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polycarbonate blend
Composites Science and Technology 174 (2019) 68–75
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Effect of the selective distribution of graphite nanoplatelets on the electrical and thermal conductivities of a polybutylene terephthalate/polycarbonate blend
T
Bianying Wen∗, Xiaolei Zheng Department of Material Science and Engineering, Beijing Technology and Business University, Beijing, 100048, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: A: Polymer-matrix composites (PMCs) B: Electrical and thermal properties C: Distribution D: Scanning electron microscopy (SEM) E: Melting mix
Controlling the functional filler selective distribution in a co-continuous polymer blend matrix is an effective way to achieve a high-performance functional composite with a relative lower filler loading. In this work, a polybutylene terephthalate/polycarbonate (PBT/PC) blend with good performance complementarity is selected as the host matrix, and graphite nanoplatelets (GNPs) with excellent conductivity and a typical two-dimensional layered structure serve as the functional filler. PBT/PC/GNPs composites with different blend ratios were prepared by using a melting mix technique. The selective segregated distribution of filler in the polymer blend and the corresponding thermal conductivity, electrical conductivity, and mechanical properties of the PBT/PC/GNPs composites were systematically investigated. The migration mechanism of GNPs in the PBT/PC blend is also discussed based on the thermodynamic calculation prediction. Kinetic experimental results indicate that the morphology of the series of PBT/PC/GNPs composites changes with the proportion of PBT and PC. A co-continuous phase morphology formed at a 50:50 vol ratio of PBT/PC. Thermodynamic analysis showed that GNPs are more likely to be distributed in the PBT phase because the interfacial tension between PBT and GNPs is lower than that between PC and GNPs. The results of the kinetic experiments and selective etching experiment further confirmed the above prediction. The thermal conductivity and electrical conductivity of PBT/PC/GNPs composites have been effectively improved with the segregated distribution of GNPs in a co-continuous PBT/PC blend. Compared to a binary PBT or PC matrix composite, the electrical conductivity of PBT/PC/GNPs ternary composite filled with 3 vol% GNPs increased by eight orders of magnitude and the thermal conductivity increased by ∼10% for the same content of GNPs. Simultaneously, the tensile strength of the PBT/PC/GNPs composite is 50 MPa, which maintains a high application value.
1. Introduction Conductive composites have become a key research topic in recent years. By incorporating functional fillers in a polymer matrix, we can not only combine the lightness, flexibility, and processability of the polymer with the magnetic properties, electrical conductivity, and other special properties of the filler but also achieve an excellent functional composite material with relatively lower costs. Great efforts focused on polymeric conductive composites have achieved fruitful achievements [1,2]. Among them, carbon-based fillers, such as carbon black (CB) [3], carbon nanotubes (CNTs) [4], carbon fibre (CF) [5], and graphene [6], have attracted considerable attention because of their have high thermal and electrical conductivities. Unfortunately, those carbon-based fillers often exhibit a highly agglomerated state in a polymer matrix. To fabricate a composite with high electrical ∗
conductivity usually requires a large volume fraction of filler content. As a result, not only is the process expensive but it is also harmful to the mechanical property of the material. Therefore, a low percolation threshold is desired for a polymeric conductive composite. An effective way to reduce the electrical percolation threshold is by using the so-called double percolation technique, in which a two-phase polymer blend is used as the polymer matrix [7]. When the filler is located preferentially in one continuous phase or in the interfacial region of the blend, the percolation threshold is often lower. For example, Göldel et al. [8] investigated a polycarbonate/poly(styrene-acrylonitrile)/multiwalled carbon nanotube (MWCNT) composite and observed that the MWCNTs were exclusively located within the polycarbonate (PC) phase, resulting in much lower electrical resistivity than PC or styrene-acrylonitrile (SAN) composites with the same MWCNT content. Cui et al. [9] researched a co-continuous polystyrene (PS) and poly
Corresponding author. E-mail address: [email protected] (B. Wen).
https://doi.org/10.1016/j.compscitech.2019.02.017 Received 17 August 2018; Received in revised form 12 February 2019; Accepted 17 February 2019 Available online 20 February 2019 0266-3538/ © 2019 Published by Elsevier Ltd.
Composites Science and Technology 174 (2019) 68–75
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modality, by using a laser flash thermal conductivity analyser (LFA467, NETZSCH Scientific Instruments Co., Ltd.) according to the ASTM E1461 standard.
(methyl methacrylate) (PMMA) blend filled with octadecylaminefunctionalised graphene (GE-ODA); their results reveal that the composite exhibits an extremely low electrical percolation threshold with a GE-ODA content of 0.5 wt% when GE-ODA is selectively located in the PS phase. Liu et al. [10] reported that MWCNTs were stably anchored at the interface of poly(L-lactide)/poly(D-lactide) grafted ethylene-acrylic ester copolymer (PLLA/EGD) blends and that the electrically conductive pathway formed at a much lower percolation threshold. The above facts illustrate that the percolation threshold can be significantly reduced when the filler is selectively distributed in the interface or in one phase of the polymer blend matrix. However, most of these studies only investigate the effect of filler selective distribution on the electrical conductivity of the composite; their corresponding thermal conductivity was of less concern. In this work, a PBT/PC blend with good performance complementarity was selected as the matrix. Graphite nanoplatelets (GNPs) with excellent conductive property and typical two-dimensional layer particles were selected as the filler. The dependence of both the thermal conductivity and the electrical conductivity of the PBT/PC/GNPs composite on the selective distribution of GNPs is systematically investigated. The migration mechanism of GNPs in PBT/PC blend is also discussed based on the predictions of thermodynamic calculations and kinetic experimental verification.
2.3.3. Scanning electron microscopy observation The morphology of the composite was observed under a scanning electron microscope (FEI Quanta FEG 250) at an accelerating voltage of 10 kV. Before observation, the specimens were first fractured in liquid nitrogen and then sprayed with gold. To obtain a clear monochrome image, the PC phase was selectively etched with dichloromethane (H2Cl2) in advance. 2.3.4. Thermogravimetric analyses Thermogravimetric analysis (TG) of samples was performed by using TA Instruments Q50. Each specimen was put in a platinum crucible and heated at a scan rate of 20 °C/min from room temperature to 800 °C under a nitrogen atmosphere. 2.3.5. Rheological property The shear viscosity of neat PBT, PC resin, and its corresponding composites were tested using a rotational rheometer (Thermo Scientific HAAKE MARS III, USA) at 240 °C under nitrogen atmosphere. Circular disk specimens with a diameter of 20 mm were placed between two parallel plates with a gap set to 1 mm. Frequency sweep analysis was performed in the range of 0.1–100 rad/s during the linear viscoelastic regime.
2. Experiment 2.1. Materials
2.3.6. Static and dynamic contact angle The contact angle was tested with a goniometer (Data Physics OCA35, Beijing Eastern–Dataphy Instruments Co., Ltd., China) by using a sessile drop technique on the surface sample, and probe liquids of deionised water and diiodomethane (CH2I2) were employed. The surface tension at ambient temperature (20 °C) was calculated based on the measured contact angle by using the Owens–Wendt geometric mean equation [11].
All the materials used in the present work are commercially available. Polybutylene terephthalate resins (PBT201-G0ST) with a density of 1.3 g/cm3 (25 °C) was obtained from Beijing Research Institute of Chemical Industry, China. Bisphenol-A PC (2805) with a density of 1.18 g/cm3 (25 °C) was purchased from Bayer Co., Ltd., Germany. The GNPs (8 μm) were supplied by Qingdao Yanhai Carbon Material Co., Ltd., China. The surfaces of the GNPs used in this study have not been modified. Triphenyl phosphite (TPPi) was provided by J&K Scientific Ltd., China.
2.3.7. Mechanical property The tensile tests were performed using a universal tensile machine (Model CMT6104, Mex Industrial Systems Co., Ltd., China) at room temperature with a crosshead speed of 50 mm/min according to GB/ T1040.2–2006 standard. The average value of tensile strength was calculated using at least five specimens.
2.2. Sample preparation PBT and PC were dried at 120 °C in a vacuum oven for at least 6 h to remove the moisture before the melting process. The PBT/PC blends (in ratios of 100/0, 70/30, 50/50, 30/70, and 0/100) filled with GNPs were prepared by melt mixing in a torque rheometer (XSS-300 Rheometer, Shanghai KeChuang Rubber Plastics Machinery Set Ltd., China) at 240 °C. Here, both polymers and GNPs were introduced directly into the mixer at a rotation speed of 60 rpm for 10 min. In addition, a two-step mixing method was also employed to prepare a series of composite materials at 240 °C and 60 rpm. The volume fraction of GNPs in the entire polymer blends was fixed at 3 vol% in all of the PBT/ PC/GNPs composites. The above melting mixed composites were hot compressed with a square mould (120 × 120 × 1 mm) by using a hot press machine (LP-S50, Labtech Ltd., Sweden) at 35 bar and 240 °C for 15 min to obtain a series of thin sheets for the preparation of test specimens.
3. Results and discussion 3.1. Structure and property of PBT/PC/GNPs ternary composites with different matrix blend ratios 3.1.1. Evolution of phase structure In our previous work [12], we showed that the transesterification reaction takes placed in PBT/PC melting blend systems, which greatly affects the phase structure of the blend and further affects the segregated distribution of the filler. This is significant for improving the performance of the blend. Therefore, the transesterification inhibitor TPPi (1 wt% with respect to the matrix) was used to suppress the transesterification reaction between PBT and PC in this work. Scanning electron microscopy (SEM) observation was performed to characterise the phase morphology of the PBT/PC/GNPs ternary composite. Fig. 1 shows the SEM images of the PBT/PC blend with 3 vol% GNPs at different PBT/PC volume ratios. The PC phase was etched by using CH2Cl2 before the images were taken; therefore, the voids left after etching indicate the position of the PC domains. A typical seaisland structure [13] for the PBT/PC (70/30) composite is observed from Fig. 1a, i.e.; the spherical PC domains are dispersed in the PBT matrix, and GNPs are embedded in the matrix. Once the PC content increases to 50 vol% in the matrix, a co-continuous structure well
2.3. Measurement and characterisation 2.3.1. Electrical conductivity The volume electrical conductivity measurement was performed using a digital ultra-high-resistance micro current tester (EST121, Beijing Hua Jing Hui Technology Co., Ltd.) under an applied voltage of 500 V, according to the GBT1410-2006 standard. 2.3.2. Thermal conductivity Thermal conductivity was measured at 25 °C, in through-plane 69
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Fig. 1. SEM images of PBT/PC/GNPs composites, with PBT/PC volume ratios of (a) 70/30, (b) 50/50, and (c) 30/70. (d) Partial enlarged image from (c).
developed, as shown in Fig. 1b. In contrast with Fig. 1a and b, Fig. 1c presents a reversed phase morphology of the composite with a matrix ratio of 30/70 for PBT/PC, in which PC becomes a continuous phase while PBT is dispersed in the PC phase with irregular larger strips. Careful identification showed that PC with smaller particles is also dispersed in the PBT domain, forming a “phase in phase” structure, as shown in the partial enlarged image of Fig. 1d. To clearly express the evolution of phase structure with different matrix blend ratios, Fig. 2 displays a schematic diagram of the phase structure of PBT/PC/GNPs composite corresponding to Fig. 1. In this figure, the yellow, white, and dark rectangular areas represent the PC domain, PBT domain, and GNPs, respectively. The “sea-island structure,” “co-continuous structure,” and “phase in phase structure” are well presented. 3.1.2. Physical property Fig. 3 shows the electrical and thermal conductivities of PBT/PC/ GNPs composites as a function of the matrix blend ratio. Obviously, the electrical conductivity of the composite changes significantly with the variation of PC content. First, note the two ends of this curve. These two endpoints actually show the electrical conductivity of the binary composite, the former being PBT/GNPs and the latter being PC/GNPs. Both PBT/GNPs and PC/GNPs display very low electrical conductivity, which indicates that the concentration of 3 vol% GNPs is below its electrical percolation threshold. Because GNPs distributed in the PBT
Fig. 3. Electrical conductivity and thermal conductivity vs PC content in PBT/ PC/GNPs composites.
amorphous region increases its effective concentration, the conductivity of the PBT/GNPs is slightly higher than that of the PC/GNPs. Overall, the trinary composite has higher electrical conductivity than that of the binary composite. A remarkable growth of electrical conductivity accompanying percolation transition is observed when the Fig. 2. Schematic presentation of PBT/PC/GNPs composites filled with 3 vol% GNPs. The PBT/PC volume ratios are (a) 70/30, (b) 50/50, and (c) 30/ 70.
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between PBT and GNPs, PC and GNPs, and PBT and PC, respectively. According to the wetting coefficient, there will be three kinds of situations in PBT/PC/GNPs ternary composite systems. If ωa > 1, GNPs will preferentially be distributed in the PC phase; if −l < ωa < l, they will preferentially be distributed at the interface of the blend; and, if ωa < −1, they will preferentially be distributed in the PBT phase. The interfacial tension γij can be derived from the surface free energy of phase i (γi ) and phase j (γj ). Two equations can be used to calculate γij , according to the harmonic-mean equation d d γ pγ p ⎞ ⎛ γ γ γ12 = γ1 + γ2 − 4 ⎜ d 1 2 d + p 1 2 p ⎟ γ1 + γ2 ⎠ ⎝ γ1 + γ2
(2)
and the geometric-mean equation
γ12 = γ1 + γ2 − 2 ( γ1d γ2d +
content of PC is at 50 vol% in the matrix. The electrical conductivity is approximately eight orders of magnitude higher than that of the twocomponent composite. This significant increase may be attributed to the selective distribution of GNPs in a co-continuous phase structure and the formation of a conductive network. With further increasing the PC content to 70 vol%, the electrical conductivity of the composite drops remarkably to 10−13 S/cm, probably as a result of the phase reversal and the destruction of the conductive network. For thermal conductivity, as shown in this figure, the trend of the curve is similar to that of electrical conductivity, but the amplitude of variation is not as significant as that of the electrical conductivity. The thermal conductivity reaches its maximum when the content of PC is at 50 vol% in the matrix, at which point thermal conductivity is increased by ∼9% and ∼12.5% compared to that of PBT/GNPs and PC/GNPs blends, respectively. This demonstrates that, in a composite system, phonon transport is more difficult than electron transport. In other words, it is more challenging to improve the thermal conductivity of the composite than the electrical conductivity. For a functional composite, its mechanical properties are also critical. Fig. 4 presents the tension strength for 3 vol% GNPs-filled PBT/ PC/GNPs composites with various PBT/PC volume ratios. The PBT/PC/ GNPs ternary composite have higher tensile strength than that of the PC/GNPs and PBT/GNPs binary systems. This is a result of the transesterification reaction product acting as a compatibilizer and enhancing the interfacial adhesion between PBT and PC. Because the tensile strength of PC is higher than that of PBT, as the PC content increases, the PC phase morphology changes from a dispersed phase to a continuous phase, and the tensile strength of the composite gradually increased. The above results demonstrate that a co-continues phase structure of the composites plays a very important role in improving its final properties.
−
γ − γT2 dγ = T1 dT T2 − T1
(4)
dγ dT
is the surface energy temperature coefficient, γTi refers to where − the surface energy at a certain temperature, T1 represents the measured temperature, and T2 represents the predicted temperature. Table 2 lists the surface energies calculated for PBT, PC, and GNPs at 20 °C and 240 °C. Using these values, the interfacial tension between the components can be calculated by Eqs. (2) and (3), and the results are listed in Table 3. Clearly, whether using the harmonic-mean equation or the geometric-mean equation, the interfacial tension γPBT − GNPs is smaller than γPC − GNPs , which indicates that the dispersion of GNPs in PBT is more favourable than in PC. Moreover, the values of the wetting coefficient calculated based on Eq. (1) (−9 and −8.3, respectively) are much smaller than −1. According to the wetting coefficient approach, this result indicates that GNPs will preferentially be distributed in the PBT phase. For a polymer blend system, it is generally accepted that the thermodynamics of wetting is not the only factor influencing the fillers' localisation; the kinetic factor, especially the melting viscosity of the polymer matrix, will also affect the final filler distribution. Fig. 5 presents the complex viscosity (η*) as a function of scan frequency at the melt mixing temperature of 240 °C for PBT, PC, and PBT/GNPs and PC/ GNPs binary composites. It shows that all of the PBT and PC and their GNPs-filled composites are insensitive to shear rate. The viscosity of neat PC (∼3100 Pa s) is significantly higher than that of neat PBT (∼260 Pa s) under the angular frequency range of 0.1–100 rad/s. The addition of GNPs further increases the viscosity of both, but it does not change the order in which the viscosity of the PC-based composite is greater than that of the PBT-based composite. This is favourable for the dispersion of GNPs in the PBT phase with less flow resistance.
3.2. Selective distribution mechanism of GNPs in PBT/PC blends 3.2.1. Theoretical analysis for selective distribution of GNPs Based on the thermodynamic theory, the selective distribution of fillers in binary polymer blends is commonly explained by the wetting coefficient (ωα ) approach [14–17]. The wetting coefficient can be adapted in a PBT/PC/GNPs ternary system as follows:
γPBT − GNPs − γPC − GNPs γPBT − PC
(3)
where γi is the surface energy of component i and γid and γi p are, respectively, the dispersive and polar contributions. The distribution of GNPs in the PBT/PC blend can be predicted by calculating the wetting coefficient as given by Eq. (1). According to the contact angle, the surface energies of PBT, PC, and GNPs at 20 °C can be calculated. The corresponding contact angles at a measured temperature of 20 °C for the three materials are listed in \ 1. However, the melting mixed temperature is 240 °C; therefore, the surface energies of PBT, PC, and GNPs at the mixing temperature of 240 °C are required. These can be calculated by using
Fig. 4. Tensile strength for PBT/PC/GNPs composites filled with 3 vol% GNPs for various PBT/PC volume ratios.
ωa =
γ1p γ2p ),
Table 1 Contact angle of probe liquids on PBT, PC, and GNPs surfaces. Probe liquid
(1)
Water Diiodomethane
where γPBT − GNPs , γPC − GNPs , and γPBT − PC are the interfacial tensions 71
Contact angle (°) PBT
PC
GNPs
88.8 38.6
94.4 42.2
68.3 27.2
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Table 2 Surface energy of PBT, PC, and GNPs at 20 °C and 240 °C. Component
PBT PC GNPs
20 °C (mN/m)
240 °C (mN/m)
γ
γd
γp
−
43.2 40.1 46.8
42.3 39.7 38.4
0.9 0.4 8.4
−0.08 [18] −0.04 [19] −0.16 [20]
dγ (mN/mK) dT
γ
γd
γp
χp
25.6 31.3 11.6
25.1 31 9.5
0.5 0.3 2.1
0.02 0.01 0.18
γ = γ d +γp and χ p =γp /γ; χ p is polarity and is independent of temperature.
In summary, all the above results demonstrate that GNPs will preferentially be distributed in the PBT phase. Is that true? The following experiments will further verify the prediction. 3.2.2. Experiment on migration and distribution of GNPs To verify the migration and distribution of GNPs in PBT/PC/GNPs composites, we investigated two processing factors: feeding sequence and mixing time. The volume ratio of the PBT/PC blend matrix was fixed at 50:50 during all of these experiments.
Fig. 5. η* as a function of frequency at the melting temperature of 240 °C for PBT, PC, and PBT/GNPs and PC/GNPs composites.
3.2.2.1. Feeding sequence. For the preparation of the PBT/PC/GNPs composite with a controlled distribution of GNPs, different feeding sequences were employed. First, PC or PBT was mixed with GNPs for 10 min to obtain a PC-GNPs or PBT-GNPs composite. Then, each previously prepared composite was further mixed for 10 min with the second resin. The prepared composites are denoted as PC-GNPs/PBT and PBT-GNPs/PC, respectively. Fig. 6 displays SEM images of PBT/PC/GNPs series composites with different feeding sequences. The feeding sequence exerts a great influence on the domain size of the PBT/PC/GNPs composite. In contrast to the neat PBT/PC blend (see Fig. 6a), the domain sizes of PC and PBT phases become smaller and more uniform after GNPs are added, because the addition of the fillers changes the rheological properties of the polymer phases. When GNPs are mixed with PBT in advance, the viscosity of the PBT/GNPs phase with the PC phase is reduced and the ability of the matrix to transmit shear stress is increased. This is favourable for obtaining satisfactory dispersion. Therefore, the domain size of the PC phase is smallest, as shown in Fig. 6b. In turn, when GNPs are mixed with PC in advance, this inhibits PC phase dispersion. The domain size of the PC phase is greater and looks rough (see Fig. 6c). When PBT, PC, and GNPs are simultaneously melting mixed, the domain size of PC falls between that of PBT-GNPs/PC and PC-GNPs/PBT composites. Fig. 7 shows the electrical and thermal conductivities of PBT/PC/ GNPs series composites prepared with different feeding sequences. The electrical conductivity of the ternary composites increases by eight orders of magnitude above that of the binary blends of PBT/PC. At the same time, the thermal conductivity increases by ∼71% compared to that of PBT/PC blends. For the ternary simultaneously melt-mixed composite, the thermal and electrical conductivities are slightly higher than those of the other two, but the difference is not obvious. This fact illustrates that the feeding sequence does not affect the fundamental double continuous phase or alter the distribution of the filler. The tensile strength of PBT/PC/GNPs series composites with different feeding sequences is shown in Fig. 8. All of the ternary composites have higher tensile strength than that of the binary PBT/PC blend. GNPs have a reinforcing effect on the PBT/PC blend due to the selective
distribution of GNPs greatly improved efficiency of the formation of force transferring network [21]. And the feeding sequence has little influence on tensile strength. This shows that the ternary composites not only improve both thermal and electrical conductivities but also maintain a high tensile strength of 50 MPa, which provides favourable conditions for its practical application.
3.2.2.2. Mixing time. Because the interfacial tension between PBT and GNPs is less than that between PC and GNPs, will GNPs migrate during the processing? Therefore, a PC-GNPs/PBT composite with various mixing times (4, 7, and 10 min, respectively) was selected to clarify this issue. The mixing time refers to the time after adding PBT into the premixed PC/GNPs melts. Fig. 9 presents the testing results for the electrical and thermal conductivities of PC-GNPs/PBT composites prepared with different mixing times. Both the electrical conductivity and thermal conductivity of the composite increase with the mixing time and the electrical conductivity changes more significantly. Therefore, we explain this phenomenon based on the electrical conductivity. Fig. 9 shows that the mixing time exerts a significant influence during the early stages of mixing. For a mixing time change from 4 to 7 min, the electrical conductivity of the composite changes from 10−16 to 10−10 S/cm, an increase of six orders of magnitude, whereas when the mixing time is further prolonged from 7 to 10 min, the electrical conductivity of the composite only increased by one order of magnitude. For this mixing system, 10 min is enough. This is very meaningful for the actual process design of a polymeric composite. What occurred during the mixing process? Fig. 10 presents the SEM images (a1, b1, and c1) of PC-GNPs/PBT composites at different mixing times and exhibits schematic diagrams for the migration process of GNPs from the PC phase to the PBT phase (a2, b2, and c2). The GNPs are distributed in the PC phase at the beginning under a mixing time of 4 min. It is difficult for GNPs to migrate from the PC phase to the PBT phase during such a short mixing time. In this case, therefore, the conductivity of the composite is lower because the GNPs have been wrapped and isolated. As the mixing time increases, more and more
Table 3 Interfacial tension and wetting coefficient of the materials at 240 °C. Component
ϒPBT − PC (mN/m)
ϒPBT − GNPs (mN/m)
ϒPC − GNPs (mN/m)
ωa
Harmonic mean Geometric mean
0.5 0.3
8 4.4
12.5 6.9
−9 −8.3
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Fig. 6. SEM images of PBT/PC/GNPs composites: (a) PBT/PC, (b) PBT-GNPs/PC, (c) PC-GNPs/PBT, and (d) PBT/PC/GNPs.
Fig. 7. Electrical conductivity and thermal conductivity of PBT/PC/GNPs composites with different feeding sequences. Fig. 8. Tensile strength of PBT/PC/GNPs composites with different feeding sequences.
GNPs migrate through the blend interface into the PBT phase, whereas some of them remained at the interface. When the mixing time reached 10 min, most of the GNPs migrated from the PC phase to the PBT phase and embedded in it to achieve thermodynamic equilibrium state [22]. At this time, GNPs distributed in the amorphous region of PBT more easily formed an effective conductive network, resulting in a significant increase in electrical conductivity. In summary, the kinetic experimental result also indicates that GNPs are preferentially selectively distributed in the PBT phase during the melt mixing, which is consistent with the evaluation by using the wetting coefficient.
photographs of etching solution bottles after PBT/PC/GNPs composites were dissolved for one week with a CH2Cl2 etching agent. In this experiment, CH2Cl2 served as an etching agent for PBT/PC/GNPs composites because it can dissolve PC effectively but cannot dissolve both PBT and GNPs. The percentage of the etched weight of the PC component was evaluated by using the following expression:
CI =
3.3. Selective etching experiment
Wi − Wf WPC
× 100%,
(5)
where wi and wf are the weights of the sample before and after solvent etching, respectively, and wPC is the original weight of the PC component in the sample before etching. According to Eq. (5), the quality of the PC components etched off accounted for 44%, 87% and 90% of the
The distribution of GNPs in the PBT/PC blend was also examined by conducting a selective etching experiment. Fig. 11 presents digital 73
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Fig. 11. Digital photograph of etching solution bottles after the PBT/PC/GNPs composites were dissolved. The PBT/PC volume ratios in the matrix are (a) 70/ 30, (b) 50/50, and (c) 30/70.
Fig. 9. Electrical conductivity and thermal conductivity of PC-GNPs/PBT composites. The mixing time refers to the time after adding PBT into the premixed PC/GNPs melts.
Fig. 12 presents the TG curves of the neat PBT, PC, and GNPs and PBT/ PC/GNPs composites with various PBT/PC volume ratios. The residual weights of PBT, PC, and GNPs at 800 °C were 4.3, 25.3, and 97 wt%, respectively. Obviously, the order of thermal stability is GNPs > PC > PBT. For the composites, on the one hand, the residual weight of the composite before etching (A1, B1, and C1) was slightly higher than that of the composite after etching (A2, B2, and C2). This is mainly attributable to the contribution of PC content after etching. On the other hand, for a case study of 50/50 PBT/PC, based on the residual weight of neat PBT, PC, and GNPs, the theoretical residual weight of the sample before etching can be calculated. Compared with the experimental values of TG analysis, the theoretical residual weight (18.8 wt%) was very close to the experimental value (17.9 wt%). This establishes the accuracy of the theoretical calculation. Therefore, if we assume that all of the GNPs were in the PBT phase, the theoretical residual weight of the sample after etching can be calculated. By comparing the theoretical residue (14.8 wt%) with the experimental value (14.7 wt%) after etching, we find that the result is similar to that before etching. The experimental result directly illustrates that GNPs are mainly distributed in the PBT.
original PC mass in the corresponding PBT/PC (70/30, 50/50 and 30/ 70) blend, respectively. Obviously, most of the PC can be etched by CH2Cl2, especially for PBT/PC at ratios of 50/50 and 30/70. As shown in Fig. 11, the clear solution and the intact sample of 70/ 30 PBT/PC clearly indicate that few GNPs are distributed in the PC phase. A slightly black solution was observed when the volume ratio of PBT/PC was 50/50 in the matrix, but the sample was still intact; this indicates that most of the GNPs are distributed in the PBT phase and that the PBT phase is self-supported based on the co-continuous structure. For the sample of 30/70 PBT/PC, the sample framework collapsed because the continuous PC phase dissolved and the solution is light black in appearance; this fact indicates that some GNPs exist in the PC phase but that most of the GNPs are segregated in the PBT phase. These experimental results prove that GNPs have a segregated distribution in PBT. To confirm the distribution of GNPs further, the PBT/PC/GNPs composites before and after etching were analysed by TG analysis.
Fig. 10. SEM images of PC–GNPs/PBT composite with mixing times (tm) of (a1) 4 min, (b1) 7 min, and (c1) 10 min, and schematic presentations of the corresponding migration process of GNPs: (a2), (b2), and (c2). 74
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[2]
[3]
[4]
[5]
[6]
[7]
[8]
Fig. 12. TG curves for PC, PBT, GNPs, and PBT/PC/GNPs composites with various PBT/PC volume ratios. (A1), (B1), and (C1) and (A2), (B2), and (C2) are before-etched and after-etched composites, respectively.
[9]
[10]
4. Conclusions [11]
The selective distribution of GNPs in PBT/PC blends and its effect on electrical, thermal, and mechanical properties of the composites have been systematically studied. Thermodynamic theory suggests that GNPs are preferentially distributes in the PBT phase with better interfacial wettability. Experimental investigation of controlling the feeding sequence and mixing time and performing selective etching experiments showed that the GNPs had a segregated distribution in the PBT phase. The thermal conductivity and electrical conductivities of PBT/PC/GNPs composites can be effectively improved with the segregated distribution of GNPs in a co-continuous structure. Compared to the binary composite, the conductivity of PBT/PC/GNPs composite filled with 3 vol% GNPs increased by eight orders of magnitude and the thermal conductivity was enhanced by ∼10%. This shows that, in a composite system, improving the thermal conductivity of the composite is more challenging than improving the electrical conductivity. Meanwhile, the composite maintains a tensile strength of 50 MPa, making it suitable for many applications.
[12] [13]
[14] [15]
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[18] [19] [20]
[21]
Acknowledgment [22]
This work was financially supported by the Project of the Innovative Research Team of Polymeric Functional Film of Beijing Technology and Business University (19008001071). References [1] J. Chen, X. Cui, K. Sui, et al., Balance the electrical properties and mechanical
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Effect of the selective distribution of graphite nanoplatelets on the electrical and thermal conductivities of a polybutylene terephthalate/polycarbonate blend
T
Bianying Wen∗, Xiaolei Zheng Department of Material Science and Engineering, Beijing Technology and Business University, Beijing, 100048, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: A: Polymer-matrix composites (PMCs) B: Electrical and thermal properties C: Distribution D: Scanning electron microscopy (SEM) E: Melting mix
Controlling the functional filler selective distribution in a co-continuous polymer blend matrix is an effective way to achieve a high-performance functional composite with a relative lower filler loading. In this work, a polybutylene terephthalate/polycarbonate (PBT/PC) blend with good performance complementarity is selected as the host matrix, and graphite nanoplatelets (GNPs) with excellent conductivity and a typical two-dimensional layered structure serve as the functional filler. PBT/PC/GNPs composites with different blend ratios were prepared by using a melting mix technique. The selective segregated distribution of filler in the polymer blend and the corresponding thermal conductivity, electrical conductivity, and mechanical properties of the PBT/PC/GNPs composites were systematically investigated. The migration mechanism of GNPs in the PBT/PC blend is also discussed based on the thermodynamic calculation prediction. Kinetic experimental results indicate that the morphology of the series of PBT/PC/GNPs composites changes with the proportion of PBT and PC. A co-continuous phase morphology formed at a 50:50 vol ratio of PBT/PC. Thermodynamic analysis showed that GNPs are more likely to be distributed in the PBT phase because the interfacial tension between PBT and GNPs is lower than that between PC and GNPs. The results of the kinetic experiments and selective etching experiment further confirmed the above prediction. The thermal conductivity and electrical conductivity of PBT/PC/GNPs composites have been effectively improved with the segregated distribution of GNPs in a co-continuous PBT/PC blend. Compared to a binary PBT or PC matrix composite, the electrical conductivity of PBT/PC/GNPs ternary composite filled with 3 vol% GNPs increased by eight orders of magnitude and the thermal conductivity increased by ∼10% for the same content of GNPs. Simultaneously, the tensile strength of the PBT/PC/GNPs composite is 50 MPa, which maintains a high application value.
1. Introduction Conductive composites have become a key research topic in recent years. By incorporating functional fillers in a polymer matrix, we can not only combine the lightness, flexibility, and processability of the polymer with the magnetic properties, electrical conductivity, and other special properties of the filler but also achieve an excellent functional composite material with relatively lower costs. Great efforts focused on polymeric conductive composites have achieved fruitful achievements [1,2]. Among them, carbon-based fillers, such as carbon black (CB) [3], carbon nanotubes (CNTs) [4], carbon fibre (CF) [5], and graphene [6], have attracted considerable attention because of their have high thermal and electrical conductivities. Unfortunately, those carbon-based fillers often exhibit a highly agglomerated state in a polymer matrix. To fabricate a composite with high electrical ∗
conductivity usually requires a large volume fraction of filler content. As a result, not only is the process expensive but it is also harmful to the mechanical property of the material. Therefore, a low percolation threshold is desired for a polymeric conductive composite. An effective way to reduce the electrical percolation threshold is by using the so-called double percolation technique, in which a two-phase polymer blend is used as the polymer matrix [7]. When the filler is located preferentially in one continuous phase or in the interfacial region of the blend, the percolation threshold is often lower. For example, Göldel et al. [8] investigated a polycarbonate/poly(styrene-acrylonitrile)/multiwalled carbon nanotube (MWCNT) composite and observed that the MWCNTs were exclusively located within the polycarbonate (PC) phase, resulting in much lower electrical resistivity than PC or styrene-acrylonitrile (SAN) composites with the same MWCNT content. Cui et al. [9] researched a co-continuous polystyrene (PS) and poly
Corresponding author. E-mail address: [email protected] (B. Wen).
https://doi.org/10.1016/j.compscitech.2019.02.017 Received 17 August 2018; Received in revised form 12 February 2019; Accepted 17 February 2019 Available online 20 February 2019 0266-3538/ © 2019 Published by Elsevier Ltd.
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modality, by using a laser flash thermal conductivity analyser (LFA467, NETZSCH Scientific Instruments Co., Ltd.) according to the ASTM E1461 standard.
(methyl methacrylate) (PMMA) blend filled with octadecylaminefunctionalised graphene (GE-ODA); their results reveal that the composite exhibits an extremely low electrical percolation threshold with a GE-ODA content of 0.5 wt% when GE-ODA is selectively located in the PS phase. Liu et al. [10] reported that MWCNTs were stably anchored at the interface of poly(L-lactide)/poly(D-lactide) grafted ethylene-acrylic ester copolymer (PLLA/EGD) blends and that the electrically conductive pathway formed at a much lower percolation threshold. The above facts illustrate that the percolation threshold can be significantly reduced when the filler is selectively distributed in the interface or in one phase of the polymer blend matrix. However, most of these studies only investigate the effect of filler selective distribution on the electrical conductivity of the composite; their corresponding thermal conductivity was of less concern. In this work, a PBT/PC blend with good performance complementarity was selected as the matrix. Graphite nanoplatelets (GNPs) with excellent conductive property and typical two-dimensional layer particles were selected as the filler. The dependence of both the thermal conductivity and the electrical conductivity of the PBT/PC/GNPs composite on the selective distribution of GNPs is systematically investigated. The migration mechanism of GNPs in PBT/PC blend is also discussed based on the predictions of thermodynamic calculations and kinetic experimental verification.
2.3.3. Scanning electron microscopy observation The morphology of the composite was observed under a scanning electron microscope (FEI Quanta FEG 250) at an accelerating voltage of 10 kV. Before observation, the specimens were first fractured in liquid nitrogen and then sprayed with gold. To obtain a clear monochrome image, the PC phase was selectively etched with dichloromethane (H2Cl2) in advance. 2.3.4. Thermogravimetric analyses Thermogravimetric analysis (TG) of samples was performed by using TA Instruments Q50. Each specimen was put in a platinum crucible and heated at a scan rate of 20 °C/min from room temperature to 800 °C under a nitrogen atmosphere. 2.3.5. Rheological property The shear viscosity of neat PBT, PC resin, and its corresponding composites were tested using a rotational rheometer (Thermo Scientific HAAKE MARS III, USA) at 240 °C under nitrogen atmosphere. Circular disk specimens with a diameter of 20 mm were placed between two parallel plates with a gap set to 1 mm. Frequency sweep analysis was performed in the range of 0.1–100 rad/s during the linear viscoelastic regime.
2. Experiment 2.1. Materials
2.3.6. Static and dynamic contact angle The contact angle was tested with a goniometer (Data Physics OCA35, Beijing Eastern–Dataphy Instruments Co., Ltd., China) by using a sessile drop technique on the surface sample, and probe liquids of deionised water and diiodomethane (CH2I2) were employed. The surface tension at ambient temperature (20 °C) was calculated based on the measured contact angle by using the Owens–Wendt geometric mean equation [11].
All the materials used in the present work are commercially available. Polybutylene terephthalate resins (PBT201-G0ST) with a density of 1.3 g/cm3 (25 °C) was obtained from Beijing Research Institute of Chemical Industry, China. Bisphenol-A PC (2805) with a density of 1.18 g/cm3 (25 °C) was purchased from Bayer Co., Ltd., Germany. The GNPs (8 μm) were supplied by Qingdao Yanhai Carbon Material Co., Ltd., China. The surfaces of the GNPs used in this study have not been modified. Triphenyl phosphite (TPPi) was provided by J&K Scientific Ltd., China.
2.3.7. Mechanical property The tensile tests were performed using a universal tensile machine (Model CMT6104, Mex Industrial Systems Co., Ltd., China) at room temperature with a crosshead speed of 50 mm/min according to GB/ T1040.2–2006 standard. The average value of tensile strength was calculated using at least five specimens.
2.2. Sample preparation PBT and PC were dried at 120 °C in a vacuum oven for at least 6 h to remove the moisture before the melting process. The PBT/PC blends (in ratios of 100/0, 70/30, 50/50, 30/70, and 0/100) filled with GNPs were prepared by melt mixing in a torque rheometer (XSS-300 Rheometer, Shanghai KeChuang Rubber Plastics Machinery Set Ltd., China) at 240 °C. Here, both polymers and GNPs were introduced directly into the mixer at a rotation speed of 60 rpm for 10 min. In addition, a two-step mixing method was also employed to prepare a series of composite materials at 240 °C and 60 rpm. The volume fraction of GNPs in the entire polymer blends was fixed at 3 vol% in all of the PBT/ PC/GNPs composites. The above melting mixed composites were hot compressed with a square mould (120 × 120 × 1 mm) by using a hot press machine (LP-S50, Labtech Ltd., Sweden) at 35 bar and 240 °C for 15 min to obtain a series of thin sheets for the preparation of test specimens.
3. Results and discussion 3.1. Structure and property of PBT/PC/GNPs ternary composites with different matrix blend ratios 3.1.1. Evolution of phase structure In our previous work [12], we showed that the transesterification reaction takes placed in PBT/PC melting blend systems, which greatly affects the phase structure of the blend and further affects the segregated distribution of the filler. This is significant for improving the performance of the blend. Therefore, the transesterification inhibitor TPPi (1 wt% with respect to the matrix) was used to suppress the transesterification reaction between PBT and PC in this work. Scanning electron microscopy (SEM) observation was performed to characterise the phase morphology of the PBT/PC/GNPs ternary composite. Fig. 1 shows the SEM images of the PBT/PC blend with 3 vol% GNPs at different PBT/PC volume ratios. The PC phase was etched by using CH2Cl2 before the images were taken; therefore, the voids left after etching indicate the position of the PC domains. A typical seaisland structure [13] for the PBT/PC (70/30) composite is observed from Fig. 1a, i.e.; the spherical PC domains are dispersed in the PBT matrix, and GNPs are embedded in the matrix. Once the PC content increases to 50 vol% in the matrix, a co-continuous structure well
2.3. Measurement and characterisation 2.3.1. Electrical conductivity The volume electrical conductivity measurement was performed using a digital ultra-high-resistance micro current tester (EST121, Beijing Hua Jing Hui Technology Co., Ltd.) under an applied voltage of 500 V, according to the GBT1410-2006 standard. 2.3.2. Thermal conductivity Thermal conductivity was measured at 25 °C, in through-plane 69
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Fig. 1. SEM images of PBT/PC/GNPs composites, with PBT/PC volume ratios of (a) 70/30, (b) 50/50, and (c) 30/70. (d) Partial enlarged image from (c).
developed, as shown in Fig. 1b. In contrast with Fig. 1a and b, Fig. 1c presents a reversed phase morphology of the composite with a matrix ratio of 30/70 for PBT/PC, in which PC becomes a continuous phase while PBT is dispersed in the PC phase with irregular larger strips. Careful identification showed that PC with smaller particles is also dispersed in the PBT domain, forming a “phase in phase” structure, as shown in the partial enlarged image of Fig. 1d. To clearly express the evolution of phase structure with different matrix blend ratios, Fig. 2 displays a schematic diagram of the phase structure of PBT/PC/GNPs composite corresponding to Fig. 1. In this figure, the yellow, white, and dark rectangular areas represent the PC domain, PBT domain, and GNPs, respectively. The “sea-island structure,” “co-continuous structure,” and “phase in phase structure” are well presented. 3.1.2. Physical property Fig. 3 shows the electrical and thermal conductivities of PBT/PC/ GNPs composites as a function of the matrix blend ratio. Obviously, the electrical conductivity of the composite changes significantly with the variation of PC content. First, note the two ends of this curve. These two endpoints actually show the electrical conductivity of the binary composite, the former being PBT/GNPs and the latter being PC/GNPs. Both PBT/GNPs and PC/GNPs display very low electrical conductivity, which indicates that the concentration of 3 vol% GNPs is below its electrical percolation threshold. Because GNPs distributed in the PBT
Fig. 3. Electrical conductivity and thermal conductivity vs PC content in PBT/ PC/GNPs composites.
amorphous region increases its effective concentration, the conductivity of the PBT/GNPs is slightly higher than that of the PC/GNPs. Overall, the trinary composite has higher electrical conductivity than that of the binary composite. A remarkable growth of electrical conductivity accompanying percolation transition is observed when the Fig. 2. Schematic presentation of PBT/PC/GNPs composites filled with 3 vol% GNPs. The PBT/PC volume ratios are (a) 70/30, (b) 50/50, and (c) 30/ 70.
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between PBT and GNPs, PC and GNPs, and PBT and PC, respectively. According to the wetting coefficient, there will be three kinds of situations in PBT/PC/GNPs ternary composite systems. If ωa > 1, GNPs will preferentially be distributed in the PC phase; if −l < ωa < l, they will preferentially be distributed at the interface of the blend; and, if ωa < −1, they will preferentially be distributed in the PBT phase. The interfacial tension γij can be derived from the surface free energy of phase i (γi ) and phase j (γj ). Two equations can be used to calculate γij , according to the harmonic-mean equation d d γ pγ p ⎞ ⎛ γ γ γ12 = γ1 + γ2 − 4 ⎜ d 1 2 d + p 1 2 p ⎟ γ1 + γ2 ⎠ ⎝ γ1 + γ2
(2)
and the geometric-mean equation
γ12 = γ1 + γ2 − 2 ( γ1d γ2d +
content of PC is at 50 vol% in the matrix. The electrical conductivity is approximately eight orders of magnitude higher than that of the twocomponent composite. This significant increase may be attributed to the selective distribution of GNPs in a co-continuous phase structure and the formation of a conductive network. With further increasing the PC content to 70 vol%, the electrical conductivity of the composite drops remarkably to 10−13 S/cm, probably as a result of the phase reversal and the destruction of the conductive network. For thermal conductivity, as shown in this figure, the trend of the curve is similar to that of electrical conductivity, but the amplitude of variation is not as significant as that of the electrical conductivity. The thermal conductivity reaches its maximum when the content of PC is at 50 vol% in the matrix, at which point thermal conductivity is increased by ∼9% and ∼12.5% compared to that of PBT/GNPs and PC/GNPs blends, respectively. This demonstrates that, in a composite system, phonon transport is more difficult than electron transport. In other words, it is more challenging to improve the thermal conductivity of the composite than the electrical conductivity. For a functional composite, its mechanical properties are also critical. Fig. 4 presents the tension strength for 3 vol% GNPs-filled PBT/ PC/GNPs composites with various PBT/PC volume ratios. The PBT/PC/ GNPs ternary composite have higher tensile strength than that of the PC/GNPs and PBT/GNPs binary systems. This is a result of the transesterification reaction product acting as a compatibilizer and enhancing the interfacial adhesion between PBT and PC. Because the tensile strength of PC is higher than that of PBT, as the PC content increases, the PC phase morphology changes from a dispersed phase to a continuous phase, and the tensile strength of the composite gradually increased. The above results demonstrate that a co-continues phase structure of the composites plays a very important role in improving its final properties.
−
γ − γT2 dγ = T1 dT T2 − T1
(4)
dγ dT
is the surface energy temperature coefficient, γTi refers to where − the surface energy at a certain temperature, T1 represents the measured temperature, and T2 represents the predicted temperature. Table 2 lists the surface energies calculated for PBT, PC, and GNPs at 20 °C and 240 °C. Using these values, the interfacial tension between the components can be calculated by Eqs. (2) and (3), and the results are listed in Table 3. Clearly, whether using the harmonic-mean equation or the geometric-mean equation, the interfacial tension γPBT − GNPs is smaller than γPC − GNPs , which indicates that the dispersion of GNPs in PBT is more favourable than in PC. Moreover, the values of the wetting coefficient calculated based on Eq. (1) (−9 and −8.3, respectively) are much smaller than −1. According to the wetting coefficient approach, this result indicates that GNPs will preferentially be distributed in the PBT phase. For a polymer blend system, it is generally accepted that the thermodynamics of wetting is not the only factor influencing the fillers' localisation; the kinetic factor, especially the melting viscosity of the polymer matrix, will also affect the final filler distribution. Fig. 5 presents the complex viscosity (η*) as a function of scan frequency at the melt mixing temperature of 240 °C for PBT, PC, and PBT/GNPs and PC/ GNPs binary composites. It shows that all of the PBT and PC and their GNPs-filled composites are insensitive to shear rate. The viscosity of neat PC (∼3100 Pa s) is significantly higher than that of neat PBT (∼260 Pa s) under the angular frequency range of 0.1–100 rad/s. The addition of GNPs further increases the viscosity of both, but it does not change the order in which the viscosity of the PC-based composite is greater than that of the PBT-based composite. This is favourable for the dispersion of GNPs in the PBT phase with less flow resistance.
3.2. Selective distribution mechanism of GNPs in PBT/PC blends 3.2.1. Theoretical analysis for selective distribution of GNPs Based on the thermodynamic theory, the selective distribution of fillers in binary polymer blends is commonly explained by the wetting coefficient (ωα ) approach [14–17]. The wetting coefficient can be adapted in a PBT/PC/GNPs ternary system as follows:
γPBT − GNPs − γPC − GNPs γPBT − PC
(3)
where γi is the surface energy of component i and γid and γi p are, respectively, the dispersive and polar contributions. The distribution of GNPs in the PBT/PC blend can be predicted by calculating the wetting coefficient as given by Eq. (1). According to the contact angle, the surface energies of PBT, PC, and GNPs at 20 °C can be calculated. The corresponding contact angles at a measured temperature of 20 °C for the three materials are listed in \ 1. However, the melting mixed temperature is 240 °C; therefore, the surface energies of PBT, PC, and GNPs at the mixing temperature of 240 °C are required. These can be calculated by using
Fig. 4. Tensile strength for PBT/PC/GNPs composites filled with 3 vol% GNPs for various PBT/PC volume ratios.
ωa =
γ1p γ2p ),
Table 1 Contact angle of probe liquids on PBT, PC, and GNPs surfaces. Probe liquid
(1)
Water Diiodomethane
where γPBT − GNPs , γPC − GNPs , and γPBT − PC are the interfacial tensions 71
Contact angle (°) PBT
PC
GNPs
88.8 38.6
94.4 42.2
68.3 27.2
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Table 2 Surface energy of PBT, PC, and GNPs at 20 °C and 240 °C. Component
PBT PC GNPs
20 °C (mN/m)
240 °C (mN/m)
γ
γd
γp
−
43.2 40.1 46.8
42.3 39.7 38.4
0.9 0.4 8.4
−0.08 [18] −0.04 [19] −0.16 [20]
dγ (mN/mK) dT
γ
γd
γp
χp
25.6 31.3 11.6
25.1 31 9.5
0.5 0.3 2.1
0.02 0.01 0.18
γ = γ d +γp and χ p =γp /γ; χ p is polarity and is independent of temperature.
In summary, all the above results demonstrate that GNPs will preferentially be distributed in the PBT phase. Is that true? The following experiments will further verify the prediction. 3.2.2. Experiment on migration and distribution of GNPs To verify the migration and distribution of GNPs in PBT/PC/GNPs composites, we investigated two processing factors: feeding sequence and mixing time. The volume ratio of the PBT/PC blend matrix was fixed at 50:50 during all of these experiments.
Fig. 5. η* as a function of frequency at the melting temperature of 240 °C for PBT, PC, and PBT/GNPs and PC/GNPs composites.
3.2.2.1. Feeding sequence. For the preparation of the PBT/PC/GNPs composite with a controlled distribution of GNPs, different feeding sequences were employed. First, PC or PBT was mixed with GNPs for 10 min to obtain a PC-GNPs or PBT-GNPs composite. Then, each previously prepared composite was further mixed for 10 min with the second resin. The prepared composites are denoted as PC-GNPs/PBT and PBT-GNPs/PC, respectively. Fig. 6 displays SEM images of PBT/PC/GNPs series composites with different feeding sequences. The feeding sequence exerts a great influence on the domain size of the PBT/PC/GNPs composite. In contrast to the neat PBT/PC blend (see Fig. 6a), the domain sizes of PC and PBT phases become smaller and more uniform after GNPs are added, because the addition of the fillers changes the rheological properties of the polymer phases. When GNPs are mixed with PBT in advance, the viscosity of the PBT/GNPs phase with the PC phase is reduced and the ability of the matrix to transmit shear stress is increased. This is favourable for obtaining satisfactory dispersion. Therefore, the domain size of the PC phase is smallest, as shown in Fig. 6b. In turn, when GNPs are mixed with PC in advance, this inhibits PC phase dispersion. The domain size of the PC phase is greater and looks rough (see Fig. 6c). When PBT, PC, and GNPs are simultaneously melting mixed, the domain size of PC falls between that of PBT-GNPs/PC and PC-GNPs/PBT composites. Fig. 7 shows the electrical and thermal conductivities of PBT/PC/ GNPs series composites prepared with different feeding sequences. The electrical conductivity of the ternary composites increases by eight orders of magnitude above that of the binary blends of PBT/PC. At the same time, the thermal conductivity increases by ∼71% compared to that of PBT/PC blends. For the ternary simultaneously melt-mixed composite, the thermal and electrical conductivities are slightly higher than those of the other two, but the difference is not obvious. This fact illustrates that the feeding sequence does not affect the fundamental double continuous phase or alter the distribution of the filler. The tensile strength of PBT/PC/GNPs series composites with different feeding sequences is shown in Fig. 8. All of the ternary composites have higher tensile strength than that of the binary PBT/PC blend. GNPs have a reinforcing effect on the PBT/PC blend due to the selective
distribution of GNPs greatly improved efficiency of the formation of force transferring network [21]. And the feeding sequence has little influence on tensile strength. This shows that the ternary composites not only improve both thermal and electrical conductivities but also maintain a high tensile strength of 50 MPa, which provides favourable conditions for its practical application.
3.2.2.2. Mixing time. Because the interfacial tension between PBT and GNPs is less than that between PC and GNPs, will GNPs migrate during the processing? Therefore, a PC-GNPs/PBT composite with various mixing times (4, 7, and 10 min, respectively) was selected to clarify this issue. The mixing time refers to the time after adding PBT into the premixed PC/GNPs melts. Fig. 9 presents the testing results for the electrical and thermal conductivities of PC-GNPs/PBT composites prepared with different mixing times. Both the electrical conductivity and thermal conductivity of the composite increase with the mixing time and the electrical conductivity changes more significantly. Therefore, we explain this phenomenon based on the electrical conductivity. Fig. 9 shows that the mixing time exerts a significant influence during the early stages of mixing. For a mixing time change from 4 to 7 min, the electrical conductivity of the composite changes from 10−16 to 10−10 S/cm, an increase of six orders of magnitude, whereas when the mixing time is further prolonged from 7 to 10 min, the electrical conductivity of the composite only increased by one order of magnitude. For this mixing system, 10 min is enough. This is very meaningful for the actual process design of a polymeric composite. What occurred during the mixing process? Fig. 10 presents the SEM images (a1, b1, and c1) of PC-GNPs/PBT composites at different mixing times and exhibits schematic diagrams for the migration process of GNPs from the PC phase to the PBT phase (a2, b2, and c2). The GNPs are distributed in the PC phase at the beginning under a mixing time of 4 min. It is difficult for GNPs to migrate from the PC phase to the PBT phase during such a short mixing time. In this case, therefore, the conductivity of the composite is lower because the GNPs have been wrapped and isolated. As the mixing time increases, more and more
Table 3 Interfacial tension and wetting coefficient of the materials at 240 °C. Component
ϒPBT − PC (mN/m)
ϒPBT − GNPs (mN/m)
ϒPC − GNPs (mN/m)
ωa
Harmonic mean Geometric mean
0.5 0.3
8 4.4
12.5 6.9
−9 −8.3
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Fig. 6. SEM images of PBT/PC/GNPs composites: (a) PBT/PC, (b) PBT-GNPs/PC, (c) PC-GNPs/PBT, and (d) PBT/PC/GNPs.
Fig. 7. Electrical conductivity and thermal conductivity of PBT/PC/GNPs composites with different feeding sequences. Fig. 8. Tensile strength of PBT/PC/GNPs composites with different feeding sequences.
GNPs migrate through the blend interface into the PBT phase, whereas some of them remained at the interface. When the mixing time reached 10 min, most of the GNPs migrated from the PC phase to the PBT phase and embedded in it to achieve thermodynamic equilibrium state [22]. At this time, GNPs distributed in the amorphous region of PBT more easily formed an effective conductive network, resulting in a significant increase in electrical conductivity. In summary, the kinetic experimental result also indicates that GNPs are preferentially selectively distributed in the PBT phase during the melt mixing, which is consistent with the evaluation by using the wetting coefficient.
photographs of etching solution bottles after PBT/PC/GNPs composites were dissolved for one week with a CH2Cl2 etching agent. In this experiment, CH2Cl2 served as an etching agent for PBT/PC/GNPs composites because it can dissolve PC effectively but cannot dissolve both PBT and GNPs. The percentage of the etched weight of the PC component was evaluated by using the following expression:
CI =
3.3. Selective etching experiment
Wi − Wf WPC
× 100%,
(5)
where wi and wf are the weights of the sample before and after solvent etching, respectively, and wPC is the original weight of the PC component in the sample before etching. According to Eq. (5), the quality of the PC components etched off accounted for 44%, 87% and 90% of the
The distribution of GNPs in the PBT/PC blend was also examined by conducting a selective etching experiment. Fig. 11 presents digital 73
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Fig. 11. Digital photograph of etching solution bottles after the PBT/PC/GNPs composites were dissolved. The PBT/PC volume ratios in the matrix are (a) 70/ 30, (b) 50/50, and (c) 30/70.
Fig. 9. Electrical conductivity and thermal conductivity of PC-GNPs/PBT composites. The mixing time refers to the time after adding PBT into the premixed PC/GNPs melts.
Fig. 12 presents the TG curves of the neat PBT, PC, and GNPs and PBT/ PC/GNPs composites with various PBT/PC volume ratios. The residual weights of PBT, PC, and GNPs at 800 °C were 4.3, 25.3, and 97 wt%, respectively. Obviously, the order of thermal stability is GNPs > PC > PBT. For the composites, on the one hand, the residual weight of the composite before etching (A1, B1, and C1) was slightly higher than that of the composite after etching (A2, B2, and C2). This is mainly attributable to the contribution of PC content after etching. On the other hand, for a case study of 50/50 PBT/PC, based on the residual weight of neat PBT, PC, and GNPs, the theoretical residual weight of the sample before etching can be calculated. Compared with the experimental values of TG analysis, the theoretical residual weight (18.8 wt%) was very close to the experimental value (17.9 wt%). This establishes the accuracy of the theoretical calculation. Therefore, if we assume that all of the GNPs were in the PBT phase, the theoretical residual weight of the sample after etching can be calculated. By comparing the theoretical residue (14.8 wt%) with the experimental value (14.7 wt%) after etching, we find that the result is similar to that before etching. The experimental result directly illustrates that GNPs are mainly distributed in the PBT.
original PC mass in the corresponding PBT/PC (70/30, 50/50 and 30/ 70) blend, respectively. Obviously, most of the PC can be etched by CH2Cl2, especially for PBT/PC at ratios of 50/50 and 30/70. As shown in Fig. 11, the clear solution and the intact sample of 70/ 30 PBT/PC clearly indicate that few GNPs are distributed in the PC phase. A slightly black solution was observed when the volume ratio of PBT/PC was 50/50 in the matrix, but the sample was still intact; this indicates that most of the GNPs are distributed in the PBT phase and that the PBT phase is self-supported based on the co-continuous structure. For the sample of 30/70 PBT/PC, the sample framework collapsed because the continuous PC phase dissolved and the solution is light black in appearance; this fact indicates that some GNPs exist in the PC phase but that most of the GNPs are segregated in the PBT phase. These experimental results prove that GNPs have a segregated distribution in PBT. To confirm the distribution of GNPs further, the PBT/PC/GNPs composites before and after etching were analysed by TG analysis.
Fig. 10. SEM images of PC–GNPs/PBT composite with mixing times (tm) of (a1) 4 min, (b1) 7 min, and (c1) 10 min, and schematic presentations of the corresponding migration process of GNPs: (a2), (b2), and (c2). 74
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[2]
[3]
[4]
[5]
[6]
[7]
[8]
Fig. 12. TG curves for PC, PBT, GNPs, and PBT/PC/GNPs composites with various PBT/PC volume ratios. (A1), (B1), and (C1) and (A2), (B2), and (C2) are before-etched and after-etched composites, respectively.
[9]
[10]
4. Conclusions [11]
The selective distribution of GNPs in PBT/PC blends and its effect on electrical, thermal, and mechanical properties of the composites have been systematically studied. Thermodynamic theory suggests that GNPs are preferentially distributes in the PBT phase with better interfacial wettability. Experimental investigation of controlling the feeding sequence and mixing time and performing selective etching experiments showed that the GNPs had a segregated distribution in the PBT phase. The thermal conductivity and electrical conductivities of PBT/PC/GNPs composites can be effectively improved with the segregated distribution of GNPs in a co-continuous structure. Compared to the binary composite, the conductivity of PBT/PC/GNPs composite filled with 3 vol% GNPs increased by eight orders of magnitude and the thermal conductivity was enhanced by ∼10%. This shows that, in a composite system, improving the thermal conductivity of the composite is more challenging than improving the electrical conductivity. Meanwhile, the composite maintains a tensile strength of 50 MPa, making it suitable for many applications.
[12] [13]
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[21]
Acknowledgment [22]
This work was financially supported by the Project of the Innovative Research Team of Polymeric Functional Film of Beijing Technology and Business University (19008001071). References [1] J. Chen, X. Cui, K. Sui, et al., Balance the electrical properties and mechanical
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