Bulk polymers in general possess low thermal conductivities around 0.1–0.5 W m−1 K−1 due to their disordered structure (Huang et al., 2018; Chen et al., 2016). A typical polymer structure is composed of both crystalline domains where the chains are regularly oriented and amorphous domains with randomly twisted and entangled chains. In polymers, heat conduction can only be supplied by phonon transfer; however, discontinuous internal interfaces and defects due to the low crystallinity of the polymers cause phonon scattering and decrease phonon transfer efficiency. The crystalline materials tend to show higher thermal conductivity as a result of the ordered structure, and the increase in the polymer chain alignment improves the thermal conductivity by helping the phonon transfer. Researchers have focused to align the polymer chains by several methods including mechanical stretching, nanoscale templating, and electrospinning. Although mechanical stretching and nanoscale templating can also increase the thermal conductivity of polymers significantly, mechanical stretching can cause ruptures in the fibers, which can cause phonon scattering, and heat stretching can lower the crystallinity (Huang et al., 2018). Nanoscale templating is a multistep technique with the need of templating agents and templates.
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Electrospinning is a widely used method for the production of nanofibers based on the use of electrostatic forces to draw charged filaments (Bhardwaj and Kundu, 2010; Greiner and Wendorff, 2007). In this technique, a polymer solution or melt is taken in a syringe with a needle, which is placed in a pump to be fed at a constant feed rate ( ). An electrostatic force is applied to the droplet at the tip of the syringe and when the applied tension beats the surface tension, a jet is ejected from the tip to the grounded collector. The solvent evaporates throughout the distance to the collector, leading to fibers with nano-scale diameters. The main factors affecting the chain morphology during electrospinning can be given as the applied voltage and jet speed. It is found out that strengthening the electric field tends to improve the polymer chain alignment, preferably along the axis of the fiber. The jet speed is another important parameter for the alignment. It is a simple and low-cost method, with the advantage of being suitable to a broad diversity of polymer types. However, the process depends on many variables, and thermal conductivity of the as-spun fibers may differ due to the whipping instability (Wei et al., 2021). Moreover, the solvents used in solvent electrospinning can be toxic, and scale-up production may not be feasible.
In a study by Zhong and coworkers, the effect of the size and anisotropic alignment of electrospun aligned Nylon-11 nanofibers on thermal transport was analyzed using high-resolution in situ wide-angle X-ray scattering (Zhong et al., 2014). It was revealed the Nylon-11 nanofibers possessed the γ-form among its polymorphs and were stable during electrospinning and hot-stretching. The thermal conductivity of the Nylon-11 nanofibers was increased more than 6 times compared with bulk case with contribution of decreasing fiber diameter and additional axial hot-stretching in improved orientation, crystal size, and phonon transfer. Ma and coworkers investigated the relation between the structure and thermal conductivity of crystalline PE nanofibers that were electrospun under a voltage between 9–52 kV (Ma et al., 2015). They reached to a maximum value of 9.3 W m−1 K−1, which is over 20 times higher compared with the bulk PE under 45 kV at 270 K, suggesting that the PE chains became more oriented under strong elongational force. It was also noted that orthorhombic crystallinity of the chains increased with increasing voltage. The improvement in the thermal conductivity of a semicrystalline polyethylene oxide (PEO) polymers using electrospinning was studied by Lu and coworkers with values between 13–29 W m−1 K−1 depending on the concentration at a constant relative humidity of 20% (Lu et al., 2017). PEO nanofibers with a crystallinity around 54% preferred a cystalline orientation of [001] lattice direction with a tendency of increasing the thermal conductivity with increased chain alignment in the amorphous region. The electrospinning of PEO increased the thermal conductivity up to 150 times compared with the bulk polymer, proving that this method can also enhance the thermal conductivity of semicrystalline polymers. Seol and coworkers studied the effect of electrospinning polyvinyl alcohol (PVA) and polyvinyl alcohol/cellulose nanocrystal (PVA/CNC) on thermal conductivity and stability (Park et al., 2019). The thermal conductivities of PVA nanofibers and PVA/CNC nanofibers with an average diameter of 200 nm were measured by suspended microdevice technique, respectively, as 1.23 and 0.74 W m−1 K−1 at room temperature, which were higher than that of bulk PVA by factors of 2.5 and 3. The enhancement was due to the increased crystallinity, chain orientation, and also hydrogen bonding for PVA/CNC.
Boron nitride (BN), an isoelectronic with carbon, is another thermal conductive material that exists in two crystalline forms, cubical and hexagonal (Abbas et al., 2013). Cubic boron nitride (c-BN) is hard and abrasive with a diamond-like structure, whereas hexagonal boron nitride (h-BN) is smooth and slippery with a white layered crystal structure similar to graphite having a high in-plane thermal conductivity around 300–600 W/mK, which is about 20–30 times that of the thickness direction. Hexagonal boron nitride nanosheets (BNNSs) and boron nitride nanotubes (BNNTs) are analogues to graphene and carbon nanotubes, respectively, but electrically insulating. They have localized electrons due to the ionic characteristic of B−N bonds, wide band gap as well as high phonon velocity that bring in low dielectric constant, high electrical insulation and high thermal conductivity, thermal stability, and elastic modulus (Zeng et al., 2017). Due to the outstanding anisotropy of BN in thermal conductivity, thermal conductivity can be improved in the orientation direction with increased orientation.
Huang and coworkers reported the fabrication of flexible PVA/BNNS/polydimethylsiloxane (PDMS) composites with improved through-plane thermal conductivity for advanced thermal management applications (Chen et al., 2017). PVA/BNNS composite fibers were electrospun and rolled to form a vertically oriented PVA/BNNS cylinder, which were impregnated by PDMS under vacuum to form PDMS/PVA/BNNS nanocomposites. The thermal conductivity of the PDMS/PVA/BNNS nanocomposites with 15.6 vol % BNNS had a through-plane thermal conductivity of 1.94 W m−1 K−1, which is 978% higher than pure PDMS. The equilibrium temperature of the LED chip integrated with PDMS/PVA/BNNS nanocomposites was measured about 33°C lower than that of neat PDMS, exposing their potential as thermal interface materials (TIMs). A high temperature thermal conductive, light and flexible nanocomposite textile composed of amino functional boron nitride nanosheets (FBNNS) and polyimide (PI) nanofibers was introduced by Lei and coworkers by green electrospinning of FBNNS and polyamic acid, followed by thermal crosslinking (Wang et al., 2018a). The thermal conductivity of the FBNNS-PI nanofibers was measured as 13.1 W m−1 K−1 at 300⁰C with 20 wt % FBNNS loading, which corresponds to 4,773% increase in comparison with pure PI mat, due to the uniformly dispersed FBNNS in the polymer matrix with vertically aligned orientation. The FBNNS-PI nanofiber mat also exhibited a cooling effect at high temperature, which makes them promising materials for high temperature thermal regulated clothing applications. In a study by Zhang et al., composites with advanced thermal and mechanical properties were prepared using electrospun polyvinylidene fluoride (PVDF) nanofibers and polydopamine (PDA)-modified boron nitride (m-BN) (Zhang et al., 2018). The thermal conductivity of the 30 wt % m-BN loaded composites increased until 7.29 W m−1 K−1, which is 98% higher than that of casting film and 21% higher than electrospun film with BN, due to the strong hydrogen bondings formed as a result of PDA modification and high orientation of m-BN. In addition, tensile strength and Young modulus for the 30 wt % m-BN loaded composites were also improved compared with pure PVDF electrospun films. These flexible composite fibrous films can be potentially used in some flexible electronic applications. A different strategy was adopted by Huang and his group to produce thermally conductive and electrically insulating PVDF/BNNS composites based on electrospinning of a solution containing PVDF and BNNS, folding, and hot-pressing as shown in (Chen et al., 2019a). The 33 wt % BNNS-containing composite film morphologically had an ordered structure and showed an in-plane thermal conductivity of 10.4 W m−1 K−1 at a thickness of 28 μm, which is 4 times higher than randomly distributed BNNS composites and 2 times higher than directly hot-pressed composites under the same conditions. The thermal conductivity could be increased up to 16.3 W m−1 K−1 when the thickness decreased to 18 μm. These films also show electrically insulating properties and proved to have a cooling capability, which pave the way to thermal management applications for them.
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Open in a separate windowDing and coworkers introduced highly thermoconductive and hydrophobic polyurethane (PU)/BNNS composite nanofibers in order to construct thermal management of textiles for personal cooling via electrospinning blends of fluorinated PU, PU, and BNNS, which were linearly oriented along nanofibers (Yu et al., 2020). The maximum in-plane and cross-plane thermal conductivities, respectively, reached to 17.9 W m−1 K−1 and 0.29 W m−1 K−1 with FPU/BNNS nanofibers with 18% BNNS electrospun at an RH of 50%. These membranes had a high water vapor transmission rate of 11.6 kg m−2 day−1, air permeability of 120 mm s−1, and a high peak heat flux of 0.38 W cm−2. In a recent study by the same group, the hierarchical and interconnected network of vascular plants was adapted to PU/BNNS nanofibrous membranes in order to design multifunctional drying and cooling textiles that allow directional water transport and unblocked heat dissipation throughout perspiration ( ) (Miao et al., 2021). Four layers of membranes were prepared by layer-by-layer assembly of the electrospun aligned PU/BNNS nanofibers with decreasing fiber diameter and capillary pores from inner to outer layers. Sixty wt % BNNS loading in the multilayer nanocomposite resulted in both good mechanical properties with high through-plane and in-plane thermal conductivities of 0.182 W m−1 K−1 and 1.137 W m−1 K−1, respectively. Together with these remarkable thermal conductivities, water transport index of 1,072% and evaporation rate of 0.36 g h−1 highlight the capability of these biomimetric textiles for personal drying and cooling.
Open in a separate windowOutlast® temperature regulating materials start working even before the moisture is created. In this way, the amount of sweat can be significantly reduced. Independent studies confirm a reduction of up to 48%*. And the highlight: If the body temperature falls again, for example due to reduced physical activity, the natural wax in the temperature regulating material clothing releases the stored heat again. Despite fluctuating external temperatures, the user can thus always enjoy a wonderfully balanced climate.
* Tests for various final applications such as clothing, shoes, and helmets conducted by C. Russ – INSIDE CLIMATE, an independent test laboratory in Munich (THG AreaView – SleepView). Details on request.
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