Supporting Information

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1 Supporting Information Highly Thermally Conductive Yet Electrically Insulating Polymer/ Boron Nitride Nanosheets Nanocomposite Films for Improved Thermal Management Capability Jin Chen, Xingyi Huang*, Bin Sun, Pingkai Jiang Department of Polymer Science and Engineering, Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, Shanghai , China; *Corresponding Author: Xingyi Huang, S-1

2 Figure S1. The TEM images of PVDF/BNNS fibers (20 wt% BNNS) S-2

3 Figure S2. SEM images of PVDF/BNNS film (20 wt% BNNS) S-3

4 Supplementary method S1. The calculation of interfacial thermal resistances To understand the mechanism of thermal conduction of BNNS based composites, the interfacial thermal resistance in random dispersed and oriented BNNS based composites were calculated using the effective medium theory (EMT) and Foygel s theory, respectively. In PVDF/BNNS composite, thermal conductivity (K) follow EMT model 1 : K = K m 3 + 2V f ( Kp Km K m ) 3 V f [ (1 α) K m K p ] (1) and α = R 1 K m (2) d where K m is thermal conductivity of the PDMS, and K p (300 W/(m K)) is thermal conductivity of the filler; V f is the volume fraction of BNNSs; R 1 is the interfacial thermal resistance between PVDF and filler; d (3 nm) is the thickness of BNNS. In random dispersed BNNSs composite, the calculated data of R 1 is m 2 K/W. The EMT model provides a reasonable interfacial thermal resistance for random dispersed BNNSs composite. However, it is unsuitable to fit the vertically aligned and interconnected BNNS based composites, where the interfacial thermal resistance is mainly between BNNSs layers. Here, Foygel s theory is adopted to analysis the interface thermal resistance. K K 0 = [V f V c (a)] t(a) (3) S-4

5 R c = 1 K 0 L [V c (a)] t(a) (4) where K 0 is pre-exponential factor in relation to the contact between BNNSs as well as the topology of the percolation cluster; Vc(a) is the critical volume fraction of BNNSs, which depends on the morphology and aspect ratio (a) of BNNSs; t(a) is the conductivity exponent that is dependent on the aspect ratio of filler; Rc is the contact resistance between BNNSs; L is the length of BNNSs. In our previous work 2, BNNSs are counted to have a majority of about 1 μm. In PVDF/oriented BNNS nanocomposite (28.9 vol%), by fitting the in-plane thermal conductivity, the values of K 0, Vc(a) and t(a) are respectively obtained to be 21, and 1.2. Then the calculated data of Rc is K/W. We assume that 5% of a BNNS layer area contact with other BNNSs and contributes to the heat conduction, then the active interface area between BNNSs can be estimated as m 2. Based on the above data, the interfacial thermal resistance (R 2 ) in PVDF/oriented BNNS nanocomposite is computed to be m 2 K/W. It is worth noting that R 1 is about one order of magnitude higher than R 2 of random dispersed BNNS based composite. S-5

6 Supplementary method S2. Finite Element Model The heat transfer processes of PVDF/oriented BNNS composite compared with commercial silicone pad were modeled in COMSOL Multiphysics software, which included a 90 o C cuboid heat source(4*4*1mm), oriented BNNS composite pad or silicone pad(10*10*0.2mm). The polymer pad was contacted with the top surface of heat source, the convective heat flux is 10 W/(m 2 K). According to the previous test results, the in-plane and through-plane TC of PVDF/oriented BNNS composite are set to be 10 and 0.5 W/(m K), and the isotropic TC of silicone pad is 3 W/(m K). Finally, the ambient temperature is 20 o C. 1. Nan, C. W., Birringer, R., Clarke, D. R., & Gleiter, H. Effective Thermal Conductivity of Particulate Composites with Interfacial Thermal Resistance J. Mater. Chem. A 1997, 81, Chen, J.; Huang, X.; Zhu, Y.; Jiang, P. Cellulose Nanofiber Supported 3D Interconnected BN Nanosheets for Epoxy Nanocomposites with Ultrahigh Thermal Management Capability Adv. Funct. Mater. 2016, 27, S-6