Maya Blue as natural coloring fillers in a multi-scale polymer-clay nanocomposite

Composites Science and Technology 71 (2011) 1685–1691

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Maya Blue as natural coloring fillers in a multi-scale polymer-clay nanocomposite Nicolas Volle a, Laetitia Challier a, Alain Burr b,⇑, Françoise Giulieri a, Sophie Pagnotta c, Anne-Marie Chaze a a

Chimie des Matériaux Organiques et Métalliques, Université Nice-Sophia Antipolis, 06108 Nice cedex 2, France Mines-ParisTech, CEMEF (UMR CNRS 7635), BP 207, F-06904 Sophia Antipolis, France c Centre Commun de Microscopie Appliquée, Université Nice-Sophia Antipolis, 06108 Nice cedex 2, France b

a r t i c l e

i n f o

Article history: Received 28 January 2011 Received in revised form 5 July 2011 Accepted 10 July 2011 Available online 23 July 2011 Keywords: A. Hybrid composites A. Nanoclays B. Mechanical properties C. Elastic properties, Filler dispersion

a b s t r a c t The purpose of this article is to understand the underlying mechanisms of the potential of nanohybrids. Two types of fillers were dispersed in a biocompatible organic matrix, poly-(hydroxyethyl acrylate) (PHEA): either a natural fibrous clay (pristine sepiolite), or a natural nanopigment (Maya Blue). In comparison with the pure polymer, the mechanical properties of the composites have been already greatly improved with the addition a small amount of Maya Blue. However the PHEA/pristine sepiolite nanohybrid gives better results than the PHEA/Maya Blue for the same ratio of fillers. The stress–strain curves (with unloading and reloading) show that the presence of indigo modifies the interactions between the filler and the matrix. These analyses demonstrate that there is a direct correlation between the filler/matrix local interactions, the dispersion of the fillers and some macroscopical mechanical properties. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Since the pioneering work of Wichterle and Lim in 1960 on crosslinked poly hydroxyethyl methacrylate (PHEMA) hydrogels [1] and many years after [2,3], these acrylate polymers have been of great interest to biomaterial scientists. Most of polymer hydrogels have many applications in biomedical engineering [4–6] due to their excellent biocompatibility and their water permeation properties. The most common example of such polymers is poly(hydroxyethyl acrylate) (PHEA) [7,8]. It is widely used as biomaterial or coating, i.e. contact lenses, intraocular lenses, hydrophilic sponges and drug delivery patches. PHEA is a biocompatible polymer with abundant hydroxyl bonds and hydrophilic properties. Moreover for composites filled by sepiolite, this elastomer can be used as an interesting matrix [9]. Sepiolite is a filler in a needle-like shape and transversal nanometric sizes. This natural clay is a crystalline hydrated magnesium silicate of theoretical unit cell formula Si12O30Mg8(OH)4(OH2)4 8  H2O [10]. This structure consists of continuous two-dimensional talclike tetrahedral sheets and discontinuous octahedral sheets (Fig. 1). Thanks to these discontinuous octahedral ribbons, periodic nanopores with a rectangular section are formed within the main structure of the fiber. When these pores are at the surface of the fiber they form channels. If the pore is embedded within the fiber they build a tunnel. ⇑ Corresponding author. E-mail address: [email protected] (A. Burr). 0266-3538/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2011.07.009

Both are running parallel to the mean axis0 of the fiber. These pores 0 have a cross-section size of 11.5 Å A  3.7 A Å admitted in the literature [11]. The length of the fibers ranges between 0.5 and 7 lm and their cross-section lies between 20–50 nm. It is particularly interesting to notice that both the tunnels and the channels are naturally filled with zeolitic water in ambient conditions (Fig. 1) [12]. Sepiolite has been widely used to reinforce different polymers such as polypropylene [13–15], epoxy [16], nylon-6 [17], polyurethane [18], polyacrylic acid [19], natural rubber [20], biocompatible materials [21,22]. Moreover pristine sepiolite has specific properties of fibrous natural clay, such as a high specific surface area (300 m2 g1). Effectively owing to the presence of pore of nanometric size, the available interface with another phase increases dramatically. Several hydroxyl groups, such as silanol (SiOH) and Mg(OH)2, are covering the clay surface. When dispersed in a polymer matrix, these groups can be involved in hydrogen bonding with the hydrophilic polymer matrix through association with OH, NH and other polar groups. Thus the filler-polymer interfacial interactions are enhanced, which in turn results in a great improvement of the material properties. Therefore many interactions between the OH-terminated pending groups of PHEA and hydrophilic groups of sepiolite are possible. Another specificity of sepiolite is that its periodic cavities can receive, under specific conditions, organic molecules. Organic molecules like indigo can replace zeolitic water inside the tunnels [23]. Thus each fiber can play the role of the matrix, encapsulating totally the molecules of indigo. This leads to the possible development of

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SiOH Mg(OH2 )2 channels

tunnels

Mg

O

Si

OH

OH2 : coordinated water H2O: zeolitic water

Fig. 1. Schematic presentation of sepiolite fiber: layer of silica extends as a continuous layer with inversion generating uniform size of tunnels and channels (11.5 Å  3.7 Å) along the fiber.

modern and original nanocomposites [24], e.g. the famous Maya Blue which is an organic-inorganic nanohybrid: nanopigment. The presence of indigo inside the pores of the fiber can lead to a pigment of a deep blue [23]. Moreover these pigments are stable in both acidic and basic environments, and are resistant to oxidation [25]. Note that Maya Blue has never been used as fillers. So it is interesting to find ways to reinforce PHEA with organomineral pigments (Maya Blue) as it is done with sepiolite. Note that the nanocomposite resulting from the synthesis between Maya Blue and PHEA is an organic-inorganic-organic nanohybrid. It is a multi-functional and multi-scale nanocomposite. This nanocomposite is multi-scale because the sepiolite is both a matrix for indigo and a filler for the acrylate elastomer. The multi-functional state is a result of the presence of one single filler that does both mechanical reinforcement and much better coloration of the PHEA. The aim of this article is to expose and compare the results of nanocomposites filled by either pristine sepiolite or Maya Blue. First it involves the control of the manufacturing process of the filled elastomers. Secondly it will be necessary to carefully inspect the interactions between the fillers and matrix before and after modifications of the sepiolite. The choice of indigo molecule has been naturally driven based upon previous studies [23,26]. We will show that the natural pigment ‘‘Maya Blue’’ is a very stable filler for this polymer. To realize these aims mechanical tensile tests and TEM observations will be performed on all basic components and on all nanohybrids. The conclusions of these analyses should lead to better ways to produce multi-scale organic-inorganic nanohybrids with multi-functional properties.

20 min in the monomer. Then the dispersions were let at room temperature for another 20 min before to be used. In order to compare the effect of mixing energies, reference mixtures were made with a magnetic stirrer (low power) (500 rpm) during 24 h. After addition of 0.05 phr of the photoinitiator, the resulting dispersion was injected into a mold consisting of two glass plates (25  75 mm) separated by a sealing ring of a given thickness (0.8 mm). This mold was then irradiated under UV light with an intensity of 60 lW cm2 (wavelength 365 nm) during about 3 h. The temperature of the mold was free to evolve inside the lab (temperature 25 °C) according to the kinetics of the reaction of polymerization. Following irradiation, each specimen was then verified in terms of polymerization quality. In this study we will be making on several kinds of samples: – pure PHEA, – PHEA/SepX which corresponds to the nanocomposite containing X phr of pristine sepiolite, – PHEA/MBY (PHEA containing Y phr of Maya Blue), – PHEA/IndZ (PHEA containing Z phr of indigo molecules). 2.4. Transmission electron microscopy Two kinds of preparations were studied. First, ultrathin sections of samples of elastomer composites were done at 100 °C with a cryo-ultramicrotome (Leica EM FCS) and quickly placed on TEM grids. Their thickness was approximately 100 nm. Secondly, a dispersion of sepiolite sonicated in water was disposed on a grid. Microscopy studies were carried out using a Philips CM 12 microscope at an accelerator voltage of 120 kV in the conventional mode.

2. Experimental 2.5. Mechanical tensile tests 2.1. Materials The 2-hydroxylethyl acrylate (HEA) (H2C = CHCO2CH2CH2OH) and the indigo (C16H10N2O2) were purchased from Aldrich. The liquid state of acrylate monomer will make easier the dispersion of fillers. Sepiolite Pangel S-9 was supplied by TOLSA (85% pure; other clays 15%). The photoinitiator, Irgacure 819, comes from Ciba. 2.2. Elaboration of the nanofillers sepiolite/indigo ‘‘Maya Blue’’ The used process to elaborate Maya Blue fillers, MB, was explained by Sonia Ovarlez et al. [23]. The mixtures of indigo/sepiolite, i.e. MB(2%), MB(5%) and MB(8%) contain respectively 2, 5, 8 wt% of indigo. 2.3. Processing of nanohybrids composites PHEA/Maya Blue and PHEA/ Sepiolite The fillers MB or pristine sepiolite were sonicated (high power US waves – BIOBLOCK 20 kHz-750 W, microprobe 3 mm) during

One single sample with a dumbbell shape was cut in each film. Therefore all tests are comparable. On the surface of every sample four marks were drawn. They were served as strain indicators by using a video tracking system. A careful experimental protocol was implemented to determine the mechanical behavior of nanocomposites. Pure PHEA and filled PHEA were submitted to uniaxial tensile tests using an electromechanical Erichsen tensile machine on 0.8 mm-thick specimens. The displacement rate was 0.5 mm s1. All tests were conduced at room temperature (25 °C). The specimens were gripped by pneumatic clamps linked to a load sensor of a maximal capacity of 200 N. A video extensometer followed in real time the longitudinal and transverse strains. The force and both deformations were simultaneously recorded in a file by a computer. Several kinds of data were obtained: 2.5.1. Equilibrium tensile stress–strain measurements This test allowed determining the current true stress, r, and the current natural strain, e, using the following definitions:

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r¼

F S

ð1Þ

where F and S were respectively the current load applied on the specimen and the current section of the specimen.

  l e ¼ lnðkÞ ¼ ln l0

ð2Þ

where l and l0 were respectively the current and the initial gauge lengths drawn on the specimen. However the initial section S0 was known. As the evolution of the strains verifies the isovolume condition, so S0  l0 = S  l, Eq. (1) gives the relation between the current true stress, the current applied load and the strain:

F S

r¼ ¼

F e e S0

ð3Þ

Tensile results were displayed in the form of Mooney–Rivlin plot. The strain energy density function (4) for an incompressible Mooney–Rivlin material was:

W ¼ C 1 ðI1  3Þ þ C 2 ðI2  3Þ

ð4Þ

where C1 and C2 were empirically determined material constants independent of k, and I1 and I2 were the first and the second invariant of the deviatoric component of the left Cauchy–Green deformation tensor. For the case of an incompressible Mooney–Rivlin material under uniaxial elongation, the reduce stress (5) differences could be calculated as:

½r  ¼

r k2  k1

¼ 2C 1 þ 2C 2 k1

ð5Þ

If this curve is horizontal, the Young Modulus will can be determined in taking the average of reduce stress. In others cases the Young Modulus will be obtained in taking the y-intercept of the curve linear regression. 2.5.2. Stress–strain tensile: loading, unloading and reloading sequences The tensile stress–strain curves were shown with successive increasing extensions. These sequences of unloading and reloading were going to reveal the intrinsic behavior of the composite. The interactions between each filler and its surrounding matrix, and the interconnections within this network will be translated by a softening of the maximum of stress experienced during each cycle. The phenomenon is known as the Mullins effect [27]. The synthesis of specimen is verified in term of polymerization, before making the mechanical tests. This particular precaution leads to reproducible

A

True stress (MPa)

3. Results and discussion 3.1. PHEA/sepiolite: process optimization 3.1.1. Dispersion effect It has been verified that PHEA is incompressible (or at least very close to). True stress, r, is calculated using Eq. (3). Thus this allows us to connect the mechanical behavior of the PHEA/sepiolite nanohybrids with the dispersion process of the fillers. The crucial point for obtaining elastomer nanocomposites remains a good dispersion of the fillers within the soft matrix. In this present study the clay particles are carefully dispersed into the monomer (HEA) before proceeding to the in situ polymerization process. Mechanical properties of samples issued from either mechanical stirring or sonicated dispersions have been compared. Fig. 2Aa tallies with pure PHEA and will be used as a reference. For the same applied strain, the stress level reached by the sample dispersed by Ultra-Sounds of high power (sonicator with a microtip) (Fig. 2Ac) is more important than the stress level reached by the sample mechanically stirred at low power (magnetic stirrer) (Fig. 2Ab). These results can be explained by analyzing TEM micrographs. The observations of sepiolite dispersion are done on the PHEA/ Sep3 samples. Fig. 2B exhibits the nanocomposite that was elaborated by mechanical stirring process (low power). The sepiolite is still aggregated in form of bundles. Indeed such a composite made from aggregates will have very poor mechanical properties. By opposition the hybrid obtained using the sonication process (Fig. 2C) shows a homogeneous dispersion of sepiolite. Almost each fiber of pristine sepiolite is isolated. This is an indication that a minimum level of power is needed to achieve a stable dispersion before polymerization. Therefore, with individual fibers, the interfacial compatibility between the whole surface of the clay and the pending OH-groups of the PHEA are significantly improved. According to this work it is shown that sonication combined with selective interactions between the fibers of sepiolite and the PHEA leads to a composite material with enhanced mechanical properties. 3.1.2. Factor shape The fiber size is another important parameter that also contributes to the final properties. The pristine sepiolite with an average length of 2 lm (Fig. 3B) and a cut sepiolite with a length evolving

B

14

C

c

(a) pure PHEA (b) PHEA/Sep3 mechanical stirring (c) PHEA/Sep3 sonication

12

results and for each sets of data; five samples were tested to get the average value.

10

b 8

a

6 4

2µm

2 0

0

100

200

300

2µm

400

Strain (%) Fig. 2. True stress versus the natural strain curves for PHEA/Sep (A) and TEM micrographs of PHEA/sepiolite nanocomposites containing sepiolite dispersed by mechanical stirring (B), by sonication (C).

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A

True stress (MPa)

8

B

a) PHEA/MB1cut b) PHEA/MB2cut c) PHEA/MB1 6 d) PHEA/MB2

C

d b a c

4

2

2µm

2µm 0

0

100

200

300

400

Strain (%) Fig. 3. Stress–strain curves vs. the sepiolite size, pristine sepiolite and cut sepiolite at different phr contents (A) and TEM micrographs of sepiolite dispersed on a grid, pristine sepiolite (B) and cut sepiolite (C).

between 0.5 and 2 lm (Fig. 3C) were used. By comparing the mechanical behavior from tensile tests (Fig. 3A) an increase of the true stress is remarked with the increase of the clay size whatever the strain. When the size of sepiolite is small, fibers do not participate efficiently to the transfer of load within the material. Thus pristine sepiolite has a shape factor more suitable. Moreover on these curves, another parameter can be observed. The gap between true stresses for the same strain level is higher for the long fibers than for the small fibers (Fig. 3A). This phenomenon can be explained by the shape factor (lengh/ diameter) which is 100 for the pristine sepiolite and only 50 for cut sepiolite. According to Jeulin and Moreaud [28] the percolation, ‘‘continuous’’ paths of fibers, appears for a shape factor of 100 at 1.1 wt% fillers. Whereas for a shape factor of 50, the fillers ratio needed to reach percolation is 9 wt%. That is why when the fibers are smaller, a less sharp gap is observed, because the percolation threshold is not achieved. Here the influence of the size of the fillers has been clearly identified. A higher shape factor leads to higher mechanical properties especially visible at low filler concentrations. To obtain an optimized nanocomposite, it is necessary to disperse the fibers of sepiolite by sonication and to have an adapted size for these needles. Moreover this study allows appreciating the interactions between pristine sepiolite and PHEA matrix.

3.2. Maya Blue behavior The Maya Blue is a natural characteristic pigment produced by the Maya people. The use of MB in Central America, mostly in

Mexico, is well documented in archeological sites between the 8th and the 16th centuries [29]. Its use is prolonged even to recent times [30]. This pigment is the result of the heating of a mixture of colorless clay – from palygorskites and sepiolites – and an organic blue dye – the indigo. Maya Blue was originally used in fresco or mural paintings. In this article we show that Maya Blue has also properties as filler. Using the same protocol as for pristine sepiolite, several kind of Maya Blue nanohybrids have been elaborated and tested. Thus we demonstrated one more time that there is a direct correlation between the filler/matrix chemical interactions, the dispersion and mechanical properties.

3.2.1. Mechanical tensile tests The stress–strain curves of tensile tests of PHEA, PHEA/Sep3, PHEA/MB3 and PHEA/Ind0.24 are shown in Fig. 4A. Samples filled and colored with pure indigo were tested in uniaxial tensile. So the addition of indigo (less than 0.24%) has no significant reinforcement of the composite with regard to the pure polymer. The results are completely different when Maya Blue or pristine sepiolite are used. Indeed mechanical properties are greatly improved in comparison with pure PHEA as the true stress is higher for any given strain level. The overall behavior of PHEA filled with 3 phr sepiolite looks similar to the one of PHEA filled with 3 phr Maya Blue. Both curves have rather similar forms. The failure strain is almost the same (approximately 310 %). Nevertheless, at equal strain, stress supported by the PHEA filled by pristine sepiolite is always higher for all rate of filler explored. Thus as filler, Maya Blue is lower than pristine sepiolite. The curves of the tensile results are displayed on the format of Mooney Rivlin type graphs (Fig. 4B). The reduced

A

B

(a) PHEA/Ind0.24 12 (b) pure PHEA (c) PHEA/MB3(8%) 10 (d) PHEA/Sep3

d c

8

b

6 4

a

2 0

0

100

200

Strain (%)

300

(a) PHEA/Ind0.24 (b) pure PHEA (c) PHEA/MB3(8%) (d) PHEA/Sep3

1.4

Reduced stress (MPa)

True stress (MPa)

14

400

1.2 1 0.8 0.6

d c

0.4

a b

0.2 0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1/λ

Fig. 4. Stress–Strain curves (A) and reduced stress vs. reciprocal elongation (B) for (a) PHEA/Ind0.24 (b) pure PHEA (c) PHEA/MB3 (8%), and (d) PHEA/Sep3.

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stress of the pure polymer exhibits a straight line ascribed to the ‘affine-phantom’ transition. The rupture of the specimen occurs before the limit extensibility of the polymer chains. By opposition the reinforced composites show an upturn in the reduced stress at large deformations. In the ‘‘1phr’’ group of composites the effects of the limit extensibility of the polymer chains can be visible at a much lower global deformation. First comment: the value of 1/k at which the upturn occurs, decreases with the amount of fillers. This reveals a strong interaction between the polymer chains and the particles. In other words, the polymer chains are cross-linked by the clay particles, leading to a network where the apparent molecular weight between the cross-linking nodes decreases with the amount of filler. The same thing is observed on nanohybrids with the modified surface of the fillers. Second comment: the Young’s Modulus of PHEA/MB3 is lower than the one of PHEA/Sep3. Therefore, the interactions between the matrix and the fillers are different and certainly weaker when the surface of sepiolite is modified. By comparing the effect of reinforcement by the Maya Blue or the pristine sepiolite we conclude that the mechanical behavior of composites was better when the sepiolite does not contain indigo. To conclude the existence of interactions between fillers and PHEA matrix is verified with the stress–strain tensile measurements. Mullins effect or ‘‘stress softening’’ [27] is a hysteresis phenomenon that appears for the nanocomposite PHEA/Sep3 (Fig. 5A). These data reveal at each strain loop a noticeable amount of stress-softening. The origin of this softening is due to:

of pure indigo have not influence on the global measurements. There is none visible loop and no difference with the monotone tensile test. So for the amount of indigo that afford the blue color in PHEA/SepMB3, there is no interaction between the matrix and the indigo. Eventually the stress softening is important in the case of elastomer filled with sepiolite, because there are several interactions between fillers and matrix. In the case of the Maya Blue, the effect is less important. To sum up, a change in mechanical properties was highlighted on MB or pristine sepiolite elastomers elaborated with the same process. To improve the manufacturing process it is necessary to have a homogeneous dispersion, what is also resulting by better interactions between the fillers and the matrix. Next, the dispersions of the fillers will be observed by TEM.

a decohesion of the matrix from the filler surface; a slippage at the interface between the filler and the matrix; a failure of the network of load paths; a sliding of the polymer chains between neighboring fibers; an effect of the distribution in length of chains or of the distribution in length between the entanglement nodes;

3.2.2. Dispersion state PHEA/Sep3 and PHEA/MB3 are observed under the electron beam of a transmission electron microscope. As revealed in Fig. 5B, the nanoclay is homogeneously dispersed in the PHEA matrix. In Fig. 5C some MB fibers are dispersed homogeneously but there are also heaps of fibers with consequent sizes (1 lm approximately). In summary, TEM studies allow to conclude that the blocking of interactions makes the Maya Blue more difficult to disperse in the PHEA than pristine sepiolite. The interactions between nanoclay and PHEA are crucial to stabilize the dispersion. Without the formation of these links, some aggregates of fillers would be formed. The consequence is translated by the decrease of mechanical properties of nanohybrids even if they have been improved in comparison with the pure PHEA. Ultimately, these interactions between the surface of the fillers and the matrix have an important impact on the mechanical behavior. Note that aggregates are observed both in sonicated MB elastomers and stirred pristine sepiolite elastomers. Aggregations should be involed in the decrease of stress for a given strain. It is now interesting to modulate the ratio of indigo fixed onto Maya Blue fillers.

or a combination of some of these phenomenons. But in every case, the stress-softening shows the existence of interactions between the fillers and polymer. These fillers are having an effect on the PHEA behavior revealed when several consecutive tensile are done. The cycles of load/unload for the PHEA filled with 3 phr Maya Blue (Fig. 5A) are now discussed. As in the case of the pristine sepiolite, loops are present which proves the existence of interactions between the matrix and the Maya Blue. However, the width of all loops is smaller. Thus PHEA/pristine sepiolite and PHEA/Maya Blue interactions are really different. The cycles of load/unload on the PHEA filled with 0.24 phr (amount of indigo in Sep/MB3)

3.2.3. Indigo ratio in Maya Blue fillers The fact to include indigo into the tunnels and the channels of sepiolite decreases the mechanical behavior, because the interactions between sepiolite and matrix are changed. Fig. 6 exhibits the influence of indigo on the mechanical properties of PHEA filled by Maya Blue (PHEA/MB3). Whatever the indigo content in the sepiolite (2, 5 or 8 wt%), the true stress (Fig. 6A) and Young’s modulus (Fig. 6B) are still higher than for the pure elastomer. The Young modulus increase about 35% with regard to pure PHEA. But, for a given sepiolite fraction, the modulus and the true stress are identical no matter how much indigo is included in the

– – – – –

A

B

True stress (MPa)

12

(a) PHEA/Sep3 (b) PHEA/MB3

10

C

a

8

b

6 4 2 0

2µm 0

50

100

150

200

250

300

2µm

350

Strain (%) Fig. 5. First and second stretching curves obtained at different deformations for PHEA/Sep3 and PHEA/MB3 (A) and TEM micrographs of PHEA/Sep3 nanocomposite cut with homogeneous dispersion (B) and PHEA/MB3 nanocomposite cut with aggregate formation (C).

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A

B

14

True stress (MPa)

10

e

1,4

d c b

1,2

True stress (MPa)

(a) pure PHEA (b) PHEA/MB3(2%) (c) PHEA/MB3(5%) (d) PHEA/MB3(8%) (e) PHEA/sep3

12

8

a 6 4 2

(a) pure PHEA (b) PHEA/MB3(2%) (c) PHEA/MB3(5%) (d) PHEA/MB3(8%) (e) PHEA/sep3

1 0,8

e

0,6 0,4 0,2

d c b a

0

0

50

100

150

200

250

300

350

400

0 0,1

0,2

0,3

Strain (%)

0,4

0,5

0,6

0,7

0,8

0,9

1/λ

Fig. 6. Stress–Strain curves (A) and reduced stress (B) for (a) pure PHEA (b) PHEA/MB3 (2%) (c) PHEA/MB3 (5%) (d) PHEA/MB3 (8%) and (e) PHEA/Sep3.

sepiolite fiber (up to 8 wt%). The stress softening and the hysteresis loop are less important for the Maya Blue fillers than for pristine sepiolite fillers (Fig. 5A). Besides, the Fig. 6B exhibits that the stress hardening appears sooner for 8 wt% indigo than for 2 wt% indigo. To conclude at this stage of our developments, the mechanical reinforcement due to the Maya Blue is lower than the one of pristine sepiolite. However colored and strengthened samples were still obtained. The main difference between the two fillers is the changes of interactions caused by the indigo which could prevent the PHEA to interact with the inside of the channels and the tunnels of the sepiolite. 4. Conclusions Nanohybrids have a good potential to replace some conventional material. But in order achieve this goal several skills are important to control the nanocomposite properties: – the micrometric scale with the length of the filler; – the means to turn these fillers initially forming bundles into clearly individual fibers; – the nanometric scale with the modification of the sepiolite surfaces involving fine chemistry inside and outside the fibers. Therefore, it is necessary to optimize the duality between manufacturing process and the chemistry to elaborate a nanomaterial with the expected mechanical properties. For example this article highlights the deep interactions between pristine sepiolite and PHEA elastomer, and the challenges to overcome to use the Maya Blue as fillers. The lack of compatibility at the interface between indigo and PHEA involves a premature failure when compared to the pure elastomer. On the other hand this study has clearly highlighted the high potential of the Maya Blue as a nanopigment to enhance some mechanical properties of bio-inspired multi-scale elastomers. Indeed the sepiolite is both a filler for the PHEA and a matrix for the indigo. It is also multi-functional because it is colored and strengthened by a single filler: ‘‘Maya Blue’’. As next challenge, and in order to optimize this nanocomposite for a given use, it will be interesting to determine the type of interactions at the fillers surface which mainly operates. More of chemical characterisations and mechanical measurements will be done simultaneously to highlight the potential of the reinforcement of nanohybrids.

Acknowledgment The authors are thankful to Tolsa S.A. for providing pristine sepiolite S-9.

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