Effect of Chemical Functionality on the Mechanical and Barrier

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Effect of Chemical Functionality on the Mechanical and Barrier Performance of Nanocellulosic Films Veronica Lopez Duran, Johannes Hellwig, Per Tomas Larsson, Lars Wågberg, and Per A. Larsson ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00452 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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ACS Applied Nano Materials

Effect of Chemical Functionality on the Mechanical and Barrier Performance of Nanocellulosic Films Verónica López Durán1,2,*, Johannes Hellwig1, P. Tomas Larsson3,4, Lars Wågberg1,2,4 Per A. Larsson1,2:*. 1

Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, SE11428 Stockholm, Sweden 2

BiMaC Innovation, KTH Royal Institute of Technology, SE-100 44, Sweden

3 4

RISE Bioeconomy, Drottning Kristinas väg 61, 114 28 Stockholm, Sweden

Wallenberg Wood Science Center, KTH Royal Institute of Technology, Teknikringen 56, 100 44 Stockholm, Sweden

Keywords: borohydride reduction, chemical modification, chlorite oxidation, ductility, nanocellulose, periodate oxidation, reductive amination, TEMPO oxidation.

Abstract In the present work, we have partially modified fibrils chemically to create a shell of derivatised cellulose that surrounds the crystalline core of native cellulose. Through the different modifications we aimed at creating a toolbox to enable the properties of CNF materials and materials containing CNFs to be tuned to meet specific material demands. In total, nine different chemical modifications using different aqueous-based procedures were used as chemical pre-treatments before CNF production through homogenisation. Eight of these modifications included periodate oxidation with an average of 27% of the anhydroglucose units in the cellulose chain being cleaved into di-aldehydes. The presence of aldehydes then facilitated a conversion to other functional groups. TEMPO oxidation was also performed, alone or combined with either periodate oxidation or periodate oxidation followed by borohydride reduction or chlorite oxidation. The nine different CNFs produced after chemical modifications were characterized by size, charge density and colloidal stability. Free-standing films were subsequently fabricated form the fibrils and characterized by 1 ACS Paragon Plus Environment

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mechanical, moisture adsorption and barrier properties, as well as by their degradation upon heating. The properties of the CNFs achieved through the different modification methods varied depending on the functional group present in the modified CNF shell. After periodate oxidation, no colloidal stability was observed. Interestingly, the periodate-oxidized fibres could be fairly easily homogenized at a concentration of 4 wt%, in contrast to 2 wt% for the other fibres. When charges were introduced onto the periodate-oxidized fibres, CNFs with a higher colloidal stability were produced but, to facilitate CNF production, the mass concentration during this step had to be decreased. After the sequential modification by periodate oxidation and borohydride reduction, a great increase in ductility in the films was observed, but, interestingly, when more charges were introduced, this increase in ductility was lost, and the mechanical strength increased. CNFs without further modification after the periodate oxidation resulted in crosslinked films with low moisture sorption and a low oxygen permeability; 0.2–0.7 mL⋅µm/ (m2⋅24h ⋅kPa) at 50% RH, and 0.3–14 mL⋅µm/ (m2⋅24h ⋅kPa) at 80% RH depending on the modification performed.

Introduction Cellulose nanofibrils (CNFs) have attracted a huge interest because of the intriguing inherent properties of this material1–3. They are nanosized fibrils with a width of 3–5 nm, large specific surface area, high strength, high stiffness and a low weight while being abundant with good biodegradability1. CNFs have found applications in nanocomposites4, barrier films5,6, flame-retardant materials7, flexible screens8, printed electronics9 and in paper and paperboard10,11. In order to produce CNFs, a chemical or enzymatic pre-treatment12,13 is usually used to ease the processing and decrease the energy consumption of the process. Examples

of

14,15

oxidation

commonly

reported

chemical

pre-treatments

16

17,18

, carboxymethylation , cationic modification

are

TEMPO-mediated

and phosphorylation7.

In earlier studies it has been shown that after periodate oxidation it is possible to produce nanocellulose films with reduced moisture sensitivity19. During periodate oxidation, the C2– C3 bond is cleaved to produce dialdehyde cellulose, which facilitates further conversion to different functional groups. Periodate oxidation, followed by borohydride reduction20 or chlorite oxidation21 , has also been shown to be effective as a pre-treatment to produce CNFs. Films prepared from dicarboxylic acid cellulose were reported to be stiff but fairly brittle22, while dialcohol cellulose provided significantly improved ductility. However, very little is known about how combinations of different functional groups and how these groups will affect the properties of CNFs and materials made thereof. 2 ACS Paragon Plus Environment

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In a recent publication from our laboratory22, handsheets were prepared from fibres containing dialdehyde cellulose due to periodate oxidation. The aldehydes were further reduced, aminated or oxidized. Moreover, TEMPO-mediated oxidation was used alone or combined with periodate oxidation or further reduction with sodium borohydride. However, the degree of modification was kept low, about 11 %, to avoid fibrillation of the fibres for the modifications introducing charges to the fibres. It was found that the chemical structure has a strong influence on the mechanical performance of handsheets made from these fibres, i.e. the shell of the modified fibrils within the fibres controls the properties of the handsheets. In some cases, the results were difficult to separate from the control due to the low degree of modification achieved and the effect that the structure of the fibre network imposes on the mechanical properties of the material. In the present work, we have prepared CNFs by homogenisation after a combination of different chemical modifications of cellulose fibres. The modifications predominantly introduced functional groups in the outer parts of the CNFs, ideally creating a shell of modified cellulose surrounding the core of native crystalline cellulose, i.e. upon water removal the dry solid material formed can be considered being an all-cellulose composite constituted by native and derivatised cellulose23. The functional group present in the core surrounding each CNF seems to be vital for the properties of the resulting material19–21, thus we also have aimed to gain a better understanding of the relationship between chemical structure and material properties by studying thin films fabricated from core-shell modified CNFs, i.e. by eliminating the influence of the macro- and microscopic network structures facilitating use of higher degrees of modification compared with our previous study. The investigated chemical modifications could potentially serve as a platform for selection of CNFs for new applications. In order to produce modified CNFs, we pre-treated cellulose fibres by periodate oxidation, partly forming dialdehyde cellulose. The dialdehydes formed were then either reduced, aminated or oxidized. For comparison, the well-studied TEMPO-mediation oxidation was also used and combined with the described modifications. The resulting fibrils were characterized in terms of size by atomic force measurements, colloidal stability and total surface charge. Finally, films prepared from the different CNFs were characterized by mechanical performance, dynamic mechanical analysis, moisture sorption, thermal gravimetric analysis and barrier properties

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Experimental Materials Chlorine-free bleached unbeaten softwood kraft pulp (K48) was supplied by SCA Forest Products (Östrand pulp mill, Timrå, Sweden). The fibres were first disintegrated and washed to have sodium as counter ion to the charges within the fibres according to an earlier reported procedure24. Sodium hydroxide and hydrochloric acid standard solutions were purchased from Merck Millipore. Sodium carbonate (≥ 99.5%), sodium metaperiodate (99%), hydroxylamine hydrochloride (99%), sodium borohydride (≥96%), 2-propanol (99.9%), 4-acetamido-2,2,6,6tetramethylpiperidine 1-oxyl (≥98.0%), sodium chlorite (80%), sodium hypochlorite solution (10–15% available chlorine), glacial acetic acid, sodium acetate (99%), sodium phosphate monobasic monohydrate (98%) and hydrogen peroxide (30 wt% in water) were purchased from Sigma-Aldrich.

Methods Chemical modifications

Periodate oxidation: Fibres were suspended in water to a concentration of 20 g/L, containing 6.3 wt% of 2-propanol as a radical scavenger, and heated to 50 °C. When the temperature reached 50°C, 1.35 g NaIO4/ g fibre was added to the suspension and allowed to react for 2 h. The container was covered with aluminium foil to protect the reactants from the light. After the reaction the pH of the fibre suspension was 3.5.

Borohydride reduction: Periodate oxidized fibres were suspended to a concentration of 8 g/L in the presence of NaH2PO4 0.01 M. Thereafter, 0.5 g NaBH4/g fibre was added to the suspension and allowed to react for 2 h at room temperature (RT)25. TEMPO-mediated oxidation: Non-modified fibres were oxidized as previously described26. In brief, fibres were dispersed in 0.1 M acetate buffer at a pH of 4.8. NaClO2 was then added followed by NaClO and 4-AcNH-TEMPO. The reaction was allowed to react at 40 °C for 48 h. After the fibres had been washed the remaining aldehydes27 were further oxidized using sodium chlorite.

Chlorite oxidation: After periodate oxidation the fibres were redispersed to a concentration of 20 g/L, and NaClO2 and H2O2 were added to a concentration of 2.5 mol per mol of 4 ACS Paragon Plus Environment

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aldehyde, previously determined by titration with hydroxylamine. The suspension was acidified with 1 M HCl to pH 5 and allowed to react for 48 h at RT.

Reductive amination: Before the amination, 250 mL of hydroxylamine 0.25 M solution and a suspension of 0.1 – 0.2 g of periodate oxidized fibres were mixed to a final fibre concentration of 10 g/l, the pH was set to pH 4 and the reactants allowed to react for 2 h at RT. Then, an in-situ reduction of the imine group was carried out by adding 100 mL of NaH2PO4 0.1 M and 0.5 g NaBH4/g fibre. The reaction was then continued another 2 h at RT. All the reactions were stopped by washing the fibres with deionised water until the conductivity was below 5 µS/cm. The pH was then adjusted to 9 and kept for 20 min, followed by washing with water to a conductivity lower than 5 µS/cm. The scheme of modifications is shown in Figure 1 and a summary of the reactions is presented in Table S1 in the Supporting information.

Figure 1. A scheme summarizing the chemical modifications of the fibres

Carbonyl content: The amount of aldehydes formed was measured by titration with hydroxylamine hydrochloride28,29. A volume of 25 mL of 0.25 M of hydroxylamine at a pH 4 5 ACS Paragon Plus Environment


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was allowed to react for 2 h with about 0.2 g of fibres, which had also been adjusted to pH 4 before washing. Thereafter, the fibres were filtered and placed in an oven at 105 °C to determine the exact amount of fibres used, and the filtrate was titrated back to pH 4 using 0.1 M NaOH. The carbonyl content was calculated from the moles of NaOH consumed. Titrations were performed in triplicate.

Total charge density: The amount of carboxylic acids was determined according to SCANCM 65:02. The fibre suspension was set to pH 2 with HCl and kept at this pH for 20 min followed by washing with water until the conductivity was below 5 µS/cm. Thereafter, 5 mL of 0.1 M HCl and 10 mL of 0.01 M NaCl were added to the fibre suspension to a total volume of 500 mL. The fibres were then titrated with 0.1 M NaOH until a conductivity of ~140 µS/cm was achieved (i.e. until there was a clear linear increase in conductivity as a function of titrant volume). After titration, the suspension was filtered and the fibres were placed in an oven at 105 °C to determine the dry weight. Two titrations were made per sample.

Fibril preparation: CNFs were prepared by fluidization using a Microfluidizer M-110 EH (Microfluidics Corp., Westwood, MA, USA) with two chambers connected in series; first by A–B passes through 400 µm and 200 µm diameter chambers at a pressure of 900 bar, followed by C–D passes through 200 µm and 100 µm diameter chambers at a pressure of 1500 bar. Highly charged fibres were passed four times through the small chambers and low charged fibres seven times through the small chambers.

Surface charge density: The surface charge (i.e. carboxylic acids)

of the CNFs was

measured using a Stabino particle charge mapping equipment (Partic Metrix Gmbh, Meerbusch Germany) according to an earlier described procedure30. The CNFs were first diluted to a concentration of 0.1 wt%, and 1 mL of the dispersion was then added to 9 mL of water. The CNFs were titrated with poly(diallyldimethylammonium chloride) (pDADMAC) with a molecular weight of 400–500 kDa and a total charge of 0.351 µeq/mL. Three measurements were performed for each sample.

Gravimetric yield: The gravimetric yield, i.e. of the colloidally stable CNFs after homogenization, was determined by Ultra Turrax mixing followed by centrifugation. The CNFs were dispersed to a concentration of 0.2 wt% and mixed at 12 000 rpm for 15 min, before being centrifuged at 4000 rpm for 1 h. The supernatant was then carefully separated 6 ACS Paragon Plus Environment

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from the sedimented pellet followed by dry content determination. The gravimetric yield of the colloidally stable CNF was then simply calculated as the mass percentage found in the supernatant.

Atomic Force Microscopy: The width (and for some grades also the length) of the CNFs produced was measured using an atomic force microscope (AFM) (Mutlimode IIIa, Veeco Instruments, CA). The individual CNFs were spin coated (3000 rpm, 40s, KW4A Chemat Technology) onto a PEI pre-coated silicon wafer and the surface topography of the spincoated samples was scanned in the tapping mode (tap150 cantilever, Bruker). After the height profile of the silicon surface and the CNFs had been obtained, a cross section profile was used to calculate the fibre dimensions (NanoScope Analysis 1.5, Bruker) by averaging a minimum of 20 data points for each sample. No compensation from the tip width was made.

Film preparation: CNFs were diluted to a concentration of 0.2 wt% and dispersed for 10 min at 12000 rpm using an IKA Ultra Turrax and then vacuum filtered through a 0.45 µm membrane (Durapore, Merck Millipore). After filtration, another membrane of the same type was placed on top of the film and the whole assembly was placed in between two paperboards and dried for 15 min at 93 °C under a reduced pressure of 95 kPa, using the dryers of a Rapid Köthen sheet former (Paper Testing Instruments, Austria). Preparation of CNF films by filtration is a common method that allows comparison with the results of different research groups.

Tensile test: Tensile tests were performed under a controlled climate of 23 °C and 50% RH using an Instron 5944 equipped with a 500 N load cell. The test pieces had a width of 5 mm, a free span of 15 mm in between the clamps and were strained at a constant rate of 1.5 mm/min. A total of ten specimens were tested per sample, those which failed at the jaw face being excluded. The Young´s modulus was calculated from the slope of the stress–strain curve in the region of 0.1–0.2% strain. The thickness of the films was measured using a digital Mitutoyo thickness gauge at ten random locations.

Dynamic mechanical analysis: A Dynamic Mechanical Analyser (DMA) (Perkin-Elmer Instruments, model DMA7e) combined with a moisture generator was used to measure the storage modulus as a function of relative humidity, i.e. at different moisture contents in the sample. The samples were first conditioned at 30 °C and 20% RH. Thereafter, the humidity 7 ACS Paragon Plus Environment


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was increased at a rate of 0.4 % units/min until the humidity reached 90%. The samples were then kept at 90% RH for 30 min. The temperature was kept at 30 °C during the whole measurement. Each sample mas measured in duplicate.

Moisture sorption measurements: Dynamic vapour sorption (DVS) measurements were performed using a Q5000SA DVS (TA instruments). The samples were first conditioned at 30 °C and 20% RH during 60 min, followed by an increase in the humidity at a rate of 0.4 %units/min until the humidity reached 90%. Thereafter, the samples were kept at 90% RH for 30 min. A microbalance continuously measured the weight of the sample and the temperature was kept at 30 °C throughout the measurement. Two samples were measured for each material. Oxygen permeability measurements: The oxygen permeability was evaluated for 5 cm2 samples using a MOCON OX-TRAN 2/21 (Mocon Inc, Minneapolis, MN, USA) according to the ASTM D3985 and ASTM F1927 standards. The permeability measurements were performed at 23 °C and 50% RH or 80% RH, using the same relative humidity on both sides of the sample, i.e. the apparatus precisely controlled the RH, according to the specifications of the standard, of both sides of the sample.

Results and discussion

Chemical modification and characterization of modified fibres and CNFs

Periodate oxidation was performed on kraft softwood fibres (containing 15–20% hemicellulose31, which, however, undergo the same chemical reactions) to achieve about 27% of modification. The choice of this value was related to a previous study which showed that at higher degrees of modification the strength of the materials formed started to decrease due to a too large decrease in crystallinity19,25. A too high degree of modification would also severely affect the gravimetric yield of the chemical modification. The aldehydes formed by the periodate oxidation allow conversion to different functional groups. In this study, the aldehydes were converted to primary alcohols, carboxylic acids or aminated with hydroxylamine and it was assumed that the degree of modification was the same after the sequential modifications. In combination with the aldehydes, alcohols and carboxylic acids, TEMPO oxidation also performed, which introduce carboxylic acids in the C6 position of 8 ACS Paragon Plus Environment

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cellulose. Both with and without TEMPO, the amount of aldehydes, i.e. degree of modification, was the same, 3.2 mmol/g, in order to better be able to compare the properties of the resulting films. However, 30 minutes more were needed to reach the same carbonyl content when carboxyl groups were present, i.e. the periodate oxidation was somewhat inhibited by the presence of carboxyl groups on the C6 carbon. This is in agreement with earlier reports that the reaction is inhibited when acidic moieties are present, causing electrostatic repulsion between them and the negatively-charged periodate ions32. No aldehydes were here detected in any of the reactions which included an additional reaction step after periodate oxidation (Samples 3–5 and Samples 8–9). The total charge density before homogenization and the surface charge after homogenization were determined and the results are shown in Table 1. The reference material (Sample 1), dialdehyde cellulose (Sample 2), dialcohol cellulose (Sample 3) and reductively aminated material (Sample 4) all showed significant differences between their total and their surface charge densities, which indicates the presence of fibril aggregates in these samples, i.e. the total charge is significantly higher than the determined surface charge. In general, it was observed that a higher total charge density resulted in a relatively higher surface charge density, which most probably is due to an increased degree of fibrillation. Particularly, when combining periodate and chlorite oxidation (Sample 5) or TEMPO, periodate and chlorite oxidation (Sample 9), resulting in a high charge density of the cellulose rich material, the total charge and the surface charge were very similar. Fall et al.33 observed the same trend when different charge densities were introduced in cellulosic fibres prior to homogenization to CNFs.



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Table 1. Carbonyl content, surface and charge density of the modified fibres and Gravimetric yield (GY) of CNFs. Surface charge

Carbonyl

Charge

content

density

(µmol /g)

(µmol/g )

1:Ref

30 ± 10

60 ± 3

28 ± 1

48

2:IO4

3170 ± 90

45 ± 4

13 ± 1

2

3:IO4+BH4

0*

36 ± 1

22 ± 2

95

4:IO4+NH2OH/BH4

0*

47 ± 2

9±1

24

5:IO4+ClO2

0*

3280±20

3190 ± 45

96

6:TEMPO

0*

960 ± 60

800 ± 60

93

7:TEMPO+IO4

3190 ± 80

870 ± 30

740 ± 5

92

8:TEMPO+IO4+BH4

0*

785 ± 20

531 ± 2

99

9: TEMPO+IO4+ClO2

0*

3560 ± 50

3400 ± 80

99

Sample

(after homogenization)

GY (%)

(µmol/g)

*Below the detection limit of the method

After periodate oxidation (Sample 2) and consecutive reductive amination (Sample 4), the gravimetric CNF yields after homogenization decreased to 2% and 24% respectively. Interestingly, despite the low charge density and removal of hemicelluloses after borohydride reduction (Sample 3) the gravimetric yield of this sample was 95%. It is suggested that dialcohol cellulose forms a shell that surrounds the native cellulose core of each CNF20, a layer that is highly hydrated, and that presumably is of benefit for the colloidal stability despite the low charge density. CNFs prepared from TEMPO oxidized fibres (Sample 6) showed a colloidally stable fraction of 93%. CNFs prepared after TEMPO oxidation followed by a periodate oxidation (Sample 7) had a colloidally stable fraction of 92%, indicating that the high charge density achieved by TEMPO oxidation was indeed able to stabilise the poorly stable dialdehyde CNFs and almost completely avoid sedimentation during centrifugation. In the case of di- and tricarboxylic acid cellulose, Samples 5 and 9 respectively, the gravimetric CNF yield in both cases was 99%.


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Morphological characterization of CNFs Figure 2 shows the dimensions of the different CNFs measured by AFM. The CNFs vary considerably in sizes. In general, the modified CNFs had typical widths between 2 and 10 nm depending on the modification. A more significant difference was found in length, where the length was basically different for every modification. The reference CNFs (Sample 1), where larger aggregates were also seen more frequently, had a length between 1000 to 1800 nm. After periodate oxidation (Sample 2), fibrils with a size between 200 and 1100 nm were observed. However, large fibre fragments were also observed (Figure S3 in the supporting information). Dialcohol cellulose CNFs (Sample 3), on the other hand, showed a decrease in length, with typical lengths between 400 nm and 800 nm. A similar size was observed when periodate oxidation and borohydride reduction were preceded by TEMPO oxidation (Sample 8). After reductive amination, relatively large aggregates were observed (Figure 2d). After TEMPO oxidation (Sample 6), the fibrils had a typical length between 1000 nm and 3000 nm. When periodate and chlorite oxidation (Sample 5) and TEMPO, periodate and chlorite oxidation were carried out (Sample 9), a decrease in the size of the CNFs was observed to a length of 200–400 nm, i.e. a length similar to that of cellulose nanocrystals. This is similar to the sizes observed by Yang et al. (2012)34 and could be the result of hydrolysis caused by the acidic conditions used during the reaction. In general, the different CNFs had a good propensity to form volume filling arrested states35, with the exception of the periodateoxidized and aminated CNFs (Sample 2 and Sample 4, respectively), which tended to phase separate and sediment.



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Figure 2. AFM images showing typical dimensions of the different CNFs, spin-coated on polymer-coated silica surfaces: a) no further chemical modification (reference); b) periodate oxidized; c) periodate oxidized and borohydride reduced; d) periodate oxidized and reductive amination with hydroxylamine; e) periodate and chlorite oxidized; f) TEMPO oxidized; g) TEMPO and periodate oxidized; h) TEMPO and periodate oxidized and borohydride reduced; i) TEMPO, periodate and chlorite oxidised.

Mechanical characterization of CNF films CNF films were prepared by vacuum filtration of dispersions of the various CNFs and the mechanical performance of these films is shown in Figure 2 (and Table S4 in Supporting information). Films prepared from CNFs from unmodified fibres (Sample 1) had a tensile stress of around 267 MPa and a strain-at-break of about 7%. A similar result was achieved after TEMPO oxidation, i.e. when introducing a carboxyl group onto the C6 position. However, a lower concentration was used during fluidisation when preparing CNFs from 12 ACS Paragon Plus Environment

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unmodified fibres, in combination with more passes through the fluidizer. Periodate-oxidized CNFs (Sample 2) showed a stress-at-break of 167 MPa and a strain-at-break of 3.5%. This decrease in both strength and ductility is presumably due to the formation of hemiacetal crosslinks between the aldehydes introduced and the abundant hydroxyl groups in cellulose, consequently hindering plastic deformation of the films19. The shortening of the fibrils can naturally also have a negative influence on the mechanical properties of the films. This effect was even more noticeable when TEMPO and periodate oxidation were combined (Sample 7), where the strain-at-break decreased to about 2%. As shown in Figure 3, dialcohol cellulose CNF films (Sample 3) had a strain-at-break of 13.6%, presumably because of the shell of dialcohol cellulose, allowing for yielding in the individual contacts between the CNFs, i.e. facilitating significant plastic deformation in the film20. However, when TEMPO oxidation, periodate oxidation and borohydride reduction were combined (Sample 8), the characteristic ductility of dialcohol cellulose decreased to 9%, although displaying a higher stress-at-break. It has been suggested36,37 that when polyacrylic acid is mixed with CNFs there is a strong interfacial interaction due to hydrogen bonding, but the relative effect of these hydrogen bonds compared with, for example, van der Waals interactions was not established. It was also suggested that these hydrogen bonds could impart restrictions on segments of the polymer chain, leading to higher stiffness and strength while reducing the ability of the polymer chains to rotate and slide past one another, which could explain the decrease in ductility observed for Sample 8. There is no doubt that these latter modification will change the interactions between the fibrils, compared with the interaction between dialcohol coreshell fibrils (Sample 3), leading to a more brittle material but the molecular interactions causing this change are still not fully established. The results also show that at the same degree of modification, dialcohol cellulose CNF films (Sample 3) and dicarboxylic acid cellulose CNF films (Sample 5) display significantly different mechanical behaviour. In fact, films formed from dicarboxylic acid CNF were very stiff, but also very brittle, without introduction of covalent cross-links. After the sequential oxidation using TEMPO, periodate and chlorite oxidation (Sample 9), the prepared CNF film had a stress-at-break of 159 MPa and a Young´s modulus of 8.6 GPa. This result is higher compared with that of Yang et al.38 who reported a tensile strength of 63 MPa and a Young´s Modulus of 4.0 GPa. It should, however, be mentioned that films from di- and tricarboxylic acid CNFs were very difficult to prepare due to a poorer retention of the short CNFs (Figure 4) on the membrane used. To fabricate films, the CNF suspension, with an original concentration of 1 wt%, had to be filtered several times to retain the particles. The results also show that a reductive amination 13 ACS Paragon Plus Environment


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of hydroxylamine showed no improvement in the mechanical properties of the films. On the contrary, it dramatically decreased the mechanical performance, giving a stress-at-break of only 85 MPa and a strain-at-break of 1.6%. According to these results, reductive amination (with hydroxylamine) does not therefore seem to be an appropriate method to pre-treat fibres if the aim is to produce strong CNF films. Since these films basically can be described as allcellulose nanocomposites, it is of interest to compare them with other CNF nanocomposites.

In the context of green composites, nanocellulose is considered an ideal candidate due to high strength

and

stiffness.

Sehaqui

et

al.39

prepared

composites

from

NFC

and

hydroxyethylcellulose (HEC). The strain of the films varied between 8 and 35%, and the stress between 200 and 50 MPa when different ratios NFC/HEC were used. Comparing these results with for example films produced after periodate oxidation and borohydride reduction (Structure 3), similar results were obtained. Furthermore, the strain-at-break in the present study can be further increased with higher degree of modification. Spoljaric et al.6 prepared composites from NFC, polyvinyl alcohol (PVA) and montmotrillonite (MMT). In order to produce NFC the fibres were first beated followed by microfluidization to achieve a solid content of 1.39 wt% suspension. Crosslinked films were also produced by adding polyacrylic acid (PAA) and mixing it with PVA followed by drying in the oven at 140 °C for 10 min. Films had a strain-at-break between 3% and 6% and it was observed that the ductility decreased as the concentration of MMT increased while a tensile strength between 110 and 180 MPa was reported. In this case, crosslinked films had higher tensile strength compared with non-crosslinked ones. These results can be compared with the results after the sequential modification of TEMPO, periodate oxidation and borohydride reduction, since both functional groups are introduced. Furthermore, in our case we use the modification not only to tune specific properties of the materials but also as a pre-treatment which facilitates the preparation of the films.


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Figure 3. Characteristic stress–strain curves of films made from chemically modified CNFs.

Hygromechanical properties Moisture has a plasticising effect on carbohydrate and therefore the effect of moisture after the different modifications was of interest. This was determined as the loss in storage modulus when the relative humidity was gradually increased. As shown in Figure S3 in the Supporting information , the storage modulus of films prepared from non-modified cellulose (Sample 1) decreases as the relative humidity increased. Films of dialcohol cellulose CNFs (Sample 3) and TEMPO combined dialcohol cellulose (Sample 8) showed a material softening already at 40% RH, the decrease in modulus being more pronounced for the latter. This could be explained by the presence of carboxylic acids, known to readily sorb moisture and cause swelling and softening of the material. Dicarboxylic acid cellulose (Sample 5) and tricarboxylic acid cellulose (Sample 9), showed a slow initial decrease in storage modulus but, as the humidity increased the loss in modulus was more pronounced due to the high charge density of these films. A decrease in storage modulus was also observed in the films made from CNFs after TEMPO oxidation (Sample 6), but in this case the rapid loss of storage modulus started at a higher RH than for di- and tricarboxylic acid cellulose. This softening can clearly be seen in Figure S4a (Supporting information), showing the loss modulus as function of relative humidity. Primary alcohols after reductive amination (Sample 4) were less sensitive to relative humidity than primary alcohols from dialcohol cellulose (Sample 3). 15 ACS Paragon Plus Environment


Page 16 of 25

They turned out to be less sensitive, displaying softening at a higher RH. CNF films produced after periodate oxidation (Sample 2) showed a limited decrease in storage modulus, even at high relative humidities. When carboxylic groups were introduced and combined with dialdehyde cellulose (Sample 7), a similar small change in storage modulus was observed and somewhat unexpected since the carboxylic acids would increase the moisture sensitivity but obviously the dialdehyde cellulose dominated the properties of the films. At high relative humidity the modulus started to decrease though. In this case the decrease in the storage modulus was more pronounced compared to the decrease observed for the dialdehyde cellulose CNF film. If the samples were kept at 90% for 30 min at the end of the RH scan, the storage modulus continued to decrease in some samples, i.e. indicating that moisture sorption equilibrium had not been reached in these samples. To clarify the dynamics detected in the DMA measurements, moisture sorption in the CNF films was studied as a function of relative humidity (Figure S4b in the Supporting information). The reference CNFs (Sample 1) showed a 10% increase in mass when the relative humidity was increased from 20 % RH to 90 % RH, and a similar change, at a similar rate, was observed for the films prepared of fibrils following TEMPO oxidation (Sample 6). After periodate oxidation alone (Sample 2) and TEMPO and periodate oxidation (Sample 7), a lower rate of adsorption was observed, and a smaller relative change in moisture content at the end of the measurement. However, when Sample 7 was exposed to high RH, a significant adsorption was eventually observed, suggesting that on extended exposure to a high relative humidity the carboxylic acids facilitate moisture sorption as expected. The highest increase in mass was observed for tricarboxylic acid cellulose (Sample 8), which could be expected due to the ability to sorb water and swelling facilitated by the carboxylic acids, and similar behaviour in the case of the dicarboxylic acid cellulose (Sample 5). For the preparation of both di- and tri-carboxylic acid periodate oxidation was performed, and therefore a decrease in crystallinity also occurred. It has been reported that a decrease in crystallinity is associated with an increase in moisture sorption40.

Thermal stability of films The thermal stability of the different CNFs was determined by TGA, to study the effect that functional groups have on the degradation of partially modified cellulose. As can be seen in Figure 4, the chemical structure greatly affects the thermal behaviour of the CNFs. The degradation of all the modified samples started at lower temperatures than the degradation of the reference. Dialdehyde cellulose CNFs (Sample 2) was less stable at lower temperatures 16 ACS Paragon Plus Environment

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than the reference, but at higher temperatures dialdehyde cellulose was more stable. This effect was also observed by Parks (1971)41. The increase in stability is related to dehydration and the formation of hemiacetal linkages, which could also increase the residue at higher temperatures. The degradation of dialcohol cellulose (Sample 3) started at 275 °C, i.e. about 25 °C before the reference, probably because primary hydroxyl groups are more easily eliminated as water. The sample aminated with hydroxylamine (Sample 4), displayed a twostep decomposition; a sudden weight loss was observed at around 200 °C, assigned to the hydroxylamine moiety which facilitates the formation of volatile compounds, followed by the later decomposition of unmodified cellulose42. The degradation of dicarboxylic acid cellulose (Sample 5) started at 250 °C, this decomposition at a lower temperature was also observed both for the TEMPO oxidized sample (Sample 6) and the tricarboxylic acid cellulose sample (Sample 9). The introduction of carboxylic acids at C6, C2 and C3 positions had the greatest effect on the thermal stability, probably due to decarboxylation during heating43. In general, a higher residue mass was obtained at higher charge density, due to the presence of counter ions.

Figure 4. Thermogravimetric analysis of chemically modified CNFs in nitrogen.

Barrier properties of films Cellulose films are good oxygen barriers under dry conditions44, and, as shown in Table 2, low oxygen permeability were obtained at 50 % RH for

all the chemically modified

materials. When the oxygen permeability was evaluated at 80% RH, a greater sensitivity was 17 ACS Paragon Plus Environment


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observed, except for dialdehyde cellulose (Sample 2), which showed the same value as at 50% RH. The presence of crosslinks in this material prevents the CNFs from separating from each other upon moisture sorption and thereby prevents the opening of diffusion paths for the oxygen molecules19. When carboxyl-containing CNFs were periodate oxidized, films displayed a relatively high oxygen permeability, probably due to a higher affinity to moisture (Figure 4) and a greater swelling, creating a shorter diffusion path. Liu, et al (2011)45 prepared composites with CNFs and montmorillonite clay . They reported an oxygen permeability of 0.45 mL⋅µm/(m2⋅24 h⋅kPa) at 50% RH for composites containing 50% CNFs and 50% clay and 35 mL⋅µm/(m2⋅24 h⋅kPa) when the films were tested at 90% RH. Spoljaric et al (2014)6 reported oxygen permeability values between 2 and 0.06 mL.µm/(m2.24h.kPa) at 50% RH and

2 – 0.05 mL.µm/(m2.24h.kPa) at 0% RH when

composites of PVA/NFC/MMT were produced, and PAA was used as crosslinker. In our case, films prepared after periodate oxidation also maintained the oxygen permeability at high RH and the values were comparable to those with 50 % of clay, moreover, in our case the films could be prepared directly after homogenization which greatly decreased and simplifies sample preparation.

Table 2. Oxygen permeability of films made of chemically modified CNFs 23 °C, 50 % RH

23°C, 80 % RH

(mL⋅µm/(m2⋅24h⋅kPa))

(mL⋅µm/(m2⋅24h⋅kPa))

1:Ref

0.7

13.1

2:IO4

0.3

0.3

3:IO4+BH4

0.4

11.4

4:IO4+NH2OH/BH4

0.4

4.3

5:IO4+ClO2

0.5

10.2

6:TEMPO

0.4

11.9

7:TEMPO+IO4

0.4

7.7

8:TEMPO+IO4+BH4

0.2

9.5

9: TEMPO+IO4+ClO2

0.5

14.0

Sample


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Conclusions Nanocellulose was produced by a combination of different chemical modifications to introduce different functional groups in the outer part of the cellulose nanofibrils, in an forming a shell of modified cellulose that surrounds the core of native crystalline cellulose. It was found that the properties and behaviour of the CNFs and films made from these fibrils are highly dependent on the functional group within the fibrils. A high charge density resulted in better colloidal stability, measured as a larger mass fraction in the supernatant after centrifugation of dilute dispersions, but, somewhat surprisingly, after a sequential modification consisting of periodate oxidation and borohydride reduction, (i.e. with no introduction of charges), an equally high gravimetric yield of 95% was observed. Furthermore, after periodate oxidation, the gravimetric yield after centrifugation decreased to only 2% (the untreated reference had 48 %) and, even without centrifugation, this CNFs tended to sediment. However, this effect was suppressed when charges were introduced in cellulose backbone. The width of the nanofibrils, measured by AFM ranged from 200 nm to micrometres. Films produced from the different CNFs were mechanically tested and it was found that a combination of periodate oxidation and borohydride reduction resulted in a high strain-atbreak. This increase in strain-at-break could not be achieved by any other of the chemical modifications studied. The presence of carboxylic acids leads to an increase in tensile strength and Young´s modulus, but a decrease in strain-at-break was also observed. The introduction of aldehydes, on the other hand, resulted in brittle films, but also a decreased in the moisture sorption rate and a maintained high modulus even at high relative humidity. Finally, the barrier properties of the different films showed values between 0.2–0.7 mL⋅µm/(m2⋅24h⋅kPa) at 50% RH, but at 80% RH the films started to swell and the oxygen permeability increased, with the exception of the more moisture-resistant films fabricated from periodate-oxidized CNFs, which still displayed a low permeability. An advantage with the methods used is that all the reactions are water based. Some of the chemicals such as periodate or TEMPO can be recycled. The waste after chlorite oxidation can, for example, be treated with sodium thiosulfate which creates non-polluting sodium sulfate and sodium chlorite. In a hypothetic commercialization of any of these chemistries, this aspect would naturally be thoroughly investigated.



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Associated content: The Supporting Information is available: Summary of chemical modifications performed on the fibres prior to homogenization, yield of the reactions, number of passes and homogenization concentration, FT-IR, NMR after the different modifications, size of the CNFs after chemical modification determined by AFM, summary of mechanical properties and hygromechanical properties. Author information: Verónica López Durán: [email protected] Per A. Larsson: [email protected]

Acknowledgements The authors acknowledge VINNOVA, the Swedish Governmental Agency for Innovation Systems, through BiMaC Innovation Excellence Centre for financial support. StoraEnso is acknowledged for performing the oxygen permeability measurements. L. Wågberg also acknowledges the financial support of the Wallenberg Wood Science Center.

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Effect of Chemical Functionality on the Mechanical and Barrier

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