Surface modification of melt electrospun polypropylene fibrous film by silicon dioxide gel for high thermomechanical properties

Abstract

In this work, a polypropylene (PP) porous film was made-up by melt electrospinning. The fibrous film was coated by a superthin layer of silicon dioxide (SiO₂) gel. The surface morphology of the SiO₂ gel was decoded by scanning electron microscopy and atomic force microscopy. The SiO₂ gel interface with film, as well as its crystallinity, were characterized by Fourier transform infrared spectroscopy, energy dispersive spectroscopy, and X-ray diffraction. The PP composite film's thermomechanical properties were studied through thermal shrinkage, thermal gravimetric analysis, dynamic mechanical analysis, and an Instron tensile machine with a heating chamber. The results showed that the coated SiO₂ gel network could effectively reduce the PP film's thermal shrinkage by 48.5% without change of crystallinity. The coated SiO₂ gel is capable of enlarging the decomposition temperature range and the storage modulus of the PP film. Meanwhile, it was discovered that, along with the increase of measured temperature, the loads on both pure PP film and PP/SiO₂ gel film decreased under a constant strain, or the tensile strain of both of them was enhanced under the same load. The solid gel network endows the PP electrospun film with relatively higher thermal safety.

Keywords

Electrospinning polypropylene sol–gel coating thermomechanical properties

Polypropylene (PP) film, a kind of technical material with countless nano- or micro-fibrils and micropores, has been widely used in industrial areas, such as for battery separators, facemasks (ion and air transfer areas),^1–5 etc. The PP products have several advantages, such as good chemical stability, super hydrophobicity, and light weight with a stable porous microarchitecture.⁶ However, the PP products also have relatively low glass transition temperatures, which may cause some risks in use. For example, some work environments may gradually enhance the ambient temperature, which would lead to the high mobility of PP macromolecules, such as battery PP separators. The heat accumulation at the interface would disorder the alignment of molecular chains after the glass transition temperature. Thermal shrinkage of PP film occurs under high temperature, which is regarded as a significant safety concern of PP film in some specific uses,⁷ primarily when it works as a battery separator.⁸,⁹

In recent studies, polyolefin combined with a kind of high melting-point material can improve the thermal safety of PP or polyethylene (PE) products.⁷,¹⁰ The materials mainly include some high melting-point inorganic particles and coatings. Kong et al.¹¹ used solution casting to prepare PP non-woven fabric filled with a silicon dioxide/polyvinylidene fluoride composite. This work proved that 4 wt% of SiO₂ filler can improve the mechanical properties, as well as the thermal safety. Similarly, Cai et al.¹² employed nano SiO₂ in a PP precursor system for melt-stretching a PP microporous membrane—2% SiO₂ content would lead to mechanical strength. Also, the elastic modulus has improved by two times, because of the 2% SiO₂, without deteriorated the pore structure or properties. Then, Li et al.¹³ investigated the thermal aging of PP/SiO₂ nanocomposite films and answered how to improve PP and SiO₂ nanoparticle interfacial bonding. The introduction of SiO₂ nanoparticles into PP products would significantly improve the composite film's mechanical and thermal properties. However, avoiding nanoparticle agglomeration in the PP system to realize even properties is the main issue to develop the excellent quality of PP nanocomposite products.

The surface coating of a PP system is a rational way to improve PP mechanical or thermal safety. There is no concern about the agglomeration of nanoparticles. A few trials have been carried out for the coating of such materials. For example, a kind of PE film manufactured from the biaxial drawing process has been coated with a SiO₂ gel layer for a thermal safety study.⁷ However, due to the low porosity of PE film, the thermal safety improvements were not considerable. In fact, there are many surface coating approaches. Among them, a sol–gel coating is an effective way to combine polyolefin with high melting-point inorganics.¹⁴ The precursors in the sol solution preparation are mostly metal-organic compounds or inorganic salts, such as metal alkoxide and aluminum oxide. Any precursors would experience a series of hydrolysis and polymerization reactions to generate a colloidal suspension, that is, a sol. The sol–gel process shows the transition of a colloidal solution into an integrated solid network in the porous structure under certain conditions. Here, the sol components in a liquid are a proper candidate to process some polyolefin materials because the solution can penetrate the micropores between the polyolefin membrane's fibrils—if the surface of the polyolefin membrane has been treated for hydrophilicity. The sol is then attached to the surface of polyolefin micro-fibrils. Under a stimulus, the sol would evolve into a crosslinked inorganic network that contains small liquid molecules. The crosslinked network can prevent the ‘sol' from being in the flowing state, which becomes a ‘solid' state, namely, a gel. A drying process can remove the liquid molecules of the gel, resulting in the formation of a porous solid material in and upon the polyolefin film.⁷ To date, the sol–gel coating process has been employed to process some functional gel matter on many kinds of materials increasingly, such as polymer,¹⁵ metal,¹⁶ and glass.¹⁷

In this work, a PP porous film was fabricated by melt electrospinning; the micro-fibrils inside the film were randomly arranged. Then the film was coated with a layer of silicon dioxide (SiO₂) gel. The coating experienced a dipping→coating→drying process. The coated film was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FT-IR), and energy dispersive spectroscopy (EDS). The thermomechanical properties were closely studied using the flame-close reaction, thermal gravimetric analysis, dynamic mechanical analysis (DMA), and temperature rise in stretching to study the effect of SiO₂ gel on PP electrospun film.

Materials and experimental methods

Materials

The PP filament (0.75 mm in diameter, Mn = 6000–15,000) was put into a melting extruder. The extruder can reach a higher heating temperature (180–200°C) than the melting point of the PP filament. At the head of the extruder, there was a liquid semi-microsphere that appeared at the spinneret. A self-established electrospinning device was connected to the spinneret, as shown in Figure 1(a). Under the electrical field's high voltage, the liquid semi-microsphere would be stretched into a bunch of microjets and split into micro- and nano-fibrils on the received metal plate to form a piece of fibril-assembly porous PP film. The electrospinning conditions were performed with an electrical voltage of 20 kV, a distance of the spinneret to the received plate of 30 cm, a diameter of the inner spinneret of 0.7 mm, and an extruding speed of 0.1 g/min.

Figure 1.

Preparation of polypropylene (PP)/SiO₂ gel film: (a) melt electrospinning for generating a thin layer of PP film; (b) preparation of SiO₂ sol; (c) drying of PP/SiO₂ gel composite film. TEOS: tetraethyl orthosilicate; DMF: dimethylformamide.

After 2–5 electrospinning hours, a piece of superthin PP film (thickness: around 8 μ $m$ ) was picked up from the received metal plate and was used in this study. The film samples were washed in ethanol followed by two rinses in distilled water for purification, and then they were dried at 40°C in an oven.

Preparation of SiO₂ sol

The developed gel should be highly thermally resistant to protect the electrospun PP film. Similar to our previous studies,⁷,¹⁰ SiO₂ sol was selected as the coating solution. The preparation of SiO₂ sol was performed by blending precursors of tetraethyl orthosilicate (TEOS) with ethanol (98% EtOH) at 20°C under high-speed stirring. Hydrochloric acid (HCl) as the catalyst was added into the blended solution, following by some deionized water and dimethylformamide (DMF) added into the system as a buffer. The molar ratio of the used liquid materials was as follows: 1 (TEOS):8 (EtOH):0.04 (HCl):3 (H₂O):1.6 (DMF). The chemical reaction formula is shown in Figure 1(b). Formula (1) in Figure 1 shows the hydrolysis process that employs HCl as the catalyst, and the reaction formula (2) in Figure 1 shows the condensation of the hydrolysis output. Both chemical reactions were ongoing simultaneously. The preparation lasted 6 hours at 40°C with a stirring speed of 200 rpm. Finally, a stable and transparent liquor was generated after continuous stirring and reaction.

Sol–gel coating on film

The prepared electrospun PP samples from the Materials section were immersed in a mixed solution of K₂Cr₂O₇+ H₂O+H₂SO₄ (density of 1.8 g/cm³, weight ratio = 4.4:71:88.5) for 1 hour of chemical reaction under 70°C. The strong acid plus oxidant modification to the PP surface would give some hydrophilic groups on the PP nanofiber, such as –OH, -COOH, -SO₃H, etc.¹⁸ Then, the surface tension for wetting the PP electrospun film and hydrophilicity would increase significantly. The modified film was washed in ethanol twice and immersed in the prepared sol for 1 hour. The wet PP films were then compressed by a circular glass bar on a smooth plate surface. The compression process was repeated two or three times. A paper tissue was used to remove the excess sol liquid on the electrospun film surface. Then, the sol-coated samples were dried at 40–50°C for 1 hour. The facile drying process can effectively remove the small liquid molecules and benefit the transformation of liquid sol to solid gel. A schematic illustration of sol–gel coating for the electrospun film is shown in Figure 1(c).

Characterizations

Morphology observation

The morphologies of electrospun PP film and coated PP/SiO₂ gel film were observed via SEM (JEOL Model JSM-6490). The scanning electron microscope was equipped with an EDS device (JEOL Model Oxford INCA 250 - Energy Dispersive Spectrometer) to determine the chemical elements on the film surface in the microzone. Meanwhile, a domestic atomic force microscope (CSPM4000, Benyuan Company) was used to test the coated gel's morphology or dimension on film that was just dried from an oven at 50°C. The sample was pasted on the atomic force microscope test stage, and the tapping mode of the probe was used during scanning. All film samples were scanned at 20°C in nitrogen for SEM observation and at ambient atmosphere for AFM scanning.

Bonding and crystalline analysis

The interaction of chemical groups between the sulfonated PP electrospun film and SiO₂ gel was characterized by FT-IR (PerkinElmer Spectrum 100 FT-IR Spectrometer, USA). The scan range was set from 4000 to 650 cm⁻¹ using the ATR-Ge (attenuated-total-reflectance) approach. The FT-IR absorption spectra of scanning were recorded using 64 scans under the resolution of 4 cm⁻¹. Furthermore, the crystallinity of PP electrospun film and the coated film were both determined by X-ray diffraction (XRD; Rigaku Smart Lab 9KW XRD system). Different from the previous work,¹⁹,²⁰ the film mode was chosen for XRD scanning, and 1 cm × 1 cm square size of the sample was placed on the test stage. The parameters were set in the XRD scanning system in advance; for example, the 2θ scan range was selected from 10° to 50°, and the scanning speed was set as 3º·min^–1 at 40 kV and 40 mA (a common condition for scanning). The XRD device was equipped with Cu Kα radiation with a wavelength of 0.154 nm.

Thermomechanical study

Shrinkage behavior

Most polyolefin materials display a feature of thermal shrinkage under high temperatures. In order to study the improvement of thermal stability, the samples of PP and PP/SiO₂ gel films were carried out using the following steps: ① the sample was put close to an alcohol burner and its reaction behavior was recorded by a camera; ② the film samples were heated from 20°C to 120°C under the vacuum condition, and the heating rate was relatively slow, which was around 5°C $\cdot$ min⁻¹, and the sample was maintained at high temperature (120°C) for 20 minutes. During this period, the film samples would deform, which depends on their thermal stability. The change of film size was used to measure this factor; an enormous shift in size means more inferior thermal stability of film when close to a high-temperature resource. Therefore, it is inferred that a coating of thermal-resistant materials on PP porous film would act as a layer of thermal shield to improve the thermal stability due to the thermal resistance of the inorganic gel layer to the PP micro fibers network during the environmental heating to the film shrinkage.

Thermomechanical analysis

In this work, the thermomechanical properties of PP electrospun film and its coated gel film were analyzed using thermogravimetric analysis (TGA; Mettler Toledo TGA/DSC-1 Thermal-analyzer, Switzerland) for the study of the coated content of gel, DMA (PerkinElmer Diamond, DMA, USA) for cyclic heating–cooling analysis, and an Instron-5566 universal test machine with a heat chamber (Instron Calibration Lab, 5566Q7582) for the study of thermos-stretching behavior. All the tests were conducted using nitrogen (N₂) as a purge gas. Temperature parameters were set for TGA from 25°C to 700°C with a heating rate of 10°C $\cdot$ min⁻¹, DMA for 25–275°C with a heating rate of 2°C $\cdot$ min⁻¹, and the Instron-5566 for temperatures from 15°C to 160°C with a stretching rate of 10 mm $\cdot$ min⁻¹. For each type of test, three tests were carried out for each fresh sample, and the average value or typical curve was selected for studying the thermomechanical performance of the coating effect on PP electrospun film.

Results and discussion

Shrinkage behavior

A natural feature is frequently elucidated that any polyolefin film is shrunk and gradually melted when it is close to a fire flame. The macromolecular chains are long and parallel with each other in the polyolefin polymer. Thermal heating endows the straight molecular chains with high mobility in spatial displacement. According to the Second Law of Thermodynamics, the enhanced entropy from heat accumulation²¹ pushes the orientation of molecules into an unstable or random state. Figure 2(a) shows a pronounced shrinkage of PP electrospun film with a black curly bead when it is close to the flame and finally is lit.

Figure 2.

Shrinkage behaviors of polypropylene (PP) and PP/SiO₂ electrospun films: (a) close-flame behavior; (b) shrinkage of film at high temperature under the vacuum condition.

Nonetheless, the coated SiO₂ gel on and in the porous film, to some extent, blocks the curly shrinkage behavior of the film. Meanwhile, owing to the adsorbed solvent molecules in the film, such as alcohol, the coated film is directly lit after a certain extent of heat accumulation related to the decomposed temperature, as shown in the highly lit image of Figure 2(a). More information on such shrinkage and the residues of burned materials can be seen in Scheme 1 in the supplementary file.

To have a quantitative study, two square frames with the size of 2 cm × 2 cm were compared with respect to their shrinkage behavior. Both were heated up to 120°C in a vacuum heating chamber for 1 hour. The marked area of 4 cm² was reduced to 1.76 cm² for the PP electrospun film, while it was reduced to 3.64 cm² for the gel-coated film under 120°C (see Figure 2(b)). The 44% compared with 92% of shrinkage using the coating layer indicates the significant improvement of PP film's thermal stability after a coating layer of solid gel. A considerable variation of PP film shrinkage (the mean value of 3.64 cm² from three test samples of 3.88, 3.45, and 3.56 cm²) discloses the uneven thickness of the collected electrospun film, which comes from the anisotropic shrinkage during heating. In contrast, the coated solid gel layer hinders such thermal shrinkage of PP electrospun film significantly.

Morphology characterization

The surface morphologies of PP and PP/SiO₂ films were detected using SEM under different magnifications, as shown in Figure 3(a). The PP electrospun film shows a porous structure with a large number of electrospun micro-fibrils and different sizes of micropores among the fibrils. The electrospun film has been immersed in SiO₂ sol, and a smooth gel solid layer on the fibrils is achieved after curing. The chemical elements inside the gel layer were analyzed using EDS (point selection mode). It was confirmed that the main components in the gel layer were Si and O (right-hand image of Figure 3(a)), indicating the existence of SiO₂ gel on the fibrils.

Figure 3.

(a) Morphologies of polypropylene (PP) (left-hand image) and PP/SiO₂ (middle image) electrospun films, and scanning electron microscopy and energy dispersive spectroscopy (EDS) (right-hand image) results. (b) Atomic force microscopy results. (Color online only.)

AFM was also used to characterize the detailed surface morphology of the gel layer; Figure 3(b) shows that the diameters of gel particles were from 5 to 20 nm using cross-section analysis under an arbitrary red dashed line. Such diameter range of gel particles is much less than the diameter of micropores among electrospun fibrils, indicating a good penetration of the SiO₂ sol into the micropores for fine coating. This improvement is an advantage over others, such as the materials in Xiao et al.⁷ Nevertheless, the dense staked gel on fibrils is good for better thermal stability but not beneficial to some specific applications, such as separators, because of the decreased porosity from the intuitive feeling. This point was consistent with the results of previous work.¹⁰

Structure and physical analysis

The weight percentage of coated gel on the PP electrospun film can be roughly measured through TGA. The TGA tests were performed from 25°C to 700°C under a vacuum condition. Theoretically, the PP macromolecules would decompose to gas beyond the melting point. The experimental data verified the decomposition behavior. From the TGA test result (Figure 4(a)), the PP film shows its decomposition temperature occurring at 430°C. Higher than this temperature, the weight percentage of the PP sample drops remarkably, indicating the splitting of macromolecules into small molecules and eventually the organic gas state.

Figure 4.

Characterized results of polypropylene (PP) and PP/SiO₂ gel films from (a) thermogravimetric analysis (TGA), (b) X-ray diffraction (XRD), and (d) Fourier transform infrared spectroscopy (FT-IR), and (c) the change of hydrophilicity from pure PP electrospun film, oxidant + acid modification of PP electrospun film to the final gel-coated PP film using the concept of the contact angle.

The melting PP sample evaporates without residue, which means that all PP macromolecules have decomposed to gas completely. However, the TGA result of the PP/SiO₂ gel weight percentage shows a different decreasing curve during the test temperature range. The curve also has a dramatic decrease in weight percentage at the same decomposition temperature of PP film. During this decomposition temperature, as shown in the inset of Figure 4(a), the measured curve of PP/SiO₂ gel displays an enlarged decomposition range compared with that of PP film. This indicated an extension of decomposition or an enlarged temperature range (from 486°C of PP to 510°C of PP/SiO₂ gel for the end of decomposition) for splitting PP macromolecules—due to the resistance of the coated SiO₂ gel network to the escape of PP decomposed molecules. After the complete decomposition of PP, the weight percentage curve displays a nearly constant line, indicating the remaining residue should be the coated inorganic materials on PP film. As a rule of thumb, the whole weight decrease contains two parts for PP/SiO₂ gel film. The first is the slowly decreasing stage with the temperature from 100°C to 430°C, namely the wet gel phase, in which small molecules of liquid that have been absorbed inside the gel network escape under increased temperature. The other (from 430°C to 510°C) is the decomposition of PP film. The 55 wt% of residual is the undecomposed SiO₂ gel, plus the 20 wt% of the beginning locked sol phase; therefore, a total of 75 wt% of coated gel accounts for the whole weight of the coated film.

The crystalline phases of PP and PP/SiO₂ gel films were analyzed through XRD, as shown in the measured result in Figure 4(b). Four characteristic peaks were found at 2θ of 14.0° (110), 16.8° (040), 17.8° (130), and 21.5° (111, 131) for the common α type of monoclinic crystal.²² There is no ß type of crystal (300) observed from the curves, which usually requires nucleating agents during PP film manufacturing for their better mechanical properties.²³ According to Equation (1),²⁴,²⁵ a calculation of crystallinity was carried out for both films using the peak area of the test data utilizing normalization. Approximately 35% of crystallinity was obtained for both PP film and PP/SiO₂ gel film

X c = \frac{X_{T} - X_{A}}{X_{T}}

(1)

where

X_{T}

represents the total area of the characterized diffractogram and

X_{A}

denotes the area related to the amorphous region. It is inferred that there is no crystalline phase to be formed during the transformation from SiO₂ sol to gel based on the same abscissas of peaks of the two scanned curves, and the coated layer of SiO₂ gel does not affect the crystallinity of PP fibrous film. The amorphous phase of SiO₂ gel benefits the thermal stability of the coating layer without a melting point. Figure 4(c) also shows the change of hydrophilicity from the pure PP electrospun film—the oxidant + acid modification of PP electrospun film to the final gel-coated PP film. Pure PP electrospun film shows an 86.6° contact angle with water drop, indicating its high hydrophobicity. With the modification to the PP electrospun film, the contact angle becomes zero, which means the surface of PP electrospun film has high hydrophilicity for better combination with the gel layer.

What would take the place of the coated SiO₂ gel layer on the PP electrospun film on their interface? This phenomenon can be reflected through FT-IR analysis, as shown in Figure 4(d). The characteristic peaks can reveal specific organic signals during the whole test range of 650–3700 cm⁻¹, such as the asymmetric stretching vibration band of -C-C- $(⊥)$ at the peak of 2916 cm⁻¹ and the symmetric stretching vibration band of -C-C- $(∥)$ at the peak of 2850 cm⁻¹. The characteristic peaks at 725 and 1464 cm⁻¹ indicated the existence of -C-C- $(⊥)$ for the symmetrical vibration and -C-C- $(⊥)$ for the out-of-plane twisting vibration of the PP crystalline phase. The other peaks at 973, 1066, and 1651 cm⁻¹ denote the Si-O-Si asymmetric/symmetric stretching vibrations, the Si-OH curve vibration, the S-O asymmetric stretching vibration, and the H-OH stretching vibration, respectively. These characteristic peaks confirm the existence of the -SO₃ group and the coated layer of SiO₂ gel on the PP film.

Thermomechanical properties

SEM was used to observe the surface morphologies of PP and PP/SiO₂ gel films after stretching at 120°C. As shown in Figures 5(a) and (b), white color lines were noted on the PP electrospun film. The reason may be ascribed to the reorientation of electrospun random fibrils along the stretching direction under high temperature. However, the white lines were covered by SiO₂ gel such that wavy surface morphology was observed instead of an orientated white physical raised back. Such stretching hardly affected the gel's coated structure, indicating the good thermal stability of coated gel in protecting electrospun PP film. Figure 5(c) compares the shrinkage of pure PP electrospun film and its SiO₂ gel-coated film, especially the action mechanism of SiO₂ to resist the thermal shrinkage of PP fibers. As a matter of fact, the shrinkage resistance of electrospun PP fibers by coated SiO₂ gel can be explained through two scales, that is, fiber scale and network scale. Theoretically, each fiber was covered with a thin layer of SiO₂ solid gel; they have different thermal shrinkage performance. When the film is under high temperature, the low rate of SiO₂ solid gel's thermal shrinkage could resist the internal PP fiber contraction. Meanwhile, the SiO₂ solid gel between fibrils also blocks the fibers from thermal shrinkage under high temperatures. In contrast, pure PP fibers would shorten and melt under increasing temperature without the coated thermal barrier because of the release of stretched strain from electrospinning to its original molecule state.

Figure 5.

Scanning electron microscopy images of the film under stretching at 120°C: (a) polypropylene (PP) film; (b) PP/SiO₂ gel film; (c) shrinkage resistance of electrospun fibers by the coated SiO₂ gel layer in terms of fiber scale and fibrous network scale.

During the increase of ambient temperature, the DMA tests of PP film would display a variation of storage modulus (E') and phase angle (D), as shown in Figures 6(a) and (b). Regarding the measured E' value, two plateaus were found as the glass-like and rubber-like states for both samples at the beginning of the heating stage. The plateaus representing the E' value decrease significantly at around 130°C for the PP film and 145°C for the gel-coated PP film. This indicates that a coating layer of thermal-resistant inorganic gel can enhance the glass transition temperature and the E' value as well, to some extent. By contrast, the phase angle (D) from Figure 6(b) manifests a content ratio of viscous material to the elastic material. A higher angle means a polymer with less elasticity. Figure 6(b) shows the measured tanD curves of both samples. At the same temperature, the PP-coated gel sample shows a relatively smaller tanD value, indicating a higher elasticity of the PP sample given by the gel network. In comparison with the pure PP sample under a certain high temperature, the stable thermal network of SiO₂ gel can prevent the PP macromolecule chains from being in the viscose state or even flow state due to the increased heat transfer from the outside environment into the PP itself. Therefore, the coated SiO₂ layer would delay the melting behavior of PP film.

Figure 6.

Dynamic mechanical analysis test results and thermomechanical behaviors of polypropylene (PP) film and PP/SiO₂ gel film: (a) storage modulus; (b) tanD; Instron-5566 tensile results of (c) measured tension force of the film under a fixed strain and (d) the length of the film stretched under a fixed tensile load, both with the increase of temperature from 15°C to 160°C.

DMA results tell us the variation of a polymer sample's viscoelasticity under a particular stretched stain with ambient temperature change, for example, the modulus or phase angle. To understand the thermomechanical properties of a polymer sample, for example, the stress and strain behaviors under various heat accumulations, as a supplementary research approach to the DMA test, a tensile test was performed on the sample with the Instron-5566 with a heating chamber. Here, each PP film sample was tailored into a rectangle sample with the size of 2.5 cm $\times$ 5 cm, and the stretching was carried out under the same stretched stain (constant 10%) during the increase of temperature from 15°C to 160°C. Figure 6(c) shows the variation of measured tension force for both film samples. It is noted that the tension force decreases with the increase of temperature and the PP/SiO₂ gel film shows a relatively higher tension force compared with the PP film. Here, the SiO₂ gel network plays a vital role in the enhanced tension force. Figure 6(d) shows the variation of film strain under the same tension force with increased testing temperature. For a particular temperature, for example, 100°C, the tested tensile force of PP/SiO₂ film is 16% larger than that of PP film under the same stain; in contrast, the tensile strain of PP/SiO₂ film is 19% less than that of PP film under the same load. Indeed, the two curves reflect that the PP film with the coated gel displays relatively less increased strain than pure PP film, owing to the hysteresis of the coated thermo-resistant gel layer to the inside of the PP electrospun film. The increased temperature would result in an enhancement of the mobility of molecule chains and a lesser crystalline phase; nonetheless, from the slope of the strain curve, it can be seen that the gel network is able to reduce the deformation of the film under high temperature when the film is under an absolute load. This behavior points to a sufficient improved stiffness, thermal stability, and safety by the SiO₂ gel network to the PP electrospun film, which coincides with the enhanced storage modulus of PP/SiO₂ film by the test results from DMA.

Conclusions

Pure polyolefin film requires modification for some specific applications owing to its poor thermal stability. This work investigated the thermomechanical properties of PP electrospun film and its coated SiO₂ gel film. The SiO₂ sol was self-made, and a sol–gel coating process ensured a well-coated gel layer formed on the PP fibrils. SEM and AFM were used to observe the morphologies of the film fibrils and coated gel, and both were in micro- and nano-scales. EDS, FT-IR, and XRD were used to verify the formed SiO₂ chemical elements and the gel interaction with the PP fibrils, and also the gel's effect on the PP crystalline phase. The TGA test indicated a high weight ratio (75 wt%) of SiO₂ gel was formed onto and inside the PP electrospun film. The decomposition temperature of the PP film was extended by 24°C more with the aid of a gel network for the resistance to the escape of decomposed molecules. Meanwhile, the SiO₂ gel network can prevent pure PP film's thermal shrinkage and improve the storage modulus with increased stiffness. With the increase of temperature, the tension force variations with fixed strain and tension strain variation with a fixed load were both decreased for PP film and its coated SiO₂ gel film, indicating that the sol–gel coating process can enhance the thermal stability and safety of PP electrospun film to some extent.

Supplemental Material

sj-pdf-1-trj-10.1177_00405175211010671 - Supplemental material for Surface modification of melt electrospun polypropylene fibrous film by silicon dioxide gel for high thermomechanical properties

Supplemental material, sj-pdf-1-trj-10.1177_00405175211010671 for Surface modification of melt electrospun polypropylene fibrous film by silicon dioxide gel for high thermomechanical properties by Jumei Zhao, Hongtao Zhou, Huizhen Ke, Xueliang Xiao, Qingqing Wang and Quan Feng in Textile Research Journal

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Integration Platform of Industry and Education of Jiangsu High Vocational Education (No. 26 of Jiangsu Vocational Education 2019), the Qinglan Project of the Jiangsu High Education Institutions of China and Jiangsu High Vocational College Academic Leaders High-end Research and Training, the Open Project Program of Fujian Key Laboratory of Novel Functional Textile Fibers and Materials, Minjiang University, China (Grant No. FKLTFM2006), the Open Project Program of Anhui Province College of Anhui Province, College Key Laboratory of Textile Fabrics, and the Anhui Engineering and Technology Research Center of Textile (Grant No. 2018AKLTF01).

ORCID iDs

Hongtao Zhou

Huizhen Ke

Supplemental material

Supplemental material for this article is available online.

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Supplementary Material

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Surface modification of melt electrospun polypropylene fibrous film by silicon dioxide gel for high thermomechanical properties

Abstract

Keywords

Materials and experimental methods

Materials

Preparation of SiO2 sol

Sol–gel coating on film

Characterizations

Morphology observation

Bonding and crystalline analysis

Thermomechanical study

Shrinkage behavior

Thermomechanical analysis

Results and discussion

Shrinkage behavior

Morphology characterization

Structure and physical analysis

Thermomechanical properties

Conclusions

Supplemental Material

sj-pdf-1-trj-10.1177_00405175211010671 - Supplemental material for Surface modification of melt electrospun polypropylene fibrous film by silicon dioxide gel for high thermomechanical properties

Footnotes

Declaration of conflicting interests

Funding

ORCID iDs

Supplemental material

References

Supplementary Material

Preparation of SiO₂ sol