Influence of fibrillation method on the character of nanofibrillated cellulose (NFC) Pöhler, T.1, Lappalainen, T.1, Tammelin, T.1, Eronen, P.2, Hiekkataipale, P.2, Vehniäinen, A.3, Koskinen, T.M.3 The Finnish Centre for Nanocellulosic Technologies 1 VTT Technical Research Center of Finland 2 Aalto University, School of Science and Technology 3 UPM-Kymmene Oyj EXTENDED ABSTRACT Increasing research efforts have been directed towards the production and analysis of the properties of nanofibrillated cellulose (NFC), aiming for industrial exploitation in various material applications. Different fibrillation methods, mechanical and chemical, yield fibrils with varying size distributions and length/diameter- ratios. Indirect methods, such as turbidity, can be used to estimate fibril size. However, direct and accurate assessment of the NFC fibril dimensions may bring a new understanding of the fibrillation mechanisms. High-resolution microscopy renders this possible. The objective of this investigation was to compare high-resolution microscopy techniques for determining fibril diameter distributions, and to produce new quantitative data on NFC fibrils prepared using different techniques. Fibril thickness and width distributions were measured by image analysis of the images from a field emission scanning electron microscope (FE-SEM), a cryogenic transmission electron microscope (cryo-TEM) and an atomic force microscope (AFM). Information obtained using the microscopy methods was complemented with an analysis of changes in the degree of polymerisation (DP) of cellulose. Nanofibrillated cellulose was prepared from hardwood kraft pulp with a mass colloider and a fluidizer. These NFC grades were then compared with chemically pre-treated and mechanically fibrillated NFC grades. The chemical pre-treatments included carboxymethylation, TEMPO-mediated oxidation and cationization. Some properties of nanocrystalline cellulose (NCC) whiskers and of commercial microfibrillated cellulose (MFC) were analysed for reference. Sample preparation for AFM and FE-SEM imaging was conducted by spin-coating /1, 2/. A scattered dry fibril monolayer on an extremely flat surface enables very accurate fibril thickness measurements with AFM. For FE-SEM, a layer of carbon was evaporated on the sample surface. In cryo-TEM the fibrils were imaged in their wet state by vitrifying a thin NFC gel layer at -180 °C. Microscope images were quantified by using both a specially developed image analysis program (AFM images) and by manual measurements (cryo-TEM and FE-SEM images). Due to differences in sample preparation, in the technical resolution of the microscopes, and in the available number of pixels in the digital images, it is anticipated that the microscopy techniques report varying absolute values for the same NFC samples. Figure 1 shows some selected AFM height images of different NFC and NCC grades. Figure 2 presents the fibril thickness and width distributions acquired with different microscope techniques. Fibril thickness (AFM) and width (FE-SEM) distributions of chemically pre-treated NFC grades were significantly narrower compared to only mechanically fibrillated grades, overlapping at small fibril thickness and width. All three microscopy methods agreed that carboxymethylation resulted in thicker/wider fibrils than TEMPO-mediated oxidation. The difference in fibril thickness/width between the mass colloider and the fluidizer NFC grades was small. The most common height value in AFM images representing TEMPO fibrils was surprisingly small, below 2 nm. This means that the number of adjacent cellulose molecule chains would be around 5, assuming that the width of cellulose molecule is ~0.4 nm. The most common fibril width of wet TEMPO fibrils was ~3 nm seen with cryo-TEM. The differences may be the result of the state of the fibrils (dry vs. wet), slightly different analysis resolution and from statistical operations (different class size in frequency calculations). Accurate measurement of the NFC fibril diameter requires high magnifications. Due to the high length versus diameter ratio of the fibrils, the whole length of a single fibril cannot be seen in the microscope images. Analysis of

polymerization degree (DP) of cellulose implied that the fluidizing process cuts the length of fibrils more than grinding (mass colloider). Carboxymethylation preserved the DP at higher level compared to TEMPO-oxidation.

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Figure 1. AFM images (area 2x2 µm) of different NFC and NCC grades: a) fluidizer NFC b) NCC whiskers from cotton, same height bar from 0-20 nm for both images; c) carboxymethylated NFC and d) TEMPO-oxidized NFC, same height bar (0-6 nm) for both images.

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Figure 2. Fibril thickness and width distributions for the different NFC grades: results from a) AFM b) FE-SEM and c) cryo-TEM image analysis. High-resolution microscopy methods agree qualitatively on the differences in fibril diameter distributions of the various NFC grades. Chemically aided fibrillation methods give more homogeneous fibril material than do pure mechanical fibrillation methods. Fluidizing and grinding (mass colloider) techniques produce fibrils with similar diameter distributions, but with different fibril length. TEMPO-mediated oxidation results in fibrils that may consist only of a few adjacent cellulose molecules. REFERENCES /1/ Ahola, S., Salmi, J., Johansson, L.-S., Laine, J., Österberg, M., Model films from native cellulose nanofibrils. Preparation, swelling and surface interactions. Biomacromolecules 9(2008):4, pp. 1273-1282. /2/ Tapper, U., Tammelin, T., Österberg, M., Pere, J., Focusing and tapping on nanofibrillated cellulose (NFC) – methodology for microscopical characterisation. Poster presentation, EPNOE 2009, Polysaccharides as a source of advanced materials, Turku, September 21-24, 2009.

Influence of fibrillation method on the character of nanofibrillated cellulose (NFC)

Tiina Pöhler, Timo Lappalainen, Tekla Tammelin, VTTPaula Eronen, Panu Hiekkataipale, Aalto University

Annikki Vehniäinen, Timo M. Koskinen, UPM

The Finnish Centre for Nanocellulosic Technologies

Espoo, Finland

Background

• There are several ways to prepare NFC

• Magnitude of fibril dimensions can be evaluated by measuring, for example, turbidity

• Do we need direct assessment of the single fibril dimensions? o Understanding fibrillation mechanismso Product safety perspective

Approach

• High-resolution microscopy renders direct assessment of fibril diameter

• Common techniques: o Atomic force microscopy (AFM)o High resolution scanning electron

microscopy (FE-SEM)o Transmission electron microscopy

(TEM)

• Operating principles and sample preparation for these techniques differo Suitability for wide range of NFC

grades

50 nm

100 nm

Objectives

• To create numeric data of fibril diameter and its distributiono for various NFC gradeso with adequate resolutiono with a method that allows image analysis

Materials and methods

Samples: Different NFC grades

• Nanofibrillated cellulose from hardwood kraft pulp

– Mechanical fibrillationo Masscolloidero Fluidizer

– Chemical pre-treatment + mechanical fibrillationo Carboxymethylationo TEMPO-mediated oxidation o Cationization

• Reference materials– Nanocrystalline cellulose (NCC) whiskers from cotton– Commercial microfibrillated cellulose (MFC)

Different mechanical fibrillation principle

Effect of fiber charge on fibrillation

Appearance of NFC gelsMasscolloider Fluidizer

Carboxymethylation TEMPO oxidation Cationization

-24 mV -39 mVZeta potential: +41 mV

NFC grades vs. commercial MFCOptical microscope images, macrostructure

Masscolloider Fluidizer

TEMPO oxidationCarboxymethylation

Commercial MFC 1

Commercial MFC 2

Sample preparation for high-resolution microscopes

• Spin-coating: a sub monolayer of individual fibrils on an extremely flat surface /1, 2/ => AFM

• Spin-coated fibril substrate + evaporation of thin conductive carbon layer=> Field-emission SEM

• Vitrification of thin NFC gel layer at -180°C=> Cryo-TEM, imaging at -196°C (LN2)

Spin-coater

/1/ Ahola, S., Salmi, J., Johansson, L.-S., Laine, J., Österberg, M., Model films from native cellulose nanofibrils. Preparation, swelling and surface interactions. Biomacromolecules 9(2008):4, pp. 1273-1282. /2/ Tapper, U., Tammelin, T., Österberg, M., Pere, J., Focusing and tapping on nanofibrillated cellulose (NFC) – methodology for microscopical characterisation. Poster presentation, EPNOE 2009, Polysaccharides as a source of advanced materials, Turku, September 21-24, 2009.

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fluid-flow/evaporation substrate

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AFM substrates and image analysis

• Fibril height is measured locally along the fibril skeletono Height value must exceed 1 nmo Non-fibrillar material is discarded based on shape factoro Pixels representing fibril crossings are included

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AFM image of silicon wafer with polyethylene imine (PEI) coating

Pixel height distribution of the substrates

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Results

AFM images of various NFC gradesImage area 2 µm x 2 µm

Masscolloider Fluidizer

Carboxymethylation TEMPO oxidation

NCC whiskers (cotton)

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Image analysis of AFM images: dry fibril thickness

Resolution in z-direction: 0.1 nm Class size in statistics: 0.25 nm

• Masscolloider ≈ Fluidizer

• Distributions of mechanical grades and grades with increased charge overlapped

Manual analysis of FE-SEM images: dry fibril width

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Pixel resolution: 1.9 nm (CM and TEMPO), 3.1 nm othersClass size in statistics: 4 nm (CM and TEMPO), 7 nm others

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• Similar information than with AFM

• Clearly different numeric data

Manual analysis of cryo-TEM images: wet fibril width

Carboxymethylation

TEMPO oxidation

Pixel resolution: 0.2-0.5 nm Class size in statistics: 1.0 nm (TEMPO), 1.5 nm (CM)

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CarboxymethylationTEMPO oxidation

• Similar information than with AFM

• Different numeric data

The length dimension

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Effect of mechanical fibrillation method on cellulose chain length

Macrostructure

• The way the mechanical fibrillation is made affects the cellulose chain length

• Fluidizer shortens the chain length more than masscolloider

Observations on fibrillation mechanisms

• Mechanical fibrillation results in wide fibril diameter distributiono Masscolloider and fluidizer NFCs had similar distributionso Distributions of mechanical NFC grades and NFC grades with increased

charge overlapo Nanoscale fibrils may still be attached to larger fibrils

• Increase in fibre charge (+/-) results in narrow fibril diameter distributiono The higher the fibre charge, the smaller the fibril diameter in NFC

• Cellulose chain length is affected by the mechanical fibrillation methodo Fluidizer shortens the cellulose chain length more than masscolloider

Conclusions of the microscopy methods

• Differences in fibril diameter distributions of the various NFC grades were detected similarly with all the used methods

• AFM and TEM suited best for NFC grades with narrow fibril diameter distributiono Representativeness

• Numeric values for fibril diameter differed between the methodso Sample preparation (wet, dry, level of adhesion, coatings needed)o Analysis of height, analysis of widtho Technical resolution of the microscopeso Available number of pixels in the digital images

Acknowledgements

• UPM and TEKES (Finnish Funding Agency for Technology and Innovation)

• Panu Lahtinen, Kyösti Valta, Lauri Kuutti, Sauli Vuoti, VTT • Eero Kontturi, Karoliina Junka, Aalto University

Thank you!