Cellulose Nanofibrils and Diblock Copolymer Complex: Micelle

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Cellulose Nanofibrils and Diblock Copolymer Complex: Micelle Formation and Enhanced Dispersibility Hong Dong, Eugene Napadensky, Joshua A. Orlicki, James F Snyder, Tanya L. Chantawansri, and Alda Kapllani ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00811 • Publication Date (Web): 20 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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ACS Sustainable Chemistry & Engineering

Cellulose Nanofibrils and Diblock Copolymer Complex: Micelle Formation and Enhanced Dispersibility Hong Dong1,2,3*, Eugene Napadensky1, Joshua A. Orlicki1, James F. Snyder1, Tanya L. Chantawansri1, Alda Kapllani1 1

US Army Research Laboratory, Macromolecular Science & Technology Branch, 4600 Deer Creek Loop, Aberdeen Proving Ground, MD 21005 2

US Army Research Laboratory, Biotechnology Branch, 2800 Powder Mill Road, Adelphi, MD 20783 3

General Technical Services, 3100 NJ-138, Wall, NJ 07719

*To whom correspondence should be addressed: E-mail: [email protected]; Phone: 301-394-2415

ABSTRACT A great challenge to the utilization of bio-derived cellulose nanofibrils (CNFs) is related to dispersion, where the hydrophilic nature makes them difficult to disperse in both organic solvents and hydrophobic polymers. In this study, an amphiphilic diblock copolymer, poly(methyl methacrylate-b-acrylic acid) (PMMA-b-PAA), which contains a short interactive block of PAA and a long hydrophobic block of PMMA, was utilized to modify the surface of CNFs covered with surface carboxylic acid groups (CNF-COOH). The PAA block binds to the surface carboxylic acid groups on the CNFs through the formation of multiple hydrogen bonds, while the hydrophobic PMMA block enables better dispersion of the CNFs as well as interfacial adhesion with the matrix polymer. The attachment of PMMA-b-PAA to the CNFs was confirmed through Fourier transform infrared spectroscopy. Micelles were observed to form from a

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dispersion of CNF-COOH/PMMA-b-PAA complex in H2O. Good dispersion with individualized nanofibrils has been achieved in dimethylformamide, dimethyl sulfoxide, ethanol, and methanol even when a low amount of block copolymer was functionalized on the CNF surface. The dispersion level of CNF-COOH/PMMA-b-PAA correlates well with the dielectric constant of the solvents, where solvents with high dielectric constants are better able to disperse the PMMA-bPAA modified nanofibrils. Keywords: cellulose nanofibrils; diblock copolymer; surface modification; hydrogen bonding interactions; micelles; dispersion.

INTRODUCTION Cellulose nanofibrils (CNFs) are in an emerging class of biomaterials, which exhibit numerous advantages in the field of nanocomposites. They are a sustainable and environmentally-friendly product derived from abundant, renewable, and biodegradable resources such as wood, plants, bacteria, and many forms of algae. Cellulose nanofibrils have mechanical properties that make them desirable as reinforcing components in low-density and high strength materials, especially when biocompatibility of the composites is required. Since the cellulose chains are connected through internal hydrogen bonding, where crystalline regions are interspersed throughout the nanofibrils, CNFs exhibit excellent mechanical properties, including high modulus and strength, and a low coefficient of thermal expansion.1-3 CNFs have shown promise as reinforcement fillers for hydrophilic polymers such as polyethylene oxide (PEO)4 and polyvinyl alcohol (PVA)5 as well as hydrophobic polymers such as poly(methyl methacrylate) (PMMA)6 and polystyrene (PS)7. Significant improvements in strength, modulus, and fracture toughness have been achieved for water-cast PEO/CNF

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composite films.4 Highly transparent and toughened PMMA nanocomposites reinforced with CNFs containing surface carboxylic acid groups have also been recently reported.6 The reinforcements were attributed to CNFs high mechanical properties, good dispersion and interfacial hydrogen bonding between cellulose and the matrix.4,6 However, challenges associated with using cellulose nanofibrils in composites still remain because of low dispersity in processing organic solvent and lack of compatibility with hydrophobic polymers. For example, cellulose nanofibrils produced by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)mediated oxidation tend to aggregate and form gel particles in common polar and nonpolar organic solvents due to the prevalence of hydroxyl groups on the nanofibril surface.8 To fully utilize the potential of cellulose nanofibrils as reinforcement in composites, the hydrophilic character of cellulose nanofibrils needs to be tuned in order to make them dispersible in organic solvents for processing and compatible with hydrophobic polymer matrices. Various surface modification methods have been explored to address this hurdle.9-11 The surface of CNFs could be modified either by physical adsorption or by chemical modification using lowmolecular-weight molecules or polymers. For example, hydrophobic molecules such as quaternary alkylammonium cations have been ionically incorporated on TEMPO-oxidized CNFs as carboxyl counterions, resulting in high degrees of nanodispersion of modified CNFs in various solvents.12 In addition to small molecules, surface modification of CNFs has been achieved by grafting approaches to covalently or ionically attach macromolecules. This type of chemical modification can increase the hydrophobicity of the nanofibrils and compatibility with hydrophobic polymer matrices. Different lengths of poly(ε-caprolactone) (PCL) chains were covalently grafted onto CNFs via ring-opening polymerization (ROP); the study showed the strongest PCL biocomposites were obtained after reinforcement with CNFs grafted with the

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longest PCL graft length.13 Amino-terminated poly(ethylene glycol) (PEG) chains were surfacegrafted onto carboxylic CNFs via ionic bonds; the PEG-grafted CNFs were dispersible in organic solvents and the mechanical properties of the composite films were remarkably improved.14 Developing facile approaches of surface modification of CNFs will certainly constitute an enabling development to support the application of CNFs in polymer nanocomposites. Herein we describe our effort to attach diblock copolymers on the nanofibril surface via physical interactions utilizing multiple hydrogen bonds. Our aim is to improve the dispersibility of these nanofibrils in organic solvents and potentially improve the interfacial adhesion with polymer matrices. Macromolecules attached to the surface of the nanofillers increase adhesion at interfaces due to entanglements, therefore introducing stronger affinity to a surrounding polymer matrix.15 Our target polymer matrix is PMMA, so an amphiphilic diblock copolymer poly(methyl methacrylate-b-acrylic acid) (PMMA-b-PAA), which contains a short interactive block of PAA and a long hydrophobic block of PMMA, was selected to modify the surface of TEMPO-oxidized CNFs. The PAA block is utilized to effectively adsorb macromolecular chains onto the CNFs by forming multiple hydrogen bonds with the carboxylic acid groups, while the hydrophobic PMMA block is employed to improve dispersion as well as interfacial entanglements with the matrix polymer.

EXPERIMENTAL SECTION Materials. Water dispersions of carboxylated cellulose nanofibrils (balanced with sodium ions, noted as CNF-COONa) were provided courtesy of the United States Department of Agriculture (USDA) Forest Products Laboratory (Madison, Wisconsin). The cellulose nanofibrils were produced from wood pulp using the TEMPO oxidation technique and sodium hypochlorite as the

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terminal oxidant.16 The dispersion of CNFs used in this study has a concentration of 1.0 wt% and a surface carboxylate content of 1.3 mmol per gram of CNF dry weight. The nanofibrils were measured to be ~ 5.5 nm in average diameter from TEM images, where the length ranged from a few hundred nanometers up to a micron (Figure S1). The diblock copolymer poly(methyl methacrylate-b-acrylic acid) (PMMA-b-PAA, Mnx103:17.0-b-2.0) was purchased from Polymer Source Inc. N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), methanol, ethanol, acetone, toluene and all other chemicals were purchased from SigmaAldrich and used as received. Deionized water with a resistance of ~18.2 MΩ·m was used in all experiments. CNF-COOH/PMMA-b-PAA Complex. The “as-produced” CNF-COONa was converted to CNF-COOH by protonation using a method adapted from Isogai’s group.8 Briefly, 1 wt% CNFCOONa was diluted to 0.1 wt% in H2O, which was adjusted to a pH~2 with 1 N HCl. The protonation of CNF-COONa at a low pH of ~2 resulted in the agglomeration of CNFs. Gel particles of CNF-COOH were isolated by centrifugation, decanting, and resuspension in H2O several times. After that, water contained in the gel particles were completely solvent exchanged with acetone by centrifugation\decanting several times. DMF was added to the CNF-COOH gel particles in acetone, and acetone was removed by evaporating the mixture using a rotary evaporator under vacuum at 50 °C. The concentration of the obtained CNF-COOH gel particles in DMF was determined to be 1.15% by dry weight. The CNF-COOH gel particles with a dry weight of 0.10 gram (8.70 g gel mass) were diluted with 30 mL DMF. PMMA-b-PAA with a weight of 0.10 gram was dissolved with the CNFCOOH gel particles in DMF. The mixture was sonicated in a water bath sonicator (Fisher Scientific, FS30H) until the gel particles dispersed. Any remaining gel particles, representing the

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unfibrillated fraction, was removed by centrifugation. To remove unattached PMMA-b-PAA, the dispersion of CNF-COOH/PMMA-b-PAA in DMF was added into THF. THF is a good solvent for PMMA-b-PAA, but allow the complexed CNF-COOH/PMMA-b-PAA to form a gel and fall out of solution. After the supernatant was removed by centrifugation, fresh THF was added and mixed with the gel particles of CNF-COOH/PMMA-b-PAA, and the mixture was then centrifuged. This procedure was repeated four to five times in order to remove any unattached PMMA-b-PAA. Dispersion in Solvents. An organic solvent was added to the gel particles of CNFCOOH/PMMA-b-PAA in THF to get ~0.2% (w/v) of nanofibrils. For the solvents with high boiling points, such as DMF and DMSO, THF was removed from the mixture by vacuum rotary evaporation at 40 oC. For the solvents with low boiling points, such as methanol and acetone, THF was removed from the mixture using solvent exchange two times via consecutive centrifugation and removal of the supernatant. After the gel particles were transferred to the organic solvent, the mixture was sonicated using a water-bath sonicator. Characterization. Transmission electron microscopy (TEM). Dispersions of the modified nanofibrils in solvents and the micelle formation of CNF-COOH/PMMA-b-PAA complexes were characterized using a JEOL 2100F transmission electron microscope (TEM) operated at 200 kV. The scanning transmission electron microscopy (STEM) dark field images were acquired with a Gatan 806 high angle annular dark field (HAADF) detector. To prepare TEM and STEM samples for imaging, the TEM grids covered with ultrathin carbon films were treated with air plasma for 30 seconds in order to increase the hydrophilicity of the carbon support film and thus prevent aggregation of the nanofibrils when dried on the grid. A droplet of the diluted dispersion or solution was cast on the grid and remained on the grid for 1 minute. Then the extra

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fluid was removed with the edge of filter paper, and the nanofibrils were stained with 2 % uranyl acetate before being air-dried. Fourier transform infrared spectroscopy (FTIR). FTIR spectra of the CNF-COOH, PMMA-bPAA and CNF-COOH/PMMA-b-PAA were collected on a Thermo Nicolet NEXUS 870 spectrophotometer. The dispersions of CNF-COOH in DMF, PMMA-b-PAA in acetone, and CNF-COOH/PMMA-b-PAA in acetone were smeared on the crystal disks (25mm, NaCl), and completely dried in an oven purged with N2. The spectra were acquired in transmission mode on the crystal disks at a spectral range of 4000-500 cm-1. Contact Angle Measurement. The hydrophobicity and surface energy of the films were evaluated by applying a static sessile drop contact angle measurement where Milli-Q water, diiodomethane, ethylene glycol, and glycerol were used as probe liquids at room temperature. Films of PMMA-b-PAA and CNF-COOH/PMMA-b-PAA were solvent cast several times on glass slides (pretreated with UV-Ozone) from solution or dispersion in DMF/THF, respectively. Then the films were dried several days in an oven at a temperature of ~40 oC under N2 purge. Films of CNF-COONa were cast on a glass slide from water dispersion followed by a similar drying protocol. During the experiments, roughly 2-3 µL droplet of liquid was deposited onto the surface of the sample. The liquid was allowed a short time to reach equilibrium and a still image was taken using a digital camera. The angles of the tangent line to the left and right of the solidliquid interface were calculated using software (Drop Image V2.6.1), and the average value was recorded as the contact angle for this particular solid-liquid combination. Typically, 7 to 20 drops per surface type, per solvent (water, diiodomethane, ethylene glycol, and glycerol) were captured and analyzed. The surface energy of each film was calculated from collected contact angle values using the Owens/Wendt method.

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RESULTS AND DISCUSSION Cellulose Nanofibrils/Polymer Complex Formation

Scheme 1. Formation of CNF-COOH/PMMA-b-PAA complex by hydrogen bonding interactions between surface carboxylic acid groups on the nanofibrils and the acrylic acid blocks in the PMMA-b-PAA macromolecular chains.

In this study, diblock copolymers composed of a short block of poly(acrylic acid) (Mn 2.0x103) and a long block of poly(methyl methacrylate) (Mn 17.0x103) were attached to the surface of nanofibrils by physical interactions, as shown in Scheme 1. The attachment of the diblock copolymer to the nanofibrils was carried out in DMF since it is a common solvent for both solubilizing PMMA-b-PAA and dispersing CNF-COOH. Our choice to use a diblock copolymer is motivated by its amphiphilic nature, such that one block may interact with the hydrophilic surface groups on the nanofibrils while another block provides the compatibility necessary for dispersion in a hydrophobic matrix. The length of PMMA block in the block copolymer is above the critical molecular weight called the entanglement molecular weight Me.17 Thus the long hydrophobic PMMA block not only improves the dispersibility of the hydrophilic 8|P a g e


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nanofibrils in an organic solvent but also allows for the optimization of physical entanglements with the hydrophobic polymer matrix in the nanocomposite. The PAA block is used to adsorb the macromolecular chains to the surface of the nanofibril. Chains of PMMA-b-PAA were attached to the surface of the nanofibrils by forming hydrogen bonds between acrylic acid from the PAA block and the surface carboxylic acid groups on CNFs.18-19 Moreover, the PAA blocks are able to bind the CNFs at multiple sites because of the repeating units of acrylic acid, exhibiting cooperative interactions by virtue of the multiple interactive groups between the polymer and the fibril. Cooperative hydrogen bonding interaction19-20 among active sites in the complex could further enhance binding strength of the PMMA-b-PAA chains to the CNFs. The success of the physical modification was assessed by FTIR spectroscopy, through monitoring the intensity of the carbonyl band at ∼1730 cm−1 as well as the appearance of other bands (Figure 1). For unmodified CNF-COOH, the peak at around 1725 cm-1 corresponds to the C=O stretching frequency of carbonyl groups in their acidic form, which is generated by acid protonation of CNF-COONa. In the spectrum of CNF-COOH/PMMA-b-PAA, the peak of the C=O stretching frequency slightly shifts to 1730 cm-1, and the intensity increases relative to invariant peaks, selection of which is discussed below, due to the carbonyl function from both PMMA-b-PAA and the nanofibril surfaces. Additionally, new peaks assigned to the C–H bond stretching vibrations of the –CH3 and –CH2 groups were observed at ∼2991 cm−1 and 2949 cm-1, respectively, which are mainly from the long polymer chains of PMMA-b-PAA. These confirm the presence of long polymer chains on the nanofibril surfaces. Other bands specific to the cellulose nanofibrils21-22 are also present in the CNF-COOH/PMMA-b-PAA spectrum such as a broad peak at 3344 cm-1 (related to OH intramolecular and intermolecular hydrogen bonds, and

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free OH stretching), in addition to sharp peaks at 1161cm-1 (COC stretching), 1060cm-1 (C-O stretching at C3 and C-C stretching), and 1035cm-1 (C-O stretching at C6). The amount of PMMA-b-AA on the surface of the nanofibrils was estimated based on the integrated peak area at ~1730 cm-1. In the spectra of CNF-COOH/PMMA-b-PAA, the intensity of the C=O stretching corresponds to the carbonyl function from both PMMA-b-PAA and CNFCOOH. To calculate the amount of C=O stretching at 1730 cm-1 contributed from CNF-COOH, the peaks at 1060 cm-1 and 1035 cm-1 from CNF-COOH spectrum, which have little overlap with absorption from PMMA-b-PAA, were integrated and used as internal standards. Based on the calculation (described in details in supporting information), 53% of C=O stretching at 1730 cm-1 is from CNF-COOH and 47% is from PMMA-b-PAA in the spectrum of CNF-COOH/PMMA-bPAA. Considering concentrations of carbonyl groups on the surface of nanofibrils (1.3 mmol/g) and in the molecular chain of PMMA-b-PAA (10.4 mmol/g), the amount of PMMA-b-PAA attached to the nanofibrils was calculated to be approximately 9.7 wt%.

PMMA-b-PAA CNF-COOH

CNF-COOH/PMMA-b-PAA

3,500

2,500 1,500 Wavenumber (cm-1)

500

Figure 1. FTIR spectra of CNF-COOH/PMMA-b-PAA, CNF-COOH and PMMA-b-PAA.

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The morphology and the size of nanofibrils before and after surface attachment of PMMAb-PAA were investigated using transmission electron microscopy (TEM). CNF-COOH is dispersible with a low concentration in DMF when assisted by low-energy sonication, but remains in the form of gel particles in ethanol even with sonication. After modification with PMMA-b-PAA, the nanofibrils were found to disperse well in DMF without sonication. Additionally, the complex is quite dispersible in ethanol with sonication, in contrast to the CNFCOOH without attachment of PMMA-b-PAA. This indicates PMMA-b-PAA remains associated with the nanofibrils in ethanol, moderating nanofibril dispersion. Figure 2 shows TEM images of CNF-COOH prepared from DMF dispersion and CNF-COOH/PMMA-b-PAA prepared from ethanol dispersion. The number-average width of CNF-COOH/PMMA-b-PAA was 5.6 nm, measured from the TEM images. Despite attachment of the diblock copolymer, there is no significant change in the width and length of the nanofibrils based on the TEM images. a

b

Figure 2. TEM images of (a) CNF-COOH dispersed in DMF, and (b) CNF-COOH/PMMA-bPAA dispersed in ethanol; all samples stained with uranyl acetate aqueous solution (2%).

Micelle Formation of CNF-COOH/PMMA-b-PAA Complex in Water

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The gel particles of CNF-COOH/PMMA-b-PAA complex in THF, after removal of the unattached polymer through centrifugation and re-dispersal, were added in small aliquots into water. Interestingly, the gel particles quickly dispersed into water by simply shaking the vial. Bubbles formed at the air and water interface, indicating typical surfactant behavior (Figure 3a inset). By contrast, gel particles of CNF-COOH in DMF added to water were not redispersed by shaking. The morphology of the CNF-COOH/PMMA-b-PAA complex isolated from water was studied using TEM. As shown in Figures 3a, spherical micelles with an average diameter of 22 nm were observed in between or attached to the nanofibrils, though some nanofibrils might be obscured due to the low contrast of cellulose, even after staining. Block copolymers with comparatively long hydrophobic and short hydrophilic blocks can form a variety of nanostructures in water or in water mixed with organic solvents.23-24 PMMA-bPAA does not dissolve in water directly owning to a long block of hydrophobic PMMA. The copolymer has to be first dissolved in a common solvent for both blocks. Then, a selective solvent, water, is added slowly to induce micellization.23 In this case, PMMA-b-PAA was dissolved in acetone first, and the solution of the polymer was added to water. It is noted that at a low concentration PMMA-b-PAA with a pH of ~4.4 tends to form spherical micelles, shown in Figure 3b. The average size of PMMA-b-PAA micelles is ~20 nm. Although the two types of micelles have similar average size, micelles observed from the CNF-polymer complex have a large size distribution whereas the micelles of PMMA-b-PAA have uniform sizes, by comparing the two TEM images (Figures 3a and 3b). Diblock copolymers form spherical micelles in a solution within a selective solvent, and the micelles can be directly adsorbed onto solid substrates25 and cellulose surfaces15. Alternatively, diblock copolymers adsorbed or grafted on the substrates from non-selective

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solvents can self-organize or self-assemble in a selective medium, resulting in the formation of so-called surface micelles.26-29 In this study, unbound polymer chains had been removed through several washes of the CNF-block copolymer complex in a good solvent (THF) for the polymer. The micelles could originate from self-organization of the PMMA-b-PAA chains on the surface of the nanofibrils. Introducing the PAA block onto the CNF surface moderates the nanofibril surface characteristics as the PMMA block provides a favorable interface for modestly hydrophobic solvents or matrices while the cellulosic nanofibril surface complexed with PAA remains hydrophilic. Compared to the baseline block copolymer, the complex with the cellulose nanofibrils increases the relative hydrophilic fraction, which otherwise is dominated by the long PMMA block. Surface-modified cellulose nanocrystals (CNCs)30-31 and CNFs32 that possess amphiphilic nature have also been used as effective stabilizers with oil/water (o/w) emulsions (so-called Pickering emulsions), in which the CNCs or CNFs adsorbs irreversibly to the o/w interface. Similarly, the CNFs with the attached block copolymer are amphiphilic and could potentially be used in the stabilization of Pickering emulsions. a

b

Figure 3. TEM images of (a) micelles of CNF-COOH/PMMA-b-PAA formed in H2O (inset: photo of bubble formation of CNF-COOH/PMMA-b-PAA dispersed in H2O), (b) micelles of PMMA-b-PAA formed by introducing the polymer/acetone solution into water. Both samples on the TEM grids were stained with 2% uranyl acetate solution before imaging to enhance image contrast again carbon support films.

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Dispersion of Modified CNFs in Organic Solvents The dispersibility of CNF-COOH/PMMA-b-PAA was tested in several organic solvents, ranging from highly polar organic solvents to non-polar solvents. The dispersion quality of the modified nanofibrils in various solvents (~0.2%) was examined by qualitative judgments of the solutions using visualization of the individualized nanofibrils deposited from the indicated solvents using TEM and STEM. The good dispersion was observed for CNF-COOH/PMMA-b-PAA in DMSO, DMF, methanol and ethanol (Figure 2b, 4a-b, S3). In these solvents, the majority of the discrete nanofibrils are observed with a similar width as compared to the pristine nanofibrils. A few micelles were occasionally found after deposition from hydrophilic solvents, such as DMF. The dispersion of the complex in above solvents was also confirmed by the presence of birefringence in the solutions using a cross polarizer. Observation of birefringence between cross-polarizers has been utilized as an indicator of dispersion at the individual cellulose nanofibril level in solutions.12,14 The stability of CNF-COOH/PMMA-b-PAA in several organic solvents and water was investigated by FT-IR study of the composition of the complex after introducing to the solvents (see supporting information for details). The polymer chains of PMMA-b-PAA remain attached on CNFs, contributing to the improvement of dispersibility of the nanofibirls in the aforementioned solvents. The dispersion of CNF-COOH/PMMA-b-PAA in THF and toluene remains poor. Aggregates containing associated nanofibril networks and bundles of nanofibrils were observed in the TEM image, shown in Figure 4d. Both individualized nanofibrils (Figure 4c) and aggregates of nanofibrils as well as a small amount of micelles (Figure S4), were observed in the images of CNF-COOH/PMMA-b-PAA dispersed in acetone. Interestingly, the dispersion level of CNF-COOH/PMMA-b-PAA in solvents (Table 1) qualitatively correlates well with the dielectric constant of the solvents. Solvents with high dielectric constants provide 14 | P a g e


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better dispersibility to PMMA-b-PAA modified nanofibrils. The polarity of the solvents is given as the dielectric constant, thus the modified nanofibrils have better dispersion in polar solvents over non-polar solvents.

Table 1. Dispersibility of CNF-COOH/PMMA-b-PAA, CNF-COOH and PMMA-b-PAA in various organic solvents. +/- indicates good/poor dispersibility. Solvents

DMSO

DMF

Methanol

Ethanol

Acetone

THF

Toluene

PMMA-b-PAA

+

+

−

−

+

+

+

CNF-COOH

−

+

−

−

−

−

−

CNF-COOH /PMMA-b-PAA

+

+

+

+

+/-

−

−

46.7

36.7

32.7

24.5

20.7

7.58

2.38

Dielectric constant ε

a

b

c

d

Figure 4. TEM Images of CNF-COOH/PMMA-b-PAA dispersions in different solvents: (a) methanol, (b) DMF, (c) acetone, and (d) toluene.

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Contact Angle and Surface Energy To examine the influence of the functionalized diblock copolymer on the surface energy of CNFs, contact angles of the smooth films supported on glass slides were measured with the static sessile drop method and four solvents: Milli-Q water, diiodomethane, ethylene glycol, and glycerol. Surface energy of the films was then calculated from contact angles and surface tensions of the solvents using the Owens/Wendt method (see supporting information for details). As shown in Figure 5, the attachment of PMMA-b-PAA to the surface of cellulose nanofibrils leads to a higher contact angle against water, indicating that modification with the polymer increases the hydrophobicity of the CNFs. The overall surface energy decreases from 61.3 mJ/m2 for CNFs to 56.8 mJ/m2 for CNF-COOH/PMMA-b-PAA (Table 2). Among the overall surface energy, the dispersive interaction component increased from 20.5 mJ/m2 to 28.6 mJ/m2 and the polar interaction component decreased from 40.8 mJ/m2 to 28.1 mJ/m2, respectively, indicating an increase in the apolar character of CNFs after complexing with the polymer. The increase in apolar component of the modified CNFs enables their amphiphilic property as described above. a

b

c

Figure 5. Contact angle of water on glass slides supported films of (a) CNFs: 30.4o, (b) PMMAb-PAA: 79.6o, and (c) CNF-COOH/PMMA-b-PAA: 39.6o.

Table 2. Surface energy parameters (mJ/m2) as calculated by the Owens/Wendt method. Sample

CNFs

PMMA-bPAA

CNF-COOH /PMMA-b-PAA

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Overall surface energy (mJ/m2)

61.3

36.5

56.8

Dispersive interaction component (mJ/m2)

20.5

32.7

28.6

Polar interaction component (mJ/m2)

40.8

3.8

28.1

The increase in surface hydrophobicity of CNFs using the diblock copolymer is relatively low compared to that of cellulose nanocrystals (CNCs) modified by multiple alkyl chains, where contact angles of water on self-standing films up to 110.5° were reported.12 One of the contributed factors is hydrophilicity of the PAA block in the polymer, which decreases the contact angles of both the pure diblock copolymer and the CNFs attached with PMMA-b-PAA. The contact angle of PMMA-b-PAA is only around 79.6o owing to the hydrophilic property of PAA. The amount of hydrophobic species attached to the surface also significantly influence the values of contact angles, since a high amount of the attached species should result in better surface coverage and thereby a higher contact angle. In the current study, the attached amount of diblock copolymer is 9.7wt% on the basis of the calculations mentioned previously. The low amount of polymer attachment is plausible since a reduced surface grafting density is often obtained by using the ‘‘grafting onto’’ method due to steric hindrance induced by already attached polymer chains.9,11 Using a PAA block instead of a monofunctional polymer as an attachment site on nanofibrils may influence the amount of surface-bound polymer. The Mn values of PAA block and PMMA block in the block copolymer are 2000 and 17000, respectively. A higher number of PAA blocks leads to more binding sites per chain, which will likely yield stronger interactions with the CNF surface. However, a higher amount of binding sites in a chain will also lead to a lower number of adsorbed chains, since one macromolecular chain can simultaneously bind to several carboxylic acid sites on the CNFs. If a low number of PMMA-b-PAA chains attach to

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the CNF-COOH, the available PMMA blocks will not be sufficient to cover the hydrophilic cellulose surface. During the investigation of surface attachment of ionic polymer chains on cellulose nanofibrils, it was found that the length of the PCL block contributes to the contact angle, where a block copolymer with a longer PCL block should be able to increase the contact angle more than one with a shorter PCL block.15 Therefore, the ratio between the interactive block and the hydrophobic block needs to be considered in order to balance between strong attachment and hydrophobicity. For PMMA-b-PAA, the shorter block of PAA and the longer block of PMMA are expected to increase the hydrophobicity of the films and more likely, further improve the dispersibility of the modified CNFs in organic solvents, especially in solvents with low polarity. Nevertheless, this study shows that the dispersibility of cellulose nanofibrils in organic solvents has been significantly improved even with a low amount of attached diblock polymers. This effect could be attributed to hydrophobic macromolecular chains utilized in the surface modification. Surface modified cellulose nanofibrils are considered to be effective nanofillers to reinforce polymer materials. One of the advantages of attaching macromolecular chains over small hydrophobic molecules is that the macromolecular chains can introduce chain entanglements to a polymer matrix giving improved adhesion or interaction at the interface with the polymer.

CONCLUSION In this study, physical modification of TEMPO-oxidized cellulose nanofibrils with amphiphilic diblock copolymer PMMA-b-PAA, which contains a short block of interactive block PAA and a long hydrophobic block of PMMA, have been explored. The attachment of PMMA18 | P a g e


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b-PAA to the nanofibrils was achieved by cooperative hydrogen bonding interactions between acrylic acid from the block copolymer and the carboxylic acid on the nanofibrils. The modification was confirmed by FT-IR analysis and the amount of the polymer on nanofibrils were estimated to be 9.7wt%. The modified nanofibrils showed surfactant behavior in water, and micelles attached on the nanofibrils were observed from TEM examination. Although the increase in surface energy and hydrophobicity of CNFs modified using the diblock copolymer was not dramatic, a good dispersion containing individualized nanofibrils was observed in DMSO, DMF, ethanol, and methanol. The dispersion level of nanofibrils of CNFCOOH/PMMA-b-PAA correlates well with the dielectric constant and polarity of the solvents, and the modified nanofibrils have better dispersion in polar solvents than non-polar solvents. The knowledge obtained through this investigation can be exploited in subsequent experiments towards the improvement of dispersibility of modified CNFs in organic solvents with low polarity as well as the reinforcement in polymer nanocomposites.

Supporting Information FTIR spectra and quantification, stability of CNF-COOH/PMMA-b-PAA in solvents, calculation of surface free energy, and TEM images of nanofibrils dispersed or aggregated in different solvents.

ACKNOWLEDGEMENTS The research reported in this document was performed in connection with contract/instrument W911QX-14-C-0016 with the U.S. Army Research Laboratory. The authors thank Dr. Alan Rudie and Mr. Richard Reiner at USDA Forest Product Laboratory (FPL) for providing the CNF 19 | P a g e



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dispersions. Dr. Dimitra N. Stratis-Cullum and Dr. Eric Gobrogge at Army Research Laboratory (ARL) are acknowledged for discussions and help in FT-IR study.

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For Table of Contents Use Only

Cellulose Nanofibrils and Diblock Copolymer Complex: Micelle Formation and Enhanced Dispersability Hong Dong, Eugene Napadensky, Joshua A. Orlicki, James F. Snyder, Tanya L. Chantawansri, Alda Kapllani

Diblock copolymer PMMA-b-PAA has been attached physically to the surface of hydrophilic cellulose nanofibrils, resulting in enhanced dispersibility of the modified nanofibrils in organic solvents.

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Cellulose Nanofibrils and Diblock Copolymer Complex: Micelle

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