# Non-tissés de nanofibres de PA-6 en structure multicouches obtenus par electrofilage

Amir Houshang Hekmati, Nabyl Khenoussi, Laurence Schacher, Dominique

Adolphe, Jean-Yves Drean

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Amir Houshang Hekmati, Nabyl Khenoussi, Laurence Schacher, Dominique Adolphe, Jean-Yves Drean. Non-tissés de nanofibres de PA-6 en structure multicouches obtenus par electrofilage. CFM 2011 - 20ème Congrès Français de Mécanique, Aug 2011, Besançon, France. hal-03422917

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# NON-TISSES DE NANOFIBRES DE PA-6 EN STRUCTURE MULTICOUCHES OBTENUS PAR ELECTROFILAGE

A. H. HEKMATI<sup>a,b</sup>, N. KHENOUSSI<sup>a</sup>, L. SCHACHER<sup>a</sup>, D. ADOLPHE<sup>a</sup> and J. Y. DREAN<sup>a</sup>

a. Laboratoire de Physique et Mécanique Textiles EAC 7189 CNRS – UHA Mulhouse, France

b. Université AZAD, Branche : Science et Recherche, Téhéran, Iran

Amir-houshang.hekmati@uha.fr

## Résumé

L'étude porte sur l'influence des propriétés morphologiques des nanofibres de PA-6 sur les propriétés mécaniques globales au sein d'un non-tissé. Des granulés de PA-6 ont été dissous dans de l'acide formique pour obtenir des solutions de polymère à 25%. Pour assurer la reproductibilité des essais durant le processus électrofilage, tous les paramètres importants ont été étudiés et les conditions optimales d'électrofilage ont été choisies. Les non-tissés électrofilés ont été retirés du collecteur (feuille aluminium) en utilisant une méthode simple sans qu'ils soient endommagés. Les microscopies MEB et AFM ont été utilisées pour observer avec précision la morphologie longitudinale et transversale des non-tissés. Tous les échantillons ont été coupés aux mêmes dimensions (5 x 15 mm<sup>2</sup>) et pesés par microbalance pour obtenir la masse surfacique de chaque échantillon. Ces derniers ont été placés sur des supports spéciaux afin de faciliter leur manipulation lors des essais de traction sur une machine MTS. Les résultats de ce travail montrent la relation entre la masse surfacique du non-tissé de PA-6 et sa résistance en traction ainsi que son module. Les micrographies MEB des facièces de rupture des non-tissés de PA-6 après essais de traction illustrent la structure multi couches des non-tissés obtenus. Cette structure multicouche est confirmée par l'observation des courbes contrainte-déformation obtenues qui montrent un phénomène de rupture échelonnée.

## Abstract

The mechanical properties of PA-6 electrospun nanowebs and the effect of nanowebs structure on these properties were studied in this research. PA-6 pellets were dissolved in formic acid to yield 25 wt.% polymer solution. To ensure the reproducibility of electrospinning process, all important parameters have been studied and the optimum of electrospinning conditions was obtained. Electrospun nanowebs were removed from the aluminum foil (collector) using a simple method without any physical damage. SEM and AFM devices were used to precisely observe the nanowebs longitudinal and cross-sectional morphology. All samples were cut in same dimension (5×15 mm) and weighted by a mechanical micro-balance to get the weight per surface unit of nanowebs. Samples were put in the special supports which were built by cardboard layers to do the tensile tests by MTS tensile testing machine. The results of this work show the relationship between weight per surface unit of PA-6 nanowebs and their tensile strength and modulus. The SEM micrographs of ruptured PA-6 nanoweb after tensile test illustrate the multilayer structure of PA-6 nanowebs. This structure could be explained by observing the nanowebs stress-strain curve. The curves show multi-steps rupture in PA-6 nanoweb that can be caused by multilayer structure of nanowebs.

**Keywords:** electrospinning, PA-6 nanofibers, nanowebs, mechanical properties, multilayer structure
## Introduction

Nowadays, PA-6 has been used widely as one of the most important thermoplastic polymers thanks to its very good properties such as mechanical properties, good thermal stability and fire resistance [1]. It has great potential to be utilized in the textile industry and nanofibers fabrication especially by the electrospinning process.

Electrospinning process [2] is the main method to produce the nanofibers and also nanowebs. Researchers have used this method for very wide variety of polymers [3]. Nanofiber and nanoweb production via electrospinning process is affected by applied voltage, distance between needle tip to collector and polymer solution concentration and viscosity [4]. Characterization of obtained nanowebs from electrospinning process is important to develop their fields of application. Morphological properties of nanofibers and nanowebs are investigated without any problem by SEM and AFM techniques but due to the small size of nanofibers and their fragility, handling nanofiber samples to mechanical characterization can be very challenging [5].

Most of studies on electrospinning have been focused on its phenomena and on the processing control parameters [6]. Some limited studies on mechanical properties of single nanofiber and nanowebs exist. E.S.P. Tan et al, T. Namazu et al and M. A. Haque et al. have worked on mechanical properties of nanofibers and nanowebs [5]. In this work we use a simple way to prepare the reproducible samples which could be handled easily for mechanical characterization using tensile testing machine. The obtained results help us to understand the details of nanowebs structure and the changes which occur during the tensile test in the nanowebs structure.

## Experimental Details

### Materials

Polyamide 6 (nylon-6) pellets M<sub>w</sub>=10,000 were obtained from Acros Organics, USA. Formic acid (90%) was purchased from Fisher Scientific, France. PA-6 polymer solution (25 wt.%) was prepared dissolving the PA-6 pellets in Formic acid. Stirring was used to expedite dissolution.

### Electrospinning Process

Polymer solution was electrospun in ambient pressure, humidity and temperature by LPMT electrospinning booth [7]. The electrospinning process was based on vertical projection. The flowing rate of polymer solution was controlled by syringe pump located outside of electrospinning cabin and the polymer solution inject into a 4 mm tube through the metallic needle vertically placed on the collector. A high voltage gradient was prepared by a high voltage system between needle tip and collector. The metallic collector is made of a copper plate covered by an aluminum foil. The gap between the collector and needle tip can be adjusted by two step-by-step motors which make possible vertical movement of collector and horizontal movement of needle. This movement system helps us to change the needle situation during the electrospinning process. The electrospinning system is insulated inside a PVC booth including a Faraday cage to avoid the electrical parasites and also for security reasons (Fig. 1). Applied voltage was 30 kV and distance between needle tip and collector was 20 cm. Polymer solution feedrate was controlled on 0.212 ml/h.
<figure><img src="image_1.png" /><figcaption>FIG. 1 – Electrospinning booth</figcaption></figure>

## Scanning Electron Microscopy (SEM)

The electrospun samples were coated by gold and analyzed by scanning electron microscopy (SEM, Hitachi S-2360N).

## Atomic Force Microscopy (AFM)

The nanofibers surface morphology were observed using AFM (Nanoscope IIIa). Images were obtained by tapping non-contact mode. Working in this mode avoids shear forces that can damage soft samples and reduce image resolution.

## Sample Preparation

After electrospinning, the SEM and AFM photos were taken to check the nanowebs homogeneity (Fig. 2). Samples were cut by special roller cutter in same dimension (5×15 mm), then, they were removed from collector properly without any physical damage. They were weighted by a mechanical micro-balance (0.000001 g). Mechanical micro-balance was used because in digital balances electrostatic parasite made mistakes in weighting. Samples were sandwiched between two cardboard layers to keep them fixed. Preparation of samples with cardboard supports helps to handle them easily for mechanical tests. Figure 3 shows the PA-6 electrospun nanoweb samples with cardboard supports.

<figure><img src="image_2.png" /><figcaption>FIG. 2 – SEM and AFM micrographs</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>FIG. 3 – Prepared samples</figcaption></figure>

Prepared samples were tested by MTS (FIG. 4) tensile testing machine (10N load-cell) by 10 mm/min rate of extension under 25 C° and 65% RH. Before the start of testing the two supporting handles of cardboards, which keep nanowebs fixed between grips, were cut.
<figure><img src="image_4.png" /><figcaption>FIG. 4 – MTS Tensile testing machine</figcaption></figure>

## Results and Discussion

The tensile strength of prepared samples was measured. Due to the fact that the weight per surface unit of samples is out of our control, eleven weight points of specimens with a regular increase in weight per surface unit among the obtained samples were selected for tensile test. The weight per surface unit of samples increases regularly 0.4 (g/m²) from sample 1 to sample 11. Between sample 11 and 12 there is 0.6 g/m² difference. Samples 12 and 13 have same difference in weight as samples 1 to 11. Table 1 shows the weight of each sample.

<table><caption>TABLE 1 – Weight per surface unit of samples</caption><thead><tr><td>Sample number</td><td>Weight g/m²</td><td>Sample number</td><td>Weight g/m²</td></tr></thead><tbody><tr><td>1</td><td>5.064</td><td>7</td><td>7.464</td></tr><tr><td>2</td><td>5.464</td><td>8</td><td>7.864</td></tr><tr><td>3</td><td>5.864</td><td>9</td><td>8.264</td></tr><tr><td>4</td><td>6.264</td><td>10</td><td>8.664</td></tr><tr><td>5</td><td>6.664</td><td>11</td><td>9.064</td></tr><tr><td>6</td><td>7.064</td><td>12</td><td>10.130</td></tr><tr><td></td><td></td><td>13</td><td>10.530</td></tr></tbody></table>

The obtained results of tensile test show a relationship between weight per surface unit of PA-6 electrospun nanoweb and tensile strength and modulus (Fig. 5).
<figure><img src="image_5.png" /><figcaption>FIG. 5 – Relationship between weight of samples, tensile strength (Force) and modulus</figcaption></figure>

Stress-strain curves show the special fracture behavior for all samples. In the other words, the rupture of PA-6 nanowebs occurs in more than one step. Figure 6 show two SEM micrographs of ruptured PA-6 nanoweb after tensile test. Figure 7 is the stress-strain curve of same sample. They could help us to explain this phenomenon.

<figure><img src="image_6.png" /><figcaption>FIG. 6 – PA-6 nanowebs SEM micrographs</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>FIG. 7 – Stress-strain curve</figcaption></figure>

Figure 6 and 7 show multistep rupture in PA-6 nanoweb that can be caused by multi layer structure of nanowebs. Multi layer structure of nanowebs is dependent on time of electrospinning and other parameters which influence the centralization of collecting area on aluminum foil (collector). The cross-section view of PA-6 nanowebs confirms their multilayer structure and its regular web formation during an electrospinning process (Fig. 8). This figure shows a PA-6 nanoweb which is electrospun for a time more than usual to understand the effect of time of electrospinning on homogeneity of nanoweb layers formation. The same distance between layers and same thickness of layers are clearly observable.
<figure><img src="image_8.png" /><figcaption>FIG. 8 – PA-6 nanoweb cross-section SEM micrograph (two different magnifications)</figcaption></figure>

## Conclusion

This work presents a simple way to characterize mechanical properties of electrospun nanowebs that can be applied to a wide range of polymers. The relationship between weight per surface unit of PA-6 electrospun nanowebs, their tensile strength and modulus was investigated. Increase of weight per surface unit of nanowebs leads to increase of maximal force and modulus of samples. The multistep ruptures of nanowebs during the tensile test which is observed in stress-strain curves explained by the SEM micrograph of ruptured samples. The micrographs show the fracture in nanowebs layer which occur one by one. For tested samples depending on time of electrospinning, the PA-6 nanowebs contain two or three layers but if we continue the electrospinning, the SEM micrograph shows the regular nanowebs layer formation which is of interest for more mechanical studies.

## Future work

Investigation of the effect of the electrospinning parameters on mechanical properties of electrospun nanowebs is the next step of this work. This sampling method helps us to prepare proper specimens for other characterization such as fatigue and bending test. This study is a preliminary work to mechanical characterization of PA-6, PA-66 and PA-6/PA-66 nano-membrane composite based on electrospun nano-nonwoven.

## References

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[7] Hekmati A H, Khenoussi N, Drean E, Schacher L, Adolphe D, "Study of rheological behavior of polymer solution used for electrospinning process" AUTEX international conference, 2009, Izmir, Turkey.