# Green Electrospinning of Highly Concentrated Polyurethane Suspensions in Water: From the Rheology to the Fiber Morphology

Bryan Gross, Guy Schlatter, Pascal Hébraud, Flavien Mouillard, Lotfi Chehma, Anne Hébraud, Emeline Lobry

► To cite this version:

Bryan Gross, Guy Schlatter, Pascal Hébraud, Flavien Mouillard, Lotfi Chehma, et al. Green Electrospinning of Highly Concentrated Polyurethane Suspensions in Water: From the Rheology to the Fiber Morphology. Macromolecular Materials and Engineering, 2024, 309 (12), pp.2400157. 10.1002/mame.202400157. hal-04720701

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# Green Electrospinning of Highly Concentrated Polyurethane Suspensions in Water: From the Rheology to the Fiber Morphology

Bryan Gross, Guy Schlatter, Pascal Hébraud, Flavien Mouillard, Lotfi Chehma, Anne Hébraud, and Emeline Lobry*

Suspension electrospinning allows the environmental-friendly fabrication of nano-micro-fibrous membranes since it is based on the processing of an aqueous particle suspension in which a hydrosoluble template polymer is added to insure the formation of a continuous fiber. Here, the case of polyurethane (PU) aqueous suspensions formulated with poly(ethylene oxide) (PEO) as the template polymer is studied. The effect of several parameters (particle size, PU/PEO ratio, PEO molar mass, and PEO concentration in the continuous phase) on particle-particle and particle-template polymer interactions that influence the rheological properties of the formulation and finally the electrospinning and the fiber morphology, is studied. The goal is to process a formulation with the highest particle content as possible. Thanks to a deep rheological investigation and the study of interactions and suspension morphology by zeta potential and diffusing wave spectroscopy, it is shown that regular fibers are efficiently produced when small particles are electrospun under favorable particle-template polymer interactions and without screening the electrostatic repulsion between particles. Finally, a fibrous membrane is obtained from a formulation with a PU/PEO weight ratio equal to 50 under very stable and efficient production conditions.

## 1. Introduction

In recent years, the field of materials science has witnessed a growing demand for sustainable and environmentally friendly manufacturing techniques. As researchers strive to develop innovative strategies, electrospinning has emerged as a versatile and powerful method for producing continuous nanofibers having an average diameter of several hundreds of nanometers dedicated to a wide range of applications such as biomedical engineering,[1-3] energy storage[4] or air/liquid filtration.[5,6] Conventional electrospinning is based on the generation of a thin jet of a polymer solution thanks to the action of an electric field of few kV per cm. During the flight of the jet between the emitting and the collecting electrodes, the solvent evaporates allowing thus the deposition of a continuous and dry nanofiber. In order to be able to tune the fiber diameter in conventional electrospinning, one of the key parameters is the viscosity of the solution which is directly affected by the molar mass of the polymer and its concentration.[7-9] Shenoy et al.[10] established that successful formation of regular fibers depends on the polymer concentration in solution C which must be superior to 2.5C~e~, where C~e~ is the concentration corresponding to the onset of entanglements above which the polymer chains are in the semi-dilute entangled regime. Additionally, other formulation factors such as the solvent nature and the solvent-polymer interactions enable to control the fiber diameter and more generally the morphology of the electrospun mat.[11]

As most of polymers are only soluble in organic and toxic solvents, their evaporation leads to environmental and health issues that must be tackled especially when electrospinning is intended at industrial production scales requiring strict safety regulations. In this regard, earlier studies described innovative and efficient alternative approaches called green-electrospinning using eco-friendlier and less toxic solvents.[12] Different strategies are considered to reduce the environmental footprint of electrospun mats including the use of benign solvents or other techniques such as melt-electrospinning,[13] or emulsions electrospinning which drastically reduced solvent

B. Gross, G. Schlatter, F. Mouillard, L. Chehma, A. Hébraud, E. Lobry Institut de Chimie et Procédés pour l'Energie, l'Environnement et la Santé, ICPEES UMR 7515, CNRS

Université de Strasbourg, Ecole Européenne de Chimie, Polymères et Matériaux

25 rue Becquerel, Strasbourg Cedex 2 67087, France

E-mail: elobry@unistra.fr

P. Hébraud
Institut de Physique et Chimie des Matériaux de Strasbourg, IPCMS UMR
7504, CNRS

Université de Strasbourg
23 rue du Loess, Strasbourg Cedex 02 67034, France

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The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/mame.202400157

© 2024 The Author(s). Macromolecular Materials and Engineering published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/mame.202400157
consumption.[14] Another method renowned for its environ-
mental friendliness and adaptability, centers on the versatile
use of aqueous suspensions of polymer particles. Solid poly-
mer particles are dispersed in water and are thus the main
compound of the final produced nanofibers.[15] However, sus-
pension of polymer particles does not fulfill the rheological
conditions providing the required chain entanglement leading
to the formation of a continuous electrospun jet. To address
this issue, a water-soluble template polymer is added to achieve
the viscoelasticity requirement for the formulation while lim-
iting its content compared to the particle suspension being
the polymer of interest.[15] High molar mass water-soluble
polymers are generally chosen as template in order to reach
the semi-dilute entangled regime at the lowest concentration
level[16] among with poly(vinyl alcohol) PVA,[17] poly(ethylene
oxide) PEO[16] or water-soluble poly(vinyl pyrrolidone) PVP.[18]
The first electrospinning from polymer suspension was reported
by Greiner et al.[19] who formulated a suspension of polystyrene
(PS) particles in the presence of a surfactant and used PVA as
template polymer. Since then, suspension green electrospin-
ning of a wide range of polymer particles has been demon-
strated including poly(styrene-co-butyl acrylate) (PS-co-BA),[19,20]
poly[(hexamethylene adipate)-block-(methoxypolyethylene
oxide)] (PHA-b-PEO),[21] poly(methyl methacrylate-co-butyl
acrylate) (PMMA-co-BA),[17,22] or polyurethane (PU).[16,23,24]
Fiber formation by suspension electrospinning depends i) on
the formulation properties of the continuous phase (polymer
template aqueous solution, molar mass, and concentration)
and ii) on the formulation of the dispersed phase comprising
polymer particles (size, concentration, thermal properties) as
well as iii) the interaction between polymers (particle-particle or
polymer template-particle). These three major points contribute
to tune the rheological properties and affect the ability to create a
coherent viscoelastic jet that can be elongated by the coulombic
forces. To succeed, viscoelastic properties should be adapted in
order to i) have a sufficient molecular entanglement to resist
the breakup of the jet and ii) to ensure the appropriate range of
resistance to deformation (shear viscosity) allowing sufficient
stretching of the jets to obtain thinner fiber. Table 1 summarizes
different studies of nanofibers obtained by suspension electro-
spinning regarding their formulation and their contribution to
establish the relationship between formulation parameters and
final nanofibers morphology. First, authors either maintained
a constant solid content (polymer particles and hydrosoluble
polymer template) while tuning the ratio between both polymers
or one polymer concentration was kept constant while the
second one varied, changing the total solid content between the
different formulations. This complicates the interpretation of
the influence of all the previous listed formulation parameters
and the interaction between polymers on the morphology of the
fibers obtained. Then, very few studies investigated thoroughly
the relationship between suspension properties and the fibers
morphology and they are not exhaustive. Finally, the weight ratio
between particle and polymer template never exceeds a value of
9. As a consequence, whereas suspension electrospinning is of
 growing interest, the effect of the rheology and physical-chemical
interactions of polymers in the formulation on the final fiber
morphology represents a relatively underexplored frontier in the
literature at high particles content in suspension. Rheological

behavior and effect of different parameters on fiber morphology
were deeply investigated for sulfur-free softwood lignin (SFLS)
colloidal dispersion electrospinning[25,26] as reported in Table 1
in the italic part. Authors propose correlation to choose appropri-
ate conditions for homogeneous nanofiber generation and the
tuning of morphology for lignin/PEO systems at high colloidal
lignin content.

The present work aims to highlight the key parameters of
the formulation for a better control of nanofibers morphology
by using suspension-electrospinning process. For this purpose,
formulations containing a commercial aqueous suspension of
thermoplastic polyurethane (PU) and water-soluble polyethy-
lene oxide (PEO) as the template polymer were studied. The
properties of the aqueous solution, forming the continuous
phase has been studied by varying the molar mass of the carrier
polymer and its weight fraction. In parallel, the properties of
the dispersed phase have been tuned to study the effect of the
PU particle size and concentration in the formulation. The
rheological properties of the PU suspension-based formulations
have been investigated in order to get more insight into the
physical-chemical interactions acting between all components
of the formulations keeping in mind the final goal being the
enhancement of the PU dispersed phase content and the control
of the nanofiber morphology. Particle-particle and particle-PEO
interactions will be discussed as a function of the particle
size and their concentration as they play a major role on the
rheological properties especially when formulations, highly
concentrated in particles, are considered.[30] This study paves
the way to a better understanding of the formulation parameters
to improve the morphology of the nanofiber mats by suspension
electrospinning by decreasing the amount of carrier polymer.

## 2. Experimental Section

### 2.1. Materials

Waterborne polyurethane (Baymedix FD103) with a solid content (s.c.) of 59 wt.% was provided by Covestro. Poly(ethylene oxide) with molar masses of 400, 900, and 2000 kDa were purchased from Sigma-Aldrich. Purified water (milliQ) was employed as solvent for all experiments.

### 2.2. Methods

#### 2.2.1. Characterization of the Waterborne Polyurethane Suspension

Two separate batches of the FD103 suspension were provided by Covestro. The two distinct batches named PU-D1 and PU-D2 were characterized, and their properties are summarized in Table 2. The average hydrodynamic diameters of PU particles were measured by Dynamic Light Scattering (ALV-CGS3), with a He-Ne LASER (wavelength 633 nm), at a scattering angle equal to 90 degrees. Suspensions were diluted to a concentration of 6.10⁻³ wt.% with purified water, which were additionally filtered through a cellulose acetate filter (pores size: 0.2 µm) to remove any remaining impurities that could interfere with the measurements. The field correlation function is deduced from the measured intensity correlation through the Siegert relation. The inverse Laplace transform of the field autocorrelation function was
<table><caption>**Table 1.** Literature overview of suspension electrospinning focusing on the relationship between formulation parameters and fiber morphology.</caption><thead><tr><th>Ref</th><th>Particle polymer</th><th>d<sub>H</sub> [nm]</th><th>Template polymer and its concentration in water</th><th>Particles to template polymer weight ratio</th><th>S.C [wt.%)</th><th>Investigated parameters</th></tr></thead><tbody><tr><td>[19]</td><td>PS</td><td>100, 200 and 335</td><td>PVA 28–99 (145 kDa)<br>PVA 56–98 (195 kDa)<br>6%w/v</td><td>4</td><td>30</td><td><ul><li>Particle size</li><li>Particle/polymer template ratio at constant PVA</li><li>Surfactant</li></ul></td></tr><tr><td>[20]</td><td>PS<br>PS-co-BA<br>PBA</td><td>85 to 170</td><td>PVA 28–99 (145 kDa)<br>6%w/v</td><td>4</td><td>30</td><td><ul><li>Particle type (T<sub>g</sub> = 7 °C to 100 °C)</li><li>Particle size</li></ul></td></tr><tr><td>[21]</td><td>PHA-b-PEO</td><td>108</td><td>PEO (5 kDa)<br>(1 to 4 wt.%)</td><td>/</td><td>16</td><td><ul><li>Particle type</li><li>PEO concentration</li></ul></td></tr><tr><td>[22]</td><td>PMMA-co-BA</td><td>65 to 112</td><td>PVA 56–98 (195 kDa)</td><td>4</td><td>/</td><td><ul><li>PVA/particles solid contents</li><li>Particle size</li><li>Surfactant</li></ul></td></tr><tr><td>[23]</td><td>PU</td><td>80</td><td>PEO 900 kDa<br>3 to 5%w/v</td><td>1 to 5.6</td><td>9.5 to 20</td><td><ul><li>PU/PEO ratio at fixed PEO concentration</li><li>PEO %w/v at fixed PU</li></ul></td></tr><tr><td>[27]</td><td>PU</td><td>/</td><td>PVA 17–88 M<sub>w</sub> = 146–186 kDa</td><td>0.1-9</td><td>15</td><td><ul><li>PU/PEO ratio at fixed total solid content</li></ul></td></tr><tr><td>[17]</td><td>MMA/BA<br>MMA/BA/AA</td><td>107 to 317<br>139</td><td>PVA 13–88 69.5 kDa, 25–88<br>92.6 kDa and 47–88<br>138.9 kDa<br>3 to 10%w/v</td><td>1-5</td><td>9 to 16.7</td><td><ul><li>PVA M<sub>w</sub> and concentration</li><li>Total solid content at fixed PVA concentration</li><li>Particle size and particle size distribution</li><li>Surface properties</li></ul></td></tr><tr><td>[16]</td><td>PU</td><td>200</td><td>PEO 900 kDa<br>4%w/v</td><td>1-6</td><td>8 to 24</td><td><ul><li>Rheology of the formulation</li><li>PU/PEO ratio at constant PEO concentration</li></ul></td></tr><tr><td>[28]</td><td>Bio PU</td><td>/</td><td>PEO<br>3, 4, 5%w/v</td><td>5-8.33</td><td>25</td><td><ul><li>Rheology of the formulations</li><li>Effect of particle size</li><li>Ratio at constant solid content</li></ul></td></tr><tr><td>[29]</td><td>PU</td><td>/</td><td>PEO 900 kDa<br>5.6%w/v</td><td>3-8</td><td>/</td><td><ul><li>Effect of PU/PEO</li></ul></td></tr><tr><td>[25]</td><td>Lignin SFLS<br>Rg = 3.5 nm</td><td>/</td><td>PEO in DMF at 1000, 2000 and 5000 kDa</td><td>/</td><td>32, 42 and 48 (in vol)</td><td><ul><li>PEO concentration (volume fraction) and M<sub>w</sub></li><li>Concentration in lignin</li><li>Viscoelastic behavior</li></ul></td></tr></tbody></table>

d<sub>H</sub> hydrodynamic diameter of particles, T<sub>g</sub> glass transition temperature, S.C. solid content, PBA states for poly(butyl acrylate), PHAcoPEO for Poly(hexamethylene adipate)-poly(ethylene oxide) block copolymers, MMA for methyl metacrylate, BA for butyl acrylate, AA for acrylic acid, Bio-PU for soy-based polyol prepolymer.

performed using the CONTIN algorithm,[31] from which the distribution of particles sizes was obtained.

The glass transition (T<sub>g</sub>) of PU was determined via differential scanning calorimetry (DSC, Mettler DSC1 STARe instrument). For this purpose, the PU suspensions were dried in an oven at 30 °C for 24 h to remove water and obtain a PU film. Samples of ≈2 mg of each PU film were crimped in aluminum pans. The DSC characterizations were carried out at a heating rate of

<table><caption>**Table 2.** Properties of the two studied batches of PU suspensions characterized.</caption><thead><tr><th></th><th>Initial solid content of FD103 PU [wt.%)</th><th>Hydrodynamic diameter [nm]</th><th>T<sub>g</sub> [°C]</th></tr></thead><tbody><tr><td>PU-D1</td><td>59</td><td>410 ± 65</td><td>-48</td></tr><tr><td>PU-D2</td><td>59</td><td>710 ± 47</td><td>-48</td></tr></tbody></table>

10 °C min<sup>-1</sup> and under nitrogen atmosphere (10 mL.min<sup>-1</sup>) from -80 to 150 °C.

Zeta potential measurements were performed with the Litesizer 500 (Anton Paar) based on electrophoretic light scattering (ELS). The method to measure the zeta potential is based on the continuous monitored phase analysis light scattering (cmPALS). The net electrophoretic mobility of the particles v<sub>E</sub> is measured and then converted into ζ potential thanks to Smoluchowski equation:[32]

$$v_E = \frac{\epsilon\zeta}{\eta} \qquad (1)$$

where ε and η are respectively the permittivity of the solution and the viscosity of the continuous phase. For the measurement, the initial suspensions presented in **Table 3** were diluted to set the PU concentration to 0.2%w/v. As a consequence, the PEO content ranged from 0.1 to 0.008%w/v.
<table><caption>**Table 3.** Studied formulations and resulting morphology of electrospun membranes (as an example D2-PEO900-4%-R8 corresponds to a formulation containing 0.4 g of PEO, 5.42 g of FD103, and 7.78 g of additional DI water).</caption><thead><tr><th>Sample name</th><th>PEO [%w/v]</th><th>PU [%w/v]</th><th>Solid content [%w/v]</th><th>PU/PEO</th><th>Fiber morphology</th><th>Fiber diameter [nm]</th></tr></thead><tbody><tr><td>D1-PEO900-4%-R4</td><td>4.0</td><td>16.3</td><td>20.3</td><td>4.1</td><td>Bead-free fiber</td><td>450 ± 105</td></tr><tr><td>D1-PEO900-4%-R8</td><td>4.0</td><td>32.7</td><td>36.7</td><td>8.1</td><td>Bead-free fiber</td><td>560 ± 85</td></tr><tr><td>D1-PEO900-4%-R10</td><td>4.0</td><td>40.9</td><td>44.9</td><td>10.2</td><td>Bead-free-fiber</td><td>640 ± 131</td></tr><tr><td>D1-PEO900-4%-R12</td><td>4.0</td><td>49.2</td><td>53.2</td><td>12.2</td><td>Bead-free fiber</td><td>731 ± 131</td></tr><tr><td>D2-PEO900-4%-R4</td><td>4.0</td><td>16.3</td><td>20.3</td><td>4.1</td><td>Non-uniform fiber</td><td>425 ± 227</td></tr><tr><td>D2-PEO900-4%-R8</td><td>4.0</td><td>32.0</td><td>36.0</td><td>8.0</td><td>Bead-free fiber</td><td>576 ± 265</td></tr><tr><td>D2-PEO900-4%-R10</td><td>4.0</td><td>40.9</td><td>44.9</td><td>10.2</td><td>Non-uniform fiber</td><td>684 ± 437</td></tr><tr><td>D2-PEO900-4%-R12</td><td>4.0</td><td>47.8</td><td>51.8</td><td>11.9</td><td>Non-uniform fiber</td><td>682 ± 388</td></tr><tr><td>D1-PEO900-2%-R8</td><td>2.1</td><td>16.7</td><td>18.8</td><td>8.1</td><td>Bead on a string</td><td>/</td></tr><tr><td>D1-PEO900-3%-R8</td><td>3.0</td><td>24.5</td><td>27.5</td><td>8.1</td><td>Non-uniform fiber</td><td>525 ± 203</td></tr><tr><td>D2-PEO900-2%-R8</td><td>2.0</td><td>16.2</td><td>18.2</td><td>8.1</td><td>Bead on a string</td><td>570 ± 458</td></tr><tr><td>D2-PEO900-3%-R8</td><td>3.0</td><td>24.2</td><td>27.2</td><td>8.0</td><td>Non-uniform fiber</td><td>403 ± 429</td></tr><tr><td>D2-PEO900-4%-R8</td><td>4.0</td><td>32.1</td><td>36.1</td><td>8.0</td><td>Non-uniform fiber</td><td>576 ± 265</td></tr><tr><td>D2-PEO400-2%-R8</td><td>2.0</td><td>16.2</td><td>18.2</td><td>8.0</td><td>Bead on a string</td><td>/</td></tr><tr><td>D2-PEO400-3%-R8</td><td>3.0</td><td>23.9</td><td>26.9</td><td>8.0</td><td>Bead on a string</td><td>/</td></tr><tr><td>D2-PEO400-4%-R8</td><td>4.0</td><td>32.0</td><td>36</td><td>8.0</td><td>Non-uniform fiber</td><td>600 ± 310</td></tr><tr><td>D2-PEO2000-2%-R8</td><td>2.0</td><td>15.9</td><td>17.9</td><td>8.0</td><td>Bead on a string</td><td>/</td></tr><tr><td>D2-PEO2000-3%-R8</td><td>3.0</td><td>24.0</td><td>27</td><td>8.0</td><td>Bead-free fiber</td><td>803 ± 210</td></tr><tr><td>D2-PEO2000-4%-R8</td><td>4.0</td><td>32.2</td><td>36.1</td><td>8.1</td><td>Bead-free fiber</td><td>912 ± 222</td></tr><tr><td>D2-PEO900-2%-R50</td><td>2.0</td><td>100.0</td><td>102.0</td><td>50.0</td><td>Bead-free-fiber</td><td>980 ± 320</td></tr></tbody></table>

### 2.2.2. PU/PEO Suspension Preparation

For each formulation, a stock solution was initially prepared by dissolving PEO powder (m = 0.2 to 0.4 g) in purified water (m = 3.05 to 8.89 g) for 24 h under magnetic stirring at ambient temperature. Subsequently, the waterborne PU suspension (m = 2.71 to 16.95 g) was added into the PEO stock solution and subjected to magnetic stirring for additional 24 h. The PU/PEO solid weight ratio ranged from 0 to 12, corresponding to a PU concentration relative to the total volume of water in the formulation varying from 0 to 49%w/v. The different formulations are reported in Table 3. In the following part, the samples were named as Dx-PEOy-z%-Rt where x refers to the suspension diameter number,

y to the molar mass of PEO, z to the weight percent of PEO relatively to the final volume of water, and Rt to the PU/PEO weight ratio. The sample name thus contains all the parameters tuned during the study.

### 2.2.3. Rheological Measurements

Rheological characterizations by steady and oscillatory shear measurements were performed with a rheometer Discovery HR30 from TA Instrument.

Steady and oscillatory shear measurements were performed using a cone-plate geometry (CP50-1) having a diameter of

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Schematic description of the suspension electrospinning of PU particles in aqueous PEO solutions.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Scheme of two formulations of equal volume concentration showing PU particles (in green) and PEO macromolecular chains (in red) in the case of the smallest PU particles ($D_1$ = 410 nm) and the largest ones ($D_2$ = 710 nm). The particle number and size scale are respected.</figcaption></figure>

50 mm and a contact angle of 1.009°. A Pelletier oven plate was used to control the temperature at 22 °C. In order to prevent water evaporation and keep the surrounding atmosphere of the cone-plate geometry saturated in water a cover cup was placed over the geometry. The viscosity measurement was measured under steady shear flow with a shear rate ranging from $10^{-3}$ s⁻¹ to $10^3$ s⁻¹. Values were considered consistent when the torque was greater than 1 µN.m. The experimental data were fitted according to the Carreau-Yasuda model to calculate the zero-shear viscosity $\eta_0$.[³³] In order to start with a controlled state of suspension structure, all oscillatory measurements were carried out after a pre-shear at a steady shear rate of $10^2$ s⁻¹ followed by a phase rest of 1 min. Oscillatory measurements were carried out from 0.01 rad.s⁻¹ to 100 rad.s⁻¹.

### 2.2.4. Diffusing Wave Spectroscopy

Diffusing Wave Spectroscopy (DWS) measurements are performed to monitor the stability of the suspensions in the concentrated regime (for volume fractions larger than 8%). The sample, placed in a rectangular cell of thickness 1 cm, is illuminated by an expanded parallel LASER beam of diameter 6 mm and of wavelength 633 nm. The backscattered diffused light was collected with a single mode optical fiber and its intensity measured with a photomultiplier tube (SO-SIPD, ALV GmbH). The intensity correlation function was computed with a multi-tau correlator (ALV-7002).

### 2.2.5. Electrospinning

The vertical electrospinning setup was composed of a cylindrical aluminum rotating collector having a diameter of 6 cm, two high voltage power supplies (Spellman) and a syringe pump (Harvard) with a 18G flat-tip needle (Figure 1). The ambient air conditions were controlled and fixed to 21+/−1 °C and 20 to 30% of relative humidity. The process conditions of the electrospinning were fixed, with a flowrate of 1 mL h⁻¹ and a spinneret to collector distance of 15 cm. The rotation speed of the collector was fixed at 100 rpm. The voltages applied respectively at the collector and the emitter were set to -4 and 12.5 kV. The collector was covered with silicon substrate to facilitate the removal of the final mat for characterization.

### 2.2.6. Scanning Electron Microscopy Characterization (SEM)

Samples were cut into pieces of 4 × 4 mm² and placed on a conductive carbon tape on the microscopy stub. Electrospun samples were sputter coated with a thin gold layer prior to observation (Q150RS, Quorum technologies) for 2 min. Scanning electron microscopy (SEM, TESCAN Vega 3 LMU) was used in high vacuum mode using an accelerating voltage of 5 kV and a working distance of 7 mm. Nanofibers average diameter was calculated by measuring 100 nanofibers distributed on two different SEM images with the ImageJ analysis software.

## 3. Results and Discussion

### 3.1. Effect of the Dispersed PU Phase Properties on the Rheology and the Fiber Morphology

The present section aims to understand how the physical-chemical properties of the dispersed phase can affect the electro-spinning conditions and the resulting fiber morphology. To thoroughly discuss this point, the state of formulations was first studied by DWS and zeta potential to understand particle-particle, particle-PEO, or PEO chains interactions. Then, the rheological behaviors of the suspensions and the formulations were studied. Finally, electrospinning tests were performed and the nanofiber morphologies were discussed according to the above-mentioned parameters to investigate the effect of the structure and rheology of PU formulations on the processing conditions and the morphology of the fibers.

The study of two batches of waterborne PU allowed us to study the effect of PU particle diameter on the shear viscosity and the fiber morphology. The mean hydrodynamic diameter measured by DLS of PU particles were $D_1$ = 410 nm for PU-D1 and $D_2$ = 710 nm for PU-D2, corresponding to a diameter ratio $D_2/D_1$ = 1.73. Thus, for a given PU weight content, the number of particles is 5.2 higher for a formulation containing the smallest particles of diameter $D_1$ than for that containing particles of diameter $D_2$ (Figure 2) and moreover, it corresponds to a particle surface ratio $S_{D1}/S_{D2} = D_2/D_1 = 1.73$ and an average particle center to particle center distance ratio $Hcc_{D2}/Hcc_{D1} = D_2/D_1 = 1.73$ (Figure 2).

The rheological properties of a formulation depend on various factors, including the properties of the continuous phase, such as PEO concentration and molar mass, as well as the properties of the dispersed phase, such as particle amounts, size, and size
<figure><img src="image_6.png" /><figcaption>**Figure 3.** a) Intensity correlation functions measured by Diffusing Wave Spectroscopy of PU particles suspensions at concentrations of 8, 16, 24, 32, 40, 48% w/v in water (dashed lines) and their equivalent with PEO at 4%w/v respectively corresponding to PU/PEO ratios of 2, 4, 6, 8, 10 and 12 (continuous lines). Black dots are adjustment of the data at 40%w/v PU particles in water, leading to average radius 340 nm. b) The same data are plotted as a function of the reduced time $t/\sqrt{\eta_0/\eta_s}$ where $\eta_0$ is the zero shear rate viscosity of the continuous phase of the studied system (i.e., pure water or the corresponding PEO solution at 4%w/v in water) and $\eta_s$ is the viscosity of pure water. All measurements were carried out at 22 °C.</figcaption></figure>

distribution while considering the interactions between both phases. Moreover, the state of PU suspensions, i.e., isolated particles, particle aggregation, or flocculation, can also affect significantly the rheological properties of the formulations. However, the state of the PU suspensions cannot be assessed from DLS measurements, as they require dilution of the samples. We thus perform Diffusing Wave Spectroscopy (DWS) experiment,[34] which is a dynamic light scattering technique suited for very turbid media. Similarly, to DLS technique, the intensity correlations functions of suspensions of PU in water (Figure 3 continuous curves) and in PEO solutions (Figure 3 dashed curves) were computed from DWS. In water, the characteristic decay time of the intensity correlation function does not depend on the PU concentration of the latex suspension (Figure 3a). Moreover, the intensity correlation curves may be fitted according to the general theory of DWS[35] and using the expression Equation (2) in ref. [36]. The adjustment is plotted in Figure 3 for the suspension of PU particles at 40%w/v in water, leading to an average particle radius equal to 340 nm, in reasonable agreement with the size measured by DLS (355 nm radius). This implies that PU particles in water are in a dispersed state, whatever the studied concentration. In PEO solutions, the particles diffusive motion is slowed down by the increase of viscosity of the suspension, and

to the exception of the most concentrated PU suspensions (of PU/PEO ratio equal to 10 and 12), the curves superimpose. The slowdown of the motion of the particles can be quantitatively taken into account by considering a time rescaled by the squared root of the viscosity of the suspension relative to water $t/\sqrt{\eta_0/\eta_s}$ (Figure 3b). It is observed that all curves, both in water and in PEO solution almost superimpose, albeit the most concentrated formulations in PU particle. Only at the largest studied PU/PEO ratios, few aggregates form that are responsible for an increased noise of the intensity correlation function close to the baseline (see the circled area in Figure 3a) induced by weaker averaging of rare fluctuations of light generated by these few aggregates. In order to get more insight into the stability of the formulations, the zeta potential was measured for PU particles in water with and without the presence of PEO at the different studied PU/PEO ratios. A zeta potential of -46.6 mV was measured for PU particles in water and a constant zeta potential of -46.3 ± 2 mV was measured whatever the PU/PEO value. Such result demonstrates that PEO chains are not able to screen the electrostatic repulsions between PU particles explaining thus that no aggregation was evidenced.

The rheological behavior was then investigated for each formulation detailed in Table 3. Figure 4 depicts the evolution of

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Steady shear viscosity of the formulations for a) the smallest PU particles (PU-D1) and b) the largest particles (PU-D2).</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Viscoelastic behavior of a) D2-PEO900-4%-R6 and b) D2-PEO900-4%-R12.</figcaption></figure>

the shear viscosity at a given continuous phase with a PEO molar mass of 900 kDa at 4%w/v for the formulations with a PU/PEO weight ratio ranging from 0 to 12 and based on the smallest particles of diameter D₁ (Figure 4a) and the largest ones of diameter D₂ (Figure 4b). For the lowest particle contents (i.e., PU/PEO ≤ 4 for D1 and PU/PEO ≤ 8 for the largest particles of diameter D₂), a classical shear-thinning behavior having a similar shape as the PEO solution is observed. For these formulations, a Newtonian plateau characterizes the flow at low shear rates then, a decrease of the viscosity, induced by PEO disentanglement, is measured above a critical shear rate. A similar behavior was already depicted for another kind of PU which was studied in a PU/PEO range between 0 and 6.[16] This typical shear-thinning behavior of polymer solutions is however not observed for the highest particle contents (i.e., PU/PEO ≥ 6 for D1 and PU/PEO ≤ 10 for the largest particles of diameter D₂). For these cases, the behavior is affected by the presence of PU particles which is characterized by a shoulder and a significant increase of the viscosity at low shear rates. Such Bingham-like behavior is the result of the formation of a stiffer inner structure[37] due to the interactions between PU particles and the PEO-based aqueous continuous phase.[26] Above a yield stress the solid-like structure

is destroyed and the curves converge to the shear-thinning behavior mostly affected by the disentanglement of PEO chains in the continuous aqueous phase. The onset from which the deviation from the typical shear-thinning to the Bingham behavior is reached at a lower PU/PEO ratio for the smallest particles. Such effect can be explained by the fact that for a given PU/PEO content the smallest PU particles have a larger surface area and a smaller particle-to-particle distance which favor the particle-particle and the particle-PEO interactions being responsible to the Bingham behavior at large PU/PEO values.

In order to get more insight into such difference in behavior between the lowest and the highest particle contents, the viscoelastic properties of D2-PEO900-R6 and D2-PEO900-R12 were obtained from oscillatory shear measurements (Figure 5). For the lowest PU/PEO ratio (i.e., PU/PEO = 6), a typical behavior of a polymer solution is observed with a storage modulus G' much lower than the loss modulus G'' that increases almost linearly with the frequency at the lowest values (Figure 5a). For the highest particle content (i.e., PU/PEO = 12), the intensity of the complex viscosity follows the same tendency than the viscosity measured at continuous flow (Figure 4b). Moreover, G' is greater than G'' for the lowest frequencies corresponding to the domain where the viscosity presents a shoulder. This demonstrates that at such high PU/PEO value, the particles within the continuous phase form a weak gel due to favorable interactions between PU particles and PEO chains.

Furthermore, the viscosity curves of the lowest PU/PEO ratios are below the one of the PEO solution (Figure 4b). To address this point, Figure 6 depicts the relative viscosity ηᵣ of the formulations (ηᵣ = η₀, formulation / η₀, PEO phase) as a function of the PU/PEO ratio for a given PEO molar mass of 900 kDa and a PEO concentration of 4%w/v in the continuous phase. For a given PU/PEO ratio, it is observed a significantly higher relative viscosity for the formulation containing the smallest particles; for instance, an increase by a factor of 18 was measured for PU/PEO = 8 when particles of diameter D₁ are chosen instead of D₂. Such phenomenon is explained by the higher surface area and the lower particle-to-particle distance which favor the PU particles-PEO chains interactions in the case of the smallest particles. For PU/PEO = 2 (i.e., corresponding to a particle concentration in the aqueous phase is of 8%w/v), there is no significant effect of the particle size on the relative viscosity. By increasing the particle content, the rheological behavior is significantly different for both formulations.

<figure><img src="image_9.png" /><figcaption>**Figure 6.** Relationship between the relative viscosity and the weight PU/PEO ratio for the smallest particles (D1-PEO900-4%) and the biggest ones (D2-PEO900-4%).</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Effect of PU/PEO weight ratio according to formulations with smaller (PU-D1/PEO) and larger (PU-D2/PEO) PU particles.</figcaption></figure>

Because the number of particles is 5.2 times higher with a total interfacial particle area 1.73 higher in case of the smallest particles, it promotes the interactions between the smallest PU particles and the PEO continuous phase, explaining thus the discrepancy in viscosity. Furthermore, Figure 6 shows a non-monotonic behavior of the relative viscosity as a function of PU/PEO ratio whatever the PU particle size. In the case of the smallest PU particles, a slight decrease of η<sub>r</sub> is observed when PU/PEO increases up to a ratio of 4, then a significant increase of η<sub>r</sub> is measured. This non-monotonic behavior is much more pronounced for the largest particles with a change in slope appearing at a higher particle content (i.e., PU/PEO = 6). This non monotonic behavior is not in agreement with the Einstein law.[38]

The decrease of the relative viscosity may be due to the mod-
ification of the PEO-PEO interactions in the continuous phase.
A thin layer of PEO may be adsorbed on the PU particles as no-
ticed for other polymers[28,39] which may lead to a rearrangement
and/or disentanglements of PEO chains in the continuous phase.

<figure><img src="image_11.png" /><figcaption>**Figure 8.** Average diameter according to the PU/PEO ratio for D1 and D2 (PEO 900 kDa 4%w/v) in the same electrospinning conditions (12.5 kV, 1 mL h⁻¹, and 15 cm).</figcaption></figure>

The discrepancy between both suspensions made of particles of different diameters is probably related to the number of particles. PEO is also adsorbed on the smallest particles but the close vicinity and the larger surface area of these particles promotes the onset of the increase of the relative viscosity at a lower particle content than what it is observed for particles of larger diameter. The composition of the aqueous liquid surrounding the FD103 PU may also explain the decrease of the relative viscosity at low PU/PEO values as it enters in each formulation. Thus, the aqueous liquid surrounding the particles of the FD103 suspension was isolated by centrifugation for study. The surface tension of the isolated liquid was characterized and a value of 52.3 mN.m⁻¹ was measured whereas a surface tension of 70.5 mN.m⁻¹ was measured for the additional deionized water entering in the formulations. Thus, the water entering in the initial FD103 suspension may include some molecules and/or surfactants that could also affect the rheology of the PEO continuous phase of the formulations explaining the discrepancy with the Einstein law at low PU/PEO content.

Such findings underline the complex rheological behavior of
the formulations and emphasize the pivotal role of particle size
and concentration in the optimization of the electrospinning pro-
cess. Thus, formulations of PU/PEO ratio equal to 4, 8, 10, and
12 were electrospun. Ratios lower than 4 were not studied as the
goal was to reach the highest PU/PEO value as possible. What-
ever the PU/PEO ratio, the process of electrospinning was stable
for long time of production. The relationship between the rheo-
logical properties and the morphology of the fibers is discussed
as a function of the PU dispersed phase properties. SEM images
of electrospun fibers at the different PU/PEO ratios are shown in
**Figure 7**. The results revealed a noticeable increase in the mean
diameter of electrospun fibers with the PU/PEO ratio for both PU
particle diameter (Figure 8). Specifically, for the smallest PU par-
ticles, the average fiber diameter increased from 450 ± 110 nm
(PU/PEO = 4) to 730 ± 130 nm (PU/PEO = 12). For the largest
PU particles, the mean fiber diameter exhibited a comparable
trend, rising from 425 ± 230 nm (PU/PEO = 4) to 680 ± 400 nm
(PU/PEO = 12). This is attributed to the increase of particle con-
centration in line with the increase of the viscosity of the formu-
lations.
<figure><img src="image_12.png" /><figcaption>**Figure 11.** SEM pictures of electrospun membranes for various PEO molar mass and concentration. Left column, $M_w$ = 400 kDa. Middle column, $M_w$ = 900 kDa. Right column, $M_w$ = 2000 kDa. First line, PU/PEO = 8 and $C_{PEO}$ = 2%w/v. Second line, PU/PEO = 8 and $C_{PEO}$ = 3%w/v. Third line, PU/PEO and $C_{PEO}$ = 4%w/v. Last line, no PU particle and $C_{PEO}$ = 4%w/v.</figcaption></figure>

ity of the continuous phase with the one of the corresponding formulation with the particles). However, and very surprisingly, the addition of particles decreases the viscosity of the continuous phase in the case of $M_w$ = 2000 kDa. Such effect may have a similar origin then what it was discussed regarding the decrease of the viscosity with the addition of PU particles at low particle con-

tent. Increasing the PEO molar mass may amplify the PU particles – PEO chains interactions. For larger $M_w$, the adsorption layer changes from trains to tails resulting in lower PEO density at the surface of the particles. The addition of PU particles in the formulation will then disturb not only the particle-particle interactions but also the polymer chains interactions in the vicinity of
<figure><img src="image_13.png" /><figcaption>**Figure 13.** a) SEM of an electrospun membrane obtained from the formulation D2-PEO900-2%-R50 with a PU/PEO ratio of 50 and a PU solid content in the formulation of 102%w/v (i.e., relatively to the volume of water) corresponding to 49.5%w/v (relatively to the total weight of the formulation). b) Steady shear viscosity of D2-PEO900-2%-R50.</figcaption></figure>

particles. Consequently, it resulted to an increase of the average fiber diameter with an improvement of the fiber homogeneity. Thanks to this in-depth study, a fibrous membrane was obtained from a formulation with a PU/PEO weight ratio equal to 50 under very stable and efficient production conditions. It must be pointed out that these guidelines are only valuable for the studied particle-template polymer system. However, this work emphasizes the need to deeply investigate the rheological behavior in regard to the complex interactions occurring in the formulation. This is of paramount importance to increase the electrospinning productivity of waterborne suspensions while controlling the fiber morphology.

## Acknowledgements

This work was funded by BPI France through the DECAPES project of the PSPC Région 2 program. This work of the Interdisciplinary Thematic Institute HiFunMat, as part of the III 2021–2028 program of the University of Strasbourg, CNRS and Inserm, was supported by IdEx: Unistra (ANR-10-IDEX-0002) and SFRI (STRAT'US project, AVR-20-SFRI-0012) under the framework of the French Investments for the Future Program.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.

## Keywords

green electrospinning, nanofibers, waterborne polyurethane

Received: April 26, 2024

Revised: July 17, 2024

Published online:

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