Controlled interactions between silanol groups at the surface of sepiolite and an acrylate matrix: Consequences on the thermal and mechanical properties

Materials Chemistry and Physics 134 (2012) 417–424

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Controlled interactions between silanol groups at the surface of sepiolite and an acrylate matrix: Consequences on the thermal and mechanical properties Nicolas Volle a , Franc¸oise Giulieri a,∗ , Alain Burr b , Sophie Pagnotta c , Anne Marie Chaze a a b c

Laboratoire de Physique de la Matière Condensé, LPMC (UMR 7336), Université Nice-Sophia Antipolis, 06108 Nice Cedex 2, France Mines ParisTech, CEMEF (UMR 7635), BP 207, F-06904 Sophia Antipolis, France Centre Commun de Microscopie Appliquée, Université Nice-Sophia Antipolis, 06108 Nice Cedex 2, France

a r t i c l e

i n f o

Article history: Received 17 June 2011 Received in revised form 24 February 2012 Accepted 3 March 2012 Keywords: A. Elastomers A. Interfaces Nanofibers Functionalization Reinforcement

a b s t r a c t Elastomer filled with fibrous clay (sepiolite) was manufactured using a hydrophilic elastomer matrix, poly 2-hydroxyethylacrylate (PHEA). The surface silanol groups located onto the channel sides of the sepiolite were functionalized with both octyltrimethoxysilane (OTMS) and 3-methacryloxypropyltrimethoxysilane (MPTMS), which form covalent bonds with the mineral surface and modify their properties. After the grafting of OTMS, PHEA is in contact with a non-polar chain, which prevents matrix–filler interactions. After the grafting of MPTMS, covalent bonds are formed between the acrylate groups of PHEA and MPTMS, which increase the matrix–filler interactions. After functionalization, there is no change in the structural and zeolitic water of the sepiolite which conserves its hydrophilic character. So, an equivalent distribution of the pristine and modified sepiolite is detected in the elastomeric matrices on transmission electron microscopy views of ultramicrotome cuts. The elastomeric macroscopic behavior is therefore related to the PHEA–sepiolite interactions. We show that the stronger the host–matrix interactions, the more important is the reinforcement effect. A direct relation between the interaction strength and the improvement of the mechanical properties was established. The control of the nature, quantity, and localization of the molecules grafted on the sepiolite surface allows us to manage the mechanical properties. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Inorganic nanofibers have the potential to reinforce elastomeric matrices. The large contact surface and the shape of these fillers exert an influence on the nanocomposite behavior. Fillers with fiber-like shapes play an important role in the mechanical, optical, and gas-barrier properties of the nanocomposite, and reinforce them at a low concentration of filler. Although carbon nanotubes exhibit exceptional properties in terms of length, conductivity, etc. [1,2], sepiolite, which is a natural clay mineral, is an attractive filler for polymers because of its acicular form, low cost, and

Abbreviations: (PHEA), poly-2-hydroxyethylacrylate; (HEA), 2-hydroxyethyl acrylate; MPTMS, 3-methacryloxypropyltrimethoxysilane; (OTMS), octyltrimethoxysilane; SepCoup, sepiolite functionalized with MPTMS; SepProt, sepiolite functionalized with OTMS; phr, parts per hundred parts of rubber. ∗ Corresponding author at: Laboratoire LPMC, UFR Sciences, Université de NiceSophia Antipolis, Parc Valrose, 06120 Nice Cedex 02, France. Tel.: +33 4 92 07 61 41; fax: +33 4 92 07 61 44. E-mail addresses: [email protected] (N. Volle), [email protected] (F. Giulieri), [email protected] (A. Burr), [email protected] (S. Pagnotta), [email protected] (A.M. Chaze). 0254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.03.011

abundance [3–5]. Furthermore, the surface of sepiolite is easily modified. Sepiolite is a magnesium silicate that has a porous fibrous structure with internal and external channels running along the length of the fiber. The ideal half-unit cell formula of sepiolite is [Mg8 Si12 O30 (OH)4 (OH2 )4 ·8H2 O] where 4H2 O and 8H2 O represent coordinated and zeolitic water molecules, respectively [6–8]. In sepiolite, SiOH groups are located every 0.5 nm along the side of external channels and Mg(OH2 )2 are the polar groups within the channels. This fibrous clay (10–15 nm × 20–30 nm × 500–5000 nm in size) was used to reinforce elastomers [3,9]. The nanosize, the compatibility, and the homogeneous distribution of the acicular fillers are crucial for the final properties of the material [2,3,9]. Concerning size and distribution, we have very recently proposed processes based on an ultrasonication wave in the near field that leads to well-dispersed sepiolite fibers of controlled size in hydrophilic elastomers [10]. A concentration of 0.5 phr of nanofillers was shown to be sufficient to reinforce the acrylate elastomers. Effective reinforcement is clearly related to good dispersion of the fillers within the matrix. However, their mutual ability to transfer the mechanical load under deformation is a direct result of the surface filler–polymer interactions. This interface control is one of

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the key points [11]. The mobility of the macromolecules in the vicinity of the filler is proposed to have a great influence on the elastomeric mechanical behavior [11–15]. It is difficult to separate the contributions of the network polymer chain, the filler dispersion state, and the filler interfacial interaction to the final macroscopic reinforcement [16,17]. For identical distributions of the fibers in the matrix, the change in the surface functions of the sepiolite will be directly related to the mechanical behavior of the composite. Therefore, our aim is to modify the characteristic of the sepiolite silanol groups and determine the impact of these changes on the final properties of a polyhydroxyethylacrylate (PHEA) matrix. PHEA is an acrylic linear polymer with polar pendant groups. These groups contribute to the hydrophilic character of the polymer and its large capacity to adsorb water. The resultant swollen state leads to the formation of a hydrogel [18]. PHEA is formed from the radical polymerization of hydroxyethyl acrylate (HEA). The acrylic functional groups enable the polymerization reaction and the hydroxyl groups give the hydrophilic properties of the monomer and polymer. Therefore, without modification, the pendant OH groups of PHEA can form H-bonds with the silanol groups of the sepiolite surface. The characteristics of the silanol groups of sepiolite can be changed using chlorosilane or alkoxysilane molecules that form covalent bonds with SiOH groups [19–27]. These covalent bonds (Si O Si C) are more stable than Si O C bonds that are easily hydrolyzed in the presence of moisture [19]. To prevent H-bonding between sepiolite silanol and PHEA, a molecule with a long non-polar hydrogenated chain, i.e., octyltrimethoxysilane (OTMS), will be grafted, thereby minimizing the polar interactions between the fillers and the matrix. The silanol groups of sepiolite will also be functionalized to increase the sepiolite–PHEA interactions by forming a network of covalent bonds that will associate the fillers and matrix. A coupling agent with two types of functionality that is able to covalently interact with the filler and to form covalent bonds with the matrix will be grafted onto the sepiolite SiOH groups to form this covalent network. As the polymerization of the matrix used here is related to the radical polymerization of the acrylate part of hydroxyethyl acrylate, we chose 3-methacryloxypropyltrimethoxysilane (MPTMS), which is a coupling agent with an acrylic moiety on one side and three methoxysilane groups on the other side. To the best of our knowledge, the grafting of MPTMS on sepiolite was never fully described; however, MPTMS has been successfully grafted onto silica [28–30], kaolinite, saponite [31], lead oxide [32], tin oxide [33], and zirconia [34] nanoparticles. The synthesis of the filled elastomer will be realized in two steps: first, the trimethoxysilane portion of the MPTMS or OTMS molecules will be grafted onto the SiOH sepiolite groups, and second, the functionalized sepiolite will be dispersed in the monomer (HEA) for polymerization. The effect of the untreated and functionalized fillers on the thermal and mechanical behavior will be reported and discussed.

2.1. Synthesis Functionalized sepiolite was obtained by mixing sepiolite (5 g), ethanol (50 mL), water (2 mL), MPTMS (the coupling agent) or OTMS (the protect agent) (3 g) for 3 h at 60 ◦ C. The solid obtained after centrifugation of this suspension was washed a time and then heated for 12 h at 110 ◦ C. Finally, the sample was washed and sonicated three times in ethanol. After centrifugation, the solid was air dried at 110 ◦ C. The fillers (pristine sepiolite or functionalized sepiolite) were sonicated (high power US waves – BIOBLOCK 20 kHz – 750 W, microprobe 3 mm) for 20 min in HEA. The dispersions were then left at room temperature for another 20 min before being used. After the addition of 0.05 phr of the photoinitiator, the resulting dispersion was injected into a mold consisting of two glass plates (25 mm × 75 mm) separated by an 0.8 mm thick sealing ring. This mold was then irradiated for 30 min under UV light with an intensity of 360 ␮W cm2 (wavelength 365 nm). The temperature of the mold was free to change within the laboratory (temperature 25 ◦ C) according to the kinetics of the reaction of polymerization. Following irradiation, each specimen was then analyzed for polymerization quality. In this study, we made several kinds of samples: (1) pure PHEA, (2) a nanocomposite with PHEA and pristine sepiolite (PHEA/Sep), (3) a nanocomposite with PHEA and sepiolite that is functionalized with MPTMS (PHEA/SepCoup), and (4) a nanocomposite with PHEA and sepiolite that is functionalized with OTMS (PHEA/SepProt). All the results presented here were monitored with a concentration of 1 phr of sepiolite in PHEA.

2.2. Infrared The functionalization of the sepiolite was studied by FTIR spectroscopy (Perkin Elmer spectrometer Paragon 1000). Sepiolite, and their mixtures with OTMS and MPTMS were characterized before and after heating by diffuse reflection infrared spectroscopy (Eurolabo Minidiff Plus diffuse reflectance device). The powder mixed with KBr (3–5%) was directly placed in the cup and the background spectrum of KBr was subtracted. All reflectance spectra were taken at a resolution of 4 cm−1 for 16–64 scans from 4000 to 400 cm−1 .

2.3. Transmission electron microscopy 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. Microscopy studies were carried out using a Philips CM 12 microscope at an accelerator voltage of 120 kV in the conventional mode.

2.4. Thermal analysis 2. Materials and methods Sepiolite Pangel S-9 was supplied by TOLSA (85% pure and 15% other clays); 3-methacryl-oxypropyltrimethoxysilane (MPTMS) and octyltrimethoxysilane (OTMS) were sourced from ABCR. Hydroxyethylacrylate (HEA) came from Aldrich. The photoinitiator, Irgacure 819, was procured from Ciba. Sepiolite was sonicated (high power US waves – BIOBLOCK 20 kHz – 750 W, microprobe 3 mm) for 20 min in water to disintegrate bundles and obtain a set of isolated fibers. The isolated fibers were dried for 12 h before being used.

Thermogravimetric analysis (TGA) was carried out using a thermogravimetric analyzer (851e from Mettler Toledo). The microbalance has a precision of ±0.1 ␮g and was kept at a constant temperature (∼25 ◦ C) during analyses to avoid the variation of weight measurements with temperature. The thermal behavior was studied to measure the thermal stability of PHEA and PHEA/Sep nanocomposites. Samples of about 20 ± 3 mg were placed in 70 ␮L aluminum pans. The samples were heated from 35 ◦ C to 950 ◦ C at a rate of 7 ◦ C min−1 under a synthetic air (N2 /O2 = 80/20) flow of 50 mL min−1 . At least three replicates were done for each sample.

N. Volle et al. / Materials Chemistry and Physics 134 (2012) 417–424

419

Si-OH

Si-O

Si-OH

Si-O + (CH3O)3Si-R

OH Si-OH Si Si-OH

-CH OH -H O

… - - - - …

Si(OH)-R

O Si O -Si-R Si-O Si R O Si-O -Si-R O

Scheme 2. Kinds of grafting reactions of trisilanol derivatives onto sepiolite silanol groups (schematic representation).

Scheme 1. Schematic representation of sepiolite.

2.5. Mechanical tests A single sample with a dumbbell shape was cut from each film. This shape enabled the use of identical test tubes regardless of the amount of studied fillers. Therefore, the test results were comparable. On the surface of each sample, 4 marks were drawn. They served as strain indicators using a video tracking system. A careful experimental protocol was implemented to determine the mechanical behavior of the 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 s−1 . All tests were conducted at room temperature (25 ◦ C). The specimens were gripped by pneumatic clamps linked to a load sensor with a maximum capacity of 200 N. A video extensometer followed the longitudinal and transverse strains in real time. The force and deformations were simultaneously recorded by a computer. The equilibrium tensile stress–strain measurements were obtained. For each data point, the five samples were tested to obtain an average value. 3. Results and discussion 3.1. Sepiolite functionalization (organo-modified clay) Micronized S-9 sepiolite is found in bundles, so the sepiolite powder was dispersed by sonication in water to disintegrate the bundles and obtain isolated fibers. This allows easier access to the SiOH groups and minimizes the association between the fibers. After sonication, the fibers were dried; we observed that these fibers were then easily dispersed again in hydrophilic solvents such as water, alcohol, and HEA. 3.1.1. Coupling of sepiolite silanol groups with the matrix chains (SepCoup) The reactivity of MPTMS [CH2 C(CH3 )C(O) O (CH2 )3 Si (OCH3 )3 ] begins with the hydrolysis of triethoxymethyl groups. We use sepiolite conserved in normal conditions after drying which leads to the presence of zeolitic water (around 10%). The hydrolysis of the methoxysilane part of OTMS and MPTMS is then favored by this water. Therefore the silanol moieties that are formed may interact with the sepiolite SiOH groups and/or condense to form siloxane bonds (Scheme 2) [35,36]. Our aim is to favor the grafting with the mineral surface and minimize the condensation of the silanol bonds of the MPTMS. Many factors, such as pH, solvents, temperature, and hydration surface, affect the way the silanederivatized molecules are absorbed, condensed, and/or interact with the substrate. All this influences the coupling effectiveness

[19,37]. The chemical interactions between the silane derivatives and silanol groups of the inorganic surface can be accelerated by acid treatment or heating [30,38]. In this work, we chose a heating process to favor the condensation step in preserving the stability of the magnesium cations in the sepiolite structure. First, an excess of MPTMS was mixed with the sepiolite in methanol at 40 ◦ C to favor MPTMS adsorption. The solid obtained after centrifugation of this suspension was washed once and observed via FTIR. Fig. 1 shows the FTIR spectra of pristine sepiolite (Fig. 1a), MPTMS (Fig. 1d), and the quickly dried mixture of sepiolite-MPTMS (Fig. 1c). The bands that correspond to the acrylate groups at 1719 (␯ C O), 1637 (␯ C C), 1454, 1405, 1321, and 1297 cm−1 and to the SiOCH3 groups at 2948 (␯ as C H), 2839 (␯ C H), and 816 cm−1 (␯ Si O) allow us to conclude both that MPTMS is adsorbed onto sepiolite and that all the trimethoxyl groups were not hydrolyzed in the quickly dried mixtures (Fig. 1c). This sample was heated for 12 h at 110 ◦ C and then washed three times by sonication in methanol to extract all the isolated or condensed MPTMS molecules that do not interact with the sepiolite SiOH groups. Fig. 1b shows that afterwards, the bands corresponding to SiOCH3 are not present. The ␯ C H bands are then located at 2984, 2962, 2926, and 2850 cm−1 , the C O band is shifted by 21 cm−1 (from 1719 to 1698 cm−1 ), and low shifts are also evident for the CH2 fingerprint whose bands are now at 1457, 1407, 1326, and 1303 cm−1 . The position of the C O band at 1698 cm−1 is due to the H-bonding of the carbonyl with sepiolite and corresponds to the MPTMS molecules located parallel to the mineral surface [15]. The resistance to three high energy washings and the evolution of the FTIR spectra lead to the proposed covalent bonding between sepiolite and MPTMS. Note that there is no evolution concerning the bands of the sepiolite zeolitic water at 3349, 3241, and 1662 cm−1 . This is consistent with internal and external channels of sepiolite full of water, as the MPTMS molecules are not incorporated into sepiolite and consequently interact only with the external SiOH surface groups of the clay. The quantity of the grafted MPTMS molecules and the nature of the bonding were determined by thermal analyses. Fig. 2 compares the thermal gravimetry of pristine sepiolite (Fig. 2Aa) and grafted sepiolite (Fig. 2Ab). Sepiolite water is removed in 4 stages of heating between 35 and 950 ◦ C: (1) zeolitic water between 35 and 150 ◦ C, (2) first coordinated water between 150 and 350 ◦ C, (3) second coordinated water between 550 and 600 ◦ C, and (4) water related to structural OH between 600 and 950 ◦ C. The structure is then destroyed and enstatite is formed (Fig. 2Aa). Fig. 2Ab corresponds to heated and washed MPTMS/sepiolite. The zeolitic water is equivalent for the pristine sepiolite and grafted sepiolite. This confirms the FTIR results and shows that the MPTMS molecules do not replace zeolitic water and are not located in the internal or external channels. MPTMS begins to degrade at a temperature higher than 300 ◦ C, which corresponds to chemical grafting (Fig. 2Bb). A difference in the weight removed from the

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a b

1719 1698

c

2850 Transmittance (% %)

2926

a d 2839

b

2948 10% c

d

3200

3000 2800 2600 Wavenumber (cm-1)

240 800

1700

1600

1500 1400 1300 Wavenumber (cm-1)

1200

Fig. 1. FTIR spectra of (a) pristine sepiolite, (b) MPTPS and sepiolite mixture after heating followed by washing three times, (c) MPTPS and sepiolite mixture before heating, and (d) MPTPS.

A 100

B

98

50

Temperature (oC) 150 250 350 450 550

Weight removaal (%)

94 92 90 88 86

a

84 82

b

80 0

200

400

600

Temperature (oC)

weigght removal ratee 10-4 (%/ C)

96

650

a

b - 2,5

-5

800

Fig. 2. (A) TG and (B) DTG curves of (a) pristine sepiolite and (b) sepiolite grafted with MPTMS.

pristine sepiolite and grafted sepiolite begins to appear at around 300 ◦ C and the excess loss, which corresponds to grafted MPTMS, is around 4%. If each silanol site reacted with the molecule, the maximum weight loss due to grafting would be around 15–16%. The cross sectional surface area of the hydrolyzed MPTMS is around 0.20–0.30 nm2 , which is the space occupied by MPTMS if the chains were organized perpendicular to the surface. Here, the size of the surface occupied by MPTMS is around 2 nm2 ; one molecule corresponds to 4–5 silanol sites. The molecule can be grafted through a maximum of 2 or 3 sites. Therefore, 1–2 sites are unoccupied and can interact with the carbonyl groups of the grafted molecule. As already suggested by the FTIR results, the grafted molecules are parallel to the sepiolite surface. Irregular grafting can also be considered. However, it is not a network of MPTMS formed on the sepiolite surface, but rather a loose monolayer. The coupling molecules form covalent bonding with some of the silanol to form SepCoup.

3.1.2. Protection of sepiolite silanol groups from matrix chains (SepProt) Our aim is to limit the access of the polar groups (OH, CO) of PHEA to the sepiolite silanol groups. The OTMS molecule comprises methoxysilane groups on one side and a linear hydrogenated chain on the other side. This non-polar chain (octyl chain) has a very weak affinity with HEA and does not form H-bonds.1 The process previously used for MPTMS was applied to graft OTMS onto the sepiolite silanol groups. Fig. 3 shows the FTIR spectra of pristine sepiolite (Fig. 3a), OTMS and sepiolite mixture before heating (Fig. 3b), and OTMS grafted onto sepiolite (Fig. 3c). The band at 2839 cm−1 is missing in Fig. 3c, which indicates the lack of trimethoxysilane groups. Consequently, the OTMS trimethoxy groups are completely

1 Flory–Huggins Interaction Parameter for PHEA: 6.044 for octane and 0.812 for ethanol (60 ◦ C).

N. Volle et al. / Materials Chemistry and Physics 134 (2012) 417–424

a

that there is no excess of grafted molecules. The quantity of water that does not interact with the sepiolite silanol groups (zeolitic and coordinated water) is the same for all samples. Consequently, the difference between the nanocomposites prepared with HEA and modified or unmodified sepiolite will be only due to changes in the silanol groups of sepiolite. The nanocomposites were prepared using the same process, under the same conditions for polymerization, and with the same ratio of sepiolite in HEA. PHEA/Sep, PHEA/SepCoup, and PHEA/SepProt were prepared with 1 phr of filler.

c

Transmitttance (% a.u)

2857

b

2839

3000

2900

W Wavenumber b

2800

((cm-11)

Fig. 3. FTIR spectra of (a) pristine sepiolite, (b) OTMS and sepiolite mixture before heating, and (c) OTMS and sepiolite mixture after heating followed by washing three times.

hydrolyzed. Moreover, the occurrence of organic molecules after high energy washing, shown by the band at 2857 cm−1 , allows us to assume the formation of siloxane bonds between sepiolite and the octyl chain. Fig. 4 compares the weight removal from sepiolite before and after grafting. The mass loss at around 300–350 ◦ C confirms chemical bonding between OTMS and sepiolite; 4 (w/w)% of molecules are grafted onto sepiolite. This does not correspond to a dense layer where molecules occupied 0.2 nm2 , but rather disperse hydrocarbon chains that occupy ∼2 nm2 . Therefore, SepProt is formed. In this configuration, as for the MPTMS grafting, some of the sepiolite silanol groups are still present, but due to the presence of octyl chains, their interactions with PHEA are impeded. 3.2. Effect of SEP/PHEA interactions on the nanocomposite behavior Note that, after functionalization of sepiolite with MPTMS or OTMS, not all the silanol groups of the surface are modified and

A 100

3.2.2. Impact of the coupled and protected SiOH groups on the composite thermal stability The thermal stability of polymeric materials is usually determined by thermogravimetric analysis (TGA), which monitors weight removal as a function of temperature. As the samples decompose in air, it is an oxidative degradation. Clays usually act as insulators and mass transport barriers to volatile products produced during thermal treatments [39–41]. In air, the barrier effect also acts to prohibit the diffusion of oxygen from air to the polymer bulk and, therefore, inhibits the oxidative process [42]. The TGA results of PHEA, PHEA/Sep, PHEA/SepCoup, PHEA/SepProt composites are shown in Fig. 6 and the data are summarized in Table 1. Fig. 6 displays portions of the typical thermograms obtained for pure PHEA. Four different steps of weight loss [43,44] that persist in the filled PHEA are detected.

Temperature (oC)

B

98 96 Weight removal (%)

3.2.1. Microstructural characterization To determine the influence of the fiber distribution in PHEA, samples of 1phr PHEA/Sep, PHEA/SepProt, and PHEA/SepCoup are cut using an ultramicrotome and observed via microscopy. Fig. 5 shows the micrographs of these samples examined using transmission electron microscopy (TEM). Good dispersion is evident in the three samples; however, the distribution is slightly less homogeneous for PHEA/SepProt. As the fibers are isolated by sonication in HEA before grafting and polymerization, the difference in behavior is related to the sepiolite aggregation during polymerization. The comparable distribution obtained for pristine sepiolite and OTMSand MPTMS-treated sepiolite implies that the difference in properties of nanocomposites will be due to other factors. In this case, the performance modifications will be related to the characteristics of the filler/PHEA interphase.

94 92 90 88

a

86 84

b

82

Derivative weight removal (%/ C)

3100

421

0

0

200

400

a -0,0004

-0,0008

-0,0012

80 0

200

400

600

600

800

Temperature (oC) Fig. 4. (A) TG and (B) DTG curves of (a) pristine sepiolite and (b) sepiolite grafted with OTMS.

b

422

N. Volle et al. / Materials Chemistry and Physics 134 (2012) 417–424

Fig. 5. TEM view of an ultramicrotome cut of filled nanocomposites (A) PHEA/Sep, (B) PHEA/SepCoup, and (C) PHEA/SepProt.

Table 1 degradation temperatures and residue mass determined by thermal analyses. Samples

Initial degradation temperature (◦ C) T15%

Degradation temperature (◦ C) Tmax

Residue mass at 600 ◦ C (%)

Pure PHEA PHEA/Sep PHEA/SepCoup PHEA/SepProt

335 348 356 350

433 432 433 434

1.2 1.8 1.9 1.9

The first step corresponds to the loss of residue monomer and oligomer volatilization, and absorbed water (below 200 ◦ C). The second step at 380 ◦ C (between 200 and 400 ◦ C) corresponds with depolymerization and consequent oligomer volatilization. The main step, at 422 ◦ C, corresponds to polymer degradation, which is delayed because of the prior depolymerization [18,45]. The more intense peak in step 3 compared to step 2 is a feature of the acrylate polymer [46]; Finally, the last mass weight loss, which occurs at 560 ◦ C, is related to the loss of carbonaceous residues (Fig. 6). We observe that the first step is similar for all the elastomer samples (5% removal; Fig. 6). The hydration and polymerization of all samples are similar. The second step of degradation is evident by comparing the temperature variation at which 15% degradation occurs. We observe that the thermal stability of the PHEA matrix in air is significantly improved by the presence of modified and pristine sepiolites (Table 1). A temperature increase of 21 ◦ C for T15% is observed in the presence of SepCoup (356 ± 1 ◦ C) as compared to PHEA (335 ± 1 ◦ C). The temperatures of degradation of PHEA/Sep (348 ± 1 ◦ C) and PHEA/SepProt (350 ± 1 ◦ C) are comparable. For the PHEA/SepProt composites, the interactions between PHEA and

Fig. 6. TGA and DTG thermograms of pure PHEA and PHEA/Sep nanocomposite samples that contain 1 phr sepiolite. Effect of the sepiolite surface treatment.

sepiolite silanol groups are prevented by the non-polar chains; therefore, the insulator effect and/or barrier effect attributed to an increase in the diffusion pathway related to clay space can explain this behavior. For PHEA/Sep and PHEA/SepCoup, there is an additional effect of the limitation of the diffusion gases, which could also be related to expansion in the molecular interactions between the sepiolite and matrix. Sepiolite acts as a “crosslinking agent” [47,48]. This effect has already been observed for sepiolite/PU and sepiolite/starch nanocomposites. We remark that the clay itself can catalyze the degradation of the matrixes which is not the case in these experiments [49,50]. As far as Tmax is concerned (step 3), regardless of the sepiolite used (organically modified or not), no significant variations of the thermo-oxidative degradation of the PHEA matrix are observed. The composites are stabilized (Table 1); however, this benefit could have been counterbalanced by the catalytic action of sepiolite on the thermal degradation pathway [5,22,47,51]. The mass loss of hydrated sepiolite between 35 and 950 ◦ C is 14%, and it is around 7% after the zeolitic water was removed (see Fig. 2a). The residue mass at 600 ◦ C accounts for the sepiolite content of the composites and is around 0.8–0.9%% for 1% PHEA/Sep, PHEA/SepCoup, and PHEA/SepProt (see Figs. 2a and 4a). An increase in the residue mass to around 0.8–0.9% is attempted, and a residue mass of 0.6–0.7% is obtained. For pure PHEA, the residue obtained at 600 ◦ C in air (1.2%) is related to the formation of carbonaceous residues. The presence of pristine or modified sepiolite does not increase the amount of the formed carbonaceous residues. In summary, a concentration of 1% of fillers leads to a minor effect on the thermal behavior of the nanocomposites. However, the temperature increase of the second step of degradation and the lack of catalytic degradation of the matrix due to sepiolite are both positive indications of the thermal stability and possible flammable properties of the nanocomposite.

N. Volle et al. / Materials Chemistry and Physics 134 (2012) 417–424

with acrylate groups that could co-polymerize with HEA during the polymerization process. The sepiolite skin will then be like a block-part of the polymer chain and form a new elastomer network with strong connections. An obvious increase in the stress of PHEA/SepCoup at all strains (Fig. 7d) compared to PHEA/Sep (Fig. 7c) is detected. Similar results are obtained with samples of 0.5 phr PHEA/Sep (Suppl.). To sum up, the interactions between MgOH2 and PHEA reinforce the nanocomposite. The physical bonds created between the sepiolite silanol groups and PHEA amplify this reinforcement effect. The integration of sepiolite into the PHEA network by chemical bonding leads to an even more pronounced effect.

7

d 6

c

Stress (MP Pa)

5

b 4 3

a

2 1 0 0

100

200 Strain (%)

300

423

400

Fig. 7. Stress–strain curves of pure PHEA (a) and nanocomposites: PHEA/SepProt (b), PHEA/Sep (c), PHEA/SepCoup (d).

3.2.3. Impact of coupled and protected SiOH groups on the mechanical tensile properties (tests) In addition to the shape of the fillers and their dispersion within the matrix, the mechanical properties of the nanocomposites depend on the interactions at the filler/matrix interface [12,17]. The sepiolite surface influences the mobility and the polymer state in the vicinity of the particle and this affects the mechanical properties of the nanocomposite. The stress–strain curves of the PHEA nanocomposites are shown in Fig. 7. The pure PHEA curve will be used as a reference (Fig. 7a). For the same applied strain (200%), the stress level reached by PHEA/Sep (2.5 MPa) (Fig. 7c) is more important than the stress level reached by pure PHEA (1.1 MPa). This improvement is attributed to the good dispersion of sepiolite in the PHEA matrix at a nanometer scale and to strong interactions at the interface between sepiolite and PHEA. The “nanometer effect” is related to the “physical crosslinks” effect. At the solid surface, intermolecular interactions related to physical adsorption are related to free energy components such as London dispersive force, Debye inductive force, Keesom orientational forces, Lewis acidbase interactions, and hydrogen bonding [52]. It is noteworthy that sepiolite has numerous silanol groups on the external edges of the fibers that can form H-bonds with the numerous alcohol pendant groups of PHEA [3,53] (Scheme 1). The important effect detected at a low concentration of sepiolite could be related to all of these interactions. Compared to those of PHEA (Fig. 7a) and PHEA/pristine sepiolite (Fig. 7c), the strain–stress curve of PHEA/SepProt (Fig. 7b) in which a large portion of the sepiolite is covered by non-polar chains is below that of PHEA/Sep and above that of PHEA. The decrease of around 20% in stress relative to PHEA/Sep highlights a strong influence of the sepiolite SiOH groups in the interactions between the PHEA matrix and pristine sepiolite. The non-polar chains cover the external surface of sepiolite but not the internal surface of the channels, which results in the maintenance of the hydrophilic properties of sepiolite because of the coordinated water coverage (Scheme 1). Therefore, the increase of around 15% in stress relative to PHEA (Fig. 7b versus a) corresponds to the interaction of PHEA chains with both the grafted non-polar chains and the external channels that are still accessible. This shows that when the silanol groups are protected, a portion of the sepiolite surface is still effective for filler dispersion and the matrix–surface filler interactions. Note that if the silanol groups are accessible and the sepiolite channels are blocked by indigo molecules, then a similar strain/stress decrease is observed (Fig. 2 in supplementary information). However, in this case, the decrease is related to a clear inhomogeneous dispersion as shown in Fig. 5 of our previous paper [10]. It is interesting to examine the influence of strength interactions between sepiolite and PHEA. Therefore, the sepiolite was grafted

4. Conclusion Previously, we have shown that the properties of PHEA–sepiolite nanocomposite are controlled by the dispersion quality, the factor form, and the quantity of sepiolite. In this study, we showed that the surface quality is another important factor for the mechanical properties; an increase in the matrix/sepiolite interactions improves the mechanical reinforcement of the elastomer composites. First, the distribution of sepiolite in the PHEA matrix is equivalent for pristine sepiolite, sepiolite modified with non polar group, and sepiolite that forms covalent bonds with HEA. Secondly, for the same amount of filler, increased interactions between the elastomer macromolecules and the filler surface augment the reinforcement. This could be related to the decrease in the surface mobility and the dragged slippage of elastomer chains over the filler surface. These factors are known to be important in terms of reinforcement. We also showed that the mechanical behavior of the nanocomposite is very sensitive to the variations in interactions between the elastomer and sepiolite surface. On the contrary, at these filler concentrations, the thermal behavior of the sepiolite/PHEA nanocomposites is not significantly influenced by the change in PHEA/sepiolite surface interactions. The control of the nature, quantity, and localization of the molecules grafted on the sepiolite surface allowed us to manage the mechanical properties. A deeper study of the mechanical comportment of the preliminary results described here will afford new information about these systems. In the future, we intend to focus on the dynamic properties. Acknowledgement The authors are thankful to Tolsa S.A. for providing pristine sepiolite S-9. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matchemphys.2012.03.011. References [1] F. Dalmas, J.Y. Cavaillé, C. Gauthier, L. Chazeau, R. Dendievel, Compos. Sci. Technol. 67 (2007) 829–839. [2] L. Bokobza, Polymer 48 (2007) 4907–4920. [3] L. Bokobza, A. Burr, G. Garnaud, M.Y. Perrin, S. Pagnotta, Polym. Int. 53 (2004) 1060–1065. [4] A.F. Vargas, V.H. Orozco, F. Rault, S. Giraud, E. Devaux, B.L. Lopez, Composites A 41 (2010) 1797–1806. [5] E. Dusquesne, S. Moins, M. Alexandre, P. Dubois, Macromol. Chem. Phys. 208 (2007) 2542–2550. [6] K. Brauner, A. Preisinger, Stuktur und Entstehung des Sepioliths, Tschermaks Min. Petr. Mitt. 6 (1956) 120–140. [7] M. Rautureau, C. Tchoubar, Clays Clay Miner. 24 (1976) 43–49. [8] C. Serna, M. Rautureau, R. Prost, C. Tchoubar, J.M. Serrarosa, Bull. Groupe Franc¸. Argiles. 26 (1974) 153–163. [9] L. Bokobza, J.P. Chauvin, Polymer 46 (2005) 4144–4151.

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