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Nanocelluloses – surface modification and use in functional materials
Michaela Salajková
Licentiate Thesis
Stockholm, Sweden, 2012
AKADEMISK AVHANDLING
som med tillstånd av Kungliga Tekniska Högskolan i Stockholm framlägges till offentlig granskning
för avläggande av teknologie licentiatexamen 16 mars 2012, kl. 13:00 i K2, Teknikringen 28.
Avhandlineg förvaras på engelska.
Wallenberg Wood Science Center Department of Fibre and Polymer Technology Royal Institute of Technology, KTH SE-100 44 Stockholm, Sweden TRITA-CHE-Report 2012:7 ISSN 1654-1081 ISBN 978-91-7501-255-1 Copyright © Michaela Salajková, 2012 Tryck: Universitetsservice US-AB, Stockholm 2012
Abstract
Cellulose nanocomposites offer interesting potential in terms of improved properties and new functionalities compared with microcomposites. Preparation from colloidal suspensions is promising, since high reinforcement content is possible and a wide range of constituents can be used. In the first study, the challenge is to form a stable suspension of well-dispersed carbon nanotubes (CNT) and nanofibrillated cellulose (NFC) in water and to prepare commingled high CNT content nanopaper structures by filtration. Various surfactants were used to modify CNT. The NFC was stabilized by charged carboxylate groups. A nonylphenol phosphate ester surfactant, NPPE, worked well for CNT and provided a stable and well-dispersed water suspension of CNT and NFC. Field emission scanning electron microscopy (FE-SEM), porosimetry and atomic force microscopy (AFM) were used to characterize nanopaper structure, and tensile properties were measured as well as surface resistivity. The processing route is water based and it is possible to prepare thin coatings as well as thicker films with a combination of low surface resistivity, flexibility in bending and high strength and toughness in tension.
As inspired by organo-modified layered silicates, the objective of the second study is to develop an environmentally friendly procedure for the surface modification of cellulose nanocrystals, CNC, using quaternary ammonium salts via adsorption. In order to obtain higher surface charge density on CNC, a new route is developed for preparation of CNC with carboxylic acid groups. Quanternary ammonium cations bearing alkyl, phenyl, glycidyl, and diallyl groups are used to modify CNC to render their surface more hydrophobic. The structure and surface hydrophobicity of unmodified and modified CNC as well as their dispersibility in organic solvent are characterized by AFM, FE-SEM, Fourier-transformed infrared spectroscopy (FT-IR), X-Ray analysis (XDR) and contact angle measurement (CAM). Future work will focus on surface-modified nanocelluloses in composite materials, in order to learn more about surface treatment effects on nanocomposite properties.
Keywords: Nanocellulose, surfactant, surface modification, papermaking process, quaternary ammonium salt
Sammanfattning
Nanokompositer från cellulosa har potential att ge starkt förbättrade egenskaper och ny funktionalitet jämfört med mikrokompositer. De ger även möjlighet till komposittillverkning från kolloidala suspensioner där man kan uppnå hög halt av förstärkningsfasen. Det är också möjligt att välja från en bred flora av lösliga och dispergerbara materialkomponenter. I första studien är utmaningen att skapa en stabil och väldispergerad suspension av kolnanorör (CNT) och nanofibrillerad cellulosa (NFC) i vatten för att genom filtrering framställa nanopapper med interpenetrerande CNT och NFC nätverk. Olika ytaktiva ämnen användes för att modifiera CNT. NFC stabiliserades genom laddade karboxylgrupper på ytan. En nonylfenol fosfatester, NPPE, fungerade bra för CNT och resulterade i en stabil och väldispergerad vattensuspension av CNT och NFC. FE-SEM, densitometri och AFM användes för att karakterisera nanopapperstruktur. Mekaniska egenskaper och ytresistivitet mättes. Processen för framställning av CNT/NFC nanopapper är vattenbaserade och det är möjligt att framställa tunna ytbeläggningar likväl som tjockare filmer. Dessa strukturer har en kombination av låg resistivitet, flexibilitet i böjning liksom hög hållfasthet och seghet i dragbelastning.
Syftet med den andra studien är att utgå från organo-modifierade skiktade silikater (leror) för att utveckla en miljövänlig ytmodifieringsmetod för nanocellulosa. För att öka ytladdningstätheten på CNC (nanokristaller från cellulosa) utvecklas ett nytt sätt att skapa karboxylgrupper på ytan. Kvarternära ammoniumsalter med alkyl, fenyl, glycidyl och diallylgrupper används för att göra ytan på CNC mer hydrofob. Ytans struktur och hydrofoba karaktär, liksom dispersionsegenskaper i organiska lösningsmedel, karakteriseras med hjälp av AFM, FE-SEM, FT-IR, XDR och kontaktvinkelmätning. Fortsatt arbete kommer att fokusera på ytmodifierad cellulosa i kompositmaterial, för att utveckla förståelsen för effekter av ytmodifiering på nanokompositers egenskaper.
List of papers This thesis is a summary of the following papers, which are appended at the end of the thesis.
Paper I
Tough and conductive nanopaper structures, based on cellulose nanofibrils and carbon nanotubes, prepared by processing of aqueous suspensions
Michaela Salajková, Luca Valentini, Qi Zhou and Lars Berglund
Manuscript.
Paper II
Hydrophobic cellulose nanocrystals modified with quaternary ammonium salts
Michaela Salajková, Lars Berglund and Qi Zhou
Manuscript.
The contributions of the author of this thesis to the above listed papers are:
Paper I Performed most of the experimental work and most of the preparation of the manuscript.
Paper II Performed all experimental work and contributed to the preparation of the manuscript.
Other relevant publications not included in the thesis:
Mechanical performance tailoring of tough ultra-high porosity foams prepared from cellulose I nanofibers suspensions
Houssine Sehaqui, Michaela Salajková, Qi Zhou and Lars A. Berglund
Soft Matter 2010, 6, 1824-1832.
Determination of Young´s modulus for nanofibrillated cellulose multilayer thin films using buckling mechanics
Emily D. Cranston, Mohamed Eita, Erik Johansson, Julia Netrval, Michaela Salajková, Hans Arwin and Lars Wågberg
Biomacromolecules 2011, 12 (4), 961-969.
A transparent hybrid of nanocrystalline cellulose and amorphous calcium carbonate nanoparticles
Denis Gebauer, Vitaliy Oliynyk, Michaela Salajková, Jordi Sort, Qi Zhou, Lennart Bergström and German Salazar-Alvarez
Nanoscale 2011, 3 (9), 3563-3566.
Table of Contents
1. Objectives ............................................................................................. 1
2. Background ........................................................................................... 1 2.1. Nanocellulose ................................................................................. 1 2.2. Surface modification ....................................................................... 3 2.3. Utilization of nanocellulose – material concept ............................... 3 2.4. Hybrid materials .............................................................................. 4
3. Experimental part .................................................................................. 5 3.1. Nanocellulose preparation .............................................................. 5 3.2. MWCNT purification (Paper I) ......................................................... 6 3.3. MWCNT suspension preparation (Paper I) ..................................... 6 3.4. MWCNT/TO-NFC hybrid preparation (Paper I) .............................. 6 3.5. Surface modification of cellulose nanocrystals (Paper II) ............... 7 3.6. Material characterization ................................................................. 7
4. Results and Discussion ...................................................................... 10 4.1. MWCNT/TO-NFC hybrids ............................................................. 10 4.2. Preparation of cellulose nanocrystals ........................................... 16 4.3. Surface modification of cellulose nanocrystals ............................. 20
5. Conclusions ........................................................................................ 25
6. Acknowledgements ............................................................................. 26
7. References ......................................................................................... 27
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1. Objectives
The overall objective of the thesis is to study surface functionalization of nanocellulose and its use in functional materials.
Environmentally friendly routes for modification and processing are of particular interest. Surface functionalization may serve different purposes, but the formation of stable nanoparticle dispersion in polar/non-polar liquids is an important goal.
In this thesis, nanocellulose was considered either as a matrix (Paper I) or as a reinforcement component (Paper II). In Paper I, cellulose nanofibrils (NFC) were combined with multi-walled carbon nanotubes (MWCNT) to obtain electrically conductive paper. The main challenge here was to overcome mutual incompatibility of these components and to obtain stable NFC-MWCNT dispersions in water. In Paper II, a new route for isolation and surface modification of cellulose nanocrystals was developed so as to later use nanocellulose as reinforcement phase in polymer matrix. Therefore the challenge is to control the surface properties of nanocellulose with the specific goal of obtaining a stable suspension of nanocellulose in organic solvents, since this is not possible with unmodified nanocrystals. The availability of a fast and simple approach to surface modification of nanocelluloses would open new possibilities for their use in functional materials.
2. Background
2.1. Nanocellulose
Cellulose is one of the most ubiquitous and abundant polymers on the planet. It is widely used in industry nowadays, but it has been used in the past for a large range of applications including ropes, sails, paper, timber for housing etc. [1].
Cellulose is a crystalline structural polysaccharide that is present in wood biomass. The basic repeat unit contains two anhydroglucose rings joined via the β-1,4 glycosidic linkage [1]. Furthermore, 30 – 40 cellulose polymer chains aggregate into microfibrils (also referred to as elementary fibrils) that are 3 – 5 nm wide. Within each of these microfibrils, there are regions, where the cellulose chains are arranged in highly ordered (crystalline) structures, and regions that are disordered [2]. As it is schematically illustrated in Figure 1, microfibrils agglomerate further into larger aggregates and these, together with hemicelluloses and lignin, form the plant cell wall (i.e. fibre) [1, 3].
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Figure 1. Hierarchical structure of wood biomass and the characteristics of cellulose microfibrils consisting of disordered (amorphous) and crystalline regions. According to reference [2].
The diversity of cellulose nanoparticle types and geometries results from the plant origin and the extraction processes from the cellulosic plant fibres, which include pretreatments, disintegration, or deconstruction processes of the hierarchical structure of the cell wall [2].
When mild chemical [4] or enzymatic [5] pretreatment followed by mechanical treatment are applied, microfibrils in the form of an aqueous suspension can be obtained, referred to as nanofibrillated or microfibrillated cellulose (NFC or MFC, Figure 2a) consisting of mixture of cellulose fibrils (3 – 5 nm wide) and fibril aggregates (10 – 20 nm wide), that are several µm long. On the other hand when a strong acid hydrolysis is used, all disordered regions are removed resulting in highly crystalline, rod-like particles called cellulose nanocrystals or whiskers (3 – 5 nm wide and 100 – 300 nm long, Figure 2b [6, 7]).
(a) (b)
Figure 2. Comparison of nanocelluloses isolated from wood using (a) mild enzymatic
pretreatment and (b) strong acid hydrolysis [8].
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2.2. Surface modification
An important aim of chemical functionalization has been the introduction of stable negative or positive electrostatic charges on the surface of nanocellulose. This is done to obtain better colloidal dispersion and to tune the surface characteristics of nanocellulose to improve compatibility, especially when used in combination with nonpolar or hydrophobic matrices in nanocomposites [9].
Because of the abundance of hydroxyl groups on the surface of cellulose, many different chemical modifications have been carried out [9]. In general, surface functionality of cellulose nanoparticles can be categorized into 3 groups: (1) native surface chemistry of the particles as a result of their extraction (for example carboxylic groups originating from TEMPO-mediated oxidation or sulfate groups as a result of sulfuric acid hydrolysis), (2) physical adsorption of surfactants or polyelectrolytes and (3) covalent modification such as esterification, etherification, silylation and polymer grafting [2].
2.3. Utilization of nanocellulose – material concept
Nanocelluloses and their unique properties such high stiffness and strength, high specific surface area, low coefficient of thermal expansion, optical transparency and self-assembly behavior has attracted great interest in utilization of these properties in bio/nanocomposites.
2.3.1. Nanocomposites based on cellulose nanocrystals
The main use of cellulose nanocrystals in nanocomposites is as reinforcements, nucleating agent, coatings and templates, responsive materials [10] or self-assembled materials [6, 11-13].
It was shown that introduction of 1 % of cellulose nanocrystals into polyurethane matrix using in-situ polymerization approach results in nanostructured composites with ultra high tensile strength and strain to failure, as well as Young´s modulus [14]. The importance of surface chemistry is demonstrated in [15], where nanocomposites with a polyurethane matrix were prepared by in-situ polymerization of unmodified and modified cellulose nanocrystals, resulting in completely different material structure. Preferential location of modified nanocrystals in the hard domains of the polyurethane matrix was observed, leading to enhancement of the segment crystallization.
Well-dispersed cellulose nanocrystals have been used to increase crystallinity of poly-lactic acid by more than 20 % [16], giving stiffer nanocomposites [17].
Liquid crystalline behavior of cellulose nanocrystals was used in the design of thin coatings and films with tunable optical properties [18]. Self-organization behavior of cellulose nanocrystals was also utilized, as a template for deposition of silica nanoparticles [19, 20].
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2.3.2. NFC nanopaper
When cellulose nanofibrils are dried from a water suspension, strong interfibril interactions are formed, as is true with conventional paper prepared from wood pulp fibres. The resulting dense and coherent NFC network has been termed nanopaper [21]. It has also been demonstrated that large sheets with a diameter of 20 cm can be prepared using a semiautomatic sheet former [22]. Nanopaper exhibits remarkable toughness and its porosity can be controlled by drying from a less polar liquid than water [21], by freeze-drying or by supercritical drying from CO2 [23]. The porosity can easily be tailored in a range of 20 – 88 % and the specific surface area can be as high as 482 m2/g [23]. Compared to conventional paper, nanopaper has higher density and better mechanical properties [24]. It is flexible in bending and can be optically transparent [25], with low coefficient of thermal expansion [25] and has excellent oxygen barrier properties [26, 27].
Many potential applications for nanocomposites based on NFC has been suggested, such as packaging [28], loud-speaker membranes [29], foldable substrates for electronic displays [25, 30] or battery membranes [31].
2.3.3. Nanocomposites and other materials based on NFC
Nanopaper fibril networks can also be used as reinforcement in nanocomposites. NFC melamin formaldehyde [32], NFC/starch [33] and NFC/hydroxyethylcellulose [34] were prepared by either impregnation of dried nanopaper [32] or by careful mixing of NFC suspension with polymer matrix [33, 34].
An interconnected network of NFC can also be used for preparation of porous materials, such as aerogels and foams. Foams of controlled densities (7 – 103 kg/m3) were prepared by freeze-drying of NFC suspensions [35]. Using a bioinspired route, mechanical properties of the foams were further tailored by the addition of xyloglucan, which has well documented effects on the mechanical behavior of plant cell walls [36]. Furthermore, NFC was used as a reinforcement in starch foams [37]. The nanosized fibrils effectively reinforced the cell wall of the foams. This would not be possible with micrometer-scale cellulose fibres, since their diameter is too large in comparison to the typical cell wall thickness in polymer foams.
High porosity aerogels were prepared from NFC suspensions by tert-butanol freeze-drying, and due to reduced capillary effects during drying, this results in aerogels with higher specific surface area compared to conventional freeze-drying from water [38].
2.4. Hybrid materials
The remarkable properties of nanocellulosic materials can be further extended by addition of inorganic entities. Hybrid materials with interesting properties can be prepared by
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taking advantage of the continuous nanofibrous NFC network or the stiffness and optical properties of rod-like cellulose crystals.
By combining cellulose nanocrystals with calcium carbonate, the most abundant biomineral, an optically transparent hybrid nanocomposite with properties similar to dentin (natural composite of hydroxyapaptite and collagen) was obtained [39].
It was demonstrated, that NFC-montmorrilonite (MTM) clay nanopaper can be prepared, resulting in hybrid materials with interesting self-extinguishing and oxygen-barrier properties [22, 27]. Cellulose nanofibrils can also be decorated with TiO2 nanoparticles, resulting in flexible, transparent and hard films [40] or with magnetic nanoparticles resulting in magnetic membranes and films [41].
Desire to broaden the possible functionalities of cellulose-based materials to electrically conductive materials led to attempts to mix carbon nanotubes (CNT) with cellulose, although previously CNT have not been combined with cellulose nanofibres from wood. Electrically conductive paper was prepared by addition of carbon nanotubes to wood pulp [29, 42, 43], bacterial cellulose [44-47] and regenerated cellulose [48, 49]. However, NFC from wood pulp offers the advantage of a much lower cost than bacterial cellulose and greater potential for large-scale applications. Moreover, use of native cellulose fibrils does not require organic solvent, as is the case of regenerated cellulose.
3. Experimental part
3.1. Nanocellulose preparation
3.1.1. TEMPO-mediated oxidation (Paper I and II)
TEMPO-mediated oxidation of wood pulp was performed according to the method described by [50]. Briefly; the softwood fibres (a commercial never-dried softwood pulp was provided by Nordic Paper; the cellulose content was 86% and the reminder was mostly hemicelluloses) were suspended in water containing TEMPO (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, free radical, 98%, Aldrich) and sodium bromide (99%, Alfa Aesar). The TEMPO-mediated oxidation was started by adding the desired amount of the sodium hypochlorite solution (14 %, GPR Rectapur, VWR) and was continued at room temperature while stirring. The pH was kept at 10 by adding sodium hydroxide (98%, Reagent, Sigma-Aldrich) no further consumption was observed. The TEMPO-oxidized cellulose was thoroughly washed with water by filtration. Afterwards, TEMPO-oxidized fibre suspension (approximately 1 wt%) was disintegrated using a homogenization process
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using Microfluidizer M-110EH (Microfluidics Ind., USA). Resulting suspension of microfibrillated cellulose (TO-NFC) was stored at 4°C before use.
3.1.2. HCl hydrolysis (Paper II)
Cellulose nanocrystals (CNC) were isolated from nanofibrils using hydrochloric acid hydrolysis. Typically, 100 g of wet TO-NFC (corresponds to 1 g of dry weight) was diluted to 316 ml of deionized water and dispersed using shear forces (Ultra Turrax T18 basic, IKA Werke GmbH & Co. KG, Germany) for 2 min at 13 500 rpm and stirred with a magnetic stirrer overnight. The fibril suspension was placed into a three-neck round bottom flask. Hydrochloric acid (37 %, ACS, Merck) was added drop-wise until a concentration of 2.5 M was obtained. The system was heated to 105°C and refluxed for 3, 5 and 7 h. The reaction was stopped by dilution with deionized water. The resulting suspension was centrifuged at 4166 g for 20 min. Precipitant was collected and diluted to approximately 100 ml of water and dialyzed against water using Spectrum Spectra/Por regenerated cellulose dialysis membrane with a molecular weight cut off of 12 000 – 14 000 for 5 – 7 days. After that, the suspensions were sonicated 2 x 5 minutes (Sonics Vibracell, VCX 750, Sonic&Materials, USA) to obtain individual nanocrystals.
3.2. MWCNT purification (Paper I)
Multi-walled carbon nanotubes (Elicarb®, purity 70 - 90%, kindly provided by Thomas Swan) were purified prior use. It is well known that 3 M nitric acid is very effective at dissolving metal particles. On the other hand 5 M hydrochloric acid is suitable to dissolve metal oxide [51]. Therefore, the crude material was stirred in 3 M nitric acid (65 %, Scharlau) and refluxed for 12 hours at 60°C. After that, MWCNT were suspended and refluxed in 5 M hydrochloric acid solution for 6 hours at 120°C [44].
3.3. MWCNT suspension preparation (Paper I)
Purified MWCNT were dispersed in water using acid phosphate ester of ethoxylated nonylphenol (STEPFAC 8170, kindly provided by Stepfan Company, Northfield, IL, USA.). Typically, 2 % solution of surfactant was used and the concentration of MWCNT was 5 mg/mL. After sonication stable homogenous suspension was obtained.
3.4. MWCNT/TO-NFC hybrid preparation (Paper I)
Suspension of TO-NFC was diluted to approximately 0.1 wt% and dispersed using shear forces (Ultra Turrax, IKA) for 5 min at 13 500 rpm. MWCNT suspension was added drop-wise until the desired MWCNT content (0.5 – 16.7 wt%) was obtained. The suspension was sonicated and stirred with magnetic stirrer overnight in order to obtain a homogenous distribution of MWCNT in the cellulose nanofibril network. After that, water
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was removed by filtration with a glass filter funnel (7.2 cm in diameter) and a filter membrane (0.22 µm GV, Milipore, USA). The obtained wet cake was dried at 93°C and vacuum for 10 – 20 min in a sheet former (Rapid Kothen, RK3A-KWT PTI). Control samples containing only TO-NFC (0 % control) and TO-NFC/4.8 wt% surfactant (4.8 % control) were prepared using the same procedure. The resulting nanopaper sheets had 7 cm diameter and thickness 25 and 50 µm.
3.5. Surface modification of cellulose nanocrystals (Paper II)
The surface of cellulose nanocrystals was modified using an adsorption of quaternanry ammonium salts. A serie of quaternary ammonium salts was used to study the versatility of the approach: stearyltrimethylmmonium chloride (C18, 97%, Chemtronica), glycidyl trimethylammonium chloride (COC, technical, Aldrich), diallyldimethylammonium chloride (C=C, 97%, Aldrich) and phenyltrimethylammonium chloride (CPh, 97%, Aldrich). A more detailed study was then done using C18 and the effect of cellulose:quaternary amine ratio was studied as follow: First, 0.1 % w/v cellulose nanocrystals suspension (CNC-3) was prepared and the pH was adjusted to 10 (in order to have the carboxyl groups on the surface of the nanocrystals fully dissociated, [52]). The desired amount of C18 (or other quaternary amine) was dissolved in water resulting in 0.1 % w/v (3 mmol/L). Both suspensions were heated up to 60°C. The CNC suspension was added drop by drop to the C18 solution to obtain molar ratios of 1:1, 1:2 and 1:4 (carboxyl content of nanocrystals:C18). The temperature was kept at 60°C for 3 hours. After that, suspensions were stirred at room temperature overnight. The NaCl and unbound C18 was washed away by dialysis. The modified nanocrystals were freeze-dried and the adsorbed amount was noted after freeze-drying and the value was used to determine the amount of C18 on the surface of cellulose. After freeze-drying, the modified nanocrystals were re-dispersed in toluene (1 wt%) using an ultrasonic treatment for 1 min. These suspensions were then centrifuged for 10 min at 20 000 g to remove possible excess quaternary ammonium salts that were aggregated on the surface of CNCs. The resulting pellet was easily re-dispersed in the desired amount of organic solvent with an ultrasonic treatment of a few tens of seconds.
For the rest of the quaternary amines, ratio 1:2 was used.
3.6. Material characterization
3.6.1. Scanning electron microscopy (FE-SEM) (Paper I and II)
Morphology of the nanopapers and cellulose nanocrystals suspension was observed using FE-SEM. In the case of the films, the specimens were fixed on a metal stub using colloidal graphite. Suspensions were deposited on freshly cleaved mica treated with poly-L-lysine
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(1 g/L) and attached onto the metal stub. Specimens were always coated with thin layer of coating consisting of gold-palladium using Cressington 208HR Sputter coater. Hitachi S-4800 scanning electron microscopy operated at a low acceleration voltage (1 – 1.5 kV) and a short working distance was used to capture secondary electron images of the surface of the specimen.
A suspension of unmodified CNC in water and a suspension of C18 modified CNC in toluene were deposited on the copper grid. A Hitachi S-4800 scanning electron microscope equipped with transmitted electron detector was used to capture transmitted electron image to further characterize the morphology of the particles.
3.6.2. Atomic force microscopy (AFM) (Paper I and II)
Atomic force microscopy was used to study detailed nanostructure of the nanomaterial (Paper I), the particle morphology and the particle length distribution (Paper II). The surfaces of the specimens were imaged with tapping-mode AFM (Multimode IIIa, Veeco Instruments, Santa Barbara, CA). Topographic (height) and phase images were recorded under controlled air conditions (23°C and 50 % relative humidity). RTESP silica cantilevers (Veeco Instruments, tip radius 8 nm and spring constant 40 N/m) were oscillated close to resonance frequency. The scan size was 1 – 5 µm2. No further image processing except flattening was made.
3.6.3. Mechanical testing (Paper I)
Mechanical properties of the nanopaper were investigated using tensile testing. Specimens of 40 mm length, 25 µm thickness and 5 mm width were tested using a Universal Material Testing Machine Instron 5566 equipped with a100 N load cell. A cross-head speed of 4 mm/min which corresponds to an initial strain rate of 10 %/min was used and the stress-strain curves were recorded. The experiment was performed under controlled air conditions of 23°C and 50 % relative humidity. Specimens were equilibrated under the same conditions for two days. The results are based on data collected from 7 specimens and the standard deviations were calculated.
3.6.4. Density and porosity measurement (Paper I)
The density of the nanopapers was determined using the mercury displacement method [53]. The samples were weighed both in air (mSample(air)) and when submerged in mercury (mSample(Hg)). The density of the samples (ρSample) was calculated from (1) and the porosity was calculated using equation (2), assuming the density of cellulose to be 1500 kg/m3 and density of carbon nanotubes to be 2100 kg/m3 [54]. Weight fractions of cellulose and MWCNT are represented by wcell and wMWCNT.
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!!"#$%& = !!"!!"#$%& !"#
!!"#$%& !"# !!!"#$%& !" (1)
!"#"$%&' % = 1 −!!"#$%&
!!"##.!!"##!!!"#$%.!!"#$%. 100 (2)
3.6.5. Surface resistivity (Paper I)
Surface resistivity of the nanopaper was measured using a Keithley 6517B Electrometer/High Resistivity Meter together with a Keithley 8089 Resistivity Test Fixture. Using this test fixture, surface resistivity in the range of 103 – 1017 Ω can be measured.
3.6.6. Conductometric titration (Paper II)
The carboxyl content of cellulose nanocrystals was determined by conductometric titration. Typically, 50 mg of cellulose nanocrystals were suspended in 50 mL of water containing 1 mM NaCl. The titration was performed using a 0.0114 M solution of NaOH (solution was standardized against oxalic acid) and the conductivity was monitored with a Mettler Toledo Conductivity meter (USA).
3.6.7. Fourier-transformed infrared spectroscopy (FT-IR) (Paper II)
The presence of carboxylic groups and the adsorption of quaternary ammines were determined using a Perkin-Elmer Spectrum 2000 FTIR equipped with a MKII Golden Gate, single reflection attenuated total reflectance (ATR) system (Specac Ltd., London, UK). The ATR crystal was a MKII heated diamond 45° ATR top plate. Samples were prepared by casting of theb CNC suspension were used.
3.6.8. X-Ray analysis
X-ray measurements were made on films prepared by casting of the nanocrystal suspensions in a small dish. Diffractograms were recorded in reflection mode in a 2θ angular range 5 – 40° by steps of 0.05°. The measurements were performed using a Philips X´Pert Pro diffractometer (model PW 3040/60). The Cu Kα radiation (λ = 1.5418 Å) generated at 45 kV and 40 mA was monochromatized using a 20 µm Ni filter. Diffractograms of rotating specimen were recorded using a position sensitive detector.
3.6.9. Contact angle measurement (CAM) (Paper II)
Contact angle measurements were carried out using an easy drop commercial instrument (CAM 200, KSV Instrument, Finland). Milli-Q water (3 µL) was added onto each sample surface using a Hamilton syringe at 25°C and the contact angle was calculated using the Sessile Drop Fitting Method. Because the C18 modified nanocrystals do not form a smooth
10
film on a substrate (dry as a powder), a model surface was prepared using spin coating. A plasma treated Si-wafer was dipped into poly-L-lysine solution (1 g/L) for 3 min. Smooth model films (approximately 30 nm thick) were prepared by spin-coating (Chemat Technology, KW-4A) the nanocrystal suspension onto the substrates at 700 rpm for 15 s followed by 2500 rpm for 60 s. Afterwards the surfaces were cleaned using nitrogen gas and the substrate was dipped into stearyltrimethylammonium chloride solution (1 g/L) for 3 min. One sample was afterwards washed with water and one with toluene and dried with nitrogen. The thickness of the model surfaces was determined using a manual nulling photoelectric ellipsometer (43702-200E Rudolph Research, Hackettstown, NJ) with a mercury lamp at a wavelength of 546.1 nm. Although this procedure leads to slightly incorrect values with respect to the absolute film thickness, it gives a quick and precise determination of the relative film thicknesses [52].
4. Results and Discussion
4.1. MWCNT/TO-NFC hybrids
4.1.1. Structural characterization
In order to prepare commingled NFC/CNT nanopaper structures from aqueous solution, it is critical to have CNT stably dispersed in the NFC water suspension. A surfactant, nonylphenol polyoxyethylene (10) phosphate ester (abbreviated as NPPE in the following), was used to modify the hydrophobic CNTs. Previous work by Heux and colleagues has shown that this surfactant can be used to coat the surface of cellulose whiskers and effectively aid their dispersion in organic solvents and poly(propylene) [55-57]. This surfactant has also been used to improve the dispersion of cellulose whiskers in polylactic acid (PLA) [58]. As shown in Figure 3a, unmodified MWCNT agglomerated and precipitated immediately after dispersion in water, while a homogenous and stable suspension of MWCNT (5 mg/mL) in water was obtained simply by adding MWCNT in 20 mg/mL NPPE water solution with sonication for 10 minutes (Figure 3b). NFC was prepared as described in the experimental section with a carboxylate content of 1.5 mmol/g of cellulose as determined by conductometric titration. This presence of carboxylate functionality contributes to the NFC dispersion and suspension stability. Without addition of NPPE, MWCNT precipitated out of the TO-NFC water suspension (Figure 3c). In contrast, a stable dispersion of MWCNT in TO-NFC water suspension (Figure 3d) was achieved by using MWCNT modified with NPPE, due to the strong repulsive dispersion force between the carboxylic group on the surface of TO-NFC and the
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Figure 3. Photograph of: MWCNT suspension (5 mg/mL) in water without (a) and with 20 mg/mL NPPE surfactant (b), MWCNT suspension mixed with TO-NFC suspension in water without (c) and with (d) NPPE surfactant (concentration is 0.1 mg/mL for TO-NFC and 0.01 mg/mL for MWCNT; corresponds to composite containing 9.1 wt% MWCNT).
phosphate group on NPPE modified MWCNT in water. The TO-NFC/MWCNT suspension was stable and formed no visible agglomerates during storage at room temperature for more than 6 months. Sodium dodecyl sulphate (SDS) and Triton x-100, common surfactants for CNT water suspension did not facilitate dispersion of CNT in NFC water suspension.
4.1.1. Morphology
The morphology of the nanopaper was observed using field emission scanning electron microscopy. The FE-SEM images are shown in the Figure 4 (a) – (d). In order to clearly distinguish the cellulose nanofibrils and the MWCNT in the composite, the surfaces of control samples containing only TO-NFC (a) and MWCNT (b) were observed. Furthermore, the distribution of MWCNT in the cellulose nanofibrils was investigated of the composites containing 9.1 wt% MWCNT. Both top and bottom surfaces were studied. It was found that MWCNT are homogenously distributed in the cellulose nanofibres matrix and that the distribution is homogenous throughout the sample. The orientation of both cellulose nanofibrils and MWCNT is random in plane.
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Figure 4. FE-SEM micrographs of (a) control sample of TO-NFC nanopapers, (b) control sample of MWCNT, (c) top surface of composite containing 9.1 wt% MWCNT and (d) bottom surface of composite containing 9.1 wt% MWCNT.
The morphology of samples containing different amount of MWCNT was also studied (Figure 5) and it was found that at 2.0 wt% MWCNT loading, the MWCNT are individually dispersed in the cellulose nanofibrils with or without very few contacts with each other (a). At 5.7 wt% MWCNT loading, much more connections between the MWNCT were observed (b) and at 9.1 wt% MWCNT, a continuous MWCNT network is formed (c). However, at 16.7 wt% MWCNT loading, agglomerates are found (d).
Figure 5. Morphology of nanopapers containing different amount of MWCNT: (a) 2.0 wt%, (b) 5.7 wt%, (c) 9.1 wt% and (d) 16.7 wt%.
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Figure 6. AFM micrographs of (a) control sample of TO-NFC nanopapers, (b) control sample of MWCNT, (c) top surface of composite containing 9.1 wt% MWCNT and (d) bottom surface of composite containing 9.1 wt% MWCNT.
Atomic force microscopy was used to observe the nanostructure of the material and the incorporation of the MWCNT into the cellulose nanofibrils matrix. Figure 6 shows AFM phase images of (a) TO-NFC, (b) MWCNT, (c) top surface of conductive nanopaper containing 9.1 wt% MWCNT and (d) bottom surface of nanopaper containing 9.1 wt% MWCNT. By comparing the dimensions of MWCNT and cellulose nanofibrils using micrographs of control samples (a) and (b), it is obvious that MWCNT are well incorporated into cellulose nanofibrils matrix.
These structural observations indicate that fairly homogeneous nanopaper structures based on NFC and CNT have been prepared using NPPE surfactant as a compatibilizer. The nanopapers prepared with SDS or Triton x-100 as the surfactants resulted in heterogenous structures with CNT agglomerates on the bottom surface of the nanopaper (image not shown). Although SDS and Triton x-100 are efficient at dispersing CNT in water, they do not provide compatibility with NFC. We are however aware of attempts to reduce use of NPPE surfactant, therefore we are going to test other surfactants having similar hydrophilic-lipophilic balance as NPPE.
4.1.2. Mechanical properties
The stress-strain behaviour in uniaxial tension of the samples is presented in Figure 7. Associated property data and the standard deviations are shown in Table 1. At low strains (<1%) the stress-strain behaviour is linear elastic. At a stress in the region of 90 – 120 MPa, there is a knee in the curve (apparent yield stress σ0.2) followed by a linear and fairly strongly strain-hardening region. This region is termed the plastic region[21].
The slope of the linear elastic region of the curves corresponds to the Young´s modulus. There is slight decrease in the Young´s modulus with increasing concentration of MWCNT. Same behaviour is observed for the tensile strength of the material. On the other hand there was significant decrease in the strain to failure at very low MWCNT loading (0.5 – 1 wt%). However, at MWCNT loading higher than 2 wt%, the strain to failure increases back to its original value and does not change significantly.
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(a) (b)
Figure 7. (a) Typical stress-strain curves of nanopapers at different MWCNT loadings
(the values indicate the volume content of NFC/MWCNT in the composite, taking porosity into account) and (b) Flexible conductive nanopaper with 9.1 wt% MWCNT.
The strain at which yield phenomenon occurs decreases with increasing MWCNT content. It was suggested [21] that the yielding is associated with the onset of interfibril debonding and nanofibril slippage that is facilitated by voids. This could also explain the slight decrease in strength of the composites with increasing MWCNT content, because the porosity of the nanopapers also increases (Table 1). Meaning that more voids are present to facilitate slippage. Moreover, there is no chemical bond between the MWCNT and cellulose nanofibrils. Thus, the presence of MWCNT, as well as increased porosity, results in reducing the density of hydrogen bonds between the fibrils. However, it should be emphasized that the nanopaper retains its mechanical properties fairly well up to 9.1 wt% of MWCNT. The material keeps its flexibility and toughness while in comparison with plastics, glass, ceramics and metals it is lightweight and can be easily folded.
Table 1. Properties of conductive nanopapers (the standard deviations are in parentheses).
MWCNT content
(weight %)
Density (kg/m3)
Porosity (%)
MWCNT content
(volume %)
Young´s modulus E (GPa)
Tensile strength σ (MPa)
Strain to failure ε (%)
0 0.889 40.7 0.0 9.26 (0.43) 239 (23) 6.2 (0.9) 0.5 0.850 43.4 0.2 9.34 (0.57) 177 (21) 3.6 (0.7) 1.0 0.906 39.8 0.4 9.15 (0.65) 199 (35) 4.8 (1.2) 2.0 0.960 36.5 0.9 8.78 (0.33) 212 (8.6) 6.1 (0.4) 4.8 0.848 44.5 1.9 8.54 (0.43) 208 (13) 6.5 (1.0) 5.7 0.859 44.0 2.3 8.25 (0.27) 205 (13) 6.6 (0.8) 7.4 0.823 46.7 2.9 7.54 (0.22) 210 (15) 7.2 (0.6) 9.1 0.820 47.3 3.5 7.72 (0.24) 183 (8.7) 6.0 (0.4)
16.7 0.731 54.3 5.8 2.60 (0.16) 662 (8.3) 6.9 (1.1)
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4.1.3. Surface resistivity
Electrical properties of nanopapers were studied by increasing the amount MWCNT in the cellulose nanofibril matrix for two different sample thicknesses (25 and 50 µm). The dependence of the surface resistivity on the MWCNT concentration is shown in Figure 8. There was a decrease in the surface resistivity of approximately two orders of magnitude at a MWCNT loading of 5.7 wt% for sample with thickness 50 µm. For sample with thickness of 25 µm, the decrease of surface resistivity is observed at MWCNT concentration 7.4 wt%. Further, an increase of MWCNT led to a decrease of the surface resistivity by almost 4 orders of magnitude with respect to the control samples containing only cellulose for samples of both thicknesses.
Two different thicknesses of conductive nanopaper were studied. There is no significant difference between the surface resistivity and in fact there is no reason to expect that there should be. Surface resistivity is determined by the ratio of DC voltage drop per unit of length to the per unit width. In other words it is the electrical resistance of the surface of the material. Therefore this value is independent of the physical dimensions (i.e., thickness and diameter) of the material. The similar values of surface resistivity of sample with different thickness suggest that the distribution of carbon nanotubes through the thickness of the specimen is homogenous as well. In case of inhomogeneous distribution of carbon nanotubes through the thickness resulting in higher concentration on the surface or bulk, the surface resistivity of thicker sample would be higher or lower respectively.
Figure 8. Surface resistivity values for TO-NFC/MWCNT nanopapers of 25 and 50 µm thicknesses.
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4.2. Preparation of cellulose nanocrystals
4.2.1. Hydrochloric acid hydrolysis
Cellulose nanocrystals were prepared by hydrochloric acid hydrolysis of cellulose nanofibrils prepared by TEMPO-mediated oxidation of wood pulp (Figure 9). The acid concentration and the temperature were kept constant while the reaction time was varied between 3 and 7 hours.
Typically, for the isolation of cellulose nanocrystals, concentrated mineral acid is used. Initial acid removes polysaccharide material closely bonded to the microfibril surface, resulting in an overall decrease of amorphous material. Subsequent hydrolysis breaks down those portions of the long glucose chains in accessible, non-crystalline regions. A degree of polymerization that plateaus is achieved; this corresponds to the residual highly crystalline regions of the original cellulose fibre. The precise physical dimensions of the crystallites depends on several factors, including the source of cellulose, the exact hydrolysis conditions and ionic strength [7]. The hydrolysis conditions have been shown to affect the properties of the resulting nanocrystals. For example, hydrochloric acid yields cellulose rods with minimal surface charge showing limited dispersibility (their aqueous suspensions tend to flocculate [59]), whereas the use of sulfuric acid provides highly stable aqueous suspensions, due to the esterification of surface hydroxyl groups resulting in sulfate groups [60]. It was also found that a longer reaction time leads to shorter nanocrystals. Specifically, a rapid decrease of the size is observed at early stages of hydrolysis and a slowing down in the later stages. This is because at the early stage of the hydrolysis, the acid diffuses preferentially into the non-crystalline portions of cellulose and hydrolyses the accessible glycosidic bonds. After these more accessible glycosidic bonds in the fibres are hydrolyzed, further reaction occurs much slowly at the reducing end and at the surface of the residual crystalline regions [61].
The dry content of the suspensions and the yield of the reactions were determined by gravimetric analysis. The yield of the hydrolysis decreased with increasing reaction time from 85, 70 and 69 % respectively.
Figure 9. Schematic illustration of acid hydrolysis of TO-NFC.
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4.2.2. Chemical structure
The presence of carboxylic groups after TEMPO-mediated oxidation and after acid hydrolysis was confirmed by FT-IR. The spectra (Figure 10 a) show characteristic bands of cellulose: the most intensive band at around 1050 cm-1 corresponds to the pyranose ring ether band [13]. After TEMPO-mediated oxidation, there is a new band around 1730 cm-1 corresponding to the C=O stretching frequency of carbonyl groups in the acidic form [59]. This band is absent in the spectrum corresponding to initial wood pulp, but is not affected by the hydrolysis.
The total carboxylate content was determined by conductometric titration. The curves obtained by titration are shown on Figure 10 b and the results are summarized in Table 2. Each of the titration curves contains three intervals: the first region (slightly decreasing part of the curve) corresponds to neutralization of the protons liberated by the added salt. The middle region (slightly increasing part of the curve) corresponds to neutralization of the carboxylic groups, during which the conductivity remains essentially unchanged. The added sodium ions (Na+) are associated as counter-ions to the carboxylic acidic groups and the dissociated protons are neutralized by the added hydroxide ions (OH-). The third region (strongly increasing part) is related to an excess of sodium hydroxide in the system causing an increase in the conductivity (Total acidic group content protocol). It was found that the surface charge of the particles was not affected by the hydrolysis.
One of the main disadvantages presented by the preparation of aqueous suspension of cellulose nanocrystals by the sulfuric acid method is that the resulting sulfate moieties at the nanocrystals surfaces are rather labile, being in particular readily removed under mild alkaline conditions [59]. Cellulose nanocrystals prepared by hydrochloric acid (HCl) or hydrobromic acid (HBr) hydrolysis have limited dispersibility in water and tend to
(a) (b)
Figure 10. (a) FT-IR spectra confirming the presence of carboxylic groups and (b) curves obtained by conductometric titration used for quantification of carboxylate content.
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Table 2. Carboxylate content determined by conductometric titration
TO-NFC CNC-3 CNC-5 CNC-7 Carboxylate content (mmol/g)
1.46 1.50 1.50 1.50
flocculate. In order to improve their dispersion in water, subsequent TEMPO-mediated oxidation has been employed to convert C6 primary hydroxyls to carboxylic acid groups, which are more chemically stable than the sulfate esters [62-64]. By using this route, i.e. TEMPO-mediated oxidation on HCl or HBr hydrolyzed cellulose, a maximum degree of oxidation (DO) value of ca. 0.20 corresponding to a carboxyl groups content of 1.2 mmol/g can be achieved [62, 63]. However, the maximum DO vaules of TEMPO-oxidized nonhydrolyzed wood pulp were higher than those of TEMPO-oxidized HCl-hydrolyzed samples [50]. Inspired from this observation, an alternate new route, HCl hydrolysis of TEMPO-oxidized cellulose nanofibrils (TO-NFC) from softwood pulp was applied to prepare cellulose nanocrystals since higher surface charge density is essential for further functionalization on the surface of CNC. TO-NFC with a carboxylate content of 1.46 mmol/g corresponding to a DO value of 0.25, was used evaluate this new route.
4.2.3. Morphology
The morphology and the size distribution of cellulose nanocrystals were investigated using atomic force microscopy (Figure 11). The length of the nanocrystals was measured using image analysis software Image J and the diameter was determined from transverse line profile of nanocrystals, or in other words as the height difference between the mica substrate and the particle (Figure 11 c). This technique was chosen to avoid the broadening effect caused by the AFM tip [65, 66]. It was found that with increasing reaction time the particle length distribution gets narrower, whereas the diameter of the particles remains almost unchanged (Table 3). There is only a slight increase in thickness of the particles in sample CNC-7, but this is most likely due to a larger amount of agglomerates present in the suspension.
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Figure 11. Atomic force microscopy images (a) and (b) used for nanocrystals size determination and (c) schematic illustration of width determination using the height difference between the nanocrystals and the mica wafer.
Figure 12. X-ray diffraction patterns of the original pulp fibres, TO-NFC, and CNCs
obtained by different hydrolysis time.
20
X-ray diffraction measurements were performed in order to verify any alternations of cellulose crystallinity due to chemical reaction (Figure 12). X-ray diffraction patterns obtained for cellulose nanocrystals, TEMPO oxidized nanofibrils and wood pulp show patterns characteristic for cellulose I crystalline structure. Two peaks centered at about 14.8 and 16.8 in the X-ray diffraction patterns, which correspond to d-spacing of 0.60 − 0.61 nm and 0.53 − 0.54 nm respectively, were separated by curve-fitting using pseudo-Voigt function [67]. The crystal sizes of the corresponding planes were calculated from full widths at half heights of the diffraction peaks by Scherrer’s equation [68] and the data are summarized in Table 3. The crystal sizes of the CNC samples did not change as compared to TO-NFC, which is comparable to the results of AFM. The crystallinity of the CNC samples is 78 – 80%, which is higher than the value for TO-NFC and the original softwood pulp due to the degradation of the non-crystalline region by HCl hydrolysis. Altogether these results suggest CNC bearing high content of carboxyl groups on the surfaces have been successfully prepared for the first time via direct HCl hydrolysis of TO-NFC.
Table 3. Properties of cellulose nanocrystals.
Nanocrystals dimensions (AFM) Crystal size (nm)a Crystallinity (%) Length (nm) Width (nm) C1 C2 CA
TO-NFC - 2.4 (0.7) 2.8 2.4 2.6 66 CNC 3h 331 (140) 2.3 (0.1) 2.6 2.4 2.5 79 CNC 5h 278 (115) 2.7 (1.1) 2.5 2.5 2.5 80 CNC 7h 288 (94) 3.6 (1.1) 2.6 2.4 2.5 78 aThe C1 and C2 are crystal sizes of the planes corresponding to a d-spacing of 0.60 – 0.61 and 0.53 – 0.54 nm, respectively. The CA is the average value of C1 and C2.
4.3. Surface modification of cellulose nanocrystals
4.3.1. Adsorption of quaternary ammonium salts
Cellulose nanocrystals were modified using 4 different quaternary ammonium salts: stearyltrimethylmmonium chloride (C18), glycidyl trimethylammonium chloride (COC), diallyldimethylammonium chloride (C=C) and phenyltrimethylammonium chloride (CPh), Figure 13. A more detailed study was performed for C18, where three different molar ratios of cellulose charged groups and C18 were studied (CNC:C18 = 1:1, 1:2 and 1:4). The samples were weighed after the freeze-drying to calculate the adsorbed amount and the amount of C18 in the modified sample and the results are summarized in Table 4.
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Table 4. Determination of C18 content after the adsorption.
CNC:C18 ratio Adsorbed amount
(mg) C18 content in the
sample (mmol) Real ratio CNC:C18
1:1 7.30 0.021 1:0.28 1:2 24.1 0.069 1:0.92 1:4 23.3 0.067 1:0.89
The adsorption isotherm of C18 has been previously studied [69]. The concentration 3 mmol/L was chosen due the fact that the molecules at this concentration form admicelles in water. Formation of admicelles enables having CNC well dispersed in water even after modification. During the re-dispersion and washing step with toluene, the admicelles are re-organized, resulting in possible monolayer surfactant coverage of the surface of CNC.
It was found that ratio of cellulose and C18 1:1 is not sufficient to provide entire surface coverage of the available groups of cellulose. On the other hand there is no difference in the surface coverage for samples 1:2 and 1:4. The coverage is approximately 90 % in both cases, which seems to be the maximum value that can be achieved with the given method. Therefore for further experiments with all quaternary amines, a molar ratio 1:2 was chosen as sufficient to obtain maximum surface coverage.
Figure 13. Schematic illustration of ion exchange reaction with stearyltrimethylmmonium chloride (1), phenyltrimethylammonium chloride (2), glycidyl trimethylammonium chloride (3) and diallyldimethylammonium chloride (4).
OHO OHO OHO OO OO OONaOH aq.pH 10
N Cl
16
N
16
N16
N
16
N Cl N Cl N Cl
O
1
2 3 4
cellulose cellulose
OO OO OO
cellulose
Na Na Na
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4.3.2. Chemical characterization
Fourier Transform Infrared Spectroscopy (FT-IR) confirmed that the surface modification of CNC was successful (Figure 14). Before the adsorption (CNC-3 curve), the spectrum of CNC shows a peak at 1730 cm-1 (corresponding to C=O stretching of carboxyl groups in their acidic form). While the spectrum of the modified nanocrystals, shows that this peak is shifted to approximately 1600 cm-1, which is assigned to the carboxylate group.
Moreover, strong bands at 2900 cm-1 and 2850 cm-1 corresponding to asymmetrical and symmetrical CH2 stretching from the long alkyl chain of stearyltrimethylammonium chloride, were observed in the spectrum of modified CNC [70]. The intensity of this peak increases from the ratio 1:1 to 1:2, but there is no difference in the intensity of the peaks between samples 1:2 and 1:4. Figure 14 b shows FT-IR spectra of CNC modified with CPh, COC, CC and C18. In all cases, the peaks corresponding to carboxyl groups were shifted peaks corresponding to carboxylates.
Modified freeze-dried nanocrystals were dispersed in acetone, THF, chloroform and toluene. In acetone and THF, very poor suspensions were obtained; particles were visible by naked eye and the suspension sediment rather quickly. Better suspension was obtained in chloroform; translucent suspension was obtained. Finally, transparent suspension was obtained in toluene for samples CNC/C18 1:2 and higher. Sample CNC/C18 1:1 gives translucent suspension in toluene. This could be due to a lower surface coverage of cellulose with C18 that does not provide sufficient hydrophobicity. On the other hand, ratio 1:4 did not further improve the quality of the suspension. These observations further supported previous hypothesis that molar ratio 1:2 between CNC and C18 is sufficient.
(a) (b)
Figure 14. FT-IR spectra of modified CNC with (a) C18 using three different molar ratios
and (b) four different quaternary ammines.
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4.3.3. Morphology
The morphology of the modified CNC dispersed in toluene was observed using FE-SEM (Figure 15). A suspension of cellulose was deposited ether on a copper grid to detect transmitted electrons or on a mica wafer to capture secondary electron images. The unmodified CNCs are well individualized in water with a typical dimensions of 300 – 500 nm in length and 3 – 4 nm in width (Figure 15 a). A good dispersion was observed for the toluene suspension of the CNCs modified with C18 (Figure 15 b). The modified nanocrystals have similar dimension in length as compared to unmodified ones, but their widths are higher (around 15 nm) due to aggregates of a few nanocrystals in the form of bundles. Concentrated suspensions (5 wt% for unmodified nanocrystals and 10 wt% for C18 modified nanocrystals) in glass vials were observed between crossed polarizers. Both suspensions showed birefrigency (Figure 15 c and d).
Figure 15. Transmitted electrons images of (a) unmodified nanocrystals in water and (b)
C18 modified nanocrystals in toluene deposited on copper grid. Secondary electron images of (c) unmodified nanocrystals in water and (d) C18 modified nanocrystals in toluene deposited on a mica wafer. The images in the upper corner of (c) and (d) show corresponding concentrated suspensions between crossed polarizers.
4.3.4. Contact angle measurement
The model surface containing unmodified CNC has a contact angle with water of 12°. The model surface prepared from C18 modified cellulose nanocrystals washed with water showed a higher contact angle with water of 48°. The model surface of C18 modified CNC washed with toluene showed on the other hand much higher contact angle with water of 71°. This shows the importance of the washing step, during which the admicelles reorganize themselves and the excess of surfactant is removed. The water drops and the AFM images of the corresponding surfaces are shown in Figure 16.
24
Figure 16. Contact angle and corresponding AFM image of model surface of (a)
unmodified CNC and (b) C18 modified nanocrystals washed with water and (c) C18 modified nanocrystals washed with toluene. The size of the AFM images in the upper corners is 5 µm.
Quaternary ammonium salts have already been used in modification of nanofibrillated cellulose to reduce the water wettability [69, 71]. In these studies, adsorption isotherms and contact angle were investigated and the presence of surfactant double layers and admicelles was confirmed. However, re-dispersion of the quaternary amine modified nanocellulose in organic solvent is reported for the first time here. Re-dispersion in organic solvent (or washing of model surface with toluene) might lead to reorganization of the micelles and the presence of monolayer of surfactant resulting in a more hydrophobic cellulose surface.
The main advantage of surface modification of cellulose nanocrystals using adsorption approach is its simplicity, flexibility, high yield and broad scale of possible functional groups that can be introduced to the surface of cellulose nanocrystals.
We present a simple and variable toolbox for the preparation and the surface modification of cellulose nanocrystals.
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5. Conclusions
The aim of this thesis was to study surface functionalization of nanocelluloses and their use in functional materials.
In Paper I, a simple method was developed for preparation of lightweight and electrically conductive nanopaper structures. The strength and toughness (work to fracture) of cellulose nanofibril networks is combined with the electrical conductivity of carbon nanotube networks in the creation of commingled nanopaper structures. TO-NFC cellulose nanofibrils were mixed with NPPE surfactant modified MWCNT to form a stable aqueous suspension. A papermaking approach was used to form the commingled nanopaper structures without the use of organic solvents or other chemical modification. It was discovered that the CNT surface treatment has to not only provide CNT dispersion in water, but also to ensure homogeneous mixing in the NFC/CNT suspension mixture. Thus, the surfactant also acts as a compatibilizer.
The development of smart paper technology includes improving the properties and adding new functionalities to paper. Electrically conductive paper is of particular interest due to the potential applications in electromagnetic interference shielding, electronic circuits, active matrix displays, high strength cables and substrates [72], and loudspeaker membranes [73].
In Paper II, a new and environmentally friendly route for isolation and surface modification of nanocellulose was developed using simple water-based adsorption of quaternary ammonium salts. It was shown that different functionalities can be introduced to cellulose, opening up new possibilities to disperse otherwise hydrophilic cellulose in non-polar solvents. This new series of modified CNC can be dried from solvent in non-agglomerated form and have the potential to form well-dispersed nanocomposites with non-polar polymers, through mixing with thermoplastic melts or liquid prepolymer/monomers. Studies of utilization of modified cellulose nanocrystals in nanocomposites and also application of this approach to cellulose nanofibrils are currently in progress.
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6. Acknowledgements
First of all, I would like to express my gratitude to my supervisor Lars Berglund for scientific guidance throughout this work. I would also like to thank my co-supervisor Qi Zhou for his assistance and help.
All my co-workers are acknowledged for their contributions and scientific discussions; I really enjoy working with you!
Lars Wågberg and Richard Olsson are acknowledged for valuable discussions.
Former and present members of the Biocomposites group, the Wallenberg Wood Science Center and the Fibre and Polymer Technology Department are gratefully acknowledged. Not only for their generous help with my research, but also for creating a nice working environment and being such great friends to me.
Richard Andersson is acknowledged for his help with the image processing as well as Christina Schütz for typographic corrections and Stacy Trey for kind help with language corrections.
The Wallenberg Wood Science Center is thanked for providing financial support.
Finally, I would like to thank my family and friends for their love and moral support.
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7. References
1. Eichhorn, S.J., et al., Review: current international research into cellulose nanofibres and nanocomposites. Journal of Materials Science, 2010. 45(1): p. 1-33.
2. Moon, R.J., et al., Cellulose nanomaterials review: structure, properties and nanocomposites. Chemical Society Reviews, 2011. 40(7): p. 3941-3994.
3. Isogai, A., T. Saito, and H. Fukuzumi, TEMPO-oxidized cellulose nanofibers. Nanoscale, 2011. 3(1): p. 71-85.
4. Saito, T., et al., Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose. Biomacromolecules, 2007: p. 2485-2491.
5. Henriksson, M., et al., An environmentally friendly method for enzyme-assisted preparation of microfibrillated cellulose (MFC) nanofibers. European Polymer Journal, 2007. 43(8): p. 3434-3441.
6. Zhou, Q., H. Brumer, and T. Teeri, Self-Organization of Cellulose Nanocrystals Adsorbed with Xyloglucan Oligosaccharide-Poly(ethylene glycol)-Polystyrene Triblock Copolymer. Macromolecules, 2009: p. 5430-5432.
7. Fleming, K., D. Gray, and S. Matthews, Cellulose crystallites. Chemistry-a European Journal, 2001: p. 1831-1835.
8. Paakko, M., et al., Enzymatic hydrolysis combined with mechanical shearing and high-pressure homogenization for nanoscale cellulose fibrils and strong gels. Biomacromolecules, 2007. 8(6): p. 1934-1941.
9. Habibi, Y., L.A. Lucia, and O.J. Rojas, Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications. Chemical Reviews, 2010. 110(6): p. 3479-3500.
10. Yi, J., et al., Temperature-induced chiral nematic phase changes of suspensions of poly(N,N-dimethylaminoethyl methacrylate)-grafted cellulose nanocrystals. Cellulose, 2009: p. 989-997.
11. Heux, L., G. Chauve, and C. Bonini, Nonflocculating and chiral-nematic self-ordering of cellulose microcrystals suspensions in nonpolar solvents. Langmuir, 2000: p. 8210-8212.
12. Elazzouzi-Hafraoui, S., J. Putaux, and L. Heux, Self-assembling and Chiral Nematic Properties of Organophilic Cellulose Nanocrystals. Journal of Physical Chemistry B, 2009: p. 11069-11075.
13. Pei, A., Q. Zhou, and L. Berglund, Functionalized cellulose nanocrystals as biobased nucleation agents in poly(L-lactide) (PLLA) - Crystallization and mechanical property effects. Composites Science and Technology, 2010: p. 815-821.
14. Pei, A.H., et al., Strong Nanocomposite Reinforcement Effects in Polyurethane Elastomer with Low Volume Fraction of Cellulose Nanocrystals. Macromolecules, 2011. 44(11): p. 4422-4427.
15. Rueda, L., et al., Isocyanate-rich cellulose nanocrystals and their selective insertion in elastomeric polyurethane. Composites Science and Technology, 2011. 71(16): p. 1953-1960.
28
16. Pei, A.H., Q. Zhou, and L.A. Berglund, Functionalized cellulose nanocrystals as biobased nucleation agents in poly(L-lactide) (PLLA) - Crystallization and mechanical property effects. Composites Science and Technology, 2010. 70(5): p. 815-821.
17. Fortunati, E., et al., Multifunctional bionanocomposite films of poly(lactic acid), cellulose nanocrystals and silver nanoparticles. Carbohydrate polymers, 2012. 87(2): p. 1596-1605.
18. Cranston, E.D. and D.G. Gray, Morphological and optical characterization of polyelectrolyte multilayers incorporating nanocrystalline cellulose. Biomacromolecules, 2006. 7(9): p. 2522-2530.
19. Qi, H., et al., Chiral Nematic Assemblies of Silver Nanoparticles in Mesoporous Silica Thin Films. Journal of the American Chemical Society, 2011. 133(11): p. 3728-3731.
20. Shopsowitz, K.E., et al., Free-standing mesoporous silica films with tunable chiral nematic structures. Nature, 2010. 468(7322): p. 422-U246.
21. Henriksson, M., et al., Cellulose nanopaper structures of high toughness. Biomacromolecules, 2008. 9(6): p. 1579-1585.
22. Sehaqui, H., et al., Fast Preparation Procedure for Large, Flat Cellulose and Cellulose/Inorganic Nanopaper Structures. Biomacromolecules, 2010. 11(9): p. 2195-2198.
23. Sehaqui, H., et al., Strong and Tough Cellulose Nanopaper with High Specific Surface Area and Porosity. Biomacromolecules, 2011. 12(10): p. 3638-3644.
24. Sehaqui, H., et al., Wood cellulose biocomposites with fibrous structures at micro- and nanoscale. Composites Science and Technology, 2011. 71(3): p. 382-387.
25. Nogi, M., et al., Optically Transparent Nanofiber Paper. Advanced Materials, 2009. 21(16): p. 1595-+.
26. Syverud, K. and P. Stenius, Strength and barrier properties of MFC films. Cellulose, 2009. 16(1): p. 75-85.
27. Liu, A.D., et al., Clay Nanopaper with Tough Cellulose Nanofiber Matrix for Fire Retardancy and Gas Barrier Functions. Biomacromolecules, 2011. 12(3): p. 633-641.
28. Spence, K.L., et al., The effect of chemical composition on microfibrillar cellulose films from wood pulps: water interactions and physical properties for packaging applications. Cellulose, 2010. 17(4): p. 835-848.
29. Nishi, Y., et al., The Structure and Mechanical-Properties of Sheets Prepared from Bacterial Cellulose .2. Improvement of the Mechanical-Properties of Sheets and Their Applicability to Diaphragms of Electroacoustic Transducers. Journal of Materials Science, 1990. 25(6): p. 2997-3001.
30. Zoppe, J.O., et al., Reinforcing Poly(epsilon-caprolactone) Nanofibers with Cellulose Nanocrystals. Acs Applied Materials & Interfaces, 2009. 1(9): p. 1996-2004.
31. Nystrom, G., et al., Ultrafast All-Polymer Paper-Based Batteries. Nano Letters, 2009. 9(10): p. 3635-3639.
29
32. Henriksson, M. and L.A. Berglund, Structure and properties of cellulose nanocomposite films containing melamine formaldehyde. Journal of Applied Polymer Science, 2007. 106(4): p. 2817-2824.
33. Svagan, A.J., M.A.S.A. Samir, and L.A. Berglund, Biomimetic polysaccharide nanocomposites of high cellulose content and high toughness. Biomacromolecules, 2007. 8(8): p. 2556-2563.
34. Sehaqui, H., Q. Zhou, and L.A. Berglund, Nanostructured biocomposites of high toughness-a wood cellulose nanofiber network in ductile hydroxyethylcellulose matrix. Soft Matter, 2011. 7(16): p. 7342-7350.
35. Sehaqui, H., et al., Mechanical performance tailoring of tough ultra-high porosity foams prepared from cellulose I nanofiber suspensions. Soft Matter, 2010. 6(8): p. 1824-1832.
36. Zhou, Q., et al., Xyloglucan in cellulose modification. Cellulose, 2007. 14(6): p. 625-641.
37. Svagan, A.J., M.A.S.A. Samir, and L.A. Berglund, Biomimetic foams of high mechanical performance based on nanostructured cell walls reinforced by native cellulose nanofibrils. Advanced Materials, 2008. 20(7): p. 1263-+.
38. Sehaqui, H., Q. Zhou, and L.A. Berglund, High-porosity aerogels of high specific surface area prepared from nanofibrillated cellulose (NFC). Composites Science and Technology, 2011. 71(13): p. 1593-1599.
39. Gebauer, D., et al., A transparent hybrid of nanocrystalline cellulose and amorphous calcium carbonate nanoparticles (vol 3, pg 3563, 2011). Nanoscale, 2011. 3(12): p. 5187-5187.
40. Schütz, C., et al., Hard and transparent films formed by nanocellulose-TiO2 nanoparticle hybrids. Manuscript.
41. Galland, S., et al., Cellulose nanofibrils decorated by inorganic nanoparticles and used in magnetic nanocomposite membranes of high toughness. Manuscript, 2012.
42. Fugetsu, B., et al., Electrical conductivity and electromagnetic interference shielding efficiency of carbon nanotube/cellulose composite paper. Carbon, 2008. 46(9): p. 1256-1258.
43. Oya, T. and T. Ogino, Production of electrically conductive paper by adding carbon nanotubes. Carbon, 2008. 46(1): p. 169-171.
44. Jung, R., et al., Electrically conductive transparent papers using multiwalled carbon nanotubes. Journal of Polymer Science Part B-Polymer Physics, 2008. 46(12): p. 1235-1242.
45. Kim, Y., et al., Transparent Conducting Films Based on Nanofibrous Polymeric Membranes and Single-Walled Carbon Nanotubes. Journal of Applied Polymer Science, 2009. 114(5): p. 2864-2872.
46. Yan, Z.Y., et al., Cellulose synthesized by Acetobacter xylinum in the presence of multi-walled carbon nanotubes. Carbohydrate Research, 2008. 343(1): p. 73-80.
47. Park, W.I., et al., Synthesis of bacterial celluloses in multiwalled carbon nanotube-dispersed medium. Carbohydrate polymers, 2009. 77(3): p. 457-463.
30
48. Zhang, H., et al., Regenerated-cellulose/multiwalled-carbon-nanotube composite fibers with enhanced mechanical properties prepared with the ionic liquid 1-allyl-3-methylimidazolium chloride. Advanced Materials, 2007. 19(5): p. 698-+.
49. Yun, S. and J. Kim, Characteristics and performance of functionalized MWNT blended cellulose electro-active paper actuator. Synthetic Metals, 2008. 158(13): p. 521-526.
50. Saito, T., et al., Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose. Biomacromolecules, 2007. 8(8): p. 2485-2491.
51. Chen, X.H., et al., Non-destructive purification of multi-walled carbon nanotubes produced by catalyzed CVD. Materials Letters, 2002. 57(3): p. 734-738.
52. Wagberg, L., et al., The build-up of polyelectrolyte multilayers of microfibrillated cellulose and cationic polyelectrolytes. Langmuir, 2008: p. 784-795.
53. Baggerund E., S.S., Lindström T., Measurement of volume fractions of solid, liquid and gas in kraft and CTMP paper at varying moisture content, in Proceedings of the international paper physics conference2003. p. 157 - 163.
54. Hussein L., U.G., Krüger M., Fabrication and characterization of buckypaper-based nanostructured electrodes as a novel material for biofuel cell applications. Physical Chemistry Chemical Physics, 2011. 13: p. 5831 - 5839.
55. Bonini, C., et al., Rodlike cellulose whiskers coated with surfactant: A small-angle neutron scattering characterization. Langmuir, 2002. 18(8): p. 3311-3314.
56. Heux, L., G. Chauve, and C. Bonini, Nonflocculating and chiral-nematic self-ordering of cellulose microcrystals suspensions in nonpolar solvents. Langmuir, 2000. 16(21): p. 8210-8212.
57. Ljungberg, N., et al., New nanocomposite materials reinforced with cellulose whiskers in atactic polypropylene: Effect of surface and dispersion characteristics. Biomacromolecules, 2005. 6(5): p. 2732-2739.
58. Bondeson, D. and K. Oksman, Dispersion and characteristics of surfactant modified cellulose whiskers nanocomposites. Composite Interfaces, 2007. 14(7-9): p. 617-630.
59. Habibi, Y., H. Chanzy, and M. Vignon, TEMPO-mediated surface oxidation of cellulose whiskers. Cellulose, 2006: p. 679-687.
60. Beck-Candanedo, S., M. Roman, and D. Gray, Effect of reaction conditions on the properties and behavior of wood cellulose nanocrystal suspensions. Biomacromolecules, 2005: p. 1048-1054.
61. Dong, X., J. Revol, and D. Gray, Effect of microcrystallite preparation conditions on the formation of colloid crystals of cellulose. Cellulose, 1998: p. 19-32.
62. Araki, J., M. Wada, and S. Kuga, Steric stabilization of a cellulose microcrystal suspension by poly(ethylene glycol) grafting. Langmuir, 2001. 17(1): p. 21-27.
63. Filpponen, I. and D.S. Argyropoulos, Regular Linking of Cellulose Nanocrystals via Click Chemistry: Synthesis and Formation of Cellulose Nanoplatelet Gels. Biomacromolecules, 2010. 11(4): p. 1060-1066.
31
64. Habibi, Y., H. Chanzy, and M.R. Vignon, TEMPO-mediated surface oxidation of cellulose whiskers. Cellulose, 2006. 13(6): p. 679-687.
65. Kvien, I., B.S. Tanem, and K. Oksman, Characterization of cellulose whiskers and their nanocomposites by atomic force and electron microscopy. Biomacromolecules, 2005. 6(6): p. 3160-3165.
66. Elazzouzi-Hafraoui, S., et al., The shape and size distribution of crystalline nanoparticles prepared by acid hydrolysis of native cellulose. Biomacromolecules, 2008. 9(1): p. 57-65.
67. Wada, M., T. Okano, and J. Sugiyama, Synchrotron-radiated X-ray and neutron diffraction study of native cellulose. Cellulose, 1997. 4(3): p. 221-232.
68. Okita, Y., T. Saito, and A. Isogai, Entire Surface Oxidation of Various Cellulose Microfibrils by TEMPO-Mediated Oxidation. Biomacromolecules, 2010. 11(6): p. 1696-1700.
69. Alila, S., et al., Adsorption of a cationic surfactant onto cellulosic fibers - I. Surface charge effects. Langmuir, 2005. 21(18): p. 8106-8113.
70. Siqueira, G., J. Bras, and A. Dufresne, New Process of Chemical Grafting of Cellulose Nanoparticles with a Long Chain Isocyanate. Langmuir, 2010: p. 402-411.
71. Xhanari, K., et al., Reduction of water wettability of nanofibrillated cellulose by adsorption of cationic surfactants. Cellulose, 2011. 18(2): p. 257-270.
72. Anderson, R.E., et al., Multifunctional single-walled carbon nanotube-cellulose composite paper. Journal of Materials Chemistry, 2010. 20(12): p. 2400-2407.
73. Xiao, L., et al., Flexible, Stretchable, Transparent Carbon Nanotube Thin Film Loudspeakers. Nano Letters, 2008. 8(12): p. 4539-4545.
