Interface Engineering toward Promoting Silanization by Ionic Liquid for

Article pubs.acs.org/IECR

Interface Engineering toward Promoting Silanization by Ionic Liquid for High-Performance Rubber/Silica Composites Zhenghai Tang,†,§ Jing Huang,† Xiaohui Wu,‡ Baochun Guo,*,†,§ Liqun Zhang,*,‡ and Fang Liu† †

Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou 510640, P. R. China State Key Laboratory of Organic/Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, P. R. China § Key Laboratory of Polymer Processing Engineering of Ministry of Education, South China University of Technology, Guangzhou 510640, P. R. China ‡

ABSTRACT: In silica-filled rubber composites, the silanization modification of silica plays a vital role in enhancing the compatibility between silica and a rubber matrix and hence the properties of the composites. In the present study, with the goal of promoting silanization reactivity and extent, we utilize a phosphonium ionic liquid (PIL) as a novel catalyst for the silanization reaction between silica and bis(3-triethoxysilylpropyl)-tetrasulfide (TESPT), a commonly used silane in the tire industry, in the styrene−butadiene rubber (SBR) matrix. Dynamic rheological measurement, bound rubber measurement, freezing point depression, and heat capacity increment together show that the addition of a small amount of PIL into a TESPT-modified SBR/ silica composite gives rise to significant improvement in the interfacial adhesion between silica and the rubber matrix, which is on account of the promoted silanization extent of silica with the catalyst of PIL. Consequently, the resulting composite prepared at room temperature with fewer parts of TESPT exhibits superior overall performance in comparison with the composites prepared by adding excessive TESPT and compounding at an elevated temperature. In particular, the energy loss during rolling of the rubber wheel is drastically decreased as a result of the improved interfacial silanization, which shows great potential in energysaving green tires. between the rubber matrix and silica.12 The ethoxy groups of TESPT react with the silanol groups of silica. Meanwhile, the tetrasulfide moiety of TESPT can graft onto rubber chains during curing; therefore the promoted dispersion of silica and enhanced interfacial adhesion are simultaneously resulted.13 Despite these benefits, one major bottleneck for the implementation of TESPT in rubber/silica composites is the low reactivity between TESPT and silica, as such a reaction takes place in highly viscous rubber melt. In this regard, excessive dosage of TESPT (e.g., ∼10 wt % relative to silica) is needed in silica-filled rubber formulations to improve the silanization extent.14 Previous studies demonstrated that the silanization reaction between silica and TESPT could be promoted under moisture15 or acid catalysts,16 as they accelerated the hydrolysis of ethoxy groups of TESPT to yield silanol groups, which subsequently underwent condensation with the silanol groups on the silica surface.10,17 Nevertheless, the improved hydrolysis of TESPT simultaneously promotes the condensation reaction between neighboring TESPT molecules to form multilayer oligomerization, which consumes TESPT and decreases the contact area between silica and the rubber matrix.17 Besides, moisture cannot be quantitative, and the process of catalysis is in an uncontrollable fashion during compounding, while acid catalysts greatly retard the curing process by consumption of curatives. Alternatively, an elevated processing temperature is adopted during mixing to improve the silanization reactivity. As suggested

1. INTRODUCTION Rubbers are acknowledged as strategically important materials due to their high entropy−elasticity and diverse irreplaceable applications. In rubber science and technology, the reinforcement of rubbers by diverse fillers such as carbon black and silica is essential because almost all the neat rubbers suffer from poor mechanical strength.1,2 Carbon black has been widely used as the most versatile reinforcing filler in the rubber industry for more than one century. Since the 1990s, silica has gained increasing importance in the manufacture of “green tires,” as it offers significant advantages over carbon black in terms of lowering rolling resistance of rubber tires. The lowered rolling resistance greatly decreases the fuel consumption and improves fuel efficiency of the tire during service.3 However, due to numerous silanol groups on the silica surface, the poor dispersion of silica in the nonpolar hydrocarbon rubbers and weak rubber-silica interfacial interaction pose the persistent obstacles for the use of silica in the rubber industry. To maximize the potentials of silica in rubber composites, especially in tires, great efforts are focused on the surface treatments of silica to enhance the compatibility between silica and rubbers. Various strategies including physically wrapping silica with a surfactant4 or polar rubbers as compatibilizers,5 treating silica by electron beam6 or plasma,7 and covalently grafting silica with a coupling agent8−10 or polymer chain11 are explored to tailor the surface properties of silica. Among them, the most convenient method for surface functionalization is to utilize the condensation reaction between the silanol groups on the silica surface and suitable silane coupling agents. Bis(3-triethoxysilylpropyl)tetrasulfide (TESPT), a bifunctional silane coupling agent, is most commonly used because it can establish molecular bridges © XXXX American Chemical Society

Received: August 26, 2015 Revised: October 2, 2015 Accepted: October 8, 2015

A

DOI: 10.1021/acs.iecr.5b03146 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

atmosphere. Then, the mixture was maintained at 120 °C for 24 h until a yellow precipitate was yielded. The product was purified by repeatedly washing it with pentane, followed by vacuum drying overnight. Preparation of SBR Composites. The formulation for SBR composites is shown in Table 1. For the preparation of SBR

by Reuvekamp et al., a minimum temperature of 130 °C was required to ensure that the silanization reaction efficiently proceeded.18 However, such a high temperature, on the other hand, will trigger the cross-linking reaction between silane and rubber, leading to premature scorch and undesired high compound viscosity.18,19 Thus, it is critically important, but still challenging, to develop an efficient and simple method to promote the silanization reactivity. In recent years, ionic liquids (ILs), as a kind of versatile molten salt, have explored a broad application prospect in the field of green catalysis due to their negligible vapor pressure, low toxicity, and excellent and tunable Lewis/Brönsted acidity.20 To date, ILs exhibit high catalytic activity and selectivity in nucleophilic substitution reaction.21,22 As an example, previous studies demonstrated that ILs could assist the sol−gel process to synthesize mesoporous silica. It argued that the anions of the ILs might behave as Lewis base catalysts to accelerate the condensation reaction among silanol groups.23,24 Very recently, Lungwitz et al. showed that highly basic ILs could be chemisorbed onto the silica surface to generate more nucleophilic silanolate anions on the silica surface.25 Along these lines, considering that the silanization reaction is essentially nucleophilic substitution through the condensation between silane and silanol groups of silica accompanied by the release of ethanol or water, we expect that the silanization reactivity between silica and silanes in rubber may be enhanced in the presence of ILs during compounding. In the present work, with the objective to overcome the low silanization reactivity between silica and TESPT in a highly viscous rubber matrix, we use a phosphonium-based ionic liquid, octadecyltriphenylphosphonium iodide (PIL), as an activator to promote the silanization reactivity in a silica-filled styrene− butadiene rubber (SBR) matrix. The effects of PIL on the dispersion of silica and the interfacial interaction between silica and SBR are investigated by dynamic rheological measurement, bound rubber measurement, freezing point depression, and heat capacity increment. The overall performance of the resulting composites is studied and correlated to the morphology and interfacial structure.

Table 1. Composition of SBR Compositesa codes

SBR

silica

TESPT

PIL

SBR/silica SBR/T-silica SBR/P-silica SBR/PT-silica-x

100 100 100 100

40 40 40 40

0 1 0 1

0 0 1 0.5, 1, 1.5, 2

a

Rubber ingredients: zinc oxide, 5; stearic acid, 1; N-cyclohexyl-2benzothiazole sulfenamide, 1.5; dibenzothiazole disulfide, 0.5; sulfur, 1.5 (the unit is g).

composites, SBR was first mixed with pristine silica for 3 min on a two-roll mill, then zinc oxide, stearic acid, TESPT, PIL, Ncyclohexyl-2-benzothiazole sulfenamide, dibenzothiazole disulfide, and sulfur were successively added within 10 min. Last, the mixture was compounded for an additional 2 min. SBR/silica composite is the control sample without TESPT or PIL. SBR/Tsilica composite refers to the SBR/silica composite modified with 1 phr TESPT but no PIL. SBR/P-silica composite represents the SBR/silica composite with 1 phr PIL but no TESPT. SBR/PTsilica-x represents the SBR/silica composite with both 1 phr TESPT and x phr PIL. During mixing, the temperature was set as 25 °C, and the roll speed ratio was 1:1.2 with a roll speed of 27.5 rpm (the lower speed roll). The nip between the two rolls was 0.5 mm. After mixing, the compounds were subjected to compression at 150 °C for the optimum curing time determined by a vulcameter. Preparation of Tread Rubber Composite. To examine the validity of PIL in industrial practice, a tread rubber composite was prepared. The basic formulation is listed as follows: SSBR1, 55 g; SSBR2, 30 g; BR, 30 g; zinc oxide, 3 g; stearic acid, 2 g; carbon black, 20 g; silica, 50 g; TESPT, 5 g; PIL, 1.5 g; paraffin, 1.5 g; N-tert-butylbenzothiazole-2-sulphenamide, 1.8 g; tetramethyl thiuram disulfide, 0.3 g; sulfur, 2.3 g. To prepare the tread rubber composite, rubbers were compounded with fillers and rubber ingredients with a two-roll mill, according to the processes described above. Last, the mixture was compounded at 145 °C for 7 min. Surface Modification of Silica. To envisage the effect of PIL on the reactivity between silica and TESPT and eliminate any confounding influence of the SBR matrix, the modification of silica with TESPT was performed in solution as follows. A total of 10 g of silica was dispersed in 300 mL of toluene in a threenecked flask; then 1.0 g of TESPT and 0.25 g of PIL were added. The silanization reaction was conducted at 80 °C for 12 h under stirring. After that, the product was subjected to several cycles of centrifugation and washing with ethanol and toluene to remove the ungrafted TESPT and PIL and then dried at 50 °C. The resultant was labeled as pt-silica. As a comparison, following a similar protocol for the preparation of pt-silica, t-silica was prepared by reacting silica with TESPT in the absence of PIL, and p-silica was prepared by mixing silica with PIL in the absence of TESPT. Characterizations. Fourier transform infrared (FTIR) measurement was carried out on a Bruker Vector 33 spectrometer. Transmission electron microscopy (TEM) for

2. EXPERIMENTAL SECTION Materials. Styrene-butadiene rubber (SBR 1502) with a styrene content of 23.5 wt % was manufactured by Jilin Chemical Industry Company, China. Solution-polymerized styrenebutadiene rubber (SSBR1, Buna VSL 5025−2HM) with 75% butadiene (50% vinyl content, 25% styrene, and extended with 37.5 phr of treated distillate aromatic extracts oil) and butadiene rubber (BR, CB24) were purchased from Lanxess Chemical Industry Co., Ltd. Solution-polymerized styrene-butadiene rubber (SSBR2, T2003) was provided by Shanghai Gaoqiao Co., Ltd. Triphenylphosphine and octadecyl iodide were purchased from Aladdin. Precipitated silica WL180 (specific surface area of 200 m2/g) was provided by NanPing Jialian Chemicals, China. TESPT was purchased from Alfa Aesar. Carbon black N375 was provided by Tianjin Dolphin Carbon Black Co., Ltd. The other chemical reagents including toluene, ethanol, cyclohexane, and pentane were analytically pure. All the rubber ingredients were industrial grade and were used as received. Synthesis of PIL. Octadecyltriphenylphosphonium iodide, PIL, was synthesized according to the literature.23 Typically, 10 g of triphenylphosphine and 14.6 g of octadecyl iodide were added into 40 mL of toluene in a 100 mL flask under a nitrogen B


Article

Industrial & Engineering Chemistry Research the ultramicrotomed samples was performed on a JEOL2100 instrument. Scanning electron microscopy (SEM) was observed using a Hitachi S-4800 instrument. A tensile test was performed on a U-CAN UT-2060 instrument according to ISO standard 372005. Dynamic rheological properties of the composites were measured on a rubber processing analyzer (Alpha, RPA 2000) at a dynamic strain amplitude from 0.56 to 200%. The frequency and temperature were set as 1 Hz and 60 °C, respectively. Curing characteristics were determined at 150 °C by a U-CAN UR-2030 vulcameter. Akron abrasion was determined in accordance with GB/T1689-1998. A rubber wheel rolling resistance test was measured on an RSS-II model rolling resistance testing machine (Beijing Wanhuiyifang Technology Co., Ltd.). The load and rotation speed were 30 kg and 800 r/min, respectively. Cross-link density (Ve) of the vulcanizates was determined by an equilibrium swelling experiment based on the Flory−Rehner equation, as described elsewhere.26 The freezing point depression of cyclohexane in vulcanizate was determined according to the literature.27 Briefly, a small piece of specimen was swollen in cyclohexane for 24 h until equilibrium swelling was reached. Then, the swollen sample was sealed in a DSC pan with an excess of solvent to prevent deswelling. The sample was first cooled to −35 °C and then heated to 35 °C at 5 °C/min. The melting point during heating was taken as the freezing temperature of the cyclohexane. The measurement was carried out on a TA Q200 differential scanning calorimeter. Heat capacity increment (ΔCp) of the composite at the glass transition temperature was obtained using a NETZSCH Instruments DSC 204 F1. Prior to the measurement, the instrument was calibrated by a sapphire sample. The sample was isothermal at −70 °C for 5 min, followed by heating to 20 °C at 10 °C/min under a nitrogen atmosphere. Thermal gravimetric analysis (TGA) was performed on a TA Q5000 thermogravimetric analyzer under a nitrogen atmosphere. The sample was heated from room temperature to 700 °C at a heating rate of 20 °C/min. The grafting density was calculated according to the literature.28 grafting density (mmol/g) =

Figure 1. FTIR spectra of pristine silica, t-silica, p-silica, and pt-silica.

the Si−OH group.29 In the spectrum of p-silica, two additional absorption peaks at 2926 and 2858 cm−1 (−CH2− stretching vibration) are observed, and the absorption peak at 956 cm−1 (stretching vibration of O−H) is decreased. A previous study showed that imidazolium and phosphonium cations could be chemisorbed onto silica surface to generate silanolate anions.25 Along this line, it is well understood that the presence of the −CH2− stretching vibration in p-silica is originated from PIL that is strongly adsorbed onto the silica surface and cannot be removed even by vigorously washing with ethanol and toluene, and the decreased absorption peak of the O−H stretching vibration is due to the generation of silanolate anions. In the spectra of t-silica and pt-silica, the absorption peaks at 2926 and 2858 cm−1 are mainly related to the −CH2− stretching vibration in the grafted TESPT. When using the intensity at 1120 cm−1 (Si−O−Si) as a reference, the absorption intensity of −CH2− in pt-silica is distinctly higher than that for t-silica. This observation suggests that a higher amount of TESPT is coupled onto silica in pt-silica; namely, the silianzation reaction is promoted in the presence of PIL. TGA measurement was performed to study the grafting amount of TESPT on the silica surface. The evolutions of the weight and corresponding derivative curve versus temperature for the silica samples are shown in Figure 2. Prior to measurement, the samples were dried at 80 °C overnight. As shown in Figure 2a, in the weight trace of pristine silica, a weight loss of about 4.0 wt % below 200 °C is ascribed to the removal of adsorbed water, and a slight weight loss in the broad temperature region between 200 and 700 °C corresponds to the dehydration of silanol groups on silica surface.29,30 In the TGA curve of psilica, a weight loss of about 3.8 wt % between 380 and 480 °C is due to the decomposition of the chemisorbed PIL. In the cases for t-silica and pt-silica, the weight loss associated with adsorbed water is much lower in comparison with pristine silica, indicating that the silica surface is tailored to be more hydrophobic upon grafting. On the other hand, both t-silica and pt-silica experience two obvious weight losses above 200 °C. The weight loss stages at around 330 and 520 °C (Figure 2b) are due to the physically adsorbed and chemically bonded species, respectively.30 Unlike with p-silica, it needs to be pointed out that little PIL is adsorbed onto the silica surface in pt-silica as indicated by the few weight loss between 380 and 480 °C associated with thermal decomposition of PIL. This may be because the grafting of TESPT onto silica reduces the silanolate anion content and weakens the attraction between PIL and silica; thus PIL can be

103 × W380 − 700 (100 − W380 − 700) × M

where M is the molecular weight of TESPT (538 g/mol) and W380−700 is the weight loss between 380 and 700 °C. Bound rubber content (BR) was measured by extracting the uncured compound with toluene for 3 days at room temperature, and the solvent was renewed every 24 h. The remnant was then dried at 50 °C in a vacuum oven to a constant weight. The BR was calculated as follows: BR% =

m0 − (m1 − m2) × 100% m0

where m0 is the weight of rubber component in the compound and m1 and m2 are the weight of compound before and after extraction, respectively.

3. RESULTS AND DISCUSSION Effect of PIL on Silanization Reaction between Silica and TESPT. Figure 1 displays the FTIR spectra of pristine silica, t-silica, p-silica, and pt-silica. In all of the samples, the absorption peaks at 1120 and 802 cm−1 are originated from the asymmetric and symmetric stretching vibration of Si−O−Si, respectively, and the peak at 956 cm−1 is due to the stretching vibration of O−H in C


Article


Figure 2. (a) Weight and (b) its derivative as a function of temperature for pristine silica, t-silica, p-silica, and pt-silica.

substitution of incoming TESPT molecules and consequently accelerates the condensation reaction with TESPT. Considering that the nature of ILs can be tuned by choosing a specific combination of cation and anion among numerous possibilities, the catalytic activity of ILs on silanization may be further optimized. The understanding of the relationship between catalytic activity and structures of ILs is currently ongoing. Dispersion of Silica and Interfacial Adhesion in SBR Composites. In polymer composites, the dispersion of filler and interfacial interactions are critical to the properties of the composites. The dispersion status of silica in the composites was investigated by TEM measurement. As shown in Figure 4, obvious silica aggregates consisting of primary silica are observed in the SBR/silica composite, which is on account of the large polarity discrepancy between silica and SBR. For the SBR/Tsilica composite, although the dispersion of silica is modestly improved, big silica aggregates are still evident. This manifests that TESPT cannot efficiently graft onto silica in the compounding process to improve the compatibility between silica and SBR. In the TEM images of SBR/PT-silica-1.5, silica presents a relatively uniform dispersion throughout the matrix with fewer aggregates. This is because the silanization extent of silica is promoted with the catalyst of PIL; concomitantly, the number of silanol groups on the silica surface is decreased, which alleviates the self-aggregation of silica. In addition, the interfacial interaction between silica and SBR is improved as the grafting density of silica increases (as discussed below), which can prevent the thermodynamically favorable reaggregation of silica during vulcanization. The dispersion of silica in the composites was then quantified by measuring and counting the diameter of silica domains in SEM graphs using ImageJ software.32 Taking the SBR/silica as an example, Figure 5a illustrates the typical SEM graph of a cryogenically fractured surface; the silica domains in the SEM graph are highlighted and measured according to the image analysis (Figure 5b). The size distribution of silica for all the composite samples is shown in Figure 5c. Compared with SBR/ silica and SBR/T-silica, the fraction of silica domains in the smaller diameter range (15−30 nm) is increased, and that in the larger diameter range (>60 nm) is decreased, providing convincing evidence that the dispersion of silica is improved with the addition of PIL. It should be noted that an excess dosage, i.e., 2 phr, of PIL leads to a poor dispersion of silica. This may be because excess PIL covers the silica surface, which inversely prevents TESPT from approaching and grafting onto silica.

nearly completely removed during washing. Taking into account the weight loss between 380 and 700 °C, the quantity of covalently grafted TESPT in t-silica and pt-silica is determined to be 3.3 and 5.2 wt %, respectively. Accordingly, the grafting density in t-silica and pt-silica is calculated to be 0.06 and 0.10 mmol/g, respectively. The grafting density in pt-silica is about 70% higher than that in t-silica, which provides further implication that PIL can promote the silanization reactivity between silica and TESPT. A dispersibility experiment provides complementary and visual evidence to disclose the surface properties of silica. To this end, the silica samples were suspended into toluene at a concentration of 1 mg/g under sonication, and then the resulting suspension was settled for 1 h. As shown in Figure 3, when a laser

Figure 3. Tyndall effect of silica, t-silica, and pt-silica dispersed in toluene.

beam passes through the suspension, only pt-silica suspension gives rise to the Tyndall effect, revealing the stable dispersion of silica in toluene, which is indicative of the hydrophobic surface of pt-silica after grafting. The hydrophobic properties of pt-silica make it more compatible with the nonpolar hydrocarbon rubber matrix. By contrast, pristine silica and t-silica cannot be stably dispersed and undergo sedimentation in the bottom of vial due to their hydrophilic nature; thus the Tyndall effect cannot be clearly observed. These results confirm that the use of PIL can promote the covalent attachment of TESPT onto the silica surface, thereby altering its surface property and imparting to it dispersibility in toluene. As illustrated in Scheme 1, a possible mechanism for the catalysis of silanization with PIL is proposed. The I− anions of PIL potentially behave as Lewis base catalysts, which react with silica to yield silanolate anions on the surface of silica.25,31 The silanolate anion is more nucleophilic, which favors the D


Article

Industrial & Engineering Chemistry Research Scheme 1. Proposed Mechanism for the Catalyst of Silanization Reaction with PIL

Figure 6. Dependence of G′ of the composites on the strain. Figure 4. TEM images of (a) SBR/silica, (b) SBR/T-silica, and (c and d) SBR/PT-silica-1.5.

for SBR/T-silica and SBR/PT-silica composites. In the SBR/ silica composite, silica aggregates severely and forms a threedimensional filler network, which traps a significant amount of rubber within the filler network. The entrapped SBR chains lose their identity as elastomers and behave as a part of rigid filler, and thus the G′ is much higher.28,33 In SBR/T-silica and SBR/PTsilica composites, the dispersion of silica is improved, and the

Dynamic rheological measurement with strain amplitude from 0.56 to 200% was carried out to investigate the silica network structure in the composites. Figure 6 shows the dependence of the elastic modulus (G′) on strain. At a small strain (30%), it appears that the G′ is increased with the incorporation of PIL, and the highest G′ is obtained in SBR/PTsilica-1.5. Considering that the filler network is broken down at a large strain, the improved G′ indicates an enhanced rubber−filler interaction. From Figure 6, it is found that the G′ for all the composites decreases dramatically upon increasing strain. This phenomenon is well-known as the “Payne effect,” expressed by the difference between G′ at small strain and large strain. Although the mechanism for the “Payne effect” is still controversial, it is commonly accepted that this behavior is mainly related to the collapse of the filler network and the release of the trapped rubber in the filler network upon application of oscillatory shear.34 Compared with SBR/silica composite, the SBR/T-silica composite exhibits a lower “Payne effect.” The magnitude of the “Payne effect” for SBR/PT-silica composites is further decreased, suggesting a more uniform dispersion of silica and weaker filler network in SBR/PT-silica composites. It is in correspondence with the conclusion that PIL has a positive effect on the dispersion of silica as a result of improved silanization reaction. Bound rubber (BR) is defined as the rubber that cannot be dissolved by a good solvent anymore after compounding, which is widely used as an indicator for rubber−filler physicochemical interactions. The BR for all the compounds is listed in Table 2. In

and SBR and consequently improves the interfacial interactions. However, the BR of SBR/PT-silica-2.0 with 2 phr PIL is somewhat decreased in comparison with SBR/PT-silica-1.5. It is well documented that the freezing point of a solvent imbibed in a swollen vulcanizate would be depressed due to the dimensional restrictions caused by the polymer mesh sizes in the nucleation process, and the magnitude of the depression of the freezing point could be used to evaluate the interfacial adhesion between the filler and polymer.37,38 The freezing curves of the composites swollen in cyclohexane are shown in Figure 7, and

Figure 7. DSC freezing curves for the composites swollen in cyclohexane.

the depression of the freezing point (ΔT) of cyclohexane is summarized in Table 2. In Figure 7, two exothermic transitions related to the melting temperature of cyclohexane are observed. The transition at about 6.0 °C corresponds to the melting of macroscopic crystals of free cyclohexane that locates at the surface of the composite, while the transition at a lower temperature is due to melting of the smaller crystal of confined cyclohexane in the swollen composite. The difference between these two melting points is defined as ΔT. Compared with the SBR/silica composite, ΔT for SBR/T-silica is slightly increased with the introduction of TESPT. It is of interest to note that ΔT for SBR/PT-silica composites is remarkably increased in comparison with the SBR/T-silica composite. This can be explained by the fact that the promoted silanization extent of silica in SBR/PT-silica facilitates the dispersion of silica and improves the interfacial interaction. As a result, the mobility of the SBR chain segment is restricted, and a tighter dimensional is formed, which consequently restricts the size of solvent cages where nucleation takes place and hinders the formation of a solvent crystalline nucleus within the composite, thus leading to a decrease of freezing point of the confined cyclohexane.37 The heat capacity increment (ΔCp) at glass transition for all the composites is determined by DSC, and the values are also summarized in Table 2. It is commonly acknowledged that the value of ΔCp is proportional to the amount of polymer participating in the glass transition.39 Compared with the SBR/ silica composite, ΔCp for the SBR/T-silica composite is increased, suggesting that a higher fraction of rubber chains is involved in the glass transition, while ΔCp for SBR/PT-silica composites is decreased with increasing PIL content. There are two opposite factors affecting the ΔCp. On one hand, the improved dispersion of filler results in the release of occluded rubber that is occluded in the voids of the filler, leading to a greater amount of rubber chains that engage in the glass

Table 2. Interfacial Interaction Parameters of the SBR Composites samples

BR (%)

ΔT (°C)

ΔCp (J/(g·K))

SBR/silica SBR/T-silica SBR/PT-silica-0.5 SBR/PT-silica-1.0 SBR/PT-silica-1.5 SBR/PT-silica-2.0

8.2 9.8 17.5 22.3 24.9 20.9

4.9 5.5 7.5 9.5 10.2 9.7

0.30 0.36 0.35 0.31 0.25 0.25

the SBR/silica compound, almost all the rubber component is extracted by toluene, and the value of BR is only 8.2%, which implies that the interaction between SBR and silica should be very weak. In the SBR/T-silica compound, the BR is slightly increased from 8.2 to 9.8%. The slight increase in the BR suggests that TESPT cannot efficiently graft onto silica, and thus the interfacial interaction is not substantially improved with only the addition of TESPT. It seems to conflict with other studies where BR is apparently improved after the modification of silica with silane. This may be because the dosage of TESPT is much larger9,12,35 and the compounding temperature is higher18,36 in other systems. Interestingly, the BR in SBR/PT-silica compounds is significantly increased with the addition of PIL. For instance, with the addition of 1.5 phr PIL, the BR of SBR/PTsilica-1.5 is tripled as compared to SBR/silica. This can be ascribed to the fact that silanization reaction is enhanced with the catalyst of PIL, which improves the compatibility between silica F


Article


PT-silica-x composites are drastically cut down by adding PIL. It can be well understood by considering that the silanol groups are converted to silanoates in the presence of PIL, which prevents the adsorption of curing agents onto the silica surface. In addition, the phosphonium cation in PIL can act as a secondary accelerator, which may also accelerate the curing rate. ML and MH are related to the shear modulus of the uncured compounds and vulcanizates, respectively, which are mainly determined by the filler network, rubber network, and rubber− filler interactions. From Table 3, compared with the SBR/silica compound, on one hand, ML for SBR/T-silica and SBR/PTsilica compounds is decreased. The decreased ML is originated from the depressed silica network as the ML in the uncured compounds is mainly governed by a filler network. On the other hand, MH exhibits an increased tendency with increasing PIL content, which is due to the strengthened interfacial interaction and increased cross-linking density, while excess PIL, i.e., 2 phr PIL, leads to a decrease in MH, which may be related to plasticizing effects of PIL and decreased interfacial interaction (as discussed above). In addition, when compared to with SBR/silica composite, ML is almost unchanged, and the MH is increased in the SBR/P-silica composite, suggesting that the dispersion of silica is nearly invariable and the cross-linking density is increased with only the addition of PIL. The cross-link density (Ve) for all the composites is also shown in Table 3. Compared with SBR/silica, the increased Ve in SBR/P-silica composite is probably because the phosphonium cation in PIL is a secondary accelerator that can enhance the cross-linking density. Moreover, Ve in SBR/T-silica and SBR/ PT-silica is further increased. Two reasons may be responsible for the enhanced Ve. TEPST acts as sulfur donor to increase the covalent cross-link density. In addition, improved interfacial interaction between silica and SBR allows silica to be giant crosslinks in the composites, which contributes to the cross-link density. Performance of SBR Composites. The representative stress−strain curves for the composites are compared in Figure 8a, and the mechanical properties are summarized in Table 4. Compared with the SBR/silica composite, the modulus (stress at 300% strain, similarly hereinafter) and ultimate strength of SBR/ T-silica composite shows only moderate improvements, suggesting that the addition of 1 phr TESPT cannot efficiently modify silica due to the low reactivity between them. The mechanical properties of SBR/P-silica are also shown in Table 4. It can be seen that the modulus of the SBR/P-silica composite is

transition. On the other hand, more rubber chains will be immobilized onto the silica surface as a result of the enhanced interfacial adhesion, which reduces the amount of rubber that can participate in the glass transition of the bulk rubber.33 In light of the above considerations, compared with the SBR/silica composite, although the improved interfacial adhesion in the SBR/T-silica composite may decrease the ΔCp, the rubber released from the occluded rubber in the SBR/silica composite can compensate for the loss in the amount of rubber chains being immobilized to the silica surface, and thus the ΔCp for SBR/Psilica is increased. Nevertheless, the interfacial adhesion is further strengthened in SBR/PT-silica composites, which causes more rubber chains to be immobilized on the silica surface, and thereby ΔCp is decreased. Vulcanization Characteristics and Cross-linking Density of SBR Composites. Vulcanization characteristics, expressed in the terms of scorch time (Tc10), optimum curing time (Tc90), curing rate (1/(Tc90−Tc10), minimum torque (ML), and maximum torque (MH) of the composite are summarized in Table 3. In the SBR/silica composite, a long marching cure with a Table 3. Curing Parameters and Crosslinking Density of the Composites samples SBR/silica SBR/Psilica SBR/Tsilica SBR/PTsilica-0.5 SBR/PTsilica-1.0 SBR/PTsilica-1.5 SBR/PTsilica-2.0

Tc10 (s)

Tc90 (s)

curing rate (10−2 s−1)

ML (dN· m)

MH (dN· m)

Ve (10−4 mol·cm3)

538 227

1416 617

1.1 2.5

5.2 6.03

24.1 31.0

3.5 6.5

511

1353

1.2

3.3

24.4

5.6

315

919

1.7

3.1

27.8

7.6

194

731

1.9

2.5

29.5

10.4

141

667

1.9

2.4

30.8

12.5

114

345

4.3

3.1

26.1

9.3

low curing rate is clearly observed. The delayed vulcanization can be explained by the fact that the acidity of the silanol groups on the silica surface trap the curing agents.40 In the SBR/T-silica composite, the delayed vulcanization behavior is somewhat alleviated with the addition of TESPT, which is because the silanol groups on silica surface are partially shielded by grafting TESPT. Moreover, Tc10 and Tc90 for SBR/P-silica and SBR/

Figure 8. (a) Representative stress−strain curves for SBR composites. (b) σ* as a function of λ−1 for SBR composites based on the Mooney-Rivlin equation. G


Article


in industrial practice, as described in the Experimental Section. Compared with the control tread rubber composite, the modulus of the tread rubber composite with 1.5 phr PIL is increased from 13.1 to 18.5 MPa, indicating that PIL can also promote the silanization reaction in industrial practice. Collectively, our present work shows significant advantages as follows: (1) The dosage of TESPT is greatly reduced by only the addition of a small amount of PIL as the catalyst. (2) The compounding can be conducted at room temperature instead of elevated temperature. (3) More importantly, the performance of the composite using PIL as a catalyst is superior to the conventional systems. It needs to be pointed out that when the PIL content is increased to 2 phr, the modulus of SBR/PT-silica-2 is inversely decreased. This may be because the excessive PIL prevents the grafting of TESPT onto the silica surface, and the redundant PIL can act as a plasticizer to decrease the modulus of the composite. Abrasion loss volume of the composites is also given in Table 4. Compared with the SBR/silica composite, the volume loss shows a progressive decrease from 1.28 to 0.63 cm3/1.61 km in the SBR/T-silica composite, and it further decreases to 0.47 cm3/ 1.61 km in SBR/PT-silica-1. When more PIL is included, the resistance to abrasion is almost unchanged. The rubber network and interfacial interaction can be evaluated using the well-known Mooney-Rivlin equation by plotting the reduced stress (σ*) against the reciprocal of the extension ratio (λ). The equation is listed as follows.44

Table 4. Mechanical Performance of the Composites samples SBR/silica SBR/Tsilica SBR/Psilica SBR/TPsilica-0.5 SBR/TPsilica-1.0 SBR/TPsilica-1.5 SBR/TPsilica-2.0

stress at 300% strain (MPa)

ultimate strength (MPa)

elongation at break (%)

loss volume (cm3/1.61km)

2.0 ± 0.1 2.9 ± 0.1

21.6 ± 0.6 26.4 ± 1.6

924 ± 12 839 ± 26

1.28 0.63

3.1 ± 0.1

21.5 ± 2.3

715 ± 18

0.83

4.4 ± 0.2

25.8 ± 1.3

705 ± 10

0.57

6.3 ± 0.3

32.9 ± 1.0

672 ± 15

0.47

8.0 ± 0.3

29.6 ± 0.6

589 ± 10

0.58

4.9 ± 0.2

30.7 ± 2.1

708 ± 14

0.55

slightly improved, while the strength is almost unchanged as compared to the SBR/silica composite. This is because PIL can only facilitate the dispersion of silica by disturbing the hydrogen bonding among silica and increase the cross-linking density, whereas the interfacial interaction is still weak as PIL cannot covalently bridge silica and SBR chains. It is of interest to note that the modulus and strength of SBR/PT-silica-x composites are significantly improved. In particular, with respect to the SBR/ silica composite, the modulus and strength of the SBR/PT-silica1 composite increases by about 220% and 52%, respectively. Such enhancements are much higher than those achieved in the composites prepared by adding excessive TESPT and compounding at elevated temperature.41−43 For example, when 6 wt % TESPT (relative to the weight of silica) was added into SBR/ silica composite, the modulus and ultimate strength were increased by 52% and 57%, respectively.41 In another study, to ensure the silanization reaction in the SBR/silica composite, 10 wt % TESPT was adopted, and the compounding temperature was maintained at 145−160 °C. About a 210% increase in modulus and 46% improvement in strength were obtained.35 For comparison, we prepared the SBR composite by adding 10 wt % TESPT into the SBR/silica composite. In the resulting composite, the modulus (8.3 MPa) is close to that for SBR/ TP-silica-1.5, while the ultimate strength (22.5 MPa) is much lower than that for SBR/TP-silica-1.5. The superior performance in SBR/PT-silica-1.5 composites with only 2.5 wt % TESPT is due to the highly efficient silanization reactivity with the catalyst of PIL, which consequently improves the dispersion of silica and strengthens the interfacial interaction. In another control, tread rubber composites were prepared to examine the validity of PIL

σ * = σ /(λ − λ−2) = 2C1 + 2C2λ−1

where σ is the stress and 2C1 and 2C2 are constants and are independent of λ. As displayed in Figure 8b, the σ* for all the composites is decreased drastically during stretching in the region of small extension ratio (λ−1 > 0.9), which is on account of the Payne effect. After a flat region, the σ* exhibits abrupt upturns, which is attributed to the finite extensibility of rubber chains bridging neighboring fillers during stretching.45 Compared with SBR/silica, both the absolute value of σ* and the value of λ−1 at which the upturn point appears are increased in SBR/Tsilica, and those in SBR/PT-silica are further increased. Such observations can be explained by the improved dispersion of silica and interfacial interaction, which favor the chain orientation between two adjacent silica particles when deformation is applied. The realization of chain orientation in rubber composites is crucial to the rubber reinforcement.46 Rolling resistance is one of the most important considerations for tread rubber used in green tires, which is expressed by the energy loss transformed from mechanical energy when tires run

Figure 9. (a) Tan δ value as a function of strain for all the composites. (b) Diagrammatic sketch of RSS-II model rolling resistance. (c) Energy loss and temperature increment of SBR/T-silica and SBR/PT-silica-1.5 composites. H


Article

Industrial & Engineering Chemistry Research Notes

through a unit distance. It is estimated that about 40% of the fuel consumption of an automobile results from the rolling resistance of tires.47 According to the “Green Tire Technology” of Michelin, a decrease of 20% in the rolling resistance of the tires will lead to a reduction in fuel consumption and CO2 emissions by an estimated 3−5%.48 The rolling resistance can be well reflected by the hysteresis of the composites, conventionally characterized by the value of tan δ at 60 °C. The lower the tan δ value, the lower the predicted rolling resistance. Shown in Figure 9a is the comparison on the tan δ for the composites. Compared with SBR/silica, tan δ for SBR/T-silica at 10% strain is decreased by 19%. Even more striking, the addition of PIL gives rise to a further decrease in the tan δ for SBR/PT-silica composites. For example, the tan δ for SBR/PT-silica-1.5 shows a decrease of 52% in comparison with SBR/silica. As a proof of concept, the energy loss of two typical composites of SBR/T-silica and SBR/PTsilica-1.5 was measured on a rotational power loss tester. The diagrammatic sketch for the measurement of rolling resistance on the RSS-II model is illustrated in Figure 9b. In Figure 9c, the energy loss of the SBR/PT-silica-1.5 composite wheel is determined to be 5.5 J/r, which is much lower than SBR/Tsilica composite (8.7 J/r), indicating a 37% energy savings. In the meantime, the temperature of the SBR/T-silica composite wheel surface is sharply increased by 100 °C within 10 min, while the temperature increment for SBR/PT-silica-1.5 composite is only 70 °C. The temperature increment is due to the heat generation and accumulation inside rubber tires caused by dynamic hysteresis loss, which will inevitably deteriorate the performance of the tire and may cause potential dangers during serving. Considering that the hysteresis loss is mainly originated from the internal frictions between filler−filler and filler−rubber,47,49 the decreased rolling resistance and heat build-up in SBR/PT-silica composites are attributed to the improved dispersion of silica and strengthened interfacial interaction.

The authors declare the following competing financial interest(s): A Chinese patent based on this work has been filed (application no. CN201510193681.2).

■

ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (2015CB654700 (2015CB654703)), National Natural Science Foundation of China (51222301, U1462116, and 51320105012), Key Project of Beijing Municipal Science and Technology Commission (D14110300230000), Natural Science Foundation of Guangdong Province (2014A030310435 and 2014A030311051), China Postdoctoral Science Foundation (2015M570710), Fundamental Research Funds for the Central Universities, and Opening Project of the Key Laboratory of Polymer Processing Engineering, Ministry of Education.

■

4. CONCLUSION In conclusion, we demonstrate that phosphonium ionic liquid (PIL) is highly efficient in catalyzing the silanization reactivity between silica and TESPT in a rubber matrix. PIL reacts with silanol groups on the silica surface to yield more nucleophilic silanolate anions, which favors the condensation reaction with ethoxy groups in TESPT and improves the silanization extent with the addition of a small amount of PIL. Compared with the SBR/T-silica composite, morphological and interfacial studies show that the dispersion of silica is improved and the interfacial interaction between silica and the rubber matrix is greatly enhanced in SBR/PT-silica composites, which is due to the promoted silanization reaction with the catalyst of PIL. As a result, the overall performance of SBR/PT-silica composites is remarkably improved. Importantly, the energy loss of the SBR/ PT-silica composite is drastically decreased, showing great potential in reducing the fuel consumption of vehicles. We envisage that the present work provides straightforward yet highly efficient interface engineering to promote the silanization reaction by using ionic liquid as a novel activator, which is expected to offer new scientific and technological opportunities for the preparation of high-performance rubber/silica composites from the perspective of the interface engineering.

■

REFERENCES

(1) Tang, Z. H.; Zhang, L. Q.; Feng, W. J.; Guo, B. C.; Liu, F.; Jia, D. M. Rational Design of Graphene Surface Chemistry for High-Performance Rubber/Graphene Composites. Macromolecules 2014, 47, 8663−8673. (2) Bokobza, L. Multiwall carbon nanotube elastomeric composites: a review. Polymer 2007, 48, 4907−4920. (3) Stockelhuber, K. W.; Svistkov, A. S.; Pelevin, A. G.; Heinrich, G. Impact of Filler Surface Modification on Large Scale Mechanics of Styrene Butadiene/Silica Rubber Composites. Macromolecules 2011, 44, 4366−4381. (4) Lee, J. Y.; Lee, T.; Kim, K.; Kim, B.; Kwag, G.; Kim, J. Y.; Ji, S.; Kim, W.; Paik, H. J. Poly(styrene-r-butadiene)-b-poly(poly(ethylene glycol) methyl ether methacrylate) as a silica dispersant in rubber compounds. Polym. Int. 2014, 63, 908−914. (5) Xu, T. W.; Jia, Z. X.; Luo, Y. F.; Jia, D. M.; Peng, Z. Interfacial interaction between the epoxidized natural rubber and silica in natural rubber/silica composites. Appl. Surf. Sci. 2015, 328, 306−313. (6) Shanmugharaj, A. M.; Bhowmick, A. K. Dynamic mechanical properties of styrene-butadiene rubber vulcanizate filled with electron beam modified surface-treated dual-phase filler. J. Appl. Polym. Sci. 2003, 88, 2992−3004. (7) Nah, C.; Huh, M. Y.; Rhee, J. M.; Yoon, T. H. Plasma surface modification of silica and its effect on properties of styrene-butadiene rubber compound. Polym. Int. 2002, 51, 510−518. (8) Lei, Y. D.; Tang, Z. H.; Guo, B. C.; Jia, D. M. SBR/silica composites modified by a polymerizable protic ionic liquid. Polym. J. 2010, 42, 555− 561. (9) Theppradit, T.; Prasassarakich, P.; Poompradub, S. Surface modification of silica particles and its effects on cure and mechanical properties of the natural rubber composites. Mater. Chem. Phys. 2014, 148, 940−948. (10) Castellano, M.; Conzatti, L.; Turturro, A.; Costa, G.; Busca, G. Influence of the silane modifiers on the surface thermodynamic characteristics and dispersion of the silica into elastomer compounds. J. Phys. Chem. B 2007, 111, 4495−4502. (11) Kim, Y. J.; Ha, S. W.; Jeon, S. M.; Yoo, D. W.; Chun, S. H.; Sohn, B. H.; Lee, J. K. Fabrication of Triacetylcellulose-SiO2 Nanocomposites by Surface Modification of Silica Nanoparticles. Langmuir 2010, 26, 7555−7560. (12) Kaewsakul, W.; Sahakaro, K.; Dierkes, W. K.; Noordermeer, J. W. M. Mechanistic Aspects of Silane Coupling Agents With Different Functionalities on Reinforcement of Silica-Filled Natural Rubber Compounds. Polym. Eng. Sci. 2015, 55, 836−842. (13) Dohi, H.; Horiuchi, S. Locating a silane coupling agent in Silicafilled rubber composites by EFTEM. Langmuir 2007, 23, 12344−12349. (14) Suzuki, N.; Yatsuyanagi, F.; Ito, M.; Kaidou, H. Effects of surface chemistry of silica particles on secondary structure and tensile properties of silica-filled rubber systems. J. Appl. Polym. Sci. 2002, 86, 1622−1629.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. I


Article


Silica Reinforced Solution Styrene Butadiene Rubber. Polym. Compos. 2013, 34, 1575−1582. (36) Wu, Y. P.; Zhao, Q. S.; Zhao, S. H.; Zhang, L. Q. The influence of in situ modification of silica on filler network and dynamic mechanical properties of silica-filled solution styrene-butadiene rubber. J. Appl. Polym. Sci. 2008, 108, 112−118. (37) Lopez-Manchado, M. A.; Valentin, J. L.; Herrero, B.; Arroyo, M. Novel approach of evaluating polymer nanocomposite structure by measurements of the freezing-point depression. Macromol. Rapid Commun. 2004, 25, 1309−1313. (38) Valentín, J.; Mora-Barrantes, I.; Carretero-González, J.; LópezManchado, M.; Sotta, P.; Long, D.; Saalwachter, K. Novel Experimental Approach To Evaluate Filler-Elastomer Interactions. Macromolecules 2009, 43, 334−346. (39) Fragiadakis, D.; Bokobza, L.; Pissis, P. Dynamics near the filler surface in natural rubber-silica nanocomposites. Polymer 2011, 52, 3175−3182. (40) Tang, Z. H.; Wu, X. H.; Guo, B. C.; Zhang, L. Q.; Jia, D. M. Preparation of butadiene-styrene-vinyl pyridine rubber-graphene oxide hybrids through co-coagulation process and in situ interface tailoring. J. Mater. Chem. 2012, 22, 7492−7501. (41) Siriwong, C.; Sae-Oui, P.; Sirisinha, C. Comparison of coupling effectiveness among amino-, chloro-, and mercapto silanes in chloroprene rubber. Polym. Test. 2014, 38, 64−72. (42) Suzuki, N.; Ito, M.; Yatsuyanagi, F. Effects of rubber/filler interactions on deformation behavior of silica filled SBR systems. Polymer 2005, 46, 193−201. (43) Castellano, M.; Conzatti, L.; Costa, G.; Falqui, L.; Turturro, A.; Valenti, B.; Negroni, F. Surface modification of silica: 1. Thermodynamic aspects and effect on elastomer reinforcement. Polymer 2005, 46, 695−703. (44) Bokobza, L.; Erman, B. A theoretical and experimental study of filler effect on stress-deformation-segmental orientation relations for poly (dimethylsiloxane) networks. Macromolecules 2000, 33, 8858− 8864. (45) Joly, S.; Garnaud, G.; Ollitrault, R.; Bokobza, L.; Mark, J. Organically modified layered silicates as reinforcing fillers for natural rubber. Chem. Mater. 2002, 14, 4202−4208. (46) Wang, Z.; Liu, J.; Wu, S.; Wang, W.; Zhang, L. Novel percolation phenomena and mechanism of strengthening elastomers by nanofillers. Phys. Chem. Chem. Phys. 2010, 12, 3014−3030. (47) Liu, J.; Lu, Y. L.; Tian, M.; Li, F.; Shen, J. X.; Gao, Y. Y.; Zhang, L. Q. The Interesting Influence of Nanosprings on the Viscoelasticity of Elastomeric Polymer Materials: Simulation and Experiment. Adv. Funct. Mater. 2013, 23, 1156−1163. (48) Rauline, R. EP patent 0 501227A1, 1992, to Michelin & Cite. (49) Medalia, A. I. Heat generation in elastomer compounds: causes and effects. Rubber Chem. Technol. 1991, 64, 481−492.

(15) Goerl, U.; Hunsche, A.; Mueller, A.; Koban, H. G. Investigations into the silica/silane reaction system. Rubber Chem. Technol. 1997, 70, 608−623. (16) Lung, C. Y. K.; Matinlinna, J. P. Aspects of silane coupling agents and surface conditioning in dentistry: An overview. Dent. Mater. 2012, 28, 467−477. (17) Vilmin, F.; Bottero, I.; Travert, A.; Malicki, N.; Gaboriaud, F.; Trivella, A.; Thibault-Starzyk, F. Reactivity of Bis[3-(triethoxysilyl)propyl] Tetrasulfide (TESPT) Silane Coupling Agent over Hydrated Silica: Operando IR Spectroscopy and Chemometrics Study. J. Phys. Chem. C 2014, 118, 4056−4071. (18) Reuvekamp, L. A. E. M.; Ten Brinke, J. W.; Van Swaaij, P. J.; Noordermeer, J. W. M. Effects of time and temperature on the reaction of TESPT silane coupling agent during mixing with silica filler and tire rubber. Rubber Chem. Technol. 2002, 75, 187−198. (19) Valentin, J. L.; Lopez-Manchado, M. A.; Posadas, P.; Rodriguez, A.; Marcos-Fernandez, A. M.; Ibarra, L. Characterization of the reactivity of a silica derived from acid activation of sepiolite with silane by Si-29 and C-13 solid-state NMR. J. Colloid Interface Sci. 2006, 298, 794−804. (20) Plechkova, N. V.; Seddon, K. R. Applications of ionic liquids in the chemical industry. Chem. Soc. Rev. 2008, 37, 123−150. (21) Hallett, J. P.; Welton, T. Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis. 2. Chem. Rev. 2011, 111, 3508− 3576. (22) Kim, D. W.; Chi, D. Y. Polymer-supported ionic liquids: Imidazolium salts as catalysts for nucleophilic substitution reactions including fluorinations. Angew. Chem., Int. Ed. 2004, 43, 483−485. (23) Marins, J. A.; Soares, B. G.; Silva, A. A.; Hurtado, M. G.; Livi, S. Electrorheological and dielectric behavior of new ionic liquid/silica systems. J. Colloid Interface Sci. 2013, 405, 64−70. (24) Trewyn, B. G.; Whitman, C. M.; Lin, V. S. Y. Morphological control of room-temperature ionic liquid templated mesoporous silica nanoparticles for controlled release of antibacterial agents. Nano Lett. 2004, 4, 2139−2143. (25) Lungwitz, R.; Linder, T.; Sundermeyer, J.; Tkatchenko, I.; Spange, S. Synthesis of chemisorbed imidazolium and phosphonium cations by reaction of ionic liquid precursors with silica. Chem. Commun. 2010, 46, 5903−5905. (26) Lei, Y. D.; Tang, Z. H.; Zhu, L. X.; Guo, B. C.; Jia, D. M. Functional thiol ionic liquids as novel interfacial modifiers in SBR/ HNTs composites. Polymer 2011, 52, 1337−1344. (27) Mora-Barrantes, I.; Rodriguez, A.; Ibarra, L.; Gonzalez, L.; Valentin, J. L. Overcoming the disadvantages of fumed silica as filler in elastomer composites. J. Mater. Chem. 2011, 21, 7381−7392. (28) Livi, S.; Giannelis, E. P. An improved process for the surface modification of SiO2 nanoparticles. Green Chem. 2012, 14, 3013−3015. (29) Li, Y.; Han, B. Y.; Liu, L.; Zhang, F. Z.; Zhang, L. Q.; Wen, S. P.; Lu, Y. L.; Yang, H. B.; Shen, J. Surface modification of silica by two-step method and properties of solution styrene butadiene rubber (SSBR) nanocomposites filled with modified silica. Compos. Sci. Technol. 2013, 88, 69−75. (30) Livi, S.; Silva, A. A.; Pereira, J.; Nguyen, T. K. L.; Soares, B. G.; Cardoso, M. B.; Gerard, J. F.; Duchet-Rumeau, J. Supercritical CO2organosilane mixtures for modification of silica: Applications to epoxy prepolymer matrix. Chem. Eng. J. 2014, 241, 103−111. (31) Karout, A.; Pierre, A. C. Silica gelation catalysis by ionic liquids. Catal. Commun. 2009, 10, 359−361. (32) Berki, P.; Nemeth, Z.; Reti, B.; Berkesi, O.; Magrez, A.; Aroutiounian, V.; Forro, L.; Hernadi, K. Preparation and characterization of multiwalled carbon nanotube/In2O3 composites. Carbon 2013, 60, 266−272. (33) Wang, M. J. Effect of polymer-filler and filler-filler interactions on dynamic properties of filled vulcanizates. Rubber Chem. Technol. 1998, 71, 520−589. (34) Bokobza, L. The reinforcement of elastomeric networks by fillers. Macromol. Mater. Eng. 2004, 289, 607−621. (35) Qu, L. L.; Yu, G. Z.; Xie, X. M.; Wang, L. L.; Li, J.; Zhao, Q. S. Effect of Silane Coupling Agent on Filler and Rubber Interaction of J


Interface Engineering toward Promoting Silanization by Ionic Liquid for

Recommend Documents