Fabrication of nanoprotrusion surface structured silica nanofibers for the improvement of the toughening of polypropylene

Abstract

The toughening of semi-crystalline polymers with inorganic nanofiller is very important in the practical applications of such polymers. In this study, we successfully fabricated the surface attaching silica nanoparticles of silica nanofibers (SiO₂@SNFs) from the calcination of electrospun poly(vinyl pyrrolidone)/tetraethyl orthosilicate/silica nanoparticle (PVP/TEOS/SiO₂) nanofibers for the toughening of polypropylene (PP). The SiO₂@SNFs had a nanoprotrusion structured surface, and the degree of surface nanoprotrusion of the silica nanofibers (SNF) can be adjusted via the incorporated SiO₂ nanoparticle content of the SiO₂@SNFs. The effects of the SiO₂ content of the SiO₂@SNFs on the crystallization behavior, relative β-form crystal content, and mechanical properties of PP were investigated with polarized optical microscopy, X-ray diffraction and notched Izod impact test methods. By comparison with SNF, the SiO₂@SNFs showed greater improvements in the impact strength and heterogeneous crystal nucleation of PP at the same loading content of filler. The impact strength of PP/SiO₂@SNFs at a loading of 2 wt% of SiO₂@SNFs with 9 phr (SiO₂/TEOS = 9/100) of SiO₂ nanoparticles was improved by about 1.9 and 1.4 times that of neat PP and PP/SNFs composite (2 wt% of SNFs), respectively. However, the crystallinity, relative β-form crystal content, and tensile strength of PP/SiO₂@SNFs were almost independent of the SiO₂ nanoparticle content of the SiO₂@SNFs. Our results demonstrated that these nanoprotrusion surface structured silica nanofibers can be used as a novel nanofiller for improving the toughening of PP.

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Introduction

The toughening of semi-crystalline polymers with inorganic nanofillers is very important for their practical applications.^1,2 Isotactic polypropylene (iPP) is the most important semi-crystalline polymer commodities and is widely used as a matrix component in automotive parts, appliances and other industrial uses. iPP has numerous desirable properties such as processability, chemical resistance, low density, and low price. However, the poor impact strength of iPP at low temperature limits its applications. Therefore, the toughness of iPP is very important in most practical applications.^3–11

In general, the rubber toughening of iPP is an effective method to improve its impact strength. However, the significant drawback of rubber toughening of iPP is the decrease in the tensile strength and Young's modulus.^3,12 There is considerable interest to simultaneously improve both the stiffness and toughness of semi-crystalline polymers. Recently, numerous researchers reported that the toughening of iPP can be achieved by the incorporation of inorganic nanoparticles such as micrometer or nanometer scale calcium carbonate (CaCO₃),^{5,6,8,13–15} silica (SiO₂) nanoparticles,¹ graphene oxide (GO),⁹ glass fiber (GF),¹⁰ and fly ash.¹¹ For example, Lin et al.¹⁵ reported that the average Izod impact strength of a 150 °C-annealed nanocomposite, containing 20 wt% (7.8 vol%) CaCO₃ nanoparticles coated with 6 wt% stearic acid, was 168 J m⁻¹ (3.5 times higher than that of iPP). Thio et al.¹⁴ investigated the mechanisms of deformation and fracture of iPP/CaCO₃ composites with various diameters of CaCO₃ (0.07, 0.7, and 3.5 μm). They found that the incorporation of 0.7 μm CaCO₃ particles into the iPP matrix led to an improvement in the Izod impact strength of iPP by up to four times. Li and Dou¹³ investigated the influence of malonic acid (MA) treatment of nano-CaO₃ on the crystallization, morphology, and mechanical properties of iPP/nano-CaCO₃ composites. They reported that the toughness of PP/MA treated nano-CaCO₃ composite was drastically improved. With the addition of 2.5 wt% MA treated nano-CaCO₃, the Izod notched impact strength reached its maximum, which was 2.89 times higher than that of pure iPP. Recently, Bao et al.⁹ reported that the incorporation of functionalized graphene oxide (GO) into iPP can improve the impact strength by almost 100% and the tensile strength by about 30% at a loading of 0.1 wt% functionalized GO. Moreover, Chen et al.¹⁰ reported the oscillatory shear injection molded GF/iPP parts with β-nucleating agent whose strength and toughness were simultaneously improved (the tensile strength was increased by 19.3 MPa and the impact toughness by two fold) compared to those of the conventional injection-molded GF/iPP parts. The oscillatory shear injection molded GF/iPP composites with β-nucleating agent had a hierarchical structure, in which the outer layer of GF/iPP was dominated by highly oriented glass fibers and shish-kebabs, while the inner layer was dominated by a large population of β-crystals and less oriented glass fibers and shish-kebabs.

For the mechanism of the toughening of iPP by inorganic particles, Kim and Michler¹⁶ proposed the concept of a ‘three-stage mechanism’ for describing the micromechanical deformation processes in various toughened and particle-filled semi-crystalline polymers. The inorganic particles serve as stress concentrators to build up a stress field around themselves. Stress concentration generates triaxial stress around the filler particles and leads to debonding at the particle–polymer interface. Voids caused by cavitation or debonding at the particle–matrix interface occurred. The triaxial tension can be locally released in the surrounding voids, corresponding to an increase in the shear component.

In addition, Gersappe¹⁷ suggested that based on the molecular dynamics simulations, the mobility of nanofillers in polymers controls their ability to dissipate energy, which would increase the toughness of the polymer nanocomposites in the case of a proper thermodynamic state of the matrix. Zhou et al.¹ experimentally investigated the effect of the mobility of non-layered nanoparticles on the toughening of polymers. They investigated the mechanical properties of crystalline iPP and amorphous polystyrene (PS) nanocomposites with untreated SiO₂ nanoparticles or grafted SiO₂ nanoparticles at a constant particle concentration of 1.36 vol% at different temperatures. Their results proved that the energy dissipation mechanism induced by nanoparticle mobility works to improve the toughness of non-layered nanoparticles–polymer composites.

The mechanical and physical properties of the semi-crystalline polymers are also intimately associated with their crystalline features, such as crystal structure, crystalline size, morphology, and crystallinity. For example, it is well known that the α-form and β-form crystals of iPP exhibit different physical and mechanical properties. β-form iPP has a higher impact toughness, ductility and heat distortion temperature than α-form iPP.¹⁸

In this study, we fabricated surface attaching silica nanoparticles of silica nanofibers (SiO₂@SNFs) from the calcination of electrospun poly(vinyl pyrrolidone)/tetraethyl orthosilicate/silica nanoparticle (PVP/TEOS/SiO₂) nanofibers for the toughening of polypropylene (PP). The SiO₂@SNFs had a nanoprotrusion structured surface. The degree of surface nanoprotrusion of the silica nanofibers (SNFs) was adjusted by the incorporated content of the SiO₂ nanoparticles. We also investigated the effects of the SiO₂ content of the SiO₂@SNFs on the crystallization behavior, crystal nucleation and mechanical properties of PP.

Experimental

Materials

Poly(vinyl pyrrolidone) (PVP, MW = 130 000) and tetraethyl orthosilicate (TEOS) were purchased from Sigma Aldrich Co. Polypropylene (PP, K8303) was provided by Beijing Yanshan Petrochemical Company, China. Hydrochloric acid solution (37 v/v%), N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO) and ethanol were purchased from Beijing Eastern Chemical Works, China. Silica (SiO₂) nanoparticles (R974) with a 20 nm diameter were purchased from PPG Industries Inc., USA.

Preparation of solutions for electrospinning

In order to prepare the solutions for electrospinning, three types of solutions were first prepared: (1) PVP solution was prepared by mixing 1.4 g PVP, 5.0 g DMF, and 2.5 g DMSO, (2) TEOS solution was prepared by hydrolyzing a mixed solution of 5 g TEOS, 1.5 g hydrochloric acid solution (HCl, 37%) and 2.0 g ethanol for 12 h, and (3) SiO₂ nanoparticle dispersed solutions were prepared by dispersing a certain amount of SiO₂ nanoparticles in a solvent mixture of 2.0 g DMF and 1.0 g DMSO. The three solutions were then mixed and stirred for about 24 h at room temperature to prepare mixed solutions for electrospinning. The weight ratios of SiO₂/TEOS were 3/100 (3 phr), 6/100 (6 phr) and 9/100 (9 phr).

Electrospinning setup

The electrospinning setup was composed of a plastic 20 ml syringe provided with a needle with an inner diameter of 0.20 mm, a syringe pump (KDS-200), a high voltage power supply (ES30P, Gamma High Voltage Research Inc., USA) and a rotating drum (diameter 5 inches). The operation voltage was 15 kV, the feed rate was 1.0 ml h⁻¹, the rotating rate of the drum was 300 rpm min⁻¹ and the distance between the needle tip and the drum was 20 cm.

Calcination process

The SiO₂@SNFs were obtained by the calcination of electrospun PVP/TEOS/SiO₂ nanofibers. The calcination process was performed in a Heavy Duty Tube Furnace (Lindberg 54453) at 325 °C for 6 h, and then at 600 °C for 1 h, in order to completely remove the organic components. In order to avoid the effect of the aspect ratio of the SiO₂@SNFs on the mechanical properties of PP, we crushed the SiO₂@SNFs using a pestle in a bowl to obtain short fibers with a length of about 5 μm.

Preparation of PP nanocomposites

The PP/SiO₂@SNFs composites containing 2 wt% SiO₂@SNFs were extruded at 180 °C twice in order to ensure uniform mixing in the molten state, and then were made into pellets at room temperature. To improve the interface bonding between the PP matrix and the SiO₂@SNFs, γ-methacryloxypropyl trimethoxy silane (KH570) was used as a surfactant to modify the SiO₂@SNFs. The amount of KH570 was about 10% of that of the SiO₂@SNFs. The samples for the impact resistance tests were prepared by an injection machine (LMM, Dynisco, Co, USA) using a mould with a size of 3.0 mm (width) × 3.0 mm (thickness) × 50.0 mm (length) and a V-shaped gap (0.6 mm depth and 3 mm length) in the center. The samples for the tensile tests were prepared using a dog bone like mould with a length (for the narrow section) of 2 mm, a width of 2 mm and a thickness of 2 mm.

Characterizations

The morphology of the nanofibers was characterized by transmission electron microscopy (TEM) (Hitachi, H-800 and Tecnai) and scanning electron microscopy (SEM) (Hitachi S-4700, Japan). The composition of the nanofibers was determined using a Fourier transform infrared spectroscope (FTIR) mounted with a variable incidence angle attenuated total reflection accessory (ATR) (Tensor27, Bruker, Germany). The crystal structure of the PP nanocomposites was determined by X-ray diffraction with Cu Kα radiation (40 kV and 20 mA, D/max2500, Rigaku Co, Japan). The wavelength of the X-ray radiation was 0.154 nm and the scan rate was 2° min⁻¹. The crystallization rate of the PP nanocomposites was estimated by differential scanning calorimetry (DSC) (Stare system DSC1, Mettler Toledo, Co, Switzerland) under nitrogen atmosphere. In order to observe the melt-crystallization process, the samples were heated at 200 °C for 5 min to remove the thermal history, and then rapidly cooled to 130 °C to perform isothermal crystallization for 1 hour. The crystalline morphology of the PP composites was observed using a polarized optical microscope (POM, Leica, Biomed) equipped with a temperature controllable hot-stage and photo camera. The tensile tests of the PP composites were carried out using a CMT 4102 tensile tester with a cross-head speed of 50 mm min⁻¹, and the Notched Izod impact tests were performed using a Resil impactor (Ceast, Italian).

Results and discussion

Formation of nanoprotrusion surface structured SNFs

The SiO₂@SNFs were produced from the calcination of electrospun PVP/TEOS/SiO₂ nanofibers. In order to understand the structure formation of the electrospun PVP/TEOS/SiO₂ nanofibers, the morphology of a single electrospun PVP/TEOS/SiO₂ nanofiber was observed by TEM, as shown in Fig. 1. The electrospun PVP/TEOS/SiO₂ nanofiber showed a core–shell structure and had a thickness of about 450 nm. The thicknesses of the core and shell were about 230 and 225 nm, respectively.

Fig. 1 TEM images of a single electrospun PVP/TEOS/SiO₂ nanofiber containing 3 phr of SiO₂ nanoparticles.

This indicates that the partially gelatinized TEOS further undergoes gelation and phase separation to form the core–shell structured nanofibers during the electrospinning process. Based on the contrast of the TEM images, we can determine that the gelatinized TEOS and PVP form the core and shell of the nanofibers, respectively.

The structure changes of the PVP/TEOS/SiO₂ nanofiber before and after calcination were characterized by the FTIR-ATR spectroscopy, as shown in Fig. 2. In the FTIR-ATR spectrum of electrospun PVP/PEOS/SiO₂ nanofibers, the peak at 1625 cm⁻¹ assigned to the –C=O stretching vibration is contributed by the PVP, and the peaks at 3355 cm⁻¹ assigned to –OH vibrations are contributed by the –Si–OH groups of the gelatinized TEOS. However, the peaks at 1625, 3355 and 948 cm⁻¹ of the electrospun PVP/PEOS/SiO₂ nanofibers completely disappear after calcination. This indicates that the PVP was completely pyrolyzed and the gelatinized TEOS was dehydrated during the calcination, and the SiO₂@SNFs were produced by the calcination of electrospun PVP/TEOS/SiO₂ nanofibers.

Fig. 2 FTIR-ATR spectra of PVP/TEOS/SiO₂ nanofibers containing 9 phr of SiO₂ nanoparticle content (A) before and (B) after calcination.

Fig. 3 shows the SEM images of electrospun and calcinated PVP/TEOS/SiO₂ nanofibers containing 9 phr of SiO₂ nanoparticle content. The electrospun PVP/TEOS/SiO₂ nanofibers have a smooth surface, as shown in Fig. 3(A). After calcination, however, the PVP/TEOS/SiO₂ nanofibers (i.e. SiO₂@SNFs) exhibit a nanoprotrusion surface morphology, as shown in Fig. 3(B). The TEM images show more clearly that some aggregated silica nanoparticles are present on the surface of the SNF, as shown in Fig. 3(C). However, it is also possible that the silica nanoparticles are embedded in the bulk of the silica nanofibers.

Fig. 3 SEM images of (A) electrospun and (B) calcinated PVP/TEOS/SiO₂ nanofibers (i.e. SiO₂@SNFs) containing 9 phr of SiO₂ nanoparticles. (C) TEM images of SiO₂@SNF containing 9 phr of SiO₂ nanoparticles.

The degree of surface nanoprotrusion of the SNFs increases with increasing content of the incorporated SiO₂ nanoparticle, as shown in Fig. 4. However, the precise density of the SiO₂ nanoparticles on the surface of the SNFs is difficult to quantitatively obtain based on the incorporated SiO₂ nanoparticle content in this study due to the uncertain distribution of the SiO₂ nanoparticles on the surface of the SNFs. In spite of this, the degree of surface nanoprotrusion of the SNFs can be adjusted qualitatively via the incorporated SiO₂ nanoparticle content in the SiO₂@SNFs, as shown in Fig. 4. The specific surface area and surface roughness of the SiO₂@SNFs are influenced by the incorporated SiO₂ nanoparticle content. Our previous study¹⁹ showed that the specific surface areas of the SiO₂@SNFs were increased to 97.65, 250.52 and 345.07 m² g⁻¹ with the incorporation of 1.47, 2.90 and 4.29 wt% of SiO₂ nanoparticles into the SNFs, respectively. The surface roughness of the SiO₂@SNFs increased with increasing content of the SiO₂ nanoparticles.

Fig. 4 SEM images of SiO₂@SNFs containing various SiO₂ nanoparticles. (A) 0 phr, (B) 3 phr, (C) 6 phr, (D) 9 phr.

Effect of SiO₂@SNFs on the crystal nucleation of PP

The effects of SiO₂@SNF on the spherulite crystal size and nucleation density of PP were observed by POM. The samples were isothermally melt-crystallized at 130 °C for 1 hour. Fig. 5 shows that the spherulite crystal size of the PP/SiO₂@SNFs composite is obviously decreased compared with that of pure PP, and slightly decreases with increasing content of the SiO₂ nanoparticles. The effect of SiO₂@SNFs on the crystal nucleation density of PP shows the opposite trend to the spherulite crystal size. These results indicate that the SiO₂@SNFs act as a nucleation agent for PP. The spherulite crystal size influences the mechanical properties of PP. In general, a small size of the spherulite crystal morphology is favourable for improving the toughness of PP. Compared with SNF, the SiO₂@SNFs showed greater effectiveness for the heterogeneous nucleation of PP.

Fig. 5 POM images of PP (A) and PP/SiO₂@SNFs nanocomposites containing 2 wt% of SiO₂@SNFs with (B) 0 phr, (C) 3 phr, and (D) 9 phr of SiO₂ nanoparticles obtained at 130 °C.

Fig. 6 shows DSC thermograms of PP and the PP/SiO₂@SNFs nanocomposites obtained during the cooling process, with a cooling rate of 10 °C min⁻¹. The curves show that the melt-crystallization temperature of PP is 120.8 °C, and the melt-crystallization temperatures of PP/SiO₂@SNFs composites are 121.5, 122.3, 122.7 and 122.8 °C when the SNFs are loaded with 0, 3, 6 and 9 phr of SiO₂ nanoparticles, respectively. The melt-crystallization temperature of PP increased with increasing SiO₂ content of the SiO2@SNFs. This demonstrates that the heterogeneous crystal nucleation of PP can be enhanced by the SiO₂ nanoparticle content of the SiO₂@SNFs.

Fig. 6 DSC thermograms of PP and PP/SiO₂@SNFs nanocomposites containing 2 wt% of SiO₂@SNFs with various SiO₂ nanoparticle contents (0, 3, 6 and 9 phr) obtained during the cooling process with a cooling rate of 10 °C min⁻¹.

Effect of SiO₂@SNFs on crystallinity and PP β-form crystal nucleation

The effects of the SiO₂@SNFs on crystallinity and PP β-form crystal nucleation were determined by XRD, as shown in Fig. 7. The diffraction peaks at 2θ = 14.2°, 16.2°, 17.0° and 18.6° were observed in the XRD profiles of PP and the PP/SiO₂@SNFs nanocomposites. The diffraction peaks at 2θ = 14.2°, 17.0° and 18.6° are assigned to the (110)_α, (040)_α and (130)_α crystalline planes of PP α-form crystals, and the diffraction peak at 16.2° is assigned to the (300)_β crystalline plane of PP β-form crystals. As shown in Fig. 7, the relative peak intensity of (300)_β obviously increases after the incorporation of the SNFs into PP. However, the relative peak intensity of (300)_β is not further increased by the SiO₂@SNFs. This indicates that the relative β-form crystal content of PP is almost uninfluenced by the nanoprotrusion surface morphology of the SNFs. The relative β-form crystal content (K_β) of PP can be calculated from the X-ray diffraction data according to the equation proposed by Turner-Jones et al.²⁰ (1)where H_α(110), H_α(040) and H_α(130) are the intensities of the diffraction peaks at 14.2°, 17.0° and 18.8° from iPP α-form crystals, respectively, and H_β(300) is the intensity of the diffraction peak at 16.2° from iPP β-form crystals.

Fig. 7 XRD profiles of PP and PP/SiO₂@SNFs nanocomposites containing 2 wt% of SiO₂@SNFs with various amounts of SiO₂ nanoparticles (0, 3, 6 and 9 phr).

The total crystallinity can be calculated by the ratio of the total areas of the crystal diffraction peaks and whole peaks. The calculated total crystallinity and K_β value are summarized in Table 1. The total crystallinity, X_w (including both α and β-form crystals), of the PP/SiO₂@SNFs nanocomposites is almost independent of the SiO₂ nanoparticle content of the SiO₂/SNFs. Even though the K_β value of the PP/SiO₂@SNFs composites is almost two times higher than that of PP, the absolute value of K_β is still small. Moreover, the K_β value of the PP/SiO₂@SNFs is almost independent of the SiO₂ nanoparticle content of the SNFs. This indicates that the relative β-form crystal content of the PP/SiO₂@SNFs composites is insensitive to the degree of surface nanoprotrusion of the SNFs. Therefore, the effect of PP β-form crystals on the toughening of PP may play a minor role in this study due to the lower crystallinity of the β-form crystals.

Table 1 Total crystallinity and K_β values of PP/SiO₂@SNFs nanocomposites containing 2 wt% of SiO₂@SNFs with various amounts of SiO₂ nanoparticles (0, 3, 6 and 9 phr)

Sample	Xw	Kβ
PP	0.43	0.07
PP/SiO2@SNFs (0 phr)	0.41	0.12
PP/SiO2@SNFs (3 phr)	0.43	0.13
PP/SiO2@SNFs (6 phr)	0.46	0.12
PP/SiO2@SNFs (9 phr)	0.43	0.14

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Effects of SiO₂@SNFs on the mechanical properties of PP

Fig. 8 shows the impact and tensile strengths of PP and PP/SiO₂@SNFs at a loading of 2 wt% SiO₂@SNFs with various contents of SiO₂ nanoparticles. The impact and tensile strengths of PP are 22.0 kJ m⁻² and 25.7 MPa, respectively. The impact strength of PP/SiO₂@SNFs is significantly increased with increasing SiO₂ nanoparticle content. For example, the impact strength of PP/SiO₂@SNFs with 9 phr SiO₂ nanoparticles (41.6 kJ m⁻²) is 1.9 times higher than that of PP, even though the SiO₂@SNFs content is only 2 wt%. However, the tensile strength of the PP/SiO₂@SNFs composites is almost independent of the SiO₂ nanoparticle content of the SiO₂@SNFs.

Fig. 8 Impact strength and tensile strength of PP and PP/SiO₂@SNFs nanocomposites containing 2 wt% of SiO₂@SNFs with various amounts of SiO₂ nanoparticles (0, 3, 6 and 9 phr) obtained at room temperature.

The surface of the SiO₂@SNFs was treated with silane (KH570) for improving the interfacial bonding between the fillers and the PP matrix. Assuming that the interfacial bonding is improved by the silane treatment, the applied stress can then be transferred from the PP matrix to the SiO₂ nanoparticles of SiO₂@SNF during tensile testing. This can lead to an increase in the tensile strength with increasing the filler content because the fillers can carry the applied stress. However, Fig. 8 shows that the tensile strength of the PP/SiO₂@SNFs composites is independent of the filler content. The interfacial bonding between the fillers and the PP matrix also depends greatly on the amount of silane. Presumably, the amount of silane used in this study is not enough to ensure strong interfacial bonding.

Recently, Zhou et al.¹ reported the mechanical properties of crystalline iPP nanocomposites with untreated SiO₂ nanoparticles and grafted SiO₂ nanoparticles (grafting of poly(dodecafluoroheptyl acrylate), PDFHA, onto the nanoparticles) at a constant particle concentration of 1.36 vol% at different temperatures. Their results showed that the grafted SiO₂ nanoparticles improved the toughening of iPP in contrast with untreated SiO₂ nanoparticles. In their case, the grafted polymer chains on the SiO₂ nanoparticles would become entangled with the matrix polymer in the course of melt mixing, leading to improved nanoparticles–matrix interaction and the fluoride facilitated the relative sliding of the grafted nanoparticles under applied stress. Their results proved that the PP composites with an energy dissipation mechanism induced by nanoparticle mobility work to improve the toughness of non-layered nanoparticles–polymer composites. In our case, the entanglements of the PP chains may be enhanced by the SiO₂@SNFs due to the rougher surface of the SiO₂@SNFs. The SiO₂@SNFs in the PP matrix show a rougher surface morphology for the nanoparticles containing 9 phr content of SiO₂ than that for the nanoparticles containing 3 phr content of SiO₂, as shown in Fig. 9. This indicates that the nanoprotrusions on the surface of the SNFs promote the interaction between the PP matrix and the fillers than SNFs.

Fig. 9 SEM images of PP/SiO₂@SNFs nanocomposites containing 2 wt% of SiO₂@SNFs with different SiO₂ nanoparticles (A): 3 phr, (B): 9 phr. The insets in (A) and (B) are SEM images showing SiO₂/SNFs containing 3 and 9 phr SiO₂ nanoparticles, respectively.

Conclusions

In this study, we successfully fabricated SiO₂@SNFs by the calcination of electrospun PVP/TEOS/SiO₂ nanofibers. The SiO₂@SNFs had a nanoprotrusion structured surface morphology. The core–shell structured PVP/TEOS/SiO₂ nanofibers were the prerequisites for the formation of the SiO₂@SNFs. The degree of nanoprotrusion on the surface of the SNFs can be adjusted via the incorporated SiO₂ nanoparticle content. The effects of the SiO₂ content of the SNFs on the crystallization behaviour, relative content of β-form crystals, and mechanical properties of PP were also investigated. The DSC results indicated that the heterogeneous crystallization nucleation was enhanced by the nanoprotrusion surface structured SNFs. However, the relative content of β-form crystals was almost independent of the SiO₂ nanoparticle content of the SiO₂@SNFs. In contrast with SNFs, the SiO₂@SNFs provided a greater improvement of the impact strength of PP at the same loading content of filler. The impact strength of the PP/SiO₂@SNFs at a loading of 2 wt% of SiO₂@SNFs with 9 phr (SiO₂/TEOS = 9/100) of SiO₂ nanoparticles was 1.9 and 1.4 times greater than that of neat PP and PP/SNFs composite with 2 wt% of SNFs, respectively. Moreover, the crystallinity, relative content of β-form crystals, and tensile strength of PP were almost independent of the SiO₂ nanoparticle content of the SNFs. An impact strength of 41.6 kJ m⁻² can be achieved for PP at 2 wt% of SiO₂@SNFs with 9 phr of SiO₂ nanoparticles, which is sufficient for industrial requirements. The nanoprotrusion surface structured SiO₂@SNFs can promote PP chain entanglements, leading to an improvement in the nanoparticles–matrix interaction. However, the PP β-form crystals play a minor role in the toughening of PP in this study due to the lower crystallinity of the β-form crystals. We demonstrated that the nanoprotrusion surface structured silica nanofibers can be used as a novel nanofiller for improving the toughening of PP.

Acknowledgements

The research was supported by the National Basic Research Program of China (2015CB654700 (2015CB674705)), the Fundamental Research Funds for the Central Universities in China (JD1407), the Beijing Higher Education Young Elite Teacher Project (YETP0493), and the Program of Beijing Excellent Talents (2013D009016000003).

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