Constructing a Multiple Covalent Interface and Isolating Disperion

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Applications of Polymer, Composite, and Coating Materials

Constructing a Multiple Covalent Interface and Isolating Disperion Structure in Silica/Rubber Nanocomposites with Excellent Dynamic Performance Junchi Zheng, Dongli Han, Suhe Zhao, Xin Ye, Yiqing Wang, Youping Wu, Dong Dong, Jun Liu, Xiaohui Wu, and Liqun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02358 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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ACS Applied Materials & Interfaces

Constructing a Multiple Covalent Interface and Isolating Disperion Structure in Silica/Rubber Nanocomposites with Excellent Dynamic Performance Junchi Zheng a, b, Dongli Han a, b, Suhe Zhao a, b, Xin Ye a, b, Yiqing Wang a, b, Youping Wu a, b, Dong Dong c, Jun Liu a, b, *, Xiaohui Wu a, *, Liqun Zhang a, b, * a

Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China b

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China c

Beijing Red Avenue Innova Co., Ltd, Beijing 100176, PR China

Address for a and b: Beijing University of Chemical Technology, Beisanhuan East Road, Beijing 100029, China. Address for c: Unit 1, 2, 3, Building 10, No. 20 Kechuang Fourteenth Street, Beijing Economic-Technological Development Area (BDA), Beijing 100176, China. The e-mail addresses of all authors:

*Correspondence to: Liqun Zhang (E-mail: [email protected]) Tel.: +86-010-64423312; fax: +86-010-64443413. Address: Beijing University of Chemical Technology, Beisanhuan East Road, Beijing 100029, China

1

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Graphic Abstract:

Abstract: Realizing and manipulating a fine dispersion of silica nanoparticles (NPs) in the polymer matrix is always a great challenge. In this work, we firstly

successfully

synthesized

N'-bis[3-(triethoxysilyl)propyl-isopropanol]

-propane-1,

N, 3-diamine

(TSPD), which was a new interface modifier, aiming to promote the dispersion of silica NPs. Through the Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance analyses (NMR) and mass spectroscopy, we verified that the TSPD contains together six ethoxy groups at its two ends. Then, we used this TSPD to modify the pure silica NPs and this modified silica was abbreviated as D-MS, which is realized by the TGA examination, scanning electron microscopy (SEM) analyses and dynamic light scattering (DLS) results. It was clearly observed that D-MS NPs are connected to one another but are not conglutinated tightly, exhibiting a novel pre-dispersed structure with around 1-2 nm certain extent of inter-particle distance. Next, we 2


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fabricated the following four elastomer nanocomposites such as pure silica/natural rubber (NR) composite (PS-NR), D-MS/NR composite (DMS-NR), bis-(γ-triethoxysilylpropyl)-tetrasulfide (TESPT) modified silica/NR composite (TS-NR) and TESPT modified D-MS/NR composite (T&DMS-NR), and found that the Payne effect is the smallest for T&DMS-NR via the combination use of the D-MS and the traditional coupling agent TESPT, attributed to its best dispersion state evidenced by the TEM results. Moreover, by measuring a series of other important mechanical performances such as the stress-strain curve, the dynamic strain dependent of the loss factor and the heat build-up, the T&DMS-NR system greatly exceeds those of the three other rubber composites. In general, this new approach provides a good opportunity to prepare a silica/rubber composite with excellent properties in mechanics strength and dynamic behavior by tailoring the fine dispersion of NPs. 1. Introduction

The demand for the development of the “green tire” with low rolling resistance is greatly pressing 1. The development of the “green tire” with excellent

performance

contributes

to

energy

conservation

and

environmental protection 2. Previous researchers indicated that the rolling resistance of tire is largely influenced by the performance of the tire tread 3, 4

, which is prepared by rubber composites. In this regard, preparing a

rubber composite with an excellent dynamic performance is the key to the 3



development of the “green tire.”5, 6 Silica is commonly used as filler for rubber reinforcement because it provides a good performance in mechanics strength and dynamic behavior for rubber composites 7-10. As a kind of inorganic nanoparticles (NPs), silica has huge amounts of hydroxyl (–OH) groups on its surface aggregate with large amounts of silica NPs

11-13

. Silica presents as an

14

. The aggregation of silica

NPs is induced by the attractive interaction between NPs 15, 16, such as the van der Waals force and the hydrogen bonds

17

. In the rubber matrix,

there is a possibility that the distance between silica NPs becomes smaller, thereby causing an increase in van der Waals attraction between the NPs. Silica NPs get close to each other under this attraction, and the attraction between silica NPs also get much greater during this approaching process. When the distance between the silica NPs is close enough, the hydrogen bonds is formed between silica NPs with the help of the hydroxyl groups on the silica surfaces

18

. Therefore, the silica NPs tend to conglutinate

tightly, resulting in forming an aggregation structure. In previous research, silica modification is a commonly used method to prevent the aggregation of silica NPs 19, 20. Silane coupling agent (SCA) was widely used in modifying inorganic NPs

21, 22

. SCA has a chance to graft on the silica surface

because of the dehydration condensation reaction between the hydroxyl groups on the silica surface and the hydroxyl groups generated after the 4


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hydrolysis of SCA

23-26

. Many kinds of surface active agent, such as

alkapolpeg (PEG), tris(2-hydroxyethyl)amine (TEOA) and N, N, N-trimethylhexadecan-1-aminium bromide (CTAB) were also proven to 19, 27-29

be very effective for silica modification

. Previous researchers

indicated that silica surface was usually covered by the surface active agent, resulting in reducing the amount of exposed hydroxyl groups

30-32

.

In sum, silica modification is an effective process to eliminate the hydroxyl groups on the surface of silica

33, 34

, resulting in preventing the

formation of silica aggregates 35. After the silica modification, the amount of silica aggregates in rubber composite is reduced, and the compatibility between rubber and silica is improved

36, 37

. Therefore, SCA and surface

active agent are commercially used to prepare silica/rubber composites with satisfactory static and dynamic mechanical performance 38. Previous researchers proposed many methods for silica modification, and the most commonly used method was blending silica and modifier together in liquid

33, 39

. In addition, some researcher tried to prepare one-step

modified silica by the hydrolysis tetramethoxysilane in the ethanol solution with the addition of modifier

18, 40, 41

. In preparing silica/polymer

composite, modifier is commonly blended with polymer and silica together at high temperature, and the silica modification is completed accompanied by the destruction of silica aggregates in the polymer matrix 42, 43

. 5



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Most kinds of silica still remains in the form of a small aggregate when silica was modified by SCA or surface active agent. This is mainly due to the absence of one agent to endow the chemically isolating function to prevent the formation of the hydrogen bonds between nearby silica NPs. Those aggregates are tightly conglutinated, resulting in a significant mutual friction under the dynamic situations

39

. As previous

researchers indicated, the rolling resistance was the macroscopic feedback of energy loss caused by internal friction in rubber composites 44

, and the mutual friction among silica aggregations was the largest

component of total internal friction in silica/rubber composites. Therefore, preparing modified silica with a structure of silica NPs isolated from each other is an effective method for realizing low rolling resistance of rubber composites 45. Induction of chemical interface between silica NPs is an effective strategy to achieve isolating dispersed silica. Accordingly, we try to design an organic compound that can react with two silica NPs and serves as support between silica NPs to constrain the distance between NPs larger than the distance of the construction of the hydrogen bonds, resulting in preventing the adsorption between silica NPs. Through the above methods, the aggregation of silica NPs is prevented, and the mutual friction between silica NPs under the dynamic loading is reduced by preventing the mutual friction among silica NPs. In addition, forming a 6


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chemical interface between silica and rubber has always been considered as the key for rubber composite with excellent performance in mechanics strength and dynamic behavior. Therefore, we try to further form a chemical interface between silica NPs and rubber molecules by adopting the sulfide-containing SCA in the presence of the chemical interface between the silica NPs. To

achieve

the

goal

N'-bis[3-(triethoxysilyl)propyl-isopropanol]

described

above,

-propane-1,

N,

3-diamine

(TSPD), which is a novel isolating-dispersion modifier, was synthesized. The chemical structure of TSPD was characterized by Fourier transform infrared spectroscopy (FT-IR), mass spectroscopy, and

1

H nuclear

magnetic resonance spectroscopy (1H NMR). In silica modification, the amount of TSPD was controlled to ensure that silica would not be completely linked into a giant network. Meanwhile, part of the active hydroxyl groups on the silica surface should be retained for further grafting of sulfide-containing SCA. Silica modified by TSPD (D-MS) was characterized through thermal gravimetry (TGA), dynamic light scattering (DLS), and scanning electron microscopy (SEM) analyses. In preparing silica/NR composites, bis-(γ-triethoxysilylpropyl)-tetrasulfide (TESPT), which is the most commonly used sulfide-containing SCA, was selected to use independently and use with D-MS. The performance of silica/NR composites was examined by transmission electron microscopy 7



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(TEM), rubber process analyzer (RPA), tension tester, and heat build-up. This work designs a novel isolating-dispersion modifier and proposes a creative strategy in preparing rubber composites with excellent performances. 2. Experiment 2.1 Materials

Natural rubber (NR) was obtained from China Hainan Rubber Industry Group Co., Ltd. (Haikou, China). The silica of K-160 (powder, particle size: 20-30 nm, BET specific surface: 160-170 m2/g) was supplied

by

Yihai

Kerry

Co.,

Ltd.

(Jiamusi,

China).

Bis(3-Triethoxysilylpropyl)disulfide (TESPT) was provided by Nanjing Capatue

Chemical

Co.,

Ltd.

(Nanjing,

China).

3-Glycidoxypropyltriethoxysilane (GPTES) and 1, 3-diaminopropane (DP) were provided by Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). 2.2 Preparation of TSPD

O

O Si O O

O

+

O Si O

OH O

NH2

Nitrogen

125 O

H 2N

H N

H N

OH

8


O

O O Si O

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(1) TSPD was synthesized through the condensation of GPTES and DP, as shown in equation (1). In brief, 1 mol GPTES and 0.5 mol DP were dropped into a flask with three necks. The mixture was reacted at 125 °C in a nitrogen atmosphere for 5 h with stirring. Then, the product was cooled down to the room temperature, and TSPD was obtained. 2.3 Preparation of silica modified by TSPD (D-MS)

We used this TSPD to perform the first-step surface modification of pure silica to achieve D-MS. Ethanol (1000 mL) and pure water (50 mL) were added to a flask with three necks. The pH of the ethanol solution was adjusted to approximately 4.5 by dropping acetic acid. Then, 5 g of TSPD (mass ratio of TSPD to silica is 5:100) was dropped into this ethanol solution and hydrolyzed for 8 h to transform ethoxy groups to hydroxyl groups. Silica (100 g) was dispersed in the solution above mentioned under stirring at 60 °C for 12 h. Then, the mixture of modified silica was dried in this flask with three necks to obtain D-MS powder. The vaporized ethanol was collected by a reflux device. For the characterization of silica modification, a portion of D-MS was extracted by ethanol for 1 day to remove the TSPD that is not grafted on the silica. Finally, the extracted D-MS was dried in an oven at 60 °C for 24 h. 2.4 Preparation of silica/NR compounds and composites

The formulation for preparing four kinds of silica/NR compounds is shown in Table 1. The whole process of preparing silica/NR compounds 9



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is divided into three stages. First, the chamber temperature of the internal mixer was set to 55 °C and the rotor speed was set to 50 rpm. Then, NR was put into the chamber of the internal mixer for plasticating. Two minutes later, silica (pure silica or D-MS) and TESPT (if needed) was added into the chamber and blended with NR. Four minutes later, activator and antioxidant was put into the chamber. After two minutes, the silica/NR mixture was obtained. Second, the chamber temperature was set to 150 °C and the rotor speed was set to 50rpm. Then four kinds of silica/NR mixture was kneaded for another 5 minutes. After this process, all of silica/NR mixture were cooled to room temperature. Finally, accelerator and curing agent was added into silica/NR mixture on an open roll mill. PS-NR compound, DMS-NR compound, TS-NR compound and T&DMS-NR compound were obtained after all processes above mentioned. Table 1. Formulation of silica/NR compounds. Materials

PS-NR /phra DMS-NR /phra

TS-NR /phra

T&DMS-NR /phra

NR Silica D-MS TESPT

100 50 0 0

100 0 52.5 0

100 50 0 5

100 0 52.5 2.5

Other additivesb

-

-

-

-

a

Parts per hundred of rubber. stearic acid 2.0 (activator), zinc oxide 5.0 (activator), N-1,3-dimethylbutyl-N'-phenylp-phenylenediamine 2.0 (antioxidant), N-Cyclohexyl-2-beozothiazole sulfenamide 2.0 (accelerator), 1,3-Diphenylguanidine 1.0 (accelerator) and sulfur 2.0 (curing agent). b

10


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The silica/NR composite was prepared by vulcanizing the silica/NR compounds. The temperature of vulcanization was set to 143 °C, and the optimum curing time (T90) of all kinds of silica/NR compounds was obtained by a valcameter. In the process of vulcanization, the silica/NR compound was placed in a flake or columnar mold. These prepared silica/NR composites should be stored for more than 1 days before testing. 2.4 Characterizations

The differences of reactive group in GPTES, DP, and TSPD were identified using a TENSOR 27 FT-IR spectrometer (Bruker Optik Gmbh Co., Germany). Before the testing, GPTES, DP, and TSPD were dropped on pellets, which were prepared by molding potassium bromide (KBr). Spectra were recorded within wave numbers ranging from 400 cm-1 to 4000 cm-1. The 1H NMR spectra was identified using an AVANCE III 400 MHz NMR Spectrometer (Bruker Daltonik GmbH Co., USA). All of samples were dissolved in the chloroform-d (CDCl3). The mass spectra was identified using a ZMD single-quadrupole mass spectrometer (Waters Co., USA). The electrospray capillary voltage was set to 3000 V. The cone voltage was set to 25 V, and the extractor voltage was set to 4 V. The source block and desolvation temperatures were set at 120 °C and 350 °C, respectively. 11



The particle size of silica samples were characterized by Malvern ZEN3600 Nano ZS Analyzer (Malvern Instruments Co., England). All kinds of silica samples were dispersed in ethanol by ultrasonic technology for 2 h. The solid content of these samples was 0.01%. The micro-morphologies of D-MS and pure silica were observed through a JEOL S-4700 SEM (JEOL CO., Japan). D-MS and pure silica were prepared for SEM observation by dropping silica particle suspensions onto silicon wafers. D-MS and pure silica were sprayed with gold to make these samples conductive before the observation. The accelerating voltage of SEM was 20 kV. The silica dispersion in rubber composites was observed by a Tecnai G2 20 TEM (FEI Co., USA). The accelerating voltage of TEM was 200 kV. A tiny slice of silica/NR composite was cut at -100 °C for TEM observation. This tiny slice was collected by the copper grids. The viscoelastic performance of all silica/NR compounds and the dynamic performance of all silica/NR composites were characterized by RPA 2000 (Alpha Technologies Co., USA). For silica/NR compounds, the strain range was 0.1-400%, the test frequency was set to 1 Hz, and the temperature of test was set to 60 °C. For silica/NR composites, the strain range was 0.1-40%, the test frequency was set to 1 Hz, and the temperature of test was set to 60 °C. The static mechanical performance of the all rubber composites were 12


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characterized by a CMT4104 electrical tensile tester (Shenzhen SANS Test Machine Co., China). For this characterization, the shape of these silica/NR composites was cut according to ASTM D638 specifications. Heat build-up (HBU) was characterized by a Goodrich flexometer (BF Goodrich Co., USA). The temperature rise at the specimen base was recorded. 3. Results and discussion

3.1 Chemical structure of TSPD

Figure 1. Structure characterization of synthesized TSPD: (a) FT-IR 13



spectra of 3-Glycidoxypropyltriethoxysilane (GPTES), 1, 3-diaminopropane (DP) and TSPD, (b) mass spectra of TSPD, (c) 1H NMR spectra of TSPD In Figure 1 (a), the spectra of GPTES presents the peak of –CH3, – CH2–, and epoxy group; meanwhile, the spectra of DP presents the peak of –CH2– and –NH2. The FT-IR spectra of TSPD has no adsorption peaks of –NH2 and epoxy group but has a peak of –NH–. TSPD was synthesized by the reaction between the epoxy group on the GPTES and the primary amine group on the DP. The group change in this reaction is consistent with the change in the absorption peak shown in Figure 1 (a). Figure 1 (b) exhibits the mass spectra of TSPD. The peak in 631 m/z is the most significant peak in the mass spectra. This peak is consistent with the molecular weight of TSPD (631 g/mol). As shown in Figure 1 (c), H atoms in TSPD are marked as a-j according to their local chemical environment. Letters marked above each peak in 1H NMR indicate that the peak corresponds to the H atoms of TSPD. The peak area, which corresponds to the number of H atoms in different local chemical environments, is calculated by the integral. For H atoms in different local chemical environment, the ratio of their signal peak area to their number is equivalent. Therefore, the chemical structure of the synthesized TSPD is consistent with the designed one. Summarizing the results of the above characterizations, we verifi 14


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that TSPD is synthesized according to our design. 3.2 Structure of silica modified by TSPD (referred to D-MS)

Figure 2. Characterization of silica modified by TSPD (D-MS): (a) TGA

curves of pure silica and D-MS, (b) particle size distributions of pure silica and D-MS, (c) and (d) SEM images of pure silica and D-MS As shown in Figure 2 (a), both pure silica and D-MS have significant weight loss in the region below 115 °C because of the removal of the adsorbed water 46. In this region, D-MS has less weight loss than pure silica, demonstrating that D-MS has less absorbed water than pure silica. This result was due to the reaction of the hydroxyl groups on the silica surface with TSPD. 15



D-MS has a more significant weight loss than pure silica in the region above 115 °C. In this region, the weight loss of the pure silica is about 2.5%. The degradation of the hydroxyl groups on the silica at high temperatures was the main reason for the weight loss of pure silica. The weight loss of D-MS in this region was mainly caused by the degradation of TSPD grafted on the silica surface. The D-MS has about 4.7% weight loss, which is significantly larger than the weight loss of pure silica in this region. Therefore, we can confirm that TSPD is successfully grafted with silica. Figure 2 (b) exhibits one peak on the curve of pure silica and two peaks on the curve of D-MS. The large silica aggregates were prone to break down into small ones in silica modification because of the grafting reaction of TSPD. Meanwhile, TSPD reacted with two different silica NPs, because TSPD had ethoxy groups at both ends. Therefore, D-MS NPs tended to interconnect with each other to form a network structure, in which silica NPs was considered as the point and TSPD was considered as the line. In our research, the amount of TSPD used in preparing D-MS was controlled to avoid the formation of giant networks consisted of silica NPs, which was harmful to the application of D-MS in preparing silica/rubber composites. In silica modification, TSPD preferentially reacted with particles in the outer layer of pure silica aggregates. The amount of TSPD grafted on silica NPs originally in the 16


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inner layer of pure silica aggregates was insufficient, when the amount of TSPD used in silica modification was controlled. The silica particles grafted with sufficient TSPD interconnected with each other to form a big spherulite with a loose structure. In contrast, the silica particles grafted with insufficient TSPD tended to exist independently. Therefore, a portion of D-MS has a smaller particle size than pure silica and another portion of D-MS possesses a larger particle size than pure silica.

Scheme 1. Utilizing TSPD in preparing D-MS 17



The schematic diagram for the preparation of D-MS is shown in Scheme 1. Meanwhile, according to the structure of TSPD, which is also shown in Scheme 1, we calculate the theoretical radius of gyration for TSPD (0.924 nm) and find that TSPD has a larger radius of gyration than TESPT (0.703 nm). Therefore, TSPD is extremely effective in constraining the distance between silica NPs larger than the distance of the construction of hydrogen bonds. As shown in Figure 2 (c), pure silica NPs are completely bonded and conglutinated tightly to form an aggregate without voids. However, numerous small voids (1-2 nm) exist among the D-MS NPs, and the inter-particle distance is almost the same as the length of the TSPD (twice the radius of gyration of the TSPD). Therefore, the structure of D-MS is looser than that of pure silica; meanwhile, D-MS possesses a novel dispersed structure, where silica particles are connected to one another but are not conglutinated tightly. We deduce that the TSPD grafted simultaneously on two silica particles serves as isolating modifier to form a chemical interface between silica NPs. As illustrated above, we successfully adopt the TSPD to perform the surface modification of pure silica NPs during the first-step, resulting in the formation of a novel pre-dispersed silica structure(D-MS), which is realized via the reaction of the two end-groups of TSPD with the surface of the silica to isolate the neighboring silica NPs. And because some 18


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active hydroxyl groups still exist on the D-MS surface attributed to the limited introduced amount of TSPD, a further interface chemical covalent bonding will be formed during the second-step of the blending process between D-MS and rubber. 3.3 Application of D-MS in rubber composites

Scheme 2. Realizing a further chemical interface between silica and

rubber by adopting TESPT We

designed

and

prepared

the

following

four

elastomer

nanocomposites such as pure silica/natural rubber (NR) composite (PS-NR),

D-MS/NR

composite

Bis(3-Triethoxysilylpropyl)disulfide

(TESPT)

(DMS-NR), modified

silica/NR

composite (TS-NR) and TESPT modified D-MS/NR composite (T&DMS-NR) to investigate the effect of silica-silica chemical interface and silica-rubber chemical interface on the static and dynamic mechanical 19



performances of silica filled rubber composites. In T&DMS-NR composite, a further chemical interface is formed between D-MS and rubber by using of TESPT, as shown in Scheme 2. 3.3.1 Viscoelastic performance of silica/NR compounds

Figure 3. (a) Storage modulus (G′)-strain curves and (b) loss factor-strain

curves of pure silica/NR (PS-NR), D-MS/NR (DMS-NR), TESPT modified silica/NR (TS-NR) and TESPT modified D-MS/NR (T&DMS-NR) 20


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As shown in Figure 3 (a), the storage modulus (G’) of all rubber compounds decrease rapidly with increasing strain, when the strain amplitude is larger than 1%. In silica/rubber compounds and composites, the

G’

under

small

strain

amplitudes

was

deformation-induced changes in filler-filler network

caused

by

the

47

. In contrast, the

filler-filler network in rubber compounds or composites was destroyed under sufficient strain amplitudes

48

. In the 1960s, A. R Payne first

proposed the relationship between the strain and viscoelastic performance of rubber compounds and composites. Therefore, in a G’-strain curve, the change in G’ is called Payne effect 47. The weaker the Payne effect, the better the dispersion of filler in rubber matrix 49. PS-NR compound has a more significant Payne effect than DMS-NR compound as shown in Figure 3 (a), indicating that D-MS has a more homogeneous dispersion than pure silica in the rubber matrix. In rubber compounds, the chemical interface formed by TSPD among silica NPs served as buffer to prevent the particles from approaching one another. Therefore, the direct interaction between silica NPs was weak, resulting in a fine dispersion of D-MS in rubber matrix. However, DMS-NR compound exhibits somewhat more significant Payne effect than TS-NR compound, which means that silica is better dispersed in TS-NR compound than in DMS-NR compound. The silica in TS-NR compound was modified by TESPT, which also contains ethoxy groups at 21



opposite ends. Therefore, the silica in TS-NR compound also possessed a chemical interface among NPs. However, the amount of TESPT used in TS-NR compound was more than that of TSPD used in DMS-NR compound, which was the major reason for the difference of silica dispersion in TS-NR and DMS-NR compounds. T&DMS-NR compound has a less significant Payne effect than TS-NR compound and the lowest Payne effect among these four kinds of silica/NR compounds, which means that T&DMS-NR compound has a more homogeneous silica dispersion than other three kinds of silica/NR compounds. The dosage of the modifiers is the same in the compounds of TS-NR and T&DMS-NR. As a pre-modified silica, D-MS exhibited better compatibility with rubber than silica modified by TESPT in the rubber mixed and could be quickly mixed and dispersed in the rubber matrix. Moreover, adding TESPT to DMS-NR compound made the dispersion of silica more homogeneous in the rubber composites. Therefore, preparing a pre-modified silica containing chemical interface between NPs is an effective method to improve silica dispersion in silica/rubber compounds. In Figure 3 (b), all of the silica/rubber compounds has almost the same strain-loss factor curve, indicating that the loss factor of rubber compounds is little affected by silica dispersion. 3.3.2 Micro-morphology of silica/NR composites

22


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Figure 4. Silica dispersion of four kinds of silica/NR composites

(vulcanized) In Figure 4, PS-NR composite has a more uneven silica dispersion than DMS-NR and TS-NR composites because PS-NR composite contains large amounts of silica aggregates. In TS-NR composite, the dispersed silica was prevented to aggregate again, because a chemical interface formed by TESPT existed between silica particles and rubber molecules. In DMS-NR composite, the chemical interface formed by TSPD among silica NPs also prevented the aggregation of NPs in silica/rubber composites. The dispersion of silica in the T&DMS-NR composite is more homogeneous than that in other three kinds of silica/NR composites. The dispersion of silica is ideal in the T&DMS-NR composite because of a 23



synergistic combination of chemical interface formed by TSPD serving as buffer between silica NPs and the chemical interface produced by TESPT between silica and rubber serving as “coupling bridge”. Therefore, using additional TESPT in D-MS/NR composite for further forming chemical interface between silica particles and rubber molecules is an easy and effective method for making the silica dispersion in rubber filled with D-MS more homogeneous. 3.3.3 Static and dynamic mechanical performance of silica/NR composites

Figure 5. (a) Stress–strain curves, (b) modulus and reinforcing index, (c) 24


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G’-strain curves and (d) loss factor-strain curves of four kinds of silica/NR composites The tensile strength of all silica/NR composites is shown in Figure 5 (a); meanwhile, the modulus at 100%, the modulus at 300% and the reinforcing index (modulus at 300% divided by modulus at 100%), are shown in Figure 5 (b). DMS-NR composite exhibits 50% higher modulus than PS-NR composite. The reinforcing index reflects the reinforcing efficiency of filler on rubber, and DMS-NR composite has a 50% higher reinforcing index than PS-NR composite. This result was attributed to the reduction of the silica aggregates in the rubber matrix by the help of the D-MS. Meanwhile, the tensile strength of DMS-NR composite is also higher than that of PS-NR composite, because D-MS was a pre-modified silica, which played a role in reducing stress-concentrated regions. The tensile strength, modulus and reinforcing index of TS-NR composite are higher than those of DMS-NR and PS-NR composites. This result was also due to the chemical interface between silica and rubber formed by TESPT, which played a significant role in improving the reinforcing efficiency of silica on rubber. T&DMS-NR composite exhibits 8%–20% higher modulus, tensile strength and reinforcing index than TS-NR composite, because silica NPs in the T&DMS-NR composite were interconnected by each other and chemically bonded to the rubber molecules.

In

the

T&DMS-NR

composite,

25


the

amounts

of


stress-concentrated regions was reduced and the reinforcing efficiency of silica on rubber was improved by the help of the chemical interface between silica NPs and the chemical interface between silica and rubber. Therefore, T&DMS-NR composite is uniformly and effectively reinforced. Hence, the chemical interface among silica NPs formed by TSPD and the chemical interface between silica and rubber formed by TESPT act synergistically to improve the static mechanical performances of silica/NR composites. The tan δ values at 60 °C is generally considered to reflect the relative magnitude of rolling resistance of rubber composites. As shown in Figure 5 (d), when the strain is larger than 1%, PS-NR composite has the highest tan δ value and T&DMS-NR has the lowest tan δ value. The rolling resistance of PS-NR composite is the highest in these four kinds of silica/NR composites because of a drastic mutual friction between silica NPs under the dynamic situations. The tan δ value in the DMS-NR composite is lower than that in the PS-NR composite. In the DMS-NR composite, the mutual friction between silica NPs decreases, because the chemical interface formed by TSPD among silica NPs served as buffer to prevent mutual friction among silica particles under the dynamic situations. The tan δ value in the TS-NR composite is lower than that in the DMS-NR composite, and T&DMS-NR composite has the lowest tan δ value in these four kinds of silica/NR composites. The mutual friction 26


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among the silica particles that were chemically fixed with the rubber macromolecules was weaken, resulting in reducing the internal friction loss. Two types of chemical interfaces existed in the T&DMS-NR composite simultaneously. The mutual friction among the silica NPs was synergistically prevented by buffer function and “coupling bridge.” The trend of the loss factor changing with strain can also be found in Figure 5 (d). As the strain increases, the tan δ value of T&DMS-NR composite has a less significant change than that of other silica/NR composite. This result means that the rolling resistance of tire prepared by the T&DMS-NR composite has minimum change, when the load of motor vehicle increases. Therefore, using D-MS and TESPT together is a novel method for preparing silica/NR composites with excellent dynamic performances. The loss factor-strain curves of all silica/NR compounds (Figure. 3(b)) have a significant increase under large strain. However, the loss factor-strain curves of all silica/NR composites (Figure. 5(d)) have a gentle increase with the stain increasing. Loss factor is the ratio of the loss modulus (G”) to the storage modulus (G’). In silica/NR compounds, the rubber molecules were not cross-linked; therefore, the G’ and the G” were mainly affected by the elastic collisions between silica particles and the mutual friction among the silica particles, respectively. In silica/NR composites, the G” was still mainly affected by the mutual friction among 27



the silica particles, but the G’ was mainly affected by rubber matrix under large strain. Therefore, the loss factor-strain curves of silica/NR composites and silica/NR compounds exhibit different variations with respect to composition. Figure 5 (c) exhibits the Payne effect of silica/NR composites. The weaker the Payne effect, the better the dispersion of filler in rubber matrix 49. This result is in good agreement with the TEM images. Figure 6 (a) shows the schematic diagram of heat build-up test. The value of heat build-up has a direct relationship with the mutual friction among all components of silica/rubber composites under the dynamic situations. As presented in Figure 5 (b), in these four kinds of silica/NR composites, T&DMS-NR composite exhibits the lowest heat build-up, meaning that T&DMS-NR composite possesses the weakest mutual friction among all components. This result indicate that the tire prepared by the T&DMS-NR composite has a fine dynamic performance.

28


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Figure 6. (a) Schematic diagram of heat build-up and (b) heat build-up of

four kinds of composites

4. Conclusions

We firstly synthesized a novel isolating-dispersed agent, namely, N, N'-bis[3-(triethoxysilyl)propyl-isopropanol]

-propane-1,

3-diamine

(TSPD). The structure of TSPD, which contained six ethoxy groups at opposite ends and a linkage comprising aliphatic groups, hydroxyl groups 29



and secondary amine groups at the center of the molecule, was consistent with the originally designed structure. We prepared a novel pre-modified silica with chemical interface (referred to D-MS) by using of this TSPD. The D-MS NPs were connected to one another but were not conglutinated tightly. As a pre-modified silica, D-MS could be quickly blended and well dispersed in rubber composites. In silica/rubber composites, the chemical interface among silica NPs served as buffer to prevent NPs from approaching one another, resulting in a homogeneous dispersion of silica NPs in the rubber matrix. This fine dispersion of silica in rubber matrix leaded to an improvement in the static mechanical performance of silica/NR composites. The addition of TESPT in D-MS filled rubber composites would bring another chemical interface between silica and rubber. These two chemical interfaces played a synergistic role in improving the static and dynamic mechanical performance for rubber composites. This novel interface design opens up a new avenue for fabricating high performance polymer nano-composites with tailored dispersion of NPs.

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Constructing a Multiple Covalent Interface and Isolating Disperion

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