Polymer 180 (2019) 121760
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Achieving high thermal conductivity and mechanical reinforcement in ultrahigh molecular weight polyethylene bulk material
Yan-Fei Huanga,b,c, Zhi-Guo Wanga, Wan-Cheng Yua, Yue Rena, Jun Leia, Jia-Zhuang Xua,∗, Zhong-Ming Lia a
College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, People's Republic of China Engineering Laboratory for the Next Generation Power and Energy Storage Batteries, Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen, 518055, PR China c Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing, 100084, PR China b
H I GH L IG H T S
with high thermal conductivity (TC) of 3.30 W/mK is achieved. • AThebulkTC UHMWPE shows an anisotropic ratio of 750%. • The formation of cylindrical crystals, highly oriented lamellae, and the monoclinic form of UHMWPE contributes to the high and anisotropic TC. • The tensile strength increases to 107.7 MPa with an increment of 158.9%. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Bulk material Structural manipulation Thermal conductivity Mechanical performance
Exploring pure polymers with high thermal conductivity (TC) is promising to overcome the side eﬀect of conventional thermally conductive polymer-based composites. However, the geometrical shape of high TC polymers was limited to the ﬁbers and/or thin ﬁlms. Here, we employed a home-made solid-state extrusion (SPE) apparatus to prepare a bulk material of ultrahigh molecular weight polyethylene (UHMWPE) with high TC. The inplane TC reached 3.30 W/mK, which was 588% higher than that of the high-pressure (HP) molded counterpart (0.48 W/mK). Moreover, the TC of SPE UHMWPE showed an anisotropic ratio as high as 750%, which favored the heat dissipation along the in-plane direction as revealed by infrared thermal imaging. The increased and highly anisotropic TC was attributed to the formation of cylindrical crystals, highly oriented lamellae, as well as the monoclinic form of UHMWPE generated during SPE processing. Beneﬁting from this unique architecture, the mechanical performance was also greatly intensiﬁed, where the ultimate tensile strength increased from 41.6 MPa for HP UHMWPE to 107.7 MPa for SPE UHMWPE. The current study opens up a new pathway to fabricate bulk polymers with simultaneous enhancement of thermal conductivity and mechanical performance, which has great potential for the applications in thermal management ﬁelds.
1. Introduction The continuing miniaturization and rapid escalation of power densities in the modern electronic devices have set an unprecedented need than ever for eﬀective thermal management to ensure system reliability. Polymers with the ease of processability, electrical insulation, and design freedom, are promising materials to be used in thermal management. However, the thermal conductivity (TC) of polymers is quite low (on the order of 0.1 W/mK) [1–5]. To tackle this issue, the most widely used strategy is to introduce thermally conductive ﬁllers
into polymer matrices . These ﬁllers could be metallic nanoparticles [7,8], carbon nanotubes [9,10], graphite , graphene [12,13], carbon ﬁber , alumina (Al2O3) [15,16], aluminum nitride (AlN) [17,18], boron nitride (BN) [19–22], beryllium oxide (BeO) [23,24], etc. In this situation, large ﬁller loading is required to achieve high TC, which inevitably causes complexity in manufacturing, increases the cost, impairs the mechanical performance, and has a negative impact on the recyclability and light weight of the ﬁnal product. Even so, the TC enhancement of polymer composites is limited as a result of large thermal interface resistance between the ﬁller and the polymer matrix
Corresponding author. E-mail address: [email protected]
https://doi.org/10.1016/j.polymer.2019.121760 Received 13 July 2019; Received in revised form 21 August 2019; Accepted 28 August 2019 Available online 29 August 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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specimen was put on a hot stage with thermal equilibrium temperature of 100 °C. Scanning Electron Microscope (SEM) Observation. The smooth cryofracture surface was etched by solution of potassium permanganate (0.5%) in a mixture of concentrated sulfuric acid (98%) and concentrated nitric acid (65%) for 72 h at ambient temperature. Subsequently, the etched surface was cleaned with dilute sulfuric acid (30%), hydrogen peroxide solution, distilled water, and acetone under ultrasonication. The cleaned and dried etched-surface was sputtered with a thin layer of gold and observed by a ﬁeld-emission SEM (Nova NanoSEM450, FEI, USA) operating at 5 kV. Structural Characterization. Two-dimensional (2D) wide-angle Xray diﬀraction (WAXD) and small-angle X-ray scattering (SAXS) measurements were carried out at the beam line BL16B1 of the National Synchrotron Radiation Laboratory, Shanghai, China. The wavelength of the light is 0.124 nm and the sample-to-detector distance was 160 mm and 5200 mm for WAXD and SAXS, respectively. The degree of orientation was calculated mathematically using a Picken's method from the (110) reﬂection of WAXD for PE [39–41]. Long period (L) was calculated using the Bragg equation, L = 2π/q*, where q* is the peak position in the 1D SAXS scattering curves [42,43]. Mechanical Performance. Tensile test of dumbbell samples with a cross section area of 2 × 6 mm2 and length of 50 mm was performed on an Instron 5967 machine (Instron, USA) at a crosshead speed of 10 mm/ min.
[10,25]. It has been reported that the TC of pure polymers is determined by the intrinsic structure factors, such as chain conﬁguration, crystallinity, crystalline form, orientation, etc.  Manipulation of the structure of polymers is thus an ideal approach to enhance TC. Molecular dynamics stimulations showed that a single polyethylene (PE) chain had an extremely high TC of ca. 350 W/mK , while the bulk PE material only showed an experimental TC of ca. 0.4 W/mK . Such a large discrepancy is ascribed to the large diﬀerence in phonon mean free path between intrachain and interchain. Having covalent bonds, lattice vibration along molecular chains is much less anharmonic than that between chains which interact via weak van der Waals force [6,28]. In the amorphous phase, polymer chains are randomly distributed and the intermolecular coupling is rather weak. This decreases phonon mean free path and brings signiﬁcant phonon scattering. The chain arrangement is relatively ordered in the crystalline phase, which brings about a long phonon mean free path and a high TC. It was proved by a recent report where the TC of UHMWPE was increased to 1.7 W/mK when the crystallinity was improved to 68.3% . Nonetheless, a large amount of polymer chain ends and entanglements exists in the crystallineamorphous interfaces, causing anharmonic scattering and lowering the TC of bulk polymers. Considering these facts, two issues should be addressed to improve the TC of pure polymers: (1) constructing more ordered structure and (2) decreasing the amounts of crystalline-amorphous interfaces. Till now, high TC has been achieved in pure polymers [30–38]. For instance, a PE nanoﬁber prepared by gel spinning showed an exceptionally high TC of 104 W/mK. A ultrahigh molecular weight polyethylene (UHMWPE) ﬁlm prepared via uni- or bi-axial stretching exhibited a TC of 65 W/mK . Such high TCs were associated with the ultra-oriented molecular chains and low number of entanglements . Although high TC was achieved, their geometrical shape was limited to the ﬁbers or thin ﬁlms, the bulk material with high TC has never been reported. To broadly expand the scope of thermal management, fabricating bulk materials with high TC is of vital importance. In this work, we proposed to prepare a highly thermal conductive UHMWPE bulk material by using a home-made solid-phase extrusion (SPE) apparatus. The TC of SPE UHMWPE was greatly enchanted to 3.30 W/mK, which was 588% higher than that of normal UHMWPE (0.48 W/mK). The TC enhancement was ascribed to the formation of highly oriented molecular chains and the reduced number of crystalline-amorphous interface. Moreover, the mechanical performance was signiﬁcantly improved. The ultimate tensile strength increased from 41.6 MPa to 107.7 MPa.
3. Results and discussion Fig. 1a depicts the in-plane and out-plane TC of SPE UHMWPE with diﬀerent extrusion temperature. Within the selected processing window, the in-plane TC of SPE samples follows an inverted U-shaped trajectory with the extrusion temperature. The peak TC reaches up to 3.30 W/mK when extrusion temperature is 120 °C, 588% higher than that of the HP counterpart (0.48 W/mK). To the best of our knowledge, our SPE UHMWPE exhibits the highest in-plane TC among all the pure bulk polymers in the reported literatures [1–5]. And this value is even higher than those of polymer composites with high ﬁller loading [44–51]. As shown in Fig. 1b, the TC of polymer/BN composites is generally 0.1–0.4 W/mK at BN loading of 5 vol%. Due to the mismatch between the phonon vibrational spectra of thermal conductive ﬁllers and the polymer matrix, the distinct percolation in TC does not exist [10,25]. As a result, the TC of numerous polymer composites remains at a low level (< 3.0 W/mK) even at the BN content of 30 vol%. Interestingly, the through-plane TC of SPE samples is dramatically lower than the in-plane TC (Fig. 1a). To quantitively evaluate the TC variance in diﬀerent directions, the anisotropic ratio was deﬁned as (TCin-plane – TCthrough-plane)/TCthrough-plane. The HP samples show almost isotropic TC with an anisotropic ratio of 7% (Fig. 1c). On the contrary, the SPE samples present a fairly high anisotropic ratio, up to 750%. It is highly desired for practical use, since the heat could dissipate along one direction without disturbing the electronic components in the other direction. To visualize the superior thermal property of SPE UHMWPE, an infrared imaging was used to record the temperature response during heating. The sample was ﬁrst placed at room temperature (25 °C) for 2 h and then quickly transferred to a hot stage with a setting temperature of 100 °C. It could be observed from Fig. 1d that the surface temperature climbs much faster for SPE120 than HP UHMWPE. Fig. 1e displays the surface temperature as a function of time, where the surface temperature of SPE120 is always higher than those of HP UHMWPE. At 120 s, the surface temperature of SPE120 is 56.1 °C, which is 7 °C higher than that of HP UHMWPE (49.1 °C). It has been well documented that the TC enhancement is of crucial relevance to the formation of eﬀective thermal conduction pathways. For polymer composites, thermal conduction pathways are generally determined by the connection and distribution of the ﬁllers. While for
2. Experimental section Materials. Commercial UHMWPE (z1800) with a viscosity-average molecular weight of ~6.1 × 106 g/mol was purchased from Samsung General Chemicals Co., Ltd., Korea. Sample Preparation. UHMWPE was consolidated into a cylindrical parison at 200 °C and 10 MPa for 20 min. The as-prepared parison was extruded through a rectangular die with the dimension of 30 × 2 mm2 using a home-made SPE apparatus under the pressure of 130 MPa and at the extrusion temperature of 110–130 °C. Extrusion rate is slow enough to ensure the suﬃcient deformation of the parison. For comparison, the UHMWPE counterpart was prepared by hot pressing under high pressure (HP) of 130 MPa at 200 °C for 20 min. For brevity, the SPE samples were denoted as SPEx, where x is the extrusion temperature. Thermal Conductivity Measurement. Thermal diﬀusivity (α) and speciﬁc heat capacity (Cp) were measured using a laser ﬂash method (LFA467, NETZSCH, Germany) at 25 °C. Density (ρ) was measured using a densimeter (XS205, Mettler Toledo, Switzerland) with ethanol as the reference (ASTM D792). The TC was determined according to the equation: TC = α × Cp × ρ. The variation of surface temperature was recorded by an infrared thermography (IR-160P, RNO, US). The 2
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Fig. 1. (a) The in-plane and through-plane TC of SPE and HP samples. (b) Comparison of in-plane TC of our SPE samples with other polymer composites reported in literatures [44–51]. (c) The anisotropic ratio of TC of SPE and HP samples. (d) Infrared thermal images and (e) surface temperature variations of SPE120 and HP UHMWPE as a function of time.
increased number of crystalline-amorphous interface causes phonon scattering, generating a low through-plane TC (Fig. 1a). As for HP UHMWPE, only randomly distributed lamellae appear (Fig. 2d). Thermal conduction path that follows the chain conformation is circuitous, which causes small phonon mean free path and strong phonon scattering. As a result, HP sample shows a low TC. WAXD measurement was conducted to inspect the structure information on the oriented lamellae of SPE UHMWPE. Arc-like diﬀractions are found in all the SPE UHMWPE (Fig. 3a). This is a typical signal for oriented crystalline structure [52–54]. Azimuthal integration of (110) plane was performed to obtain the degree of orientation according to the Picken's method . As shown in Fig. 3b, the degree of orientation is 0.97–0.99 for SPE UHMWPE. The structure of the SPE sample is investigated from skin to core (Figure S1 and S2), and a homogeneous crystalline structure is found along the thickness direction, which is probably a result of the uniform processing ﬁeld provided by SPE technique. While for HP UHMWPE, only isotropic rings (Fig.S3) are detected with the orientation degree of 0 (Figs. S3 and 3d). As
neat polymers, the thermal conduction pathway is governed by its internal morphology and structure. To ﬁgure out the reason behind the TC enhancement of SPE UHMWPE, crystalline morphology along different directions was carefully observed by SEM (Fig. 2a). Along the inplane direction, one can easily observe the cylindrical crystals, which are composed by large amounts of spherulites closely packed along the extrusion direction. They bring more crystal-to-crystal contacts to largely reduce the phonon dissipation, being helpful for phonon transfer along the extrusion direction. It is an important factor to enhance the in-plane TC of SPE UHMWPE. The well-oriented lamellae arranged along the in-plane direction are observed in the area without the cylindrical crystals, as shown in Fig. 2b. They are induced by the high shear force of the SPE processing. When observing from the through-plane direction, the lamellae exhibit a woven pattern (Fig. 2c). Besides the shear force along extrusion direction, the compression stress in the transverse direction also forces on the molecular chains. The lamellae are thus squeezed along the transverse direction, showing a staggered pattern. Accordingly, the 3
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Fig. 2. SEM images of SPE120 (a, b) along in-plane direction, (c) along through-plane direction, and (d) HP UHMWPE sample.
− monoclinic reﬂections, namely, (200)m, and ( 2 01)m, as shown in Fig. 3c. It was reported that the monoclinic form of PE showed higher orientation than the orthorhombic form [58,59], and was usually formed when the PE melt was stretched by high stress beyond the yield point . On basis of this fact, the formation of monoclinic form may be another factor for the high in-plane TC of SPE samples. To acquire the packing structure of lamellae, SAXS measurement was performed as shown in Fig. 4. For the sample detected from the inplane direction, a pair of meridional scattering maxima appears along the equator direction (Fig. 4a1, 4b1, and 4c1). Four-leaf-clover patterns are observed when detecting from the through-plane direction (Fig. 4a2, 4b2, and 4c2). It illustrates that the lamellae present two periodic alignment, in well agreement with the SEM observation. The 1D intensity proﬁles are acquired by circularly integrating 2D SAXS patterns
reported, phonon propagation is more readily along the chain skeleton than between the inter-chains coupled by weak van der Waals interaction. High degree of orientation of SPE sample along the in-plane direction accounts for the remarkably enhanced in-plane TC and the highly anisotropic TC for SPE UHMWPE. Fig. 3c displays 1D WAXD curves of SPE samples circularly integrated from 2D WAXD patterns. For SPE110 and SPE120, two strong diﬀraction reﬂections of orthorhombic structure of PE are observed: (110) and (200), with (110) being the stronger reﬂection [55,56]. A weak diﬀraction peak appears, representing the (210) plane. A shoulder appears at q value of 13.68 nm−1, which represents the (001) plane of the monoclinic form . Hereinafter, a suﬃx, m, is used to diﬀerentiate the reﬂections of monoclinic structure from the orthorhombic structure. Diﬀerent with SPE110 and SPE120, SPE130 shows two more
Fig. 3. 2D WAXD patterns of (a) SPE110, SPE120, and SPE130 (from left to right); (b) degree of orientation for SPE and HP samples; (c) 1D WAXD curves of SPE110, SPE120, and SPE130. 4
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Fig. 4. 2D SAXS patterns of (a1) SPE110, (b1) SPE120, and (c1) SPE130 along the in-plane direction and (a2) SPE110, (b2) SPE120, and (c2) SPE130 along the through-plane direction. The corresponding 1D SAXS curves along the (d) in-plane and (e) through-plane direction.
Fig. 5. Schematic diagram of the heat transfer in SPE and HP UHMWPE.
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Fig. 6. (a) Ultimate tensile strength and (b) elongation at break of HP UHMWPE and SPE UHMWPE under diﬀerent extrusion temperatures.
HP UHMWPE to 77.5, 107.7, and 85.3 MPa for SPE110, SPE120, and SPE130, respectively (Fig. 6a). In particular, the increment of the tensile strength is 159% for SPE120. The enhanced mechanical properties are ascribed to the formation of oriented and thick lamellae. Although elongation at break reduces after SPE processing (Fig. 6b), it is much higher than that of thermal conductive polymer composites and is adequate for engineering applications. The well-balanced mechanical properties and thermal conductivity expand the application of SPE UHMWPE to some thermal management ﬁelds with harsh stress condition.
(Fig. 4d and e). The long spacing of SPE120 (42.4 nm) is much higher than that (26.9 nm) of SPE110, indicating the formation of much thicker lamellae in SPE120 (Note that the crystallinity of the two samples is comparable). The existence of thicker lamellae reduces the number of crystalline-amorphous interface, thus decreasing the number of scattering sites for phonons. This explains the higher in-plane TC of SPE120 than SPE110 (Fig. 1). As for SPE130, the long spacing is not obtained, probably because the lamellae stack in a relatively disordered way. This results in the decline of the in-plane TC (Fig. 1). The above results imply that 120 °C is an optimized extrusion temperature to achieve high in-plane TC of SPE UHMWPE. At low extrusion temperature, the mobility of molecular chains is rather limited to pack into the crystalline region. As a result, the thickness of lamellae is small. At high extrusion temperature, the crystalline region starts to melt (the meting point of UHMWPE is 138 °C, Fig. S4), giving a highly deformable melt that is easy to go through the SPE die. The oriented molecular chains are prone to relax due to the high temperature. The long spacing of SPE samples along the through-plane direction is shown in Fig. 4e. The long spacing is smaller in the through-plane direction than the in in-plane direction. This is understandable considering the fast crystallization rate along the through-plane direction due to the fast solidifying rate. The reduced thickness of lamellae increases the number of crystalline-to-amorphous interface and thus is related to the low through-plane TC. From the above morphological and structural characterization, the mechanism behind the high and anisotropic TC of SPE UHMWPE is revealed, as shown in Fig. 5. In the in-plane direction, the formation of large quantities of cylindrical crystals reduces the number of crystalline-amorphous interfaces as well as phonon scattering sites (Fig. 5a1). Thus, the in-plane TC of SPE UHMWPE is greatly enhanced (Fig. 1a). Meanwhile, a vast number of oriented lamellae are formed along the inplane direction (Fig. 5a2), which also plays a vital role in improving inplane TC, given the fact that heat is more readily to propagate through chain-skeleton than interchain direction. Besides, the monoclinic structure formed in the SPE UHMWPE (Fig. 5a4) shows higher orientation than orthorhombic form (Fig. 5a5), which brings more transmission channels for phonon along the chain-skeleton direction. It also contributes to the increased in-plane TC of SPE UHMWPE. In the through-plane direction, almost all the heat transfers perpendicular to the stack direction of cylindrical crystals (the black arrow, Fig. 5a1), causing enormous interface heat resistance. On the other hand, lamellae in through-plane panel are less oriented than those in inplane direction (Fig. 5a2 vs. 5a3). It increases the number of phonon scattering sites to go against phonon dissipation. As for HP UHMWPE, the low TC is due to the formation of randomly distributed spherocrystals (Fig. 5b1), disordered lamellae (Fig. 5b2), and orthorhombic structure of PE (Fig. 5b3). Except for high TC, it is encouraging to ﬁnd that the mechanical performance of SPE UHMWPE is signiﬁcantly enhanced, as shown in Fig. 6. The ultimate tensile strength increases greatly from 41.6 MPa for
4. Conclusions A highly thermal conductive UHMWPE bulk material was prepared by structural manipulation through a home-made SPE apparatus. The in-plane TC reached up to 3.30 W/mK, 588% higher than that of the controlled HP counterpart (0.48 W/mK). The SPE sample also showed high anisotropic ratio of TC ranging from 290% to 750%. We attributed the largely improved and highly anisotropic TC of the SPE sample to the formation of cylindrical crystals and highly oriented lamellae. The mechanical properties of SPE UHMWPE were also signiﬁcantly enhanced due to such a structure architecture. In particular, the ultimate tensile strength of SPE UHMPWE increased from 41.6 MPa to 107.7 MPa. The integration of the greatly improved thermal conductivity and mechanical properties endows the SPE UHMWPE with great potential to be used in thermal management applications. Acknowledgements The authors gratefully thank the ﬁnancial support from the National Natural Science of China (51533004, 51773136, 51761145112) and the Fundamental Research Funds for Central Universities. We express sincere thanks to the beamlines BL16B1 of the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China) for the kind help on WAXD and SAXS measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymer.2019.121760. References  G.H. Kim, D. Lee, A. Shanker, L. Shao, M.S. Kwon, D. Gidley, J. Kim, K.P. Pipe, High thermal conductivity in amorphous polymer blends by engineered interchain interactions, Nat. Mater. 14 (3) (2015) 295–300.  J. Hu, Y. Huang, Y. Yao, G. Pan, J. Sun, X. Zeng, R. Sun, J.B. Xu, B. Song, C.P. Wong, Polymer composite with improved thermal conductivity by constructing a hierarchically ordered three-dimensional interconnected network of BN, ACS Appl. Mater. Interfaces 9 (15) (2017) 13544–13553.  V. Singh, T.L. Bougher, A. Weathers, Y. Cai, K. Bi, M.T. Pettes, S.A. McMenamin, W. Lv, D.P. Resler, T.R. Gattuso, High thermal conductivity of chain-oriented
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