Significant Enhancement of Thermal Conductivity in Polymer

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Significant enhancement of thermal conductivity in polymer composite via constructing macroscopic segregated filler networks Hongju Zhou, Hua Deng, Li Zhang, and Qiang Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07947 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 12, 2017

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ACS Applied Materials & Interfaces

Significant enhancement of thermal conductivity in polymer composite via constructing macroscopic segregated filler networks Hongju Zhou,ab Hua Deng,a* Li Zhang,a Qiang Fua*

a

College of Polymer Science and Engineering, State Key Laboratory of Polymer

Materials Engineering, Sichuan University, P.R. China

b

Patent Examination Cooperation Center of the Patent Office, SIPO, Sichuan, P.R.

China

*Corresponding authors. Tel.: +86 28 8546 0953 (H. Deng). Tel./fax: +86 28 8546

1795 (Q. Fu). E-mail addresses: [email protected] (H. Deng), [email protected] (Q. Fu).

KEYWORDS: segregated structure, polymer composites, thermal conductivity, electrical conductivity, EMI shielding ability

ABSTRACT：The low efficiency of thermal conductive filler is an unresolved issue in the area of thermal conductive polymer composites. Although it is known that minimizing phonon or electron interfacial scattering is the key for achieving high thermal conductivity, the enhancement is generally limited by preparation methods that

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can yield the ideal morphology and interfaces. Herein, low temperature expandable graphite (LTEG) is added into a commercial impact modifier (Elvaloy®4170), which is then coated onto poly(butylene terephthalate) (PBT) particles with various sizes at millimeter scale between their melting temperatures. Thus, macroscopic segregated filler networks with several considerations are constructed: high LTEG loading leads to short distance between fillers and robust filler network; continuous Elvaloy-LTEG phase leads to continuous filler network and good interaction among filler and matrix leads to good interfacial interaction. More importantly, the rather large size of PBT particles provide the filler networks with low specific interfacial area, which minimizes the interfacial scattering of phonons or electrons. Relative to homogeneous composites with an identical composition, the thermal conductivity is enhanced from 6.2 to 17.8 W/mK. Such an enhancement span is the highest compared with results reported in the literature. Due to possible “short-cut” behavior, much higher effectiveness can be achieved for current system than literature results when Elvaloy-LTEG phase is considered as filler, with the effectiveness even exceeding the upper limit of theoretical calculation for highly loaded Elvaloy-LTEG phase with relative large PBT particle sizes. This could provide some guidelines for the fabrication of highly thermal conductive polymer composites as well as multi-functional polymer composites.


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1. Introduction Thermal conductive polymer composites have been a very active research area recently. As thermal management in electronic devices becoming increasing more important to meet the demand of ever decrease in device size as well as complexity in structure.1-2 Neat polymer is often lack of thermal conductivity with a typical value of below 0.4 W/mK, 3-5 highly thermal conductive fillers are added into polymer matrix to enhance their thermal conductivity. However, these highly conductive fillers with intrinsic thermal conductivity of typically above 100 W/mK often only lead to little increase in thermal conductivity for final composites 5, far below their expectations. It was proposed that the scattering of phonons at interfaces, low thermal conductivity of polymer matrix as well as enormous specific surface area of various nanofillers could be responsible 6-7.

Theoretically, the most ideal scenario for thermal conductivity enhancement in polymer composites is described by rule of mixture (or parallel model ) 8, where the filler with rather high thermal conductivity and polymer matrix with rather low thermal conductivity contribute independently to the overall thermal conductivity of final composites. Meanwhile, the worst scenario is described by series model 8, where the enhancement in thermal conductivity is only confined locally in polymer matrix. Nanofillers filled polymer composites are often reported to be between these two scenarios, with data approaching series model 5. It is thought that the discontinuous filler network, rather large interfacial thermal resistance and short distance phonon



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needs for transportation (unlike in electrical conductive polymer composites, where electron can “jump” between local networks), are responsible. Therefore, continuous and robust filler networks with minimum amount of specific surface area as well as small interfacial thermal resistance are needed to achieve efficient contribution from these fillers. To tackle these three issues mentioned above, good interfacial interaction between filler and matrix, continuous network or thermally conductive phase, short distance between highly thermal conductive fillers are needed. Several strategies have been employed in literature 9, including enhancing the thermal conductivity of polymer matrix 10-11, surface modification of fillers 12-15, hybrid fillers 16-21, orientation of fillers 22

, and selective distribution of fillers 23-26. However, the range of enhancement and

efficiency achieved for various fillers through using these methods is still inadequate, especially for achieving highly thermal conductive polymer composites. Therefore, more study is needed to realize high thermal conductivity, while maintaining high efficiency for these highly thermal conductive fillers.

Meanwhile, the morphological control of various functional fillers in polymer matrix have been demonstrated as crucial for various functionalities, such as: electrical conductivity 27-30, EMI shielding 31-33, strain sensing 34-35, dielectric properties 36-37, etc., since these functionalities of polymer composites depends heavily on the morphology of these functional fillers. These morphological control methods include polymer blends 24, 38-39, segregated structure 40, orientation 41, mixed filler

23, 39

, annealing

42-43

,

etc. Nevertheless, these methods are shown to have limited effect on the thermal conductivity of polymer composites in the literature 44-45. As one of these morphological


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control methods, the formation of segregated architecture is widely reported to be able to reduce the electrical percolation threshold, thus improving electrical conductivity. In such segregated architecture, fillers are distributed only at the interfaces between polymer granules often with nano or micrometer domain size. This leads to selective localization of rather large amount of filler only at the interface between polymer granules, triggering the formation of continuous network at rather low overall filler concentration 31, 46-47. However, the enhancement in thermal conductivity is still limited, due to poor interfacial interaction between filler and polymer, and construction of poor thermally conductive network 25, 48-51, which fails to provide the “short cut” for thermal transportation. Then, these dispersed highly thermally conductive fillers are more contributing their thermal conductivity locally for the final composites, which is described by series model. Another issue for these studies containing nano or micro meter size domain is their rather large specific surface area or interfacial area, where phonons could have significant scattering, thus, the overall thermal conductivity is hindered. Therefore, more study is still needed on realizing issues including: good interfacial interaction between filler and matrix, continuous network or thermally conductive phase, short distance between highly thermal conductive fillers or robust filler network and reduced specific surface area of thermal conductive networks within polymer matrix.

Thus, system consisting of poly (butylene terephthalate) (PBT) and Elvaloy, a terpolymer consisting of ethylene, butyl acrylate and glycidyl methacrylate, with low temperature expandable graphite (LTEG) as thermal conductive filler is studied.



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Selective distribution of filler is achieved through blending higher melting temperature PBT pellets with diameter in the range of mm, with lower melting temperature Elvaloy containing LTEG, between their melting temperatures to allow the structure of PBT pellets intact and rather low specific interfacial area between PBT and Elvaloy-LTEG phase

52-53

. Thus, highly loaded continuous Elvaloy-LTEG phase can provide the

system with highly thermal conductive and continuous phase with low specific interfacial area. The good interaction between Elvaloy & LTEG, and Elvaloy & PBT is thought to enhance the interfacial interaction and wetting between filler and matrix in the system. Therefore, above three issues are all considered. Electrical conductivity and EMI shielding ability of these composites could be significantly altered through selectively localizing functional filler. PBT particle size and LTEG concentration in Elvaloy is altered to control the filler distribution. Resulted thermal conductivity, electrical conductivity as well as EMI shielding performance are systematically investigated. The mechanism responsible for the changes in these functionalities is investigated and discussed. Analytical modelling is used to analyze the thermal conductivity observed.

2. Experimental Section

2.1. Materials. PBT (Crastin® ST820 BK503), with a density of 1.22 g/cm3 ; Elvaloy®4170, a terpolymer consisting of ethylene, butyl acrylate and glycidyl methacrylate, which is a reactive elastomeric copolymer, often used as impact modifier for PBT or poly(ethylene terephthalate) (PET), with a density of 0.94 g/cm3, melt flow


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rate of 8 g/10 min (190 oC/2.16 kg) and melting temperature of 72 °C, were used. Both PBT and Elvaloy were supplied by Dupont Chemical Co. The original LTEG (ADT KP801) is supplied by Shijiazhuang ADT Carbonic Materials (China). Such LTEG is a sulfur-free EG prepared by intercalation of acetic acid. The expansion ratio is 230 mL/g, and the initial expanding temperature for the original LTEG is about 150 °C. The original LTEG exhibits a density of 2.20 g/cm3 with a particle size of around 180 µm. These properties are given by respective manufacturers.

2.2. Samples Preparation. These Elvaloy-LTEG/PBT composites were fabricated with a three-step process. Firstly, the original LTEG was placed in mufﬂe furnace to undergo rapid heating at 600 °C for 60 s, where they will experience explosive expansion. Subsequently, these LTEG powder were put into a high speed rotating mixer (25000 rpm, 3 min, supplied by Linda Mechanical Co., Ltd. China) to produce fine LTEG powders.

Secondly, Elvaloy-LTEG 20 vol. %, Elvaloy-LTEG 40 vol. %, Elvaloy-LTEG 60 vol. % composites were prepared through mixing as-prepared LTEG powders with Elvaloy pellets (pre-dried for 10 h under vacuum at 50 °C) with an internal mixer (XSS300, Qingfeng Mold Factory, China) at 170 oC, 60 rpm for 10 min. 20 vol. %, 40 vol. % and 60 vol. % represent LTEG powder content in these Elvaloy-LTEG composites. These composites are noted as Elvaloy-20%, Elvaloy-40%, Elvaloy-60%, respectively.

To prepare PBT particles with different diameters, the size of orginal PBT particles was reduced to two smaller ones through twin-screw extrusion by using different rates



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during pelitizing. These particles are noted as M-PBT, S-PBT and L-PBT (the original PBT particles), where M, S and L stands for medium, small and large, respectively. Another smaller sized PBT particle was obtained by melt-spining using high pressure capillary rheometer, named SS-PBT. The cross section of L-PBT, M-PBT and S-PBT is ellipse, with average section area of 24.94 mm2, 12.08 mm2, 5.68 mm2, respectively. The length of above three PBT particles is about 3 mm. The length of SS-PBT particles is not uniform, with average value around 4.5 mm. The cross section of SS-PBT is circle with a diameter of 0.47 mm. The aspect ratio of L-PBT, M-PBT, S-PBT and SSPBT is 1.26, 1.68, 2.45 and 6.19, respectively.

At last, Elvaloy-LTEG/PBT composites containing 50vol. % PBT with segregated architecture were fabricated by blending above Elvaloy-20%, Elvaloy-40% and Elvaloy-60% composites with PBT in the internal mixer at 170 °C, 30 rpm for 5 min, respectively. This temperature is used to prohibit the deformation of PBT particles as well as selectively melt Elvaloy-LTEG phase during mixing process, thus, allowing coating of Elvaloy-LTEG melt onto PBT particles. Respective Elvaloy-LTEG/PBT composites were also mixed in above internal mixer at 240 °C, 30 rpm for 5 min to compare with above composites. Specimens for different measurements were prepared with thickness around 4mm by hot pressing (10 MPa, 240 °C) to keep the network architecture intact.

To investigate the effect of LTEG concentration in Elvaloy-LTEG phase on the properties of above discussed composites, Elvaloy-40% and Elvaloy-60% was blended


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with 75 vol. % and 83.3 vol. % M-PBT in the internal mixer at 170°C, 30 rpm for 5 min, to fabaricate Elvaloy-40%/M-PBT 25-75 and Elvaloy-60%/M-PBT 10-50 composites, respectively. Comparing with above Elvaloy-20%/M-PBT 50-50, the overall LTEG concentration in these three composites is kept at 10vol. %, with different filler concentration in the coating phase.

To investigate the influence of PBT particle size on the functionalities of these Elvaloy-LTEG/PBT composites. Elvaloy-40%/L-PBT, Elvaloy-40%/M-PBT, Elvaloy40%/S-PBT and Elvaloy-40%/SS-PBT composites containing 10, 20, 80, 90 vol. % of Elvaloy-40% were then fabricated through blending Elvaloy-40% and PBT in internal mixer at 170 oC, 30 rpm for 5 min, respectivity. For example, 10-90 indicates that the volume fraction of Elvaloy-40% phase is 10 vol. %.

2.3. Characterization. 2.3.1. Morphology Observations. Morphological study on Elvaloy-LTEG/PBT composites was carried out with optical microscope and scanning electron microscopy (SEM, JSM-5900, JEOL, Tokyo, Japan), whose acceleration voltage is 5 kV.

2.3.2. Thermal Conductivity Measurements. The thermal conductivity of these composites was studied with a thermal constant analyzer (Hot Disk TPS 2500, Sweden) by exerting a certain electrical power on the samples. The transient plane source (TPS) method was used to obtain the thermal conductivity of a given material. The fundamental principle for measurement in TPS method is the Fourier Law of heat conduction. Specimens having diameter around 25 mm, with thickness around 4 mm



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were obtained by compression molding.

2.3.3. Electromagnetic Interference Shielding Effectiveness (EMI SE). An E5071C ENA series network analyzer (Agilent Technologies) was used to evaluate the EMI shielding effectiveness of above composites. It is conducted with a coaxial test cell (APC-7 connector) together with an Agilent N5230 vector network analyzer. Specimens having diameter of 12mm and thickness of 3mm were put into the sample holder. Such measurement was conducted between 8.2 − 12.4 GHz (X band) frequency range.

2.3.4. Surface Tension. KRUSS DSA100 in sessile drop mold was used to measure the contact angle of compression molded specimens. Contact angle was obtained on 3 mL of a given wetting solvent at around 23°C. 3 replicates were produced to obtain the mean value for each group of specimens. Surface tension, dispersion as well as polar components for above materials can be determined from such measurement.

2.3.5. Electrical Conductivity. The measurement method used for electrical conductivity was slightly modified according to previous report 53. The direct current (DC) electrical conductivity was obtained using two points method at room temperature. Silver paint was applied onto these specimens to guarantee good contact. Thus, contact resistance could be considered as negligible. The electrical resistance was characterized at voltage of 10 V or above, with a Keithley 6487 picoammeter. Conductivity is calculated using:  = l/Rwt, where R, l, w, t, and is the measured resistance, length, width and thickness of the specimen, respectively. Conductivity below 10-6 S·cm is


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considered as insulating due to the limit of current set-up.

2.3.6. Infrared thermal imaging. The infrared thermal imaging was measured by a Fluke infrared thermal imager (Ti 27). The samples having a diameter of 12 mm with thickness of 3mm were first put onto a hot stage (Linkam GS315) which was heated to 80 oC already, then infrared thermal images were taken immediately and every 40 seconds subsequently.

3. Results and Discussion

3.1. Effect of PBT particle size. The experimental procedure is summarized in Figure 1. The optical photos of PBT particles are shown in Figure 2. It is obvious that the size of L-PBT, M-PBT, S-PBT and SS-PBT particles decreases in sequence, and their aspect ratio increases as specified in the experimental part.

Figure 1. Sketch for preparation process of Elvaloy-LTEG/PBT composites having segregated network.



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Figure 2. (above) Optical images of L-PBT, M-PBT, S-PBT, SS-PBT, and (below) Elvaloy-LTEG/PBT composites with diferent PBT particle sizes and LTEG concentration in Elvaloy-LTEG phase.

The cross section of Elvaloy-LTEG/PBT composites is also shown in Figure 2. The white section represents PBT phase while the black sections represents ElvaloyLTEG phase. It is clearly demonstrated that typical segregated architecture or selective filler distribution has been achieved, where PBT particles are coated and surrounded with Elvaloy-LTEG phase. The size of PBT phase decreases and the number of PBT


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phase increases with decreasing size of PBT particle. Meanwhile, it is speculated that nearly all LTEG are dispersed in Elvaloy phase due to the white color of PBT phase, which indicates that LTEG are not migrated to PBT phase during processing. Thus, the distribution of LTEG can be effectively controlled by selectively disperse LTEG in Elvaloy phase as well as regulating the morphology and scale of the segregated architecture through using PBT particles with different size. As shown in Figure 3, SEM was used to further study such segregated structure. It can be observed that the boundary between these two phases is clear in these composites. Such phenomenon further verifies that Elvaloy-LTEG phase is not migrated into PBT phase, which agrees well with Figure 2.

Figure 3. SEM micrographs of the segregated network in Elvaloy-LTEG/PBT composites with L-PBT, M-PBT, S-PBT and SS-PBT, respectively. The concentration marked on the left is the overall LTEG concentration in the composites.



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It is understood that the interfacial wetting between Elvaloy-LTEG phase and PBT could influence the degree of coating on PBT particles, therefore, have important influence on the final properties of these composites. Thus, such issue is studied with contact angle measurement. By employing harmonic-mean and geometric-mean equations 53-54, the interfacial energies (see Table 1) between these two phases can be determined using the surface tensions from respective components. The harmonicmean equation (eq1) is often thought to be valid among low energy materials and the geometric-mean equation (eq2) is often thought to be valid among high energy materials and low energy materials:

  1d  2d

 12   1   2 -4 

d d  1   2

1  2  p



p



1   2  p

p

 12   1   2  2(  1  2   1  2 d

d

p

p

(1)

(2)

where γ1, γ2 are the surface tension for 1 and 2 component; γ1p, γ2p are the polar part of surface tension for components 1 and 2; and γ1d, γ2d are the dispersive part of surface tension for 1 and 2 component. For composites containing segregated filler architecture, the wetting coefficients obtained are positive as shown in Table 1. This is caused by the fact that the interfacial energy between Elvaloy-LTEG and PBT increases with increasing filler concentration of Elvaloy-LTEG, indicating worse wetting between Elvaloy-LTEG and PBT at higher LTEG content.

To understand the overall thermal conductivity of these composites better, the thermal conductivity of these composites without PBT particles is characterized first.


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Figure S1 shows the variation in thermal conductivity for Elvaloy-LTEG composites as a function of LTEG concentration. The thermal conductivity increases with increasing volume fraction of LTEG up to 60 vol. %, with thermal conductivity reaching 20 W/mK while filled with 60 vol. % LTEG. The slope of the curve increases till 40 vol. % and then slightly decreases. These composites are then used to coat PBT particles to act as continuous phase in the composites.

Table 1. Surface tension and interfacial energy of PBT, Elvaloy-LTEG specimens with various LTEG concentrations. Surface tension (mN/m) Name Total (γ)

Dispersive part (γd)

Polar part (γp)

Interfacial energy with PBT(mN/m) Method used for calculation harmonic

geometric

PBT

32.47

0.44

32.43

/

/

Elvaloy20% Elvaloy40% Elvaloy60%

27.69

0.21

27.25

0.36

0.10

28.21

0.25

26.85

1.12

0.76

27.66

0.1

26.79

1.29

1.00

The dependence of thermal conductivity of Elvaloy-LTEG/PBT composites on the overall filler concentration is shown in Figure 4. The thermal conductivity of these composites greatly increases with increasing overall filler concentration. By comparing composites with the same overall filler concentration but different filler distribution, the thermal conductivity of Elvaloy-LTEG/PBT 50-50 240oC is much lower due to higher interfacial thermal resistance caused by rather homogeneous filler distribution. Unlike Elvaloy-LTEG/PBT 240oC composites, Elvaloy-LTEG/PBT 50-50 composites



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containing PBT particle in different sizes show significantly higher thermal conductivity than Elvaloy-LTEG composites at the same filler concentration, especially the thermal conductivity of Elvaloy-60%/L-PBT composites is close to that of Elvaloy60% composites with the filler concentration of the former is only half of the latter. This might be caused by the higher effective filler concentration due to excluded volume effect or possible “short-cut” effect provided by the continuous filler network, the detailed mechanism will be discussed in the following part. Additionally, it is interesting to note that the segregated structure containing L-PBT and M-PBT displays much higher thermal conductivity than PBT particles with smaller sizes, especially when the overall LTEG concentration is 30 vol. %. It is thought that reducing PBT particle size would result in less robust coating phase due to increased specific surface area of PBT particles. Thus, less continuity of LTEG thermal conductive network is obtained with smaller PBT particles, which leads to much lower thermal conductivity. On the other hand, the aspect ratio of PBT particles increases with decreasing size of PBT particles as shown in Figure 2. Higher aspect ratio of PBT particles could more easily cut off the thermal conductive phase, namely Elvaloy-LTEG phase. Thus, hindering heat conduction and leading to lower thermal conductivity. Therefore, larger PBT particle sizes lead to higher thermal conductivity.


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Figure 4. Thermal conductivity of Elvaloy-LTEG/PBT 50-50 composites as a function of overall LTEG concentration and size of PBT particles. Please note that 10 vol. %, 20 vol. % and 30 vol. % are the overall LTEG concentration in the composites, and Elvaloy-LTEG/PBT 240

o

C is homogeneous composites with filler randomly

distributed.

Moreover, comparing Elvaloy-LTEG/PBT 240oC with composites containing segregated architecture, the thermal conductivity enhancement caused by segregated architecture can be calculated. The thermal conductivity is largely enhanced by introducing such segregated architecture, especially for Elvaloy-60%/L-PBT and Elvaloy-60%/M-PBT composites, the enhancement reaches as high as 187.09% and 166.13 %, respectively. This verifies that LTEG network in such segregated



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architecture are more effective at dissipating heat. Meanwhile, such large range of thermal conductivity from 6.2 to 17.8 W/mK for composites with identical composition has never been reported to the best of our knowledge. Therefore, a systematic comparison is made by summarizing results in literature as shown in Table 2. It can be noted that the range of thermal conductivity achieved for Elvaloy-60%/L-PBT 50-50 and Elvaloy-60%/M-PBT 50-50 composites are much larger than the results reported in literature, where methods including hybrid fillers, segregated network by mechanical grinding, filler surface modification and filler selective distribution have been used. This illustrates that such method used in this study is a very effective way to largely enhance the thermal conductivity of polymer composites.

To confirm the thermal conductivity observed above, infrared thermal images are taken for composites containing overall 30wt.% LTEG as shown in Figure 5 and S1. These specimens are placed onto a hot surface, the sample surface temperature increases with increasing exposure time. Specimens with higher thermal conductivity illustrate lower surface temperature due to better heat dissipation in the sample. This is confirmed in Figure S2, where the higher thermal conductivity phase (Elvaloy-LTEG) illustrates lower surface temperature, and lower thermal conductivity phase (PBT phase) illustrate higher surface temperature.


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Figure 5. The infrared thermal images of Elvaloy-60%/PBT composites.

As shown in Figure 5, Elvaloy-60%/PBT 50-50 composites containing L-PBT and M-PBT clearly demonstrate much lower surface temperature due to their higher thermal conductivity than the rest. This agrees well with the trend shown in Figure 4. Moreover, the temperature distribution on the surface for Elvaloy-60%/PBT 50-50 composites containing PBT particles with different size is not uniform, especially for L-PBT and M-PBT. This is caused by the unevenly distributed filler with macroscopic segregated network, where the part with lower surface temperature could be the part with more filler networks, thus better heat dissipating ability.



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Table 2. A comparison of the enhancement in thermal conductivity achieved for polymer composites between various methods. 14, 24-25, 50, 55-62

Methods

Segregated Structure

CoolingPressing

Hybrid Fillers

Filler Surface Modification

Filler Selectively Distribution

Article

Polymer Composites

Filler Content

Thermal Conductivity

Enhancement (%)

Before

After

20vol. %

1.82

4.5

147.25

20vol. %

1.82

4.3

136.26

30vol. %

6.2

17.8

187.09

30vol. %

6.2

16.5

166.13

This paper

4170-40%/L-PBT 50-50

This paper This paper This paper

4170-40%/M-PBT 50-50 4170-60%/L-PBT 50-50 4170-60%/M-PBT 50-50

Ref. 50

PP/AlN

20vol. %

0.46

0.5

14.13

Ref. 62

Graphite/PP

21.2vol.%

1.65

5.4

227.2

Ref. 25

UHMWPE/GNPs

20vol. %

4.05

4.65

14.81

Ref. 57

PVDF/Barium Titanate/SiC

60vol. %

1.05

1.67

59.05

Ref. 58

Epoxy/GNP/SWNT

20wt. %

2.75

3.35

21.82

Ref. 59

Epoxy/BNNS/AgNP s

25vol. %

1.6

3.05

90.62

Ref. 14

PVA/h-BN

20vol. %

5.5

7.6

38.18

Ref. 60

Epoxy/CNF/SiC

20vol. %

1.09

1.15

5.50

Ref. 61

Epoxy/EG

20wt. %

4

5.8

45.00

Ref. 55

PS/Si3N4

20vol. %

0.9

1.9

111.11

Ref. 56

PPS/GNPs

20wt. %

1.275

1.85

45.10

Ref. 24

PS/PVDF/SiC

23.1vol.%

0.86

1.88

118.60

To compare the electrical conductivity with thermal conductivity of these composites, Figure 6 demonstrates the electrical conductivity as a function of overall LTEG volume fraction in Elvaloy-LTEG, Elvaloy-LTEG/PBT 50-50 and Elvaloy-


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LTEG/PBT 50-50 240oC composites. Nearly all composites show higher electrical conductivity with increasing overall filler concentration. At the same filler concentration, the conductivity of Elvaloy-LTEG/PBT 50-50 composites are higher than Elvaloy-LTEG composites, which indicates that segregated architecture is beneficial for constructing conductive networks. Such effect is especially obvious for composites containing 20 vol. % LTEG. Comparing with Elvaloy-LTEG/PBT 50-50 240oC composites, segregated architecture shows similar electrical conductivity, except for 10 vol. % LTEG content. In addition, segregated architecture with L-PBT and MPBT particles illustrates higher electrical conductivity than composites containing other PBT particles, especially when the overall filler concentration is 30 vol. %. This agrees with the trend for thermal conductivity of these composites. For Elvaloy-LTEG/PBT 50-50 240oC composites containing 10 vol.% LTEG, much higher electrical conductivity is observed comparing with the rest of these composites. It is opposite to the normal phenomenon observed for polymer composites, where segregated filler network provide higher electrical conductivity than homogeneously distributed filler network. Such behaviour is confirmed by the better EMI shielding ability for that particular composition shown in Figure S3a. However, the mechanism is still unclear. It might be caused by the enhanced dispersion of LTEG at that particular ratio between filler, compatibilizer and polymer matrix. Thus, the formation of conductive network is observed at rather low filler content. Meanwhile, the thermal conductivity for Elvaloy-LTEG/PBT 50-50 240oC composites containing 10 vol.% LTEG is still quite low comparing with the rest of these composites containing the same filler content,



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indicating quite different behaviour between thermal conductivity and electrical conductivity. It is thought that thermal conductivity requires more robust filler network for the transportation of phonons comparing with electrical conductivity, where the reduced interfacial area in these macroscopic segregated networks is beneficial for enhancing thermal conductivity. In general, it also shows that the selective distribution of LTEG has significant influence on the electrical conductivity of these composites. With identical composition, 8-9 orders of magnitude difference in electrical conductivity could be achieved.

Figure 6. Electrical conductivity of Elvaloy-LTEG/PBT composites. 10%, 20% and 30% are the overall LTEG concentration in these composites.

Figure S3 shows the EMI SE of Elvaloy-LTEG/PBT 50-50 and ElvaloyLTEG/PBT 50-50 240oC composites in X-band (8.2-12.4 GHz). As well known, EMI SE is closely related to the filler concentration, electrical conductivity and


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microstructure of the composites. It has been reported that segregated architecture can enhance the EMI shielding effectiveness of the composites, because incoming electromagnetic waves could be trapped in the segregated structure by multiple reflection 31, thus, improving the shielding effectiveness. Meanwhile, higher electrical conductivity often leads to enhanced EMI SE. As shown in Figure S3a, higher EMI SE is obtained for Elvaloy-20%/PBT 50-50 240oC composites due to its higher electrical conductivity. It is interesting to observe that Elvaloy-20%/S-PBT 50-50 is able to illustrate similar EMI shielding ability with much lower electrical conductivity. Meanwhile, Elvaloy-LTEG/PBT 50-50 composites shows much better EMI ability than Elvaloy-LTEG/PBT 50-50 240oC as LTEG loading increases to 20 vol. % and 30 vol. %, which is mainly attributed to the increased electrical conductivity and segregated structure of Elvaloy-LTEG/PBT 50-50 composites. Moreover, it should be noted that EMI SE of Elvaloy-40%/PBT 50-50 and Elvaloy-60%/PBT 50-50 composites fabricated with L-PBT and M-PBT are obviously higher than other smaller PBT particles, especially the EMI SE of Elvaloy-40%/M-PBT 50-50 and Elvaloy-60%/MPBT are the highest at the same filler concentration, reaches as high as 55dB and 90dB, respectively. Comparison with the results in literatures was shown in Table S1, Elvaloy60%/PBT 50-50 composites containing L-PBT and M-PBT are among the highest EMI SE achieved for polymer composites. Issues including the formation of segregated structure, high filler content and relative large PBT particle size are thought as responsible. PBT particle in such size range might be able to provide adequate chamber for electromagnetic wave to generate resonance. Thus, much enhanced EMI ability



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could be observed.

Figure 7. OM and SEM micrographs of Elvaloy-LTEG/M-PBT composites with different LTEG concentration of Elvaloy-LTEG. (a), (e) Elvaloy-20%/M-PBT 50-50 composites; (b), (f) Elvaloy-40%/M-PBT 25-75 composites; (c), (g) Elvaloy-60%/MPBT 10-50 composites; (d), (h) Elvaloy-20%/PBT 50-50 240oC composites. The overall filler concentration of the above composites is 10 vol. %.

3.2. The effect of LTEG content in the coating phase. Several composites are fabricated containing M-PBT with different coating phase: Elvaloy-20%, Elvaloy-40% and Elvaloy-60%, where the overall filler concentration is kept at 10 vol. % by controlling the ratio between two phases. Thus, the overall filler distribution is changed through such method. Figure 7 shows optical micrographs and SEM images of ElvaloyLTEG/M-PBT composites. These images clearly demonstrate that segregated architecture has been constructed and the coating phase becomes thinner with increasing filler concentration in Elvaloy-LTEG phase. In Elvaloy-60%/M-PBT 10-50 composites, the interface between these two phases is still quite obvious. For Elvaloy-


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20%/M-PBT 50-50 240oC composites, LTEG are dispersed homogeneously in PBT/Elvaloy blends.

As shown in Figure 8a, comparing with Elvaloy-20%/M-PBT 50-50 240oC composites, the thermal conductivity of other four composites with segregated architecture is obviously higher. Elvaloy-40%/M-PBT 25-75 composite demonstrates the highest thermal conductivity. Meanwhile, the electrical conductivity of these composites containing segregated architecture increases with increasing filler concentration in the coating phase as shown in Figure 8b. This indicates that even though the coating phase became thinner, the continuity of conductive network is still intact. Furthermore, Elvaloy-40%/M-PBT 25-75 composite shows the highest EMI SE in Figure S4, which agrees with the trend for thermal conductivity. The mechanism responsible for such phenomenon might arise from three issues, including the concentrate degree of filler distribution, the thickness of coating phase and the strength of interface interaction. With increasing filler concentration in coating phase, the concentrate degree of filler distribution increases, which benefits the thermal conduction and EMI SE. However, at the same time, the coating phase became thinner and the strength of interfacial interaction between PBT and Elvaloy-LTEG phase decreases. This leads to negative effect on thermal conduction and EMI SE. Therefore, the combination and interplay of these three issues leads to higher thermal conductivity and EMI SE for Elvaloy-40%/M-PBT 25-75.



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Figure 8. (a) thermal conductivity, (b) electrical conductivity characterizations of Elvaloy-LTEG/M-PBT composites with different filler concentration in coating phase: Elvaloy-20%, Elvaloy-40% and Elvaloy-60%.

3.3. Theoretical modelling on thermal conductivity. To further understand the thermal conductivity variation of these Elvaloy-LTEG/PBT composites with different PBT particle sizes in this study, the issue needs to be solved for the preparation of high performance thermal conductive polymer composites is the effectiveness of these filler on the final thermal conductivity. Therefore, the effectiveness of these filler needs to evaluated and compared with the results in literature to understand the phenomenon discussed above better. There are two basic models describing the upper and lower bound for the thermal conductivity of these composites5: rule of mixture and series


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model, respectively. For the rule of mixture model, or parallel model, it is assumed that each phase contributes independently to the overall composites conductivity, which is proportionally to its volume ratio: kc  kf f  k m  m

(4)

Where kc , k f , k m represents the thermal conductivity of the composites, filler, matrix, respectively. And the volume fraction of filler and matrix is  f and  m , respectively. In such parallel model, perfect contact between fillers with a fully percolating network is assumed, which maximize the contribution of conductive phase. For series model, it is assumed that no contact between fillers, and the contribution of filler is locally confined. Series model is described as follows:

kc 

1 (( m / k m )  (f / kf ))

(5)

As shown in Figure 9, results from current system are compared with literature as well as theoretical calculation using parallel model and series model. The thermal conductivity for LTEG and neat matrix are used as 300 W/mK65 and 0.26 W/mK (measured using the tool described in this paper), respectively. While LTEG is considered as filler, the current study is more effective at enhancing the final thermal conductivity than the results in literature. Such effect is more pronounced for composites with Elvaloy-60% based on L-PBT and M-PBT particles, while the composites based on smaller PBT particles or have less LTEG in Elvaloy phase illustrates less effectiveness. However, all data are well below the upper limit, which



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indicates the effectiveness of incorporating highly thermal conductive nano or micro sized fillers into polymer matrix is still limited. The rather large specific surface area of these fillers are thought as responsible, as thermal conductivity is often reduced due to the scattering of phonons at the interface between filler and polymer matrix. Meanwhile, Elvaloy-LTEG phase could also be considered as “filler”. Such highly thermal conductive macroscopic “filler” could provide effective enhancement in thermal conductivity due to their low specific surface area and good interfacial interaction with matrix.

Figure 9. The effective contribution from these LTEG in these composites to the final thermal conductivity. Comparison is made between current results and some data from literature53,

63-64

. Reproduced with permission from Reference 53, Copyright 2016

Elsevier. Reproduced with permission from reference 63, Copyright 2013 Elsevier. Reproduced with permission from Reference 64, Copyright 2013 Springer-Verlag.


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Figure 10. By considering Elvaloy-LTEG as “filler”, the effective contribution from these “filler” containing different amount of LTEG to the final thermal conductivity to these composites. (a) Elvaloy-20%/PBT composites; (b) Elvaloy-40%/PBT composites; (c) Elvaloy-60%/PBT composites.

As shown in Figure 10, composites based on Elvaloy-20% and Elvaloy-40% have quite effective contribution to the final thermal conductivity, with their value



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approaching the upper limit. For Elvaloy-60% containing M-PBT and L-PBT, the value is even slightly exceeding the upper limit, which might be related to several issues: reduced specific surface area by using particles with relative large size, increased relative filler concentration through constructing segregated structure, good interfacial interaction between particle and coating, as well as between LTEG and polymer matrix. More importantly, the possible “short-cut” pathway provided by the continuous filler concentrated phase for thermal conduction might be responsible for the high efficiency observed. Another issue looks quite like the characteristic of “short-cut” behaviour is the difference in effectiveness due to PBT particles size is more pronounced for Elvaloy-60% based composites comparing with Elvaloy-20% and Elvaloy-40%, as “short-cut” behaviour is more obvious while the difference in thermal conductivity between two phase is larger. In such system, it can be also considered that the contribution of the dispersed and isolated PBT phase to the overall thermal conductivity is quite inadequate due to the large difference in thermal conductivity between PBT and Elvaloy-60% phase (0.26 and 20 W/mK, respectively), as most of the thermal conductivity will be contributed by Elvaloy-60% phase. Such phenomenon is more obvious for systems containing larger PBT particles due to their smaller specific interface area, thus less scattering of phonons at the interface. Such “short cut” like behaviour is also confirmed in infrared thermal images shown in Figure 6, where composites containing L-PBT and M-PBT show that most of their sample surface with better capability to dissipating heat comparing with the rest of these composites. Thus, the overall ability of dissipating heat is improved for the composites containing L-PBT


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and M-PBT. However, detailed mechanism is still unclear and needs further investigation. These macroscopic “filler” can be used as a new method to effectively enhance the thermal conductivity of these composites. Such efficiency is much higher than traditional fillers. Systems with high viscosity (such as UHMWPE or high performance engineering plastics), where compression molding is often used as processing methods could adopt such method to obtain enhanced thermal conductivity.

4. CONCLUSIONS

To prepare polymer composites with macroscopic segregated architecture, LTEG is dispersed in Elvaloy phase firstly, then, Elvaloy-LTEG is coated onto PBT particles with different particle sizes well below the melt temperature of PBT. Thus, segregated architecture is constructed with LTEG only distributed in Elvaloy phase. The thermal conductivity of Elvaloy-LTEG/PBT composites is largely enhanced by introducing such segregated architecture. Especially the thermal conductivity of Elvaloy-60%/LPBT and Elvaloy-60%/M-PBT reaches 17.8 W/mK and 16.4 W/mK, respectively, versus 6.2 W/mK for homogeneous Elvaloy-60%/PBT composites, and 0.26 W/mK for neat polymer matrix. Such enhancement span for thermal conductivity is the highest comparing with literature results to the best of our knowledge. Moreover, the EMI shielding ability reaches as high as 90 dB for these composites containing segregated filler network in X band. The distribution of LTEG altered by different filler content in the coating phase has important influence on a number of functionalities for these composites. Theoretical method is also used to analyze the thermal conductivity for



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Elvaloy-40%/PBT composites with different PBT particle sizes. It indicates that L-PBT and M-PBT particles are beneficial for stronger thermal paths and higher thermal conductivity. Meanwhile, comparing with calculation using series model and parallel model, higher effectiveness can be obtained for current system than literature results while EG or Elvaloy-LTEG phase is considered as filler. Such effectiveness is particularly high when Elvaloy -LTEG is considered as filler, with the effectiveness even exceeding the upper limit of these calculations for Elvaloy-60%/M-PBT and Elvaloy-60%/L-PBT. Possible “short-cut” behavior is thought as responsible for this. In general, the construction of segregated structure has important influence on various functionalities of Elvaloy-LETG/PBT composites. Current study could provide some guidelines for the fabrication of highly thermal conductive polymer composites as well as multi-functional polymer composites.

Associated Content

Supporting Information

Infrared thermal images of Elvaloy-60%/L-PBT 50-50 before and after heating for different durations. Thermal conductivity of Elvaloy-LTEG composites as a function of volume fraction of LTEG. EMI shielding of Elvaloy-LTEG/PBT composites as a function of frequency. Comparison of EMI SE of composites in this work with some literature results.

Author Information


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Corresponding authors

*E-mail: [email protected] (H. Deng), Fax: +86-28-85460953, Tel.: +86-28-8546 0953.

*E-mail: [email protected] (Q. Fu), Fax: +86 28 8546 1795, Tel.: +86 28 8546 1795.

Notes

The authors declare no competing financial interest.

Ackonwledgements

We express our sincere thanks to the National Natural Science Foundation of China for financial support (51421061). H. Deng would like to thank State Key Laboratory of Polymer Materials Engineering (sklpme 2017-2-01), the Innovation Team Program of Science & Technology Department of Sichuan Province (2014TD0002) and Sichuan Province for financial support (2013JQ0008). We would like to acknowledge Prof. Wei Yang from Sichuan University for his permission to allow us to use the Fluke infrared thermal imager.

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Table of Contents 82x41mm (300 x 300 DPI)


Significant Enhancement of Thermal Conductivity in Polymer

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