The Anisotropic Self-Assembly Effect of Silica Hairy Nanoparticles: toward Sustainable Rubber Nanocomposites for Tyre Application

doi:10.21203/rs.3.rs-4681711/v1

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The Anisotropic Self-Assembly Effect of Silica Hairy Nanoparticles: toward Sustainable Rubber Nanocomposites for Tyre Application

https://doi.org/10.21203/rs.3.rs-4681711/v1

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With the aim to identify possible strategies to reduce the environmental impact of tyre, we propose the use of advanced rubber materials based on silica hairy nanoparticles (HNPs), which consist of a core of silica surrounded by end-grafted rubber chains. The rational has been to impart unique surface reactivity to the silica NPs, usually used as reinforcing fillers, for coupling with the non-crosslinked host elastomer as well as the subsequent ability to employ coupling agents other than the conventional sulfur-bridged bis-alkoxy silanes. SiO₂ HNPs, having polybutadiene (PB) chains as polymeric brushes, were prepared with different degrees of PB grafting density and embedded in elastomer to produce rubber nanocomposites (NCs), following the standard tyre formulation but without using silane coupling agents. A deep investigation on the relation between the mechanical properties of the NCs materials and HNPs self-assembling behavior was performed, showing a high degree of structural order and a significant percolative HNPs network. In fact, silica HNPs can build anisotropic energy-dissipation structures which act as additional cross-linking junctions reducing the chain relaxation and consequently enhancing the chain deformation.

The presence of self-assembled fillers domains induces a reduction in the mobility of the filler and consequently in the energy consumption due to the deformation of the microstructure of the rubber composite, with its breakage and subsequent recovery, not completely reversible. For this, the present study supports the use of HNPs as reinforcing filler for producing more sustainable rubber materials.

silica

hairy nanoparticles

rubber nanocomposites

rolling resistance

green tyre

In rubber nanocomposite (NC), the main contribution of filler nanoparticles (NPs) is to enhance the mechanical properties of the material [1–5] and in particular the reinforcement. Moreover, the current open challenge of the tyre industry is to identify possible design strategies to reduce the environmental impact throughout the life cycle of tyre by means of both the use of environmentally friendly materials [1, 6–10], and the development of innovative products able to reduce energy consumption and CO₂ emissions [11, 12]. In fact, the significant impact of tyres on energy consumption is due to resistive forces acting on it when vehicles move: lower value of resistance means better acceleration, higher speed with lower fuel consumption and CO₂ emission [13], [14]. The force resisting the motion of the tyre when it rolls on a road surface is called rolling resistance, defined as the longitudinal force oriented in the opposite direction with respect to the rolling movement [15]. This force depends on many factors related to road, tyre, and operating conditions, and among them, the most important is the cyclic deformation of tyre, producing a loss of the internal mechanical energy on the material. This phenomenon is also known as “hysteresis” and it is related to both viscoelastic and mechanical properties of the rubber NCs. Hysteresis can be minimized by tailoring the filler-filler and filler-rubber interactions on the nanoscale of rubber material, favoring a good filler dispersion and distribution with an effective interaction of NPs with rubber matrix, producing an extended percolative network [16, 17]. Actually, a strong immobilization of the polymer chains close to the NPs surfaces within their network increases the mechanical reinforcement producing lower Payne effect [18], and consequently low hysteresis. Despite the performing and established practice to add silica in rubber NCs as a reinforcing filler, in the current context of petroleum resource depletion and of a more sustainable economy, alternatives fillers are considered to develop a more sustainable tyre, while offering high mechanical performances (reinforcement, adhesion, tensile strength) [19, 20].

To achieve this objective, two fundamental starting points were considered. Firstly, the rubber NC mechanical properties increases with very large contact area between filler and polymer, especially, when the particles self-assemble to form the aligned domains, as we reported previously in the case of anisotropic filler such as the rod-like silica and sepiolites [6, 16]. These domains consist of organized and self-assembled NPs, with rubber layer strongly bonded on the NPs silica surface, having the thin layers around filler particles overlap and build up bridges between the fillers, enhancing the amount of overlapping rubber layers. This produces not only the reinforcement of rubber matrix but also the Payne effect reduction with decreased energy dissipation. In fact, the presence of self-assembled fillers domains induces a reduction in the mobility of the filler and consequently in the energy consumption due to the deformation of the microstructure of the rubber composite, with its breakage and subsequent recovery, not completely reversible. Secondly, when NPs self-assemble in anisotropic structures such as strings, sheets, or connected structures, within polymer matrix, significantly improve a range of polymer NCs properties, e.g., thermal conduction, mechanical toughness, selective permeation of gases etc. with profound application potentials in materials.

Starting to this, our strategy has been to exploit the effect of silica fillers self-assembly in anisotropic structures with purpose to obtain sustainable rubber materials having lower Payne effect and consequently low hysteresis. To induce the formation of the self-assembled anisotropic NPs superstructure, we embedded in rubber matrix the silica hairy NPs (HNPs) [21], which consist of a core of silica surrounded by end-grafted polymer chains, ranging from low to high molecular weight.

The objective is to impart unique surface reactivity of silica NPs for coupling with the non-crosslinked host elastomer as well as the subsequent ability to employ coupling agents other than the conventional sulfur-bridged bis-alkoxy silanes [22, 23].

Although some examples on the impact of HNPs on the mechanical properties of polymer-based NCs are reported in the literature [24–29], there are very few examples relating to elastomers. Bonnevide et al. reported the synthesis of SiO₂ NPs decorated with grafted polyisoprene (PI), polybutadiene (PB) and styrene-butadiene rubber (SBR) brushes through a grafting-from approach [30] and, in particular, investigated the morphologies of PI-grafted NPs aggregates in PI and SBR matrices, showing evidences of self-assembly process [31]. However, no investigation on the relation between the mechanical properties of the NCs materials and HNPs self-assembling behavior was performed.

Recently, we reported on the preparation and characterization of SiO₂ HNPs [32], having PB chains as polymeric brushes, able to self-assembly in anisotropic superstructures in a one-component system (containing only HNPs). Therefore, the present study focuses on the use of these HNPs as reinforcing filler in SBR tyre formulation instead of conventional nano-silica. Rubber NCs were prepared by melt mixing the elastomer matrix and SiO₂ HNPs [32] with different degrees of PB grafting density, without using silane coupling agents.

The specific organization of SiO₂-HNPs in the polymer matrix were inspected by transmission electron microscopy (TEM) and related to the mechanical properties of rubber NCs investigated through dynamic-mechanical tests, and also not conventional measurements such as Large Amplitude Oscillatory Shear (LAOS). These analyses reveal that moderately strong rubber − filler interactions exist in SBR NCs, which results in a high degree of structural order and a significant percolative HNPs network. Silica HNPs can build anisotropic energy-dissipation structures which act as additional cross-linking junctions reducing the chain relaxation and consequently enhancing the chain deformation.

We expect the present study to be an important contribution, since very scant information has been reported in the literature regarding the use of HNPs as reinforcing filler for rubber materials.

2.1 Materials

SiO₂ NPs and HNPs

Tetraethyl orthosilicate (TEOS, ≥ 99%), 25% aqueous ammonia solution (NH₄OH), 37% aqueous hydrochloric acid and 3-amminopropyltriethoxysilane (APTES, ≥ 98%) were purchased from Sigma-Aldrich. Ethanol (≥ 99.8%) was purchased from Honeywell. Toluene (99%) was purchased from Alfa-Aesar. Polyvest MA75 (PB) (MW ~ 3000 g/mol) was purchased from Evonik.

Compounding

SBR was Europrene SOL R C2525 obtained by Versalis (26% styrene; 24% vinyl; 50% butadiene); stearic acid was Stearina TP8 from Undesa; sulfur was from Zolfoindustria; zinc oxide (ZnO) was from Zincol Ossidi; N-cyclohexyl-2-benzothiazole sulfenamide (CBS) was Vulkacit CZ/C from Lanxess.

2.2 Synthesis of SiO₂ HNPs

SiO₂ NPs and HNPs were synthesized according to the procedure already reported [32] Briefly, for the synthesis of the SiO₂ core was performed by Stöber method, and successively, SiO₂ NPs (labelled SiO₂-ST) were functionalized by APTES (labelled SiO₂-APTES), as reported in Supporting Information, SI. For the preparation of silica HNPs with different degrees of PB grafting density 0.4680 g, 0.9360 g or 1.8720 g of PB were dissolved in toluene (5 mL), added to the SiO₂-APTES dispersion and heated at reflux under stirring (300 rpm) for 24 h. NPs were then centrifuged and washed with toluene, then dried at 85°C overnight obtained SiO₂-HNPs with PB percentage of functionalization of about 3, 4 and 6% (namely, SiO₂-HNP_X, where X is the PB wt%),.

2.4. Preparation of SiO₂ HNPs NCs

Rubber materials were prepared by blending technique in a Brabender Plasti-Corder lab station internal mixer having 65 mL mixing chamber and using 0.6 filling factor. The compounding procedure was performed as reported in the Scheme 1, by using different temperatures (145, 90 and 50°C), considering the thermal stability of the reactants, during 3 steps, as following.

1° step

at 145°C, SBR was masticated by applying a 60 rpm rotor speed, while 35 parts per hundred rubber (phr) of SiO₂-HNP_X were added in three successive portions. Filler amounts were calculated considering the degree of functionalization. In order to avoid the undesired reaction between ZnO and silica silanols, only after two minutes from the last filler addition, ZnO (2 phr) and stearic acid (2 phr); were added.

2° step

the obtained compound was reloaded in the internal mixer operating at 90°C at 60 rpm. CBS (3 phr) and sulfur (1 phr) were then added and mixed for 2 min.

3° step: composites were processed in a two-rolling mill at 50°C for 3 min.

Herein, uncured samples are called SiO₂-HNP_X/SBR. Reference materials, prepared with 35 phr of SiO₂-ST and SiO₂-APTES, are called SiO₂-ST/SBR and SiO₂-APTES/SBR, respectively. At the end, vulcanization process was performed in a hydraulic press at 170°C and 100 bar for 30 min, producing vulcanized samples, namely V-SiO₂-ST/SBR, V-SiO₂-APTES/SBR and V-SiO₂-HNP_X/SBR.

2.5. Physico-chemical, morphological, and spectroscopic characterization of SiO₂ HNPs

The degree of functionalization of SiO₂-APTES and PB grafting of SiO₂-HNP_X was estimated by CHNS (temperature of the combustion column = 1150°C, reduction column = 850°C) and TGA analyses. TGA (Fig. S1 of SI) was carried out on a Mettler Toledo TGA/DSC1 STARe under constant air flow (50 mL min^-1), by first heating in the range 30–150°C (heating rate = 5°C min^-1), then an isotherm at 150°C for 10 min, and finally heating up to 1000°C (heating rate = 10°C min^-1). In detail, TGA was used to determine the surface functionalization of both SiO₂-APTES and SiO₂-HNPs, according to Eq. (1):[33, 34]

SiO₂ powders were also investigated by TEM analysis, performed on a JEOL JEM 2200 FS operating at 200 kV mounting two CEOS Cs correctors (CETCOR, CESCOR), a JEOL JED-2300 Si(Li) EDX (energy dispersive X-ray spectroscopy) detector, a Gatan 4K UltraScan 1000 camera and a HAADF (high angle annular dark field) detector. Regarding the sample preparation the material was ground into a fine powder and suspended in toluene by sonication. Finally, the suspension was dropped onto a carbon-coated 400-mesh TEM grid and the solvent in excess removed by drying under air. Images at different magnifications were obtained by TEM in bright-field parallel imaging mode.

2.6 Morphological characterization of uncured and cured HNPs NCs

All SiO₂-HNP_X/SBR vulcanized composites were sectioned at -130°C with a Leica EM FCS cryo-ultramicrotome, obtaining 50 nm nominally thick sections. TEM analyses at different magnifications were carried out on the cryo-sections by using a Zeiss EM 900 microscope (80 kV).

2.7 Determination of crosslinking density

To evaluate the cross-linking density, V-SiO₂-HNP_X/SBR NCs were treated in swelling experiments, starting from samples of 10x10x1 mm³, after weighting the samples to define the initial mass (m₀).

The procedure was the following: samples were immersed in 10 mL of toluene at room temperature in closed vials, replacing the solvent daily with fresh toluene to eliminate all the extracted fractions within 72 h. Thus, the swollen samples were dried for 24 hours at room temperature to obtain dried samples. Both samples were weighted, and the mass of swollen samples is defined as m_S, while that of dried samples is as m_S.

To determine the volumetric fraction of the swelled rubber, namely V_R, the following Eq. (2 ) [35–37] was applied:

$$\:{V}_{R}=\frac{({m}_{D}-f\bullet\:{m}_{0})\bullet\:{\rho\:}_{p}^{-1}}{\left({m}_{D}-f{m}_{0}\right)\bullet\:{\rho\:}_{p}^{-1}+{m}_{SO}\bullet\:{\rho\:}_{s}^{-1}}$$

where m_SO corresponds to the solvent weight in the swollen sample (i.e. m_SO = m_S – m_D), ρ_p is the SBR density whose value is 0.98 g·cm^− 3, while ρ_s is the density of toluene solvent, i.e. 0.87 g·cm^− 3, and the filler fraction in the composites is the f parameter, determined from TGA.

Starting from V_R, it is possible determine the cross-linking density ν (mol/cm³) by using the Flory-Rehner Eq. (3):

$$\:\nu\:=\frac{ln\left(1-{V}_{R}\right)+{V}_{R}+\chi\:\bullet\:{{V}_{R}}^{2}}{-2\bullet\:{\rho\:}_{p}\bullet\:{V}_{S}\bullet\:{{V}_{R}}^{1/3}}$$

where χ is defined as the Flory solvent-polymer interaction term, corresponding to toluene-SBR system in this case, and V_S is the molar volume of toluene, whose values are equal to 105.91 and 0.49, respectively [38, 39].

2.8 Dynamic-mechanical analysis of uncured and cured HNPs NCs

Before curing, Constant Volume Rubber Sample Cutter (CUTTER 2000, Alpha Technologies) were used to obtain samples of both the uncured and cured NCs having both specific dimensions and mass: 3.5 cm diameter, ≈ 0.2 cm thickness and 6 ± 0.3 g weight.

Rubber Process Analyzer (RPA2000, Alpha Technologies) were utilized for the materials curing by considering the specific set-up parameters: ±1° of oscillation angle, at 170°C and 15.65 kPa, for 30 min a time nedeed to reach the curing plateau, corresponding of about 30 min.

RPA analysis allows to obtain parameters such as minimum torque (M_L), maximum torque (M_H), and the time required for the start of vulcanization process, known as scorch time, and here measured as t_s2, which is the time elapsed from the test start to that in which the value of torque exceeds the M_L value of 2 units.

The strain sweep tests were carried by using the same rheometer in the shear stress mode, at 70°C and 1 Hz from 1–100% elongation for unvulcanized samples, at 70°C and 10 Hz from 1 to 10% elongation for vulcanized ones, in order to obtain the storage modulus (G′).

We also report the values of G’ at 0.4% as G′₀ and G’ at 10% as G′_∞, as good indicators of the rubber materials reinforcement, Payne effect and hysteresis, respectively.

LAOS measurements were performed on uncured samples, with the same instrument, at 90°C and 0.5 Hz from 1 to 800% elongation.

3.1. SiO₂-HNP_X synthesis and characterization

The synthesis of SiO₂-HNP_X fillers were performed according to the procedure reported in our previous paper [32]. However, SiO₂-HNP_4 and SiO₂-HNP_6 fillers were synthetized and here characterized for the first time (Fig. 1), to discuss the effects of the particles and self-assembly into rubber NCs.

The sketch of SiO₂-HNP_X structure is shown in Fig. 1a. The surface functionalization of fillers was estimated by a TGA/CHNS combined approach: CHNS elemental analysis of SiO₂-ST was performed to determine the amount of residual surface ethoxy-groups, while TGA (Fig. S1) to determine the surface functionalization of both SiO₂-APTES and SiO₂-HNP_X samples. The resulting functionalization amounts are reported in Fig. 1b. The effective grafting of PB chains onto SiO₂-APTES NPs was confirmed by FTIR-ATR and NMR analyses, as previously described (data not reported) [40]. As supported by TEM micrographs (Fig.s 1c and S2), the SiO₂-HNPs consist of SiO₂ core particles mainly spherical with 27 ± 5 nm diameter, covered by a thin PB uniform shell. The reported diameter is the number average of at least 100 NPs of at least 5 different images (the uncertainty is the standard deviation).

3.2 SiO₂-HNP/SBR NCs: curing behavior and swelling experiments

Rheological analysis and swelling experiments were performed to evaluate the effect of HNPs on curing process of rubber materials. Firstly, the variation of viscosity over the time was measured with the torque requested to keep the rotor at a constant rate, i.e. the vulcanization curves for SiO2-HNP_X/SBR NCs (Fig. 2) and compared with NCs containing pristine SiO₂ and SiO₂-APTES.

The vulcanization characteristics of V-SiO₂-HNP_X/SBR NCs (Table S1 of SI) are expressed in terms of: i) higher maximum torque (M_H); ii) lower minimum torque (M_L); iii) scorch time (t_S2) [19]. For all the samples, t_S2 (time to 2 units of torque increase above M_L), is about 5.0 min, which is the standard time needed in the rubber industry [22] for moulding tyre. In all NCs samples, the M_L values, associable to the compounds viscosity and to the filler dispersion in the uncured sample, are in the range of 2.4–1.8 dNm, indicating a good NPs dispersion in rubber. In particular, the V-SiO₂-HNP_4/SBR and V-SiO₂-HNP_6/SBR NCs show the lowest M_L, likely due to the presence of higher density of PB polymer chains on the HNPs surface, inducing a better connection between filler and polymer and, consequently, a more efficient dispersion. The crosslink efficiency and stiffness of the rubber, as indicated by M_H values, are satisfactory precisely thanks to the incorporation of SiO₂ HNPs in SBR matrix, also considering the contribution of filler network [17, 19]. In particular, the enhanced M_H of V-SiO₂-HNP_4/SBR and V-SiO₂- HNP_6/SBR NCs with respect to both V-SiO₂-ST/SBR and V-SiO₂-APTES/SBR suggests significant interaction between silica and rubber in modified SiO₂-HNP_X cured composites, modulated by the polymer shell presence. In detail, the high Δ(M_H-M_L) values for V-SiO₂-HNP_X/SBR samples confirm the high reactivity of HNPs with the sulfur-based vulcanization compounds, mediated by PB shell. With respect to these samples, the NCs containing silica or silica functionalized only with APTES show slightly lower values of about 2–4 dNm. Therefore, the NCs containing SiO₂-HNPs with higher polymer shell grafting density exhibit not only better dispersibility and surface chemistry but also the increased interaction with the sulfur-based compounds, despite the absence of the silanization agent, leading to an efficient vulcanization process. On the contrary, the low M_H value obtained for SiO₂-ST/SBR indicates a low vulcanization grade, probably due to the presence of large amounts of acidic silanol groups on the surface of the SiO₂ NPs.

Swelling and extraction experiments in toluene were performed on cured rubber samples to evaluate the effect of the filler on the degree of crosslinking in the samples. The values of crosslink density (ν) for the V-SiO₂-HNP_X/SBR are summarized in Fig. 3 and reported in Table 1.

Table 1

Values of crosslink density ν for V-SiO₂-HNP_X/SBR.
Sample	Crosslink density ν (mol/cm³)
V-SiO₂-ST/SBR	4.88 × 10^− 6
V-SiO₂-APTES/SBR	6.65 × 10^− 6
V-SiO₂-HNP_3/SBR	4.97 × 10^− 6
V-SiO₂-HNP_4/SBR	7.99 × 10^− 6
V-SiO₂-HNP_6/SBR	14.28 × 10^− 6

As expected, V-SiO₂-ST/SBR NC presents the lowest value of ν, suggesting that i) the vulcanization efficiency is low probably due to the surface acidic silanol groups, and ii) the high aggregation and low compatibilization of filler NPs prevents the formation of effective filler-rubber interactions. Thus, the crosslink density of the reference sample is relatively low. The functionalization with amino-silane improves the crosslinking density in the rubber NC due to the improved dispersion of SiO₂-APTES NPs and their good interaction with the rubber matrix.

Considering the case of HNPs, by increasing the PB grafting density of SiO₂ HNPs the crosslinking density increases accordingly, reaching its maximum value for V-SiO₂-HNP_6/SBR, of about 14.28×10^− 6 mol/cm³, more than three times higher than the value of the reference sample and more than double the value of V-SiO₂-APTES/SBR. This high degree of crosslinking is indicative of a high interaction between silica filler and polymer matrix in modified SiO₂-HNP_X cured composites, due to the presence of the polymer shell, as confirmed by the high M_H obtained by vulcanization curve. On the other hand, the increase of crosslink density observed seems not to correlate with a different curing mechanism as normally observed when modifying the curing chemistry. The swelling restriction can be then associated with the presence of a structured filler, which can boost the immobilized rubber by occlusion, in line with both mechanical tests and morphological analysis, as reported below (Paragraph 3.3).

These outcomes may be explained by referring to the peculiar surface properties of HNPs for improved coupling with the non-crosslinked host elastomer, confirming the high reactivity of HNPs.

3.3 SiO₂-HNP/SBR NCs: dynamic and mechanical behavior

The investigation of the dynamic-mechanical behaviour of the composites performed by RPA allowed to evaluate the effect of filler networking and of SiO₂-HNPs and polymer interaction.

Storage modulus G’ for SiO₂-HNP_X/SBR NCs before (Fig. S3) and after vulcanization (Fig. 4) was measured. Curves of loss modulus G’’ and loss tangent (tanδ) of vulcanized samples are reported in Fig. 5. Values of G′(6%), Δ(G′(0.4%)-G′(10%)), and tanδ(6%), representative of the reinforcement, Payne effect and hysteresis respectively [1] were in-dept evaluated (Table 2).

A pronounced nonlinear dependence of the modulus, known as Payne effect [18] is observed in all cases. In fact, the G′ value at low strain decreases rapidly by increasing the strain amplitude. In details, before curing (Fig. S3), the storage modulus at low and high strain resulted very similar for all the composites, except for the reference material. After curing (Fig. 4), a remarkable G’ increase either at low or at high strain was observed except for the sample SiO₂-ST/SBR, due to very high filler aggregation [18, 41]. In fact, the V-SiO₂-ST/SBR reference material shows a relatively flat curve, with the lowest G’ and Δ(G′(0.4%)-G′(10%)), as reported in Table 1. This indicates that bare SiO₂-ST NPs are too aggregated to result in effective percolation of the filler network [13, 42]. After silica functionalization with APTES, a strong increase of Payne effect occurs in V-SiO₂-APTES/SBR NCs, while the reinforcement is still very low. This effect was already observed in case of sol-gel in situ derived silica NC [43]. The presence of APTES as surface modifier expectedly enhances G’ at low deformation at the expenses of a low G’ at high deformation because of the strong filler-filler interactions deriving from the polar modifier, hence giving a high Payne effect.

Table 2

Values of G'(6%), Δ(G′(0.4%)-G′(10%)) and tanδ(6%) for V-SiO₂-HNP_X/SBR.
Sample	G′(6%) (MPa)	Δ(G′(0.4%)-G′(10%)) (MPa)	tanδ (6%)
V-SiO₂-ST/SBR	1.168 (± 0.05)	0.102 (± 0.02)	0.169 (± 0.01)
V-SiO₂-APTES/SBR	1.070 (± 0.05)	0.246 (± 0.02)	0.165 (± 0.01)
V-SiO₂-HNP_3/SBR	1.163 (± 0.05)	0.238 (± 0.02)	0.170 (± 0.01)
V-SiO₂-HNP_4/SBR	1.189 (± 0.05)	0.222 (± 0.02)	0.139 (± 0.01)
V-SiO₂-HNP_6/SBR	1.300 (± 0.05)	0.173 (± 0.02)	0.165 (± 0.01)

On the contrary, NCs samples containing HNPs show higher reinforcement and lower Payne effect with respect to V-SiO₂-APTES/SBR. This points to the positive impact of filler functionalization with macromolecules, such as PB, with respect to that of small silane molecules, such as APTES, on the filler-rubber interaction. This suggests a strong immobilization of the polymer chains close to the nanofiller surfaces or within their network, which increases the mechanical reinforcement of the composites.

Evidently, after grafting the macromolecules on the filler surface, the interaction of the NPs with the polymer chains improves as well as the cross-linking coupling. In particular, V-SiO₂-HNP_6/SBR sample clearly displays the best reinforcing performance, having the highest value of G’(6%) (1.300 MPa) and the lowest Δ(G′(0.4%)-G′(10%)) (0.173 MPa) compared to all other samples.

The G’’ curves of vulcanized samples (Fig. 5a) allow to study the viscous contribution to the mechanical response of the materials. A broad maximum in the G’’ curve is present at around 1% of strain for all NCs. This maximum indicates the maximum strain at which a NC sustains the deformation without undergoing a breakdown of the filler network [17]. The high value of G’’ for the sample V-SiO₂ -HNP_6/SBR may thus be rationalized by considering that higher loss moduli correspond to the breakdown of a stronger filler network due to the presence of higher amounts of rigid rubber and crosslinking density. This is consistent with the higher values of reinforcement as indicated by G’ curve of V-SiO₂-HNP_6/SBR.

The curves of tanδ (Fig. 4b), obtained from the ratio between the storage and loss moduli, describe the dissipation of energy during the deformation of the sample, a parameter also known as hysteresis. Values of tanδ (6%) (Table 3) are representative of the hysteresis of the samples and can be associated to the rolling resistance. The lowest values of tanδ across the whole deformation is disclosed by V-SiO₂-HNP_4/SBR NC. Thus, the SiO₂-HNP_4 filler appears to significantly improve rolling resistance while maintaining a very high value of reinforcement. Compared to V-SiO₂-HNP_4, NCs with SiO₂-HNP_X having both higher (X = 6) and lower (X = 3) polymer surface coverage show a slightly increased hysteresis, suggesting that the intermediate coverage might compensate between filler-filler aggregation and dissipative effects due to the un-crosslinked chains of the polymer shell.

Tensile stress/strain tests (Fig. 6) were performed to study the performance of the materials at high strain, allowing to determinate the elongation at break (E_B) and stress at break (S_B), as reported in Table 3. V-SiO₂-ST/SBR NC shows the lowest values of stress at all strain values, supporting the conclusion that high SiO₂-ST aggregation prevented the formation of the filler network. In V-SiO₂-HNP_X/SBR NCs, the slope of the stress/strain curves consistently increases by increasing the PB grafting density.

Table 3

Values of S_B and E_B* for V-SiO₂-HNP_X/SBR NCs compared with reference materials.
Sample	S_B (MPa)	E_B (%)
V-SiO₂-ST/SBR	2.1 ± 0.2	700 ± 100
V-SiO₂-APTES/SBR	4.6 ± 0.3	550 ± 10
V-SiO₂-HNP_3/SBR	4.8 ± 0.3	570 ± 50
V-SiO₂-HNP_4/SBR	3.7 ± 0.1	480 ± 10
V-SiO₂-HNP_6/SBR	5.3 ± 0.2	460 ± 20

*Each value is the average of three tested specimens.

In detail, V-SiO₂-HNP_6/SBR possess the highest stress across the strain range, indicating the highest degree of reinforcement [16], as well as the highest value of S_B (5.3 ± 0.2 MPa) and the lowest EB (460 ± 20%) suggesting that the reinforcing filler network provided by the HNPs self-organization increases the amount of rigid rubber in the sample. This is consistent with the results obtained from dynamic-mechanical analysis.

The study of the large-strain nonlinear mechanical behavior of NCs was performed by LAOS analysis.

The aim is to study the nonlinear stress response of NCs to gain additional information on the nonlinear behavior of the tested materials [44]. In fact, although linear oscillatory shear analysis can provide useful information, the theory is valid only at small deformations, while most applications involve large and rapid deformations. LAOS can provide insight into the correlation of microstructural differences of the tested materials, such as filler aggregation, distribution and self-assembly, with their mechanical behavior [45–51]. The relative intensity of the third harmonic contribution to the elastic modulus, namely I_3/1, has been shown to be a significant indicator of the nonlinear mechanical behavior of filled rubber [46, 50]. Leblanc et al.[49] showed that the behavior of the I_3/1 curve at large strain contains information on the aggregation state of the filler. I_3/1 curves of the uncured NCs obtained from LAOS analysis are reported in Fig. 7.

A qualitative analysis of the behavior of the curves clearly shows that SiO₂-ST/SBR NC exhibits the largest “bump” starting at a strain of 400%, as well as NC containing SiO₂-APTES although with a less pronounced behavior. The presence of “bumps” in the I_3/1 curve of V-SiO₂-ST and V-SiO₂-APTES around the threshold of 400% strain is indicative of the breakdown of filler aggregates due to high filler-filler interactions. Conversely, SiO₂-HNP_X/SBR samples exhibit smoother curves, suggesting an improved filler/rubber interaction and reduced filler aggregation. This is a significant indication that the polymeric shell of HNP fillers promote the filler-rubber interaction with a positive effect on improving interphases interface and avoiding the filler agglomeration and aggregation, that is detriment for materials hysteresis.

The analysis of Nie [41] suggests a quantitative approach to resolve filler network from polymer network contributions to nonlinear response of LAOS analysis. In a silica/SBR system, the authors found a filler network contribution centred around 10% deformation and a polymer contribution at 150% deformation and above. The filler network contribution was attributed to the presence of an overlapping layer or rubber bound at the interphase, which can be pulled apart at deformations higher than typical Payne effect [41]. The direct application of Nie equation did not fit data obtained in the current work. However, it was possible to rank the compounds containing HNPs by comparing the slopes of the curves in the medium oscillatory strain regime (Table 4)

Table 4

Laos curve slopes in the medium oscillatory strain regime for SiO₂-HNP_X/SBR.
Sample	Curves Slope 100–200%
SiO₂-APTES/SBR	0.0430
SiO₂-HNP_3/SBR	0.0429
SiO₂-HNP_4/SBR	0.0396
SiO₂-HNP_6/SBR	0.0364

The analysis shows a decrease of slope from reference NC with SiO₂-APTES to that containing SiO₂-HNP_6. This suggests an increased network stability in the strain range considered. In other words, the density of polymer hair on the NPs surface seems to have a role of network stabilizer at deformations higher than those measured by Payne effect.

3.4 Morphological characterization

To highlight the role of the PB shell grafted on silica in enhancing the functional properties of the NCs, results from dynamic-mechanical analysis were correlated to dispersion and distribution of silica evidenced at local scale by TEM observations. TEM images of the vulcanized NCs filled with bare and APTES functionalized SiO₂ NPs, taken as reference samples, are reported in Fig. 8.

Sample V-SiO₂-ST/SBR (a, a’ and a’’ in Fig. 8) evidences the presence of very large micrometric clusters, resulting in a very poor filler dispersion. Such SiO₂-ST clusters appear very compact, without detectable presence of rubber matrix between the particles. The high hydrophilicity of SiO₂-ST NPs, attributable to the large amount of surface silanol groups, is responsible for the large extent of aggregation. The lack of a filler network is consistent with the mechanical behaviour of this sample, i.e. low reinforcement and Payne effect.

The images of Fig. 8b, b’and b’’, related to V-SiO₂-APTES/SBR sample, show filler clusters smaller in size and an improved overall filler dispersion. As expected, silica surface silanization enhances the affinity with the rubber matrix. Notwithstanding this, the dark colour of NPs clusters indicates that very little if any rubber is present within them. Moreover, they are more spherical than those observed in V-SiO₂-ST/SBR without evidence of NPs self-organization.

The samples V-SiO₂-HNP_3/SBR and V-SiO₂-HNP_4/SBR (a, a’ and b, b” in Fig. 9), containing HNPs, exhibit a better dispersion with only a partial aggregation of filler particles which form more elongated and preferentially oriented aggregates. At higher magnifications (a’’ and b’’ in Fig. 9) enclosed rubber between filler particles is visible. The observed anisotropic structures reveal the effective self-organization of NPs having a polymeric shell [24, 31].

Therefore, the existence of a “fillers network” where NPs interact with each other not only directly but also through the bridging polymer chains is corroborated by TEM images that highlight a rubber layer enveloping the HNPs arranged along a well-defined orientation. This filler organization takes place regardless of the mixing process used to prepare the composites and even if no procedure was performed to purposely drive the particle arrangement, as reported elsewhere [11].

Although some Authors previously reported on the HNPs organization [52, 53] for NCs based on polymer matrices with high Tg, such as polystyrene (PS) filled with PS-grafted HNPs, there is a lack of information on the preparation of HNPs having low Tg elastomers as hairs [34, 40], such as PB [54, 55] or PI [24, 27]. To the best of our knowledge, the direct relationship between the final mechanical properties of rubber NCs and the synergistic interaction created within the rubber matrix by PB shell surrounding the SiO₂-HNPs has not been methodically explored.

Finally, TEM images of the sample V-SiO₂-HNP_6 (c, c’ and c’’ in Fig. 9) show large micrometric superstructures, analogous to those observed in V-SiO₂-ST/SBR sample, but less dense. Nevertheless, as for V-SiO₂-HNP_3/SBR and V-SiO₂-HNP_4/SBR, these superstructures reveal the presence of enclosed rubber material in between the HNPs. However, in this case HNPs clusters appear more spherical in the shape [21]. Nonetheless, “string-like” anisotropic structures of HNPs, like those observed in matrix-free conditions [32], are clearly visible at higher magnification (c’ in Fig. 9). It is worth noticing that the presence of such interconnected structures coupled to a higher content of crosslinks (Fig. 3), arising from the higher grafting degree of PB chains, correlates with the observed improvement of the NC mechanical properties.

To sum-up, TEM analysis thus confirms that grafting PB chains onto spherical SiO₂ NPs leads to a controlled distribution of the particles within the rubber and to a significant filler-matrix interaction, which in turn steer the mechanical properties of the ensuing NC material, i.e. higher reinforcement and lower Payne effect as well as improved elongation and stress at break.

In particular, the NC prepared with SiO₂-HNP_6 NPs characterized by the greatest amount of PB shell, presents the highest reinforcement (highest G’ (6%) value) due to the strong filler-filler interaction mediated by polymeric shell and a very low Payne effect (lowest DG’ value) without a remarkable increase in hysteresis (tanδ value) because the so-modified NPs are able to interact also with the polymer matrix. On the other hand, V-SiO₂-HNP_4/SBR exhibits a better tuning between reinforcement effect and hysteresis (lowest tanδ and G’’ values), that suggests a better equilibrium between filler-filler and filler-matrix interactions. The polymer amount on the NPs surface, therefore, determines the balance between the two types of interactions and consequently the morphological distribution of the particles in structures of anisotropic shape, when filler-polymer interactions are prevalent, or alternatively of spherical shape, when filler-filler interactions dominate.

We report a complete study of the interplay between polymer chain and HNPs assemblies in industrial model HNPs/SBR NCs. We demonstrate that the silica networking inside the polymeric matrix can be improved in the point of view of environmental impact by using the polymer coupling functionalization on silica fillers. By controlling the texturization of specific oligomers at the filler surface in a given matrix, spherical colloidal silica assemble into anisotropic superstructures that can exhibit colloidal properties typical of structured fillers, but with the advantage of assembling the structure in-situ, improving overall filler dispersion, but keeping a high level of immobilized rubber that is necessary for achieving the desired NC properties. In other words, it is demonstrated that interfacial polymer coating of HNPs can tune the matrix chain confinement and thus, they appear to be a relevant way to reduce the energy dissipation of a compound for tyre use, either decreasing delta G’ or decreasing tanδ value, both strong predictors of the rolling resistance coefficient which determine vehicle energy consumption and CO₂ emissions.

HNPs are able to build anisotropic energy-dissipation structures, as supported by LAOS analysis, maintaining elasticity play and for this could be an important role in the design of tough soft materials. The tuning of NPs surface chemistry allows in principle to design ad hoc modification system for every possible dienic rubber formulation, just by optimizing the choice of oligomer and degree of modification.

These results are corroborated by both the mechanical and morphological characterization for V-SiO₂-HNP_X/SBR NCs. In particular, V-SiO₂-HNP_6/SBR NC possesses the highest reinforcement, largest stress at break and lowest elongation at break, while V-SiO₂-HNP_4/SBR NC the optimum balance between hysteresis and reinforcement.

Funding

This research received no specific funding.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was in the frame of the European Institute of Innovation and Technology - EIT Raw Materials Doctoral Programme.

Laura Tripaldi thanks CORIMAV (“Consortium for the Research of Advanced Materials between Pirelli and Milano Bicocca University) for its support within the PCAM European Doctoral Program.

Supplementary data

Supplementary data associated with this article can be found, in the online version, at…………

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Scheme 1 is available in the Supplementary Files section.

No competing interests reported.

SIHNPsNCmanuscript1.06.24.docx
Scheme.png
Scheme 1. Representation of the rubber NCs preparation with ex-situ functionalized SiO₂-HNP_X (where X is the PB wt%).

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The Anisotropic Self-Assembly Effect of Silica Hairy Nanoparticles: toward Sustainable Rubber Nanocomposites for Tyre Application

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Version 1

Abstract

Figures

1. Introduction

2. Experimental Section

2.1 Materials

2.2 Synthesis of SiO₂ HNPs

2.4. Preparation of SiO₂ HNPs NCs

2.5. Physico-chemical, morphological, and spectroscopic characterization of SiO₂ HNPs

2.6 Morphological characterization of uncured and cured HNPs NCs

2.7 Determination of crosslinking density

2.8 Dynamic-mechanical analysis of uncured and cured HNPs NCs

3. Results and Discussion

3.1. SiO₂-HNP_X synthesis and characterization

3.2 SiO₂-HNP/SBR NCs: curing behavior and swelling experiments

3.3 SiO₂-HNP/SBR NCs: dynamic and mechanical behavior

3.4 Morphological characterization

4. Conclusions

Declarations

References

Scheme

Additional Declarations

Supplementary Files

Status:

Version 1

The Anisotropic Self-Assembly Effect of Silica Hairy Nanoparticles: toward Sustainable Rubber Nanocomposites for Tyre Application

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental Section

2.1 Materials

2.2 Synthesis of SiO2 HNPs

2.4. Preparation of SiO2 HNPs NCs

2.5. Physico-chemical, morphological, and spectroscopic characterization of SiO2 HNPs

2.6 Morphological characterization of uncured and cured HNPs NCs

2.7 Determination of crosslinking density

2.8 Dynamic-mechanical analysis of uncured and cured HNPs NCs

3. Results and Discussion

3.1. SiO2-HNP_X synthesis and characterization

3.2 SiO2-HNP/SBR NCs: curing behavior and swelling experiments

3.3 SiO2-HNP/SBR NCs: dynamic and mechanical behavior

3.4 Morphological characterization

4. Conclusions

Declarations

References

Scheme

Additional Declarations

Supplementary Files

Status:

Version 1

2.2 Synthesis of SiO₂ HNPs

2.4. Preparation of SiO₂ HNPs NCs

2.5. Physico-chemical, morphological, and spectroscopic characterization of SiO₂ HNPs

3.1. SiO₂-HNP_X synthesis and characterization

3.2 SiO₂-HNP/SBR NCs: curing behavior and swelling experiments

3.3 SiO₂-HNP/SBR NCs: dynamic and mechanical behavior