PB)-b-PS triblock copolymer by controlling the size of silica nanoparticles with electron beam irradiation

Composites Science and Technology 70 (2010) 215–222

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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech

Preparation and properties of nanocomposites based on PS-b-(PS/PB)-b-PS triblock copolymer by controlling the size of silica nanoparticles with electron beam irradiation Mahendra Thunga a,b,*, Amit Das b,**, Liane Häußler b, Roland Weidisch a,b, Gert Heinrich b a b

Institute of Material Science and Technology (IMT), Friedrich Schiller University Jena, Löbdergraben 32, Jena 07743, Germany Leibniz-Institute of Polymer Research Dresden, Hohe Strasse 6, D-01069 Dresden, Germany

a r t i c l e

i n f o

Article history: Received 2 July 2009 Received in revised form 23 September 2009 Accepted 24 September 2009 Available online 30 September 2009 Keywords: A. Hybrid composites A. Polymer Electron beam irradiation E. Sol-gel methods A. Nano composites

a b s t r a c t In this work, silica particles are synthesized in a pre-cross-linked triblock copolymer with styrene-b-(styrene-co-butadiene)-b-styrene (LN4) chain architecture by in situ sol–gel method. Using the proposed method, an easy access to control size and distribution of silica nanoparticles, generated inside the polymer matrix, was achieved by varying the cross-linking density of the polymer network with the aid of electron beam (EB) irradiation. The morphological investigations from atomic force microscopy and small angle X-ray scattering measurements reveal that this technique not only allows to control the size of silica particles but also is helpful in restoring the microphase separation in LN4. Dynamic–mechanical and stress–strain behavior also suggested that the reinforcement effect of the sol–gel silica in the cross-linked elastomers was increasing with decreasing the particle size. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction In the recent years, the development in the field of nanocomposites and nanostructured materials has been vigorous; they have attracted extensive scientific interest as a special class of materials both in terms of fundamental and practical applications. As the molecular level interaction between heterogeneous phases in polymer based nanocomposites leads to a synergistic combination of properties, an increase in such interactions can be attained only though a super fine dispersion of inorganic nanoparticles in organic matrix. In order to enhance the performance and properties of elastomeric materials, it is very common to incorporate some filler particles like carbon black, precipitated silica, clay, etc. into the rubber matrix. Apart from carbon black, silica is also important reinforcing filler since it provides a unique combination of transparency, tear resistance, abrasion resistance and additional properties [1]. Moreover, in recent years significant attention has been given to the use of precipitated silica as a filler in tyre industry in order to increase the performance of tyres [2–4]. In general, silica is incorporated in * Corresponding author. Present address: Department of Materials Science and Engineering, 2220 Hoover Hall, Iowa State University, Ames, IA 50011, USA. ** Corresponding author. Address: Institute of Material Science and Technology (IMT), Friedrich Schiller University Jena, Löbdergraben 32, Jena 07743, Germany. E-mail addresses: [email protected], [email protected] (M. Thunga), [email protected] (A. Das). 0266-3538/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2009.09.014

the rubbers through mechanical mixing. However, due to the presence of strong inter particle hydrogen bonding a fine dispersion of silica particles is very difficult to achieve [5]. In order to overcome this problem some types of silane coupling agents is used with some specific processing conditions for attaining fine dispersion of silica particles within the elastomer matrix. Other than using coupling agents in mechanical mixing, sol–gel processing is also a versatile technique which makes it possible to produce wide variety of hybrid materials with a fine dispersion of silica particles and provides existing materials with novel properties. The main advantage of this technique lies in the formation of inorganic oxide frameworks starting from molecular precursors at ambient temperature. The applicability of this method is quite simple and it was used as a direct method for modifying many commercially available polymers [6–10]. It was reported by Yuan and Mark [11] that a control over the molecular level interactions at nanoscales can be directly tuned by controlling the size of the silica particles during in situ sol–gel processing. They showed that the pH level of the reaction medium plays a vital role in the hydrolysis and the condensation process of the alkoxysilanes which ultimately governs the size of silica particle. In another work, it was described that the sizes and size distributions of in situ silica fillers can be controlled by the concentration of base catalyst (more specifically, diethylamine) in a single-step or multi-step procedure. Similar type of investigations aiming to control the size and dispersion of in situ generated silica particles by varying the cross-linking

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density in the base material was already investigated by Ikeda and Kohjiya [8] and Breiner and Mark [12] In the pathway of controlling the nanostructural parameters, they varied the density of cross-linking either by varying the Mw of the base material or by changing the concentration of the cross-linking agent. The most significant drawbacks in both these techniques are, either it is necessary to synthesis new materials with well defined Mw for attaining desired silica particle size or to go through complicated chemical curing process which demands varieties of toxic curing ingredients, long cure times and high energy consumption. However, the EB-irradiation technique used in the present work for cross-linking the nanostructured block copolymers not only provides an easy access to restore the primary properties of structured materials like strength, stiffness, toughness and transparency of the materials but also is useful in varying the cross-linking density very easily. On the other hand, the toxic nature that will be evolved in the materials during conventional chemical curing process can be completely avoided. In addition to that, it requires short treatment time, avoids involvement of radioactive sources and helpful in eliminating some expensive additives. Concerning the elastomers from block copolymers, the unique phase behavior of block copolymers from the phase separated structure has made them as potential materials for thermoplastic elastomers. However, the temperature range within which the block copolymers can act as glassy materials is limited by the glass transition temperatures of the microphases. In diene based block copolymers, it was found that cross-linking is an effective method for enhancing the physical and mechanical properties [13]. Inclusion of nanoparticles within this microstructure can further provide a tool for scaling the material properties at nanometer level. Depending on the ratio between the domain size of the microstructure in block copolymer to the size of nanoparticles, the physical properties can be eminently modified without affecting the overall material behavior. Therefore, by varying the size of silica particles that are synthesised in situ via sol–gel technique can easily control this ratio. The present work is a novel approach for preparing block copolymer based nanocomposites with a control synthesis of sol– gel silica by electron beam irradiation. We aim to find out the role of high energy EB cross-linking of S–(SB)–S block copolymer on the growth of silica nanoparticle during in situ sol–gel reaction. The interrelation between the three-dimensional cross-linked network structure and the growth of silica particles is studied. Further, the significance of cross-linked microstructure of block copolymer in controlling the morphology of silica fillers that contribute for enhancing the ultimate mechanical properties is discussed. 2. Experimental The synthesis of S–(SB)–S triblock copolymer (LN4), provided by BASF AG Co., is described by Knoll and Niessner [14]. LN4 is linear triblock copolymer and the molecular characterization is summarized in Table 1. The samples were prepared as plates with 1 mm thickness by compression moulding at 200 °C for 5 min followed by annealing the sample to room temperature prior to the release of the pressed plates. The compression moulded plates were irradiated in the air at room temperature with absorbed doses of 0, 50, 100, 150, 300, and 500 kGy by an electron beam accelerator ELV-2 at the Leibniz Table 1 Molecular characterization of LN4. Material Mn (g/mol)

LN4

3. Results and discussion

Mw/Mn Total PS–(SB)–PS S/B ratio in Morphology PS (wt.-%) the random content copolymer (wt.-%) block

119,000 1.30

66

Institute of Polymer Research Dresden. The beam parameters are maintained at an absorbed dose of 2 kGy per pass with an average dose rate of about 10 kGy/h. To estimate the gel fraction of the cross-linked polymer the sol– gel extraction was carried out in an extractor–container with uninterrupted reflux (Soxhlet apparatus). The solvent used is toluene and the temperature was maintained above the boiling point of solvent. After every 9 min, the solvent overflows and the whole extraction took 15 h to complete. The gel was dried in a vacuum drier at 50 °C for 24 h. In order to generate silica particles inside the polymer matrix tetraethylorthosilicate (TEOS) was used as a silica precursors. The cross-linked sample sheets were subjected to swelling in the TEOS solution for 72 h. After removal from the TEOS solution, the rubber was dipped into 10% n-butyl amine aqueous solution for 24 h to get silica particles by the condensation reaction of TEOS. The obtained sheet was dried for several days at room temperature. Finally, the sheet was again dried in a vacuum oven for 8 h at 70 °C. The Atomic Force Microscopy (AFM) specimens were prepared from the bulk sample of 1 mm thickness. The fresh cut samples were made with Leica RM 2155 microtome (Leica, Nussloch, Germany) equipped with a diamond knife at a cryo-temperature below the glass transition temperature of PB phase in the block copolymer. The AFM measurements were performed in tapping mode by a Dimension 3100 NanoScope IIIa (Veeco, USA). The scan conditions were chosen according to Magonov et al. [15] (free amplitude >100 nm, set-point amplitude ratio 0.5) in order to get stiffness contrast in the phase image. The Small Angle X-ray Scattering (SAXS) measurements were performed using a homemade three pinhole collimation system with a Rigaku 2D rotating anode generator (Cu Ka radiation, k = 0.1542 nm). The data were corrected for absorption and background scattering obtained in transmission geometry at room temperature. The raw SAXS data were fitted with the form factor of the sphere model with Gaussian size distribution using the software modules supplied by NIST [16]. Dynamic–mechanical analysis (DMA) has been done using an ARES-LS2 rheometer. Measurements have been carried out on test specimens with dimensions of 30  8  1 mm3 in torsion mode using torsion rectangular geometry tools to characterize the glass transition temperature of the triblock copolymers. The temperature sweep test was carried out between 80 and 120 °C at a frequency of 1.0 rad/s and 0.01% strain amplitude. Elastic storage modulus (G0 ) and viscous loss modulus (G00 ) as a function of temperature have been measured. Tensile tests were performed with a ZWICK 1456 universal testing machine according to ISO 527 (specimen type 5A) at a crosshead speed of 50 mm/min. Multisens mechanical equipment was used to neglect the slippage of the specimens from the clamps and the compliance of the testing machine and thus to exactly evaluate the strain of five samples by measuring the elongation in the gauge length only. From this elongation and the applied load during the testing the engineering strain (e) and engineering stress (r) were calculated, respectively. Due to limited availability of samples, the tests were conducted on small size dumbbell shape specimens with the sample length of 40 mm. The thermogravimetric analysis (TGA) was conducted with Q5000-TA Instruments under nitrogen atmosphere from 40 to 750 °C with a heating rate of 10 °C/min.

16/68/16

1:1

Wormlike

3.1. Influence of cross-linking in LN4 The molecular parameters of the triblock copolymer LN4 are shown in Table 1. The samples at different cross-linking dosage

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are represented as LN4–X and the samples after synthesis of silica inside the cross-linked matrix are designated as LN4–Si–X where Si and X stand for silica and cross-linking dosage respectively. In this section, cross-linking of triblock copolymer LN4 is discussed on the basis of gel content obtained after irradiation of the samples at different dosages ranging from 0 to 500 kGy. On irradiation, first the cleavage of C–H bond takes place in the polybutadiene phase (PB) which is followed by generation a macro radical on the carbon atom of diene unit. This free radical could react with the neighboring chain to form a cross-link, which further results in another free radical for propagating of self-cross-linking in LN4 during irradiation. Fig. 1a depicts the gel fraction of LN4 at different radiation dosages. The gel content is increasing up to 150 kGy and on further increase of the radiation dose the gel content does not change significantly. The increase in gel content with the increase in the radiation dose is mainly attributed to the formation of a random three dimensional cross-link network [17]. Beyond an optimal radiation dose i.e. above 150 kGy, the marginal increase in the gel content up to 92.7% is ascribed to the balance of various competitive reactions (air oxidation, ether formations and chain scissioning) [18]. However, no traces of chemical reactions are observed over the surface of the sample, and the transparency in pure LN4 at 1 mm thickness was observed to be restored even after irradiation at 500 kGy. It was reported earlier that the cross-linking in copolymers consisting an aromatic ring like styrene segments which are covalently bonded to butadiene segments is mainly controlled by the wt.-%

100

92.7%

Gel content (%)

80



60

r ¼ 2C 1 þ 40

2C 2 k

 
1 k  2 1 þ 2:5/PS þ 14:1/2PS k

ð1Þ

and, C1 is expressed as [24] 20

a

0 0

100

200

300

400

500

Dose (kGr)

LN4-0 kGy LN4-50 kGy LN4-100 kGy LN4-150 kGy LN4-300 kGy LN4-500 kGy

1.5

2

σ N / (λ - 1/λ ))(1+2.5φ s+14.1φs ) (Mpa)

of polystyrene segments (PS) in the copolymer chain architecture [19]. Manion and Burton [20] have showed that the aromatic ring attached to the polymer chain can increase the resistance to radiation damage by serving as ‘energy shrinks’. Such influences are expected to be similar in both styrene–butadiene random copolymer (SBR) and styrene–butadiene–styrene (SBS) triblock copolymers. However, in pure LN4 as the PB segments are in the form of SBR middle block which is again constrained between PS outer blocks in SBS triblock china architecture, this special chain architecture could further enhance the high-energy transfer from PB segments to PS segments during irradiation. Thus, the materials were observed to be stable even at high EB doses and on the other hand high irradiation dose is required for preserving the microstructure in LN4. Fig. 1a, the gel fractions with respect to crosslinking dosage, reveals that the micro structure is expected to be constrained between the cross-linked network only above 150 kGy of irradiation dosage. Therefore, in order to enhance the material behavior without loosing the structural properties from microphase separation, the sol–gel reaction was conducted only on sample with cross-linking dosage above 150 kGy. Initially, the cross-linking density at different EB doses in LN4 was calculated. The influence of variation of cross-linking dosage on mechanical properties of pure LN4 was investigated by stress–strain experiment in tensile mode and these results are used for deducing the cross-linking density in a quantitative manner. It was observed that the tensile behavior of LN4 was strongly influenced by the cross-linking dosage and a detail description of the mechanical properties will be discussed later in this article. The molecular weight between the cross-links (Mc), which specify the cross-linking density, is evaluated by fitting the tensile data of all the samples with Mooney-Rivlin model for rubber elasticity. The Mooney-Rivlin model is mathematically represented by Eq. (1) [21,22]. In this equation the PS phase in LN4 is considered as filler inside the elastomeric matrix [23].

1.0

2C 1 ¼

qRT

ð2Þ

Mc

where r is the tensile stress, /PS is the fraction of PS acting as filler, q is the density of the polymer, R is the gas constant, T is absolute temperature. As shown in Fig. 1b the tensile data is fitted with Eq. (1) and the Mc is calculated by substituting the C1 value from Eq. (1) in Eq. (2) [24]. The Mc values for all the samples are listed in Table 2 and a decreasing trend in Mc was observed with the increase in irradiation dosage. Such observation leads to conclude that the increase in cross-linking density is in proportion with the irradiation dosage. The decreasing trend of Mc between effective cross-links was also calculated in terms of cross-linking density (Vc) by employing Flory and Rehner equation, Here Vc includes both chemical and physical interactions and it is determined by Eq. (3) [25,26].

2

0.5 Table 2 physical and chemical cross-linking densities of irradiated LN4.

0.0

b 0.2

0.4

1/λ

0.6

0.8

Fig. 1. (a) Gel content values for LN4 irradiated at 0, 50, 100, 150, 300 and 500 kGy, (b) Moony-Rivilin plot for cross-linked LN4 irradiated at different dosage. Represents a systematic increase in the rN with the increase in irradiation dose.

Sample

V  105 (g mol/cc)

Mc (kg/mol)

0 50 100 150 300 500

– – – 0.22 1.658 3.786

64.12 31.98 22.52 15.41 8.6 4.4

218

Vc ¼

M. Thunga et al. / Composites Science and Technology 70 (2010) 215–222



 2

1 lnð1  v r Þ þ v r þ v1 v r 2V v r1=3

ð3Þ

where v1 is the polymer–solvent interaction parameter, V the molar volume of the solvent, vr the volume fraction of the rubber in the swollen gel, vr was calculated using the following relation

mr ¼

ðDs  F f Aw Þq1 r 1 ðDs  F f Aw Þq1 r þ As qs

ð4Þ

where vr, Ds, Ff, Aw, As, qr and qs are volume fraction of rubber, deswollen weight of the sample, fraction insoluble, sample weight, weight of the absorbed solvent corrected for swelling increment, density of rubber and density of solvent respectively. The calculated Vc values at different cross-linking doses were summarized in Table 2 and the increasing trend of cross-linking density with the irradiation dose was once again verified by applying Flory and Rehner equation. A variation in cross-linking density can directly influence on the amount of imbibed solvent by the cross-linked LN4 during swelling in TEOS. As these swollen gels at different cross-linking densities were used to generate silica particles by sol–gel reaction (a details description about the synthesis procedure is given in the Section 2), it is expected that the swelling volume could significantly influence the growth of silica particles, therefore for revealing such expectations AFM and SAXS techniques are used to investigate the morphology of the composites. 3.2. Morphological investigations of LN4–Si composites 3.2.1. Atomic force microscopy (AFM) As the silica particles are generated by hydrolysis of TEOS inside these swollen gels (Fig. 2), the structural growth of silica particles was expected to be strongly dependent on the cross-linking density or swelling volume of these gels. Therefore, the influence of the cross-linking density on the growth of the silica particles is studied by the AFM images. The AFM micrographs of pure LN4 after cross-linking at 500 kGy is shown in Fig. 3a and the morphologies of LN4–Si-150, LN4–Si-300 and LN4–Si-500 composites are shown in Fig. 3b–d respectively. In our early communications the physical and mechanical properties of uncrosslinked LN4 was studied and correlated to its morphology [27a,b]. It was observed that LN4 with symmetric S–(SB)–S triblock copolymer architecture shows thermoplastic elastomeric nature which arises from weakly segregated wormlike microstructure. From the AFM micrograph (Fig. 3a) it can be observed that LN4-500 possesses a weak phase separated structure even after cross-linking up to 500 kGy. However, after the sol–gel reaction, spherical shaped silica particles are generated inside the block copolymer matrix which can be clearly seen from Fig. 3b–d. Further, the size and distribution of the silica particles are strongly influenced by the characteristics of cross-linked network. Interestingly, the size of the silica particle is decreasing with the increase of the cross-linking density from 150 to 500 kGy. Hence, the cross-linking density is observed to be a crucial parameter in governing the size of silica nanoparticles generated in LN4 during sol–gel reaction. The decrease in the size of the silica particle was quantitatively investigated by modeling SAXS data and is discussed in the next section.

Hydrolysis

≡ Si − OC2 H5 + H2O

Condensation

≡ Si − OC2 H 5 + HO − Si ≡ ≡ Si − OH + HO − Si ≡

≡ Si − OH + C2 H5OH

≡ Si − O − Si ≡ + C2 H5OH ≡ Si − O − Si ≡ + H2O

Fig. 2. Sol–gel reaction of TEOS to form silica particles.

3.2.2. Small angle X-ray scattering (SAXS) The local morphology observed from AFM micrographs is reconfirmed and quantified by studying the bulk morphology using SAXS data. Fig. 4a shows the corresponding raw SAXS data for samples examined in the previous section along with pure LN4. From Fig. 4a it can be observed that for both LN4 and LN4-500, the SAXS pattern appears to be identical with a single Bragg reflection arising from their weakly segregated microphase separated structure. Appearance of similar scattering pattern in both samples reveals that cross-linking of PB phase in LN4 has insignificant influence on the microstructure. However, synthesis of silica inside crosslinked network, causes a dramatic increase in the low-q scattering. The distinct variation in low-q scattering with the increase in the cross-linking density in the composites is ascribed to the increase in vol.-% of silica generated in side LN4 during sol–gel reaction. The persistent phase separation in LN4–Si-500 even after sol–gel reaction results in showing a pronounced peak that is exactly at the same peak position as of pure LN4 and LN4-500 indicated with dotted line in Fig. 4a. This result from the bulk sample provides the evidence that the phase separation is completely preserved even after sol–gel reaction. In order to obtain a quantitative picture about size and vol.-% of the silica particle, the SAXS data is modeled using the form factor of sphere with Gaussian size distribution as shown in Fig. 4b–d [16]. The vol.-% of silica generated inside LN4 at different cross-linking densities can be obtained by normalising the low-q scattering. On the other hand, fitting the form factor of the model with the experimental data results in attaining average size of silica particle. The fit parameters from the composites revealed that, cross-linking of LN4 is showing strong influence on both average particle size and vol.-% of silica inside LN4. A decrease in silica particle size from 200 to 30 nm accompanied with a decrease in vol.-% of silica from 22 to 17 vol.-% can be observed with the increase in irradiation dosage from 150 to 500 kGy. Such decreasing trend in the size and vol.-% of silica with the increase of cross-linking dosage is attributed to the enhanced network constrains attained by increasing the number of cross-links per unit volume, i.e. cross-linking restricts the growth of silica particles during sol–gel reaction. As a matter of fact, the observed monodispersed silica particle morphology in Fig. 3b–d is mainly due to the homogeneous nature in the cross-linking network. Such uninformed cross-linking network can only be archived by EB irradiation. Therefore adopting EB technique instead of chemical crosslinking methods in the present work is playing an efficient role in controlling the morphology of composites. 3.3. Dynamic–mechanical analysis (DMA) The dynamic mechanical properties of LN4, LN-500 and LN4–X– Si composite (LN4-150–Si, -300–Si, -500–Si) are measured over a broad temperature range (80 to 120 °C) for studying the interaction between heterogeneous phases. The temperature dependency of loss modulus (G00 ) and storage modulus (G0 ) are shown in Figs. 5a and 5b respectively. In our earlier communication, the phase behavior and phase miscibility between PS and PB phases in LN4 was quantitatively investigated from their dynamic mechanical properties [27a,b]. As LN4 is a triblock copolymer with heterogeneity in the molecular architecture from PS and PB chain segments, the thermodynamic incompatibility between PS and PB segments from their respective microdomains result in showing two distinct glass transition temperatures. From Fig. 5a, it can be observed that after cross-linking, the Tg-PB is shifted towards higher temperature from 42 °C to 31 °C in case if LN4-500. A considerable shift in Tg-PB to higher temperatures was also observed in LN4-150 and LN-300 (not shown here). The increase in Tg-PB with respect to cross-linking dosage is attributed to the increase in degree of cross-linking. Cross-links in PB phase hinder the co-operative

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M. Thunga et al. / Composites Science and Technology 70 (2010) 215–222

Fig. 3. AFM images of LN4 and LN4–Si composites with scale in micrometers. (a) LN4-500, (b) LN4–Si-150, (c) LN4–Si-300, and (d) LN4–Si-500.

4

10

3

10

2

10

1

10

0

LN4 LN4-500 LN4-Si-150 LN4-Si-300 LN4-Si-500

I (arb unit)

I (arb unit)

10

a 10

10

4

10

3

10

2

10

1

10

0

LN4-Si-150 sphere model

b

-1

0.01

0.01

0.1

10

2

10

1

10

0

LN4-Si-300 sphere model

I (arb unit)

I (arb unit)

10

3

c 0.01

0.1

q (A°-1)

0.1

q (A°-1)

q (A°-1)

10

3

10

2

10

1

10

0

LN4-Si-500 sphere model

d 0.01

0.1

q (A°-1)

Fig. 4. SAXS investigations for (a) LN4 and LN4–Si composites, (b) sphere model with Gaussian size distribution fitted with raw SAXS data for LN4–Si-150, (c) LN4–Si-300, and (d) LN4–Si-500 kGy.

segmental motions and require higher temperature for segmental relaxation. On the other hand, the Tg-PB in composites also showing

a shift towards high temperatures i.e. the Tg-PB of LN4-500–Si is staying at a higher temperature with reduced peak intensity than

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Tg shift

G'' ( Mpa)

-42 °C

10

8

10

7

10

6

-31 °C

LN4 LN4-500 LN4-Si-150 LN4-Si-300 LN4-Si-500

a -60

-40

-20

0 20 40 Temperature °C

60

80

100

9

10

LN4 LN4-500 LN4-Si-150 LN4-Si-300 LN4-Si-500

8

G' ( Mpa)

10

7

10

6

10

b -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 Temperature °C

Fig. 5. Dynamic mechanical property profile for weakly segregated block copolymer LN4 and LN4–Si composites at different cross-linking dosage (a) loss modulus (G0 0 and (b) storage modulus (G0 ).

100

Weight (%)

80

T Min-nanocomposites

LN 4 LN 4-Si-150 LN 4-Si-300 LN 4-Si-500

T M in -LN4

60 40 20

23. 6 19.8 16.2

0

0.7 100

200

300

400

500

600

700

800

900

Temperature °C Fig. 6. Thermo gravimetric analysis (TGA) for LN4 and LN4–Si composites: weight loss curves.

LN4-500. Such observations are attributed to the interactions between silica and polymer chains that hinder the relaxation process. These interactions were described as the physical interaction by Ikeda et al. [28], they showed that these interactions are mainly

due to the entrapment of the polymer chains into the silica. During the conversion of TEOS to silica nanoparticle in sol–gel reaction, initially the silica particles are in the form of swollen gel of SiO2 network and the neighbor polymer chains to surface of this swollen gel could get entrapped over the surface during the condensation process. Such interactions can be clearly illustrated in terms of variation in the amplitude of the Tg-PB peak in DMA. As shown in Fig. 5a the amplitude of the Tg-PB is increasing along with slight shift in peak position towards high temperature with the increase in the cross-linking dosage in the composites (LN4–Si-150 to LN4– Si-500). Such trend in Tg-PB peak can be corroborated with the variation in size and vol.-% of silica particles in the block copolymer matrix which have a direct influence in controlling the interaction between silica particles and polymer chains. As observed from the model parameters from SAXS data, the reduction in vol.-% of silica content is comparatively small to that of reduction in size of the particle, i.e. the vol.-% of silica is decreasing from 24 to 15 vol.-% whereas the particle size is decreasing from 200 to 30 nm. Thus it can be confirmed that the surface area of the silica particles that are interacting with neighboring polymer chains will be very high in case of LN4-500–Si when compared to LN4–Si-150. So a high interaction between silica particles and block copolymer chains can be expected in LN4-500–Si. On the other hand the microstructure in LN4 can also show a considerable influence on the Tg behavior. The physical entanglements which arise from entropic and enthalpic interplay between PS and PB chains in LN4 are expected to get disentangled during swelling in TEOS. However, as this disentanglement is influenced by the applied irradiation dosage. i.e. at high cross-linking dose (LN4–Si-500) the phase separation of LN4 is completely preserved whereas at low cross-linking doses i.e. in LN4–Si-150 and LN4–Si-300 very less traces of phase separation can be observed from the AFM micrographs (Fig. 3b and c). Similarly, the elastic modulus G0 was also observed to be significantly influenced by the variation in the interaction between silica and polymer chains. From Fig. 5b it can be observed that over the whole temperature window LN4-500–Si is showing high modules then other materials. This can be attributed to the reinforcement effect caused by the confinement of finely dispersed Si particles within the densely cross-linked LN4 with strong particle–polymer interaction. In the glassy zone i.e. below Tg-PB, in spite of high silica content in LN4-150–Si and LN4-300–Si when compared to LN4500–Si, they show a substantial reduction in the elastic modulus. The major reason behind such behavior could be due to the disordering of the microphase separation in LN4 as explained above. In the intermediate temperature range i.e. in between 0 and 40 °C, which is named as the rubbery zone of PB-rich phase. In this regime G0 of pure LN4 and LN4-500 are observed to be strongly dependent on temperature. Where as an increase in the modulus with a plateau like behavior can be seen for the composites. The elastic modulus in this regime was significantly influenced by the change in size and content of silica particles that are controlled by the cross-linking densities. At high temperature range i.e., above the Tg-PS (100–150 °C) a rubbery plateau which characterize the cross-link network can be seen and the observed variations in the plateau modulus could be due to collective contribution from cross-linking density and reinforcement from silica in the block copolymer. 3.4. Thermogravimetric analysis (TGA) The thermal stability and the mode of decomposition of the composites at different cross-linking dosage were comparatively studied with respect to pure LN4. The main aim of this task is to accurately identify the wt.-% of silica inside LN4 at different cross-linking densities. From Fig. 6 it can be observed that the temperature for minimum weight loss is reaching early for composites

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when compared to pure LN4 and also a nominal decrease in the weight of composites (LN4-150–Si, LN4-300–Si, LN4-500–Si) can be observed in between 120 and 380 °C. The slight decrease in the weight could be due to gradual evaporation of the entrapped solvent (TEOS, n-butylamine, water, etc.) which were used during synthesis of silica in cross-linked LN4-matrix. However, above this temperature the stability of LN4 is observed to be enhanced by the incorporation of silica. The enhanced thermal stability for composites is attributed to the presence of interaction between the silica particles with the block copolymer chains. The residual wt.-% of the composites obtained after 750 °C is shown in Fig. 6. As the nanosilica particles are quite stable without loss in weight until 750 °C, the residual weight in the composites correspond to the wt.-% of silica generated inside polymer matrix during sol–gel reaction. The observed wt.-% of silica content at different crosslinking dosages are exactly in accordance with the SAXS results and the decreasing trend in silica content with the increase in cross-linking dosage was reconfirmed consistently. 3.5. Mechanical properties The stress–strain behavior of the composites is compared with pure and cross-linked LN4 for investigating the efficiency of the phase separation, cross-linking and silica filling on the ultimate mechanical properties. From the stress–strain behavior, LN4 with a wormlike structure reveals elastomeric nature of the curves (Fig. 7). One can see that LN4 is yielding at low strains followed by a continuation of strain hardening which resembles like cured rubber. From Fig. 7 it can be observed that there is a strong influence of cross-linking dosage on the tensile behavior of LN4. The strength of the material was enhanced over the whole strain window with the increase in the cross-linking dosage. Here at this stage, the enhancement in the stress–strain behavior in pure LN4 was attained easily by selective cross-linking of PB phase with the aid of EB irradiation. However, cross-linking did not show a significant impact on yield strength. Whereas, with the incorporation of silica particles inside LN4-matrix, a strong increase in the yield strength along with an overall enhancement in the tensile strength in both LN4-500–Si and LN4-300–Si can be observed (as the phase separation is preserved only above 150 kGy of irradiation dosage, we focused our studies only on samples with cross-linking dosage above 150 kGy). On the other hand LN4-150–Si is showing a poor tensile response than that of pure LN4 (not shown here) which is ascribed to the disordered PS–PB microphases formed during

30

LN4-150

LN4-300-Si

LN4

LN4-300

Stress (MPa)

24

LN4-500-Si

LN4-500

221

sol–gel reaction. Hence, the phase separation aided with the in situ generated silica particles is necessary for attaining reinforcement effect in block copolymer based composites. In some of our ongoing studied, a similar wt.-% of commercial silica (20 phr = 16.6 wt.%) is added in LN4 via conventional melt mixing methods for mixing nanosilica with average particle size of 40 nm. As the phase separation in LN4 is strongly effected by the processing conditions during melt mixing the enhancement in the tensile strength in terms of yield stress and stress at 100% strain are staying less then that of the in situ sol–gel materials. For illustrating the reinforcement (RE) effect in the composites, the RE has been calculated for the investigated materials [29]. It is an approximation of the difference in modules at 100% elongation, RE can be mathematically represented as:

RE ¼ ðM 100;filled  M 100;pure Þ=wt:  % of filler

ð5Þ

It was observed that the RE values are 0.13 and 0.17 for LN-300– Si and LN-500–Si respectively. The increasing trend in RE with cross-linking dosage is attributed to the variation of morphology of composites in terms of size and wt.-% of silica particles. As a matter of fact, larger surface area from silica filler will provide a high interaction with the polymer matrix. Therefore, in spite of low wt.-% of silica in LN4-500–Si (16.2%) then LN4-300–Si (19.8%), a high RE value is mainly due to the smaller silica particle size. Further, it can be observed that with the enhancement in toughness, the strain at break is constantly decreasing. Such reduction in the ductility of the materials is attributed to the enhancement in the rigidity of the elastic network due to high crosslinking density. 4. Conclusion An easy access for controlling the size of the silica nanoparticles in triblock copolymer (LN4) through in situ sol–gel reaction was achieved by varying the cross-linking density using EB-irradiation technique. The molecular characteristics of the three-dimensional cross-link network are found to be crucial in controlling the growth of silica particles within the block copolymer matrix during sol–gel reaction. The microphase separation in LN4 was completely preserved by cross-linking above 150 kGy, whereas below that the microstructure gets disentangled through swelling in TEOS during sol–gel reaction. Presence of microphase separation even after sol– gel reaction along with silica nanoparticles in LN4-300–Si and LN4500–Si contributes in reinforcing effect for the hybrid composites. Application of this new technique in conventional elastomer can provide an access to tailor the physical and mechanical properties of their hybrid composites through varying the size of the nanoparticles. Thus, the proposed method in preparing hybrid composites was observed to be quit effective and easy, where controlling the size and content of the inorganic fillers by varying the cross-linking density inside the organic polymer matrix can scale the desired properties.

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Acknowledgements 12 LN4-Pure Crosslinked LN4 Nanocomposites

6

0 0

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120

180

240

300

360

420

480

540

600

Strain (%) Fig. 7. Stress–strain behavior of pure LN4 along with the enhancement in the tensile behavior by cross-linking and filling with in situ sol–gel Si-particle.

Authors gratefully acknowledge the financial support from German Research Foundation (DFG), Germany. References [1] Wagner MP. In: Morton M, editor. Rubber technology. New York: Van Nostrand Reinhold; 1987. p. 86. [2] Noordermeer JMW. Recent developments in rubber processing, leading to new applications such as the ‘‘green tyre”. Macromol Sympos 1998;127:131. [3] Byers JT. Fillers for balancing passenger tire tread properties. Rubber Chem Technol 2002;75:527.

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[4] Okel TA, Waddell WH. Silica properties/rubber performance correlation. Carbon black-filled rubber compounds. Rubber Chem Technol 1994;67:217. [5] Kohjiya S, Ikeda Y. Reinforcement of general-purpose grade rubbers by silica generated in situ. Rubber Chem Technol 2000;73:534. [6] Wang SP, Xu J, Mark JE. Shear and biaxial extension measurements of reinforcement from in-situ precipitated silica. Rubber Chem Technol 1991;64: 746. [7] Ikeda Y, Kameda YJ. Preparation of ‘‘green” composites by the sol–gel process: in situ silica filled natural rubber. Sol–Gel Sci Technol 2004;31:137. [8] Ikeda Y, Kohjiya S. In situ formed silica particles in rubber vulcanized by the sol–gel method. Polymer 1997;38:4417. [9] Murakami K, Osanai S, Shigekuni M, Ito S, Tanahashi H, Kohjiya S, et al. Silica and silane coupling agent for in situ reinforcement of acrylonitrile–butadiene rubber. Rubber Chem Technol 1999;72:119. [10] Sun D, Zhang R, Liu Z, Huang Y, Wang Y, He J, et al. Polypropylene/silica nanocomposites prepared by in-situ solgel reaction with the aid of CO2. Macromolecules 2005;38:5617. [11] Yuan QW, Mark JE. Reinforcement of poly(dimethylsiloxane) networks by blended and in-situ generated silica fillers having various sizes, size distributions, and modified surfaces. Macromol Chem Phys 1999;200:206. [12] Breiner JM, Mark JE, Beaucage G. Dependence of silica particle sizes on network chain lengths, silica contents, and catalyst concentrations in in situreinforced polysiloxane elastomers. J Polym Sci B Polym Phys 1999;37:1421. [13] Clough R. Radiation resistant polymers, encyclopedia of polymer science and technology. New York: Wiley; 1989. 15:666. [14] Knoll K, Niessner N. Styrolux and styroflex – from transparent high impact polystyrene to new thermoplastic elastomers. Macromol Sympos 1998;132: 231. [15] Magonov SN, Elings V, Whangbo MH. Phase imaging and stiffness in tappingmode atomic force microscopy. Surf Sci 1997;375:385. [16] Kline SR. Reduction and analysis of SANS and USANS data using IGOR Pro. J Appl Cryst 2006;39(6):895.

[17] Kudoh H, Celina M, Malone GM, Clough RL. Radiation initiated copolymerization of allyl alcohol with acrylonitrile. Radiat Phys Chem 1996;48:55. [18] Vijaybaskar V. Electron beam crosslinking of nitrile rubber with special reference to mixed crosslinking system, PhD thesis, IIT-Kharagpur India 2005. [19] Basheer R, Dole M. The radiation crosslinking of block copolymers of butadiene and styrene. Macromol Chem 1982;183:2141. [20] Manion JP, Burton M. Radiolysis of hydrocarbon mixtures. J Phys Chem 1952;56:560. [21] Mooney MJ. A theory of large elastic deformation. Appl Phys 1940;11: 582. [22] Rivlin RS. Rheology. New York: Academic press; 1956. Vol. I. [23] Guth E. Theory of filler reinforcement. J Appl Phys 1945;16:20. [24] Holden G, Bishop ET, Lege NR. Thermoplastic elastomers. J Polym Sci C Polym Lett 1969;26:37. [25] Flory PJ, Rener JH. Statistical mechanics of cross-linked polymer networks II swelling. J Chem Phys 1943;11:521. [26] Mark J. Experimental determinations of crosslink densities. Rubb Chem Technol 1982;55:762. [27] (a) Thunga M, Satapathy BK, Staudinger U, Weidisch R, Abdel-Goad M, Janke A, et al. Dynamic mechanical and rheological properties of binary S–(S/B)– striblock copolymer blends. J Polym Sci Part B Polym Phys 2008;46:329; (b) Thunga M, Satapathy BK, Staudinger U, Weidisch R, Abdel-Goad M, Janke A, Knoll K. Influence of molecular architecture of S–S/B–S triblock copolymers on rheological properties. J Polym Sci Part B Polym Phys 2006;44:2776. [28] Ikeda Y, Tanakaa A, Kohjiya S. Reinforcement of styrene–butadiene rubber vulcanizate by in situ silica prepared by the sol–gel reaction of tetraethoxysilane. J Mater Chem 1997;7:1497. [29] Hashim AS, Azahari B, Ikeda Y, Kohjiya S. The effect of bis(3-triethoxysilylpropyl) tetrasulfide on silica reinforcement of styrene–butadiene rubber. Rubber Chem Technol 1998;71:289.