Preparation and properties of dynamic crosslinked styrene butadiene rubber

2.1 Materials

Commercial BR were supplied by PetroChina Co., Ltd. SBR 1502 E (styrene content = 23.3 wt%, vinyl content = 76.7 wt%) was obtained from Sinopec-China Petroleum Co. Ltd. LA (LA, 98 %) were obtained from Shanghai Meryer Chemical Technology Co., Ltd. 2,2′-Azobis(2-methylpropionitrile) (AIBN, 98 %) were supplied by Shanghai Macklin Biochemical Technology Co., Ltd. Toluene was analytically pure and purchased from Chongqing Chuandong Chemical Co., Ltd. Dichloromethane, and anhydrous ethanol were also analytically pure, supplied by Chengdu Cologne Co., Ltd.

2.2 Preparation

The BR is properly mixed by a two-roller refiner, and added different content of LA in sequence (0.5 %, 1 %, 1.5 %, 2 %, 3 % compared to the molar mass of vinyl in the BR) and AIBN (16.7 % compared to the molar mass of LA) for 14 min. After mixing for 14 min, the compound is obtained. Leave it for 24 h to eliminate internal stress. Set the temperature to 130 °C, measure the positive curing time with a rotorless vulcanizer, and vulcanize with a plate vulcanizer to obtain cross-linked BR by thermally initiating a “simulated click” reaction. After 24 h at room temperature, experimental test strips were prepared as required. The preparation process is shown in Figure 1. The components are named: BLA_0.5, BLA₁, BLA_1.5, BLA₂, and BLA₃.

By adding different contents of ZnCl₂ to the uncured BLA₂ (the molar ratios of LA and ZnCl₂ were 80:1, 60:1, 40:1, and 20:1, respectively) and blending for 16 min, the Zn²⁺-O coordination bond was obtained. The network was placed for 24 h to relieve internal stress, and the temperature was set to 130 °C. The sample names are: BLA-(Zn²⁺)-(LA:Zn²⁺ = 80:1),BLA-(Zn²⁺)-(LA:Zn²⁺ = 60:1), BLA-(Zn²⁺)-(LA:Zn²⁺ = 40:1), BLA-(Zn²⁺)-(LA:Zn²⁺ = 20:1).

2.3 Characterization

1. Fourier transform infrared spectroscopy (FTIR) analysis: the ATR transmission method of the American Nicolet Nexus670 FTIR instrument was used to perform infrared test and characterization on the surface of BR and BLA with different components and different content.

2. Raman spectroscopy (Raman) analysis: the Raman spectrometer produced by HORIBA (HORIBA Scientific Lab RAM HR Evolution) was used to characterize the bond structure of the polymer chain. The laser wavelength was 785 nm, and the measurement range of the Raman spectrum was 300 cm⁻¹–1100 cm⁻¹.

3. Curing time test: weigh about 4 g of the mixed uncrosslinked rubber, use a rotorless vulcanizer (MD-3000 A, Taiwan High Speed Rail), set the temperature to 130 °C, and test the curing time.

4. Thermogravimetric analysis (TGA): the sample of rubber composite was tested by thermogravimetric analyzer (Q50, Waters, USA). Test conditions: nitrogen flow rate is 100 mL/min, scanning speed is 10 °C/min, and temperature is 20–1050 °C.

5. Swelling test: calculate the sol fraction, swelling rate, and cross-linking density of SBR and BR by using equilibrium swelling experiment [26, 27]. First of all, the initial mass was taken as m ₁ , the sample was soaked in toluene for 48 h. Then, the sample was taken out, the surface was dried quickly, and the swelling mass of the sample was m ₂ . Finally, the sample was placed in a vacuum oven and dried at 60 °C until constant weight. After drying, the mass of the sample was measured as m ₃ . Three samples were measured for each component.

   The crosslink density (V_CLD) was calculated by Flory–Rehner formula [28, 29]:

   (1) V C L D = − 1 V s [ ln ( 1 − V R ) + V R + χ V R 2 V R 1 / 3 ]

   V _R is the volume fraction of rubber in the swelling sample, and its formula is as follows:

   (2) V R = m 3 m 3 + ( m 2 − m 3 ) ρ ρ s

   V _S is the molar volume of the solvent (toluene is 105.9 mL/mol at room temperature), ρ is the density of the polymer (measured 1502 E SBR density: 0.901 g/cm³; BR density: 0.905 g/cm³), ρ_s is the solvent density (toluene density: 0.866 g/cm³).

   The swelling rate is calculated as follows:

   (3) m 2 − m 3 m 3 × 100 %

   The sol fraction is calculated as follows:

   (4) m 1 − m 3 m 1 × 100 %

6. X-ray photoelectron spectroscopy (XPS) analysis: the elements in the rubber film were determined by X-ray photoelectron spectroscopy (Thermo Scientific K-Alpha, Thermo). The excitation source was Al Kα rays (0.6eV).

7. Dynamic mechanical analysis (DMA): SLA and BLA were measured by dynamic mechanical analyzer (Q800 type, TA) under the following conditions: double cantilever deformation mode, frequency of 1 Hz; the amplitude is 8 μm; temperature scanning range is −125 °C–80 °C; the heating rate is 5 °C/min.

8. Mechanical performance test: universal testing machine (TSE104B, Shenzhen Wance) was used to test the tensile properties, the test standard was GB/T 528–1998, the sample was dumbbell shape, the tensile speed was 100 mm/min. At least 5 samples were tested in each group and the arithmetic average was taken. The stress-strain cycle test was performed on the sample, the tensile strain was 100 %, and the tensile speed was 100 mm/min. After each cycle, the sample was placed for 15 min before the next cycle test.

9. Polarizing microscope: the sample was cut open with a blade, and the fracture was photographed with a polarizing microscope (Axio Scope Al A, Zeiss, Germany). Then the fracture was fully contacted and placed in an oven with a set temperature for a certain time, and then the repair of the fracture after self-healing was photographed.

10. Self-healing test: the samples were cut into two pieces with a blade, and then the sample sections were joined together, and applied a certain force for 6 min. Then the mechanical properties of the samples under different healing temperatures and healing times were tested. The formula for calculating rubber self-healing efficiency is as follows [25, 30]:

    (5) η = σ ( h e a l e d ) σ ( i n i t i a l ) × 100 %

    σ_(initial) and σ_(waken) are the tensile strength before and after healing, respectively.

11. Scanning electron microscope (SEM): the surface of the prepared cross-linked BLA was sprayed with gold, and the surface morphology characteristics were observed with a scanning electron microscope (Zeiss Merlin, Zeiss, Germany). Then, the elemental distribution on the surface was analyzed with energy spectrum analyzer (Oxford Instruments, UK).

3.1 Structural characterization

Figure 2A is a photograph of the sample before and after swelling with toluene for 48 h. When immersed in toluene for 48 h, BR was completely dissolved to form a viscous polymer solution, while BLA only swelled after soaking for 48 h, indicating that LA and BR can also be cross-linked by hot pressing after blending. This is also demonstrated by the vulcanization time-torque (S´) curves of the BLA samples (Supplementary Figure S1). As shown in Supplementary Figure S2, BLA had the same results after immersion in other common solvents. The crosslink density gradually increased with increasing LA content (Figure 2B). Further ATR-FTIR spectroscopy, Raman spectroscopy and XPS tests were applied to confirm the structural features of BLA and BLA-Zn.

Figure 2:

Structural characterization of BLA and BLA-(Zn²⁺). A) Photos of samples before and after swelling for 48 h; B) crosslinking density (VCLD) of BLA; C) IR absorption spectra of BR, BR + LA, BLA; D) IR absorption spectra of BLA, BLA-(Zn²⁺); E) Raman spectrum of BLA; F) SEM image and element distribution of BLA-(Zn²⁺).

In the infrared spectrum (Figure 2C), about 726 cm⁻¹ is attributed to the out-of-plane bending vibration of = CH, and after cross-linking, the peak here does not change significantly, and the characteristic peak of C–S bond may be related to the surface of = CH The external bending vibration peaks overlap each other, so the existence of C–S bonds cannot be judged, but the existence of C–S bonds is detected in the subsequent XPS analysis (Figure 3). It can be clearly seen from the BLA infrared spectrum of Figure 2C that a characteristic peak appears at 1699 cm⁻¹, which is the stretching vibration peak of C=O, which proves that LA is successfully inserted into the double bond of BR. Supplementary Figure S3 shows the decrease of the C=O characteristic peak from 1710 cm⁻¹ to 1690 cm⁻¹ for different LA contents. It indicates that the C=O characteristic peak is red-shifted with the increase of LA content, which is due to the hydrogen bonding of C=O with –OH. Therefore, the addition of LA can generate reversible hydrogen bond in BLA and confer BR self-healing function (Supplementary Figure S5).

Figure 3:

XPS survey spectra A) and S2p spectra B) of BLA.

Supplementary Figure S3 shows the wavenumber change of the C=O characteristic peak under different LA contents. It can be seen from the figure that when the LA content is 0.5 %, the C=O characteristic peak is at 1710 cm⁻¹, while the LA content is 0.5 %. When the content increased to 3 %, the wavenumber decreased to 1690 cm⁻¹, indicating that with the increase of LA content, the characteristic peak of C=O shifted to red, which was caused by the hydrogen bond between C=O and –OH.

Figure 2D shows the infrared absorption spectra of BLA and BLA-(Zn²⁺) under the same LA content. It can be seen from the figure that 1699 cm⁻¹ is the stretching vibration peak of C=O in BLA, and 1690 cm⁻¹ is the stretching vibration peak of C=O in BLA-(Zn²⁺) and the characteristic peak of C=O in BLA-(Zn²⁺) are shifted to lower wavenumbers compared with the characteristic peak of C=O in BLA, and have a red shift, while 1264 cm⁻¹ The stretching vibration peak of C–OH on the carboxyl group at basically disappeared in BLA-(Zn²⁺), indicating that Zn²⁺ successfully formed a coordination bond with O in the carboxyl group.

As shown in Figure 2E, the Raman spectrum of BLA has the absorption peak of S–S bond at 510 cm⁻¹ and the absorption peak of C–C bond at 991 cm⁻¹. The S–S bond is generated because a part of the LA in the reaction opens the five-membered ring through thermal initiation, and the formed sulfur radical reacts with the sulfur radical of another LA to form a reversible covalent S–S bond. The hydrogen bond interaction, through the introduction of double reversible bonds, enables BLA to obtain self-healing function, which indicates that self-healing rubber can still be prepared from LA in BR.

Figure 2F is the SEM image of BLA-(Zn²⁺) and the energy spectrum of surface scanning. It can be seen from the figure that the surface of BLA-(Zn²⁺) is smooth and smooth, and there is no Zn²⁺ aggregate, which is also measured from the energy spectrometer. The existence of three elements, S, O, and Zn, and the distribution of the three elements is uniform, which also shows that Zn²⁺ exists in the BLA-(Zn²⁺) material after forming a coordination bond with O.

The DMA of BLA shows the relationship between the storage modulus, tan δ and temperature dependence of BLA under different LA contents (Figure 4). From the storage modulus-temperature curve of Figure 4A, it can be seen that when the temperature is lower than about −120 °C, BLA is in a glass state, showing a higher modulus, while as the temperature increases, the rubber molecular chain The exercise capacity is enhanced, so the modulus decreases, and with the increase of LA content, the modulus increases from 4813 MPa to 6272 MPa. Furthermore, it is observed from Figure 4B that the peak of the tan δ-temperature curve decreases from 0.17 to 0.16 with the increase of LA, and the temperature corresponding to the peak increases from −98 °C to −95 °C, resulting in an increase in T_g. Since the crosslinking density increases with the increase of LA content, the movement of the rubber molecular chains is hindered, which is consistent with the swelling experimental results.

Figure 4:

Storage modulus curves A) and tan δ curves B) of BLA with different content LA.

BLA was tested by TGA to analyze its thermal stability. Figure 5 shows the thermal weight loss curves of BLA under different LA contents. It can be seen from the figure that the initial decomposition temperature of SLA is around 330 °C, and the temperature of the maximum weight loss rate is around 450 °C. Still have excellent thermal stability.

Figure 5:

TGA curves of BLA with different content LA.

3.2 Mechanical properties

Figure 6A shows the stress-strain curves of BLA with different LA contents. It can be seen from the figure that with the increase of LA content, the tensile strength of BLA increases and the elongation at break decreases. When the LA content is 0.5 %, the tensile strength of BLA is 1.12 MPa, and the elongation at break is 937 %. With the increase of LA content, the tensile strength can reach up to 2.21 MPa, and the corresponding elongation at break is 519 %. The increase in tensile strength is due to the increase in LA content, which provides more disulfide bonds to interact with hydrogen bonds, which increases the cross-linking density of BLA, thereby forming more cross-linking points, which restricts the movement of the rubber molecular chain. As a result, its tensile strength increases and its elongation at break decreases.

Figure 6:

Mechanical properties of BLA and BLA-(Zn²⁺). A) Stress–strain curve of BLA with different content LA; B) the first cyclic loading–unloading stress–strain curves of BLA with different content LA at room temperature; C) the first, the second and the third circle stress–strain curves of BLA₂; D) stress–strain curve of BLA₂ with different content Zn²⁺; E) the first cyclic loading–unloading stress–strain curves of BLA₂ with different content Zn²⁺ at room temperature; F) the first, the second and the third circle stress–strain curves of BLA-(Zn²⁺)-LA:Zn²⁺ = 40:1.

It can be observed from Figure 6B that the stress during stretching is greater than that during unloading, resulting in a hysteresis phenomenon, which is due to the disentanglement of some BR molecular chains during the stretching process. As the LA content increases, the strain recovery is greater, which is due to the increase in the degree of cross-linking and the better the resilience of the rubber. From Figure 6C, it can be seen that the stress-strain curve of the three cycles does not change much, indicating that in more BLA still has good resilience in the secondary stretching, which indicates that it has the characteristics of permanently cross-linked rubber.

Figure 6D shows the stress-strain curves of BLA-(Zn²⁺) with different Zn²⁺ contents. It can be observed that the addition of Zn²⁺ makes the mechanical properties higher than that of BLA. With the increase of Zn²⁺ content, the tensile strength of BLA-(Zn²⁺) increases and the elongation at break decreases. When the molar ratio of LA to Zn²⁺ was 80:1, the tensile strength was 2.52 MPa and the elongation at break was 543 %, while when the molar ratio was 20:1, the tensile strength increased to 3.76 MPa, and the elongation at break was 3.76 MPa. reduced to 484 %. This is because the metal coordination bond formed by Zn²⁺ with oxygen in the carboxyl group of LA has high bond energy and good kinetics.

Figure 6E is the first loading–unloading stress-strain curve of BLA-(Zn²⁺) material with different molar ratios of LA to Zn²⁺, (Figure 6F) is the first three times that the molar ratio of LA to Zn²⁺ is 20:1 stress–strain curves during cyclic stretching. It can be seen from the figure that BLA-(Zn²⁺) also has excellent resilience, and its stress–strain curves still basically overlap during repeated loading–unloading processes.

3.3 Self-healing properties

Through the above test analysis, BLA contains dynamic disulfide bonds and hydrogen bonds, which will lead to excellent self-healing properties. Figure 7B is the optical microscope image of BLA-(Zn²⁺) before and after self-healing. It can be seen that the cracks generated in BLA-(Zn²⁺) can be obviously repaired after being treated at 65 °C for 24 h. From the stress–strain curves of BLA and BLA-(Zn²⁺) at different healing temperatures and times (Figure 7D and E), it can be seen that the healing efficiency increases as the healing temperature and healing time increase. The healing efficiencies of BLA and BLA-(Zn²⁺) reached only 37 % and 48 %, respectively, at a temperature of 40 °C. When the temperature was raised to 85 °C, the healing efficiencies reached 73 % and 82 %, respectively (Figure 7F). As the temperature increases, the movement of the molecular chain becomes easier, and the material undergoes self-healing through the synergistic effect of dynamic bond breaking and recombination, and as time goes on, more dynamic bonds participate in the reaction, so it has a higher repair rate. Comparing the self-healing efficiency of BLA and BLA-(Zn²⁺) at different temperatures (Figure 6D), BLA-(Zn²⁺) has better repairing effect than BLA, which is due to the coordination of metal ions with oxygen atoms as well as disulfide and hydrogen bonds synergy between them.

Figure 7:

Self-repair mechanism of BLA-(Zn²⁺). A) Photos of cut BLA-(Zn²⁺) sample and stretched after healing; B) the optical microscopy images of cut BLA-(Zn²⁺) sample after healing at 65 °C for 24 h; C) schematic illustration of BLA-(Zn²⁺) healing process; D) the stress-strain curves of BLA₂ after healing at various temperatures for 24 h; E) the stress-strain curves of BLA-(Zn²⁺) after healing at various temperatures for 24 h; F) healing efficiency of BLA and BLA-(Zn²⁺) after healing at various temperatures for 24 h.