Polypyrrole (PPy) chemical synthesis with xylan in aqueousmedium and production of highly conductingPPy/nanofibrillated cellulose films and coatings

Claudia Sasso • Nicolas Bruyant • Davide Beneventi •

Jerome Faure-Vincent • Elisa Zeno • Michel Petit-Conil •

Didier Chaussy • Mohamed Naceur Belgacem

Received: 25 May 2011 / Accepted: 29 July 2011 / Published online: 9 August 2011

� Springer Science+Business Media B.V. 2011

Abstract Polypyrrole was chemically synthesised by

using, for the first time, Birchwood xylan as additive,

and ammonium peroxydisulfate (APS) as oxidant. The

impact of additive concentration, polymerisation time

and reagents concentration on PPy conductivity was

studied. It was shown that, once fixed the pyrrole (Py)/

APS and Py/xylan optimal ratios, the best conductivities

(26 S/cm) were obtained for short polymerisation times

(30 min) and increased reactants concentration. Mor-

phological analysis of PPy particles, Py depletion

kinetics and oxido-reduction potential measurements

of the solutions provided interpretation elements on the

impact of the polymerisation time on PPy pellet

conductivity. Furthermore, optimised PPy particles

obtained with xylan (PPyx) were mixed with nanofibr-

illated cellulose (NFC) in order to obtain freestanding

films. Their electrical and handling performances were

evaluated at increasing PPy weight fraction in the

samples. The conductivity mechanism of the most

conductive sample (in comparison with a low perform-

ing sample) was investigated by measuring the conduc-

tivity as a function of temperature (4–350 K) and two

transport regimes were identified. Selected formulations

were finally used to produce conducting PPy/NFC

coatings on non-absorbent (glass) and absorbent (copy

paper) substrates. The impact of NFC in the percolation

of PPy particles, then in the coating conductivity, was

investigated.

Keywords Polypyrrole � Xylan � Nanofibrillated

cellulose � Conductivity � Composite film

Introduction

Depletion of raw materials and socio-economical

concerns, in the last decades, made the valorization of

waste and by-products a key challenge for citizen and

scientific communities (Polprasert 2007). The contri-

bution to a sustainable development ranges from the

reduction of polluting emissions, to the biomass and

waste transformation in valuable materials, as reus-

able or new products. Industrial processes generating

a huge amount of wastes, such as agro-food and pulp

and paper industries (Demir et al. 2005; Mahro and

Timm 2007), are particularly involved.

Xylan and xylan derivatives are promising wood

issued waste materials in term of potential industrial

C. Sasso � D. Beneventi (&) � D. Chaussy �M. N. Belgacem

LGP2, UMR 5518 CNRS-Grenoble-INP-CTP,

461 Rue de la Papeterie, DU, BP 65,

38402 St. Martin d’Heres, France

e-mail: davide.beneventi@pagora.grenoble-inp.fr

C. Sasso � E. Zeno � M. Petit-Conil

Centre Technique du Papier, DU, BP. 251,

38044 Grenoble CEDEX 9, France

N. Bruyant � J. Faure-Vincent

CEA, Institut Nanosciences et Cryogenie, LEMOH UMR

5819 CEA/CNRS/UJF, 38054 Grenoble, France

123

Cellulose (2011) 18:1455–1467

DOI 10.1007/s10570-011-9583-2

extraction and exploitation (Puls et al. 2005; Yang

et al. 2005). They can be effectively recovered, in

fact, both by eliminating non-cellulosic components

from the paper pulp during the phases of chemical

pulping/bleaching (Timell 1967; Wallberg et al.

2006; Ali and Sreekrishnan 2001) or by the ultrafil-

tration of H2O2 treated liquor resulting from the

alkaline extraction of wood (Glasser et al. 2000).

Recently, progresses in xylan recovery were reported

(Fuhrmann and Krogerus 2009; Sixta 2011), highlight-

ing the possibility of a large scale production. If this goal

is achieved, xylans would be an excellent example of

waste/by-product valorization and successful sustain-

able conversion of the pulp mill industry.

Usually, xylans physical and chemical properties as

well as their biocompatibility make them suitable as

drug carriers, dietary fibers, and food additives, for

instance (Moure et al. 2006). Anyway, the complexity of

their structure and their varied chemical composition

make xylans interesting for the development of new

materials and investigations aimed at the synthesis of

hydrogels (Gabrielii and Gatenholm 1998), cationic

polysaccharides (Schwikal et al. 2005), amphiphilic

molecules (Ebringerova et al. 1998), thermoplastic

polymers (Jain et al. 2000), and composites (Amash and

Zugenmaier 1998; Saxena et al. 2009) prospered in the

last years.

Actually, the efficiency of other wood derivatives,

such as carboxymethyl cellulose (CMC) and ligno-

sulfonates in improving PPy electric conductivity

(Sasso et al. 2008; Yang and Liu 2009) and cellulose

and nanofibrillated cellulose (NFC) in conferring film

forming properties to conducting PPy/cellulose-based

composites have been recently demonstrated (Sasso

et al. 2010, 2011; Rußler et al. 2011).

Indeed, organic electronics seems to be a promis-

ing application field for wood derivatives, due to their

chemical composition, mechanical properties and

availability. They can be used as substrate for printed

electronics, i.e. paper (Eder et al. 2004) and film

forming products (Shah and Brown 2005), or enter

actively in the synthesis of conducting composites,

i.e. carboxymethylcellulose (Mahmud et al. 2005)

and hydroxypropyl cellulose (Amaike and Yamamoto

2006). Recently, among the most promising wood

derivatives for organic electronics, nanofribrillated

cellulose (NFC), has drawn the attention due to its

optical and film forming properties (Nogi and Yano

2008). For instance, NFC was mixed with resins to

obtain transparent films for the production of organic

light emitting diodes (Okahisa et al. 2009), or

coupled with conducting polymers to have self

standing composites with enhanced electrical and

mechanical properties (Nystrom et al. 2010).

In a recent work, a first suggestion of xylan as

additive for pyrrole polymerisation was given by our

research group (Sasso et al. 2010). Moreover, in the

same work, it was demonstrated that NFC would

slightly interfere with the percolation of PPy particles

in the film, thus leading to PPy/NFC self standing

films with outstanding conductivities (Sasso et al.

2010). To continue the research on these two axes,

the present paper focuses on the optimization of PPy

chemical synthesis (in water) in the presence of

Birchwood xylan and the elaboration of PPy/NFC

conducting composites.

Experimental

Materials

Py was distilled before use and stored at 277 K.

Birchwood xylan (Aldrich) (molecular weight: 28,000

g/mol; [–COOH] = 0.46 mmol/g) and ammonium

peroxydisulfate, (NH4)2S2O8, (Aldrich) were used as

received. NFC was produced from eucalyptus pulp as

described in a previous work (Sasso et al. 2010).

Pyrrole polymerisation

Two distilled water solutions were prepared. In the

first, xylan was solubilised by heating at 90 �C during

20 min. After room temperature cooling, Py was

added. In the second, APS was dissolved. Both

solutions were conditioned at 276 K. Then the

Py/xylan solution was poured slowly in the reactor

containing the oxidant, and the polymerisation was

carried out for a fixed time. The reaction was stopped

by adding 20 mL of methanol into the polymerisation

medium. The PPy dispersions were then subjected to

three centrifugation cycles (10,000 rpm, 10 min).

Finally, PPy particles were collected.

Xylan concentration, polymerisation time and

reagents concentration were sequentially varied in

order to evaluate the impact of the different param-

eters on Py polymerisation (Table 1). The Py/APS

molar ratio was kept constant throughout all the

1456 Cellulose (2011) 18:1455–1467

123

experiments (Py/APS = 5), whereas, the xylan con-

centration was varied, and the value allowing the

maximum in conductivity was set. Then, using

previously fixed parameters (Py/APS ratio, Py and

xylan concentration), the polymerisation time was

changed from 5 min to 24 h, and the time leading to

the best conductivity was established. Finally, once

xylan concentration, and polymerisation time fixed,

the reagents concentration was modified using con-

stant Py/APS and Py/xylan molar ratios (i.e. 5 and

11.6 9 103, respectively). A starting reagents con-

centration, namely C0 (Table 1) was chosen. Then,

multiples and submultiples of C0 (n*C0) were used to

synthesise PPy.

PPy particles thus obtained were conditioned in

form of pellet or water dispersions. To produce pellets,

PPy particles were vacuum-dried in a Buchy oven at

313 K for 12 h, and pressed with an hydraulic press

(608 MPa). Water dispersions were obtained by red-

ispersing never-dried PPy particles in 6–10 mL of

deionised water, and submitting them to three ultra-

sound cycles of 1 min (Branson sonifier, 250 W).

During this treatment, the beaker containing PPy

particles was dip into an ice bath in order to prevent

excessive heating and PPy degradation.

Films and coating formation

PPy synthesised in the presence of an optimized

xylan concentration (i.e. temperature: 276 K, Py:

8.334 g/L, xylan: 0.3 g/L, polymerization time: 2 h)

was used for the elaboration of conductive films and

coatings. PPy dispersions were mixed to a NFC

(20 g/L) gel at different PPy/(PPy ? NFC) weight

ratio (Table 2) under mechanical stirring. NFC gel

mixing with the PPy dispersion caused a drop in NFC

concentration down to 8–2 g/L, depending on the

PPy/(PPy ? NFC) ratio, and the typical progressive

disruption of the NFC gel (Lowys et al. 2001; Paakko

et al. 2007). Nevertheless, as demonstrated in a

previous study (Sasso et al. 2010), PPy/NFC disper-

sions displayed excellent film forming ability and

films were obtained by dispersions casting on a

Teflon mould (2.4 9 2.4 cm) and air drying over-

night (ca. 17 h). The formulations A, B, E, and P1

were selected to obtain conducting coatings on a non-

absorbent and an absorbent surface, namely glass and

copy paper. They were produced by means of a

manual slot coater with a nominal slot thickness of

120 lm (air drying overnight).

Characterisation

The xylan carboxylic groups were dosed by poten-

tiometric titration using NaOH (0.01 mol/L) as

neutralising solution. The redox potential of the

oxidant and oxidant mixed xylan solutions was

measured by an Ag/Cl electrode (WTW, inoLAB

750). Py depletion during polymerisation (kinetics)

was controlled by high performance liquid chroma-

tography (HPLC) as described in a previous work

(Sasso et al. 2011).

Morphological analysis of PPy particles (collected

from the polymerisation bath and air-dried) and of PPy/

NFC films and coatings were carried out by scanning

electron microscopy (SEM—Fei Quanta 200), and field

effect electron microscopy (FE-SEM—Zeiss Ultra 55).

PPy/NFC coating on glass morphology was evaluated

by optical microscopy.

Electrical characterisation was carried out on PPy

pellets, films and coatings with a four probe ohm-

meter (Jandel Universal Probe). Results are an

average of at least five measurements.

The charge transport of PPy/NFC films was

investigated by evaluating the conductivity as a

function of temperature. To this purpose, in plane

Table 1 Experimental conditions for pyrrole polymerization

APS concentration

(g/L)

Py concentration

(g/L)

Xylan concentration

(g/L)

Time Temperature

(K)

5.67 8.334 Variable from 0.1 to 0.7 2 h 276

5.67 8.334 0.3 Variable from 5 min to 24 h 276

2.835–34.02 (C0: 5.67) 4.167–50.004 (C0: 8.334) 0.15–1.8 (C0: 0.3) 30 min 276

Reactants concentration is referred to the polymerization medium, after mixing reactants solutions. Py to APS molar ratio was kept

constant throughout the experiments (Py/APS = 5)

Cellulose (2011) 18:1455–1467 1457

123

DC conductivity was measured using the four-probe

method (four parallel golden contacts were deposited

on the surface of the film by evaporating gold metal

under high vacuum). The measurements were carried

out in a He gas flow Oxford cryostat allowing

measurements from 4 K and up to 350 K.

Results and discussion

Py polymerisation

The impact of dopant concentration on PPy pellet

conductivity is presented in Fig. 1. The conductivity

reached a maximum of 10 S/cm at a dopant concen-

tration of 0.3 g/L. By increasing xylan amount up to

1 g/L, a five times conductivity decrease was

observed. This means that xylan acted as dopant

until a certain amount, then the further insertion of

dopant molecules within PPy chains created a steric

barrier for the charge carrier movement between PPy

particles. This behaviour is typical for high molecular

weight molecules such as wood derivatives (Sasso

et al. 2011) and surfactants (i.e. sodium bis

(2 ethylhexyl) sulfosuccinate, sodium dodecylben-

zenesulfonate) (Omastova et al. 2004; Kudoh et al.

1998).

Once the optimal dopant concentration fixed, the

influence of polymerisation time on PPy pellet

conductivity was evaluated. Figure 2 shows PPy

pellet conductivity as a function of polymerisation

time for systems containing xylan and CMC (for

comparison) as dopants. The best conductivities were

obtained for short polymerisation times. Particularly,

when xylan was used, by decreasing the time from

2 h to 30 min, conductivity values sharply increased

from 10 to 17 S/cm. For the system containing CMC,

the optimal polymerisation time was found after 2 h

of polymerisation.

To explain this difference, redox potential measure-

ments of oxidant and oxidant/dopant solutions were

carried out. From Table 3, it could be observed that

xylan and CMC had a different impact on the electro-

chemical properties of the systems: the APS/xylan

solution redox potential (652 mV) was slightly higher

than that of the APS/CMC homologue (635 mV). As

suggested by Chen et al. (1995), systems with high

redox potential would rapidly oxidise Py, and the

Table 2 Experimental

conditions for PPy/NFC

films and coatings, with

corresponding thickness

PPy synthesis conditions:

Py/APS molar ratio: 5, Py:

8.334 g/L (C0), xylan:

0.3 g/L, corresponding to a

Py/xylan molar ratio of

11.6 9 103

PPy/(PPy ?

NFC) ratio (g/g)

Reference Film thickness (lm) Thickness coating

on glass (lm)

Thickness coating

on paper (lm)

0.35 A 0.19 4.5 14.5

0.5 B 0.33 3 13.5

0.6 0.28

0.7 0.37

0.8 E 0.55 4.3 17.5

1 P1 0.42 9 14.5

Fig. 1 PPy pellets conductivity as a function of xylan

concentration (polymerisation time: 2 h, temperature: 276 K,

Py: 8.334 g/L)

Fig. 2 PPy pellets conductivity as a function of the polymer-

isation time (temperature: 276 K, Py: 8.334 g/L, xylan: 0.3 g/L,

i.e. Py/xylan molar ratio: 11.6 9 103). The PPy doped CMC

curve is get from Sasso et al. (2011)

1458 Cellulose (2011) 18:1455–1467

123

polymerisation time leading to the best conductivities

would be short. Anyway, for those systems, PPy

degradation would occur fastly too, and a rapid loss of

conductivity would be noticed. As a consequence, the

optimal polymerisation time would be the balance

between polymer development and its degradation

(associated to the overoxidation, i.e. the formation of

C=O groups due to water and secondary oxidant species

attack; Sasso et al. 2011; Thieblemont et al. 1994;

Novak 1992). Indeed, the different impact of xylan and

CMC on the redox properties of the systems as well as

their doping activity was mainly determined by the

presence of carboxylate anions (Kuwabata et al. 1990).

Xylan and CMC present different carboxylate anions

availability. This is principally associated to the charge

amount (xylan: 0.5 mmol/g; CMC: 2.4 mmol/g), its

distribution along the chain, and the dopant molecular

weight (xylan: 28,000 g/mol; CMC: 90,000 g/mol).

Despite the lower COOH content, the lower molecular

weight confers to xylan an easier approach to the PPy

molecules thus making it a more efficient dopant than

CMC.

To investigate if the variation in conductivity due to

the polymerisation time is associated to a morphology

difference, FESEM analysis of PPy particles with-

drawn at different extent of polymerisation (from

5 min to 24 h) were carried out. Contrarily to what

expected, no differences in morphology were noticed

between PPy particles obtained after 30 min or 24 h

of polymerisation (Fig. 3). PPy particles present a

general cauliflower aspect (Omastova et al. 2003)

since 5 min of polymerisation (here not shown):

their morphology did not change during the reac-

tion, for both undoped and doped samples. This

could be associated to the kinetics of monomer

consumption (Fig. 4). Sharp monomer depletion

was noticed in the first minutes of polymerisation,

then, a slightly Py consumption was measured. This

means that, after the initial major formation of PPy

chains, their morphology did not considerably

Table 3 Redox potentials of Py polymerisation systems

Redox potential (mV)

276 K (±1)

APS 689

APS ? CMC 635

APS ? xylan 651.5

Fig. 3 SEM micrographs: a PPy doped xylan, polymerisation

time: 30 min (temperature: 276 K, Py: 8.334 g/L, xylan: 0.3 g/L);

b PPy doped xylan, polymerisation time: 24 h (same

polymerisation conditions as before); c undoped PPy, poly-

merisation time: 30 min (temperature: 276 K, Py: 8.334 g/L);

d undoped PPy, polymerisation time: 24 h

Cellulose (2011) 18:1455–1467 1459

123

change during the polymerisation (Sasso et al.

2011). The monomer consumption is associated to

the Py polymerisation mechanism (Fig. 5). It deals

with a polycondensation in which PPy chains were

fastly developed in the first minutes of polymeri-

sation (i.e. when the reagents are highly concen-

trated) thus leading to the quite immediate

formation of PPy insoluble clusters. Then, after

the PPy cluster precipitation, the reaction proceeds

slowly, and no modification in PPy chain structure

was observed. Moreover, SEM micrographs show

that PPyx particles are smaller than the undoped

ones (Fig. 3). This suggested that xylan acted also

as dispersing agent.

Fig. 4 Monomer depletion

as function of time for

polymerisation systems, in

the presence or not of the

dopant (temperature:

276 K, Py/ASP molar ratio:

5, Py/xylan molar ratio:

11.6 9 103)

Fig. 5 Py polymerisation mechanism

1460 Cellulose (2011) 18:1455–1467

123

After having evaluated the impact of polymerisa-

tion time on PPyx pellet conductivity, the influence of

reagents concentration was investigated. Figure 6

shows the conductivity of PPy pellets as a function

of the reagents concentration in the polymerisation

bath. The PPy pellet conductivity increases from 7 S/cm

for the concentration C0/2 (half the starting concen-

tration C0, see Table 1) to 26 S/cm for the concen-

tration 4*C0 (fourfold the starting concentration C0),

then it decreases. As suggested by Lei et al. (1992),

PPy chains obtained from lower reactants

concentration were shorter and with a higher amount

of defects. Particularly, shorter chains were highly

reactive and they preferably underwent side reac-

tions. As a result, many defects in the form of

carbonyl groups and double bonds between carbon

and nitrogen, –C=N–, were introduced in the PPy

chains. These defects deal with the decrease of the

conjugation length and, consequently, with the low-

ering of conductivity. For higher reactant concentra-

tion (6*C0), a decrease in conductivity was observed.

It was supposed that, after a certain concentration,

despite an initial great development of the conjuga-

tion system, the higher redox potential of the

polymerisation medium would induce defects in the

PPy chains, thus interrupting the conjugation and

lowering the conductivity (Chen et al. 1995).

PPy films

PPy particles obtained with a Py concentration of

8.334 g/L and a polymerisation time of 2 h (temper-

ature: 276 K, xylan concentration: 0.3 g/L), were

used to produce PPy/NFC composite films with

increasing PPy/(PPy ? NFC) ratio (Fig. 7). High

ratios were used with the aim of favouring conduc-

tivity (conferred by PPy) on the mechanical

Fig. 6 PPy pellets conductivity as a function of the reactants

concentration (polymerisation time: 30 min, temperature:

276 K, Py/APS molar ratio: 5, Py/xylan molar ratio:

11.6 9 103)

Fig. 7 PPy/NFC film

conductivity at increasing

amount of PPy in the films.

At the bottom; PPy/NFC

films pictures

Cellulose (2011) 18:1455–1467 1461

123

properties (conferred by NFC). As expected, it was

observed that, when increasing PPy in the film

formulation, the conductivity increased. Particularly,

in the PPy/(PPy ? NFC) range considered, the film

conductivity increased quite linearly with the relative

amount of conducting particles. Normally, in con-

ducting polymer/polymer composites, the composite

conductivity versus filler amount (dispersed ran-

domly in the matrix) follows an S curve, where the

sharp change in convexity determines the percolation

threshold (Fournier et al. 1997; Yeetsorn et al. 2008).

Even if the ratio range considered in our case would

not allow to conclude about the percolation threshold,

the upper part of the S curve was expected to appear

from our results (i.e. quite constant conductivity with

the increase of PPy in the composite). Since this was

not the case under the tested conditions, it was

supposed that the isotropy of the composite, deriving

from the multilayered structure of PPy/NFC films

(Sasso et al. 2010), would make the classical

percolation models unsuitable for our system.

Concerning the handling properties, when NFC

was used, uniform self-standing films were obtained

(Fig. 7, bottom, A, B, E), while films prepared

without NFC were brittle and fragmented (Fig. 7,

bottom, P1). Particularly, when increasing the NFC

amount, the films were the more and more flexible.

For instance, if samples B and E are compared, the

former was bendable and easy to handle, while the

latter was brittle and rigid.

Conductivity as a function of temperature

In order to deeply understand the conducting mech-

anism of the PPy/NFC films, the variation of the

conductivity as a function of the temperature was

investigated for sample B and E (Fig. 8). The curves

could be fitted using a model based on transport

mechanisms related to granular systems above metal

insulator transition (Variable-Range Hopping (VRH)

model; Mott and Davis 1971). Indeed, according to

this model, the conductivity follows the equation

rðTÞ ¼ r0eT0Tð Þ

c

where r0 is the conductivity at 0 K

and T0 is the Mott temperature. To determine the c

coefficient, W ¼ T d ln r Tð Þð Þð ÞdT was plotted versus tem-

perature (Fig. 9). The slope of W in log–log plot

determines the value of c. Within the tested temper-

ature range, W has a negative temperature coefficient,

indicating that the system is in the insulating regime.

More remarkable is that two different regimes can be

clearly identified: from 4 K to approx. 50 K, c ¼ 12

and above approx. 50 K, c ¼ 14. The c ¼ 1

4regime

corresponds to the Mott’s VRH conduction in three

dimensions among localized states for non-interact-

ing charge carriers. The c ¼ 12

behaviour corresponds

to the Efros-Shklovskii’s approach of the VRH model

(Shklovskii and Efros 1984), where the coulomb

interactions (between the electron and the hole left

behind) becomes dominant and therefore leads to a

build-up of a coulomb gap in the density of states

near the Fermi level. The characteristic temperature

of this model is T00. The crossover between the two

Fig. 8 Resistivity variation as function of temperature for

samples B and E

Fig. 9 Log-log plot of W versus T (see text for details) for

samples B and E. Solid (dashed) lines represent c ¼ 12

c ¼ 14

� �

temperature regime

1462 Cellulose (2011) 18:1455–1467

123

regimes occurs at temperatures between 50 and 65 K

noted Tcross.

Both samples show doped semiconductor behav-

iour without any visible metallic component. This

indicates that our systems are well above the Metal–

Insulator transition. The room temperature conduc-

tivity under vacuum of sample E is 20% above the

one of sample B whereas this difference is 35% under

ambient conditions. This indicates a higher sensitivity

of sample E to doping by oxygen and water and this

is in agreement with the fact that sample E contains

more PPy particles than sample B.

The adjustment parameters of the temperature

dependence are given in Table 4. The lower temper-

ature dependence of sample B, which can be seen on

Fig. 8, is clearly confirmed by the smaller values of

T0 and T00, thus indicating a larger localization length

of carriers in sample B (Nalwa 1997). These char-

acteristics can be consistently explained with a higher

doping of sample E which compensates the lower

mobility of carriers leading to higher conductivity of

sample E over sample B.

PPy coatings

PPy/NFC dispersions, at different NFC content

(A [ B [ E [ P1: no NFC), were deposed on non-

absorbent (glass) and absorbent (copy paper) sur-

faces. Conductivity plots shown in Fig. 10 illustrate

that the deposition on non-absorbent surface (glass)

led to conductivities quite an order of magnitude

higher than those obtained on absorbent surface

(copy paper). Indeed, the substrate topography and its

chemical nature affect the coating drying mechanism

and consequently its conductivity (Winther-Jensen

et al. 2007; Denneulin et al. 2008). Smooth, non-

absorbent substrates led to the formation of homo-

geneous PPy/NFC films (Fig. 11), in which charge

carriers could move without any other interference

than the material (PPy and NFC) intrinsic resistivity.

On the contrary, rough and absorbent substrates

allow the PPy/NFC dispersion penetration, thus

leading to an irregular film, in which substrate upper

fibres create an obstacle for film homogeneity

(Fig. 12a, b). In this case, charge carriers movement

Table 4 Adjustment

parameter for samples B

and E with Efros-

Shklovskii’s model under

Tcross and Mott VRH above

Tcross

T00 (for

T \ 50 K)

T0 (for

T [ 50 K)

T0/T00 r0 (for

T \ 50 K)

r00 (for

T [ 50 K)

Sample B 241 ± 2 K 24,540 ± 150 K 102 6.74 ± 0.06 S/cm 81.0 ± 0.4 S/cm

Sample E 412 ± 1 K 58,200 ± 1,100 K 141 11.05 ± 0.05 S/cm 200 ± 4 S/cm

Fig. 10 PPy/NFC coatings

conductivity at increasing

amount of PPy in the

formulation

Cellulose (2011) 18:1455–1467 1463

123

is supposed to be more difficult, and the conductivity

is lower.

As expected, the conductivity increased with the

PPy content in the samples. But, surprisingly, the

100% PPy samples (totally composed by conducting

particles) presented a lower conductivity than the

NFC mixed ones (conducting and insulating parti-

cles). As it is shown in Figs. 11 and 12, in the

samples P1s, PPy did not form a continuous layer on

the substrate surface, but it was organised in sepa-

rated clusters. This is more evident for coating on

glass, where the separation between PPy clusters was

so clear that no percolation was possible between

them, and null conductivity was measured (Fig. 11,

P1). For coating on paper, on the contrary, the rough

absorbent surface induced some constraints in PPy

dispersion drying, thus allowing the formation of

contacts between clusters. In this way, percolation

paths were formed during electrical measurements

and conductivity was revealed. As it could be

expected from previous studies (Aulin et al. 2010),

the coatings were the more and more homogeneous

and continuous as the amount of the NFC increased

in the formulation, particularly for paper substrate.

Conclusions

In this work, the efficiency of Birchwood xylan as

enhancing conductivity additive in Py polymerisation

was studied. It was demonstrated that xylan concen-

tration, polymerisation time and reactants concentra-

tion have a considerable impact on PPy pellet

conductivity. Particularly, it was shown that by

decreasing the polymerisation time from 2 h to

30 min, the conductivity increased quite twofold,

more evidently than for other wood derivatives used

as dopants (i.e. CMC). Moreover, the same results

could be obtained by multiplying four times the

reactants concentration in the bath.

The highest conductive PPy particles were mixed

with NFC to prepare self-standing films and coatings

(on absorbent and non absorbent surface). It was

shown that PPy/NFC film conductivity increased with

Fig. 11 PPy/NFC coatings on glass at increasing PPy amount in the composite

1464 Cellulose (2011) 18:1455–1467

123

the PPy content in the films, inversely to the handling

properties. Conduction mechanism in PPy/NFC com-

posites (samples E and B) was found to follow a

classical electronic transport model of doped semi-

conductor. Below 50 K, the conduction is in agree-

ment with the Efros-Shklovskii’s VRH approach,

while above 50 K, a three-dimensional conduction

mechanism dominates (Mott’s VRH model).

Coatings were produced on non-absorbent and

absorbent substrates. Coatings on glass (non-absor-

bent) were demonstrated to be one order of magni-

tude more conductive that those on paper. By

producing films and coatings at different NFC

contents, the efficiency of NFC in helping PPy

particle percolation was demonstrated. Fragmented

films composed by rigid PPy platelets, were obtained

in the absence of NFC. When NFC was mixed to PPy

particles, it act as binder and flexible and conducting

films/coatings were obtained.

Acknowledgments This work was supported by a CIFRE

grant from the French ‘‘Association Nationale de la Recherche

et de la Technologie’’, CTP and CTPi members and by the

National Research Agency through the Myosotis project

(ANR-08-NANO-012-01).

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