Effect of nanoparticles on fibril formation and mechanical performance of olefinic block copolymer (OBC)/polypropylene (PP) microfibrillar composites

Abstract

The morphology and properties of olefin block copolymer (OBC)/polypropylene (PP) microfibrillar composites (MFCs) were tuned by adding nano silica (SiO₂) and carbon nanotubes (CNTs) respectively. When the two nanoparticles (NPs) located in the OBC matrix, refined microfibrils were maintained and the final diameters of the PP microfibrils slightly increased compared to the un-modified MFCs; when the NPs located in the dispersed PP component, they exerted a profound impact on the deformation of PP during the fibrillation process. The CNTs can still lead to refined PP microfibrils while addition of SiO₂ induces a transition of PP morphology from fibrillar to nodular and even to a spherical structure as its content increases from 0.3 wt% to 1 wt%. The rheological and mechanical properties were also influenced by the type and selective dispersion of NPs. In particular, if the fibrillar morphology of PP was kept in the ternary composites, a small amount of nanoparticles could lead to synergetic enhancement, with polypropylene microfibrils, on the tensile properties of OBC. This work provides a better understanding of the role of NPs in the fiber formation process and an alternative way to control the morphological evolution of MFCs.

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1. Introduction

Microfibrillar composites (MFCs) are in situ generated from polymer blends during polymer processing, in which the polymer matrix is reinforced with high aspect ratio microfibrils of a different polymer.^1–6 The fabrication process of the composites usually contains three steps.^7,8 First, melt blending (extruding) is performed to disperse the minor phase in the matrix. A second step, namely the fibrillation step, includes drawing the obtained extrudates to fibrillate the dispersed component. During this step, the fibrillar morphology depends on the ratio between shear force and interfacial force.^9,10 Finally, the semi-finished oriented blend is processed by injection or compression molding between the melting temperatures of the two polymers. The mechanical properties of the MFCs can be improved noticeably since the dispersed phase in situ generates reinforcing microfibrils during processing. Therefore, the development of MFCs provides an alternative and promising way to fabricate fiber reinforced composites. The processing method is convenient with low cost. Especially, MFCs do not require the dispersion of preformed reinforcing fibers (e.g. natural fibers, aramid fibers and carbon fibers) into the matrix because the microfibrils are formed in situ, thus avoiding the dispersion of the agglomerated fibers. Another prominent advantage of MFCs is the method provides a technology to complete recycle the waste plastic mixtures where both low melting temperature general polymers and high melting temperature plastics co-exist and cannot be easily separated.¹¹ Even more interesting is that, if the matrix can crystallize, high interfacial bonding of the fibrils could be realized via the formation of a transcrystalline layer of the matrix around the fibrils, avoiding complicated and costly surface modifications of the nanoparticles.^12–15

Normally, the fibrillar structures are tuned by the viscosity ratio of the two polymers, interfacial interactions and external processing parameters, like post stretch ratios. Recently, nanoparticles (NPs) are also used to modify the fibrillar structures.^16–22 Due to the differed shapes and topological structures of different NPs, the viscoelastic property of the melt fluid is also varied, which plays an important role in determining the morphological evolution of the dispersed component. Based on the review of the related literatures, the adding of NPs could facilitate or hinder the fibrillation process depending on the used NPs. Although all the authors have delivered some explanations on the deformation mechanism of fibrillated polymer in their respective researches, it is also critical to comprehensively compare different NPs at the same time. In this way, one could seek for more reliable and intuitional proofs on exactly how NPs affect the fibrillation process (positive or negative) in different circumstances. Regrettably, such comparative study has not been reported yet.

Among the NPs used to modify MFCs, nanosilica (SiO₂) and multiwall carbon nanotubes (CNTs) are commonly used as isotropic and anisotropic fillers respectively and have different impact on viscoelastic property of polymer melt. In our previous study, in our previous study, the diameters of PP fibrils were proved to be independent of PP content (less than 20 wt%) and 10 wt% of PP displayed balanced reinforcement of tensile strength and flexibility on the matrix elastomer. Therefore here, the weight ratio is fixed at 90/10 so as to focus on how the nanoparticles affect the MFCs.^23,24 In this work, the two NPs were added into the blends and the effect of the fillers on the formation of fiber structures was thoroughly investigated and compared via controlling the content and location of the fillers.

2. Materials and methods

2.1 Materials

OBC was supplied by Dow Chemical Company. The commercial type is 9500 with an octene content of 12.9%. The hard and soft segment is 25% and 75% respectively. PP powder is the commercial product of Baota Petrochemical Group, China and has a melt flow index of 20 g/10 min. Hydrophobic silica nanoparticles (Aerosil R974) with a primary diameter of 12 nm were kindly supplied by Chengdu Taly industrial chemical Co., Ltd of China. Multi-walled carbon nanotubes (CNTs), under the trade name of NC 7000 was produced by Nanocyl S.A (Belgium). The average diameter and length is 9.5 nm and 1.5 μm respectively.

2.2 Sample preparation

The OBC/PP/NPs ternary nanocomposites were fabricated using one-step and two-step method respectively. For the one-step method, nanoparticles were blended with PP powders and OBC directly in the extruder equipped with a convergent die at 50 rpm. The temperature profile from hopper to die of the extruder was 110, 180, 180 and 175 °C. The extruded melt with the OBC/PP/NPs 90/10/X (X = 0.3, 0.5 and 1) w/w/w composition was drawn using an adjustable taken-up device. For the two-step method, PP powders and NP were preliminarily mixed by stirring in high speed, followed by further mixing in a Hakke internal mixer at a temperature of 180 °C and a rotor speed of 50 rpm for 10 min. The premixed composites of PP and NP were then compounded with OBC using the above extrusion-stretching method. The sample notations and composition of the composites are presented in Table 1.

Table 1 Sample notation and composition of OBC/PP MFCs nanocomposites

Samples	OBC	PP	CNTs	SiO2	I(II)
0.3C-I(II)	90	10	0.3	0	One-step (two-step)
0.5C-I(II)	90	10	0.5	0	One-step (two-step)
1C-I(II)	90	10	1	0	One-step (two-step)
0.3S-I(II)	90	10	0	0.3	One-step (two-step)
0.5S-I(II)	90	10	0	0.5	One-step (two-step)
1S-I(II)	90	10	0	1	One-step (two-step)

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2.3 Flow behavior of PP/NP binary composites

The apparent viscosity was measured by an Advanced Capillary Rheometer (RH7, Germany). The diameter of the capillary die was 1 mm, and its L/D ratio was 30 : 1. The testing temperature was set at 180 °C. The shear rates varied from 10 to 2000 s⁻¹.

2.4 Shear rheological measurements

The linear viscoelastic behaviors of pure OBC, PP/NPs blends and OBC/PP/NPs microfibrillar composites were studied using the parallel-plate geometry, with a diameter of 25 mm and 1 mm between the plates. The strain amplitude was set as 1% with the frequency sweep performed in the range of 0.06–628 rad s⁻¹. For PP/NP binary composites, the testing temperature was 180 °C. Whereas for the microfibrillar composites, the test was performed at 140 °C, which was above OBC's melting point and yet low enough to prevent PP microfibrils from melting.

2.5 Scanning electron microscope (SEM)

The scanning electron microscopy (SEM) experiments were performed using an FEI Inspect F SEM instrument with an acceleration voltage of 20 kV. The selective dispersion of NP in the microfibrillar composites was determined from the cross-section of the extruded strip. To expose the PP component and distinguish it from the matrix, the cryo-fractured samples were etched chemically in xylene at 70 °C for 30 min to slightly remove OBC matrix, followed by washing and drying. The overall view of PP morphology was also observed by SEM after OBC matrix was totally removed by hot xylene at 80 °C for 3 hours.

2.6 Atomic force microscopy (AFM)

The cross-section of the extruded strip was cryomicrotomed using a Leica microtome at −80 °C. The samples were observed with an atomic force microscope (AFM) operating in tapping mode using an instrument with a SPI4000 Probe Station controller (SIINT Instruments, Japan) at room temperature. Phase contrast images were collected.

2.7 Thermogravimetric analysis (TGA)

The OBC matrix of the MFCs was completely removed by xylene and the remaining PP fibrils were collected for the TGA test (Q500 TA Instruments) at nitrogen atmosphere. 5 mg of the testing samples were heated to 700 °C at 10 °C min⁻¹. In addition, the TGA results of MFCs without the etching process were also carried out for comparison.

2.8 Tensile test

The tensile properties of the MFCs were measured at room temperature using an Instron 4302 universal tensile test. Tensile measurements were made on dumbbell shape specimens 4 mm wide, 2 mm thick with a gauge length of 20 mm. A crosshead speed of 50 mm min⁻¹ was applied to determine the tensile properties.

3. Results and discussion

3.1 Dispersion of NP in OBC/PP blends

In this article, the ternary MFCs were fabricated through two strategies. Therefore, it is critical to determine how the NPs were dispersed in the two situations. From the theoretical point of view, the localization of nanoparticles in OBC/PP blends can be qualitatively predicted by using the wetting parameter ω. According to Young's equation, the wetting coefficient ω,²⁵

If ω > 1, NP would be preferentially localized in the PP phase; if ω < −1, NP are predicted to disperse in the OBC phase; if the value of ω is between −1 and 1, NP favor to locate at the interface of the two polymers. In eqn (1), γ is the interfacial energy between the two polymers or polymer and filler, which can be estimated by the harmonic-mean equation and geometric-mean equation, respectively.²⁶

where γ₁ and γ₂ are the surface tensions of OBC and PP respectively. γ^d₁ and γ^d₂ are the dispersion parts of the surface tensions. γ^p₁ and γ^p₂ represent the polar parts. Generally, these values are determined by a contact angle test at room temperature. Surface tensions at processing temperature (180 °C) could be calculated by extrapolating the values at room temperature using the following equations.^27,28where γ₀ represents the surface free energy at 0 °C and T_c is the critical temperature (T_c is 1000 K for polymers).²⁹

The values for the surface tensions (listed in Table 2) of OBC/PP, SiO₂ and CNTs were taken from literatures.^26,30,31 Based on eqn (4), the values of OBC and PP at our processing temperature 180 °C were calculated using the data in ref. 43, where the values were given at 190 °C. Besides, the surface tensions of SiO₂ and CNTs are assumed to be independent of temperature.^29,32 The wetting coefficients (ω) are thus calculated using the harmonic-mean equation and geometric-mean equation respectively (see Table 3).

Table 2 Surface tension at 180 °C

Materials	Surface tension (mN m^−1)
	Total (γ)	γ^d	γ^p
OBC	17.76	15.84	1.92
PP	22.03	21.52	0.51
SiO2	13.1	13.1	0
CNTs	27.8	17.6	10.2

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Table 3 Calculated ω at 180 °C

Calculation method		Interfacial energies (mN m^−1)			ω (mN m^−1)
		γOBC/NP	γPP/NP	γOBC/PP
SiO2	Harmonic-mean equation	2.18	2.56	1.68	−0.23
	Geometric-mean equation	2.0	1.55	0.89	0.57
CNTs	Harmonic-mean equation	5.75	9.16	1.68	−2.02
	Geometric-mean equation	3.32	6.35	0.89	−3.4

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According to the two methods, the ω value of SiO₂ lies between −1 and 1, indicating that the hydrophobic silica particles prefer to locate at the interface of the two polymers; for CNTs, the ω value is smaller than −1, which indicates they will be only present in the OBC phase. However, if the viscosity of the melt is high and the processing time is relatively short, the thermodynamic equilibrium state is difficult to be achieved and kinetic factors like blending procedures, blending time, NP content and shear strength may also dominate the particle distribution.³³ In our system, NPs were located in the OBC and PP component for the one- and two-step method respectively at the beginning of the extrusion process. There are only small differences in surface energy and polarity of the polymers and NPs, it would be difficult for NPs to migrate from one phase to another. After extruding, NPs would prefer to maintain the original dispersion state.

To confirm the theoretical result, the selective dispersion of NP is proved by SEM, AFM and TGA. Fig. 1 displays the micrographs of the cross-section of the extruded strip. When the composites are compounded by two-step methods, both SiO₂ and CNTs can be observed inside the microfibrillar PP phase; whereas when added by one-step, the NPs are mainly dispersed in the OBC matrix. Unfortunately, the OBC matrix was partially etched to expose the PP fibrils, it is difficult to discriminate the nanoparticles from the matrix. To overcome the drawbacks, AFM phase images are also obtained as illustrated in Fig. 2. The round dark areas represent the cross-section of PP fibrils of higher modulus. For the composites compounded by one-step, CNTs (pointed by the red arrow) are mainly dispersed in the OBC matrix which is in agreement with the wetting parameter. For SiO₂, instead of aggregating at the interface of the two polymers based on the thermodynamic prediction, SiO₂ also is located in the OBC phase because the viscosity of OBC is higher than PP and the mixing time is limited, making it difficult for SiO₂ to migrate from OBC toward PP. When the nanoparticles were mixed by the two-step method, most of NPs stayed in PP component with several CNTs or tiny SiO₂ aggregates dispersed outside PP. Because at the beginning of the extrusion process, the content of NPs in PP component is relative high (from 3 wt% to 10 wt%), the migration of NPs from PP to OBC is inevitable and the migration was expected to be more evident at higher NP loadings.

Fig. 1 SEM micrographs of the cryo-fractured surface of the strip extrudate, after the matrix was gently etched by xylene.

Fig. 2 AFM phase images of the cryo-fractured surface of the strip extrudate (the red arrows point at dispersed CNTs and SiO₂).

To roughly estimate the actual filler content in the MFCs, a TGA test was conducted on the samples of the composites with 0.5 wt% SiO₂ and CNTs. As shown in Fig. 3, the OBC matrix was totally removed by solvent, after which the remaining PP component was cleaned and dried for the TGA test. The residual carbon content of 0.5C-II-E and 0.5S-II-E is around 6%, which is higher than that of 0.5C-I-E and 0.5S-I-E. This proves that the pre-mix of nanoparticles with PP realizes the selective location of both SiO₂ and CNTs in the PP component. Theoretically, the residual content of 0.5C-I-E and 0.5S-I-E is lower than its un-etched (NE) counterpart since the nanoparticles are supposed to disperse in the OBC matrix which was etched away before the TGA test. The possible reason may be that when OBC was dissolved in the xylene solvent, some of the dispersed nanoparticles would be absorbed on the surface of the PP phase. That results in the relative higher residual percentage (3%) of 0.5C-I-E and 0.5S-I-E.

Fig. 3 The thermal gravimetric curves of the nanocomposites filled with 0.5% of nanoparticles. NE: none etched samples (extruded strip); E: the extruded samples are etched by xylene until OBC matrix is totally removed. The residual samples are tested.

3.2 Morphology of PP

The effect of the discussed selective dispersion of NP on the morphology of PP was fully displayed by SEM. Fig. 4 displays the micrograph of PP microfibrils and the diameter distributions. The average diameters of the microfibrils are illustrated in Fig. 5. Before the nanoparticles were added, PP could be transformed into tiny micro and even nano fibrils with an average diameter of 100 nm. The addition of nanoparticles, whether SiO₂ or CNTs, results in the increase of the fibril diameters. However, the different mixing strategies exert different influences on the final morphology of PP. When using the one-step method, the addition of the two nanoparticles lead to a slight increase of the fibril diameters. Generally, refined PP fibrils with a high aspect ratio were obtained for the two nanoparticles. Specifically speaking, for neat OBC/PP microfibrillar composites, the average diameters of PP was around 100 nm. After the addition of 0.3 wt% CNTs, the diameters of PP fibrils are almost invariable. When the content of CNTs reaches 0.5 wt%, the diameters of 0.5C-I increased dramatically to 200 nm. Similar to CNTs, the addition of SiO₂ also brings out the diameter increment but the diameters are smaller than that filled with CNTs.

Fig. 4 SEM micrographs and diameter distributions of OBC based MFCs with different filler content.

Fig. 5 The average diameters of the MFCs with different filler loadings. (a) CNTs-MFCs (b) SiO₂-MFCs.

When added by the two-step method, the PP fibrillar morphology varied dramatically for the two NPs. When the CNTs are loaded as high as 1 wt%, the continuous morphologies of PP are still obtained. The average diameters were smaller than that of the one-step composites. When SiO₂ was mixed by the two-step method, it induced a transition of PP morphology from fibrillar to nodular and even spherical structure as its content increased from 0.3 wt% to 1 wt%.

To explain how the fillers with different appearances and different mixing strategies affect the final morphology, a schematic diagram is drawn in Fig. 6. When the ternary composites were fabricated by the one-step method (CNTs-I and SiO₂-I), the addition of the nanoparticles always increased the viscosity of the matrix and the decreased viscosity ratio of dispersed phase and matrix can promote the fibrillation of PP.^16,17 On the other hand, the nanoparticles are also confirmed to hinder the deformation of the dispersed phase.^34–36 Due to the stronger hindrance effect of CNTs, CNTs-I displayed slightly larger average diameters than SiO₂-I. As the loading of NPs increased, the hindrance effect became more drastic and resulted in slightly rise of fibril diameters. Overall, the one-step method results in refined PP microfibrils with large aspect ratio.

Fig. 6 Morphological evolution of PP with different filler loadings via one or two step method.

However, the morphological evolution of PP in the two-step strategy turns more complicated. According to the published literature, it has been confirmed that the effect of NP on the structure and properties of MFCs is quite complex. Especially when the composites are fabricated in two steps where NP is first compounded with the fibril-forming component, the viscoelastic response of the binary composites is dramatically different compared to the neat polymer and the microfibrils even fail to be formed at higher loadings.³⁷ Therefore, it is necessary to figure out how the selective dispersion of SiO₂ or CNTs affects the rheological properties. Fig. 7(a) shows the relationship between the apparent viscosity and the shear rate for neat OBC, neat PP, and PP/NP compounds at 180 °C. Over the whole shear rate range, the viscosity of PP/NP is higher than neat PP except for PP/3SiO₂ display lower viscosity. The reduction of melt viscosity may be caused by slip between polymer matrix and filler.^38,39 Generally, the deformation of dispersed melt drops depends mainly on the interfacial interaction, viscosity ratio and other mixing parameters. A lower viscosity ratio (η_d/η_m < 1) facilitate the fibrillation of the dispersed phase in a blend.^17,23,37,40 Here, although the addition of NP could increase the melt viscosity of PP, the final viscosity of PP nanocomposites is still lower than neat OBC, implying η_d/η_m is smaller than 1. According to the mentioned theory, PP/NP hybrid melt is possible to deform into microfibrils. It should also be noted PP/CNTs compounds display higher viscosity than PP/SiO₂, which can be clearly seen inthe dynamic rheometer test. That may be caused by the entanglement of CNTs fibers. At higher shear rate, the viscosity of PP/CNTs drops dramatically due to the de-bundled CNTs clusters. Whereas PP/SiO₂ is less sensitive to an external shear field and the shear thinning effect takes place at larger shear rates compared to the PP/CNTs nanocomposites with the same filler loading. The different shear response of the two nanocomposites would also influence the deformation of PP/NP melt in the OBC matrix. When PP melt was hot stretched by the pinched roll, CNTs were prone to slip and align along the drawing direction. In other words, the incorporation of CNTs into PP did not hinder the melt deformation and the hybrid microfibrils of PP/CNTs were thus obtained. Whereas for SiO₂-II, voids arose from poor adhesion between particle agglomerates and the PP matrix as indicated in Fig. 7(b).^41–43 The defect would cause the delamination of the PP melt from the SiO₂ agglomerates, leading to rough surface of PP fibrils. At 1 wt%, SiO₂ would be easier to be agglomerated in some areas, where the defect could even result in the breakup of the PP melt. Therefore, some of the PP melt failed to form microfibrils with a large aspect ratio, instead only elongated ellipsoids or spheres were formed.

Fig. 7 Rheological tested by Advanced Capillary Rheometer and strain-controlled dynamic rheometer respectively: (a1) apparent viscosity and (a2) complex viscosity of PP/NP; SEM photograph of (b1) PP/3CNTs and (b2) PP/3SiO₂, the red circles indicate the voids between PP and SiO₂.

3.3 Rheological behavior

Normally, the addition of fillers can alter the rheological properties of polymers significantly. Such effects are related to the filler's concentration, shape, deformability, level of dispersion, and the degree of interaction between the polymer and the fillers.⁴⁴ Thus, the study of the viscoelastic behavior of polymer composites can provide some insight into their microstructures.

Fig. 8 displays the rheological responses of molten OBC and OBC/PP/SiO₂ or CNTs compounds at 140 °C. The pure OBC had a classic viscoelastic response and at low frequencies, OBC's storage modulus (G′) was highly dependent on the frequency. The addition of 10 wt% in situ generated PP microfibrils increased the OBC melt's storage modulus by 3 orders of magnitude and a plateau region appeared at low frequencies. The weakened frequency-dependence indicates the transition in the melt's viscoelastic behavior from a liquid-like to a solid-like state.¹ Such an increase in the melt's elastic behavior suggests the formation of a physically entangled network, which would have been created by the topological interactions of the PP microfibrils. The creation of such a network is a typical characteristic of the composites with deformable and long (that is, high aspect ratio) fibers.⁴⁵ After adding NPs, MFCs displayed the similar G′ plot to OBC/PP MFCs (90-10). For better understanding of the influence of CNTs and SiO₂ on the morphology, the storage modulus at a frequency ω = 0.0628 rad s⁻¹ vs. nanofiller loadings is shown in Fig. 9. It is shown that the composites mixed by the one-step method displayed higher G′ for OBC/PP/CNTs. For 0.5C-I and 1C-I, the storage modulus is higher than 0.5C-II and 1C-II, respectively. In Fig. 6, we pointed out that plenty CNTs were dispersed around the PP fibrils. The CNTs with a large aspect ratio worked as bridges to connect the adjacent PP fibrils, making the physically entangled network more effective to restrict the chain mobility of OBC. Whereas for OBC/PP/SiO₂, the smooth surface of SiO₂ without surface modification failed to connect the PP fibrils at such lower loadings.⁴⁶ Therefore, the SiO₂-MFCs-I failed to show the synergetic enhancement as CNTs-MFCs-I did. For 1S-II, some of the PP/SiO₂ hybrid melt failed to transform into microfibers and the physical network is jeopardized. Therefore, the G′ was even lower than 0.5S-II, indicating the rheological property was influenced by not only filler content, but also the fibrillar morphologies of PP.

Fig. 8 Storage modulus (G′) of neat OBC and its MFCs without (90-10) and with nanoparticle filling: (a) OBC/PP/CNTs nanocomposites (b) OBC/PP/SiO₂ nanocomposites.

Fig. 9 Storage modulus (G′) as a function of nanofiller loading content and mixing strategies: (a) OBC/PP/CNTs nanocomposites (b) OBC/PP/SiO₂ nanocomposites.

3.4 Mechanical properties

Fig. 10 shows the stress–strain curve of the MFCs and the tensile properties are concluded in Table 4. A remarkable climb in OBC's tensile strength was achieved by adding the in situ generated PP fibrils. The adding of CNTs contributes to the further mechanical enhancement. Clearly, when CNTs were mixed by two-step method, the as-formed hybrid microfibrils obtained further improved mechanical performance than pure PP fibrils. Therefore, the final mechanical properties of the nanocomposites were promoted. When CNTs were added by the one-step method, the CNTs dispersed in OBC matrix help PP fibrils to construct more improved networks as indicated by the rheological test. Synergetic enhancement of tensile properties was achieved from CNTs and PP fibrils.

Fig. 10 Stress–strain curves of OBC/PP MFCs filled with nanoparticles: (a) OBC/PP/CNTs nanocomposites (b) OBC/PP/SiO₂ nanocomposites.

Table 4 Mechanical properties of OBC and OBC/PP MFCs filled with nanoparticles. Note (a) the maximum tensile strain was controlled at 1000%

Sample	Tensile strength, MPa	Elongation at break, %	Sample	Tensile strength, MPa	Elongation at break, %
OBC	3.8 ± 0.09	1000(a)
90/10	4.7 ± 0.04	1000(a)
0.3C-I	6.1 ± 0.17	1000(a)	0.3S-I	5.8 ± 0.13	1000(a)
0.3C-II	7.3 ± 0.18	1000(a)	0.3S-II	5.7 ± 0.21	1000(a)
0.5C-I	7.8 ± 0.17	1000(a)	0.5S-I	6.1 ± 0.09	532.5 ± 37.81
0.5C-II	8.2 ± 0.13	1000(a)	0.5S-II	7.3 ± 0.15	1000(a)
1C-I	8.1 ± 0.11	810.8 ± 26.34	1S-I	5.9 ± 0.16	398.6 ± 43.15
1C-II	8.3 ± 0.28	1000(a)	1S-II	6.2 ± 0.13	1000(a)

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When SiO₂ was added by the two-step method with 0.3 and 1 wt%, the reinforcing effect is negligible due to the low silica loading and destructed PP fibrillar morphologies respectively. Only when the silica was loaded at 0.5 wt%, the balanced PP strength and its microfibrillar morphology resulted in the best mechanical performance. The flexibility of 0.5S-I and 1S-I dropped dramatically because the aggregation of silica particles and the poor de-wetting with the OBC matrix.

4. Conclusion

The nanoparticles (SiO₂, CNTs) modified MFCs based on OBC/PP are fully investigated. Depending on the mixing process, the selective dispersions of nanoparticles in OBC and PP are obtained via a one- and two-step method, respectively. Influenced by the type of nanoparticles and filler content, the MFCs display differed microstructures and properties. When the nanoparticles are added by the one-step method, PP fibrils with a large aspect ratio could be obtained. Synergetic mechanical improvement is realized for CNTs because dispersed CNTs in the OBC matrix can work as bridges to connect the adjacent PP fibrils and form a more developed physical network. When the particles are added by the two-step method, CNTs could still lead to refined PP fibrils and further promote the mechanical performance of the MFCs. Whereas for SiO₂, plenty bumps appeared in the PP fibrils and nodular-like PP, instead of large aspect ratio fibrils, were in situ generated. The synergetic reinforcing effect is negligible due to the coarse morphological structure.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant no. 51421061).

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