Glass-Fiber Networks as an Orbit for Ions: Fabrication of Excellent

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Glass Fiber Networks as an Orbit for Ions: Fabrication of Excellent Antistatic PP/GF Composites with Extremely Low Organic Salts Loadings Senlin Gu, Leon Zhu, Claude Mercier, and Yongjin Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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ACS Applied Materials & Interfaces

Glass Fiber Networks as an Orbit for Ions: Fabrication of Excellent Antistatic PP/GF Composites with Extremely Low Organic Salts Loadings

Senlin Gu,a Leon Zhu,b Claude Mercier,b Yongjin Li a*

a. College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Rd., Hangzhou 310036, People’s Republic of China b. Solvay Co. Ltd, Yindu Rd., Shanghai 201100, People’s Republic of China

Keywords: Polypropylene; glass fiber; organic salts; antistatic performance; composites

*

Corresponding author.

E-mail: [email protected] (Yongjin Li) ; TEL: +86-571-28867026

1

ACS Paragon Plus Environment


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Abstract: Polypropylene (PP)/glass fiber (GF) composites with excellent antistatic performance were prepared by a simple process melt blending PP with GF and a small amount of organic salts (OS). Two types of OS: tribuyl(octyl)phosphonium bis(trifloromethanesulfonyl)imide (TBOP-TFSI) and Lithium bis(trifloromethanesulfonyl)imide (Li-TFSI) with equivalent anions were used as antistatic agents for the composites. It was found that the GF and OS exhibited significant synergistic effects on the antistatic performance of the composites as well as the mechanical properties. On the one hand, the incorporation of GF significantly enhanced the electric conductivity of the composites at a constant OS loading. On the other hand, the two types of OS improved the interfacial adhesion between the GF and the PP matrix, which led to an enhancement of the mechanical properties. The investigation indicated that OS had specific interactions with the GFs and were absorbed exclusively on the GF surface. The GF network in the PP matrix provided perfect orbits for the movement of ions, inducing the excellent antistatic performance exhibited by the PP/GF composites at an OS loading of as low as 0.25 wt% when the GF formed a network in the PP matrix.

2


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1. Introduction Polypropylene (PP) has been widely used in applications such as packing materials, electric appliances and automobile parts. Polypropylene has a very low moisture absorption and exhibits high electric insulation, and is therefore easily charged by static electricity, where accumulated electrostatic charges can generate discharges and even create the danger of explosions.1 However, glass fiber (GF)-reinforced PP composites combine the comprehensive properties of the reinforcing GF and the PP matrix. External forces can be transferred to the glass fibers via the interface between the matrix and the reinforcing fibers, so the interface between PP and GF significantly influences the properties of the final composites. Extensive investigations have been carried out on improvements of the interfacial adhesion between the GF and PP matrix.2-6 The GFs themselves are insulating, and the antistatic investigation of the GF has also attracted attention for easy handling and for the further fabrication of antistatic polymer/GF composites.7, 8 Considerable research work has been done on antistatic PP materials by incorporating conductive fillers and antistatic agents.9-15 But few investigations have been carried out up to now to enhance the antistatic performance of PP/GF composites. It is clearly important to fabricate antistatic PP/GF composites because such composites can be applied as internal dust-free parts and also applied as industrial components that require static electricity dissipation. Compared to traditional antistatic agents such as conductive fillers and water-absorption-type antistatic agents, organic salts (OS), some of which are the room-temperature ionic liquids (RTILs), exhibiting low toxicity, relatively low melting points, negligible vapor pressure, high thermal and chemical stabilities, high ionic conductivity, and a broad electro-chemical potential window.16-21 Moreover, the long alkyl chains of the OS may have certain specific interactions or Van der Waals force interactions with the polymer molecular chains. Therefore, OS are candidates for use as antistatic agents for PP owing to their high ionic conductivity, and some ionic liquids have successfully been applied as antistatic agents for PP.9, 10 Ding et al. have reported a blend of PP/ 1-n-tetradecyl-3-methylimidazolium bromide ([C14mim]Br), where the addition of the ionic liquid enhanced the anti-electrostatic ability of PP.9 In this previous work, [C14mim]Br exhibited a superior antistatic ability compared to commercial antistatic agents, and good integrity properties were also found when the weight ratio of PP to [C14mim-IL] was 100/3. The presence of [C14mim-IL] significantly improved the physical properties of PP because of its good hydrophilic 3



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properties

and

surface

wettability,

but

such

antistatic

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performances

are

strongly

moisture-dependent, and therefore no antistatic performance was achieved under very dry conditions. On the other hand, OS based on bis(trifluoromethanesulfonyl)imide (TFSI) have some advantages over their counterparts based on imidazolium and pyridinium. First, these OS are thermally more stable, which is important for melt processing at high temperature. Second, these OS have no acidic protons, and such structures avoid the formation of carbene and exhibit better stability than the imidazolium-based OS. Finally, these OS exhibit a higher electrochemical stability, and therefore can be used as electrolytes with excellent electrochemical performance.22, 23

The OS based on bis(trifluoromethanesulfonyl)imide have been used as safety electrolytes

(nonvolatile and nonflammable) for high-energy devices such as Li batteries24-26 and double-layer capacitors.27 In this study, we fabricated high-performance antistatic PP/GF composites by melt compounding

PP,

GF

and

bis(trifluoromethanesulfonyl)imide

TFSI-based

OS.

Two

[Li-TFSI]

types and

of

OS,

lithium

phosphonium

bis(trifluoromethanesulfonyl)imide [TBOP-TFSI], were used to compare the effect of the cations on the antistatic performance of PP composites. Specifically, TBOP-TFSI is a typical RTIL while Li-TFSI has a higher melting temperature of about 230°C. Significant synergistic effects were observed with the binary incorporation of GF and OS for both antistatic performance and the interfacial adhesion. In particular, it was found that the formation of a GF network has a critical role in the dissipation of the electric current. It was considered that the GF network in the PP matrix provides perfect orbits for the movements of ions.

2. Experimental section 2.1 Materials PP used was commercially available from Sumitomo Co. Ltd (Japan) with the trade name of AH561. Both tribuyl(octyl)phosphonium bis(trifloromethanesulfonyl)imide (TBOP-TFSI) and Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI), were supplied by Solvay Co. ltd (Belgium). The short glass fibers were kindly provided by Jushi Co. Ltd (China) with a type of 568H. All the samples were used as received. 2.2 Preparation of PP/ILs and PP/GF/ILs blends 4


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All components were kept dried under vacuum at 80°C for 12h before processing. The binary and ternary composites were prepared by direct mixing of PP, GF, and OSs in a batch mixer (Haake Polylab QC), at 50 rpm and 190°C for 5 min. After melt-mixing, samples were hot-pressed at 200°C and 10 MPa into 500-µm-thick films, followed by the cold-pressing at room temperature. The obtained sheets were used for the following characterization. The GF contents and the OSs loadings in this work were all calculated based on only the amount of matrix PP by weight. The specimens for the tensile tests were prepared by injection molding. 2.3 Characterization Electrical conductivity was measured by an ultrahigh resistivity meter, with a piece of URS probe electrode (Model MCP-HT450) at 100 V. The sample thickness was about 500µm. Tensile tests were carried out using an Instron universal material testing system (Model 5966) at 23°C using the standard dumbbell shaped samples with the crosshead speed of 5 mm/min. The microstructure of cross-fractured surface of samples was obtained using field emission scanning electron microscopy (FESEM, SEM-JSM 6700). An acceleration voltage of 3kV was used for the samples and the fractured surface was coated with a thin layer of gold before the SEM observation. Dynamic mechanical analysis (DMA, TA-Q800) was carried out in multi-frequency strain mode. The dynamic loss (tanδ) was determined at 5 Hz and a heating rate of 3 °C/min, at 0~150 °C. Differential scanning calorimetry measurements were carried out by a differential scanning calorimeter (DSC, TA-Q2000). The samples were first heated to 200°C and held for 5 min to eliminate previous thermal history. Both the cooling and heating rates were 10 °C/min. The experiments were conducted under a continuous high purity nitrogen atmosphere. The Fourier transform infrared spectroscopy (FTIR) measurements were carried out in transmittance mode on grinding samples by FTIR spectroscopy (FTIR, Bruker Tensor). The FTIR spectra were recorded at a resolution of 2 cm-1, and 64 scans from 4000 to 400 cm-1 were averaged. Thermo gravimetric analysis (TGA, TA-Q500) was carried out at a heating rate of 20 °C/min from room temperature to 550°C, in a high purity N2 atmosphere. Rheology behavior was carried out by rotated rheometer (Anton Paar Co. Ltd. Austria) with 5



a type of MCR 302 and a modal of parallel-plate. The dynamic storage modulus (G’) and dynamic loss tangent (tanδ=G’’/G’) were evaluated as a function of frequency at 190°C with a strain amplitude of 15%. A polarized optical microscope (POM, Olympus BX51-P) with a hot stage unit was used to study the morphologies of PP/GF blends. All the samples were heated to 200°C.

3. Results 3.1 Antistatic performance of PP/GF/OS composites

Surface resistivity（ Ω/sq）

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14

10

PP/TBOP-TFSI composites PP/Li-TFSI composites PP/GF/TBOP-TFSI composites PP/GF/Li-TFSI composites

13

10

Insulative

12

10

Static dissipative

11

10

10

10

9

10

8

10 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

OS loading（ wt%） Fig.1. Surface resistivity of binary PP/OS blends and ternary PP/GF/OS composites with 30 wt% of GF as a function of OS loadings.

Figure 1 shows the electrical conductivity of binary PP/OS blends and ternary PP/GF/OS composites possessing with 30 wt% of GF as a function of OS loading. For comparison, neat PP is insulating with an electrical resistivity greater than 1013 Ω/sq. For binary PP/OS blends (i.e., no GF), a decrease in the resistivity is observed as the OS loadings become greater than 2 wt%. Generally speaking, antistatic materials typically have a resistivity less than 1012 Ω/sq. As shown in Figure 1, the antistatic PP materials can only be achieved with OS loading of more than 3 wt% for the binary PP/OS blends. It should also be noted that excess OS bleeding was observed for the TBOP-TFSI-incorporated PP sample during the melt mixing, indicating immiscibility between the TBOP-TFSI and the PP matrix. This is clearly observed from the SEM images of the binary 6


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PP/OS blends (as shown in Figure S1), wherein TBOP-TFSI forms large aggregates in the PP matrix with even 1 wt% OS loading. No OS bleeding phenomenon is found in the Li-TFSI blend system, however, which is attributed to the high melting point of Li-TFSI. In fact, Li-TFSI is also incompatible with the PP matrix, as evidenced by the existence of large domains in Figure S1. It is therefore concluded that the sole incorporation of OS into a binary system is not applicable for the fabrication of antistatic PP materials. In contrast, it is clear from Figure 1 that the ternary PP/GF/OSs composites exhibit vastly different conductivity behaviors from those of the binary PP/OS blends. At a constant 30 wt% GF content, the addition of OS induces a significant enhancement of the electrical conductivity for both TBOP-TFSI and Li-TFSI systems compared to the samples without OS. The electrical conductivity of PP/GF/TBOP-TFSI and PP/GF/Li-TFSI composites with 0.25 wt% OS are found to be 1011 and 1010 Ω/sq, respectively, which are much lower than the PP binary blends at the same OS loading. Furthermore, these conductivity values for the ternary blends are even one order of magnitude higher than the standard for antistatic materials, even at such low OS loadings. For the ternary composites with increasing loading of Li-TFSI, the electrical conductivity value levels off after a loading of about 0.5 wt% and the surface resistivity is in the range of 109 Ω/sq, indicating excellent antistatic performance with the incorporation of Li-TFSI. For the ternary composites with increasing loadings of TBOP-TFSI, the electrical conductivity increases and only a small amount of TBOP-TFSI is adequate for the fabrication of antistatic PP/GF composites. However, it is clear that the electric conductivity of the Li-TFSI ternary samples is always higher than that of the TBOP-TFSI ternary samples at the same loading contents. It should be emphasized that no excess OS were observed during the melt compounding and the storage of the composites, indicating that the GFs can stabilize the OSs in the

ternary

composites.

On

the

other

hand ， the

surface

resistivity

values

PP/GF/Li-TFSI(70-30-0.7) and PP/GF/TBOP-TFSI(70-30-0.7) samples are 5.53×109

of and

5.32×1011 after 6 months storage at room temperature, respectively (as shown in Table S1 which displays the electrical resistivity change with the time). The values are almost same as those measured for the newly prepared samples, indicating the excellent antistatic durability of the PP/GF/OS composites. This is very important for the real application as the permanent antistatic materials.

7


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Surface resistivity（ Ω/sq）


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14

10

PP/GF/TBOP-TFSI composites PP/GF/Li-TFSI composites

13

10

12

10

Insulative Static dissipative

11

10

10

10

0

10

20

30

40

GF content（ wt%） Fig.2. Surface resistivity of PP/GF/OS composites as a function of GF contents with 1wt% OS.

It is demonstrated in Figure 1 that the percolation threshold of the OS can be drastically decreased with the incorporation of GF. It is therefore important to elucidate the effects that the GF content has on the electrical conductivity at a constant OS loading. Figure 2 shows the surface resistance of PP/GF/Li-TFSI and PP/GF/TBOP-TFSI ternary composites at a constant 1 wt% loading of OS. Please note that the 1 wt% loading of OS was calculated based only on the PP content. It is found that the conductivity increases with increasing GF content in the ternary composites with less than 30 wt% GF. For both OS-type systems, the highest electrical conductivity occurs at the GF contents ranging from 20–30 wt%. A further increase of the composite GF content greater than 30wt% leads to a decrease in the electrical conductivity in both OS-type systems. It should also be noted that Li-TFSI exhibits a superior antistatic performance function than that of TBOP-TFSI. The differences of existing between the optimum antistatic properties of the two OS types in the composites can be attributed to the various interactions between the OS, the GF and the matrix. The GF/Li-TFSI interaction is stronger than the GF/TBOP-TFSI interaction, but TBOP-TFSI has a stronger interaction with the PP matrix because of the long alkyl chains in TBOP-TFSI. Such long alkyl chains can entangle with the segmental chain of the PP on the molecular level. Moreover, the superior distribution of the GF in the PP/GF/TBOP-TFSI composites compared to that of the PP/GF/Li-TFSI composites can accelerate the formation of a GF network in the matrix. A detailed investigation of the interactions between 8


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the GF and the OSs will be demonstrated in Section 4. 3.2 Mechanical properties of the ternary PP/GF/OS composites Figure 3 plots the tensile strength of the ternary PP/GF/OS composites as a function of the OS loadings with the PP/GF weight ratio of 70/30. It is clear that the addition of either OS type leads to a significant enhancement in the tensile strength, which indicates the interfacial strengthening induced by the OS for the PP matrix and the GF, this is discussed in section 3.3. It is also found that the greatest strengthening occurs at an OS loading in the range of 0.5–1.0%, as based on the PP matrix. It should be noted that such loadings are adequate for an excellent antistatic performance of these composites. Moreover, it is found that TBOP-TFSI exhibits better effects than Li-TFSI for the enhancement of the tensile strength. This can be attributed to the long alkyl groups present in TBOP, which have stronger interactions with the PP matrix.

42

Tensile strength (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 TBOP-TFSI

38 36

Li-TFSI

34 32 30 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

OS loading（ wt%） Fig. 3. Tensile strength as a function of OS loadings with the PP/GF ratio of 70/30.

3.3 SEM measurements for the ternary PP/GF/OS composites Figure 4 shows SEM images of the cryo-fractured surfaces of the PP/GF/TBOP-TFSI and PP/GF/Li-TFSI ternary composites with varying OS content. For the PP/GF composite with no OS，a clear gap is observed between the GF and the matrix, indicating the incompatibility existing between the GF and the PP matrix. However, with the increased addition of OS in the composites, this gap between the GF and matrix gradually disappears. This phenomenon is very clear at the 5 wt% OS loadings for both TBOP-TFSI and Li-TFSI. This result indicates that both TBOP-TFSI 9



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and Li-TFSI improve the interfacial adhesion between the PP and the GF. It is also clear from Figure 3 that TBOP-TFSI is more effective for improving the compatibility between the GF and the matrix compared to Li-TFSI. For the sample with 5 wt% TBOP-TFSI, the GF is embedded in the PP matrix and the GF surface is not smooth. We consider that the long alkyl chains of TBOP-TFSI have stronger interactions with the PP matrix and the TBOP-TFSI is more compatible with the PP matrix than Li-TFSI. A detailed investigation will be further demonstrated in the following Sections 3.4 and 4. Obviously, the interface enhanced with the addition of OS contributes to the improvements in the mechanical properties found in Figure 3. The tensile strength of the composites with included TBOP-TFSI is higher than that of the composites with included Li-TFSI at a constant OS loading.

Fig. 4. SEM images of cross-fracture surface of PP/GF（A）； PP/GF/Li-TFSI(70-30-1)（B）； PP/GF/Li-TFSI(70-30-3) （C）； PP/GF/Li-TFSI(70-30-5)（D）； PP/GF/TBOP-TFSI(70-30-1) （B’）；PP/GF/TBOP-TFSI(70-30-3) （C’）； PP/GF/TBOP-TFSI(70-30-5)（D’）. 10


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3.4 Dynamic mechanical analysis (DMA) measurements Figure 5 shows the storage modulus and tan(δ) curves of the neat PP, PP/GF binary, and PP/GF/OSs ternary composites. The addition of GF significantly enhances the storage modulus of PP owing to the strengthening effects of GF for the PP over the entire temperature range. The ternary composites with Li-TFSI have an almost equivalent storage modulus with that of the binary PP/GF composites, which means that Li-TFSI exhibits little effect on the modulus of the composites. However, the TBOP-TFSI OS exhibit significant strengthening effects on the binary PP/GF composites. An addition of only 5wt% of TBOP-TFSI induces a much higher storage modulus in the entire temperature range compared to that of binary PP/GF composites. Specifically, the storage modulus of the PP/GF composites is 2.7 GPa at room temperature, while that of the PP/GF/TBOP-TFSI composites is 3.2 GPa. The strengthening function of TBOP-TFSI clearly originates from the compatibility effects of TBOP-TFSI for the GF and the PP matrix, as shown in Figure 3. The relaxation peak of the neat PP sample in Figure 5B at about 8 °C is attributed to the Tg (glass transition temperature) of PP. The simple addition of GF does not change this relaxation peak, indicating the immiscibility between PP and GF. An improved miscibility can be observed in the ternary PP/GF/OS composites, as evidenced by the almost nonexistent Tg relaxation peak in Figure 5B for both the PP/GF/Li-TFSI and the PP/GF/TBOP-TFSI composites. Shida et al.28, 29 have investigated interfacial adhesion using of the tan(δ) of the Tg peak for shortfiber-reinforced polymer composites. Therein, the relationship of interfacial adhesion and the maximum values of tan(δ) for the composites (tanδmax)c and the polymer matrix (tanδmax)m could be described by:

( ) = ( ) − Where the subscripts c, m, and f denote the composite, matrix, and fiber, respectively; and Vf is the volume fraction of the filler. Using the value of α, the degree of the interaction between the fibers and the matrix can be estimated. In the present case, it was calculated that the value of α for the TBOP-TFSI OS (αTBOP-TFSI) is 0.062 and that for the Li-TFSI OS (αLi-TFSI) is 0.046. The fact that αTBOP-TFSI is larger than αLi-TFSI indicates the stronger interfacial adhesion for the TBOP-TFSIincorporated composites. These results are consistent with both the SEM results and the storage modulus data. 11



0.20

（A）

（B）

NEAT-PP PP/GF(70-30) PP/GF/TBOP-TFSI (70-30-5) PP/GF/Li-TFSI (70-30-5)

4

0.15

Tan( δ )

E'(GPa)

3 2 1

0.10

0.05

NEAT-PP PP-GF(70-30) PP/GF/TBOP-TFSI (70-30-5) PP/GF/Li-TFSI (70-30-5)

0 0

25

50

75

100

125

0.00

150

0

Temperature( °C)

25

50

75

100

125

150

Temperature( °C)

Fig.5. Storage modulus (A) and the loss factor of PP/GF/OS composites (B) as a function of temperature by DMA.

3.5 Differential scanning calorimetry (DSC) measurements Figure 6 shows the DSC endotherms and exotherms of the NEAT-PP, PP/GF and PP/GF/OSs composites as a function of temperature. The melting peak temperature of PP/GF (70-30) is similar to that of neat PP, which means the immiscibility between GF and PP matrix. In contrast, the melting peak temperature decreased obviously with the incorporation of OSs (Figure 6(A)), indicating the improved the interface compatibility between GF and PP matrix by OSs. In particular, TBOP-TFSI exhibits higher effects on the melting peak of PP than Li-TFSI, which is consistent with SEM results. On the other hand, it is clear from Figure 6(B) that PP has higher

（A）

Heat Flow (a. u.)

NEAT-PP

PP/GF(70-30)

PP/GF/Li-TFSI(70-30-3)

PP/GF/TBOP-TFSI(70-30-3)

Heating 140

150

160

170

（B）

Cooling NEAT-PP PP/GF(70-30) PP/GF/Li-TFSI(70-30-3)

PP/GF/TBOP-TFSI(70-30-3)

Exo

Heat Flow (a. u.)

crystallization rate in the PP/GF/Li-TFSI composites than in the PP/GF/TBOP-TFSI composites.

Exo

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 23

180

120

Temperature (oC)

130

Temperature (oC)

Fig.6. DSC endotherms (A) and exotherms (B) of neat PP，PP/GF and PP/GF/OS composites as a function of temperature.

12


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4. Discussion It is very interesting to find that the GF and the OS exhibit synergistic effects for the antistatic PP composites, where the GF enhances the electrical conductivity of the PP/OS blends and the OS improve the compatibility between the PP and the GF. In particular, the maximum electrical conductivity was achieved with a GF content of 20–30 wt% at a constant OS loading. Additionally, the OS, and especially TBOP-TFSI, strengthen the interfacial adhesion between the PP and the GF. Figure 7 shows the molecular structure of the two OS. Both OS have the same anion [TFSI-] but different cations. TBOP-TFSI has long alkyl chain in its cation. It is therefore important to elucidate the synergistic effects mechanism of the binary incorporation of GF and OS.

Octyltributylphosphonium bis(trifluoro-methanesulfonyl)imide (TBOP-TFSI)

Lithium bis(trifluoro-methanesulfonyl)imide (Li-TFSI) Fig.7. The molecule structures of octyltributylphosphonium bis(trifluoro-methanesulfonyl)imide and Lithium bis(trifluoro-methanesulfonyl)imide

We first consider the special interactions between the GF and the added OS, because both the GF and OS have very high polarities. For these measurements, we simply ground together the GF with 5 wt% OS, using either TBOP-TFSI or Li-TFSI, and these prepared GF/OS mixtures were used for further characterization. Figure 8 shows the FTIR spectra of the GF/OS mixtures, where the absorption peak at 869 cm-1 for the neat GF sample is assigned to the −Si−OH stretching vibrations of GF. This peak shifts to lower wavenumbers for the two GF/OS mixture samples, where the absorption peaks for the GF/Li-TFSI and GF/TBOP-TFSI samples are at 863 and 866 cm-1, respectively. Such peak shifts after grinding can be attributed to the strong Lewis acid–base interaction between the cations and the electron-donating oxygen atoms of −OH in the GF.30 The fact that the GF/TBOP-TFSI sample exhibits less shifting than the GF/Li-TFSI sample indicates 13



Transmittance (%)

（A）

Transmittance (%)

that the Li-TFSI exhibits stronger interactions with the GF than TBOP-TFSI.

Glass fiber

( Si-OH) -1 869cm Li-TFSI

( Si-OH)-1 863cm

GLass fiber/ Li-TFSI grind

1000 950

900

850

800

750

700

650

600

（B）

Glass fiber

( Si-OH) -1 869cm TBOP-TFSI

( Si-OH) -1 866cm

Glass fiber/TBOP-TFSI grind

1000 950

-1 Wave number (cm )

900

850

800

750

700

650

600

-1 Wave number (cm )

Fig.8. FTIR of Glass fiber with Li-TFSI (A) and TBOP-TFSI (B) for the wavenumber range 600−1000 cm−1.

The specific interactions between the GF and OS can be further confirmed by TGA of neat OS of both types and the GF/OS mixtures, as shown in Figure 9. It is seen that the maximum degradation temperature (Tmax) of the TBOP-TFSI shifts from 393 °C for the neat sample to 431

°C for the ground GF/TBOP-TFSI mixture sample. The significant shift to a higher degradation temperature indicates again the specific interactions between the GF and TBOP-TFSI. The same situation was observed for the GF/Li-TFSI system, where the Tmax of the neat Li-TFSI sample is

120

（A）

100 80 60 40 20 0 100

PURE TBOP-TFSI Grinding GF/TBOP-TFSI PURE Li-TFSI Grinding GF/Li-TFSI

200

300

400

Temperature( °C)

500

Deriv.Weight Change(%/°C)

416 °C and is shifted to 435 °C after grinding Li-TFSI with GF.

Weight(%)

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2.0 1.6

（B）

1.2

PURE TBOP-TFSI Grinding GF/TBOP-TFSI PURE Li-TFSI Grinding GF/Li-TFSI

0.8 0.4 0.0 200

250

300

350

400

450

500

550

Temperature( °C)

Fig.9. TGA (A) and DTG (B) curves for OS and GF/OS mixture samples.

It should be noted that PP is not miscible/compatible with either TBOP-TFSI or Li-TFSI because of the OS bleeding phenomena that occurs with even 3 wt% loading of OS and also the 14


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very coarse phase structure. Combining the results exhibiting the specific interactions between GF and OS and the incompatibility between OS and PP, it is rational that almost all of the OS were absorbed onto the surface of the GF in the ternary composites. This is further confirmed by the fact that almost no OS aggregations were observed in the PP matrix in the ternary composites (as shown in the SEM images of the ternary PP/GF/OS composites in Figure S2). Once we presume the absorption of the OS on the surface of the GF in the PP matrix, it is easy to explain the significant enhancement observed in the electrical conductivity with the addition of GF at a constant OS loading. The incompatibility between OS and PP leads to the aggregation of OS and also the bleeding of the excess OS in the PP matrix. Such aggregation leads to the lack of an ionic path network and the materials therefore exhibit a low electrical conductivity even with a high OS loading, as seen in Figure 1. However, the addition of GF leads to the surface absorption of both the anions and cations of the OS because of the specific interactions between the GF and OS, where the GF has a large aspect ratio and easily forms a network in the PP matrix. In other words, the GF presents a perfect orbit for the movement of the ions in the composites. Therefore, the GF not only diminishes the bleeding phenomenon of OS and the aggregation of OS in the PP matrix at high OS loadings, but also enhances the electrical conductivity. In particular, with increasing GF content in the composites, the fibers form a contact network in the PP matrix，wherein the OS ions on the surface of the GFs form a perfect ionic network. Therefore, one can achieve the best electrical conductivity with ternary composites. Both rheology analysis and optical microscopy (OM) used to detect the formation of the GF network in the PP matrix, as shown in Figures 10 and 11, respectively. The distinct plateau31-33 in the storage modulus curve at 30 wt% GF indicates the formation of GF networks in the melt state (as shown in Figure 10). The complex viscosity measurements of the PP/GF binary composites lead to the same conclusion, as evidenced by the complex viscosity curves of the binary composites as a function of frequency in Figure S3. Furthermore, the OM images provide a direct observation of the GFs in the PP matrix, where it is obvious the GFs contact each other and a network forms at GF content of about 20–30 wt% (shown in Figure 11). Therefore, we observed the best antistatic performance with 20–30 wt% GF at a constant OS content.

15



5

10

4

10

G' (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3

10

NEAT-PP PP/GF(90-10) PP/GF(80-20) PP/GF(70-30)

2

10

-1

10

0

1

10

10

2

10

Angular Frequency ω (rad/s) Fig.10. Dynamic storage modulus G’ as a function of ω for PP/GF composites at 190°C.

Fig.11. OM images of PP/GF(95-5)（A）；PP/GF/(90-10)（B）；PP/GF(80-20) （C）；PP/GF(70-30) （D）with a scale bars of 100µm.

Figure 12 shows the schematic diagram of the detailed phase structure of the ternary PP/GF/OS composites. The insulated network formed by GF has no contribution to the antistatic performance of the polymer matrix, while the sole addition of OS in the PP cannot alone form a sufficient conductive pathway. The simultaneous incorporation of GF and OS, however, leads to the OS absorption onto the surface of the GF and, simultaneously, GF forms three-dimensional (3D) network in the PP matrix when the GF content is higher than 20 wt%. Therefore, the GF 16


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network acts as an orbit for the OS ionic conductivity and an effective electrical conductive pathway occurs. One can achieve very good antistatic performance even with OS loadings as small as 0.25 wt% based on the PP matrix. It should also be mentioned that TBOP-TFSI exhibits better interactions with the PP matrix than Li-TFSI because of its long alkyl chains, which is evident from the SEM images of the composites. Such interactions may confine the movement of both cations and anions, which explains the observed higher electrical conductivity in the Li-TFSI system than in the TBOP-TFSI system. However, it is also clear that the interactions of TBOP-TFSI with both the GF and the PP matrix can strengthen the interface between the GF and the PP matrix. Therefore, a higher storage modulus was achieved over the entire temperature range for the ternary PP/GF/TBOP-TFSI composites than was found for the ternary PP/GF/Li-TFSI and binary PP/GF composites. Once the 3D GF network forms, a further increase of GF content leads to a higher GF network density, thus, the ion concentration on the GF surface of decreases and the electrical conductivity decreases even with a constant OS loading. This is clearly observed in Figure 2 wherein the sample with 40 wt% GF content exhibits lower conductivity than the sample with 20–30 wt% GF content at a constant OS loading.

Fig.12. Schematic diagram of glass fiber network and the surface absorption of OS on the surface of GF.

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5. Conclusion In this work, we successfully fabricated PP/GF composites with both excellent antistatic performance and enhanced interfacial adhesion via the incorporation of OS. It was demonstrated that GF and OS exhibited ideal synergistic effects in the composites. The strong interactions existing between the GF and OS induced the surface absorption of OS onto the fibers, and the fiber networks provided orbits for the ionic movements of the OS. Moreover, TBOP-TFSI contains the long alkyl chains and that strengthened the interfacial adhesion between GF and the PP matrix. Such compatibility effects improved the mechanical properties of the composites, but confined the movements of the ions under the electric fields. Therefore, the TBOP-TFSI exhibited inferior antistatic effects compared to Li-TFSI for the PP/GF composites. This work paves a new route for the fabrication of GF- reinforced polymer composites with multi-functional capabilities.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS Publication website. SEM images for neat PP and binary PP/OS blends, viscosity of the PP/GF binary composites, and antistatic durability data for the ternary PP/GF/OS composites.

AUTHOR INFORMATION Corresponding Author *E-mail: (Y.L.) [email protected]. Fax: +86 571 28867899. Telephone: +86 57128867026. Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21674033, 51173036).

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For table of content use only

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Glass-Fiber Networks as an Orbit for Ions: Fabrication of Excellent

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