On the temperature dependence of the thermal conductivity of linear amorphous polymers

3D-Networks Based Polymer Composites for Multifunctional Thermal Management and Electromagnetic Protection: A Mini Review

The rapid development of miniaturized, high-frequency, and highly integrated microelectronic devices has brought about critical issues in electromagnetic compatibility and thermal management. In recent years, there has been significant interest in lightweight polymer-based composites that offer both electromagnetic interference (EMI) shielding and thermal conductivity. One promising approach involves constructing three-dimensional (3D) interconnection networks using functional fillers in the polymer matrix. These networks have been proven effective in enhancing the thermal and electrical conductivity of the composites. This mini-review focuses on the preparation and properties of 3D network-reinforced polymer composites, specifically those incorporating metal, carbon, ceramic, and hybrid networks. By comparing the effects of different filler types and distribution on the composite materials, the advantages of 3D interconnected conductive networks in polymer composites are highlighted. Additionally, this review addresses the challenges faced in the field of multifunctional thermal management and electromagnetic protection materials and provides insights into future development trends and application prospects of 3D structured composites.

The Effect of Mechanical Elongation on the Thermal Conductivity of Amorphous and Semicrystalline Thermoplastic Polyimides: Atomistic Simulations

Over the past few decades, the enhancement of polymer thermal conductivity has attracted considerable attention in the scientific community due to its potential for the development of new thermal interface materials (TIM) for both electronic and electrical devices. The mechanical elongation of polymers may be considered as an appropriate tool for the improvement of heat transport through polymers without the necessary addition of nanofillers. Polyimides (PIs) in particular have some of the best thermal, dielectric, and mechanical properties, as well as radiation and chemical resistance. They can therefore be used as polymer binders in TIM without compromising their dielectric properties. In the present study, the effects of uniaxial deformation on the thermal conductivity of thermoplastic PIs were examined for the first time using atomistic computer simulations. We believe that this approach will be important for the development of thermal interface materials based on thermoplastic PIs with improved thermal conductivity properties. Current research has focused on the analysis of three thermoplastic PIs: two semicrystalline, namely BPDA-P3 and R-BAPB; and one amorphous, ULTEM^TM. To evaluate the impact of uniaxial deformation on the thermal conductivity, samples of these PIs were deformed up to 200% at a temperature of 600 K, slightly above the melting temperatures of BPDA-P3 and R-BAPB. The thermal conductivity coefficients of these PIs increased in the glassy state and above the glass transition point. Notably, some improvement in the thermal conductivity of the amorphous polyimide ULTEM^TM was achieved. Our study demonstrates that the thermal conductivity coefficient is anisotropic in different directions with respect to the deformation axis and shows a significant increase in both semicrystalline and amorphous PIs in the direction parallel to the deformation. Both types of structural ordering (self-ordering of semicrystalline PI and mechanical elongation) led to the same significant increase in thermal conductivity coefficient.

State-of-the-Art, Opportunities, and Challenges in Bottom-up Synthesis of Polymers with High Thermal Conductivity

In contrast to metals, polymers are predominantly thermal and electrical insulators. With their unparalleled advantages such as light weight, turning polymer insulators into heat conductors with metal-like thermal conductivity is...

Crosslinking Effect on Thermal Conductivity of Electrospun Poly(acrylic acid) Nanofibers

The thermal conductivity (k) of poly(acrylic acid) (PAA) nanofibers, which were electrospun at various electrospinning voltages, was measured using suspended microdevices. While the thermal conductivities of the as-spun PAA nanofibers varied depending on the electrospinning voltages, the most pronounced 3.1-fold increase in thermal conductivity in comparison to that of bulk PAA was observed at the electrospinning voltage of 14 kV. On the other hand, a reduction in the thermal conductivity of the nanofibers was observed when the as-spun nanofibers were either thermally annealed at the glass transition temperature of PAA or thermally crosslinked. It is notable that the thermal conductivity of crosslinked PAA nanofibers was comparable to that of crosslinked bulk PAA. Polarized Raman spectroscopy and Fourier transform infrared spectroscopy verified that the k enhancement via electrospinning and the k reduction by the thermal treatments could be attributed to the conformational changes between gauche and trans states, which may be further related to the orientation of molecular chains. In contrast, hydrogen bonds did not contribute significantly to the k enhancement. Additionally, the suppression of k observed for the crosslinked PAA nanofibers might result from the shortening of single molecular chains via crosslinking.

Effects of CNTs on thermal transitions, thermal diffusivity and electrical conductivity in nanocomposites: comparison between an amorphous and a semicrystalline polymer matrix

Two series of polymer nanocomposites (PNCs) based on amorphous styrene-butadiene rubber (SBR) and semicrystalline linear low-density polyethylene (PE) matrices were filled with 2-15 wt% carbon nanotubes (CNT) and were studied by employing calorimetry, dielectric spectroscopy and laser flash analysis. The electrical conductivity, σ, increased with CNT loading and similar values were exhibited for the two matrices, uniquely depending on the concentration of the CNTs, suggesting practically no effects of the crystalline fraction (CF) on σ. For both types of matrix, a fraction of the polymer was found to be immobilized (rigid amorphous fraction, RAF). For the amorphous SBR, the RAF in PNCs originates uniquely from the presence of the filler (RAFfiller up to 0.19 wt). On the other hand, for the semicrystalline PE, the RAF is significantly larger (0.4-0.6 wt) due to the severe contribution of the RAF around the crystals (RAFcrystal). The thermal diffusivity, α, is quite low in both types of PNCs and exhibits higher values in the semicrystalline matrix (PE-based PNCs). Our results suggest that in these PNCs, heat transport mechanisms are activated mainly in the crystalline domains, more so with the additive contribution of the RAFcrystal. In the amorphous SBR-based PNCs, heat transport is facilitated mainly by CNTs, whereas the RAFfiller is found to be a good measure of the thermal resistance behavior of CNT/polymer interphases and consequently, of thermal diffusivity. Direct correlation of the results obtained by the three techniques with each other revealed the systematic dependence of α on the amount of RAF in each matrix; the α(RAF) trends, however, are different for the two matrices. Furthermore, the results suggest that the two RAFs exhibit different structural characteristics, e.g. the RAFcrystal exhibits a more ordered structure than the RAFfiller; this issue is still an open debate in the literature.

Effects of Covalent Functionalization of MWCNTs on the Thermal Properties and Non-Isothermal Crystallization Behaviors of PPS Composites

In this study, a PPS/MWCNTs composite was prepared with poly(phenylene sulfide) (PPS), as well as pristine and covalent functionalized multi-walled carbon nanotubes (MWCNTs) via melt-blending techniques. Moreover, the dispersion of the MWCNTs on the PPS matrix was improved by covalent functionalization as can be seen from a Field-Emission Scanning Electron Microscope (FE-SEM) images. The thermal properties of the PPS/MWCNTs composites were characterized using a thermal conductivity analyzer, and a differential scanning calorimeter (DSC). To analyze the crystallization behavior of polymers under conditions similar with those in industry, the non-isothermal crystallization behaviors of the PPS/MWCNTs composites were confirmed using various kinetic equations, such as the modified Avrami equation and Avrami-Ozawa combined equation. The crystallization rate of PPS/1 wt % pristine MWCNTs composite (PPSP1) was faster because of the intrinsic nucleation effect of the MWCNTs. However, the crystallization rates of the composites containing covalently-functionalized MWCNTs were slower than PPSP1 because of the destruction of the MWCNTs graphitic structure via covalent functionalization. Furthermore, the activation energies calculated by Kissinger’s method were consistently decreased by covalent functionalization.

Effect of the Grafting Reaction of Aluminum Nitride on the Multi-Walled Carbon Nanotubes on the Thermal Properties of the Poly(phenylene sulfide) Composites

In this study, the PPS/MWCNTs/AlN composite was prepared with poly(phenylene sulfide) (PPS), covalent functionalized multi-walled carbon nanotubes (fMWCNTs), and aluminum nitride (AlN) via melt-blending techniques. The AlN is a fascinating non-oxidizing ceramic material having the highest thermal conductivity among the ceramic materials. In order to introduce the functional groups on the surface of the AlN particles, a silane coupling agent was used as it is able to graft with the functional groups on the covalent functionalized MWCNTs. The silanization reaction of the AlN was confirmed qualitatively and quantitatively by FT-IR (Fourier Transform Infrared Spectroscopy), and XPS (X-ray Photoelectron Spectroscopy). The grafting reaction of the AlN particles on the MWCNTs was confirmed using UV⁻Vis (Ultraviolet-Visible Spectroscopy), FE-SEM (Field-Emission Scanning Electron Microscopy) and FE-TEM (Field-Emission Transmission Electron Microscopy) images. The grafting reaction was accomplished by observing the change of the transmittance, the morphology of the AlN particle bonded to the MWCNTs. For the morphological changes of the fractured surface of the PPS/MWCNTs/AlN composites by FE-SEM, the hybrid filler was homogeneously dispersed on the PPS matrix when the AlN particle was grafted on the MWCNTs. The homogeneous distribution of the hybrid filler acts as a heat transfer path, which led the higher thermal properties, such as thermal conductivity, thermal resistance, and melting temperature than those of not grafted MWCNTs.

The influence of the interface between mica and epoxy matrix on properties of epoxy-based dielectric materials with high thermal conductivity and low dielectric loss

Two strategies, including in situ grafting chemicals and polymer coating micas, were used to improve the interface between mica and epoxy matrix. The epoxy composites prepared by strategy I exhibited better overall performance.

Structure-induced enhancement of thermal conductivities in electrospun polymer nanofibers

Polymers that are thermally insulating in bulk forms have been found to exhibit higher thermal conductivities when stretched under tension. This enhanced heat transport performance is believed to arise from the orientational alignment of the polymer chains induced by tensile stretching. In this work, a novel high-sensitivity micro-device platform was employed to determine the axial thermal conductivity of individual Nylon-11 polymer nanofibers fabricated by electrospinning and post-stretching. Their thermal conductivity showed a correlation with the crystalline morphology measured by high-resolution wide-angle X-ray scattering. The relationship between the nanofiber internal structures and thermal conductivities could provide insights into the understanding of phonon transport mechanisms in polymeric systems and also guide future development of the fabrication and control of polymer nanofibers with extraordinary thermal performance and other desired properties.

Ultrafast thermal analysis of surface functionalized gold nanorods in aqueous solution

The thermal conductivity and heat capacity of surfactant and polyelectrolyte coatings of gold nanorods (GNRs) in aqueous solution are investigated by transient absorption, following femtosecond pumping of the longitudinal localized surface plasmons. Surfactant and polyelectrolyte layer thicknesses are measured by dynamic light scattering (DLS). The GNRs are initially coated with a bilayer of the quaternary ammonium surfactant cetyltrimethylammonium bromide (CTAB). The rate of change of the absorption of gold nanorods in aqueous solution varies with the probe laser wavelength due to the shift in the plasmon resonance created by heating of media around the particles. The cooling dynamics of gold nanorods are best measured by tuning the pump-probe laser wavelength to the absorption peak of the sample. The heat capacity of the surfactant layer is 2.0 ± 0.3 J cm(-3) K(-1); the thermal conductivity of the surfactant layer drops from 0.24 to 0.18 W m(-1) K(-1) at solution concentrations above the CTAB critical micelle concentration (cmc). Layer-by-layer polyelectrolyte coatings using poly(acrylic acid) (PAA) and polyallyamine hydrochloride (PAH) increase the thermal conductivity and heat capacity of the surface layer. PAH-terminated layers have increased thickness, thermal conductivity, and heat capacity relative to PAA-terminated layers; this effect is attributed to greater water penetration into PAH-terminated surface layers.

Biocompósitos de acetato de celulose e fibras curtas de Curauá tratadas com CO2 supercrítico

Neste trabalho foram desenvolvidos biocompósitos baseados em acetato de celulose e fibras curtas de Curauá tratadas com dióxido de carbono supercrítico. O tratamento das fibras resultou na extração parcial de lignina, sendo este um método interessante pois não resulta em rejeitos químicos. Duas séries de biocompósitos, uma delas plastificada com ftalato de dioctila (DOP) e outra com citrato de trietila (TEC), foram preparadas por extrusão. Para ambas ocorreu a fibrilação e distribuição uniforme das fibrilas. Como conseqüência, os biocompósitos apresentaram maior capacidade calorífica, menor condutividade térmica e maior coeficiente de expansão térmica em comparação ao acetato de celulose plastificado. O tratamento das fibras com CO2 supercrítico intensificou as variações destas propriedades. Dentre os plastificantes, o DOP mostrou-se ligeiramente mais eficiente, resultando em materiais com menores valores de Tg e de módulo de Young. A adição das fibras teve um impacto relativamente baixo sobre o módulo (10%), porém houve uma perda significativa da resistência ao impacto. O conjunto de resultados permite concluir que estes biocompósitos apresentam potencial de aplicação como isolantes térmicos, sendo que os plastificados com TEC apresentam como vantagem o fato de todos seus componentes serem biodegradáveis.

Temperature dependence of thermodiffusion in aqueous suspensions of charged nanoparticles

Measurements of particle flows driven by temperature gradients are conducted as a function of temperature on aqueous suspensions of polystyrene nanoparticles and proteins of T4 lysozyme and mutant variants of T4 lysozyme. The thermodiffusion coefficients are measured using a microfluidic beam deflection technique on suspensions with particle concentrations on the order of 1 vol %. At T < or ~ 20 degrees C, all of the nanoparticles studied migrate to the hot regions of the fluid; i.e., the thermodiffusion coefficient is negative. At higher temperature, T > or ~ 50 degrees C, the thermodiffusion coefficient is positive with a value consistent with the predictions of a theoretical model originally proposed by Derjaguin that is based on the enthalpy changes due to polarization of water molecules in the double layer.

Transport of nanoscale latex spheres in a temperature gradient

We use a micrometer-scale optical beam deflection technique to measure the thermodiffusion coefficient D(T) at room temperature (approximately 24 degrees C) of dilute aqueous suspensions of charged polystyrene spheres with different surface functionalities. In solutions with large concentrations of monovalent salts, < or approximately = 100 mM, the thermodiffusion coefficients for 26 nm spheres with carboxyl functionality can be varied within the range -0.9 x 10(-7) cm2 s(-1) K(-1) < D(T) < 1.5 x 10(-7) cm2 s(-1) K(-1) by changing the ionic species in solution; in this case, D(T) is the product of the electrophoretic mobility mu(E) and the Seebeck coefficient of the electrolyte, S(e) = (Q(C)* - Q(A)*)/2eT, D(T) = -S(e) mu(E), where and are the single ion heats of transport of the cationic and anionic species, respectively. In low ionic strength solutions of LiCl, < or approximately = 5 mM, and particle concentrations < or approximately = 2 wt %, D(T) is negative, independent of particle concentration and independent of the Debye length; D(T) = -0.73 +/- 0.05 x 10(-7) cm2 s(-1) K(-1).

Thermal transport in au-core polymer-shell nanoparticles

Thermal transport in aqueous suspensions of Au-core polymer-shell nanoparticles is investigated by time-resolved measurements of optical absorption. The addition of an organic cosolvent to the suspension causes the polystyrene component of the polymer shell to swell, and this change in the microstructure of the shell increases the effective thermal conductivity of the shell by a factor of approximately 2. The corresponding time scale for the cooling of the nanoparticle decreases from 200 ps to approximately 100 ps. The threshold concentration of cosolvent that creates the changes in thermal conductivity, 5 vol % tetrahydrofuran in water or 40 vol % N,N-dimethylformamide in water, is identical to the threshold concentrations for producing small shifts in the frequency of the plasmon resonance. Because the maximum fraction of solvent in the polymer shell is less than 20 vol %, the increase in the effective thermal conductivity of the shell cannot be easily explained by contributions to heat transport by the solvent or enhanced alignment of the polystyrene backbone along the radial direction.