Enhancing the thermal conductivity (TC) of polymeric materials for thermal management applications has attracted attentions because of their beneficiary features such as light weight, anti-corrosive, low cost, flexibility and controllable electrical conductivity. Since phonons are the dominant heat carriers in insulating materials, creating pathways for better phonon transfer and decreasing the phonon scattering inside the matrix are the major strategies for TC enhancement. TC of bulk polymers is much less than their single chains because of the chains entanglement that increases the phonon scattering. Therefore, any approaches that decreases the entanglement of chains or enhances their alignment can be used for TC improvement. Traditionally, TC of insulating materials have been enhanced by incorporation of thermally conductive fillers. Formation of a continuous network of these fillers and their alignment can enhance TC even further in the desired direction. The network of fillers can be achieved at high content of fillers that is accompanied with sacrificing other properties such as mechanical properties and results in high cost of final products. As a result, alternative approaches that can form such a network at low content of fillers have attracted attentions.
Here, we present three different approaches, which were utilized for TC enhancement of different systems. First, induced co-continuous morphology of an immiscible polymer blend, blend of high density polyethylene (HDPE) and poly (methyl methacrylate) (PMMA) was used for localization of carbon nanofibers (CNFs). The co-continuous morphology of immiscible polymer blends has been previously used for formation of continuous network of electrically conductive fillers and electrical conductivity (EC) enhancement. This method, known as double percolation method, requires both the composition of polymers and fillers reach to percolation threshold above which they form co-continuous phase morphology and continuous network of fillers, respectively. Being inspired by this method and considering the involved parameters in tuning the morphology and distribution of fillers, we could show that processing temperature that affect the viscosity ratio of components and distribution of fillers is a key role for EC and TC enhancements. To investigate the effect of temperature on the morphology and distribution of fillers, two different temperatures of 150 and 230 °C were used for processing the blend with different contents of CNFs. The samples that were processed at 230 °C showed finer morphology and higher EC than the processed samples at 150 °C for all the content of CNFs. While, the samples processed at 150 °C showed higher TC and coarser morphology. The difference in the trends of TC and EC is because of their different mechanisms. The finer morphology and better distribution of fillers at 230 °C is accompanied with formation of more interfaces that increases the interfacial phonon scattering and decreases TC consequently. To our knowledge, this is the first study investigating the effect of processing temperature on the location of fillers in an immiscible polymer blends and its effect on TC and EC of the composite obtained.
Secondly, a series of isotropic thermally conductive composites were fabricated by incorporating xylitol crystals into aligned boron nitride (BN) aerogel (BNA) that was formed by ice-template method. As it was mentioned earlier, both the continuous network of fillers and their alignment in the desired direction can further increase the efficiency of TC enhancement. BN is a two dimensional (2D) ceramic filler that is electrically insulating and has high TC. Hence, it is a good candidate for fabricating the composite which should be exclusively thermally conductive to be used in electronic devices. Similar to the other 2D fillers, BN has anisotropic TC with high TC in the in-plane direction. Therefore, TC will be efficiently increased by aligning the in-plane direction of BN parallel to the heat flow direction. There are numerous methods for aligning 2D fillers in the horizontal direction, whereas their alignment in the vertical direction is not that easy. Ice-template method is a simple and low cost method for vertical alignment of fillers along the direction of ice crystal growth. Therefore, by employing this method, BNA with vertically aligned BN walls was fabricated. Besides the filler network, the filling agent has a key role in the TC of the final composite as well. Sugar alcohols (SAs) have relatively high TC compared to polymers and this is because of their high crystallinity. In this study the fabricated BNA went through carbonization (CBNA) first to decrease its hydrophilicity and increase its structure integrity and then molten xylitol was infiltrated into it. Xylitol crystals were solidified perpendicular to the BN walls, creating crystal packs between BN walls. As a result the xylitol crystal packs offset the anisotropic TC of the scaffold. These results offer new insights into isotropic thermally conductive composites that can be used for next generation of heat dissipating materials. Similarly, this scaffold was filled with erythritol, another SA, and the effect of scaffold on the phase change properties and TC of obtained composite was investigated. In addition to TC enhancement and gaining isotropic TC, BN scaffold improved the subcooling effect, shape and thermal stability, and the ability of the erythritol for releasing heat during crystallization process.
At last, a filler-free approach was used for TC enhancement of a polymer-based system. Here, the effects of intermolecular interactions and engineering such interactions on mechanical flexibility, optical transparency and TC of the system were investigated. For this purpose, sodium carboxymethyl cellulose (SCMC), a well-known hydrophilic biopolymer, and xylitol, were selected as the matrix and the filler, respectively. Increasing the content of xylitol up to 50 wt% resulted in enhanced TC of up to 1.75 times of neat SCMC. Besides, the mechanical flexibility and optical transparency were also improved. The achieved enhancements are attributed to the newly formed hydrogen bonding that is due to presence of numerous hydrophilic functional groups in the both components. Formation of new hydrogen bonding between SCMC and xylitol was accompanied with formation of homogenously distributed thermal network throughout SCMC. Therefore, engineering the interchain interactions can be an alternative filler-free approach for enhancing TC of polymeric-based materials.