Polymerization Topochemistry of Cellulose Nanocrystals: A Function

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Polymerization Topochemistry of Cellulose Nanocrystals: a Function of Surface Dehydration Control Chen Tian, Shiyu Fu, Youssef Habibi, and Lucian A Lucia Langmuir, Just Accepted Manuscript • DOI: 10.1021/la503990u • Publication Date (Web): 11 Nov 2014 Downloaded from http://pubs.acs.org on November 17, 2014

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Polymerization Topochemistry of Cellulose Nanocrystals:

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a Function of Surface Dehydration Control

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Chen Tian, † Shiyu Fu* † Youssef Habibi,≠ Lucian A. Lucia* ‡, § † State Key Laboratory of Pulp & Paper Engineering, South China University of Technology, Guangzhou, Guangdong 510640, P.R. China ≠ Department of Advanced Materials and Structures, Public Research Centre Henri Tudor, L-185, Luxembourg ‡ Key Lab of Pulp & Paper Science and Technology of the Ministry of Education, Qilu University of Technology, Jinan, Shandong 250353, China § The Laboratory of Soft Materials & Green Chemistry, Departments of Chemistry & Forest Biomaterials, North Carolina State University, Raleigh, NC 27695, U.S.A.

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KEYWORDS: Cellulose Nanocrystals; Solvent-exchange; Freeze-drying; Ring Opening Polymerization; Polycaprolactone

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ABSTRACT

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The activation (dehydration) of cellulose nanocrystals (CNCs) toward surface “brush” polymerization

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is accomplished either by freeze-drying or solvent-exchange. However, the question of which one of

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these protocols to choose over the other is generally open-ended. The current study attempts to shed

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light on this question by installing a standard polymer, polycaprolactone (PCL), onto the surface of

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both freeze-dried and solvent-exchanged CNCs by ring-opening polymerization (ROP), and

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examining the differences in polymerization and final product properties. The work is the first to

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demonstrate that the efficiency of surface polymerization and final product properties are in fact

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influenced by the protocols. The differences between the two sample PCL-grafted CNCs were

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investigated by X-ray photoelectron spectroscopy (XPS), elemental analysis, gel permeation

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chromatography (GPC), and contact angle measurements. The freeze-dried samples had significantly

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reduced PCL surface density.

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(SECNC-g-PCL), however, was lower than that of either pure CNCs or freeze-dried PCL-grafted

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CNCs (FDCNC-g-PCL).

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surface to provide enhanced reactivity, an effect that was not as apparent for FDCNC-g-PCL. The

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solvent-exchanged CNCs tended to have more porous, nano-textured surfaces that were tended to be

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more responsive toward brush polymerization. In addition to the physical dissimilarities in surface

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morphology and surface accessibility contributing to topochemical differences between the two

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species, it was also found that dispersibility, aggregation, and thermal stability were different.

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INTRODUCTION

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Cellulose nanocrystals (CNCs) are widely applicable nanomaterials because of excellent mechanical

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properties and morphologies that lend them to consideration in a number of areas, but particularly as

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reinforcement fillers in composites. They tend to behave quite well in this role because they are

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brick-like, robust, and dispersible nanostructures that may potentially find niches within the paper

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industry, electronics, cosmetics, and biomedicine.1, 2 Features that enhance their prospects as a filler

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are their large surface area, high crystallinity, and good stiffness. Their Young’s modulus ranges

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from 130 to 160 GPa, depending on the crystal form, which in addition to their abundance,

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renewability, and environmental friendliness, make them superior filler candidates.1-3

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However, their hydrophilic nature results in especially poor compatibility within hydrophobic

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composites. Another complication for their inclusion in composites is that strong hydrogen bonds

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can form during their preparation leading to irreversible agglomeration.3

The crystallinity of the solvent-exchanged PCL-grafted CNCs

It was determined that solvent-exchange sufficiently modified the CNC

Thus, dispersion of CNCs


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in hydrophobic matrices or a non-polar solvent is nearly impossible.

Overcoming these latter

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challenges has been typically done by installing, for example, appropriate polymers on the surface that

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endow it with a “brush”-like appearance and allow for homogeneous (co-continuous) filler/matrix

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mixing to optimize final mechanical properties.4,5

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Although the chemistry of surface brush polymerization of CNCs is well established, the factors that

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control subsequent surface reactivity are not. The surfaces of CNCs are generally activated toward

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surface brush polymerization by either freeze-drying or solvent exchange.6

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these methods enhances the surface polymerization chemistry through the removal of water. It may

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be argued that given the lack of data on the differences between the preparation methods, either is

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acceptable.

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nanocellulose and demonstrated that freeze-drying was more efficient.7 Yet, a preponderance of the

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work done in CNC surface polymerization tends to use the method established by Habibi et al. in

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which an exchange of never-dried CNCs from water to acetone is performed, later followed to toluene

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where the polymerization chemistry is done.5, 8, 9

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by Lee & Bismarck7 to be superior could also be used to dehydrate for subsequent polymerization, but

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researchers have claimed that residual water during the drying lead to side-reactions such as

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homopolymerization and transesterification providing less than optimal yields.

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Thus, the goals of this work were to provide a rigorous accounting of the physical and chemical

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differences between the two preparation protocols and topochemistries, their chemical bases, and

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conclusions for their application within the biomaterials community. To address these latter goals,

It is known that either of

However, Lee & Bismarck studied the effect of the dehydration procedure on bacterial

The simple freeze-drying method that was claimed


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solvent-exchanged and

freeze-dried

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the reactivity of

CNCs towards

esterification with

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poly(carpolactone) (PCL), a well-studied surface brush polymer for CNCs, was investigated.

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EXPERIMENTAL SECTION

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Materials. Cotton linters with a moisture content of 8% were provided by Fumin Chemical Fiber Co.

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Ltd. (Shandong, China). Sulfuric acid (98%), toluene (anhydrous, 99.8%), acetone (99%),

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dichloromethane (99.5%), and methanol (99%) were all of laboratory grade and purchased from

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Sigma-Aldrich. Sodium hydroxide, sodium bromide and other chemicals were analytical reagent grade

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obtained from Guangzhou Chemical Reagent Factory (Guangzhou, China). ε-caprolactone (ε-CL) and

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Sn(Oct)2 (98%) were from Aladdin Industrial, Inc. (Shanghai, China). Toluene and acetone were dried

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over metal sodium and potassium permanganate, respectively, and then distilled before use. ε-CL was

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dried 48 h over calcium hydride and distilled under reduced pressure. Other chemicals were used as

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received unless otherwise noted.

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Generation of CNCs. CNCs were generated by acid hydrolysis of cotton linters according to a

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procedure previously employed.4

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sulfuric acid (64 wt-%) and hydrolyzed at 45 °C for 90 min under constant stirring (500 rpm).

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Approximately 500 mL of deionized water was then added to quench the reaction.

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washed with de-ionized water through several successive centrifugations at 10,000 rpms at 20 °C for

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10 min, replacing the supernatant with deionized water each time. Dialysis using distilled water was

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performed to remove residual free acid until pH of the eluent was 7. The aqueous suspension of

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CNCs was obtained by ultra-sonication for 5 min using an ultrasonic homogenizer (KBS-1200, China)

Approximately 3 g of cotton linters (dry weight) was mixed with

The slurry was


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at an output power of 900 W. The suspension was filtered through a 400 mesh filter cloth to remove

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unhydrolyzed fibers and stored at 4 °C.

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Ring-opening polymerization (ROP) of caprolactone onto CNCs. Prior to polymerization, CNCs

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were neutralized using a 1 wt-% NaOH solution for 48 h to neutralize the sulfate groups at the surface

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of CNCs. To modify never-dried CNC (SECNC) from water, 0.5 g (dry weight) of purified CNC

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was solvent exchanged with acetone into toluene to ensure complete removal of water and acetone.

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The volumes of both acetone and toluene used for each step were 100 mL.

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ultra-sonicated for at least 2 min during each step to completely disperse the CNCs and then

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centrifuged at 10,000 rpm for 5 min, while the excess solvent was removed prior to re-dispersion in

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the next batch of solvent.

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solvent. After the final solvent exchange step into toluene, the suspension was introduced into a

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dried three-neck flask with a magnetic stirrer.

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remove any residual water. Approximately 5.0 g ε-CL and 2 wt-% (with respect to the monomer) of

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Sn(Oct)2 were slowly added under a constant nitrogen flow. The mixture was heated to 100°C for 24

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h. After cooling, the modified CNCs were recovered by precipitation with methanol. Subsequently,

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the product was washed several times using dichloromethane using centrifugation/re-dispersion and

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Soxhlet extracted for 48 h in dichloromethane to remove homo-PCL and unreacted ε-CL. The

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obtained product was dried in vacuo for 48 h at 45 °C, and the resulting PCL-grafted SECNC is

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denoted hereafter as SECNC-g-PCL.

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In a separate experiment, CNCs were dispersed in water and freeze-dried (ALPLA 1-2 LD PLUS,

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CHIRST, Germany). These CNCs are denoted as freeze-dried CNCs (FDCNC) throughout the study.

The mixture was

This centrifugation/re-dispersion step was repeated three × for each

Two toluene distillation steps were carried out to


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Approximately 0.5 g of FDCNC was dried in vacuo at 60 °C for 48 h to remove residual water and

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then dispersed into toluene directly using an ultrasonic homogenizer while the reaction was conducted

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as previously described above (vide supra). After 24 h, the modified CNCs were purified and dried

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in vacuum for 48 h at 45 °C; they are denoted as FDCNC-g-PCL.

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The reaction scheme for the ROP of ε-CL onto FDCNCs and SECNCs is represented in Scheme 1.

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Scheme 1. Proposed generic reaction scheme for the ROP of ε-CL onto CNCs.

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AFM. AFM images were recorded on a Bruker Multimode 8 (America) atomic force microscope. A

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droplet of the nanocellulose suspensions (0.01wt-%) was spread onto a fresh mica substrate. All

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analyses were conducted in tapping mode under ambient conditions. Commercial probes had spring

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constants of 20e-80 N/m and resonance frequencies of approximately 298-335 kHz. The image

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processing was done using Nanoscope IIIa Multimode software.

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SEM. SEMs of different nanocellulose samples were obtained using a high-resolution field emission

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gun scanning electron microscope (Nova, NanoSEM 430, FEI Company) operating at an accelerating

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voltage of 15 kV.

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which various magnification levels were used to obtain images.

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characterize the morphology changes of CNCs after esterification with PCL.

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FT-IR. The FT-IR traces of neat CNCs and all CNC samples grafted with PCL were collected using

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KBr discs on a Vector 33 spectrometer (Bruker, Germany). Samples were dried, weighed, while their

Neat and modified CNCs were coated with gold on an ion sputter coater, after SEM images were used to


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mass ratio to KBr was kept constant at 1:100 when making the discs. Spectra were recorded over a

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spectral window ranging from 400 to 4000 cm-1.

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XRD. XRD patterns were collected on a D/max-IIIA X-ray diffractometer (Japan).

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high-intensity monochromatic nickel-filtered Cu Kα radiation was generated at 40 kV and 40 mA.

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Diffractograms were collected in a 2θ range from 4° to 60° at a rate of 1°/min and resolution of 0.04°

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at room temperature. The crystallinity (Cr) was obtained according to Eq. 1.

 IA  Cr = 1 −  × 100%  IA + Sp 

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The

(1)

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Where IA is the amorphous integrated area, and Sp is the sum of the crystalline regions for the 101,

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101 and 002 peaks.10

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TGA. The thermal properties of CNC and CNC-g-PCL were obtained using a TA Instrument Q500

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thermogravimetric analyzer (TGA Q500, TA, USA). All tests were conducted under a nitrogen

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atmosphere at a flow rate of 25 mL/min. Samples of approximately 10 mg were heated in an

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aluminum crucible to 600 °C at a constant heating rate of 10 °C/min.

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displayed as weight percent (%) versus temperature.

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DSC. DSC analysis was performed using a TA Instruments Q200 apparatus.

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procedure was applied covering a temperature range from -10 to 150 °C. The temperature gradient

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and respective heating rates employed may be described as follows: ambient temperature to 150 °C at

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a heating rate of 10 °C/min, isotherm for 10 min at 150 °C, 150 to -10 °C at a cooling rate of 10

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°C/min, isotherm for 10 min at -10 °C, and ending at a heating cycle from -10 to 150 °C at 5 °C/min.

The TGA thermal curves are

A heat/cool/heat


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Elemental analysis. Elemental analysis for C, H, O and S was carried out on a PANalytical Axios

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X-ray fluorescence instrument (PANalytical, Netherlands).

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encapsulated in tinfoil and interrogated with an Lr Ka X-ray source at a power of 4 kW.

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X-Ray photoelectron spectroscopy (XPS). Powdered CNCs before and after PCL grafting were

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analyzed using an Axis Ultra DLD instrument (Kratos Analytical, UK) with A1 Ka X-ray source.

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The electron flood gun for charge compensation was operated at 150 W. The aperture slot used in

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the scanning process was 700 × 300 µm. Spectra were recorded at a pass energy of 160 eV for

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survey scans and 40 eV for high resolution scans. The vacuum obtained in the analysis chamber was

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greater than 5×10-9 torr. The relative amounts of different bound carbons were calculated from the

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high-resolution scans of the C1s spectra, using a Gaussian curve-fitting program. The spectra were

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modified by setting the C–C contribution in the C1s emission to 285.0 eV.

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Contact angle. Contact angle measurements were used to analyze the hydrophobic properties of

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CNCs before and after esterification. They were performed with water at RT on a Dataphysics

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OCA40 Micro instrument (Dataphysics Ltd., German). Samples were ground in an agate mortar

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until a thin powder was obtained. Then smooth surface samples were obtained by compressing the

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powder under 10 bar pressure using an IR press. A 5-10 µL volume droplet of ultrapure water was

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distributed on the surface of the sample. During the experiment, images of the droplet profile were

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recorded from which the contact angle was determined using the angle of intersection between a

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baseline and a circle. A Young-Laplace profile was used to analyze the resulting images.

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Dispersion properties. To test re-dispersion ability, SECNC and FDCNC (0.1 g each) were

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re-dispersed in distilled water and toluene under ultra-sonication at 900 W for 5 min to form 0.1 wt-%

Approximately 5 mg of sample was


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suspensions. The dispersibility of the obtained graft polymers in toluene and DMSO, respectively,

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was tested according to the following method: the samples were dispersed in toluene or DMSO at a

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concentration of 0.1 wt-% followed by magnetic stirring for 24 h at room temperature. The stability

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of the resulting suspension was carefully observed and compared.

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Specific surface areas and pore sizes. Nitrogen adsorption/desorption experiments were performed

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to determine the specific surface area and pore size of SECNCs and FDCNCs using

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Brunauer−Emmett−Teller (BET) (ASAP 2020). The cellulosic samples were first degassed at 393 K

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for 5 h before the analysis of N2 adsorption at 77.355 K. BET analysis was carried out at 77.355 K.

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Pore size distribution was determined from N2 desorption following a BJH model assuming a

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cylindrical pore shape for the samples.

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Gel permeation chromatography (GPC). GPC of homo-PCL was performed using a PL-GPC-50

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apparatus (Agilent 1100, America).

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measurement was conducted at 40 °C. A Differential Refractive Index (DRI) detector was used for

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the molecular weight analysis.

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dissolved in 2 mL of THF (HPLC grade). The solution was filtered through a 0.45 µm disposable

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filter (Nylon 66) into an HPLC vial and finally placed into the autosampler.

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RESULTS AND DISCUSSION

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Grafting efficiency of PCL onto cellulose nanocrystals. CNCs were grafted with PCL by the

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chemical method of ROP.

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solvent-exchanged and freeze-dried CNCs were immediately quite evident. An adequate elucidation

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of the differences in grafting efficiency of SECNCs and FDCNCs required monitoring the changes in

THF was used as solvent, running at 1 mL/min.

The

Before determination, approximately 2 mg of homo-PCL was

Interestingly, the differences in grafting profiles between


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chemical groups and surface chemical compositions of CNCs before and after PCL-modification using

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FT-IR (Fig. 1) and XPS (Fig. 2).

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Figure 1. FT-IR spectra of (a) FDCNC; (b) SECNC; (c) FDCNC-g-PCL; and (d) SECNC-g-PCL.

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The FT-IR spectra among FDCNC, SECNC, FDCNC-g-PCL and SECNC-g-PCL are shown in Figure

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1.

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absorption bands in the vicinity of 1160 cm-1 and 3400 cm-1.11

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corresponding to the bending vibration of OH in CNCs also appeared. Similar peaks in FDCNC and

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SECNC spectra indicated that either freeze-drying or solvent-exchange did not affect the chemical

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features of CNCs.

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Compared to unmodified CNCs, FT-IR measurements of the grafted products showed new bands

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appearing at 1734 cm-1 and 1458 cm-1.

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group of PCL and the accompanying bending vibration of CH2, respectively.12 Shoulder peaks at

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2905 cm-1 (C–H stretching of CH2) also appeared in the spectra of the grafted samples.

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the intensity of signals at 1645 cm-1 weakened, indicating that part of the hydroxyls in CNCs reacted

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with ε-CL. To confirm that these signals originated from the grafted PCL rather than the homo-PCL

The C–O–C and OH groups in FDCNC and SECNC (Figures 1a and 1b) clearly showed Meanwhile, bands at 1645 cm-1

These new bands were easily attributable to the ester carbonyl

Furthermore,


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mixed with CNCs, all the samples were treated with dichloromethane extraction for 48 h to remove

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homo-PCL.

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used as the starting material for the ROP, the intensities of the signals at 1734, 1458 and 2905 cm-1

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were much lower than that of SECNC-g-PCL, observations that were likely attributable to a

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diminished surface grafting event for FDCNCs. Yet, no significant decrease in the signal near 3400

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cm-1 could be observed (grafting should decrease the content of hydroxyl groups). Relative to

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surface hydroxyl groups, the high density of inaccessible hydroxyl groups within CNCs limits the

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visibility of any signal decrease.

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XPS was then performed to confirm the FT-IR data.

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about surface CNC reactivity.

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atomic ratios in the samples before and after modification. Surface elemental compositions and

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atomic O/C ratios were determined using the wide scan spectra (Figure 2a), while the components of

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carbon atoms were evaluated using high-resolution carbon spectra (Figure 2b, c, d and e). The

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determination of different carbon bonds in the samples illustrated the different chemical valences of

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the carbons existing in the cellulose structure. In the high-resolution carbon spectra, the carbon

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signal can be resolved into four Gaussian component peaks:13 C1 contribution (285.0 eV) corresponds

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to the C–C or C–H bonds; C2 contribution (286.5 eV) corresponds to the C–O bonds; C3 contribution

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(288.0 eV) corresponds to the O–C–O bonds; and C4 contribution (289.0 eV) corresponds to the O–

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C=O bonds.

In general, it was observed from Figures 1c and 1d that when freeze-dried CNC were

Thus, the results provide indirect information

The grafting could be determined by observing the different carbon


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(a)

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(c)

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(e)

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Figure 2. XPS spectra of FDCNCs and SECNCs and their corresponding PCL grafted samples (a): wide scan spectra; and deconvolution of the C1s peaks of (b) FDCNCs, (c) SECNCs, (d) FDCNC-g-PCL, (e) SECNC-g-PCL.

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In the wide spectra (Figure 2a), the major components in the samples before and after esterification

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were carbon and oxygen, with a trace of sulfur from the residual sulfate groups from the acid

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hydrolysis. The atomic O/C ratio of cellulose is 0.83 in theory if there is no C–C component

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present.14

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2b and Table S1), while SECNCs showed a lower O/C ratio (0.67) with an increase in the C–C

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component (Figure 2c). The rise in the C–C component indicated that the surfaces of both FDCNCs

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and SECNCs were no longer devoid of extraneous carbon contamination. Due to high surface areas

However, FDCNCs showed an O/C ratio of 0.72 with a trace of a C–C component (Figure


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and accessible hydroxyl groups, the relative amount of accumulated extraneous carbon contamination

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during preparation and storage was substantially higher than the theoretical amount calculated for

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cellulose, leading to decreases in the O/C ratio for both SECNCs and FDCNCs. Meanwhile, the

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difference in the C–C value for the CNCs from the two treatment procedures suggested that their

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surface properties were different. The higher C–C value of SECNCs compared to FDCNCs indicated

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that after solvent-exchange, CNCs might experience more carbonaceous surface contamination in the

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form of adventitious carbon. Other than FDCNCs obtained from water, SECNCs were exposed to

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acetone and toluene, which consisted of more carbonaceous constituents than water, leading to a

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higher C–C content. Moreover, due to the higher surface area of SECNCs, the relative content of

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adsorbed contamination on the surface of SECNCs was higher than FDCNCs.

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the O/C ratio decreased from 0.67 to 0.49 for SECNC-g-PCL, while it decreased from 0.72 to 0.62 for

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FDCNC-g-PCL (Table S1). The decrease in the quantity was a clear indication of the decoration of

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the CNC surfaces with PCL chains.13

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scans of the C1s signal as shown in Figure 2d and e. In SECNC-g-PCL, the intensity of the C–C/C–

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H ratio substantially increased from 7.39% to 40.70%, indicative of PCL chain grafting. Successful

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esterification could also be proven by the increase of O–C=O from 4.87% to 11.95%, which could be

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assigned to the contribution of the carboxyl groups of PCL and the ester bonds formed after

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polymerization. However, the changes of C–C/C–H and O–C=O of FDCNW-g-PCL were not as

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drastic as compared to SECNC-g-PCL. This result suggested that solvent exchange gave rise to a

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higher grafting efficiency.

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nanocellulose derivatives may have suffered increased carbon surface contamination from adventitious

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carbon.

After esterification,

At the same time, this result was confirmed by high-resolution

Albeit, the elevated C–C level in SECNCs indicated that the


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Elemental analysis also gave a very good illustration of the differences in grafting between the species.

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The carbon contents and O/C mass ratios of the cellulosic materials are shown in Table 1. The mass

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ratios of O/C in FDCNCs and SECNCs were 1.24 and 1.68, respectively, both higher than the

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theoretical value of 1.11. This increase suggested the likelihood of sulfate ester groups on the surface

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of CNCs. Additionally, the increased O/C mass ratio might also be attributed to the absorbance of

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oxygen-containing heteroatom impurities such as oxidized oligo-glucose species on the surface of

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CNCs.14 The presence of adsorbed oxidized impurities is further confirmed by the presence of an O–

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C=O contribution to the C1s peaks in the XPS signals of the CNCs.

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Elemental analysis was also used to quantify the grafting state of CNCs with PCL, using PCL content

269

(WPCL) as a calibration point in the modified samples. The PCL content in the cellulose derivative was

270

calculated according to the following equation using carbon content before and after esterification:

271

(100 − WPCL ) × Wc / before + WPCL × 63.15 = Wc / after

(2)

272 273

where WC/before and WC/after corresponded to the mass percentage of carbon in nanocellulose before and

274

after modification, as determined by elemental analysis. Meanwhile, 63.15 represented the mass

275

percentage of carbon in the PCL polymer, calculated from the chemical formula of PCL. All values

276

present in Equation (2) are calculated as percentages. The corresponding PCL contents were evaluated

277

to be 21.9% and 41.6% for FDCNC-g-PCL and SECNC-g-PCL, respectively. Therefore, the reduced

278

PCL content in FDCNC-g-PCL versus SECNW-g-PCL indicated that the grafting of PCL onto CNCs

279

via the solvent exchange preparation protocol was enhanced relative to freeze-drying.

280

To determine the PCL density grafted onto CNCs, results from the MWs of homo-PCL in the grafting

281

reaction from Table 1 were used. Both modified samples had similar number average molecular


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282

weights (Mn), while the PDI of SECNC-g-PCL was slightly higher than that of FDCNC-g-PCL. The

283

poor control over the polymerization of FDCNCs, as demonstrated by the higher PDI, could be due to

284

residual water in the freeze-drying treatment, in line with what was obtained from the Lönnberg et al.

285

study.15 Compared with an increase of the PCL contents for SECNC-g-PCL and FDCNC-g-PCL,

286

similar molecular weights for the two samples suggested that the grafted PCL density of modified

287

CNCs was significantly reduced for the freeze-dried species relative to their solvent-exchanged

288

counterparts.

289

Dispersibility of CNCs before and after modification in different solvents. The visual appearance

290

of CNC preparations in water (Figure 3) was different based on the treatment method used. CNCs

291

after dialysis could be well dispersed in water (Figure 3a). Elemental analysis revealed the presence

292

of 1.22% of sulfur in CNCs, indicating that sulfuric acid induced surface esterification to yield

293

cellulose sulfates. These sulfate groups contributed to negatively charged surfaces that demonstrated

294

sufficient repulsive forces to improve suspension stability.16 However, the suspension of re-dispersed

295

CNCs obtained after freeze-drying displayed a white turbidity (Figure 3c), while the re-dispersed

296

suspension of CNCs after solvent-exchange (Figure 3b) was almost transparent.

297

protocol resulted in strong hydrogen bonds between CNCs, leading to large aggregations of the

298

colloidal suspension.15

299

freeze-drying because of less hydrogen bonds and aggregations in the suspension.

300

sulfate groups on the surface of SECNCs were also favorable for re-dispersion. Thus, in comparison to

301

freeze-drying, the solvent-exchange protocol maintained the stability of the CNCs during the

302

re-dispersion process.

The freeze-drying

SECNCs did not suffer such aggregative tendencies arising from Moreover, the


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(b)

Page 16 of 30

(c)

303

(d)

(e)

304 305 306

Figure 3. CNCs dispersed in water (a): after dialysis; (b): after solvent-exchange; (c): after freeze-drying and in toluene; (d): after solvent-exchange; (e): after freeze-drying.

(a)

(b)

(c)

(d)

307 308 309

Figure 4. Cellulose derivatives dispersed in different solvents (a): FDCNC-g-PCL in DMSO; (b): SECNC-g-PCL in DMSO; (c): FDCNC-g-PCL in toluene; (d): SECNC-g-PCL in toluene.

310

Similar to the dispersions in water, less hydrogen bonding and aggregation, as well as sulfate groups

311

(1.03% of sulfur) led to a white and stable suspension of SECNCs in toluene, with no visible

312

aggregation (Figure 3d). However, FDCNCs dispersed in toluene accumulated as white precipitate at

313

the bottom of the vial quickly after ultra-sonication, due to their strong hydrogen bonding and

314

electrostatic character. After grafting, the dispersibility of SECNC-g-PCL and FDCNC-g-PCL was

315

monitored in two solvents of different polarities, i.e., toluene (dipole moment 0.36 D) and DMSO

316

(dipole moment 3.96 D). As shown in Fig. 4, FDCNC-g-PCL suspensions precipitated much more


Page 17 of 30

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317

quickly than SECNC-g-PCL in both toluene and DMSO. PCL is a hydrophobic polymer with good

318

solubility in both toluene and DMSO. Successful grafting of PCL onto the CNC surfaces increased

319

their hydrophobicity and allowed them to remain stable in non-polar solvents such as toluene (Figure

320

4d) and polar solvents such as DMSO (Figure 4b). However, accumulation of precipitate in the

321

FDCNC-g-PCL system that was observed both in toluene and DMSO (Figure 4a and c) suggested that

322

the FDCNC-g-PCL system tended to precipitate likely because of the lack of a sufficient density of

323

PCL chains grafted onto the CNC surface.

324

Morphology and aggregation. To maintain the structure of CNCs and increase grafting during the

325

surface chemical modification, it was necessary to control their self-aggregation tendency.

326

shows the morphology of CNCs after dialysis in deionized water.

327

hydrolysis were rod-like, with a broad middle and two tapered ends. The nanorods averaged 19 nm in

328

width and 207 nm in length (average of 100 CNC images), which were well dispersed with few

329

aggregates. After solvent-exchange with toluene, fine nanorods with an average diameter of 23 nm

330

were still visible (Figure 5b), indicating that solvent-exchange did not modify the morphology of the

331

material. AFM images of SECNCs demonstrated that there was some degree of particle aggregation,

332

which was most likely due to weaker repulsive forces between the particles and electric dipole

333

differences between hydroxyl and toluene molecules.

334

the FDCNCs (Figure 5c) showed a high degree of particle aggregation, with an average diameter

335

increasing to 25 nm. The observed aggregates were highly elongated, indicating that FDCNCs

336

aggregated in an endwise manner and the average length of FDCNCs increased from 207 nm to 320

337

nm. This observation was in accordance with findings by Jiang et al.17

338

were likely due to strong intersheet and intrasheet bonding originating from the high density of

Figure 5a

CNCs obtained from acid

However, compared with SECNCs (Figure 5b),

The observed aggregates


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Page 18 of 30

339

hydroxyl groups at the surface of the cellulose chain molecules. The strong bonding favored the

340

formation of hydrogen bonds, which could not be prevented by ultra-sonication.18 These aggregates

341

reduced the total accessible reactive surface of the CNCs.

342

FDCNCs exhibited lower reactivity and accessibility as a result of strong aggregation.

343

After ROP, the nanorod fine structure of CNCs did not appear to have been affected by the grafting, a

344

result that is evidenced in Figures 5d and 5e. Some aggregates were observed in the topography

345

images of FDCNC-g-PCL (Figure 5e), but may be ascribed to poor dispersibility of FDCNC-g-PCL in

346

DMSO. Compared to FDCNC-g-PCL, few visible aggregates of SECNC-g-PCL were observed

347

(Figure 5d).

348

(Figure S1). These results were in accordance with their dispersion properties, suggesting that more

349

PCL has been grafted onto the surface of SECNCs relative to FDCNCs.

Therefore, in relation to SECNCs,

The nanocrystals were individualized and well dispersed as shown in the phase images

(a)

(c)

(b)

350

(d)

(e)

351


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352 353

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Figure 5. AFM topography images of cellulosic materials (a: CNCs in water; b: SECNC; c: FDCNC; d SECNC-g-PCL; e: FDCNC-g-PCL). (b)

(a)

354

(c)

(d)

355 356 357

Figure 6. FE-SEM images of cellulosic materials (a: FDCNC; b: SECNC; c: FDCNC-g-PCL; d: SECNC-g- PCL).

358

SEM micrographs of the cellulosic materials are displayed in Figure 6.

359

freeze-drying of aqueous suspension led to a locally aggregated compact structure with little open pore

360

volume among the crystals (Figure 6a). It was difficult to discern individual nanocrystals from

361

agglomerated structures, a result very consistent with the low value of 26 m2/g obtained from BET

362

surface area analysis (Table 1).

363

Table 1. Surface and crystalline properties of CNCs before and after esterification.

FDCNCs

BET surface area (m2/g) 26

Pore size (nm)

CrI (%)

6

74.1

Carbon content (%) 41.20

Mass ratio of O/C 1.24

The CNCs from

Mna (g/mol)

PDI

-

-


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Page 20 of 30

364

SECNCs 152 22 68.5 34.32 21 3 72.8 46.00 FDCNC-g-PCL SECNC-g-PCL 32 16 63.1 46.31 a: Average molecular weight of homopolymer as determined by GPC.

1.60 1.03 0.98

13700 14200

1.57 1.23

365

The aggregates were attributable to the separation of the cellulose nanoparticles from the ice crystals

366

formed during the initial freezing phase of the freeze-drying. As a consequence, FDCNCs were

367

densely packed in sheet-like structures and adhered to each other through strong hydrogen bonds.19

368

The strong aggregation and low surface area led to a restriction of the surface accessibility during

369

polymerization. Thus, compared to FDCNCs, SECNCs formed an open, porous nanoscaled structure

370

after solvent-exchange from toluene (Figure 6b). The BET surface area of SECNCs was substantially

371

higher, going from 26 m2/g to 152 m2/g (Table 1) with a pore diameter of 22 nm. This loose and

372

porous structure significantly increased the accessibility of surface hydroxyl during esterification, thus

373

leading to better grafting density of PCL in SECNC-g-PCL, a result in accordance with elemental

374

analyses and molecular weight determinations.

375

After grafting, the nanorod structure of FDCNC-g-PCL was retained and some of the rods aggregated

376

(Figure 6c). Yet, a few chains of PCL were grafted onto their surfaces. The low density of PCL

377

chains lead to the distinct observation of a brush-like structure in the SEM image. However, when

378

SECNC-g-PCL was synthesized, the original nano-rod structure was lost (Figure 6d). Instead, the

379

modified cellulose resembled a low-crystalline cellulose ester structure with a smoother surface

380

texture. Meanwhile, the surface area and pore size of the SECNC-g-PCL decreased, as a result of

381

PCL surface coverage. This observation was consistent with the high signal intensity arising from

382

carbonyl bonds shown in the FT-IR spectra.

383

their cellulose hexanoate system that was produced by the esterification of bacterial nanocellulose.7

Lee & Bismarck also reported similar observations in


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384

Crystallinity and transformation. X-ray diffraction was used to study the effect of esterification on

385

the crystallinity of FDCNCs and SECNCs.

386

is shown in Figure 7a.

387

characteristic X-ray diffraction patterns of cellulose I (Figure 7a). The major diffraction peaks at 2θ

388

= 14.8°, 16.5°, 22.8° and 34.5° in Figure 7a were assigned to the crystal planes of 101, 101 , 002 and

389

040 in the crystal structure of cellulose I.20

390

affect the crystal structure of cellulose. After the grafting reaction, both of the esterified CNCs

391

prepared in this study maintained the same diffraction profile, as shown in Figures 7c and d,

392

confirming that the crystalline structure of CNCs was unaltered by the surface modification treatments.

393

However, characteristic peaks of PCL, theoretically predicted at 21.3°, were not found for the

394

modified samples because a strong diffraction peak of cellulose I occurred at the nearby diffraction

395

angle of 22.8°.21

396

was also done. The crystallinity of the cellulosic materials was calculated using the proportion

397

between the amorphous area (2θ = 18°) and crystal areas (101, 101 , 002).22

398

characteristic peaks were fitted by a Gaussian-Lorentz function using JADE 5 software.

(a)

The crystalline structure of neat CNCs and grafted CNCs

Both of the unmodified FDCNCs and SECNCs crystals displayed

The XRD spectra proved that acid hydrolysis did not

Quantitative characterization of the crystalline region before and after esterification

The areas of these

(b)

399 400 401

Figure 7. (a) XRD spectra of the neat and PCL modified CNCs (a: FDCNCs; b: SECNCs; c: FDCNC-g-PCL; d: SECNC-g-PCL). (b) DSC thermogram of the neat and PCL modified CNCs.

402

The crystallinity was determined from Eq. 1 and is shown in Table 1. Before polymerization, FDCNCs


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Page 22 of 30

403

showed a higher crystallinity of 74.1° compared with SECNCs. This higher crystallinity could

404

partially be explained by the agglomeration of nanocellulose crystals caused by the strong hydrogen

405

bonds during the freeze-drying process. After esterification, SECNCs-g-PCL showed a reduction in

406

crystallinity from 68.5° to 63.1° (Table 1), indicating that the crystal structure of SECNCs was

407

damaged by the grafting and became more amorphous.

408

by PCL chains during the esterification resulted in the decrease of hydrogen bonding density between

409

cellulose crystals, which partially destroyed the crystalline structure of cellulose nanocrystals.

410

However, the decrease in the crystallinity of FDCNCs-g-PCL was not so apparent, implying that fewer

411

PCL chains were grafted on the surface of FDCNCs and the crystal structure did not change too much.

412

This result was in good agreement with the morphology analysis of FDCNCs-g-PCL.

413

To more deeply elucidate the properties of the grafted polymers, the melting and crystallization

414

transition of PCL grafted from CNCs were investigated by DSC. Habibi et al.23 reported that when

415

grafted PCL chains engaged in a crystalline structure at the surface of CNCs, the DSC thermogram

416

displayed a defined melting endotherm around 50 °C, while the endotherm was not observed for

417

unmodified CNCs. As shown in Figure 7b, homo-PCL exhibited a sharp peak at 59.9 °C, which can

418

be assigned to melting of the crystalline portions of PCL. Meanwhile, both FDCNCs and SECNCs

419

displayed gentle DSC curves without any obvious melting endotherm.

420

FDCNC-g-PCL and SECNC-g-PCL displayed a low magnitude melting endotherm, confirming the

421

presence of PCL on the CNC surface. However, compared to homo-PCL, the melting peaks of

422

modified samples were extremely weak, and shifted to lower temperature, especially for

423

FDCNC-g-PCL. These results, in combination with the XRD analysis, likely indicated that the

424

structure of the CNCs limited the crystallization of PCL, and that the attached PCL chains may exist in

The reduction of hydroxyl groups substituted

After grafting, both


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Langmuir

This was in accordance with results obtained by Habibi et al.23

425

a low-crystalline form.

The

426

observation was also borne out by SEM morphological studies of modified cellulose (Figure 6d) that

427

resembled a structure having a smooth surface texture. Moreover, compared to FDCNW-g-PCL, an

428

increase in melting temperature was observed for SECNC-g-PCL, suggesting that more PCL chains

429

were grafted onto the surface of SECNCs.

430

Thermal stability. The thermal stability for CNCs and their derivatives are shown in Figures 8a and b

431

and Table S2. The process of thermal decomposition and the residual composition of the CNCs are

432

influenced by many physical and chemical factors including temperature, crystallinity and size of the

433

cellulose sample, type of atmosphere, and other parameters.24

434

degradation temperatures (T1), peak pyrolysis temperatures (Tmax), and weight loss indicated different

435

thermal stability of CNCs under the two preparation protocols.

436

All of the samples showed a small initial mass loss around 100 °C as a result of the evaporation of

437

adsorbed moisture, followed by major mass loss due to pyrolysis (Figure 7a and b).

438

decomposition processes were observed in the TGA and DTG curves of CNCs after dialysis which

439

began at a low T1 of 110 °C. Tmax of the two peaks were 224 °C and 335 °C, respectively. These data

440

agreed with the work conducted by Wang who analyzed the thermal degradation of CNCs obtained

441

from sulfuric acid hydrolysis.16

442

dehydration phase in the cellulose chains at the lower temperature and was then propagated to the

443

cellulose crystal interiors, which were not in direct contact with the sulfate groups, to form a solid

444

residue.25

445

residues that slowly degraded and formed char products. Compared to the thermal degradation of

The differences in the onset of

Two

Acid sulfate groups in the CNCs were able to catalyze the primary

Afterwards, the second phase occurred at the higher temperature of 335 °C to yield solid


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Page 24 of 30

446

cotton linters at a Tmax of 342 °C (Table S2), the thermostability of CNCs decreased as a result of the

447

presence sulfate groups. After they were neutralized using a NaOH solution, both FDCNCs and

448

SECNCs exhibited a single decomposition process (Figure 8a and b), illustrative of the removal of

449

most sulfate groups from the surface of CNCs. However, Tmax decreased from 342 °C for cotton

450

linters to 289 °C, 271 °C, and 224 °C for SECNCs, FDCNCs, and un-neutralized CNCs, respectively,

451

showing decreasing thermal stabilities because of the presence of residual sulfate groups present even

452

after neutralization. This result agreed with previous work that suggested that a low amount of

453

sulfate groups still remain on the surface of CNCs. T1 occurred at 179 °C and 228 °C for FDCNCs

454

and SECNCs, respectively, indicating lower thermostability of FDCNCs compared to SECNCs. A

455

decrease in thermostability for FDCNCs would also negatively affect the grafting of PCL.

(a)

(b)

456 457 458 459

Figure 8. TG and DTG-curves of cellulosic materials (CNCs: dried using freeze-drying with no further treatment; FDCNCs: neutralized before freeze-drying; SECNCs: neutralized before solvent-exchange).

460

The surface chemical modification of CNCs using ε-caprolactone remarkably changed the thermal

461

stability. Both of FDCNC-g-PCL and SECNC-g-PCL had a two-step thermal degradation process.

462

For SECNC-g-PCL, the first step of pyrolysis started at 187 °C and the second at 300 °C. Compared

463

with the only decomposition step of SECNCs and PCL (Figure 8b), the bimodal shape of the DTG

464

curves with two different Tmax of SECNC-g-PCL confirmed the presence of both CNCs and PCL


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Langmuir

465

(Table S2). T1 could be attributed to the CNCs themselves, whereas the second to the esterified PCL

466

chains. T1 and Tmax decreased from 228 °C and 290 °C for SECNCs to 187 °C and 280 °C,

467

respectively.

468

degradation of grafted-PCL, which could behave as a “medium” to accelerate the thermal degradation

469

as opposed to ungrafted CNCs where there was no “medium”. The presence of residual sulfate

470

groups that can be released to form sulfuric acid in situ and catalyze the degradation might also be an

471

explanation.

472

FDCNC-g-PCL also exhibited two pyrolysis processes with two different sets of Tmax at 278 °C and

473

345 °C, close to the Tmax of SECNC-g-PCL, indicating that a low level of PCL chains grafted onto the

474

surface of FDCNCs. However, FDCNC-g-PCL exhibited the same onset of degradation temperatures

475

as FDCNCs (Table S2), highlighting the fact that the thermal stability of FDCNCs did not change

476

drastically after esterification. This result also suggested a lower content of PCL on the surface of

477

FDCNCs relative to SECNCs. A lower surface grafting density of PCL left the structure of CNCs

478

more intact and consequently did not affect the thermal stability nearly as much.

479

Figure 8a also shows different char residues for the cellulosic samples in the TGA curves.

480

observed that the char residues in the TGA curves decreased from 35.91% and 29.35% for SECNCs

481

and FDCNCs to 18.52% and 18.99% for SECNC-g-PCL and FDCNC-g-PCL, respectively.

482

shown in Figure 8a and Table S2, no residue was retained at 600 °C for PCL, indicating that the final

483

char residues of the pyrolysis after esterification were all derived from the cellulose residue.

484

Accordingly, the yields of PCL chains after grafting could be calculated from the char residues of

485

cellulosic materials. The values for SECNC-g-PCL and FDCNC-g-PCL were 48.4% and 29.9%,

The decreased thermal stability of SECNC-g-PCL was probably a consequence of the

It can be

As


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486

Page 26 of 30

respectively. These results are slightly higher than those calculated from elemental analyses.

(a)

(b)

487

(c)

(d)

488 489 490

Figure 9. Sessile contact angle images of the cellulosic materials (a): FDCNCs; (b): SECNCs; (c): FDCNC-g-PCL; (d): SECNC-g-PCL).

491

Hydrophobicity of CNCs before and after esterification. An estimation of the change in

492

hydrophobicity of CNCs as a result of esterification was obtained by contact angle (CA)

493

measurements using water.

494

the low contact angle (31.8°, Figure 9b) and the coincident rapid absorption of water. As shown in

495

Fig. 9a, the contact angle of FDCNCs (51.9°) was higher than that of SECNCs, which was a

496

consequence of the large sheet-like aggregations arising during the freezing process. Moreover, due

497

to the higher surface area and more porous structure as shown in BET surface area measurements, the

498

water drop on the surface of SECNCs was adsorbed, leading to a lower contact angle for SECNC.

The high hydrophilic character of SECNCs was well demonstrated by


Page 27 of 30

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Langmuir

499

After esterification, the lower contact angle values of grafted CNCs were higher than those of pure

500

CNCs, with a slower water absorbance (Figures 9c and d). This transition of hydrophilic to

501

hydrophobic was ascribed to the coverage of CNCs with a hydrophobic polymeric brush.26

502

Nevertheless, the increased value of CA for SECNC-g-PCL (82.3°) versus FDCNC-g-PCL (62.2°)

503

clearly indicated that the grafting of hydrophobic PCL was more successful onto SECNCs than

504

FDCNCs.

505

CONCLUSIONS

506

The present study demonstrated that the dehydration protocols applied to CNCs significantly affected

507

their resultant processability and final properties as demonstrated by PCL grafting.

508

were solvent-exchanged to toluene were more responsive to the modification as compared to the

509

freeze-dried species. Indeed, the PCL content grafted onto solvent-exchanged CNCs was 41.6%

510

compared to 21.9% for freeze-dried CNCs. Additionally, the contact angle of solvent-exchanged

511

CNCs increased from 31.8° to 82.3°, while that of freeze-dried CNCs increased only to 62.2°. The

512

thermal stability of solvent-exchanged CNCs both before and after modification was also higher,

513

although the crystallinities for both species were not appreciably altered by either dehydration protocol.

514

The freeze-dried CNCs exhibited strong aggregation relative to the solvent-exchanged CNCs which

515

decreased accessibility of surface hydroxyl functionalities.

516

solvent-exchanged CNCs provided more open, porous nano-textured structures that were more

517

susceptible to heightened polymerization reactivity.

518

ASSOCIATED CONTENT

The CNCs that

From a structural perspective,


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Page 28 of 30

519

Supporting Information. Surface carbon atoms proportions obtained from the deconvolution of the

520

C1s peak from XPS analysis, thermal data obtained from the TGA plots, and AFM phase images of

521

the cellulosic samples before and after esterification with PCL is available gratis at http://pubs.acs.org.

522

AUTHOR INFORMATION

523

Corresponding Authors: * E-mails: [email protected]; [email protected]

524

Notes: The authors declare no competing financial interest.

525

ACKNOWLEDGMENT

526

The financial support of the National Natural Science Foundation of China (No. 31170549), the Major

527

State Basic Research Development Program (No. 2010CB732206), and the Scientific Research

528

Foundation of Guangdong Educational Commission, China (No. 2013KJCX0016) are gratefully

529

acknowledged.

530

professorship with LAL allowed parts of this work to be realized.

531

A.-C. Albertsson for suggestions to improve the quality of the MS.

532

REFERENCES

533 534

(1)

Habibi, Y.; Lucia, L. A.; Rojas, O. J. Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications. Chem. Rev. 2010, 110, 3479-3500.

535 536

(2)

Eichhorn, S. J.. Cellulose Nanowhiskers: Promising Materials for Advanced Applications. Soft Matter 2011, 7, 303-315.

537 538

(3)

Moon, R. J.; Martini, A.; Nairn, J.; Simonsen J.; Youngblood, J. Cellulose Nanomaterials Review: Structure, Properties and Nanocomposites. Chem. Soc. Rev. 2011, 40, 3941-3994.

539 540 541

(4)

Cao, X.; Habibi, Y.; Lucia, L. A. One-Pot Polymerization, Surface Grafting, and Processing of Waterborne Polyurethane-Cellulose Nanocrystal Nanocomposites. J. Mater. Chem. 2009, 19, 7137-7145.

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