# Conductive and thermoactivated flax yarns developed by in situ polypyrrole polymerizations: Interactions with carbohydrate polymers

Alaa Ismael, Sofiane Guessasma, Antoine Bozek, Sylvie Durand, Patrice

Papineau, Antoine Le Duigou, Mickael Castro, Johnny Beaugrand, Ana
Villares

► To cite this version:

Alaa Ismael, Sofiane Guessasma, Antoine Bozek, Sylvie Durand, Patrice Papineau, et al.. Conductive and thermoactivated flax yarns developed by in situ polypyrrole polymerizations: Interactions with carbohydrate polymers. Progress in Organic Coatings, 2025, 198, pp.108894. 10.1016/j.porgcoat.2024.108894. hal-04848079

HAL Id: hal-04848079

https://ubs.hal.science/hal-04848079v1

Submitted on 25 Sep 2025

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Distributed under a Creative Commons Attribution 4.0 International License
Conductive and thermoactivated flax yarns developed by in situ polypyrrole
polymerizations: Interactions with carbohydrate polymers

Alaa Ismael a*, Sofiane Guessasma a, Antoine Bozek a, Sylvie Durand a, Patrice Papineau a, Antoine le

Duigou b,c, Mickael Castro b, Johnny Beaugrand a*, Ana Villares a

a UR1268 BIA, INRAE, F-44316, Nantes, France

b Univ. Bretagne Sud, UMR CNRS 6027, IRDL, F-56100 Lorient, France

c Institut Universitaire de France (IUF), Paris, France

*Corresponding authors: alaa.ismael@inrae.fr and johnny.beaugrand@inrae.fr

**Abstract**

In this study, a straightforward strategy is presented to functionalize flax yarns with conductive polymers, adding electrical properties for the development of bio-based multifunctional materials. One of the applications aimed is smart electronics. For this purpose, the new materials need to maintain as much as possible their initial characteristics, including fiber composition and mechanical properties, with the addition of new valuable ones, such as electrothermal. Specifically, in this study, a wide range of results are obtained after two different polymerization processes, with either polypyrrole (FY-PPy) or polypyrrole/silver (FY-PPy-Ag). A full description of these materials is provided and includes the quality of the polymerization, the chemical composition analysis, and the mechanical properties before and after functionalization, such as the longitudinal tensile modulus, tensile strength, elongation at break, and hygroscopic properties. Finally, voltage-controlled experiments were performed to determine the electrothermal properties of the modified yarns. We observe higher electrical conductivity and thermal conductivity for FY-PPy than for FY-PPy-Ag. ( add )

The maximum temperature increase caused by the Joule effect at 18 V is +31.5°C and +16.1°C for FY-PPy and FY-PPy-Ag, respectively. (chemical composition does not change)

**Keywords**

Joule effect – Hygroscopic effect – Hysteresis – Cellulose – Electrical functionalization
# 1. Introduction

Plant fiber-reinforced composites are viewed as good candidates for replacing synthetic fibers in materials applications (Vinod et al., 2020). Furthermore, glass and carbon fibers are extensively used in composites as reinforcing components (Le Duigou et al., 2011) and present high levels of environmental concern due to their manufacturing processes, distribution, and end-of-life unsolved issues. Owing to their reinforcing capabilities in composites, natural fibers such as flax (Goudenhooft et al., 2019), hemp (Mieczyslaw Scheibe & Bledzki, 2023), jute (Baley et al., 2020b), sisal (Mieczyslaw Scheibe & Bledzki, 2023) and cotton (Wankhede et al., 2022) have gained popularity as alternatives to synthetic fibers. These fibers have outstanding qualities, including low density, a high specific strength-to-stiffness ratio, high toughness, low abrasiveness, and unique ecological features such as high abundance, biodegradability, renewability, recyclability, and CO2 neutrality. Moreover, their manufacture requires less energy than glass fibers (Le Duigou et al., 2011).

However, plant fibers have also low electrical conductivity (Zhang et al., 2017). This is a consequence of the chemical composition of fiber bundles, which are composed of a mixture of biopolymers. Specifically, cellulose fibrils are the main component of flax fibers and are embedded in a matrix of non-cellulosic polymers called the middle lamella (ML) (Richely et al., 2022). These non-cellulosic polymers are either polysaccharides, like hemicellulose (Gautreau et al., 2022) and pectins (Lahaye et al., 2020), or lignin (Neutelings, 2011). ML has a key role in the properties of plant fibers, by maintaining the structural integrity of plant tissues, allowing cell-cell adhesion (Melelli et al., 2022; Zamil & Geitmann, 2017). Accordingly, the development of electroconductive natural fibers has gained interest because of the high demand for improving the electrical capabilities of reinforced composites; thus, fibers have been modified through the incorporation of conductive materials, such as carbon nanotubes (CNTs) (Tripathi et al., 2016), silver (Ag) and zinc oxide (ZnO) structures (Costa et al., 2018), graphene platelets, and graphene oxide (GO) nanosheets (Zhu et al., 2023). These novel functionalization pathways enable the development of smart textiles and wearable technologies by combining the natural tactility of flax fibers with enhanced conductive properties that can be
integrated into a variety of antistatic and electronic applications, such as supercapacitors (Zhang et al., 2017), piezoresistive devices (Costa et al., 2018; Kee et al., 2016), and flexible conducting wires (Mohan & Bhattacharyya, 2020; Zhu et al., 2023). Another strategy to improve the electrical properties of natural fibers is continuous functionalization with conducting organic polymers, such as polypyrrole (PPy), polyaniline (PANI), polythiophene (PTH), and their derivatives. These polymers have many advantages over other conducting materials through the combination of the electronic properties of metals and the engineering properties of polymers, and are often referred to as 'synthetic metals' (Wang et al., 2010). These polymers feature a chemical structure that enables the delocalization of the conjugated electrons, formed by the overlapping of carbon p-orbitals, along their backbone (Lekpittaya et al., 2003). As a result, the charge carriers move between and along the polymer chains, creating a material that is remarkably conductive (Malinauskas, 2001).

Among these conductive polymers, PPy is commonly utilized in the electronic industry, such as in energy storage materials or sensors, owing to its high conductivity, ease of synthesis, strong environmental stability, and biocompatibility (Wang et al., 2010). According to the literature, PPy has been recently grafted onto cotton textiles via an *in situ* polymerization method; here, a thin coating layer of PPy can be deposited onto yarn via chemical oxidization of the pyrrole monomer using an oxidizing agent (Firoz Babu et al., 2012). This oxidative polymerization involves the withdrawal of an electron from the pyrrole monomer by the oxidant, with the formation of a radical cation; then, two radical pyrrole monomers are recombined and deprotonated to form the dimer. The process is repeated to produce a polymer chain of PPy.

In the present study, we exploit the superior tensile properties of flax and its environmental footprint among the other plant fibers for functionalization and development of electrothermal materials. Two oxidizing agents, ferric chloride ($FeCl_3$) (Chen et al., 2005) and silver nitrate ($AgNO_3$), are used to polymerize PPy on flax yarns *in situ*. The electrothermal hybrid flax yarn is supposed to achieve the capacity to transform part of the electrical energy into heat via the Joule effect (Tao et al., 2022). A procedure for functionalizing yarns with continuous electroconductive material is
presented. The addition of electrothermal functionalities on the composition and structure of the
raw flax material is known to play an important role in the mechanical behavior of flax (Lefeuvre et
al., 2014), and the effects of this addition were also investigated. Moreover, flax, like any
lignocellulosic material, is sensitive to humidity due to its biochemical composition (Célino et al.,
2014b); thus, the hygroscopic properties of flax were studied after coating. This property has dual
prospects depending on the application; in some composite industries, it is considered a limitation
and natural weakness (Faruk et al., 2012), whereas it is considered a driving force in autonomously
self-shaping materials that respond to moisture gradients in hydromorphic biocomposites (HBCs) (Le
Duigou & Castro, 2015). Finally, the obtained thermal and electrical conductivities are further
explained. Combining the mechanical and electrical properties of flax yarn forms electroresponsive
composite materials, facilitating new application fields, such as biobased sensors or actuators.

## 2. Materials and methods

### 2.1. Materials

Continuous flax yarn (FY) from SAFILIN (France) was used with a low twist (320 turns per meter) and a linear density of 68 Tex. Pyrrole (Py), ferric chloride ($FeCl_3$), silver nitrate ($AgNO_3$), L-cysteine, inositol, and each monosaccharide standard, rhamnose (Rha), fucose (Fuc), arabinose (Ara), xylose (Xyl), mannose (Man), galactose (Gal), glucose (Glc), glucuronic acid (GlcA), and galacturonic (GalA), were all laboratory grade and purchased from Sigma Aldrich (Burlington, Massachusetts, United States).

### 2.2. Functionalization

A 13 cm long flax yarn segment was cut from the bobbin and strained vertically under a consistent force of 0.13 N between two metallic clamps at a distance of 9 cm. The strained yarn was then immersed in a solution containing 0.4 M pyrrole monomer and 0.4 M oxidant $FeCl_3$ in a Teflon-based
system for one hour of reaction at room temperature under a fume hood; in the solution, the molar ratio of Py:Fe³⁺ was kept constant at 1:1. Under the same temperature conditions, another in situ polymerization of polypyrrole was also investigated by replacing FeCl₃ with 0.4 M AgNO₃ at a 4:5 molar ratio of Py:Ag⁺, as shown in equations 1 and 2; these equations were created using ChemDraw 23.0. When AgNO₃ was used, the reaction lasted up to 24 h at room temperature. For each process, molar ratio has been optimized in order to obtain the highest conductivity. The volumetric ratio of solid content (FY) to liquid content was 1:300 for both methods. After the reaction, the yarns were washed three times with deionized water and three times with ethanol (96%) and then dried under a strain of 0.13 N for 24 h under the fume hood, in ambient conditions. The flax yarns coated with PPy polymerized using FeCl₃ are referred to as FY-PPy, and those coated with PPy polymerized using AgNO₃ are referred to as FY-PPy-Ag. The referred untreated sample is designated FY.

$$ (1) \quad \chemfig{*6(---(-N_H)---(-N_H)---)} + \text{Fe}^{3+} \rightarrow \left[ \chemfig{*6(---(-N_H)---(-N_H)---)} \right]_n + \text{Fe}^{2+} + \text{HCl} $$

$$ (2) \quad \chemfig{*6(---(-N_H)---(-N_H)---)} + \text{Ag}^+ \rightarrow \left[ \chemfig{*6(---(-N_H)---(-N_H)---)} \right]_n + \text{Ag}^0 + \text{HNO}_3 $$

**Scheme 1.** (1) Chemical polymerization of the pyrrole monomers in the presence of ferric chloride as an oxidant to form a polypyrrole polymer, Fe²⁺ and HCl

(2) Chemical polymerization of the pyrrole monomers in the presence of silver nitrate as an oxidant to form a polypyrrole polymer, Ag⁰ and HNO₃
## 2.3. Characterization techniques

### 2.3.1. Surface analysis of the flax yarns

Scanning electron microscopy (SEM) was carried out using a field emission gun scanning electron microscope Phenom Pure G6 Desktop (Thermo Fischer Scientific, Quattro S, Waltham, MA, USA) at an acceleration voltage of 10 kV and at a pressure of 1 Pa. SEM images were captured using a low vacuum (LV) detector. The samples weren't metallized prior to observation, and kept at a strain of 0.13 N by clamping at their two extremities.

### 2.3.2. Chemical composition

#### Qualitative investigation by infrared spectroscopy

Infrared spectra were collected in the mid-infrared domain (4000–400 cm⁻¹) using a dry-air-purged Thermo Scientific Nicolet iS50 Fourier transform infrared (FTIR, Thermo Scientific, Courtaboeuf, France) instrument equipped with a mercury cadmium telluride (MCT) detector and cooled to 77 K. The spectral resolution was 4 cm⁻¹, and the resulting spectra is an average of 256 scans. An attenuated total reflectance (ATR) accessory Smart iTX - ATR diamond accessory (A = 2.54 mm²) at a 45° angle of incidence, with a single reflection, was used. The data were reprocessed using OMNIC 9.0 software. The spectra were corrected by an elastic baseline and min-max normalization. Each sample has been analyzed in triplicate.

#### Quantitative investigation

Flax yarns were ground under cryogenic conditions into a fine powder using a SPEX 6700 Freezer Mill (Horiba Scientific, Kyoto, Japan) and then dried at 40°C in a vacuum oven containing P₂O₅ for 48 h. All biochemical composition tests were carried out in triplicate, and the results are presented as a percentage of the dry mass.
* Monosaccharide composition using gas chromatography

Quéméner et al.'s (Quéméner et al., 2007) approach was used to detect and quantify the neutral monosaccharides after acid hydrolysis and conversion of the monomers into alditol acetates. Internal inositol and external monosaccharides (rhamnose, fucose, arabinose, xylose, mannose, galactose, and glucose) were used respectively as an internal standard and as calibration standards. Chromatography was performed on a TraceGOLD™ TG-225MS GC column (30 × 0.32 mm ID) (TRACE GC Ultra Thermo Scientific™; temperature 205°C, H₂ carrier gas). Colorimetry with meta-hydroxydiphenyl was used to assess uronic acids (galacturonic and glucuronic acids) in acid hydrolysates (Blumenkrantz & Asboe-Hansen, 1973).

* Lignin content

Lignin content was determined using spectrophotometry via the cysteine-assisted sulfuric acid (CASA) technique (Lu et al., 2021). This method was adapted to remove the polypyrrole signal prior to the final absorption reading. In detail, powder samples (10 ± 0.01 mg) were dissolved in a 1 mL solution of 1 g/L L-cysteine in 72% sulfuric acid for one hour at 24°C with continuous stirring at 500 rpm using a Bioer Mixing Block Model MB-102 (Bioer, Potsdam, Germany). The suspension was then filtered using glass filters with a porosity of 1 µm and placed into vials. A UV 3100 PC spectrophotometer from VWR (Radnor, Pennsylvania, United States of America) was used to measure the absorbance at λ=283 nm after the tenfold dilution with distilled water was filtered. The total lignin content was calculated using Beer–Lambert's law. The accuracy of the method by adding a filtration step has been verified and no differences appeared on a test sample of flax yarn.

* Elemental analysis of protein and polymer contents

### Protein Content

An elemental analyzer (Vario MICRO cube, Elementar Analysen systeme, Hanau, Germany) coupled with the automated dry combustion method known as the Dumas method (Serrano et al., 2013) was used to determine the total carbon and total nitrogen present in the FY samples by using 5 ± 1 mg of
dry matter in each replicate. The protein content was determined by considering a nitrogen-protein
conversion factor of 6.25.

Polypyrrole coating content

The polypyrrole content was calculated from the nitrogen content (%N) via the same method used
for the proteins, with the assumption that the protein content was not modified after
functionalization. This assumption was verified by controlling the protein content of the flax yarns
before and after being placed in the same acidic solvent (ferric chloride at pH=2) as the
polymerization process. A calibration curve was plotted as the %N as a function of the % mass of PPy.
More precisely, elemental analysis was performed on the FY and polypyrrole polymers at different
known masses. Two values, %N<sub>flax</sub> and %N<sub>ppy</sub>, were obtained and corresponded to the nitrogen
content in the flax yarn and in the polypyrrole, respectively.

Silver Content

Silver content analysis was performed on the FY-PPy-Ag samples. In accordance with the methods of Musino et al. (Musino et al., 2020), 5.00 ± 0.01 mg powder samples were digested with 60 mL of an aqua regia mixture of HCl/HNO₃ (3:1 %V) for one hour. Then, 140 mL of deionized water was added to the suspension for dilution. The resulting suspension was analyzed using atomic absorption spectroscopy (AAS) with an ICE 3300 from Thermo Fisher Scientific spectrometer (Waltham, MA, USA). A calibration curve was plotted by measuring a digested sample of silver standard solution (1 mg.mL⁻¹, Chem-Lab NV, Belgium) at various concentrations ranging from 0.5 to 10.0 ppm.

2.3.3. Longitudinal tensile tests

The flax yarn was tensile tested on an MTS (London, UK) with a 1 kN load cell, a 70 mm gauge length
and a displacement rate of 1 mm.min⁻¹ according to Charlet et al. (Charlet & Béakou, 2011). The yarn
samples were aligned on frames under regulated conditions (23 ± 1.5°C, 55% relative humidity (RH)).
The average apparent diameter was measured for each yarn using BX-51 optical microscopy
(Olympus, Tokyo, Japan), where 6 images were collected for each strained yarn (at a strain of 0.13 N),
and 5 measurements per image were recorded using ImageJ software. The average apparent (effective) longitudinal tensile modulus and the strain range are calculated from the mean apparent (effective) cross-sectional area considering circular idealization. Each batch was subjected to at least 30 repetitions.

### 2.3.4. Hygroscopic properties

Gravimetric measurements of water sorption and desorption isotherms were conducted as a function of %RH. A IGAsorpt-HT dynamic vapor sorption device (DVS) was used (Hiden Isochema, Warrington, UK). In the hermetic reactor, 15 mg of flax tow was placed in a microbalance. The temperature and %RH inside the reactor were controlled. The sorption/desorption sequence was set to increase from 0% to 90% RH and then to decrease from 10% to 0% RH. The sample mass was continuously monitored for each step until equilibrium was reached, determined by a mass variation less than 1% over a 60 min window. All DVS results were an average of two repetitions. The percentage change in mass, or equilibrium moisture content (EMC), were recorded as a function of equilibrium RH at constant temperature.

### 2.3.5. Conductivity measurements

The electrical resistance was measured with a multimeter (Fluke 115, Everett, Washington, USA). The measurements were recorded along a clamped yarn of a fixed length of 9 cm. The surface electrical conductivity of each yarn was calculated using the formula (equation 3), with the assumption that the cross section of the yarn was fully circular. A direct current source meter (Tenma 72-2705, UK) was connected to the strained yarn to apply a certain voltage where the corresponding current was recorded using a multimeter. A graph of the current as a function of the applied voltage is plotted to determine the electrical behavior of the conductive yarn, i.e., ohmic or non-ohmic behavior. Twenty measurements were performed for each sample under regulated conditions at 23 ± 1.5°C.

$$ \text{Conductivity (}\sigma\text{)} = \frac{1}{\rho(\text{Resistivity})} = \frac{L}{R*A} \qquad (3) $$
where σ is the conductivity (S/m), L (m) is the length of the yarn between the two electrodes, R is the resistance (Ω) measured by the multimeter, and A (m²) is the cross-sectional area of the yarn.

### 2.3.6. Quantification of the Joule effect

The increase in temperature of the strained yarn at a fixed length of 9 cm and a specific voltage (0–18 V) was determined using an infrared (IR) thermal camera with a high thermal resolution (20 mK) (VarioCAM® HD, 1,024 × 768 – XGA, infraTec, Dresden, Germany) and a frame rate of 30 Hz. The camera was equipped with a close-up lens 0.2, allowing a pixel size of 17 µm. A direct current source meter (Tenma 72-2705, UK) was connected to the strained yarn. The change in temperature (ΔT) was evaluated as a function of voltage (V). By applying Ohm's law (equation 4), the resistance was determined from the slope of the curve, with the assumption that the cross section of the yarns was fully cylindrical. The thermal conductivity (equation 5) had a substantial effect on the Joule heating performance, which is proportional to the energy transfer power. According to Joule's law (equation 6), the power is proportional to the conductance and input voltage. Twenty repetitions were performed for each batch at ambient temperature, and the following equations can be used:

$$U = RI \qquad (4)$$

$$k = \frac{PL}{A*\Delta T} \qquad (5)$$

In equation 5, the thermal conductivity k (W.m⁻¹.°k⁻¹) is a function of the power of energy transfer, where P is the energy transfer power (W), L is the thickness of the material (m), A is the area (m²), and ΔT is the temperature change with respect to the initial temperature of the yarn at 0 V and is calculated as T=Tᵢ - T₀ᵥ, with Tᵢ (°k) is the temperature measured for an applied voltage "i".

$$P = U^2G \qquad (6)$$
Equation 6 shows Joule's law, where P is the power of energy transfer (W), U is the input voltage (V), and G is the conductance ($Ω^{-1}$) (Tao et al., 2022).

### 3. Results and discussion

The *in situ* polymerization of PPy on flax yarn involves the oxidation of pyrrole in the presence of an oxidizing agent, resulting in the formation of a pyrrole chain on the surface of the flax yarn. In this work, we investigated ferric chloride and silver nitrate as oxidizing agents to determine the optimal balance between the functional properties (Joule effect) and mechanical preservation of yarn. The two functionalization scenarios require a systematic examination and characterization of the properties of the functionalized flax yarns.

#### 3.1. Surface texture

SEM images (Fig. 1) clearly show the distinct morphological features of the FY samples compared with those of the functionalized samples (FY-PPy and FY-PPy-Ag). The FY samples exhibited a smoother and more uniform surface (Fig. 1A & 1B). In the FY-PPy samples (Fig. 1C & 1D), surface roughness is observed on the bulk fragments of PPy and its coating on the yarn surface. Similarly, the SEM images of FY-PY-Ag (Fig. 1E & 1F) exhibit surface roughness along with the presence of compact clusters and likely deposition of Ag structures.

<figure><img src="image_2.png" /></figure>
**Fig. 1.** SEM images of the surface of the yarns at large and zoomed scales, respectively; FY (A&B), FY-PPy (C&D), and FY-PPy-Ag (E&F) yarns.

## 3.2. Effect of functionalization on the chemical composition of flax yarn

### 3.2.1. Infrared spectroscopy results

FTIR spectra provide insight into the effects of the presence of polypyrrole and possible degradation by the acidic media involved during yarn modification. In both scenarios, by distinguishing several bands associated with the stretching and vibrations of functional groups, the biochemical composition of the flax yarn correlates to the biopolymers present in the flax, such as cellulose, hemicelluloses, and lignins. Moreover, through FTIR, distinct bands corresponding to the functional groups of the polymer chain of PPy (Fig. 2b) can also be determined. The peaks in the range of 1245-895 cm⁻¹ (Fig. 2a) of FY are attributed mainly to cellulose and hemicelluloses (refer to the supplementary information) and remains the same in the spectra of the modified yarns (FY-PPy, FY-PPy-Ag). However, a band at 1506 cm⁻¹ presented in the FY spectra, which is attributed to the C=C aromatic stretching vibration of lignin (supplementary information), was not detected after yarn modification (FY-PPy, FY-PPy-Ag). Similarly, the C=O band at 1730 cm⁻¹, which is linked to acetylated hemicellulosic fractions of ester pectin and lignin, was not observed in the spectra of the treated samples (Fig 2a). Furthermore, the combination of the flax and coated PPy was also determined from the spectra of FY-PPy and FY-PPy-Ag through the detection of the 1550 and 1450 cm⁻¹ bands, which correspond to pyrrole ring stretching (Turczyn et al., 2020), and the 1300 cm⁻¹ band attributed to the C-N vibrations of pyrrole. These bands match the peaks of the same functional groups found in the
PPy and PPy-Ag spectra (Fig. 3b). Notably, a shift in the N-H peak of pyrrole from 1525 cm⁻¹ to 1550 cm⁻¹ was observed in both the FY-PPy and FY-PPy-Ag samples. This could be explained by the

<figure><img src="image_3.png" /></figure>

formation of hydrogen bonds between the N-H groups of pyrrole and the OH groups of cellulose (Shi et al., 2014). These bonds between the fibers and the polymers may not be the only bonds and others could also be formed. This aspect needs further investigation. Overall, the FTIR spectra serve as valuable qualitative data and provide two main deductions: (1) the biochemical composition related to cellulose, which is the main constituent of flax, was retained after modification, and (2) new bands corresponding to the polymer also appeared in the spectra of the treated yarns. Further quantitative biochemical research was conducted to determine the dry mass of associated monosaccharides, proteins and lignin present in the yarn samples.

<caption>**Fig. 2.** IR spectra of FY, FY-PPy and FY-PPy-Ag yarns (a). IR of Polypyrrole (PPy), and polypyrrole accompanied by silver structures (PPy-Ag) (b).</caption>

### 3.2.2. Chemical composition of the functionalized yarns

Quantification of the polymer deposited onto the yarn is the first part of the chemical composition study. For this purpose, the nitrogen content was quantified by exploiting the presence of nitrogen in the pyridine ring of PPy. The %wt PPy is linearly correlated to the nitrogen content (%) using a
calibration curve showing the %wt PPy as a function of %N (supplementary information), with the assumption that the endogenous %wt dry matter of protein already in the flax is 2% (supplementary information). Our results are in agreement with those of the literature (Bismarck et al., 2002). The percentage of protein remained constant after the yarns were treated with acidic solvent (supplementary information), and it is assumed to also be retained after *in situ* polymerization to determine the polypyrrole content. Thus, the %wt PPy in each functionalization scenario was determined to be 25.5 ± 2.7% in FY-PPy and 16.5 ± 4.3% in FY-PPy-Ag samples. Additionally, in sample FY-PPy-Ag, the w% of Ag was measured as 26.1 ± 2.3% of the total dry mass (Fig. 3) using atomic absorption fused spectroscopy (FAAS). After the %wt PPY and Ag were subtracted, the remaining %wt value of the flax was considered the total biochemical composition of the plant.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Chemical composition in total dry mass of each FY, FY-PPy, and FY-PPy-Ag.</figcaption></figure>

Biochemical composition of the functionalized flax yarns

Table 1 shows the distribution of the biopolymer families within each yarn after extracting the mass of the functional material (PPy and PPy-Ag) following normalization in %wt of total biochemical composition. For the reference sample FY, the total biochemical composition is correlated mainly with cellulose (85.2 %wt) (Table 1). Furthermore, the remaining minor components with major
impact on the properties are the non-cellulosic polymers (NCPs), which are mainly hemicelluloses and pectin (7.4 %wt), lignin (3.6 %wt) and proteins (2.0 %wt). This distributed %wt among the biochemical components is in the same range as the data reported in the literature (Bourmaud et al., 2018).

The biochemical composition normalized for FY-PPy shows a major decrease in uronic acids (-72%) attributed to pectin, which is found mainly in the middle lamella (ML) and primary cell walls (PCW), and a loss of lignin content (-67%), which is also related to the ML and PCW (Akin et al., 1996). Therefore, the individualization of the fiber bundles is induced within the yarn. The partial removal of these constituents is related to the acidic treatment used during *in situ* polymerization. For FY-PPy-Ag, the same trend showing a loss in uronic acids (-94%) and lignin (-89%). The greater extent of loss, in the case of FY-PPy-Ag, could be explained by the long exposure time of the chemical treatment during the functionalization of flax. On the other hand, all increasing contents cannot be properly analyzed because they are a consequence of the analyzing method. By normalizing the data to 100% of the total biochemical composition, the reductions have to be proportionally balanced by an overestimation of the other component. However, only a decrease in the composition of the flax can appear during polymerization, where the raw material is lost, reduced or transformed into bonds with the coating, and increasing in the initial mass of glucose (Glc) or hemicellulose (non-Glc monosaccharides) (Table 1) are only the result of normalization of the data.

<caption>Table 1</caption>

Neutral monosaccharides, acid monosaccharides and other biopolymers of the different samples are expressed as relative %m in 100% of the total biochemical composition (normalized). The values are significantly different (p<0.05). Glc= glucose, Uronic acids = sum of galacturonic and glucuronic acids.

<table><thead><tr><th rowspan="2">Samples</th><th colspan="2">Neutral monosaccharides</th><th colspan="2">Acid monosaccharides</th><th colspan="2">Non polysaccharide biopolymers</th></tr><tr><th>non Glc monosaccharides</th><th>Glc</th><th>Uronic acids</th><th>Lignin</th><th>Protein</th></tr></thead><tbody><tr><td>FY</td><td>7.4 ± 0.6</td><td>85.2 ±</td><td>1.8 ± 0.2</td><td>3.6 ± 0.2</td><td>2.0 ± 0.0</td></tr></tbody></table>
<table><tr><td></td><td></td><td>5.0</td><td></td><td></td><td></td></tr><tr><td>FY-PPy</td><td>9.0 ± 1.6</td><td>86.6 ±<br>5.0</td><td>0.5 ± 0.2</td><td>1.2 ± 0.2</td><td>2.7 ± 0.2</td></tr><tr><td>FY-PPy-Ag</td><td>9.6 ± 0.8</td><td>85.9 ±<br>1.9</td><td>0.1 ± 0.1</td><td>0.4 ± 0.1</td><td>3.5 ± 0.2</td></tr></table>

### 3.3. Mechanical properties of the functionalized flax yarns

Fig. 4 shows a typical longitudinal tensile stress/strain curve of the yarns subjected to longitudinal tensile loading. The three curves display two distinct regions. The initial region occurs within the engineering strain range below 0.25%, showing a non-linear region with a increasing small slope value; this is likely attributed to the reorientation of the fibers within the yarn aligning with the direction of the applied load during tensile loading (Baley et al., 2020a). The second region is defined as an elastic linear region with a maximum steep slope between 0.25% and 1.7% strain. Following

<figure><img src="image_5.png" /></figure>

this linear section, a "rupture point" is observed, where a significant and major break occurs. The apparent tensile modulus, tensile strength, and elongation at break of the reference sample, FY, shown in Table 3, are in agreement with the literature on flax yarns (Baley et al., 2020b).

<caption>**Fig. 4.** Typical stress-strain curves for FY, FY-PPy, and FY-PPy-Ag under longitudinal tensile loading.</caption>

Fig. 5 shows a whisker box statistical plot of each of the mechanical properties, including the apparent tensile modulus, maximum stress at first break and maximum strain at first break, for the
yarns subjected to longitudinal tensile loading before and after functionalization. Compared with the
untreated samples (12.3 GPa), the functionalized yarns (FY-PPY and FY-PPy-Ag) had a lower median
apparent tensile strength (9.7 and 8.2 GPa, respectively). Similarly, the median of the maximum
stress decreased from 205 MPa in the untreated samples to 149 and 111 MPa respectively in the
treated samples of FY-PPy and FY-PPy-Ag. The strain at first break also significantly decreased, from a
median of 1.76 % in the untreated samples to 1.60% (FY-PpY) and 1.49% (FY-PPy-Ag). These results
indicate that chemical treatments adversely affect the mechanical integrity of the flax yarns, whereas
the treatment used for FY-PPy-Ag has a greater effect on the mechanical properties of the yarns. The
tensile features of natural plant fibers are mainly related to the biochemical composition and
crystallinity index of cellulose (Padovani et al., 2019); thus, the decreases in the apparent tensile
modulus, tensile strength, and elongation at break, 23%, 24%, and 6%, respectively, for FY-PPy and,
more markedly, 22%, 38% and 53%, respectively, for FY-PPy-Ag can be explained by the partial loss of pectin and lignin upon functionalization under acidic conditions, as the biochemical composition revealed (Table 2). These constituents are found mainly in the middle lamella (ML) and primary cell wall (PCW) of the flax fibers. ML is responsible for the cohesion of unitary fibers by cementing the primary cell walls of adjacent cells together and playing the role of the interfacial matrix within the yarn(Melelli et al., 2020; Ray F. Evert, 2006). Some studies have demonstrated how significant the ML plays a central role in the mechanical behavior of the plant (Bourmaud et al., 2013); hence, any partial removal of the ML causes decohesion fibers (Beaugrand et al., 2017) and consequently results in a loss of macroscopic coherence and weakening of the mechanical behavior; this occurs in our case for the FY-PPy and FY-PPy-Ag samples. Notably, a difference in the loss of mechanical properties between the two samples (FY-PPy and FY-PPy-Ag) is observed and related to differences observed in the biochemical composition; this can be explained by different exposure times for both

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Statistical representation of the mechanical properties of flax yarn before and after functionalization (FY, FY-PPy, FY-PPy-Ag). a) Apparent tensile modulus, b) Maximal stress at 1st break and c) Maximal strain at 1st break.</figcaption></figure>
functionalization scenarios.

3.4. Moisture sorption–desorption behavior

Similar to any other natural lignocellulosic material, flax yarn has a moisture content that fluctuates
depending on the relative humidity of its surroundings due to its biochemical composition (Almeida
et al., 2018; Rawat & Khali, 1999). This humid sensitivity can be used as a driving force of the
actuation mechanism for hydromorphic composites (Le Duigou & Castro, 2015). The effects of the
coating and biochemical modification are shown in the sorption/desorption isotherms of Fig. 5. As
expected, the isotherms follow Park's model for natural fibers (Park, 1986), where a sigmoidal
association occurs between the equilibrium moisture content (EMC) and relative humidity (RH) for
the three different samples. Each sigmoidal curve is divided into three distinct zones based on
different mechanisms. The first zone is attributed to Langmuir's mode (RH < 10%) (Rogers, 1985),
where water is primarily absorbed chemically via hydrogen bonds formed with the hydroxyl groups
of amorphous cellulose and hemicelluloses, as well as the carboxyl groups of pectins found in the cell
walls and middle lamella of the yarn. In the second zone, these sorption sites are saturated, and in
accordance with Henry's law, the physical sorption of water occurs in the range of 10-65% RH; here,
water diffuses within the macropores of the flax fibers, and its water uptake varies linearly with
increasing RH during formation (Célino et al., 2014). The third zone is located beyond 65% RH; the
third mechanism occurs where the water is loosely bonded and the concentration substantially
increase, resulting in an accumulation of water weakly inside the microstructure. The
sorption-desorption curves of the plant fibers are not superimposable (Salmén & Larsson, 2018);
thus, their absolute difference is called hysteresis and was calculated for each zone related to the
water adsorption mechanism. For the FY sample, the hysteresis in the chemical, physical, and loosely
bonded zones are 1.2, 51.9 and 20.8, respectively, (Table 3), and the maximum EMC reached at 90%
RH is 14.7%; these values are all in agreement in the literature (Almeida et al., 2018). The hysteresis
values in the chemical zone decrease by 0.3 for both the FY-PPy and Fy PPy-Ag samples. This
reduction is expected with the chemical modification that occured upon *in situ* polymerization and
already
in biochemical
analysis.
the physical
bonded zones,
increases to
for the FY-PPy
3) because

<figure><img src="image_9.png" /></figure>

modification related to the partial loss of ML is responsible for cohesion. Therefore, the spacious rearrangements of fibers occur, and the accessibility of water adsorption in the micro- and macropores is increased. However, in the presence of silver particles in FY-PPy-Ag, the hysteresis in both the physical and loosely bonded zones, is reduced by 20% in comparison with that of the FY sample. This is caused by the hydrophobic nature of the metallic silver particles, which reduce the adsorbing affinity of the flax (see supplementary information). Consequently, the presence of silver leads to a decrease in the moisture affinity. Hence, the two in situ polymerization methods result in different hydroscopic properties for the functionalized yarn; these results are interesting and could be exploited depending on the final application desired for a certain hygroscopic property.

detailed above
composition
However, in
and loosely
the hysteresis
60.7 and 40.0
sample (Table
the structural
**Fig. 5.** Sorption-desorption isotherms at room temperature corresponding to FY, FY-PPy and FY-PPy-Ag.

<caption>Table 3</caption>

Hysteresis (absolute difference in area) of each desorption sorption of FY, FY-PPy, and FY-PPy-Ag within the three zones of water interaction (chemical, physical, and loosely bonded zones).

<table><thead><tr><th rowspan="2"></th><th colspan="4">Hysteresis</th></tr><tr><th>Chemical<br>(0–10% RH)</th><th>Physical<br>(10–65% RH)</th><th>loosely bonded<br>(65–90% RH)</th><th>Total<br>(0–90% RH)</th></tr></thead><tbody><tr><td>FY</td><td>1.2 ± 0.5</td><td>51.9 ± 1.2</td><td>20.8 ± 1.5</td><td>73.9 ± 3.2</td></tr><tr><td>FY-PPy</td><td>0.3 ± 0.1</td><td>60.7 ± 0.1</td><td>21.0 ± 0.3</td><td>81.9 ± 0.7</td></tr><tr><td>FY-PPy-Ag</td><td>0.3 ± 0.3</td><td>40.0 ± 0.5</td><td>15.6 ± 0.5</td><td>55.9 ± 0.6</td></tr></tbody></table>

### 3.5. Electrothermal properties

Fig. 6 shows the I–V curves of FY-PPy and FY-PPy-Ag. A linear ohmic relationship is shown between the current and voltage for both samples where the slope of each curve was calculated; the slope corresponds to the conductance of each batch, with values of 1.4 ± 0.3 mS and 0.8 ± 0.2 mS for FY-PPy and FY-PPy-Ag, respectively. From this linear relationship, the surface electrical conductivity was calculated by assuming that the cross section of the yarn was circular (Fig. 6). The FY-PPy samples have better electrical conductivity (826 ± 163 S.m⁻¹) along the yarn than the FY-PPy-Ag samples (505 ± 132 S.m⁻¹) despite the presence of silver particles. This contrast in conductivity is related to the amount of PPy polymer coating on each sample, which is 25.5% by mass in FY-PPy and 16.5% in FY-PPy-Ag; the conjugated system of the PPy polymer permits electron delocalization and continuous charge carrier mobility along its continuous polymeric chains (Malinauskas, 2001).

Using infrared recording (Fig. 7), thermal images were extracted at different voltages applied to the yarns. For both FY-PPy and FY-PPy-Ag, the temperature increased with the applied voltage, whereas FY did not experience any temperature variation. The evolution of temperature is different between both conductive samples (FY-PP and FY-PPy-Ag), and the average maximum temperature is reached at 18 V is for FY-PPy and FY-PPy-Ag, respectively. Fig. 8a and 8b show the exponential evolution of
temperature as a function of increasing applied voltage for both samples. For each voltage applied, the distribution of the temperature of the repetitions is related to the nature of the flax fiber (slight variability), which consequently leads to the variability of the coupling of the polymer to the flax; this impacts the homogeneity of the coating along the yarns, where the average maximum temperatures of 54 ± 8°C and 31 ± 9°C are reached at an applied voltage of 18 V for FY-PPy and FY-PPy-Ag, respectively, as shown in the infrared image in Fig. 7. The maximum temperature achieved for FY-PPy-Ag is not homogeneously distributed along the yarn. This is attributed to the inhomogeneous deposition of the silver particles. The lower conductance of the FY-PPy-Ag sample correlates to the lower change in temperature ΔT obtained at each voltage than that of sample FY-PPy. The thermal conductivity of both samples was calculated according to equations 4 and 5. FY-PPy has a higher thermal conductivity (8583 J·S⁻¹·m⁻¹·°C⁻¹) than FY-PPy-Ag (5616 J·S⁻¹·m⁻¹·°C⁻¹), as shown in Fig. 9. These results indicate that the latter has lower energy transfer. Interestingly, a large difference is observed between the two different methods of polymerization. Finally, FY-PPy has a more efficient Joule effect than FY-PPy-Ag under a certain induced voltage;

Furthermore, an evident difference is observed between the two different methods of polymerization. We observe that FY-PPy produces a Joule effect with better yield/output than FY-PPy-Ag under a certain induced voltage. This difference is related to the difference in methods used to polymerize the polymer on the surface of the yarn in situ. The percentage of polypyrrole in FY-PPy (25.5%) is 1.5 times greater than that in FY-PPy-Ag (16.5%). Thus, the final material is more conductive because of the greater presence of polymeric chains, enhancing channeling of the current among the yarn. Moreover, the surface roughness observed in the SEM images highlights the homogenous coating in FY-PPy compared with that in FY-PPy-Ag due to the presence of silver particles. Therefore, our hypothesis of an increase in conductivity resulting from the inclusion of Ag in the coating is not valid. Finally, the homogeneity of the coating material is considered an important factor in homogenous heat flow during the Joule effect.
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** IV curves for FY-PPy and FY-PPy-Ag with equations of y= 0.0014x – 0.0006 and y=0.0008x, respectively.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Infrared (top part) and optical images (lower part) of FY, FY-PPy and FY-PPy-Ag; these are recorded by a thermal camera while applying different voltages on the left axis (from 0 to 18 V). Blue correlates to low temperatures, red correlates to medium temperatures, and purple correlates to high temperatures.</figcaption></figure>
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 8.** Whisker plots showing the change in temperature as a function of voltage for FY-PPy (a) and FY-PPy-Ag (b) while different voltages are applied on the left axis (from 0 to 18 V).</figcaption></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 9.** Surface electrical conductivity (a) and thermal conductivity (b) of FY-PPy and FY-PPy-Ag at the maximum applied voltage (18 V).</figcaption></figure>
4. Conclusions

The functionalization of flax yarns with PPy and PPy-Ag composites using two different in situ
polymerization methods was successful and it is a significant advancement to control the moisture
desorption of multifunctional materials that can be used in the development of electroresponsive
hygromorphic composites based on natural fibers (Le Duigou et al., 2021). The amount of coated
material was quantified with a final functionalized material with 25.5%wt of PPy (FY-PPy), and
another material functionalized with 16.5%wt of PPy and 26.1%wt of silver (FY-PPy-Ag).

A comprehensive study of the composition-structure-properties relationships derived from this study showed that the acidic treatment used in both methods could impact the structural cohesion of the fibers within the yarn by hydrolyzing some of the biochemical components, such as pectin and lignin in the middle lamella of bundles, that were still present in the yarn. These biochemical and structural changes accounted for the decreasing trend in the mechanical properties of both samples for FY-PPy, and moreover for FY-PPy-Ag which had the poorest mechanical properties among the samples studied. In addition, the hygroscopic characteristics were altered. This is also a consequence of the modification of the biochemical composition, particularly pectins content, and the nature of the added material; here, the silver particles reduced the three sorption mechanisms, as shown in the FY-PPy-Ag samples. Furthermore, the removal of the middle lamella led to the liberation, decohesion, and defibrillation of the flax fibers within the yarn, consequently enhancing hygroscopic accessibility, particularly in the physically and loosely bonded regions.

In terms of the new electrothermal properties, the Joule effect occurred upon applying a
certain voltage where the change in temperature reached ΔT 31.5 ± 5.7°C for FY-PPy and ΔT 16.1 ±
8.0°C for FY-PPy-Ag at an applied voltage of 18 V. Silver did not result in any appreciable
improvement in neither electrical nor thermal properties and was accompanied by an
inhomogeneous distribution of temperature along the conductive yarns.
In future work, research will be conduct to understand the type of chemical bonds formed between the polymer and the flax. The Joule effect at a ΔT of 31.5°C with 75% biomass (in the case of FY-PPy) provides a new approach for the fabrication in "smart topics," such as triggering the desorption in some actuators like hygromporph biocomposites (HBCs), or could be used as sensors for electrical monitoring.

## Acknowledgements

The authors gratefully acknowledge the funding received from ANR (Agence Nationale de la Recherche), France, for the REDeSIGN project funding ANR-21-CE10-0011. We would like to thank Nathalie Guichard from Nantes University, Sylvie Chevallier from ONIRIS-Nantes and Laurent Chaunier from BIA-Nantes for their technical assistance.

## References

Akin, D. E., Gamble, G. R., Morrison, W. H., Rigsby, L. L., & Dodd, R. B. (1996). Chemical and structural analysis of fibre and core tissues from flax. *Journal of the Science of Food and Agriculture, 72*(2), 155-165. https://doi.org/10.1002/(SICI)1097-0010(199610)72:2<155::AID-JSFA636>3.0.CO;2-X

Almeida, G., Rémond, R., & Perré, P. (2018). Hygroscopic behaviour of lignocellulosic materials: Dataset at oscillating relative humidity variations. *Journal of Building Engineering, 19*(February), 320-333. https://doi.org/10.1016/j.jobe.2018.05.005

Baley, C., Bourmaud, A., & Davies, P. (2021). Eighty years of composites reinforced by flax fibres: A historical review. *Composites Part A: Applied Science and Manufacturing, 144*, 106333. https://doi.org/10.1016/j.compositesa.2021.106333

Baley, C., Gomina, M., Breard, J., Bourmaud, A., & Davies, P. (2020a). Variability of mechanical properties of flax fibres for composite reinforcement. A review. *Industrial Crops and Products, 145*(September 2019), 111984. https://doi.org/10.1016/j.indcrop.2019.111984

Baley, C., Gomina, M., Breard, J., Bourmaud, A., & Davies, P. (2020b). Variability of mechanical
properties of flax fibres for composite reinforcement. A review. *Industrial Crops and Products*, 145(March). https://doi.org/10.1016/j.indcrop.2019.111984

Beaugrand, J., Guessasma, S., & Maigret, J. E. (2017). Damage mechanisms in defected natural fibers. *Scientific Reports*, 7(1), 1–7. https://doi.org/10.1038/s41598-017-14514-6

Bismarck, A., Aranberri-Askargorta, I., Springer, J., Lampke, T., Wielage, B., Stamboulis, A., Shenderovich, I., & Limbach, H.-H. (2002). Surface characterization of flax, hemp and cellulose fibers; Surface properties and the water uptake behavior. *Polymer Composites*, 23(5), 872–894. https://doi.org/https://doi.org/10.1002/pc.10485

Blumenkrantz, N., & Asboe-Hansen, G. (1973). New method for quantitative determination of uronic acids. *Analytical Biochemistry*, 54(2), 484–489. https://doi.org/https://doi.org/10.1016/0003-2697(73)90377-1

Bourmaud, A., Ausias, G., Lebrun, G., Tachon, M.-L., & Baley, C. (2013). Observation of the structure of a composite polypropylene/flax and damage mechanisms under stress. *Industrial Crops and Products*, 43, 225–236. https://doi.org/https://doi.org/10.1016/j.indcrop.2012.07.030

Bourmaud, A., Beaugrand, J., Shah, D. U., Placet, V., & Baley, C. (2018). Towards the design of high-performance plant fibre composites. *Progress in Materials Science*, 97(July 2017), 347–408. https://doi.org/10.1016/j.pmatsci.2018.05.005

Caringella, R., Patrucco, A., Simionati, M., Gavignano, S., Montarsolo, A., Mossotti, R., Zoccola, M., Tonin, C., Fabris, R., & Floria, L. (2018). Electrically conducting linen fabrics for technical applications. *Textile Research Journal*, 88(2), 144–154. https://doi.org/10.1177/0040517516676067

Célino, A., Fréour, S., Jacquemin, F., & Casari, P. (2014). The hygroscopic behavior of plant fibers: A review. *Frontiers in Chemistry*, 1(JAN). https://doi.org/10.3389/fchem.2013.00043

Charlet, K., & Béakou, A. (2011). Mechanical properties of interfaces within a flax bundle Part I:
Experimental analysis. *International Journal of Adhesion and Adhesives*, 31(8), 875-881.
https://doi.org/10.1016/j.ijadhadh.2011.08.008

Chen, A., Wang, H., & Li, X. (2005). *One-step process to fabricate Ag – polypyrrole coaxial nanocables* [{. 1(C), 1863–1864. https://doi.org/10.1039/b417744d

Costa, S. M., Ferreira, D. P., Ferreira, A., Vaz, F., & Fangueiro, R. (2018). Multifunctional Flax Fibres Based on the Combined Effect of Silver and Zinc Oxide (Ag/ZnO) Nanostructures. *NANOMATERIALS*, 8(12). https://doi.org/10.3390/nano8121069

Faruk, O., Bledzki, A. K., Fink, H. P., & Sain, M. (2012). Biocomposites reinforced with natural fibers: 2000-2010. *Progress in Polymer Science*, 37(11), 1552-1596.
https://doi.org/10.1016/j.progpolymsci.2012.04.003

Firoz Babu, K., Dhandapani, P., Maruthamuthu, S., & Anbu Kulandainathan, M. (2012). One pot synthesis of polypyrrole silver nanocomposite on cotton fabrics for multifunctional property. *Carbohydrate Polymers, 90*(4), 1557-1563. https://doi.org/10.1016/j.carbpol.2012.07.030

Gautreau, M., Durand, S., Paturel, A., Le Gall, S., Foucat, L., Falourd, X., Novales, B., Ralet, M. C., Chevallier, S., Kervoelen, A., Bourmaud, A., Guillon, F., & Beaugrand, J. (2022). Impact of cell wall non-cellulosic and cellulosic polymers on the mechanical properties of flax fibre bundles. *Carbohydrate Polymers*, 291(May). https://doi.org/10.1016/j.carbpol.2022.119599

Gautreau, M., Kervoelen, A., Barteau, G., Delattre, F., Colinart, T., Pierre, F., Hauguel, M., Moigne, N. Le, Guillon, F., Bourmaud, A., & Beaugrand, J. (2021). *Non-Woven Composite Following Physical Pre-Treatments*.

Goudenhooft, C., Bourmaud, A., & Bailey, C. (2019). Flax (Linum usitatissimum L.) fibers for composite reinforcement: Exploring the link between plant growth, cell walls development, and fiber properties. *Frontiers in Plant Science*, 10(April), 1-23. https://doi.org/10.3389/fpls.2019.00411

Holstov, A., Farmer, G., & Bridgens, B. N. (2016). Implementing hygromorphic wood composites into
responsive building skins. *11th Conference on Advanced Building Skins, October*, 1–11.

Kee, S., Kim, N., Kim, B. S., Park, S., Jang, Y. H., Lee, S. H., Kim, J., Kim, J., Kwon, S., & Lee, K. (2016).
Controlling Molecular Ordering in Aqueous Conducting Polymers Using Ionic Liquids. *Advanced
Materials*, 28(39), 8625–8631. https://doi.org/10.1002/adma.201505473

Lahaye, M., Falourd, X., Laillet, B., & Le Gall, S. (2020). Cellulose, pectin and water in cell walls determine apple flesh viscoelastic mechanical properties. *Carbohydrate Polymers*, 232, 115768.
https://doi.org/https://doi.org/10.1016/j.carbpol.2019.115768

Le Duigou, A., & Castro, M. (2015). Moisture-induced self-shaping flax-reinforced polypropylene biocomposite actuator. *Industrial Crops and Products*, 71, 1–6.
https://doi.org/https://doi.org/10.1016/j.indcrop.2015.03.077

Le Duigou, A., Davies, P., & Baley, C. (2011). Environmental impact analysis of the production of flax fibres to be used as composite material reinforcement. *Journal of Biobased Materials and Bioenergy*, 5(1), 153–165. https://doi.org/10.1166/jbmb.2011.1116

Le Duigou, A., Fruleux, T., Matsuzaki, R., Chabaud, G., Ueda, M., & Castro, M. (2021). 4D printing of continuous flax-fibre based shape-changing hygromorph biocomposites: Towards sustainable metamaterials. *Materials and Design*, 211, 110158.
https://doi.org/10.1016/j.matdes.2021.110158

Lefeuvre, A., Bourmaud, A., Morvan, C., & Baley, C. (2014). Elementary flax fibre tensile properties: Correlation between stress–strain behaviour and fibre composition. *Industrial Crops and Products*, 52, 762–769. https://doi.org/10.1016/j.indcrop.2013.11.043

Lekpittaya, P., Yanumet, N., Grady, B. P., & Rear, E. A. O. (2003). *Resistivity of Conductive Polymer – Coated Fabric*. 2–9.

Lu, F., Wang, C., Chen, M., Yue, F., & Ralph, J. (2021). A facile spectroscopic method for measuring lignin content in lignocellulosic biomass. *GREEN CHEMISTRY*, 23(14), 5106–5112.
& Beaugrand, J. (2019). Beating of hemp bast fibres: an examination of a hydro-mechanical treatment on chemical, structural, and nanomechanical property evolutions. *Cellulose*, **26**(9), 5665–5683. https://doi.org/10.1007/s10570-019-02456-3

Park, G. S. (1986). Transport Principles---Solution, Diffusion and Permeation in Polymer Membranes. In P. M. Bungay, H. K. Lonsdale, & M. N. de Pinho (Eds.), *Synthetic Membranes: Science, Engineering and Applications* (pp. 57–107). Springer Netherlands. https://doi.org/10.1007/978-94-009-4712-2_3

Qiu, T., Xie, H., Zhang, J., Zahoor, A., & Li, X. (2011). The synthesis of Ag/polypyrrole coaxial nanocables via ion adsorption method using different oxidants. *Journal of Nanoparticle Research*, **13**(3), 1175–1182. https://doi.org/10.1007/s11051-010-0109-x

Quéméner, B., Bertrand, D., Marty, I., Causse, M., & Lahaye, M. (2007). Fast data preprocessing for chromatographic fingerprints of tomato cell wall polysaccharides using chemometric methods. *Journal of Chromatography A*, **1141**(1), 41–49. https://doi.org/10.1016/j.chroma.2006.11.069

Rawat, S. P. S., & Khali, D. P. (1999). Studies on adsorption behaviour of water vapour in lignin using the Brunauer-Emmett-Teller theory. *Holz Als Roh - Und Werkstoff*, **57**(3), 203 – 204. https://doi.org/10.1007/s001070050040

Ray F. Evert, S. E. E. (2006). *Esau's Plant Anatomy: Meristems, Cells, and Tissues of the Plant Body: Their Structure, Function, and Development* (3rd editio).

Richely, E., Bourmaud, A., Placet, V., Guessasma, S., & Beaugrand, J. (2022). Progress in Materials Science A critical review of the ultrastructure, mechanics and modelling of flax fibres and their defects. *Progress in Materials Science*, **124**(July 2020), 100851. https://doi.org/10.1016/j.pmatsci.2021.100851

Rogers, C. E. (1985). Permeation of Gases and Vapours in Polymers. In J. Comyn (Ed.), *Polymer Permeability* (pp. 11–73). Springer Netherlands. https://doi.org/10.1007/978-94-009-4858-7_2
Salmén, L., & Larsson, P. A. (2018). On the origin of sorption hysteresis in cellulosic materials.

*Carbohydrate Polymers*, **182**(November 2017), 15–20.

https://doi.org/10.1016/j.carbpol.2017.11.005

Serrano, S., Rincon, F., & Garcia-Olmo, J. (2013). Cereal protein analysis via Dumas method: Standardization of a micro-method using the EuroVector Elemental Analyser. *JOURNAL OF CEREAL SCIENCE*, **58**(1), 31–36. https://doi.org/10.1016/j.jcs.2013.04.006

Tao, X., Tian, D., Liang, S., Jiang, P., Zhang, L., & Fu, F. (2022). Research Progress on the Preparation of Flexible and Green Cellulose-Based Electrothermal Composites for Joule Heating Applications.
*ACS Applied Energy Materials*. https://doi.org/10.1021/acsaem.2c02171

Tripathi, K. M., Vincent, F., Castro, M., & Feller, J. F. (2016). Flax fibers–epoxy with embedded nanocomposite sensors to design lightweight smart bio-composites. *Nanocomposites*, **2**(3), 125–134. https://doi.org/10.1080/20550324.2016.1227546

Turczyn, R., Krukiewicz, K., Katunin, A., Sroka, J., & Sul, P. (2020). Fabrication and application of electrically conducting composites for electromagnetic interference shielding of remotely piloted aircraft systems. *Composite Structures*, **232**(December 2019).
https://doi.org/10.1016/j.compstruct.2019.111498

Vinod, A., Sanjay, M. R., Suchart, S., & Jyotishkumar, P. (2020). Renewable and sustainable biobased materials: An assessment on biofibers, biofilms, biopolymers and biocomposites. *JOURNAL OF CLEANER PRODUCTION*, **258**. https://doi.org/10.1016/j.jclepro.2020.120978

Wang, L., Wang, X., & Lin, T. (2010). Conductive coatings for textiles. *Smart Textile Coatings and Laminates: A Volume in Woodhead Publishing Series in Textiles*, 155–188.
https://doi.org/10.1533/9781845697785.2.155

Wankhede, B., Bisaria, H., Ojha, S., & Dakre, V. S. (2022). A review on cotton fibre-reinforced polymer composites and their applications. *PROCEEDINGS OF THE INSTITUTION OF MECHANICAL
https://doi.org/10.1177/14644207221143876

Zamil, M. S., & Geitmann, A. (2017). The middle lamella—more than a glue. *Physical Biology, 14*(1), 15004. https://doi.org/10.1088/1478-3975/aa5ba5

Zhang, Y., Mao, T., Wu, H., Cheng, L., & Zheng, L. (2017). Carbon Nanotubes Grown on Flax Fabric as Hierarchical All-Carbon Flexible Electrodes for Supercapacitors. *ADVANCED MATERIALS INTERFACES, 4*(9). https://doi.org/10.1002/admi.201601123

Zhu, Y., Zhao, B., Lei, L., Cheng, Z., Yu, T., & Li, Y. (2023). Facile construction of a flexible smart core-sheath flax yarns with temperature-responsive resistance for ultra-fast fire-alarm response. *CHEMICAL ENGINEERING JOURNAL, 471*. https://doi.org/10.1016/j.cej.2023.144718