Cellulose Based Composites (New Green Nanomaterials) || Composites of Nanocellulose and Lignin-like Polymers

185

9Composites of Nanocellulose and Lignin-like PolymersJustin R. Barone

9.1Introduction

Polymeric nanocomposites are defined as polymer matrices filled with a nanometer-sized filler. These materials are usually combinations of rigid nanoclay fillers inmuch less rigid polyolefins [1]. There has been more recent interest in usingbiobased reinforcements such as nanocrystalline cellulose. Nanocrystalline cellu-lose can assume several forms depending on the source and processing method.Most is produced through acid hydrolysis. Native cellulosic sources produce roundnanocrystals that have diameters up to ∼10 nm when derived from wood and cottonand >10 nm when derived from algae or tunicate [2–7]. Tunicate nanocellulose(NC) can be up to micrometers in length, which is an order of magnitude longerthan those obtained from other sources [8]. Bacterial cellulose is as long as thatderived from tunicate but has a rectangular cross section of 10 nm thicknessand 50 nm width [4]. A combination of shear and enzymatic processing producesnanocrystals with a bimodal distribution of D= 5 and 15 nm [6]. The modulus ofcellulose nanocrystals is greater than 100 GPa and combined with the high aspectratio makes the reinforcement potential very high [9–11]. Polymeric matricesincorporating NC as the reinforcing phase include synthetic and biobased thermo-plastics [12–18]. Results are mixed with positive reinforcement occurring only forpolymeric matrices of low modulus. Thermosetting matrices have also been used,most notably phenol-formaldehyde, the most common wood adhesive [19–24]. NCseems to be the best reinforcement for itself with cellulose-based polymer matricesor amorphous cellulose able to be positively reinforced [25–29].

The use of NC in nanocomposites is leveraging the same technology that plantsuse to attain strength and rigidity: ordered cellulose in a more disordered poly-meric matrix. The plant cell wall is made by in situ polymerization of phenolicmonomers (lignin) into a web of polysaccharides (cellulose and hemicellulose),a process that is catalyzed by polyphenoloxidases (laccases) or peroxidases [30].Typical lignin monomers are coniferyl alcohol, coumaryl alcohol, and sinapyl alco-hol. In this highly structured fiber composite material, semicrystalline cellulosefibrils provide mechanical strength, while the relatively hydrophobic lignin acts

Cellulose Based Composites: New Green Nanomaterials, First Edition.Edited by Juan P. Hinestroza and Anil N. Netravali.c© 2014 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2014 by Wiley-VCH Verlag GmbH & Co. KGaA.

186 9 Composites of Nanocellulose and Lignin-like Polymers

as a stiffening matrix and hemicellulose as the coupling agent between the two[31, 32]. Regardless of whether or not covalent bonds are present between ligninand cellulose, studies from a wide range of perspectives provide clues about thestrong interactions between lignin and polysaccharides. Environmental scanningelectron microscopy (SEM) of the self-assembly of a lignin model compoundon a cellulose substrate indicates that the polysaccharide matrix in plant cellscan serve as a template for lignin structure formation by cooperative interaction[33]. In a molecular dynamics calculation study, both lignin monomer coniferylalcohol and its β-O-4-linked trimer are observed to adsorb onto the surface ofmodel cellulose microfibrils by electrostatic-like interactions [34]. Enthalpy pre-dictions of the adsorption of aromatic compounds on microcrystalline cellulosecomplemented with gas chromatography show that the interaction depends onthe roughness of the cellulose surface and the conformational adaptability of thearomatic compound [35]. Furthermore, the existence of covalent linkages involvingether bonds between glucuronoarabinoxylans (GAX) and the lignin monomer isestablished by 13C NMR and xylanase-based fractionation [36]. This associationis also seen in the physical influence of hemicellulose softening on the overallviscoelastic response of wood pulp, suggesting that the hemicellulose componentxylan is strongly associated with the lignin. Hydrophobic aggregates of GAX andlignin-like dehydrogenation polymers (DHP) of coniferyl alcohol are observed byexamining the interactions between GAX and DHP in sorption experiments [37].Isolated lignin is shown to nucleate the aggregation of xylan in aqueous solutions[38]. These studies indicate that the polysaccharide components can be intrinsicallylinked to, or can have a profound effect on the structure of, the polyphenoliccomponents.

Polymerization of coniferyl alcohol has been used to understand the role of ligninin the construction of plant cell wall [39]. The polymerization of lignin monomerin the presence of bacterial cellulose [40], arabinoxylan [37, 41], and pectin [42]has been studied to elucidate the chemical linkages within lignin macromoleculesand the formation of lignin–polysaccharide bonds [32]. Studying the potentialnoncovalent and covalent interactions between lignin-like polymers and celluloseis enormously interesting because it shows that some of the organization of theplant cell wall can be established in vitro and possibly good mechanical propertiescan be obtained from the resulting composite.

In this study, we attempt to biomimic the plant cell wall by enzymaticallypolymerizing phenol and coniferyl alcohol into a solution of polysaccharide. Themajor difference between these two monomers is the para-substituted propa-nyl on coniferyl alcohol. The polysaccharide is 2,2,6,6-tetramethylpiperidinyloxy(TEMPO)-radical-oxidized NC, which has nanometer dimensions and a surfacechemistry similar to that of hemicellulose. The findings propose a simple way tosynthesize a nanocomposite material using biobased components and also providevaluable insight into the nature of interactions between polyphenols (PPs) andcellulose.

9.2 Experimental 187

9.2Experimental

9.2.1Materials

Horseradish peroxidase (HRP, type I, 25–125 units (mg solid)−1), 30% hydrogenperoxide, 99% phenol, 98% coniferyl alcohol, phosphate buffer (0.1 M, pH= 7.5),TEMPO, sodium bromide, and 13% sodium hypochlorite solution were purchasedfrom Sigma-Aldrich and used without further purification. Cellulose fibers wereobtained from kraft pulp of 88% brightness provided by Weyerhauser Co. (FederalWay, WA).

9.2.2Preparation of TEMPO-Oxidized Nanocellulose

Never-dried cellulose wood pulp (10 g) was suspended in 500 ml deionized water(DI-H2O) in a three-neck flask. TEMPO (0.06 g, 0.0385 mmol) and sodium bromide(2.4 g, 0.0235 mmol) were dissolved in the three-neck flask. TEMPO-mediatedoxidation was initiated by adding 13% sodium hypochlorite solution drop by dropat room temperature. Oxidation was controlled by maintaining 5 mmol of NaClOper gram of dry cellulose. The solution was stirred at 500 rpm, and the pH wasmaintained at 10± 0.5 by continuous addition of 0.1 M NaOH. The process wascomplete when all NaClO was consumed and pH was stable. Ethanol was addedat a ratio of 3 : 1 to quench the reaction. The resulting suspension was vacuumfiltered and thoroughly washed with DI-H2O. Moisture content was determinedby thermogravimetric analysis (TGA) and DI-H2O was added to the oxidizedcellulose to make a 0.3% solution. The solution was sonicated with a Sonics®ultrasonic processor (Model GE 505) for 20 min within a controlled temperaturerange of 5–10 ◦C followed by centrifugation at 12 000g for 15 min. The supernatantwas a liquid crystalline solution of charged rigid rods of 3–5 nm diameter and580± 330 nm long [43]. Henceforth, references to NC are TEMPO-oxidized NC.

9.2.3Enzymatic Polymerization of Phenol and Coniferyl Alcohol

The procedure is based on previous work that produced thermally stable PPs withan average molecular weight of 2000–6000 g mol−1 [44–53]. Phenol or coniferylalcohol, HRP, and NC were added to a mixture of 10 ml of methanol and 10 ml ofphosphate buffer. The amount of phenol, coniferyl alcohol, and NC solution wereadded to produce nanocomposites of 4 : 1 PP or polyconiferyl alcohol (PCA) : NC and0.67 : 1 PCA/NC. Hydrogen peroxide (28 μl) was added to the mixture every 15 minfor 20 times at 20 ◦C under air. After 24 h, the mixture was washed with deionizedwater and filtered through Whatman 50 filter paper. The obtained nanocompositewas dried under vacuum. Pure PP and PCA were produced the same way without


NC. Control samples of phenol, coniferyl alcohol, and NC were processed in thesame manner without HRP.

9.2.4Scanning Electron Microscopy (SEM)

The dried polymers were gripped on two ends with forceps and gently pulledapart. The fracture surfaces after tensile break were visualized using a LEO (Zeiss)1550 Schottky field emission SEM. Each sample was coated with 3.5 nm Au/Pdbefore visualization. The electron beam intensity was kept at 5 kV and the workingdistance was 5–10 mm.

9.2.5Fourier Transform Infrared (FTIR) Spectroscopy

Fourier transform infrared (FTIR) analysis was performed with a Thermo Nicolet6700 with a diamond attenuated total reflectance (ATR) crystal. Pressure wasapplied to each sample to ensure good sample/crystal contact. A resolution of4 cm−1 was used over 64 scans. Background spectra were obtained before eachanalysis with blanks run between samples to ensure that there was no crystalcontamination.

9.2.6Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) was performed using a TA InstrumentsDSC Q100, 4–6 mg sample size, under nitrogen atmosphere. A two-cycle analysiswas used. In the first cycle, the sample was equilibrated at 30 ◦C then heated to150 ◦C at a rate of 10 ◦C min−1, equilibrated at 150 ◦C for 2 min, then air cooled to30 ◦C. In the second cycle, the sample was heated to 300 ◦C at a rate of 10 ◦C min−1.

9.2.7Thermogravimetric Analysis (TGA)

TGA was performed using a TA instruments Q500 TGA with 4–6 mg sample sizeunder nitrogen atmosphere. The heating cycle proceeded from room temperatureto 600 ◦C at 10 ◦C min−1. Results are reported as dTGA or first derivative of weightloss with temperature.

9.2.8Nanoindentation

Nanoindentation experiments were performed at room temperature using aHysitron Triboindenter (Minneapolis, MN) with a Berkovich diamond 142.3◦ three-sided pyramidal tip under load control (LC) at 2000 μN peak load or displacement

9.3 Results and Discussion 189

control (DC) at 1000 nm maximum displacement. The reduced modulus, Er, wasdetermined according to

Er =𝑆√𝜋

2√

Ac

(9.1)

where Ac was the contact area and S was the unloading stiffness, determined as theinitial slope of a polynomial function fit over 95 to 20% of the unloading curve [54].

9.3Results and Discussion

9.3.1Nanocomposite Morphology

The fracture surface of PP is predominantly flat with about 10 vol% 10 μm longellipsoidal particles evenly distributed throughout. Fracture of these large ellipsoidsreveals that they are composed of smaller ∼0.1 μm spherical particles. A 4 : 1 PP/NCcomposite has a high volume fraction of uniformly sized 0.1 μm spheres evenlydistributed throughout the fracture surface [55]. So in the presence of NC, large-scale aggregation of PP spheres is prevented. PCA fracture surface also appearsflat at low magnification. However, at high magnification, it reveals very smallspherical particles about 0.05–0.1 μm in diameter as shown in Figure 9.1a. Insome regions of the fracture surface, larger spheres of about 1–3 μm are observedas shown in Figure 9.1b. The spherical particles do not appear to be a largefraction of the volume and reside inside the sample. In Figure 9.2a, a 4 : 1 PCA/NCnanocomposite has a very high volume fraction of more uniformly sized sphericalparticles on its surface. At the fracture surface, the spheres look similar and theNC phase can be identified as shown in Figure 9.2b. Fractured spheres indicate

PCA10 μm WD = 6 mm EHT = 5.00 kV Date:17 oct 2008

Signal A = InLensMag = 20 KX Photo NO. = 6632PCA

10 μm WD = 7 mm EHT = 5.00 kV Date:17 oct 2008

Signal A = InLensMag = 1.00 KX Photo NO. = 6637

(a) (b)

Figure 9.1 Scanning electron micrographs of (a) polyconiferyl alcohol (PCA) surface (2K×)and (b) PCA fracture surface (1K×).


PCA composite surface PCA composite xsect10 μm

WD = 7 mm EHT = 5.00 kV Date:17 Oct 2008

Signal A = InLensMag = 200 KX Poto NO. = 6639

2 μm WD = 5 mm EHT = 5.00 kV Date:17 Oct 2008

Signal A = InLensMag = 10.00 KX Poto NO. = 6656

(a) (b)

Figure 9.2 Scanning electron micrographs of 4 : 1 polyconiferyl alcohol/nanocellulose (NC)nanocomposite (a) surface (2K×) and (b) fractured cross section (10K×).

good interaction between PCA and NC. The spheres in the 4 : 1 PCA/NC compositeare about 0.2–0.8 μm in diameter. Unsubstituted phenol monomer produces apolymer with some 0.1 μm spherical particles that aggregate into 10 μm ellipsoidalparticles. Adding a propanyl substituent produces a large distribution of sphericalparticles from about 0.05–3 μm in diameter. Polymerizing unsubstituted phenol inthe presence of NC prevents large-scale aggregation and increases the amount ofspherical morphology. Polymerizing propanyl-substituted phenol in the presenceof NC lowers the spherical size distribution and again increases the amount ofspherical morphology.

Recent work on lignin structure, coniferyl alcohol polymerization and organiza-tion on solid substrates, and coniferyl alcohol polymerization in the presence ofcellulose and soluble hemicelluloses shows several common features that may begeneric to lignification [33, 37, 40–42, 56–61]. It has been demonstrated that actuallignin fragments aggregate into particles with a cross-sectional area of 0.048 μm2,which would give aggregates of about 0.22 μm in diameter [57]. ‘‘Supermodules’’ ofDHP that are 5–20 nm in diameter are composed of layers of 2 nm diameter ‘‘mod-ules’’ and aggregation is surface dependent with DHP less ordered on rough goldsurfaces than on smooth graphite surfaces [61]. The layered module concept hasbeen advanced by showing that DHP forms larger aggregates of average diameter0.43 μm with a wide distribution of diameters when deposited from solution ontomica but a very narrow distribution when deposited onto graphite [58]. The DHPdeposited onto the oriented graphite surface shows elliptical particles about 1 μmlong similar to the PP described here. This observation may indicate that PP iselongated when dried on the fibrous cellulose filter paper because the large cellulosefibers would have directionality. The same DHP spheres order on cellulose acetatesurfaces, which would be chemically similar to a cellulose/hemicellulose surface[33]. DHP spheres of 1–7 and 0.25–0.50 μm are observed when polymerized inthe presence of cellulose and cellulose and pectin, respectively [56]. We observe0.1 μm diameter spheres that coalesce into 10 μm diameter spheres in the absence


of oxidized NC but remain at 0.1 μm when concurrently polymerized with oxidizedNC. We polymerize our PP in a solution of randomly oriented rigid rods of oxidizedNC. Therefore, the PP may organize on the NC, but there would be no long-rangeordering or elongation similar to that observed previously because of the lackof a large surface area substrate [58–60]. The presence of NC prevents furtheraggregation of PP/PCA spheres beyond a certain size indicating that the PPs mustinteract strongly with the NC. The presence of NC also produced PP/PCA spheresof narrow size distribution.

The spherical shape results from the amphiphilic nature of the DHP, lignin, or PPas it polymerizes, that is, it goes from totally hydrophilic to hydrophilic/hydrophobicas it loses –OH groups through formation of ether bridges. Hydrophilic portions ofthe molecule point out toward water, whereas hydrophobic portions point inwardaway from water forming the thermodynamically favorable sphere, such as micelles[41, 60, 62, 63]. In the plant cell wall, lignin exists as very small aggregates, thatis, on the order of 10 nm, similar to one in vitro study [61], whereas other in vitrostudies show much larger aggregates. The observation of large aggregates in vitrohas been attributed to experimental conditions based on the hypothesis that, invivo, lignin is polymerized in limited space with slow rates of monomer additionaround a multitude of other polymers [56]. A peculiarity found in the literatureis the order of magnitude difference in the DHP ‘‘supermodules’’ observed bytwo studies performing the same experiments [58, 61]. The interesting questionabout lignin sphere size in the plant cell wall is if it is indeed limited by spatialconsiderations or the thermodynamics of polymerization and assembly.

9.3.2Nanocomposite’s Thermal Properties

PP has no discernible glass-transition temperature, Tg, but a small shoulder at∼166 ◦C may be indicative of a subtle high temperature relaxation process asshown in Figure 9.3. PCA has a very discernible Tg at 57 ◦C. NC does show asmall Tg at 56 ◦C but previous work has shown isolated cellulose to have no Tg

over a several hundred degree Celsius temperature range [36]. The measured Tg

in this study could arise from features that develop during the NC film formationprocess not native to actual cellulose. A composite of PP and NC shows at most asmall shoulder at about 150 ◦C. At 4 : 1 PP/NC, a simple rule of mixtures analysiswould predict a Tg of 144 ◦C so the shoulder may be a real transition albeit notthe main relaxation. PCA has Tg = 57 ◦C. At ambient humidity, isolated ligninhas Tg = 70–100 ◦C depending on the isolation method [36, 64]. The observedTg of PCA is consistent with these values and the slightly lower value may berepresentative of less ring–ring bonding in PCA and more side chain–side chainbonding. Clearly, the role of the propanyl substituent is to add molecular flexibilityand free volume to the polyphenolic polymer. For the PCA/NC nanocomposites, theTg is higher than that for pure PCA and the small NC transition, with lower PCA inthe composite yielding a higher Tg. Here the observed low NC Tg is not influencingthe PCA–NC nanocomposite Tg. For PP–NC, the two phases interact differently


0−0.5

−0.4

−0.3

−0.2

−0.1

0

50 100

Temperature, T (°C)

Het flow

,Q (

W g

−1)

PCA

PP

4:1 PCA/NC

4:1 PCA/NC

0:67:1 PCA/NC

NC

200

Figure 9.3 Differential scanning calorimetry (DSC) second heating cycle showing glass-transition temperature (Tg) for polyphenol (PP), polyconiferyl alcohol, nanocellulose, andnanocomposites.

than in PCA–NC. PCA–NC nanocomposites appear to elicit the more native highTg of cellulose. The observed Tg = 119 ◦C of the 0.67 : 1 PCA/NC nanocomposite,which has a near native polyphenolic/cellulose ratio, is consistent with the Tg oflow-moisture-content softwood or higher moisture content hardwood although thePCA/NC ratio is more similar to softwood [64].

The nanocomposites also possess different thermal degradation behavior. AddingNC to PP increases its thermal degradation temperature, Td, and decreases its rateof thermal degradation as shown in Figure 9.4 [55]. PCA has the highest Td ofabout 378 ◦C. Nanocomposites of PCA and NC show intermediate thermal stabilitybehavior to PCA and NC but PP and NC show synergistic behavior. The high Td

of PCA raises the Td of the nanocomposites over that of NC. So the role of thepropanyl substituent is to also influence the thermal stability.

9.3.3Nanocomposite’s Mechanical Properties

Nanoindentation experiments show that the addition of NC imparts exceptionalrigidity to enzymatically polymerized PP. Table 9.1 lists the reduced modulus, Er,and hardness, H, of PP and a 4 : 1 PP/NC nanocomposite.

The addition of NC to enzymatically polymerized PP increases physical propertiesby an order of magnitude. Similar experiments on actual wood plant cells observehigher average Er = 12–15 GPa and H = 0.2–0.8 GPa [65] and Er = 20 GPa andH = 0.4 GPa [66]. The variability in these studies also contains an experimentalcomponent. Nanoindentation results vary with the type of tip used, where the


1000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

150 200 250 300

Temperature, T (°C)

d(%

W)/

dT

350 400 450 500

PCA

PP

4 : 1 PCA/NC

4 : 1 PP/NC

0.67 : 1 PCA/NC

NC

Figure 9.4 First derivative of weight loss with temperature from thermogravimetric analy-sis, dTGA, of polyphenol, polyconiferyl alcohol, nanocellulose, and nanocomposites.

Table 9.1 Nanocomposite nanoindentation results.

Sample Er LC (GPa) Er DC (GPa) H LC (GPa) H DC (GPa)

PP 0.25± 0.17 0.49± 0.39 0.018± 0.011 0.021± 0.0034 : 1 PP:NC 4.37± 2.37 3.29± 1.15 0.204± 0.150 0.164± 0.085

plant cell wall is sampled relative to the cellulose fiber axis, and whether theexperiment is load controlled or displacement controlled. However, in regions ofthe plant cell wall where lignin content is higher, the Er and H values are lowerand more consistent with the results in this study, although PP/NC 4 : 1 has higherpolyphenolic content than actual lignocellulosic material [65].

9.3.4Nanocomposite’s Structure

FTIR spectroscopic and solubility studies on PP–NC nanocomposites formed byenzymatically polymerizing phenol onto the NC and simply mixing an alreadypolymerized phenol with NC showed a high degree of hydrogen bonding betweenthe components [55]. Covalent interactions between NC and PP are identified ascarbon–carbon and ether bonding between phenols and glucose rings. Lignin isbelieved to be linked with the polysaccharide through two types of linkages, one anester-type combination between lignin hydroxyls and carboxyls of uronic acid onhemicellulose and the other an ether-type linkage through the lignin propanyl andthe hydroxyls of cellulose [32, 37]. A prevalence of hydrogen bonding is enough torender the nanocomposite insoluble in a variety of solvents.


16000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

1400 1200 1000

Wavenumber (cm−1)

Ab

so

rba

nce

800 600

CA

PCA

PP

Figure 9.5 Fourier transform infrared (FTIR) spectra of coniferyl alcohol and its HRP-catalyzed polymer.

Phenol and coniferyl alcohol have similar polymerization features notably inether and carbon–carbon bonding between rings. Shifts and reductions in ν(OH)at 3220 cm−1, 𝛿(C–OH)ip at 1370 cm−1 (note some contribution from 𝛿s(CH3) herealso), and ν(C–O) at 1220 cm−1 are indicative of ether bonding between phenolrings as shown in Figure 9.5. Changes in ν(C=C) aromatic doublets around 1600and 1480 cm−1 and aromatic deformations 𝛿(C–H) at 810, 746, and 687 cm−1 areindicative of the carbon–carbon bonding between rings [55, 67]. Relative to theOH on phenol, PCA contains a propanyl substituent at the para position and amethoxy substituent at the ortho position. The methoxy group does not participatein polymerization and serves as an internal control [33, 40–42]. The 𝛿as(CH3)appears at 1466 cm−1 in coniferyl alcohol (CA) and 1463 cm−1 in PCA and bothare of the same intensity without normalization. The propanyl substituent can beconsidered an asymmetrically substituted olefin and as such ν(C=C) appears as asharp absorbance at 1658 cm−1 for CA and as a wide absorbance at 1653 cm−1 forPCA. The large shoulder at 1270 cm−1 is assigned to ν(C–O) on the phenol andcompletely disappears on polymerization. The primary alcohol remains intact afterpolymerization evidenced mainly from the ν(C–O) at 1085 cm−1. Compared to PP,PCA polymerizes similarly except for additional polymerization across the C=C onthe propanyl.

To compare structural changes in PCA with NC addition, nanocomposite FTIRspectra are normalized to 1 at 1590 cm−1. Similarly, to compare structural changesin NC with PCA addition, nanocomposite FTIR spectra are normalized to 1 at1022 cm−1 and the results are summarized in Figure 9.6. In Figure 9.6a, the mostsignificant change is the loss of the 1134 and 1085 cm−1 absorbances with theaddition of NC. The shift and change in the CA peak from 1123 to 1135 cm−1 inFigure 9.5 occurs because the C=C bond in the propanyl substituent opens to form


1800

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so

rba

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1600 1400 1200 1000

Wavenumber (cm−1)

800 600 1800 1600

(a) (b)

1400 1200 1000

Wavenumber (cm−1)

800 600

PCA0.6:1 PCA/NC4:1 PCA/NC

NC0.67:1 PCA/NC4:1 PCA/NC

Figure 9.6 FTIR spectra of (a) polyconiferyl alcohol and its nanocomposites with nanocel-lulose normalized to 1 at 1590 cm−1 and (b) nanocellulose and its nanocomposites withpolyconiferyl alcohol normalized to 1 at 1022 cm−1.

a β-O-4 linkage and the absorbance changes are consistent with new ether bonding,the formation of a secondary alcohol, and a change in the environment of theprimary alcohol on propanyl. The presence of NC prevents β-O-4 bonds betweenconiferyl alcohol molecules during polymerization. In Figure 9.6b, the remnantsof absorbances at 1134 and 1085 cm−1 indicate some β-O-4 bonding and primaryalcohol remains in the PCA phase. Loss of the ν(C–OH) shoulders on NC at 1100and 994 cm−1 indicates bonding at these sites but the subtlety does not make itclear if it is physical or covalent.

On the basis of recent work, our observations suggest that the described abbre-viated system has similarities to other in vitro replications of lignification. A modelof the proposed assembly and organization in the nanocomposites is shown inFigure 9.7. It has been shown that cellulose has an influence on DHP structure

~1 nm ~1 nm~580 nm

Phenol monomers adsorb

to nanocellulose surfacePhenol covalently attaches to

nanocellulose and itselfPolyphenol spheres with hydrophobic core

and hydrophilic surface same size as

nanocellulose rods; non spherical polyphenol

ties entire structure together

Hydrophilic portion

stays out in H2O

OHOH

OH

OHOH

OH

OHOH

OH

OO

O

O

OOOH OHOH

Figure 9.7 Proposed model of polyphenol organization, polymerization, and assembly inthe presence of nanocellulose.


PP NC

PCA NC

Figure 9.8 Appearance of 4 : 1 polyconiferyl alcohol/nanocellulose and polyphenol:nanocellulose composites.

[33] and that soluble hemicelluloses, which exist in the actual plant cell wall, act asdispersants and organizers for DHP during polymerization [56]. The latter workinsinuates that ordered cellulose is not needed as a template to organize ligninbecause it happens in the presence of a soluble polysaccharide. The acids andaldehydes on the hemicelluloses act as aqueous dispersants. Ordering appearsto have happened on short length scales as evidenced by scattering experiments,which is consistent with ordering on the soluble polysaccharides [56]. The orderingis ‘‘layering’’ in the DHP globules or ‘‘modules.’’ However, long-range order doesnot exist. Our system utilized oxidized cellulose, also with plentiful acid (in theform of COO−) and some aldehyde groups on surface cellobiose. So it resembleshemicellulose in surface chemistry and we observe small unaggregated polypheno-lic spheres. The acid and aldehyde groups allow the NC to be dispersed in water forprocessing and cause PP dispersion and initiation sites for polymerization just likehemicellulose does for lignin in vivo. PP spheres grow to be 10–40% of the length ofthe NC rods. On the basis of SEM analysis, the sufficiently oxidized NC shows thepotential to create nanocomposites with plant-cell-wall-like structure achieving inti-mate mixing and providing increased thermal and mechanical properties. Indeed,the PCA/NC nanocomposite visually resembles wood as shown in Figure 9.8.

9.4Conclusions

Phenol and coniferyl alcohol were polymerized at various ratios into solutionsof oxidized NC. The polyphenolic phase existed as spherical clusters similar toprevious reports of DHP morphology. The presence of oxidized NC during poly-merization highly influenced the PP sphere size and therefore material morphologyand properties giving rise to an intimately associated mixture at the nanoscale inter-acting physically and chemically. Nanocomposites had high thermal stability andmechanical properties.

References 197

Acknowledgments

The generous support of VT-ICTAS and Farm Pilot Project Coordination, Inc. isgreatly appreciated. I would like to thank Dr Scott Renneckar and his researchgroup for helping in obtaining and preparing the NC samples. I would also liketo thank Ahmad Athamneh, Zhuo Li, and Cally Zanarini for preparation andcharacterization of the samples.

References

1. Giannelis, E.P., Krishnamoorti, R.,and Manias, E. (1999) Polymer-silicatenanocomposites: model systems forconfined polymers and polymer brushes.Adv. Polym. Sci., 138, 107–147.

2. Beck-Candanedo, S., Roman, M., andGray, D.G. (2005) Effect of reaction con-ditions on the properties and behavior ofwood cellulose nanocrystal suspensions.Biomacromolecules, 6, 1048–1054.

3. Braun, B., Dorgan, J.R., and Chandler,J.P. (2008) Cellulosic nanowhiskers.Theory and application of light scat-tering from polydisperse spheroidsin the Rayleigh-Gans-Debye regime.Biomacromolecules, 9, 1255–1263.

4. Klemm, D., Schumann, D., Kramer, F.,Hebler, N., Hornung, M., Schmauder,H.-P., and Marsch, S. (2006) Nanocellu-loses as innovative polymers in researchand application. Adv. Polym. Sci., 205,49–96.

5. Li, Q. and Renneckar, S.H. (2009)Molecularly thin nanoparticles fromcellulose: isolation of sub-microfibrillarstructures. Cellulose, 16, 1025–1032.

6. Pkk, M., Ankerfors, M., Kosonen, H.,Nyknen, A., Ahola, S., Sterberg, M.,Ruokolainen, J., Laine, J., Larsson,P.T., Ikkala, O., and Lindstrom, T.(2007) Enzymatic hydrolysis com-bined with mechanical shearing andhigh-pressure homogenization fornanoscale cellulose fibrils and stronggels. Biomacromolecules, 8, 1934–1941.

7. Saito, T., Nishiyama, Y., Putaux, J.-L.,Vignon, M., and Isogai, A. (2006)Homogeneous suspensions of indi-vidualized microfibrils from TEMPO-catalyzed oxidation of native cellulose.Biomacromolecules, 7, 1687–1691.

8. Elazzouzi-Hafraoui, S., Nishiyama, Y.,Putaux, J.-L., Heux, L., Dubreuil, F., andRochas, C. (2008) The shape and sizedistribution of crystalline nanoparticlesprepared by acid hydrolysis of nativecellulose. Biomacromolecules, 9, 57–65.

9. Dufresne, A., Cavaille, J.Y., andHelbert, W. (1997) Thermoplasticnanocomposites filled with wheat strawcellulose whiskers. Part II: effect of pro-cessing and modeling. Polym. Compos.,18, 198–210.

10. Hsieh, Y.-C., Yano, H., Nogi, M., andEichhorn, S.J. (2008) An estimation ofthe Young’s modulus of bacterial cellu-lose filaments. Cellulose, 15, 507–513.

11. Sakurada, I., Nukushina, Y., and Ito, T.(1962) Experimental determinationof the elastic modulus of crystallineregions in oriented polymers. J. Polym.Sci., 57, 651–660.

12. Angles, M.N. and Dufresne, A. (2001)Plasticized starch/tunicin whiskersnanocomposite materials. 2. Mechanicalbehavior. Macromolecules, 34, 2921–2931.

13. Azizi Samir, M.A.S., Alloin, F.,Sanchez, J.Y., and Dufresne, A.(2004) Cellulose nanocrystals rein-forced poly(oxyethylene). Polymer, 45,4149–4157.

14. Favier, V., Chanzy, H., and Cavaille,J.Y. (1995) Polymer nanocompositesreinforced by cellulose whiskers. Macro-molecules, 28, 6365–6367.

15. Lu, Y., Weng, L., and Cao, X. (2006)Morphological, thermal and mechanicalproperties of ramie crystallites-reinforcedplasticized starch biocomposites. Carbo-hydr. Polym., 63, 198–204.

16. Svagan, A.J., Azizi Samir, M.A.S.,and Berglund, L.A. (2007) Biomimetic


polysaccharide nanocomposites of highcellulose content and high toughness.Biomacromolecules, 8, 2556–2563.

17. Svagan, A.J., Azizi Samir, M.A.S., andBerglund, L.A. (2008) Biomimetic foamsof high mechanical performance basedon nanostructured cell walls reinforcedby native cellulose nanofibrils. Adv.Mater., 20, 1263–1269.

18. Svagan, A.J., Hedenqvist, M.S., andBerglund, L.A. (2009) Reduced watervapour sorption in cellulose nanocom-posites with starch matrix. Compos. Sci.Technol., 69, 500–506.

19. Goetz, L., Mathew, A., Oksman, K.,Gatenholm, P., and Ragauskas, A.J.(2009) A novel nanocomposite filmprepared from crosslinked cellulosicwhiskers. Carbohydr. Polym., 75, 85–89.

20. Iwamoto, S., Nakagaito, A.N., andYano, H. (2007) Nano-fibrillation of pulpfibers for the processing of transpar-ent nanocomposites. Appl. Phys. A, 89,461–466.

21. Kohler, R. and Nebel, K. (2006)Cellulose-nanocomposites: towardshigh performance composite materials.Macromol. Symp., 244, 97–106.

22. Nakagaito, A.N. and Yano, H. (2005)Novel high-strength biocompositesbased on microfibrillated cellulose hav-ing nano-order-unit web-like networkstructure. Appl. Phys. A, 80, 155–159.

23. Nakagaito, A.N. and Yano, H. (2008)The effect of fiber content on themechanical and thermal expansionproperties of biocomposites based onmicrofibrillated cellulose. Cellulose, 15,555–559.

24. Nakagaito, A.N. and Yano, H. (2008)Toughness enhancement of cellulosenanocomposites by alkali treatment ofthe reinforcing cellulose nanofibers.Cellulose, 15, 323–331.

25. Johnson, R.K., Zink-Sharp, A.,Renneckar, S.H., and Glasser, W.G.(2009) A new bio-based nanocomposite:fibrillated TEMPO-oxidized celluloses inhydroxypropylcellulose matrix. Cellulose,16, 227–238.

26. Matsumura, H. and Glasser, W.G.(2000) Cellulosic nanocomposites. II.Studies by atomic force microscopy.J. Appl. Polym. Sci., 78, 2254–2261.

27. Matsumura, H., Sugiyama, J.,and Glasser, W.G. (2000) Cellu-losic nanocomposites. I. Thermallydeformable cellulose hexanoates fromheterogeneous reaction. J. Appl. Polym.Sci., 78, 2242–2253.

28. Roman, M. and Winter, W.T. (2006)Cellulose nanocrystals for thermoplasticreinforcement: Effect of filler surfacechemistry on composite properties,in Cellulose Nanocomposites: Processing,Characterization, and Properties, (eds.K. Oksman and M. Sain), AmericanChemical Society, Washington, DC., pp.99–113.

29. Soykeabkaew, N., Sian, C., Gea, S.,Nishino, T., and Peijs, T. (2009) All-cellulose nanocomposites by surfaceselective dissolution of bacterial cellu-lose. Cellulose, 16, 435–444.

30. Huttermann, A., Mai, C., andKharazipour, A. (2001) Modificationof lignin for the production of newcompounded materials. Appl. Microbiol.Biotechnol., 55, 387–394.

31. Fratzl, P., Burgert, I., and Gupta, H.S.(2004) On the role of interface polymersfor the mechanics of natural polymericcomposites. Phys. Chem. Chem. Phys., 6,5575–5579.

32. Mohanty et al. (2005). Natural fibers,biopolymers, and biocomposites: Anintroduction, in Natural Fibers, Biopoly-mers, and Biocomposites, (eds. A.K.Mohanty, M. Misra, and L.T. Drzal),Taylor and Francis Group, pp. 1–36.

33. Micic, M., Radotic, K., Jeremic, M.,and Leblanc, R.M. (2003) Study of self-assembly of the lignin model compoundon cellulose model substrate. Macromol.Biosci., 3, 100–106.

34. Houtman, C.J. and Atalla, R.H. (1995)Cellulose-lignin interactions: a computa-tional study. Plant Phys., 107, 977–984.

35. Perez, D.D.S., Ruggiero, R., Morais,L.C., Machado, A.E.H., and Mazeau, K.(2004) Theoretical and experimentalstudies on the adsorption of aromaticcompounds onto cellulose. Langmuir,20, 3151–3158.

36. Salmen, L. and Olsson, A.-M. (1998)Interaction between hemicelluloses,lignin and cellulose: Structure-property

References 199

relationships. J. Pulp Pap. Sci., 24,99–103.

37. Barakat, A., Winter, H.,Rondeau-Mouro, C., Saake, B.,Chabbert, B., and Cathala, B. (2007)Studies of xylan interactions and cross-linking to synthetic lignins formed bybulk and end-wise polymerization amodel study of lignin carbohydrate com-plex formation. Planta, 226, 267–282.

38. Westbye, P., Kohnke, T., Glasser, W.G.,and Gatenholm, P. (2007) The influenceof lignin on the self-assembly behaviourof xylan rich fractions from birch (Betulapendula). Cellulose, 14, 603–613.

39. Guan, S.-Y., Mlynar, J., and Sarkanen, S.(1997) Dehydrogenative polymerizationof coniferyl alcohol on macromolecularlignin templates. Phytochemistry, 45,911–918.

40. Touzel, J.-P., Chabbert, B., Monties, B.,Debeire, P., and Cathala, B. (2003)Synthesis and characterization ofdehydrogenation polymers in Glu-conacetobacter xylinus cellulose andcellulose/pectin composite. J. Agric.Food Chem., 51, 981–986.

41. Barakat, A., Chabbert, B., andCathala, B. (2007) Effect of reactionmedia concentration on the solubilityand the chemical structure of ligninmodel compounds. Phytochemistry, 68,2118–2125.

42. Lairez, D., Cathala, B., Monties, B.,Bedos-Belval, F., Duran, H., andGorrichon, L. (2005) Aggregationduring coniferyl alcohol polymeriza-tion in pectin solution: a biomimeticapproach of the first steps of lignifica-tion. Biomacromolecules, 6, 763–774.

43. Saito, T., Kimura, S., Nishiyama, Y., andIsogai, A. (2007) Cellulose nanofibersprepared from TEMPO-oxidation ofnative cellulose. Biomacromolecules, 8,2485–2491.

44. Ayyagari, M., Akkara, J.A. and KaplanD.L. (eds) (1998) Solvent-enzyme-polymer interactions in the molecularweight control of poly(m-cresol) syn-thesized in nonaqueous media. ACSSymposium Series 684: Enzymes inPolymer Synthesis, ed. R.A. Gross,Kaplan, D.L., Swift, G., American Chem-ical Society: Washington, DC.

45. Ayyagari, M., Akkara, J.A., and Kaplan,D.L. (1996) Enzyme-mediated polymer-ization reactions: peroxidase-catalyzedpolyphenol synthesis. Acta Polym., 47,193–203.

46. Dubey, S., Singh, D., and Misra, R.A.(1998) Enzymatic synthesis and variousproperties of poly(catechol). EnzymeMicrob. Technol., 23, 432–437.

47. Felby, C., Hassingboe, J., and Lund, M.(2002) Pilot-scale production of fiber-boards made by laccase oxidized woodfibers: board properties and evidence forcross-linking of lignin. Enzyme Microb.Technol., 31, 736–741.

48. Felby, C., Thygesen, L.G., Sanadi, A.,and Barsberg, S. (2004) Native lignin forbonding of fiber boards-evaluation ofbonding mechanisms in boards madefrom laccase-treated fibers of beech(Fagus sylvatica). Ind. Crop Prod., 20,181–189.

49. Oguchi, T., S-i, T., Uyama, H., andKobayashi, S. (1999) Soluble polyphenol.Macromol. Rapid Commun., 20, 401–403.

50. Oguchi, T., S-i, T., Uyama, H., andKobayashi, S. (2000) Enzymatic synthe-sis of soluble polyphenol. Bull. Chem.Soc. Jpn., 73, 1389–1396.

51. Reihmann, M. and Ritter, H. (2006)Synthesis of phenol polymers usingperoxidases. Adv. Polym. Sci., 194, 1–49.

52. Uyama, H. and Kobayashi, S. (2003)Enzymatic synthesis of polyphenols.Curr. Org. Chem., 7, 1387–1397.

53. Uyama, H., Kurioka, H., Sugihara, J.,and Kobayashi, S. (1996) Enzymatic syn-thesis and thermal properties of a newclass of polyphenol. Bull. Chem. Soc.Jpn., 69, 189–193.

54. Oliver, W.C. and Pharr, G.M. (1992) Animproved technique for determininghardness and elastic modulus usingload and displacement sensing inden-tation experiments. J. Mater. Res., 7,1564–1583.

55. Li, Z., Renneckar, S.H., and Barone, J.R.(2010) Nanocomposites prepared by insitu enzymatic polymerization of phenolwith TEMPO-oxidized nanocellulose.Cellulose, 17, 57–68.

56. Cathala, B., Rondeau-Mouro, C.,Lairez, D., Bedos-Belval, F., Durand, H.,Gorrichon, L., Touzel, J.-P.,


Chabbert, B., and Monties, B. (2005)Model systems for the understanding oflignified plant cell wall formation. PlantBiosys., 139, 93–97.

57. Gilardi, G. and Cass, A.E.G. (1993)Associative and colloidal behavior oflignin and implications for its biodegra-dation in vitro. Langmuir, 9, 1721–1726.

58. Micic, M., Jeremic, M., Radotic, K.,and Leblanc, R.M. (2000) A com-parative study of enzymatically andphotochemically polymerized artifi-cial lignin supramolecular structuresusing environmental scanning electronmicroscopy. J. Colloid. Interface Sci., 231,190–194.

59. Micic, M., Jeremic, M., Radotic, K.,Mavers, M., and Leblanc, R.M. (2000)Visualization of artificial ligninsupramolecular structures. Scan, 22,288–294.

60. Micic, M., Radotic, K., Jeremic, M.,Djikanovic, D., and Kammer, S.B.(2004) Study of the lignin model com-pound supramolecular structure bycombination of near-field scanningoptical microscopy and atomic forcemicroscopy. Colloid Surf., B: Bioinint, 34,33–40.

61. Radotic, K., Simic-Krstic, J., Jeremic, M.,and Trifunovic, M. (1994) A study of

lignin formation at the molecular levelby scanning tunneling microscopy.Biophys. J., 66, 1763–1767.

62. Tokareva, E.N., Fardim, P., Pranovich,A.V., Fagerholm, H.-P., Daniel, G., andHolmbom, B. (2007) Imaging of woodtissue by ToF-SIMS: critical evaluationand development of sample prepara-tion techniques. Appl. Surf. Sci., 253,7569–7577.

63. Zimmerman, T., Thommen, V.,Reimann, P., and Hug, H.J. (2006)Ultrastructural appearance of embeddedand polished wood cell walls as revealedby atomic force microscopy. J. Struct.Biol., 156, 363–369.

64. Kelley, S.S., Rials, T.G., and Glasser,W.G. (1987) Relaxation behaviour ofthe amorphous components of wood.J. Mater. Sci., 22, 617–624.

65. Konnerth, J., Gierlinger, N., Keckes, J.,and Gindl, W. (2009) Actual versusapparent within cell wall variability ofnanoindentation results from wood cellwalls related to cellulose microfibrilangle. J. Mater. Sci., 44, 4399–4406.

66. Jakes et al. (2007).67. Gunzler and Gremlich (2002).

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Cellulose Based Composites (New Green Nanomaterials) || Composites of Nanocellulose and Lignin-like Polymers