Effects of crosslinking and chain degradation on the thermal conductivity of polymers

Prediction of Thermal Conductivities of Rubbers by MD Simulations-New Insights

In this article, two main approaches to the prediction of thermal conductivities by molecular dynamics (MD) simulations are discussed, namely non-equilibrium molecular dynamics simulations (NEMD) and the application of the Green–Kubo formula, i.e., EMD. NEMD methods are more affected by size effects than EMD methods. The thermal conductivities of silicone rubbers in special were found as a function of the degree of crosslinking. Moreover, the thermal conductivities of thermoplastic polyurethane as function of the mass fraction of soft segments were obtained by those MD simulations. All results are in good agreement with data from the experimental literature. After the analysis of normalized heat flux autocorrelation functions, it has been revealed that heat in the polymers is mainly transferred by low-frequency phonons. Simulation details as well as advantages and disadvantages of the single methods are discussed in the article.

Beyond homogeneous dispersion: oriented conductive fillers for high κ nanocomposites

Rational design of structures for regulating the thermal conductivities (κ) of materials is critical to many components and products employed in electrical, electronic, energy, construction, aerospace, and medical applications. As such, considerable efforts have been devoted to developing polymer composites with tailored conducting filler architectures and thermal conduits for highly improved κ. This paper is dedicated to overviewing recent advances in this area to offer perspectives for the next level of future development. The limitations of conventional particulate-filled composites and the issue of percolation are discussed. In view of different directions of heat dissipation in polymer composites for different end applications, various approaches for designing the micro- and macroscopic structures of thermally conductive networks in the polymer matrix are highlighted. Methodological approaches devised to significantly ameliorate thermal conduction are categorized with respect to the pathways of heat dissipation. Future prospects for the development of thermally conductive polymer composites with modulated thermal conduction pathways are highlighted.

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.

Radiation Effects on Thermal Properties of Polymers. I. Polyethylene (contd.)

The information about radiation effects on the thermal properties of polymeric materials is of special interest as: (1) the temperature regime of technical structural units operated in the radiation fields is determined in many respects by the values of the material thermal constants and the rate of their degradation (2) the temperature and dose dependencies of heat conductivity and, especially, heat capacity characterize the polymer structure. The data on radiation effects on the thermal conductivity, heat capacity and diffusivity, linear thermal expansion, density, degree of crystallinity, heat and temperature of phase transitions were presented in the first part of this paper [ Nucl. Instr. Meth. in Phys. Res. B 185 (2001) 116–122] for polyethylene of high and low density, as a well-known and typical partly-crystalline polymer that is very convenient for investigation of the radiolysis processes. This part of the review examines such problems as reversible radiation effects on heat capacity, analytical models of the radiation-induced changes and anisotropy effects on the thermal conductivity, accumulated radiation energy (‘heat defect’) and important problems of specific action of different kinds of radiation on the thermal properties of the polymer and dose-rate effects, especially under vacuum.