Full length articleImproved interfacial properties for largely enhanced thermal conductivity of poly(vinylidene fluoride)-based nanocomposites via functionalized multi-wall carbon nanotubes
Graphical abstract
Introduction
Polymer materials provide multiple excellent mechanical properties, such as light weight, easy molding, chemical and fatigue resistance, which have been widely applied as structural materials, electrical insulation materials, corrosion-resistant materials and other functional materials [[1], [2], [3]]. The thermal properties of polymer materials are vital parameters which have not been widely noted. Currently, the known polymer materials are mostly poor thermal conductors with low thermal conductivity (no more than 0.5 W/(m·K)), which limits their applications in many fields, including heat exchangers, electronic and electrical materials, fuel cells and transportation systems [[4], [5], [6]]. Therefore, improving the thermal conductivity of polymer materials is a key factor that needs to be addressed. With this in mind, various thermal conductive fillers were added to polymeric matrix for achieving thermal conductive composites [[7], [8], [9], [10]]. Gu et al. [7] fabricated the micrometer boron nitride/polyimide (mBN/PI) composites by thermal imidization and hot pressing, which presented a thermal conductivity of 0.696 W/(m·K) with 30 wt% mBN. The reduced graphene oxide (rGO) foam embedded with graphene nanoplatelets (GNPs) was blended with epoxy (EP) matrix, and the thermal conductivity of GNPs/rGO/EP composite containing 0.1 wt% rGO and 20.4 wt% GNPs reached 1.56 W/(m·K) [8].
Polyvinylidene fluoride (PVDF) is a kind of thermoplastic polymer with outstanding performance, including excellent chemical and thermal stability, pollution resistance, good mechanical property, wide processing temperature range and facile film formation. In recent years, PVDF has been extensively applied in chemical industry, electricity and electronics, water treatment, foods and other fields [[11], [12], [13]]. Some studies have focused on the thermal conductivity improvement of PVDF [[14], [15], [16], [17], [18], [19]]. In our previous works [14,15], an electric field was used for ordered arrangement of graphene (GE) to prepare oriented GE/PVDF composite membrane. A 226% enhancement of thermal conductivity was achieved in the composite membrane when the aligned GE loading was as high as 20 wt% [14]. The magnetic carbon nanotube (mCNT) was obtained by coating iron oxide particles, making PVDF composite with 15 wt% aligned mCNT under low magnetic field achieved 0.507 W/(m·K) thermal conductivity [15]. The core-shell SiC/SiO2 whiskers were blended with PVDF matrix, and the thermal conductivity of composite was increased 7 times than that of pure PVDF under 20 vol% loading [16]. However, the effective enhancement in thermal conductivity for the polymer composites is still limited.
As an innovative filler, nano-filler has attracted increasing attentions. Carbon nanotubes (CNTs) exhibit excellent mechanical and electrical properties, good chemical and thermal stability, high thermal conductivity and specific surface area, which are promising in ameliorating the thermal conductivity of polymer composites [[20], [21], [22], [23], [24], [25], [26], [27]]. The performance of composite material depends not only on the reinforcement, but also on the interfacial properties between fillers and matrix. Therefore, surface chemical modification of nano-fillers is crucial for improving the performance of composites [28,29]. Zhang et al. [21] prepared nanocomposites using polyvinylpyrrolidone (PVP) treated CNTs, and it was found that PVP obviously increased the dispersion of CNTs. The thermal conductivity of PVDF/CNT/PVP composite was 0.63 W/(m·K) with CNT content of 10 wt%. Hung et al. [30] studied the graphitic nanosheets (GNPs)/polymer-based composites and found that there was heat transfer resistance at the interface between GNPs and polymer matrix. By pretreating GNPs with nitric acid, the interfacial adhesion between GNPs and matrix can be significantly improved, thus enhancing the thermal conductivity of the composites. Xu et al. [31] found that the composite prepared by filling the epoxy resin with silane treated AlN particles increased the thermal conductivity by 97% compared to the untreated composite. It is noteworthy that although many attempts have been made, the improvement of thermal conductivity still lags far behind expectation, due to the insufficient reduction of interface thermal resistance, resulting in limited amelioration of interfacial property. In the work of Zhang et al. [18], 1 wt% graphene oxide (GO) was introduced into PVDF/CNT composites for achieving enhanced thermal conductivity, however, the interface thermal resistance of PVDF/CNT/GO composites exhibited almost the same as that of PVDF/CNT composites (they were both 5.5 × 10−7 m2·K/W).
To explore an efficient strategy for largely improving the thermal conductivity of PVDF-based composites, in this study, MWCNTs were acidified with H2SO4/HNO3 followed by covalently functionalized with triethoxyvinylsilane (YDH-151), containing a group similar to the matrix. This method of introducing matrix-like groups onto fillers could significantly improve the interfacial compatibility between fillers and matrix, and greatly reduce the interfacial thermal resistance. Then the PVDF-based nanocomposite with excellent thermal conductivity could be prepared. The effects of functional modification of MWCNTs on dispersibility, interfacial properties, thermal stability, mechanical properties and thermal conductivity of PVDF nanocomposites were investigated. The experimental results were verified by a classical theoretical model, and the mechanism on thermal conductivity enhancement of composites was proposed.
Section snippets
Materials
PVDF (FR904) was supplied by 3F New Materials Technology Co., Ltd. (Shanghai, China). Multi-wall carbon nanotubes (MWCNTs) with 10–30 μm in length and 10 nm in diameter were obtained from Nanoon (Beijing, China). The purity of MWCNTs was more than 95%. Sulfuric acid (98%), nitric acid (68%) and hydrochloric acid (36%) were purchased from Tianjin Jiangtian Chemical Technology Co., Ltd. (Tianjin, China). Triethoxyvinylsilane (YDH-151, 97%) was purchased from Huaweiruike Chemical Co., Ltd.
Characterization of MWCNTs before and after functionalization
The infrared spectra of MWCNTs are presented in Fig. 1b. As shown, the characteristic peak of p-MWCNTs is not obvious due to the homonuclear diatomic atoms (i.e. CC and CC) show no infrared activity. Compared with p-MWCNTs, the strong absorption peaks of a-MWCNTs at 3430 cm−1, 1637 cm−1 and 1413 cm−1 are COH stretching vibration peak, CO stretching vibration peak and COH bending vibration peak, respectively, indicating oxygen-containing groups on a-MWCNTs. It proves that p-MWCNTs are
Conclusion
Functionalized s-MWCNTs were prepared through optimizing acid treatment and functionalization of YDH-151 to introduce vinyl groups on the surface of MWCNTs. Then, s-MWCNTs/PVDF nanocomposites were further prepared. The effects of YDH-151 functionalization on the dispersibility of MWCNTs, the interfacial properties with PVDF matrix, the tensile strength and thermal conductivity of PVDF nanocomposites were investigated. The results showed that the YDH-151 modified s-MWCNTs exhibited good
Notes
The authors declare no competing financial interest.
Acknowledgments
We thank for the financial support of the Science and Technology Project of Tianjin (Grant No. 12ZCZDSF02200) and the Innovation Service Platform Project of Desalination and Comprehensive Utilization (Grant No. CXSF2014-34-C).
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