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Oct 3, 2016 - Paving the Thermal Highway with Self-Organized Nanocrystals in Transparent Polymer Composites. Liwen Mu†‡, Tuo ... Tel: 1-(330)972-6...
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Pave Thermal Highway with Self-Organized Nanocrystals in Transparent Polymer Composites Liwen Mu, Tuo Ji, Long Chen, Nitin Mehra, Yijun Shi, and Jiahua Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10451 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 9, 2016

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Pave Thermal Highway with Self-Organized Nanocrystals in Transparent Polymer Composites Liwen Mu,a,b Tuo Ji,a Long Chen,a Nitin Mehra,a Yijun Shib and Jiahua Zhu a* a

b

Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325 USA

Division of Machine Elements, Luleå University of Technology, Luleå, 97187, Sweden

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ABSTRACT Phonon transfer is greatly scattered in traditional polymer composites due to the unpaired phonon frequency at polymer/filler interface. A key innovation of this work is to build continuous crystal network by self-organization and utilize it as “thermal highway” that circumvents the long-existing interfacial thermal barrier issue in traditional composites. By tuning the molecular diffusion rate of dicarboxylic acids (oxalic acid, malonic acid and succinic acid), different crystal structures including skeletal, dendrite, diffusion limited aggregates and spherulite were synthesized in PVA film. These continuous crystal structures benefit the efficient phonon transfer in the composites with minimized interfacial scattering and lead to a significant thermal conductivity enhancement by up to 180% compared to pure polymer. Moreover, the transparent feature of these composite films provides additional benefits in display applications. Post heat treatment effect on the thermal conductivity of the composite films shows a time dependent behavior. These uniquely structured polymer/crystal composites are expected to generate significant impacts in thermal management applications.

KEY WORDS: Polymer composites, crystal network, thermal conductivity, transparent, phonon

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INTRODUCTION Heat dissipation is becoming a crucial barrier in the continuous process of electronic devices and systems miniaturization.1-3 Thermal management of devices and systems directly determines their efficiency and life time. In recent years, the rise of mobile device and touch screen applications has driven new research and development efforts into materials compatible with transparent and flexible design requirements.4-7 Indeed, thermal interfacing materials, inserted between heat source and heat sink, are playing increasingly important roles in emerging fields such as micro-/nano-electronics,89

3D chip stacking architecture10 and flexible electronics.1, 11 Polymer is a natural fit to meet the requirements of transparency and flexibility.

However, the intrinsic low thermal conductivity (TC) of most polymers, typically >Vg) led to unstable diffusion field and thus more complicated crystal structures were formed, such as dendrite, DLA and spherulite. A few benefits could be expected from these polymer/crystal composites. Firstly, most of the crystals are transparent, which makes the goal of transparent thermally-conductive films become possible; secondly, the mechanical property of the composite films could be modified benefitting from the embedded crystal network; thirdly, the crystal network is structurally continuous

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without breakage in between, which is essentially important for efficient phonon transport without scattering across polymer/crystal interface.

Figure 3. Transmittance of pure PVA and PVA composites filled with OA, MA and SA at different loadings (at 700 nm). The number above the columns indicates the weight percentage. The whole spectrum from 400-1100 nm is in Figure S5. The photographs were taken by placing a film on UAkron logo with film thickness of 0.3-0.4 mm. The optical transparency PVA and PVA/DCA composite film was characterized by UV-Vis spectrometer and the transmittance at 700 nm is plotted in Figure 3. The PVA gives the highest transmittance of 90%, while the transmittance decreases after introduction DCA crystals in the film. Overall, PVA/MA provides the highest transparency followed by PVA/SA and PVA/OA. Larger DCA concentration leads to a decreased transparency of the composite films due to their more condensed crystal structures. The relatively higher optical transparency of PVA/MA could be attributed to the sparsely distributed large crystals.

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Figure 4. Strain-stress curves of (a) PVA/OA, (b) PVA/MA and (c) PVA/SA composites. Summarization of (d) tensile strength and (e) elongation at break of different composites. HT means the samples after heat treatment at 120 oC for 5 hours.

The tensile properties of these composite films were also characterized, Figure 4. The strain-stress curves were plotted in Figure 4(a-c). The tensile strength and elongation at break were summarized in Figure 4(d) and (e), respectively. Generally, the addition of DCA crystals in PVA film decreases the tensile strength and the strength decreases with increasing DCA concentration. However, the enhancement of elongation-at-break was clearly observed in PVA/MA and PVA/SA systems. These results indicate that the addition of DCA interrupted the physical entanglement of PVA chains and introduced free space in the composites that causes the reduction of tensile strength. As a result of post heat treatment at 120 oC for 5 hours, the tensile strength of both PVA (Figure S6) and PVA/DCA-10-HT (Figure 4a-c) increases sharply accompanied with a significant drop of elongation at break. In other words, these films become brittle after heat 12 ACS Paragon Plus Environment

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treatment. The enhanced mechanical strength of PVA could attribute to the intensified hydrogen bonding between PVA chains since no chemical reactions occurred during the thermal treatment as evidenced by FT-IR in Figure S7. However, esterification reactions occurred between PVA and DCA during thermal treatment, which has been evidenced by the newly formed ester groups at 1250-1270 cm-1 in Figure S8-S10. Such esterification reactions occurred between PVA and DCA lead to the cross-linking of PVA chains and thus increase the tensile strength.

Figure 5. (a) TC of pure PVA and PVA/DCA composites with different filler loadings at 20 oC, (b) effect of temperature and (c) heat treatment time on thermal conductivity of PVA composites with 10% DCA loading. Heat treatment is performed at 120 oC. Thermal conductivity of pristine PVA and PVA/DCA composites was measured at 20 oC, Figure 5(a). Pristine PVA gives relatively low TC of 0.25 and the addition of DCA crystals increases the TC dramatically. For instance, the TC value reaches to 0.51 W/mK by adding 10% OA in PVA. Even higher TC values of 0.70 and 0.65 W/mK are obtained with 10% of SA and MA, respectively. TC value of PVA/OA and PVA/SA follows a similar trend that the highest TC value is achieved at 30% OA(SA) and then decreased afterwards. PVA/MA gives relatively stable TC value over the entire range of filler loading from 10-40%. The significantly enhanced TC value at low crystal loading reveals the exceptional capability of crystal network in transfer heat (phonon). However, 13 ACS Paragon Plus Environment

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the drop of TC at large crystal loading (>30%) clearly points out the fact that crystal loading is not the only factor that determines the TC. In general, larger DCA loading will lead to a more branched crystal structure, as seen in Figure 2. At lower crystal loading, the increased crystals dominates the phonon transfer and thus the thermal conductivity increases with increasing crystal loadings. However, each branch in the crystal behaves as a phonon scattering center that decreases the phonon transfer efficiency. Therefore, reduced TC was observed when filler loading increases to 40%. Besides, it is well accepted that thermal conductivity is positively related to the modulus of polymers.49 However, in the PVA/DCA composites, the DCA crystal structure dominates the heat transfer. Therefore, even though the modulus of the composites is decreased after adding DCA, the thermal conductivity still increases. This result confirms the significant contribution of the continuous crystal network in heat transfer. The temperature dependent TC of polymer generally follows: temperature;

T3 at low temperature; and

temperature (L: mean free path; : average velocity of phonons;

T-1 at high at intermediate

: specific heat).50

51

The testing temperature in this work fall in the intermediate temperature range and therefore the TC is affected by and

. The increase of temperature increases both

but decreases L. The gradual increase of TC with increasing temperature, Figure

5(b), reveals the dominant contributions of specific heat and phonon velocity in determining TC at the testing range from -20 to 40 oC. The relationship between TC and post heat treatment time is also studied, Figure 5(c). The TC of pure PVA seems less affected by the post heat treatment process. However, all the PVA/DCA composites show a similar pattern that TC increases in the

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first hour and then continuously decreases with extending heat treatment time. It is worth noting that the heat transfer in PVA/DCA composites is contributed by three parts: PVA, DCA crystal and PVA/DCA interface. The heat treatment firstly softens the polymer chains and forms a good interfacial contact with crystal network. In addition, the hydrogen bonding between -OH (PVA) and -COOH (DCA) groups at the interface will be strengthened by heating. Significantly enhanced TC by hydrogen bonding was reported by other researchers.33 These two major factors promote the heat transfer across the PVA/DCA interface and as a result increase the overall TC of the composites. Longer heat treatment time causes negative effect on the crystals as evidenced by XRD and SEM, Figure S11. The crystal peaks disappeared after 5 hours and the SEM results confirmed this observation. The loss of crystals removes the thermal transport high way in the composites and results in a reduced thermal conductivity. However, the esterification reaction occurred at the PVA/DCA interface (Figure S8-S10) leads to the cross-linking of PVA chains by DCA molecules and partially compensates the TC loss through strengthened interfacial covalent bonding. Considering all the above facts, it is reasonable to observe the slightly reduced TC value after long period of heat treatment.

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Figure 6. Plot of thermal conductivity and thermal conductivity enhancement of different composite materials. 1: epoxy/BN (10 wt%);52 2: epoxy/AlN (11 wt%);53 3: epoxy/Si3N4 (20 vol%);54 4: epoxy/Al(OH)3 (10 vol%);55 5: epoxy/Al2O3 (10 vol%);56 6: epoxy/SiO2 (10 vol%);56 7: epoxy/BN nanosheet (10 vol%);57 8: PVA/BN (10 vol%);58 9: PVA/BN nanotube (10 wt%);59 10: PVA/PAA (10 wt%);60 11: PVA/PAP (20 mol%);61 12: PS/SiC (7.5 vol%);62 13: PS/CdS (6 wt%);63 14: PVC/CdS (6 wt%);63 15: PI/ZnO (10.1 vol%);64 16: PVDF/Zn (10 vol%);65 17: PVDF/PS/SiC (7 vol%);66 18: PVDF/SiC (10 vol%); 67 19: PVDF/BaTiO3 (10 vol%);67 20: LDPE/AlN (30 vol%);68 21: PVA/OA (10 mol%/9.3 wt%/6.0 vol%); 22: PVA/MA (10 mol%/10.6 wt%/8.0 vol%); 23: PVA/SA (10 mol%/11.8 wt%/9.3 vol%). *Electrically conductive filler is not listed in this figure. To illustrate the effectiveness of the self-organized crystal network in enhancing TC, the absolute TC and percentage of thermal enhancement by adding filler have been compared with literature results, Figure 6. Compared to the literature reports on thermally conductive polymer composites, the PVA/DCA in this work gives both higher absolute TC value and larger percentage of thermal enhancement. The superior heat transfer property is attributed to the continuous crystal network, along which the phonon can be efficiently transferred without scattering. While in traditional composites as listed in Figure 6, interfacial thermal barrier is always existed and phonon is greatly scattered at 16 ACS Paragon Plus Environment

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the polymer/filler interface. A breakthrough of TC value in traditional composites is practically impossible as long as the interface exists.

CONCLUSIONS To sum up, different morphologies of continuous crystal networks were successfully controlled in polymer composites by tuning molecular diffusion rate. These composites show unprecedented thermal conductivity compared to traditional composites. Moreover, optical transparency and mechanical strength of these composite films are well remained. Up to 180% of thermal conductivity enhancement was obtained in PVA/MA composites with only 10 mol% MA loading. Larger crystal loading leads to a more branched crystal structure, where phonon scattering becomes dominant and negatively affects heat transfer. The effect of post heat treatment on the thermal conductivity is time dependent. Short heat treatment increases thermal conductivity by strengthened interfacial interaction, while long heat treatment would damage the crystal structure and consequently decrease the thermal conductivity. ASSOCIATED CONTENT Supporting Information UV-Vis spectra, FT-IR-ATR, XRD pattern, optical photos, and SEM surface morphology of the PVA/DCA composite films. AUTHOR INFORMATION Corresponding Author * J. Zhu. E-Mail: [email protected]. Tel: 1-(330)972-6859. 17 ACS Paragon Plus Environment

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Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Acknowledgement is made to the Donors of the American Chemical Society Petroleum Research Fund (#55570-DNI10) and Swedish Kempe Scholarship Project (JCK-1507) for support of this research. Partial support from the start-up fund of The University of Akron is also acknowledged. The authors appreciate the technical support from Ms. Wenchen Li and Dr. Lingyun Liu with optical microscopy.

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53. Peng, W.; Huang, X.; Yu, J.; Jiang, P.; Liu, W. Electrical and Thermophysical Properties of Epoxy/Aluminum Nitride Nanocomposites: Effects of Nanoparticle Surface Modification. Composites, Part A 2010, 41 (9), 1201-1209. 54. Shimamura, A.; Hotta, Y.; Hyuga, H.; Kondo, N.; Hirao, K. Effect of amounts and Types of Silicon Nitride on Thermal Conductivity of Si3N4/Epoxy Resin Composite. J. Ceram. Soc. Jpn. 2015, 123 (1441), 908-912. 55. Shi, Z.; Fu, R.; Agathopoulos, S.; Gu, X.; Zhao, W. Thermal conductivity and Fire Resistance of Epoxy Molding Compounds Filled with Si3N4 and Al(OH)3. Mater. Des. 2012, 34, 820-824. 56. He, H.; Fu, R.; Han, Y.; Shen, Y.; Song, X. Thermal conductivity of Ceramic Particle Filled Polymer Composites and Theoretical Predictions. J. Mater. Sci. 2007, 42 (16), 6749-6754. 57. Yu, J.; Mo, H.; Jiang, P. Polymer/boron nitride nanosheet composite with High Thermal Conductivity and Sufficient Dielectric Strength. Polym. Adv. Technol. 2015, 26 (5), 514-520. 58. Ahn, H. J.; Eoh, Y. J.; Park, S. D.; Kim, E. S. Thermal Conductivity of Polymer Composites with Oriented Boron Nitride. Thermochim. Acta 2014, 590, 138-144. 59. Terao, T.; Zhi, C.; Bando, Y.; Mitome, M.; Tang, C.; Golberg, D. Alignment of Boron Nitride Nanotubes in Polymeric Composite Films for Thermal Conductivity Improvement. J. Phys. Chem. C 2010, 114 (10), 4340-4344. 60. Xie, X.; Li, D.; Tsai, T.-H.; Liu, J.; Braun, P. V.; Cahill, D. G. Thermal Conductivity, Heat Capacity, and Elastic Constants of Water Soluble Polymers and Polymer Blends. Macromolecules 2016, 49 (3), 972-978. 61. Kim, G.-H.; Lee, D.; Shanker, A.; Shao, L.; Kwon, M. S.; Gidley, D.; Kim, J.; Pipe, K. P. High Thermal Conductivity in Amorphous Polymer Blends by engineered Interchain Interactions. Nat. Mater. 2015, 14 (3), 295-300. 62. Gu, J.; Lv, Z.; Wu, Y.; Zhao, R.; Tian, L.; Zhang, Q. Enhanced Thermal Conductivity of SiCp/PS Composites by Electrospinning-hot Press Technique. Composites, Part A 2015, 79, 8-13.

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63. Mathur, V.; Patidar, D.; Sharma, K. Effect of nano CdS dispersion on Thermal Conductivity of PS/PVC and PS/PMMA Polymeric Blend Nanocomposites. Appl. Nanosci. 2015, 5 (5), 623-628. 64. Yorifuji, D.; Ando, S. Enhanced Thermal Conductivity over Percolation Threshold in Polyimide Blend Films Containing ZnO Nano-Pyramidal Particles: Advantage of Vertical Double Percolation Structure. J. Mater. Chem. 2011, 21 (12), 4402-4407. 65. Zhou, W.; Wang, Z.; Dong, L.; Sui, X.; Chen, Q. Dielectric Properties and Thermal Conductivity of PVDF Reinforced with Three Types of Zn Particles. Composites, Part A 2015, 79, 183-191. 66. Cao, J.-P.; Zhao, X.; Zhao, J.; Zha, J.-W.; Hu, G.-H.; Dang, Z.-M. Improved Thermal Conductivity and Flame Retardancy in Polystyrene/Poly(vinylidene fluoride) Blends by Controlling Selective Localization and Surface Modification of SiC Nanoparticles. ACS Appl. Mater. Interfaces 2013, 5 (15), 6915-6924. 67. Li, Y.; Huang, X.; Hu, Z.; Jiang, P.; Li, S.; Tanaka, T. Large Dielectric Constant and High Thermal Conductivity in Poly(vinylidene fluoride)/Barium Titanate/Silicon Carbide Three-Phase Nanocomposites. ACS Appl. Mater. Interfaces 2011, 3 (11), 4396-4403. 68. Zhu, B. L.; Wang, J.; Zheng, H.; Ma, J.; Wu, J.; Wu, R. Investigation of Thermal Conductivity and Dielectric Properties of LDPE-matrix Composites Filled with Hybrid Filler of Hollow Glass Microspheres and Nitride Particles. Composites, Part B 2015, 69, 496-506.

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Table of Content

Pave Thermal Highway with Self-Organized Nanocrystals in Transparent Polymer Composites Liwen Mu,a,b Tuo Ji,a Long Chen,a Nitin Mehra,a Yijun Shib and Jiahua Zhu a*

Self-organized nanocrystals in polymer enable efficient phonon transport along crystal network, minimize phonon scattering at polymer/filler interface and excel in thermal transport.

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