch granules used for preparing SNC, the lower the water uptake and reinforcing effect (except for potato starch). It is postulated that SNC prepared from higher amylose content starch might release loosely bonded amylose chains during preparation and/or soaking in water and thus prevent SNC to participate in the formation of a reinforcing network.

     

Thermoplastic corn starch reinforced with cotton cellulose nanofibers

This work evaluates the use of cotton cellulose nanofibers (CCN) as a reinforcing agent to prepare thermoplastic corn starch (TPS) matrix plasticized with 30 wt % of glycerol. The nanocomposites were filled with 0.5–5.0 wt % of CCN on a dry‐starch basis. The dried nanofibers were resuspended through the use of an ultrasonicator and then introduced in the fixed water formulation for obtaining TPS. The nanocomposites were compounded in a corotating twin‐screw extruder. Scanning transmission electron microscopy (STEM), field emission gun (FEG), X‐ray diffraction (XRD) and thermogravimetric analysis (TGA), in air atmosphere, were used to characterize nanofibers, neat TPS, and nanocomposites. The results showed that the nanofibers had needlelike structure with an average length of about 135 ± 50 nm and an average diameter of about 14 ± 4 nm. The addition of CCN was effective to enhance the mechanical properties of neat TPS in compositions above 2.5 wt %, although some agglomeration could be observed. The resulting nanocomposites showed good structural stability, because the amylopectin transcrystallization phenomena on the surface of nanofibers had not occurred. Only a slight decrease in the crystallinity index and a minor increase in the water absorption in relation to neat TPS were observed. An increase in the thermal stability of TPS nanocomposites with respect to neat TPS was verified, but it was independent of the CCN content. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Nanocomposites of poly(propylene carbonate) reinforced with cellulose nanocrystals via sol‐gel process

Cellulose nanocrystals (CNCs) organogels were first produced from aqueous dispersion through solvent exchange of CNCs to acetone via a simple sol‐gel process. After mixing the organogels with poly(propylene carbonate) (PPC) in dimethylformamide followed by solution casting, green nanocomposites were obtained with CNCs well dispersed in PPC polymer matrix which was confirmed by scanning electron microscopy observations. Differential scanning calorimeter analysis revealed that glass transition temperature of the nanocomposites was slightly increased from 34.0 to 37.4°C. Tensile tests indicated that both yield strength and Young's modulus of CNCs/PPC nanocomposites were doubled by adding 10 wt % CNCs. However, poor thermal stability of PPC occurred after incorporating with CNCs due to the thermo‐sensitive sulfate groups located on the surface of CNCs. Furthermore, PPC became more hydrophilic because of the inclusion of CNCs according to the water contact angle measurement. The enhanced mechanical and hydrophilic properties, coupled with the inherent superior biocompatibility and degradability, offered CNCs/PPC composites potential application in biomedical fields. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40832.     

Cellulose Nanocrystals/ZnO as a Bifunctional Reinforcing Nanocomposite for Poly(vinyl alcohol)/Chitosan Blend Films: Fabrication,Characterization and Properties

In this study, cellulose nanocrystals/zinc oxide (CNCs/ZnO) nanocomposites were dispersed as bifunctional nano-sized fillers into poly(vinyl alcohol) (PVA) and chitosan (Cs) blend by a solvent casting method to prepare PVA/Cs/CNCs/ZnO bio-nanocomposites films. The morphology, thermal, mechanical and UV-vis absorption properties, as well antimicrobial effects of the bio-nanocomposite films were investigated. It demonstrated that CNCs/ZnO were compatible with PVA/Cs and dispersed homogeneously in the polymer blend matrix. CNCs/ZnO improved tensile strength and modulus of PVA/Cs significantly. Tensile strength and modulus of bio-nanocomposite films increased from 55.0 to 153.2 MPa and from 395 to 932 MPa, respectively with increasing nano-sized filler amount from 0 to 5.0 wt %. The thermal stability of PVA/Cs was also enhanced at 1.0 wt % CNCs/ZnO loading. UV light can be efficiently absorbed by incorporating ZnO nanoparticles into a PVA/Cs matrix, signifying that these bio-nanocomposite films show good UV-shielding effects. Moreover, the biocomposites films showed antibacterial activity toward the bacterial species Salmonella choleraesuis and Staphylococcus aureus. The improved physical properties obtained by incorporating CNCs/ZnO can be useful in variety uses.     

In situ dispersion and polymerization of polyethylene cellulose nanocrystal-based nanocomposites

This study describes a novel method of forming a nanocomposite comprising cellulose nanocrystals (CNCs) as the reinforcing filler and a high-density polyethylene (PE) matrix. The method involves covalent attachment of a metallocene catalyst, 1,1′-bis(bromodimethylsilyl)zirconocene dibromide 1 , to the hydroxyl-rich surfaces of the CNCs and subsequent slurry polymerization with excess alumoxane (MMAO-12) as the cocatalyst. Polymerization proceeds with activities reaching 500 kg mol⁻¹ atm⁻¹ h⁻¹, while the CNCs are simultaneously dispersed to afford robust, well-dispersed nanocomposites. Films of these composites (about 7-vol % CNC) showed excellent dispersal of the filler (optically translucent; no CNC aggregation observed by atomic force microscopy). The composites (about 7-vol % CNC) also revealed an increase in Young's modulus (10–100%) and comparable yield strength relative to commercially produced PEs. The experimental simplicity of this approach suggests that our method could be scaled beyond the present laboratory scale and extended to reinforce other polyolefin grades. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48500.