HNTs nanocomposites using an electron transferring interaction method

Accepted Manuscript Title: Preparation of high performance NBR/HNTs nanocomposites using an electron transferring interaction method Authors: Shuyan Yang, Yanxue Zhou, Peng Zhang, Zhuodi Cai, Yangping Li, Hongbo Fan PII: DOI: Reference:

S0169-4332(17)32020-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.030 APSUSC 36564

To appear in:

APSUSC

Received date: Revised date: Accepted date:

3-5-2017 28-6-2017 4-7-2017

Please cite this article as: Shuyan Yang, Yanxue Zhou, Peng Zhang, Zhuodi Cai, Yangping Li, Hongbo Fan, Preparation of high performance NBR/HNTs nanocomposites using an electron transferring interaction method, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation of high performance NBR/HNTs nanocomposites using an electron transferring interaction method

Shuyan Yang*, Yanxue Zhou, Peng Zhang, Zhuodi Cai, Yangping Li, Hongbo Fan† (School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan City, 523808, P.R. China)

Highlights 

The electron transferring interaction is provoked between electron-rich -CN groups and electron-deficient aluminium atoms of halloysite nanotubes (HNTs).



This interaction facilitates a well dispersion of HNTs in Nitrile-butadiene Rubber (NBR) matrix even at high HNTs incorporation.



Due to the special interaction, NBR/HNTs nanocomposites show high light transmittance and good mechanical properties.

Abstract: Interfacial interaction is one of the key factors to improve comprehensive properties of polymer/inorganic filler nanocomposites. In this work, a new interfacial interaction called electron transferring interaction is reported in the nitrile-butadiene rubber/halloysite nanotubes (NBR/HNTs) nanocomposites. The X-ray photoelectron spectroscopy (XPS) and in-situ controlling temperature Fourier transform infrared spectroscopy (FTIR) have confirmed that electrons of electron-rich -CN groups in NBR can transfer to the electron-deficiency aluminum atoms of HNTs, which packs a part of NBR molecules onto the surface of HNTs to form bound rubber and stabilize the homogeneous dispersion of HNTs with few agglomeration as revealed by scanning electron microscope (SEM) and dynamic mechanical analysis (DMA) performances, even at high HNTs addition, resulting in high light transmittance. The tensile strength of NBR/30wt%HNTs nanocomposites is about 291% higher than pure * †

Corresponding author, E-mail: [email protected] Corresponding author, E-mail: [email protected]

NBR, without sacrificing the elongation at break. Keywords: Nanocomposites; Interface; Mechanical properties; Infrared (IR) spectroscopy; Electron transferring interaction

1. Introduction Acrylonitrile butadiene rubber (NBR) is the random polymerization of acrylonitrile and butadiene and its important applications, such as the excellent oil- or fuel-resistant and long running under high temperature properties, in modern industries strongly depend on the ratio of acrylonitrile and butadiene monomer units. However, due to the poor mechanical properties of pure NBR, reinforcement technology has been essential for NBR industry. Since the advantages of nano-filler have been realized in polymer field[1], NBR/inorganic filler nanocomposites have draw tremendous attention not only in academic, but also in rubber industries for the unique relation between micro-structure and macroscopic properties[2-9]. The nanoscale material with at least one dimension in the nanometer range is a bridge between isolated atoms or small molecules and bulk materials, the types of which in the inorganic filler fields can be divided as carbon black[7], carbon nanotubes[2], montmorillonite[10], silica[8, 9], halloysite nanotubes (HNTs)[11-15], graphene[16], etc.. Among these fillers, HNTs is a unique and facile nature mineral with the chemical formulation of Al2[Si2O5](OH)4·nH2O, the outer and inner diameters of which are about 10~50 nm and 5~20 nm, respectively, with the lengths in a wide range from 0.02 to ＞30μm[17], and has been widely investigated not only in rubber industry[15], but also in drug delivery[13, 14] field in the last decades[17, 18]. By modifying HNTs with sodium hydroxide[19], sulphuric acid aqueous solution[20], alcoholic solution of boric acid[21], rubber aging-resistant[15] or phtalocyanine pigments[22], people found that the thermal stability and the mechanical properties of rubber/HNTs nanocomposites increased and the flammability and fire hazard decreased, with respect to the rubber/unmodified HNTs nanocomposite. Others found that the incorporation of neat HNTs into NBR by using a two-roll mill could get a

homogenous dispersion of HNTs in NBR/HNTs nanocomposites at low HNTs content[23], leading to a decrease in the cure time and scorch time and a slight increase in tensile strength. However, the interactions between NBR and unmodified or modified HNTs in these articles have not yet been studied in details. Due to the electron-rich feature of the nitrile group (-CN), i.e., Lewis base, some works suggested that the nitrile group would make a hydrogen bond with the silanol groups of silica, resulting in strong NBR-silica interactions [24-28]. As a result, the addition of NBR into styrene-butadiene rubber (SBR)/silica [25-27] or natural rubber(NR)/silica composites [24] could facilitate the well dispersion of silica in rubber matrices, leading to an increase in the bound rubber fraction, modulus, tensile strength, heat buildup and crack resistance. By comparing to the functionalized alumina nanoparticle by trimethoxyvinylsilane [28], Faghihi came to a conclusion that the mechanical properties of NBR/as-received alumina nanoparticle nanocomposites were much better than that of NBR/functionalized alumina nanoparticle, which Faghihi attributed to hydrogen bonds between hydroxyl groups presented on the surface of nanoparticle and nitrile groups of NBR. Nevertheless, compared with silanol groups-rich on the surface of silica nanoparticles, fewer silanols and aluminols are exposed in the edges of HNTs [29], which would make it much more difficult to form hydrogen bonds with NBR. On the other hand, HNTs is an electron-deficiency substance as the metal atoms of HNTs such as aluminum are capable to accept foreign electrons into their empty orbits [29-33]. As a consequence, HNTs can interact with electron-rich groups such as the N and O atoms in the benzoxazolyl group via electron transferring interaction [29-31]. In this work, we reported a series of NBR/as-received HNTs nanocomposites with varied HNTs contents prepared by solution casting methods in order to mix the two compositions into a better dispersion state. X-ray photoelectron spectroscopy (XPS) and Fourier Transform infrared spectroscopy (FTIR) were performed to confirm the electron transferring interaction between HNTs and nitrile groups of NBR, which was ascribed to be responsible for the well homogenous dispersion of HNTs, high

light transmittance

and

good

mechanical

properties

of

NBR/HNTs

nanocomposites. 2. Experimental section 2.1 Materials and samples preparation NBR with acrylonitrile content of 34 wt% (35LM) was supplied by South Korea Kumho petrochemical Co., Ltd. HNTs was purchased from Sigma-Aldrich and used as-received. Dicumyl peroxyide(DCP), 99.0%, was provided by Aladdin Co., Ltd. Ethyl acetate was AR grade and used without further purification. The weight fraction of HNTs was normalized to that of NBR (100wt%), while the weight fraction of DCP was set at 1.5wt% with respect to that of NBR. NBR/HNTs nanocomposites with different weight fraction of HNTs were marked as NBR/x wt%HNTs, here, x refers to the weight fraction of HNTs. Different weight fraction of HNTs was added into the ethyl acetate solution in a flask with vigorous agitation, and then the mixing solutions were ultrasonically treated for 10 min. After that, NBR and DCP were dissolved in the mixture and stirred vigorously for 24h. The NBR/HNTs nanocomposites films were obtained by pouring the mixtures into a homemade-mould and evaporating ethyl acetate in a vacuum at 50°C until constant weight. Finally, the solvent-free NBR/HNTs nanocomposites films were vulcanized between two stainless steels at 160°C under 9 MPa for 10 min. 2.2. Characterization XPS spectra of NBR, HNTs and NBR/20wt%HNTs were recorded by using an X-ray photoelectron spectrometer (LVAC-PHI 1800, Ulvac-Phi Company) with an aluminum (mono)

K

source (1486.6 eV). The aluminum K  source was operated at

15 kV and 10 mA. All core level spectra were referenced to the C 1 s neutral carbon peak at 284.7 eV. The FTIR analysis was conducted by a Bruker Tensor 27 spectrometer. Spectra were taken from 4000 cm-1 to 400 cm-1 with the resolving power of 2 cm-1. The in-situ controlling temperature FTIR was competent to explore the electron transferring

interactions between electron-rich groups and electron-deficiency metal atoms [29, 34]. The test was carried out on a Bruker Tensor 27 spectrometer equipped with a heating furnace under the heating rate 5°C /min from 40 to 240°C. Spectra were taken from 4000 cm-1 to 400 cm-1 with the resolving wavenumber of 2 cm-1. To understand the morphology, such as the dispersion state and interfacial cohesion, in NBR/HNTs nanocomposites, scanning electron microscopy (JSM-6701F, Japan Electron Optics Laboratory Co., Ltd.) tests of samples were performed with an accelerating voltage of 5.0 kV. The fracture surface was taken from the broken cross section, then stuck on a metal plate by conducting resin and plated with a thin layer of gold before any observations. Dynamic mechanical analysis (DMA) was carried out in a TA instruments (Q800, USA) with a heating rate of 3°C/min from -60~80°C at a constant frequency of 10Hz . The samples with sizes of 10 mm (length) x 4mm (width) x 0.2mm (thickness) was performed under a tension mode with controlled strain by 0.1%. The light transmittances of the neat NBR and NBR/HNTs nanocomposites films with nearly 0.2 mm thickness were measured from 200 to 900 nm using an Ultraviolet visible spectrophotometer (UV-2550, Shimadzu Co., Ltd., Japan). Finally, the mechanical properties such as tensile modulus at 100% elongation, tensile strength, and elongation at break were measured according to ISO/DIS37-1994 specifications. Instron3367 electron tensile testing machine was used with the crosshead speed of 500 mm/min. The mechanical properties data were the mean value of three times measurements. 3. Results and discussion 3.1. Evidence for electron transferring interaction between NBR and HNTs It is well known that XPS analysis is capable of detecting the variation of binding energy of special atoms under different chemical environment [18]. The information of XPS curves and precise binding energies of characteristic atoms are disclosed in Fig.1. As one can be seen from Fig.1(A), the binding energy of N1s in NBR is about 399.3eV[35]. When blending with 20wt% HNTs, the binding energy of N1s moves to the higher level, about 0.2 eV higher than that of pure NBR. As we all

know, the binding energy of a core-level electron depends on the chemical environment surrounding it intensely. Generally, the core-level binding energy of a certain atom increases as the electron density around the atom decreases [36]. The higher binding energy of N1s in NBR/20wt%HNTs reveals that the electron clouds of N atom decreases. On the other hand, in Fig.1(B), the binding energy of Al2p of NBR/20wt% HNTs shifts towards lower binding energy[11, 12], as compared to that of pure HNTs, implying that the electron cloud of Al2p of NBR/20wt%HNTs becomes dense, which is in accordance with the change trend of the N atom and suggests that interactions may be taken place between NBR and HNTs[11, 12, 18, 37]. The shift of the induced dipole moment of certain groups would be evident when interactions between these groups are presented, which can be detected using FTIR as the infrared absorption is very sensitive to the change in dipole moment [36]. As shown in Fig.2, the absorptions around 912cm-1 and 540cm-1 are ascribed to the stretching vibration of Al-OH [37] and the deformation of Al-O-Si [18] in the pure HNTs, respectively. However, for the FTIR spectrum of NBR/20wt%HNTs, the stretching vibration of Al-OH evolves to 915cm-1, while there is about 4 cm-1 red shift for the deformation of Al-O-Si, which is an another evidence of interactions between -CN groups and Al atoms of HNTs. To further investigate the interactions between NBR and HNTs, in this work, the in-situ controlling temperature FTIR technology was taken to explore the electron transferring interactions between electron-rich groups and electron-deficiency metal atoms [29, 34]. The evolution in FTIR spectrum of NBR/20wt%HNTs during heating is presented in Fig.3. The peak around 2235 cm-1 is assigned to the stretching vibration of -CN group with one lone pair electron at the relatively low temperature. With increasing the measurement temperature, the intensity of the peak decreases and the peak position blue shifts monotonically, which is the strong proof of electron transferring interactions between -CN groups and Al atoms[29, 34]. As discussed in previous works[38-40], the inner lumen of HNTs is only 15nm in diameter and full with air, which prohibits other small molecules to enter unless one uses a vacuum pump to remove air from the lumen. Thereby, the microstructure of NBR/HNTs

nanocomposites can be illustrated in Fig.4 as follow: when blending NBR with HNTs in the good solvent, the aggregated HNTs would be dispersed into nanotubes one by one in the polar solvent with the aid of ultrasonically treatment, resulting in a perfect mixing. As the solvent evaporates, NBR molecules would deposit onto the surface of the single nanotube. The -CN groups have lone electron pairs and the capability of donating electrons. On the other hand, the Al atoms on the surface defects or the edge of HNTs can accept lone electron pairs based on the electron-deficiency feature of Lewis acid. As a consequence, electron transferring interactions take place between -CN groups and Al atoms, as confirmed in Fig.3. The NBR molecule plays the role of the electron donor while the Al atom of HNTs acts as the electron acceptor in the present system. 3.2. The morphology of NBR/HNTs nanocomposites SEM technology is powerful in exploring the dispersion state of inorganic fillers and the cohesion details of the interfacial regions in polymer matrix [9]. The SEM graphs of NBR/HNTs nanocomposites were taken from the fracture surface of the corresponding nanocomposites, as shown in Fig.5. When 5wt%HNTs was incorporated into NBR, only a few single HNTs can be found within the observed fracture surface, without any aggregation, as shown in Fig.5(A), indicating a well dispersion of HNTs in the NBR matrix. After the incorporation of HNTs reaches 20wt% (Fig.5(B)), more and more single HNTs appear and distribute randomly, only a few conglomerations in the NBR/20wt%HNTs nanocomposite. When the addition of HNTs continues to increase up to 30wt%, the single HNTs dispersion is still the predominance in the NBR matrix, however, the most difference from Fig.5(B) is that the conglomeration of HNTs becomes denser in Fig.5(C), suggesting the dispersion of HNTs is getting worse. For better understanding the interfacial adhesion between NBR and HNTs, we selected a few nanotubes in Fig.5(B) to be analyzed with a magnification factor of 5000, as disclosed in Fig.5(D). The surface of HNTs becomes much rougher, one can see that some NBR molecules deposit on the surface of HNTs. Moreover, the edge region between HNTs and NBR is unclear and the whole nanotube embeds into the

NBR matrix, which suggests a good wetting between HNTs and NBR and is consistent with the results from XPS and FTIR measurements in the form of a small scheme inserting Fig.5(D). 3.3 Dynamic mechanical analysis of NBR/HNTs nanocomposites The DMA measurement permits us to investigate the main chains motion at the transition interval from the glassy state to the rubbery state, even including secondary or tertiary transitions [9, 41]. The temperature dependence of storage modulus ( E ' ) and loss factor ( Tan



) of NBR and NBR/HNTs nanocomposites are depicted in Fig.6

and Fig.7, respectively. It can be seen that

E

'

decreases only a little when 5wt%

HNTs is added, which may be ascribed to the weak interaction between NBR and HNTs. In this case, only a few interaction points are established so that HNTs may act as a “plasticizer” for NBR, allowing NBR chains to slip on the surface of HNTs[9]. After that, the

E

'

increases slightly at low HNTs content while changes moderately

at high loading at the temperature before the transition from the glassy state to the rubbery state. Two reasons may be responsible. First, the rigidity of HNTs may contribute to the modulus increase according to the principle of simple superposition, as compared to NBR. Second, the interactions between NBR and HNTs would introduce a “rigid” interfacial layer with reduced segment dynamics, especially at high HNTs content, which will be discussed later. Fig.7 illuminates the loss factor versus temperature of all samples, and their parameters, such as glass transition temperature (Tg),

tan 

and bound rubber fraction,

are listed in Table1. From Fig.7 and Table1, the Tg values of the present samples decease slightly with HNTs, which depends on the intensity of intermolecular interaction [9, 42, 43] and indicates the electron transferring effect is a weak interaction, which is confirmed by slight changes of corresponding groups in FTIR measurements. The maximum

tan 

decreases slightly when HNTs is not more than

10wt%. As HNTs content reaches 20wt% or more, know,

tan 

tan 

drops sharply. As we all

is an indicator of “free” segments participating in the intermolecular

internal friction. The lower tan



, the less “free” segments become. Thereby, with

increasing HNTs, a part of NBR molecules have been trapped onto the surface of

HNTs by means of electron transferring interactions as proved above. Furthermore, to our interest, the tan



peak of NBR follows the normal distribution, however, for

NBR/HNTs nanocomposites, in the temperature interval from 0~60°C, a broad shoulder emerges and non-normal distribution of

tan 

peak can be observed,

indicating NBR molecules have been turned into bound rubber partially[44]. By fitting tan



curves with the aid of Origin8.0 software under the Gauss method, we can

predict the bound rubber of each nanocomposite without any assumption, as shown in the inserting graph in Fig.7 as an example. About 11.3% bound rubber is found in the NBR/5wt%HNTs. Continuing to add HNTs from 10wt% to 30wt%, the bound rubber increases up to 18.9%, 25.4% and 28.7%, respectively, suggesting more and more NBR molecules attach to the surface of nanotubes, which is also responsible for the decrease in the maximum

tan 

value. Moreover, the increase in bound rubber

becomes moderate after the incorporation of HNTs exceeding 20wt%, which is closely associated with the slight conglomeration of HNTs at high HNTs content proved by SEM measurement. 3.4 The UV-vis light transmittance of NBR/HNTs nanocomposites The light transmittance of NBR or NBR/HNTs nanocomposites films with the thickness of ~200μm were determined by UV-vis spectrophotometer as shown in Fig.8. It can be seen that all the testing samples are highly transparent within the visible light wavelength range (400-760 nm) and the light transmittance is moderately decreased due to the addition of HNTs. The maximum reduction value of NBR/HNTs nanocomposites within the visible light wavelength range is about 30% as compared to the pure NBR films, but the maximum transmittance value of NBR/30wt%HNTs nanocomposite film is still about 45%. As we discussed before, the electron transferring interaction is favorable to construct a bound rubber layer surrounding HNTs, which would facilitate the compatibility between NBR and HNTs, leading to a homogeneous dispersion and few aggregation of HNTs even at 30wt%. As a consequence, NBR/HNTs nanocomposites films exhibit high light transmittance within the visible light wavelength range. 3.5 The mechanical properties of NBR and NBR/HNTs nanocomposites

The stress-strain behaviors of NBR and NBR/HNTs nanocomposites are shown in Fig.9 and details of mechanical properties, such as 100% modulus, tensile strength and elongation at break, are also listed in Table2. From Table2, the mechanical properties of pure NBR are very poor, which is useless in practice and is why needs to be reinforced by nano-fillers. The addition of HNTs can enhance the 100% modulus and tensile strength of NBR/HNTs nanocomposites monotonically, along with a larger elongation at break. When the incorporation of HNTs is about 30wt%, the tensile strength of NBR/30wt%HNTs nanocomposites is about 4 times of that of pure NBR. This can be explained as follow [8, 9]: When an exerted stress is applied to NBR/HNTs nanocomposites, the stress runs along the rubber chain. With the aid of electron transferring interaction and bound rubber, the applied stress can pass to the HNTs effectively and NBR molecules slip along the surface of nanotubes without decohesion, allowing the network to relax to a more perfect regime and changing the local stress condition by means of stress homogenized distribution. As a result, this reinforcing effect of the formed interfacial region becomes more effective to influence the modulus and tensile strength of NBR/HNTs nanocomposites, leading to higher 100% modulus, tensile strength and elongation at break. 4. Conclusion NBR/HNTs nanocomposites have been prepared and details of properties, such as interfacial adhesion, dispersion state, bound rubber, light transmittance and mechanical properties, are investigated thoroughly. By mixing NBR and HNTs in the good dispersion state, electrons of -CN groups, a kind of electron-rich group and electron donor candidate, can transfer to the aluminum atoms of HNTs, an electron acceptor, which has been proved by XPS, transmission FTIR and in-situ controlling temperature FTIR. The electron transferring interaction can bring in segment motion reduction layer onto the surface of HNTs effectively, leading to an increase in bound rubber with HNTs and facilitate a homogeneous well-dispersion of HNTs in NBR matrix. As a result, NBR/HNTs nanocomposites are still transparent even at 30wt% HNTs. Finally, the 100% modulus, tensile strength and elongation at break of

NBR/HNTs nanocomposites are much higher than that of pure NBR, indicating a well reinforcing effect of HNTs for NBR.

Acknowledgements The authors gratefully acknowledge the financial support from The National Natural Science Foundation of China (No.51303026) and Open Research Funds of Guangdong Key Laboratory of High Performance and Functional Polymer Materials (No. 20151003).

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Fig.1. The binding energies of special atoms in NBR and NBR/20wt%HNTs

Fig.2. The FTIR shifts of aluminum groups in HNTs and NBR/20wt%HNTs

Fig.3. In situ FTIR spectra of NBR/20wt%HNTs during heating

Fig.4. The scheme for illustrating the microstructure of NBR/HNTs nanocomposites

Fig.5. The morphology of fracture surface of: (A) NBR/5wt%HNTs; (B) NBR/20wt%HNTs; (C) NBR/30wt%HNTs and (D) NBR/20wt%HNTs (with a magnification factor of 5000)

Fig.6. Temperature dependence of storage modulus for NBR and NBR/HNTs nanocomposites

Fig.7.

Tan 

versus temperature for NBR and NBR/HNTs nanocomposites

Fig.8. The UV-vis light transmittance of NBR and NBR/HNTs nanocomposites

Fig.9. The stress-strain behaviors of NBR and NBR/HNTs nanocomposites

Table 1 The data from DMA measurement of NBR and NBR/HNTs nanocomposites Code

Tg(°C)

tan 

NBR

-3.06

1.53

---

NBR/5wt%HNTs

-3.59

1.43

11.3%

NBR/10wt%HNTs

-2.35

1.38

18.9%

NBR/20wt%HNTs

-3.89

0.83

25.4%

NBR/30wt%HNTs

-3.65

0.68

28.7%

max

Bound rubber fraction

Table2 The mechanical parameters of NBR and NBR/HNTs nanocomposites 100%

modulus Tensile

strength Elongation at break

Sample (MPa)

(MPa)

(%)

NBR

1.08

1.40

164

NBR/5wt%HNTs

1.37

2.44

237

NBR/10wt%HNTs 1.33

2.52

266

NBR/20wt%HNTs 1.60

3.35

306

NBR/30wt%HNTs 3.30

5.48

197