Article pubs.acs.org/IECR
Toward an Optically Transparent, Antielectrostatic, and Robust Polymer Composite: Morphology and Properties of Polycarbonate/ Ionic Liquid Composites Chenyang Xing,† Xin Zheng,† Liqun Xu,‡ Jijun Jia,‡ Jie Ren,‡ and Yongjin Li*,† †
College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Road, Hangzhou 310036, People’s Republic of China ‡ Shangyu Java Macromolecular Material Company, Ltd., Shangyu City, Hangzhou 312367, People’s Republic of China S Supporting Information *
ABSTRACT: Transparent polymeric materials with high ductility and antistatic properties have attracted much attention. A room temperature ionic liquid (IL), tributyl(methyl)ammonium bis(trifluoromethane)sulfonylimide ([tbmam+][Tf2N−]), was integrated into polycarbonate (PC), to fabricate optically transparent antielectrostatic composites by melt processing. The morphology and physical properties of the composites were investigated. Differential scanning calorimetry and dynamic mechanical analysis indicated the depression of the glass transition temperatures of the PC/IL composites, compared with that of neat PC. This indicated the interaction between PC and the IL. Transmission electron microscopy indicated that the IL was highly compatible with PC, at low IL loadings. This resulted in enhanced antielectrostatic properties, and significantly improved elongation at break of the PC/IL composites. The PC/IL composites maintained the high transmittance of PC. In addition, the PC/IL composites with high clearance and antielectrostatic properties could be achieved by large-scale continuous melt extrusion, indicating that the method to fabricate both optically transparent and antielectrostatic composite is feasible for industrial application.
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INTRODUCTION Optically transparent polymeric materials based on polycarbonate (PC) have attracted much attention because of their applications in optical devices and displays. Most investigations concerning PC composites have concentrated on improving their mechanical and thermal properties and maintaining their high transmittance.1−5 One approach to enhancing their mechanical and/or thermal properties is the integration of nanofillers. This can generate additional functionality, such as photocatalytic properties6 and flame retardancy.7 To maintain high transmittance, nanofillers should be homogeneously dispersed in the polymer matrix, and the dispersed size should be less than the shortest wavelength of visible light, according to Rayleigh’s law.8 The difference in refractive indices of the matrix and particles is also important. If both are equal (refractive index matched), the scattering intensity is zero and the transmittance is independent of particle size. As an optical transparent polymer, however, just like most polymers, PC is insulative. Its surface can cause static-charge accumulation, possibly leading to fires or explosions in practical applications under dry conditions. Such insulating transparent materials can absorb airborne dust and bacteria, which can influence their practical appearance and industrial applications. Few studies have considered improving the antielectrostatic properties of PC while maintaining its high transparency. Room temperature ionic liquids (RTILs) exhibit low toxicity, low melting points, negligible vapor pressure, high thermal and chemical stabilities, high ionic conductivity, and a broad electrochemical potential window.9−11 RTILs have been extensively studied as “green” organic solvents, and as safe © 2014 American Chemical Society
electrolytes in solar cells, batteries, supercapacitors, and other electrochemical devices.12−15 Ionic liquids (ILs) have also recently been integrated into polymers.16−40 ILs are a new generation plasticizer, with higher efficiency and a lower bleeding (immigration of plasticizer) rate than traditional plasticizers. The effective plasticization of ILs has been investigated in poly(vinyl chloride) (PVC),16 poly(methyl methacrylate) (PMMA),17,18 and poly(vinylidene fluoride) (PVDF).19 On the other hand, ILs have also been found to be good compatibilizers for inorganic fillers and polymer matrixes, enhancing their interfacial compatibility and the subsequent dispersion of fillers, especially for carbon nanotube (CNT)20−26 and montmorillonite fillers.27 The compatibilizing effects are associated with the interactions of the IL with both polymers and nanofillers. Because of their high ionic conductivity, ILs have been applied as antistatic agents in polymers to resolve the problem of the static-charge accumulation on the surface. Pernak et al. reported the antielectrostatic effect of imidazolium-based ILs and provided a reference to evaluate their antielectrostatic abilities.28,29 The antielectrostatic capability of ILs for a certain polymer depends much upon the compatibility between the IL and the polymer. This is because a strong interaction usually causes confined ion motion in constrained IL molecules, which results in poor antielectrostatic properties. Poor compatibility may lead to the Received: Revised: Accepted: Published: 4304
December 3, 2013 February 18, 2014 February 20, 2014 February 20, 2014 dx.doi.org/10.1021/ie404096b | Ind. Eng. Chem. Res. 2014, 53, 4304−4311
Industrial & Engineering Chemistry Research
Article
vacuum oven at 100 °C, PC/IL pellets were injected into standard dumbbell-shaped samples. Characterization. Thermal properties were investigated by differential scanning calorimetry (DSC, TA-Q2000). The heat flow and temperature of the instrument were initially calibrated with sapphire and indium, respectively. Samples were first heated to 260 °C and held there for 10 min to eliminate previous thermal history. The samples were then cooled to 40 °C, followed by the heating again to 260 °C. Both the cooling and heating rates were 10 °C/min, and the experiments were conducted under a continuous high purity nitrogen atmosphere. Second heating traces of samples were recorded to ensure reproducibility. Transmission electron microscopy (TEM) was performed using a Hitachi HT-7700 instrument at 80 kV. Samples were first ultramicrotomed to obtain a section of thickness 70−100 nm. Dynamic mechanical analysis (DMA, TA-Q800) was carried out in multifrequency strain mode. The dynamic loss (tan δ) was determined at 5 Hz and a heating rate of 3 °C/min, at 40− 210 °C. Transmittances were measured using a UV−visible spectrometer (PerkinElmer, Lambda 750) at 200−700 nm. The sample thickness was about 300 μm. 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 300 μm. Tensile tests were carried out using an Instron universal material testing system (Model 5966) at 23 °C, with a gauge length of 18 mm and crosshead speed of 20 mm/min. Thermogravimetric analysis (TGA, TA-Q500) was carried out at a heating rate of 20 °C/min from room temperature to 650 °C, in a high purity N2 atmosphere.
bleeding of the IL, microphase separation of the IL inside the matrix, and thus deterioration of the mechanical properties. Ding et al. reported a polypropylene (PP)/1-n-tetradecyl-3methylimidazolium bromide ([C14mim]Br) composite, and found that the addition of an IL enhanced the antielectrostatic ability of PP.30 Lu et al. reported the polystyrene (PS)/1-butyl3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) nanofiber by electrospinning.41 The PS/[BMIM][PF6] composite exhibited both superhydrophobicity and conductivity. This was attributed to the intrinsic hydrophobicity and conductivity of [BMIM][PF6]. We recently reported the modification of PVDF with an imidazolium-based IL.19 The IL significantly improved the physical properties of PVDF because of their specific interaction. The prepared PVDF/IL films were optically transparent, were tough, and exhibited antielectrostatic properties. In the current study, we have incorporated an IL, tributyl(methyl)ammonium bis(tri fluoromethane)sulfonylimide ([tbmam+][Tf2N−]), into PC, to fabricate optically transparent antielectrostatic composites. [tbmam+][Tf2N−] is highly compatible with PC at low IL loading, and leads to improved surface conductivity and ductility and no adverse effects on the transparency of PC.
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EXPERIMENTAL SECTION Materials. The PC commercially obtained was Sabic Lexan with a type of 141R. Its molecular weight and polydispersity were determined by gel permeation chromatography to be Mw = 51 460 and Mw/Mn = 1.6, respectively. The IL was tributyl(methyl)ammonium bis(tri fluoromethane)sulfonylimide ([tbmam+][Tf2N−]), with a trade name of SA2, and was supplied by Shangyu Java Macromolecular Material Co., China. Samples were used as received. Sample Preparation. The PC and IL were dried in a vacuum oven at 100 °C overnight before processing. PC/IL composites were prepared by the direct mixing of PC and the IL in a batch mixer (Haake Polylab QC), with a twin screw at 50 rpm and 260 °C for 5 min. Sample compositions are shown in Table 1. After melt-mixing, samples were hot-pressed at 260
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RESULTS AND DISCUSSION Miscibility of PC and the IL. The effect of the IL on the glass transition temperature (Tg) of PC was studied by DSC and DMA. Figure 1 shows the DSC endotherms of neat PC and the PC/IL composites at IL loadings of 1−7 wt %. The Tg
Table 1. Compositions of PC/IL Composites sample neat PC PC/IL composite PC/IL composite PC/IL composite PC/IL composite PC/IL composite PC/IL composite PC/IL composite
(1 (2 (3 (4 (5 (6 (7
wt wt wt wt wt wt wt
%) %) %) %) %) %) %)
mass of PC (g)
mass of IL (g)
50 50 50 50 50 50 50 50
0 0.5 1 1.5 2 2.5 3 3.5
°C and 14 MPa into 300-μm-thick films, which were then coldpressed at room temperature. The obtained sheets were characterized directly. A twin-screw extruder (Haake Polylab QC) with a rodlike die and dicing cutter was also used to prepare PC/IL composites. IL (3 wt %) was integrated into the PC. The processing temperatures from the input region (zone 1), middle region (zone 2), output region (zone 3), and die were 260, 260, 240, and 260 °C, respectively. The rotation speed of the twin screw was 50 rpm. After quenching in ice−water and drying in a
Figure 1. DSC endotherms of neat PC and PC/IL composites at IL loadings of 1−7 wt %. 4305
dx.doi.org/10.1021/ie404096b | Ind. Eng. Chem. Res. 2014, 53, 4304−4311
Industrial & Engineering Chemistry Research
Article
for neat PC is about 145 °C. The Tg of PC in the PC/IL composites decreases upon incorporating small amounts of the IL. For the sample with 3 wt % IL, the Tg is 137 °C. The decreased Tg indicates the plasticization effect of the IL on PC. Similar phenomena have been observed in other polymer/IL systems, such as PMMA/IL blends17,18 and PVDF/IL blends.19 Small IL molecules are fully incorporated into the PC molecular chains. The Tg remains almost constant when the IL content is >4 wt %. Figure 2 shows Tg values of PC as a function of IL loading, measured by DSC. Tg values of the PC/ IL composites gradually decrease with increasing IL concentration, and then level off to a broad minimum, before slightly increasing at higher IL concentration. For binary polymer−diluent systems, the change in Tg produced by blending polymer 1 and diluent 2 can be described by classical and statistical thermodynamics:42 ln Figure 2. Tg of PC/IL composites as a function of IL loading. Open circles and squares represent data from DSC and DMA, respectively. The solid line is calculated from eq 1.
Tg12 Tg1
⎤ ⎡ϕ ϕ = β1⎢ 1 ln ϕ1 + 2 ln ϕ2 ⎥ r2 ⎦ ⎣ r1
(1)
where Tg1 and Tg12 are the Tg’s of component 1 and the mixture of 1 and 2, respectively. ϕ1 = r1n1/(r1n1 + r2n2) and ϕ2 = r2n2/ (r1n1 + r2n2) are the volume fractions of components 1 and 2, respectively. r1 = v1/v0 and r2 = v2/v0, where v1 and v2 are the molar volumes of components 1 and 2, respectively, and v0 is the unit lattice volume. For the polymer−diluent system, it is convenient to set v2 = v0. The nondimensional coefficient β1 is defined by β1 =
zR M1uΔCp p
(2)
where z, R, M1u, and ΔCpp are the lattice coordination number, gas constant, molecular weight of the repeating unit, and isobaric specific heat of polymer 1, respectively. Using the data43 z = 4.0, M1u = 254 g/mol, ΔCpp = 388.2 J/(kg K), ρ1 =1.2 g/cm3, and ρ2 =1.253 g/cm3, the theoretically predicted Tg as a function of IL concentration can be calculated from eq 1, and is shown in Figure 2. Predicted Tg values are in excellent agreement with the experimental DSC values at IL loadings of 1−3 wt %, but deviate when the IL loading exceeds 3 wt %. It is believed that PC is miscible with the IL at loadings of
