Titanium Dioxide

polymers Article

Preparation of Microporous Polypropylene/Titanium Dioxide Composite Membranes with Enhanced Electrolyte Uptake Capability via Melt Extruding and Stretching Shan Wang 1 , Abdellah Ajji 2 , Shaoyun Guo 3 and Chuanxi Xiong 1, * 1 2 3

*

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China; [email protected] CREPEC, Chemical Engineering Department, Ecole Polytechnique, Montreal, QC H3C 3A7, Canada; [email protected] The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China; [email protected] Correspondence: [email protected]; Tel.: +86-27-8738-7481

Academic Editor: Wei Min Huang Received: 22 February 2017; Accepted: 16 March 2017; Published: 17 March 2017

Abstract: In this work, a blending strategy based on compounding the hydrophilic titanium dioxide (TiO2 ) particles with the host polypropylene (PP) pellets, followed by the common membrane manufacture process of melt extruding/annealing/stretching, was used to improve the polarity and thus electrolyte uptake capability of the PP-based microporous membranes. The influence of the TiO2 particles on the crystallinity and crystalline orientation of the PP matrix was studied using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and infrared dichroic methods. The results showed that the TiO2 incorporation has little influence on the oriented lamellar structure of the PP-based composite films. Investigations of the deformation behavior indicated that both the lamellar separation and interfacial debonding occurred when the PP/TiO2 composite films were subjected to uniaxial tensile stress. The scanning electron microscopy (SEM) observations verified that two forms of micropores were generated in the stretched PP/TiO2 composite membranes. Compared to the virgin PP membrane, the PP/TiO2 composite membranes especially at high TiO2 loadings showed significant improvements in terms of water vapor permeability, polarity, and electrolyte uptake capability. The electrolyte uptake of the PP/TiO2 composite membrane with 40 wt % TiO2 was 104%, which had almost doubled compared with that of the virgin PP membrane. Keywords: polypropylene; microporous membrane; titanium dioxide; lamellar orientation; stretching; electrolyte uptake

1. Introduction In recent years, microporous polypropylene (PP) membranes have been widely used as separators for fabricating lithium batteries [1–5]. These separators can guarantee the rapid transport of the ionic charge carriers and keep the negative and positive electrodes apart to avoid an internal short [2]. The nonpolar properties of PP can offer excellent chemical stability, mechanical strength, and other advantages; however, they also lead to extremely low wettability with liquid electrolytes such as ethylene carbonate (EC) and propylene carbonate (PC), which consequently adversely affects battery performance [3,4]. Therefore, it is of much significance to develop novel PP membranes with higher polarity and thus an enhanced ability to uptake and retain the polar electrolyte.

Polymers 2017, 9, 110; doi:10.3390/polym9030110

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To achieve this goal, different strategies have been carried out to improve the membrane surface, i.e., increasing the amount of surface polar groups and maximizing hydration. These methods include plasma treatment, surface coating with hydrophilic materials, radical grafting hydrophilic functional groups, surface polymerization of hydrophilic monomers, and so on [6–16]. However, some inherent drawbacks of these methods have been recognized. For example, temporary hydrophilicity is an unavoidable problem in the cases of plasma treatment and surface coating; radical grafting and/or polymerization methods are often complicated and require additional steps in the membrane preparation, thus increasing the cost [17,18]. Accordingly, the above techniques are greatly limited in applications on an industrial scale. Very recently, a blending strategy, i.e., physically mixing the host polymer with a hydrophilic modifier, subsequently followed by the conventional membrane fabrication process, is proposed and considered as a potentially promising method for manufacturing the polyolefin membranes with enhanced polarity, due to its feasible, large-scale, and cost-effective characteristics [17–19]. Commonly, the fabrication of PP microporous membranes involves the melt extrusion of precursor films followed successively by the high-temperature annealing and cold and hot stretching procedures [20–26]. It has been well established that the generation of a row nucleated lamellar structure in precursor films plays a key role in determining the porous structure and thus the permeability performance of PP membranes [20–22]. Thereby, the key point of the blending strategy is to reduce the influence of the modifier on the crystalline morphology of the PP precursor film as much as possible. Saffar et al. have developed the hydrophilic PP-based membranes from the blends of PP and PP-grafted maleic anhydride (PP-g-MAH) or acrylic acid (PP-g-AA) using a commercial drying process [18]. The results showed that a 2 wt % modifier was optimal for membranes to realize good hydrophilicity while causing minimal change to the crystalline structure compared to the neat PP film. In comparison with their counterparts of hydrophilic polymers or oligomers, modifiers of hydrophilic inorganic fillers possess some extra advantages in terms of thermal, mechanical, and electrical properties [27,28]. However, to the best of our knowledge, little work has been carried out to investigate the influence of inorganic fillers on the structure and properties of PP membranes. In this work, hydrophilic titanium dioxide (TiO2 ) particles of submicron size were chosen and compounded with host PP pellets at different ratios to improve the wettability and retention of the liquid electrolytes. The influence of the TiO2 particles on the crystalline structure and deformation behavior of PP-based precursor films obtained by melt extrusion was systematically investigated. Then, the PP/TiO2 composite films were annealed and cold- and hot-stretched into microporous membranes. The porous structure, water vapor permeability, water contact angle, electrolyte uptake, and mechanical properties of the PP/TiO2 composite membranes were thus comprehensively studied. 2. Materials and Methods 2.1. Materials A commercial polypropylene homopolymer (Pro-fax 6523) purchased from LyondellBasell Industries (Rotterdam, The Netherlands) was chosen as the polymer matrix. The resin has a density of 0.9 g/cm3 and a melt flow rate (MFR) value of 4.0 g/10 min (230 ◦ C, 2.16 kg). Titanium dioxide (TiO2 , Ti-Pure R-960) from DuPont Company (Wilmington, DE, USA) were used as the filler. According to the manufacture, the particles have an average size of 0.50 µm. 2.2. Film and Membrane Preparation PP/TiO2 composites with different TiO2 loadings were prepared with an intermeshing co-rotating twin screws extruder (Leistritz Corporation, Allendale, MI, USA, ZSE 18 HPe). A temperature profile ranging from 160 to 200 ◦ C was set along the extruder, and the screw speed was 60 rpm. The extruded strands were quenched by cold water and subsequently pelletized and dried for the following film extrusion.

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3 of 12 To prepare preparethe theprecursor precursorfilms, films,cast castextrusions extrusionsofof thepre-dried pre-driedPP/TiO PP/TiO2 composites composites were were To the 2 performed using a 45 mm single extruder (Killion Corp., Indianapolis, IN, USA) equipped with a slit using of a 45 single extruder Corp., Indianapolis, IN, USA) equipped with e precursor films,performed cast extrusions themm pre-dried PP/TiO2 (Killion composites were (adie width of 25 cm25and thickness of 0.8 of mm) a set of cooling rolls. The screw 12 adie slit(Killion (a width cmaand aIN, thickness 0.8 and mm) a set of cooling rolls. The speed screw was speed 5 mm single extruder Corp., of Indianapolis, USA) equipped withand a slit rpm, and the die and roller temperatures were 220 °C and 50 °C, respectively. A draw ratio of 30 ◦ ◦ 12mm) rpm,and andathe and roller temperatures were 220 C and 50 C, respectively. A draw ratio m and a thicknesswas of 0.8 set die of cooling rolls. The screw speed was 12 (ratio of the roll speed to the die exit velocity) was used to produce films with a thickness of around of 30 were (ratio220 of the the die exit A velocity) was of used nd roller temperatures °C roll and speed 50 °C,to respectively. draw ratio 30 to produce films with a thickness of 27 μm. An air knife was applied right at the die exit to supply air toair thetofilm for cooling. The around 27 µm.used An air knife wasfilms applied at the die to supply the surface film surface for cooling. ed to the die exit velocity) was to produce withright a thickness ofexit around virgin PP film was also made as a control using the same processing condition. virgin PP film was also a control the same was applied rightThe at the die exit to supply air made to the as film surfaceusing for cooling. Theprocessing condition. The as-prepared precursor films were cut into a 70 mm 80mm mmrectangular rectangularshape shapealong alongthe the The as-prepared precursorcondition. films were cut into a 70 mm ×× 80 lso made as a control using the same processing extrusion direction, and were annealed at 145 °C for 10 min in a hot oven. Then, the annealed ◦ at 145 C for 10along min in d precursor films extrusion were cut direction, into a 70 and mm were × 80 annealed mm rectangular shape thea hot oven. Then, the annealed samples samples were cold- and hot-stretched into microporous membranes usingmachine an Instron machine were coldand hot-stretched into microporous membranes using an Instron (ElectroPlus and were annealed at 145 °C for 10 min in a hot oven. Then, the annealed (ElectroPlus E3000, Instron, Canton, MA, USA) equipped with an environmental chamber. The was cold E3000, Canton, MA, USA) equipped an environmental chamber. The cold stretching - and hot-stretched intoInstron, microporous membranes using anwith Instron machine stretching was performed at 25 °C with an extension ratio of 35% at a speed of 100 mm/min, while ◦ performed at 25 C with ratio ofchamber. 35% at a The speed of 100 mm/min, while the hot stretching Instron, Canton, MA, USA) equipped withan anextension environmental cold the hot stretching was◦ C performed at 130 °C ratio with of an 60% extension ratio of of 50 60% at a speed of as-obtained 50 mm/min. was performed at 130 with an extension at a speed mm/min. The ormed at 25 °C with an extension ratio of 35% at a speed of 100 mm/min, while The as-obtained stretched membranes were subsequently annealed at the 135porous °C for structure. 90 s to fix the stretched were subsequently annealed at mm/min. 135 ◦ C for 90 s to fix as performed at 130 °C withmembranes an extension ratio of 60% at a speed of 50 porous structure. etched membranes were subsequently annealed at 135 °C for 90 s to fix the 2.3. Characterization 2.3. Characterization Crystallinity (Xc ) of the precursor films were analyzed using a TA Instruments Q1000 differential Crystallinity (X(DSC, c) of TA the Instruments, precursor films a TA scanning calorimeter New were Castle,analyzed DE, USA).using Samples wereInstruments heated fromQ1000 40 to ◦ ◦ differential scanning calorimeter (DSC,XTA Instruments, New Castle, DE, USA). Samples were 200 C at a heating rate of 10 C/min. was calculated using a fusion heat of 209 J/g for fully c Xc) of the precursor films were analyzed using a TA Instruments Q1000 heated from 40 to 200 °C at a heating rate of 10 °C/min. X c was calculated using a fusion heat of 209 crystalline PP [29]. g calorimeter (DSC, TA Instruments, New Castle, DE, USA). Samples were J/g for fullydiffraction crystalline (XRD) PP [29].patterns of the films were recorded with a Philips X’Pert-MRD X-ray 00 °C at a heating rate of 10 °C/min. Xc was calculated using a fusion heat of 209 X-ray diffraction (XRD) patterns of the filmsusing wereNi-filtered recordedCu with a Philips(λX’Pert-MRD diffractometer (PANalytical, Almelo, The Netherlands) Kα radiation = 0.154 nm). ne PP [29]. ◦ ◦ diffractometer (PANalytical, Almelo, The Netherlands) using Ni-filtered Cu Kα radiation (λ = 0.154 Theofspectra werewere collected in the diffraction angle X’Pert-MRD (2θ) range of 5 –32 . on (XRD) patterns the films recorded with a Philips nm).Infrared The spectra wereproperties collected in (2θ) range 5°–32°. 65 Fourier transform dichroic of the the diffraction films were angle measured using of a Spectrum Nalytical, Almelo, The Netherlands) using Ni-filtered Cu Kα radiation (λ = 0.154 Infrared dichroic properties of the films were measured using a Spectrum Fourier spectrometer Corp., Waltham, MA, USA) equipped with a65wire grid transform polarizer. re collected in theinfrared diffraction angle (2θ) (PerkinElmer range of 5°–32°. infrared spectrometer (PerkinElmer Corp., Waltham, MA, USA) equipped with a wire grid For each specimen, two polarized spectra, namely parallel and perpendicular to the extrusion direction, ic properties of the films were measured using a Spectrum 65 Fourier transform − 1 1. polarizer. For with each64 specimen, two polarized parallel and perpendicular to − the collected at aequipped resolution of 4spectra, cm innamely the range of 4000–600 cm ter (PerkinElmer were Corp., Waltham, MA,scans USA) with a wire gridwavenumber extrusion direction, were with 64 scans at a resolution of 4 cm−1 in the wavenumber range The orientation factor (f ) iscollected defined as follows: specimen, two polarized spectra, namely parallel and perpendicular to the −1 of 4000–600 cm . The orientation factor (f) is defined as follows: were collected with 64 scans at a resolution of 4 cm−1 in the wavenumber range f = (D − 1)/(D + 2) (1) f = (D − 1)/(D + 2) (1) e orientation factor (f) is defined as follows:

f = (D − 1)/(D + 2)

DD =A = // A///A /A⏊

(1)

(2) (2)

where A⏊Aare parallelparallel and perpendicular respectively. whereAD A=//and and are absorbance the absorbance and perpendicular to thedirection, extrusion direction, ⏊ the A ///A (2) to the extrusion // − 1 −1 For PP, absorbance at the wavenumber of 998 cm was of chosen to calculate the orientation of the respectively. For PP, absorbance at the wavenumber 998 cm was chosen to calculate the are the absorbance parallel and perpendicular to the extrusion direction, crystalline orientationphase of the[21]. crystalline phase [21]. P, absorbance at the wavenumber of 998 cm−1 was chosen to calculate the The out using using an anInstron Instron3365 3365machine machine(Instron, (Instron,Canton, Canton,MA, MA, USA) Thetensile tensile tests tests were carried out USA) at ystalline phase [21]. at roomtemperature. temperature.The Theinitial initialgrip gripdistance distancewas wasset setto to 50 50 mm. mm. Films Films with with a 25 mm room mm width width were were s were carried out using an Instron 3365 machine (Instron, Canton, MA, USA) at stretched stretchedalong alongthe theflow flowdirection directionatataaspeed speedofof100 100mm/min. mm/min. The initial grip distance was set to 50 mm. Films with a 25 mm width were The The surface surface and and cross-section cross-section morphologies morphologies of of the the precursor precursor films films and and membranes membranes were were low direction at a speed of 100 mm/min. examinedby byaafield fieldemission emissionscanning scanningelectron electronmicroscope microscope(FESEM-Hitachi (FESEM-HitachiS4700, S4700,Tokyo, Tokyo,Japan) Japan)atat examined nd cross-section morphologies of the precursor films and membranes were anaccelerating accelerating voltage voltage of of 22 kV. kV.To Toprepare preparethe thecross crosssection, section,the thespecimen specimenwas wasimmersed immersedinto intothe the an emission scanning electron microscope (FESEM-Hitachi S4700, Tokyo, Japan) at epoxyfollowed followedby byan anultrathin ultrathinsectioning sectioningininliquid liquidnitrogen. nitrogen.All Allthe thesamples sampleswere weregold-coated gold-coatedfor for15 15ss epoxy age of 2 kV. To prepare the cross section, the specimen was immersed into the beforeobservations. observations. before n ultrathin sectioning in liquid nitrogen. All the samples were gold-coated for 15 s Thewater watervapor vapor transmission (WVTR) the membranes were measured using a The transmission ratesrates (WVTR) of theof membranes were measured using a MOCON ◦ MOCON PERMATERAN-W Model 101C.KBefore at 37.8measurement, °C. Before measurement, calibration of the cell was PERMATERAN-W Model 101 K at 37.8 calibration of the cell was performed por transmission rates (WVTR) of the membranes were measured using a performed with humidity the relative humidity 60% in the lower chamber. with the relative kept aroundkept 60% around in the lower chamber. ERAN-W Model 101 K at 37.8 °C. Before measurement, calibration of the cell was Thewater watercontact contactangle angle(WCA) (WCA)of ofthe themicroporous microporousmembrane membranewas wasdetermined determinedby bythe thesessile sessile The relative humidity kept around 60% in the lower chamber. dropmethod methodusing using a VCA Optima machine Products, Inc., Billerica, MA, USA) with a drop a VCA Optima machine (AST (AST Products, Inc., Billerica, MA, USA) with a precision act angle (WCA) of the microporous membrane was determined by the sessile precision and PC advanced PC technology. During a 1 was μL droplet lowered onto the camera andcamera advanced technology. During testing, a 1 testing, µL droplet loweredwas onto the membrane g a VCA Optima machine (AST Products, Inc., Billerica, MA, USA) with a membrane surface with a microsyringe and equilibrated for 10 s before WCA observation. Each d advanced PC technology. During testing, a 1 μL droplet was lowered onto the reported WCA was an average of at least five measurements. with a microsyringe and equilibrated for 10 s before WCA observation. Each an average of at least five measurements.

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surface with9,a110 microsyringe and equilibrated for 10 s before WCA observation. Each reported WCA Polymers 2017, 4 of 12 was an average of at least five measurements. To uptake, thethe stretched microporous membrane was soaked in the in liquid To measure measurethe theelectrolyte electrolyte uptake, stretched microporous membrane was soaked the electrolyte for 12 h,for and ratiothe of the weight compared to the dry to membrane was calculated liquid electrolyte 12the h, and ratio of thegain weight gain compared the dry membrane was according expression Uptake (%)Uptake = (W −(%) W 0=)/W W are the weights of the calculated to according to expression (W 0− × W0100%, )/W0 ×where 100%, W where 0 and W are the weights 0 andW membrane before before and after the liquid respectively. of the membrane andabsorbing after absorbing the electrolyte, liquid electrolyte, respectively. 3. Results and and Discussion 3. Results Discussion 3.1. Structures of PP/TiO2 Composite Films 3.1. Structures of PP/TiO2 Composite Films PP/TiO2 composite films were melt extruded using a draw ratio of 30, the same as that for PP/TiO2 composite films were melt extruded using a draw ratio of 30, the same as that for the the virgin PP film. Unfortunately, continuously cast extrusion and acquisition of flat and uniform virgin PP film. Unfortunately, continuously cast extrusion and acquisition of flat and uniform PP/TiO2 composite films failed when the TiO2 content reached 50 wt % (i.e., 13.3 vol %). This can be PP/TiO2 composite films failed when the TiO2 content reached 50 wt % (i.e., 13.3 vol %). This can be well understood since the drawability of the composite melts decreases sharply when filler loading well understood since the drawability of the composite melts decreases sharply when filler loading approaches a threshold value, which is typically about 16 vol % for the spherical particles [30]. In this approaches a threshold value, which is typically about 16 vol % for the spherical particles [30]. In regard, the TiO2 content is controlled within 40 wt % in this study. Figure 1a,b present the SEM this regard, the TiO2 content is controlled within 40 wt % in this study. Figure 1a,b present the SEM surface morphologies of the PP/TiO2 composite films with 10 and 40 wt % TiO2 , respectively. It can surface morphologies of the PP/TiO2 composite films with 10 and 40 wt % TiO2, respectively. It can be observed that the TiO2 particles are homogeneously dispersed in the PP matrix, even at a high be observed that the TiO2 particles are homogeneously dispersed in the PP matrix, even at a high loading of 40 wt %. The amount of the particles located on the surface increases and their distance loading of 40 wt %. The amount of the particles located on the surface increases and their distance decreases with increased TiO2 content. It is speculated that, in addition to the selection of appropriate decreases with increased TiO2 content. It is speculated that, in addition to the selection of submicron-sized TiO2 particles that possess a relatively low surface energy, the elongation flow during appropriate submicron-sized TiO2 particles that possess a relatively low surface energy, the the cast extrusion process also helps to reduce the possibility of particle agglomeration. Considering elongation flow during the cast extrusion process also helps to reduce the possibility of particle that PP/TiO2 microporous membranes are fabricated by uniaxial stretching their precursors, a good agglomeration. Considering that PP/TiO2 microporous membranes are fabricated by uniaxial dispersion of TiOprecursors, benefit an evenly distributed stress in the stretched films and thus 2 particles will stretching their a good dispersion of TiO 2 particles will benefit an evenly distributed astress uniform pore distribution in the obtained membranes. in the stretched films and thus a uniform pore distribution in the obtained membranes.

(a)

(b)

Figure 1. Surface morphologies of PP/TiO2 composite films with the TiO2 content of (a) 10 wt % and Figure 1. Surface morphologies of PP/TiO2 composite films with the TiO2 content of (a) 10 wt % and (b) 40 wt %. (b) 40 wt %.

It has been known that PP film having a large quantity of oriented lamellae greatly favors the It has of been known PP film having astretching large quantity ofWith oriented lamellaecrystallinity greatly favors formation voids whenthat subjected to uniaxial [20,21]. this regard, and the formation of voids when subjected to uniaxial stretching [20,21]. With this regard, crystallinity crystalline orientation are considered as two key parameters for evaluating the pore formation and crystalline are considered as twoTable key parameters evaluating the pore formation capabilities of orientation the PP-based precursor films. 1 lists theformelting temperature (Tm) and capabilities of the PP-based precursor films. Table 1 lists the melting temperature (T ) and crystallinity m crystallinity (Xc) of the PP and PP/TiO2 composite films analyzed by DSC. For the virgin PP film, the (X of the PP/TiO films analyzed DSC.that For the virgin film, the T the and Xc are2 2 composite Tmc )and Xc PP areand 164.4 °C and 41.8%, respectively. It isby found both the TPP m and Xc form PP/TiO ◦ C and 41.8%, respectively. It is found that both the T and X for the PP/TiO composite films 164.4 c 2 composite films with different TiO2 loadings do not showmsignificant variations compared to those with different TiO loadings do not show significant variations compared to those of the virgin PP film. 2 of the virgin PP film. This may suggest that TiO2 particles have little influence on the crystallization This may suggest that TiO particles have little influence on the crystallization of the PP matrix during 2 extrusion process wherein the elongational flow is applied. of the PP matrix during the the extrusion process wherein the elongational flow is applied.

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Table 1. Melting temperature (Tm) and crystallinity (Xc) for the polypropylene (PP) and PP/TiO2 Table 1. Melting temperature (Tm ) and crystallinity (Xc ) for the polypropylene (PP) and PP/TiO2 composite films. composite films.

TiO2 content (wt %) Tm (°C) TiO20 Content (wt %) T m (◦ C) 164.4 10 164.1 0 164.4 164.1 20 10 164.8 164.8 30 20 163.9 30 163.9 40 40 164.2 164.2

Xc % Xc % 41.8

41.5 41.8 41.5 42.0 42.0 42.1 42.1 41.6 41.6

Figure 2 shows the XRD results for the PP and PP/TiO2 composite films. Distinct peaks centered 2 shows the XRD PP and PP/TiO films. PP Distinct centered 2 composite at theFigure 2θ angles of 14.0°, 16.9°,results 18.5°,for andthe 25.4° can be observed for the virgin film, peaks corresponding ◦ , 16.9◦ , 18.5◦ , and 25.4◦ can be observed for the virgin PP film, corresponding at the 2θ angles of 14.0 to the α-form crystallographic planes (110), (040), (130), and (060), respectively; however, diffraction to the α-form crystallographic planes (110), (040),which (130),are andalso (060), however, diffraction peaks of the (111/041) planes (2θ = 21.3°, 21.8°), therespectively; characteristics of the α-form PP, ◦ , 21.8◦ ), which are also the characteristics of the α-form PP, peaks of the (111/041) planes (2θ = 21.3 are very weak [31]. This behavior has been previously reported in the case of an oriented PP sample, are very weak This behavior has been previously reported in structure the case ofinan oriented sample, suggesting that[31]. the specimen possesses a highly oriented lamellar terms of itsPP crystalline suggesting that theCompared specimen possesses a highly oriented lamellar difference structure in of its crystalline morphology [32]. to the virgin PP film, no distinctive interms the diffraction angles morphology [32]. Compared to the virgin PP film, no distinctive difference in the diffraction angles is found for the PP/TiO2 composite films, except for an additional peak appearing at around 27°, ◦ is found for the PP/TiO composite films, except for an additional peak appearing at around 2 which is a characteristic reflection of the TiO2 particles [33], indicating that the incorporation of 27 the, which is a characteristic reflection the TiO [33], indicating that the of the 2 particles TiO2 particles does not change theof crystal form of the PP matrix. Moreover, theincorporation diffraction peak of TiO particles does not change the crystal form of the PP matrix. Moreover, the diffraction peak of 2 (111/041) planes is still relatively weak compared to other diffraction peaks for these composite (111/041) planes is still relatively weak compared to other diffraction peaks for these composite films. films. Consequently, it is speculated that the addition of TiO 2 particles has little effect on the crystal Consequently, it is speculated that the addition of TiO particles has little effect on crystal structure 2 films may still possess structure of the PP matrix, and the PP/TiO2 composite an the orientated lamellar of the PP matrix, and the PP/TiO composite films may still possess an orientated lamellar structure. 2 structure.

Figure 2. X-ray diffraction patterns for the PP and PP/TiO2 composite films with different TiO2 Figure 2. X-ray diffraction patterns for the PP and PP/TiO2 composite films with different concentrations. TiO2 concentrations.

To quantitatively analyze the influence of TiO2 particles on the PP crystalline orientation, To quantitatively analyze the PP/TiO influence of TiO2 particles on studied. the PP crystalline infrared infrared dichroic properties of the 2 composite films were As clearlyorientation, shown in Figure 3a, dichroic properties of the PP/TiO composite films were studied. As clearly shown in Figure 3a, 2 −1 the absorbance at band 998 cm ascribed to the crystalline vibration in the parallel direction is much − 1 the absorbance at band 998perpendicular cm ascribed direction to the crystalline vibration the parallel is much stronger than that in the for all the films, in suggesting thedirection presence of PP stronger than that in the perpendicular direction for all the films, suggesting the presence of PP crystalline orientation (fc) along the extrusion direction. Based on the variations of the band strength crystalline extrusion direction. Basedinon the variations of the band strength c ) along the at 998 cm−1orientation in the two (f directions, fc was calculated as shown Figure 3b. It is seen that fc decreases −1 in the two directions, f was calculated as shown in Figure 3b. It is seen that f decreases at 998 cm c c slightly from 0.50 to 0.44 as the TiO2 loading increases from 0 to 40 wt %. This is contrary to the slightly 0.50 0.44 as the TiO2 loading increases from 0for towhich 40 wt the %. oriented This is contrary to the commonfrom results fortothe particulate-filled polymer composites, crystallization common for disturbed, the particulate-filled for which the oriented crystallization would beresults seriously even whenpolymer adding composites, a small amount of spherical particles [34,35]. It is would be seriously disturbed, even when adding a small amount of spherical particles [34,35]. It is thus thus suggested that the heterogeneous nucleation effect of TiO2 particles on PP is negligible suggested that the heterogeneous nucleation effect of TiO particles on PP is negligible compared to the 2 compared to the effect of elongation-induced crystallization during the cast extrusion process. Previous studies have demonstrated that row-nucleated lamellar structure is characteristic of the

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effect of elongation-induced crystallization during the cast extrusion process. Previous studies have 6 of 12 demonstrated that row-nucleated lamellar structure is characteristic of the crystalline morphology for PP cast films when crystalline more than 0.3 [20].orientation Therefore, it reasonably inferred crystalline morphology for PPorientation cast filmsis when crystalline is ismore than 0.3 [20]. that all PP/TiO composite films in this study dominantly possess a row-nucleated lamellar structure 2 Therefore, it is reasonably inferred that all PP/TiO2 composite films in this study dominantly possess to that inlamellar the virgin PP film.similar to that in the virgin PP film. asimilar row-nucleated structure Polymers 2017, 9, 110

(a)

(b)

Figure and PP/TiO PP/TiO22 composite Figure 3. 3. (a) (a) Polarized Polarized IR IR spectra spectra for for the the PP PP and compositefilms filmsininthe the range range of of 1020 1020 to to −1 − 1 Crystalline PPPP as as function of of TiO 2 content. 950 950 cm cm . (b) ; (b) Crystallineorientation orientation(fc(f) cof) of function TiO 2 content.

3.2. 3.2. Deformation Deformation Behavior Behavior of of PP/TiO PP/TiO22 Composite Composite Films Films For For manufacturing manufacturing PP PP microporous microporous membranes, membranes, the the stretching stretching strategy strategy also also greatly greatly affects affects the the porous structure and thus the final performance of the membranes [25]. Therefore, it is important to porous structure and thus the final performance of the membranes [25]. Therefore, it is important investigate thethe deformation behavior of of thethe composite films to investigate deformation behavior composite filmstotoattain attaina adeep deepinsight insightinto intothe the pore pore formation during stretching. Figure 4 compares the stress–strain curves of the annealed PP formation during stretching. Figure 4 compares the stress–strain curves of the annealed PP and and PP/TiO different TiO 2 concentrations. Similar to that of the virgin PP film, the PP/TiO2 2composite compositefilms filmswith with different TiO concentrations. Similar to that of the virgin PP film, 2 stress–strain curves of all PP/TiO 2 composite films show two distinct yields at strains of about the stress–strain curves of all PP/TiO 2 composite films show two distinct yields at strains of about 10%–30% 10%–30% and and around around 80%–100%, 80%–100%, respectively. respectively. Moreover, Moreover, itit is is observed observed that that no no apparent apparent necking necking phenomenon happens during stretching for all films. Such behavior is also referred to as hard phenomenon happens during stretching for all films. Such behavior is also referred to aselastic hard behavior, which is closely connected to the presence of a row-nucleated lamellar structure in the elastic behavior, which is closely connected to the presence of a row-nucleated lamellar structure crystalline morphology of the as-prepared films [36]. It is claimed that the first and second yields in the crystalline morphology of the as-prepared films [36]. It is claimed that the first and second correspond to the tovoid creation in the amorphous region and thetheoccurrence yields correspond the void creation in the amorphous region and occurrenceofofthe the lamellar lamellar fragmentation, respectively, and the deformation between the two yields is related to the lamellar fragmentation, respectively, and the deformation between the two yields is related to the lamellar separation. that lamellar lamellar separation separation occurs occurs when when the the PP/TiO PP/TiO2 separation. The The result result is is therefore therefore indicative indicative of of that 2 composite stress. OnOn thethe other hand, it can be observed thatthat the compositefilms filmsare aresubjected subjectedtotouniaxial uniaxialtensile tensile stress. other hand, it can be observed yield stresses of the PP/TiO 2 composite films show a declining tendency as the TiO2 content the yield stresses of the PP/TiO 2 composite films show a declining tendency as the TiO2 content increases. Similar phenomena increases. Similar phenomena have have been been reported reported by by other other researchers researchers for for particulate-filled particulate-filled polymer polymer composites, and it is mainly attributed to the occurrence of debonding of the filler from the polymer composites, and it is mainly attributed to the occurrence of debonding of the filler from the polymer matrix submicron-sized TiO matrix [37,38]. [37,38]. In In this this work, work, no no surface surface modification modification was was carried carried out out on on the the submicron-sized TiO22 particles, thus leading to a weak interfacial interaction in favor of debonding. Consequently, it particles, thus leading to a weak interfacial interaction in favor of debonding. Consequently, it suggests suggests both the separation lamellar separation and the debonding the TiOfrom 2 particles from the PP that boththat the lamellar and the debonding of the TiO2ofparticles the PP matrix take matrix take place when uniaxially stretching the annealed PP/TiO 2 composite films, which is verified place when uniaxially stretching the annealed PP/TiO2 composite films, which is verified by the SEM by the SEM observations observations that follow. that follow.

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Figure 4. Stress–strain curves of the annealed PP and PP/TiO2 composite films with different Figure 4. Stress–strain curves of the annealed PP and PP/TiO2 composite films with different TiO 2 concentrations. TiO concentrations. 2

Figure 4. Stress–strain curves of PP/TiO the annealed PP and Membranes PP/TiO2 composite films with different 3.3. Microstructure and Properties of the 2 Composite 3.3. Microstructure and Properties of the PP/TiO2 Composite Membranes TiO2 concentrations. Given the similar deformation behaviors among the PP and PP/TiO2 composite films, PP/TiO2 Given the similar deformation behaviors among the PP and PP/TiO2 composite films, PP/TiO2 3.3. Microstructure and Properties of the PP/TiO 2 Composite microporous membranes were prepared following an Membranes optimized stretching strategy of 35% cold microporous membranes were prepared following an optimized stretching strategy of 35% cold extensionGiven and the 60%similar hot extension used for the virgin PPand filmPP/TiO as we previously reported [24]. deformation behaviors among 2 composite films, PP/TiO2 extension and 60% hot extension used for the virgin PP the filmPP as we previously reported [24]. Figure 5a,b Figure 5a,b show the surface morphologies of the PP/TiO 2 composite membranes loaded with 40 wt microporous membranes were prepared following an optimized stretching strategy of 40 35% show the surface morphologies of the PP/TiO2 composite membranes loaded with wtcold % TiO2 . % TiO 2 . Similar to the commercial PP microporous membrane, the PP/TiO 2 composite membrane extension and 60% hot extension used for the virgin PP film as we previously reported [24]. Similar to the commercial PP microporous membrane, the PP/TiO2 composite membrane possesses Figurea5a,b show the surface morphologies the PP/TiO2 with composite membranes loaded 40 wt possesses large quantity of slit-like shapedofmicropores a typical dimension ofwith about 100 nm a large quantity of slit-like shaped micropores with a typical dimension of about 100 nm in length % TiO 2. Similar PP microporous membrane, PP/TiO2 composite in length and 30–40 to nmthe in commercial width (analyzed by the image analysisthe software). This typemembrane of pore might and 30–40 nmainlarge width (analyzed by the image analysis with software). This type ofof pore might be well possesses quantity of slit-like shaped micropores a typical about 100 nm be well connected with the plentiful oriented stacked lamellae in the dimension composite film, which can be connected with the plentiful oriented stacked lamellae in the composite film, which can be separated in length and 30–40 nm in width (analyzed by the image analysis software). This type of pore might separated when the film is subjected to external stress, thus leading to the voids. In addition, whenbethe film is subjected to external leading to the voids. In addition, another type of well connected with the plentiful stress, orientedthus stacked lamellae in the composite film, which can be another type of micropore with a larger size ranging from 200 to 500 nm can be found separated the film subjected to 200 external stress, thusbe leading the voids. In addition, micropore withwhen a larger size is ranging from to 500 nm can foundtohomogeneously distributed homogeneously distributed inwith the aPP/TiO 2 composite membranes. Specifically, these larger of micropore larger size ranging frommicropores 200 to 500generally nm can appear be found in theanother PP/TiOtype membranes. Specifically, these larger nearby 2 composite micropores generally appear nearby TiO 2 particles, which can be reasonably attributed to the homogeneously distributed in the PP/TiO 2 composite membranes. Specifically, these larger TiO2 particles, which can be reasonably attributed to the interfacial debonding between the TiO2 interfacial debonding between the TiO2 particles and PP matrix the film is subject to micropores appear TiO2 particles, which canwhen be reasonably attributed to external the particles and PP generally matrix when the nearby film is subject to external stress [39]. The presence of a large amount stressinterfacial [39]. Thedebonding presence between of a large of micropores induced interfacial theamount TiO2 particles and PP matrix whenby thethe film is subject debonding to external can of micropores induced by the interfacial debonding can also be observed from the cross-section stress [39]. The presence of a large amount of micropores induced by the interfacial debonding can also be observed from the cross-section morphology as shown in Figure 5c. morphology as shown in Figure 5c.

also be observed from the cross-section morphology as shown in Figure 5c.

(a)

(a)

(b)

(b)

(c)

(c)

Figure 5. (a,b) Surface and (c) cross-section morphologies of PP/TiO2 composite membranes with composite membranes membranes with Figure 5. (a,b) Surface and (c) (c) cross-section cross-section morphologies morphologies of of PP/TiO PP/TiO22 composite 40 wt % TiO2.

40 wt % TiO22..

Figure 6 shows the water vapor permeability of the microporous PP/TiO2 membranes as a

Figure the vapor permeability of microporous 2 membranes function66ofshows TiO2 content. Excitingly, except for an initially slight decline at 10PP/TiO wt % compared to the as a Figure shows the water water vapor permeability of the the microporous PP/TiO 2 membranes as virgin PP membrane, improvements in the permeability can be found for the PP/TiO 2 function of TiO 2 content. continuous Excitingly, except for an initially slight decline at 10 wt % compared to the a function of TiO2 content. Excitingly, except for an initially slight decline at 10 wt % compared composite membranes when theimprovements TiO2 loading ranges from 10 to 40 wt %.be The results could be virgin PP membrane, continuous in the permeability can found for the PP/TiO to the virgin PP membrane, continuous improvements in the permeability can be found for the2 understandable by taking of theranges dual roles TiO particles. On the one hand, composite membranes when consideration thewhen TiO2the loading fromof10 to 2 40 wt40%.wt The results could be PP/TiO2 composite membranes TiO2 loading ranges from 10 to %. The results could incorporation of the TiO 2 particles causes a slight decrease in the PP crystalline orientation understandable by taking consideration of the dual roles of TiO2 particles. On the one hand, (as shown in Figure 3), thus reducing the proportion of the micropores coming from the lamellar incorporation of the TiO2 particles causes a slight decrease in the PP crystalline orientation (as shown in Figure 3), thus reducing the proportion of the micropores coming from the lamellar

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be understandable by taking consideration of the dual roles of TiO2 particles. On the one hand, incorporation of the TiO2 particles causes a slight decrease in the PP crystalline orientation (as shown in Figure 3),2017, thus reducing the proportion of the micropores coming from the lamellar separation Polymers 9, 110 8 of 12 and its contribution to permeability. On the other hand, increasing the TiO2 content increases the separation and its contribution to permeability. the other hand, increasing thethe TiOTiO 2 content amount of larger micropores originating from the On interfacial debonding between 28 particles Polymers 2017, 9, 110 of 12 increases the amount of larger micropores originating from the interfacial debonding between and PP matrix, and this helps to improve permeability. For the PP/TiO2 composite membranethe at high 2 particles and PP matrix, and this helps to improve permeability. For the PP/TiO2 composite separation and its contribution to permeability. On the increasing the TiO content TiO2 TiO loadings, the latter would play a dominant role andother thushand, lead to the increase in 2permeability membrane at high TiOof 2 loadings, the latter would play a dominant role and thus lead to the increase increases larger micropores originating from the composite interfacial debonding between theabout on the whole.the Inamount this work, the permeability of the PP/TiO membrane reaches 2 in permeability on the whole. and In this work, the permeability of the PP/TiO 2 composite membrane TiO 2 particles and PP matrix, this helps to improve permeability. For the PP/TiO 2 composite 5 2 4.5 ×reaches 10 g/m /day when the2/day TiO2when loading is 40 wt %, which is enhanced by nearly 33% compared to about 4.5 ×TiO 105 2g/m the TiO 2 loading is 40 wt %, which is enhanced by nearly 33% membrane at high loadings, the latter would play a dominant 2 role and thus lead to the increase that of the virgin PP membrane, with a value of 3.38 × 105ofg/m compared to that of the virgin PP membrane, with a value 3.38 ×/day. 105 g/m2/day. in permeability on the whole. In this work, the permeability of the PP/TiO2 composite membrane reaches about 4.5 × 105 g/m2/day when the TiO2 loading is 40 wt %, which is enhanced by nearly 33% compared to that of the virgin PP membrane, with a value of 3.38 × 105 g/m2/day.

Figure 6. Water vapor transmission rate (WVTR) of the PP/TiO2 composite membranes as a function

Figure 6. Water vapor transmission rate (WVTR) of the PP/TiO2 composite membranes as a function of TiO2 content. of TiO2 content. Figure 6. Water vapor transmission rate (WVTR) of the PP/TiO2 composite membranes as a function

Static water contact angle (WCA) can be used as an indicator to reflect the polarity of the of TiO2 content. membranes thus the wettability with Figure to 7 shows water contactof the Static waterand contact angle (WCA) canthe beliquid usedelectrolytes. as an indicator reflectthethe polarity angle of PP/TiO 2 composite membranes with different TiO 2 concentrations. The virgin PPangle membranes andwater thus the wettability with the electrolytes. Figureto7 shows contact Static contact angle (WCA) canliquid be used as an indicator reflect the water polarity of the microporous membrane an initial water angle as highFigure as 114°, due virgin tothe thewater hydrophobic membranes and thusmembranes thehas wettability with thecontact liquid electrolytes. 7 The shows contact of PP/TiO with different TiO PP microporous 2 composite 2 concentrations. nature as well as the surface roughness. The incorporation of◦TiO TiO22 particles obviously decreases angle of PP/TiO 2 composite membranes with different concentrations. The virgin PP membrane has an initial water contact angle as high as 114 , due to the hydrophobic naturethe as well WCA of the PP/TiO 2 composite membranes, and the WCA decreases to 99° when the TiO2 loading is microporous membrane has an initial water contact angle as high as 114°, due to the hydrophobic as the surface roughness. The incorporation of TiO2 particles obviously decreases the WCA of the 40 wt %. of the roughness. WCA can be explained by theof presence of many Ti–OH groups on the nature asThe welldecrease as the surface incorporation TiO obviously decreases thewt %. PP/TiO composite membranes, andcould theThe WCA decreases to 99◦2 particles when the TiO loading is 40 2der TiO22 particles at the2surface, which interact well withdecreases the watertothrough van Waals forces WCA of the PP/TiO composite membranes, and the WCA 99° when the TiO 2 loading is The decrease of thebonding WCA can be explained by the presence of many Ti–OH groups on the TiO2 particles andwt hydrogen [40]. 40 %. The decrease of the WCA can be explained by the presence of many Ti–OH groups on the

at the surface, which could interact well with the water through van der Waals forces and hydrogen TiO2 particles at the surface, which could interact well with the water through van der Waals forces bonding [40]. and hydrogen bonding [40].

Figure 7. Water contact angle (WCA) of the PP/TiO2 composite membranes as function of TiO2 content.

The electrolyte uptake of separators is considered an important parameter in battery Figure 7. Water contact angle (WCA) of the PP/TiO2 composite membranes as function of TiO2 content. Figure 7. Water contact angle (WCA) of the PP/TiO membranes as function of TiO2 content. performance. Figure 8 presents the electrolyte uptake values of PP/TiO2 composite membranes with 2 composite different TiO 2 concentrations. As expected, adding TiO2 particles cause obvious improvements in the The electrolyte uptake of separators is considered an important parameter in battery electrolyte uptake for8the PP-based membranes. It canvalues be observed that2 the electrolyte uptake ofwith the performance. Figure presents the electrolyte uptake of PP/TiO composite membranes different TiO2 concentrations. As expected, adding TiO2 particles cause obvious improvements in the electrolyte uptake for the PP-based membranes. It can be observed that the electrolyte uptake of the

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The electrolyte uptake of separators is considered an important parameter in battery performance. Figure 8 presents the electrolyte uptake values of PP/TiO2 composite membranes with different TiO2 concentrations. As expected, adding TiO2 particles cause obvious improvements in the electrolyte Polymers 2017, 9, 110 9 of 12 uptake for the PP-based membranes. It can be observed that the electrolyte uptake of the PP/TiO2 composite membranes increases from 56% to 104%, when the TiO2 the loading 0 tofrom 40 wt0 %. PP/TiO 2 composite membranes increases from 56% to 104%, when TiO2 ranges loadingfrom ranges to This means the electrolyte uptake capacity of the PP membrane can be virtually doubled by adding 40 wt %. This means the electrolyte uptake capacity of the PP membrane can be virtually doubled the adding TiO2 particles to 40 wt %. It is believed that the presence of plentiful hydrophilic characteristics by the TiOup 2 particles up to 40 wt %. It is believed that the presence of plentiful hydrophilic of the TiO2 particles, as well some larger from larger the interfacial debonding, contribute characteristics of the TiO2 asparticles, as micropores well as some micropores from both the interfacial to the improvement in the electrolyte uptake for the PP/TiO composite membranes compared to the 2 debonding, both contribute to the improvement in the electrolyte uptake for the PP/TiO2 composite virgin PP membrane. membranes compared to the virgin PP membrane.

Figure 8. 8. Electrolyte Electrolyte uptake uptake of of the the PP/TiO PP/TiO2 composite membranes as a function of TiO2 content. Figure 2 composite membranes as a function of TiO2 content.

The mechanical properties of the composite membranes with different TiO2 concentrations The mechanical properties of the composite membranes with different TiO2 concentrations were were measured as shown in Table 2. The virgin PP membrane has a tensile strength of about 118.4 measured as shown in Table 2. The virgin PP membrane has a tensile strength of about 118.4 MPa, MPa, which is similar to that of the commercial PP separators. It is found that incorporation of TiO2 which is similar to that of the commercial PP separators. It is found that incorporation of TiO2 particles particles slightly decreases the tensile strength from 118.4 to 104.6 MPa, reduced by about 12%, slightly decreases the tensile strength from 118.4 to 104.6 MPa, reduced by about 12%, when the TiO when the TiO2 content increases from 0 to 40 wt %. The result is different from previous work by2 content increases from 0 to 40 wt %. The result is different from previous work by Lei et al., which Lei et al., which demonstrated that the mechanical strength of PP microporous membranes can be demonstrated the mechanical strength of PP microporous membranes byThe the improved by that the addition of nano-sized (20–60 nm) silica particles up can to be 10 improved wt % [28]. addition of nano-sized (20–60 nm) silica particles up to 10 wt % [28]. The difference may be attributed difference may be attributed to the influence of the particle size. According to Fu et al., the to the influence of the According to Fu et al.,particle the composite is usually composite strength is particle usuallysize. reduced with increasing loadingstrength when the particlereduced size is with increasing particle loading when the particle size is larger than 80 nm [41]. Considering larger than 80 nm [41]. Considering that the TiO2 particles have an average size of 0.5 μm andthat no the TiO particles have an average size of 0.5 µm and no surface treatment in this work, interfacial 2 surface treatment in this work, interfacial debonding between the filler and the PP matrix easily debonding between the filler and the PPvoids matrix occurs, consequently leading to and morethus voids occurs, consequently leading to more in easily the stretched composite membranes a in the stretched composite membranes and thus a reduction in the tensile strength compared to reduction in the tensile strength compared to the virgin PP membrane. Besides, the elongationthe at virgin PP membrane. Besides, the elongation at break decreasestoslightly from 92.7% for the virgin break decreases slightly from 92.7% for the virgin PP membrane 84.5% for composite membranes PP membrane to 84.5% for composite membranes when the TiO content is up to 40 wt %. However, 2 when the TiO2 content is up to 40 wt %. However, it should be mentioned here that all the it should be mentioned here that all the composite membranes still possess excellent mechanical composite membranes still possess excellent mechanical properties, which conform well to the properties, which conform well to the battery separators [2]. battery separators [2]. Table 2. Mechanical properties of PP/TiO2 composite membranes with different TiO2 concentrations.

TiO2 content (wt %) 0 10 20 30 40

Tensile strength (MPa) 118.4 115.5 112.8 108.3 104.6

Elongation at break (%) 92.7 88.3 85.6 86.4 84.5

4. Conclusions In this work, the hydrophilic TiO2 particles of submicron size were used and compounded with the host PP pellets to improve the polarity and thus electrolyte uptake capability. The PP/TiO2

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Table 2. Mechanical properties of PP/TiO2 composite membranes with different TiO2 concentrations. TiO2 Content (wt %)

Tensile Strength (MPa)

Elongation at Break (%)

0 10 20 30 40

118.4 115.5 112.8 108.3 104.6

92.7 88.3 85.6 86.4 84.5

4. Conclusions In this work, the hydrophilic TiO2 particles of submicron size were used and compounded with the host PP pellets to improve the polarity and thus electrolyte uptake capability. The PP/TiO2 composites at different ratios were fabricated into microporous membranes following the melt extruding/annealing/stretching procedure, the same as that for the virgin PP. The results of the combination of DSC, XRD and FTIR suggest that the addition of TiO2 particles has little influence on the oriented lamellar structure of the PP-based composite films. Both the lamellar separation and interfacial debonding occur when the PP/TiO2 composite films are subjected to uniaxial tensile stress, consequently leading to two forms of micropores in the stretched membranes. Compared to the virgin PP membrane, the PP/TiO2 composite membranes at high TiO2 loadings show significant improvements in terms of the water vapor permeability, polarity, and electrolyte uptake capability. Acknowledgments: Financial support from the National Science Foundation of China (grant No. 51673154) is greatly acknowledged. Shan Wang also thanks the financial support of Chinese Scholarship Council (CSC). Author Contributions: Shan Wang, Abdellah Ajji, Shaoyun Guo, and Chuanxi Xiong conceived and designed the experiments; Shan Wang performed the experiments; Shan Wang and Abdellah Ajji analyzed the data; Shaoyun Guo contributed analysis tools; Shan Wang and Chuanxi Xiong wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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