Thermal Conductivity Of Amorphous Polymers Research Articles

Empirical universal approach to describing the thermal conductivity of amorphous polymers: Effects of pressure, radiation and the Meyer–Neldel rule

In this study, we propose and validate a universal temperature-dependent model for characterizing the thermal conductivity of amorphous polymers over a wide temperature range. Our approach captures key features in the thermal conductivity data, including a plateau, an inflection point, and the subsequent increase and saturation with rising temperature. Importantly, this model proves effective not only for pristine amorphous polymers but also for polymers subjected to external influences. We investigate the temperature-dependent thermal conductivity of amorphous polymer materials under various external conditions, such as hydrostatic pressure, radiation exposure, and the incorporation of fillers. Our analysis reveals novel insights into the dual-channel heat transfer mechanisms within amorphous polymers. Specifically, we observe a linear relationship between the logarithm of the “coherence” conductivity pre-factor and the characteristic energy, consistent with the Meyer–Neldel rule governing thermal conductivity. This research advances our understanding of thermal transport in amorphous polymers and underscores the applicability of the proposed universal model in describing complex thermal behavior across different conditions.

Thermal conductivity of amorphous polymers and its dependence on molecular weight

Thermal conductivity is an important transport property governing the performance of polymers in non-isothermal conditions. Nevertheless, its dependence on molecular weight M has not been the subject of as much attention as other properties of polymeric materials. We determine the thermal conductivity of polystyrene and polyisobutylene for a wide range of molecular weight by measuring the density, heat capacity and thermal diffusivity. Using coarse-graining and reverse mapping methods, we were able to produce molecular melts to study the thermal conductivity of polystyrene using molecular dynamics simulations over a similar range of molecular weight. We find satisfactory agreement between the experimental and simulation results. However, all of our results show that thermal conductivity depends only slightly on molecular weight up the entanglement limit and it is independent thereafter. Our results put into question the few previous experimental studies on this topic by showing that the previously accepted proportionality to M does not hold. Our findings could have significant implications for the understanding of complex phenomena such as anisotropic thermal conductivity in polymers subjected to flow.

Size Effects in the Thermal Conductivity of Amorphous Polymers

Manipulating thermal conductivity through nanoengineering is of critical importance to advance technologies, such as soft robotics, artificial skin, wearable electronics, batteries, thermal insulation, and thermoelectrics. Here, by examining amorphous polymers, including polystyrene, polypropylene, polyethylene, and ethylene vinyl alcohol, using molecular dynamics simulations, we find that the thermal conductivities of amorphous polymers can be reduced below their amorphous limit by size effects. Size-dependent thermal transport in amorphous materials is decomposed into crystalline, crystalline-to-amorphous, and amorphous regimes. In the amorphous regime, the mean free path of propagating heat carriers can range from tens of nanometers to more than 100 nm, contributing 16%--36% of the total thermal conductivity. A two-channel model that combines no size effect (i.e., difusons and locons) and size effect (i.e., propagons) is proposed to account for size-dependent thermal conductivity. We also find that the presence of charged molecules in polymers can significantly affect the thermal conductivity and its size effects due to electrostatic interactions. This work provides insights into the thermal conductivity of amorphous polymers that will have a broad impact on the nano- and chemical engineering of polymers for various energy-related applications.

Thermal Transport in Ampholytic Polymers: The Role of Hydrogen Bonding and Water Uptake

The low thermal conductivity of amorphous polymers typically prevents their usage in thermal management applications. Therefore, increasing their intrinsic thermal conductivity poses an exciting sc...

Thermal-conductivity measurement by time-domain thermoreflectance

Abstract

Dimensional crossover of heat conduction in amorphous polyimide nanofibers

ABSTRACTThe mechanism of thermal conductivity in amorphous polymers, especially polymer fibers, is unclear in comparison with that in inorganic materials. Here, we report the observation of a crossover of heat conduction behavior from three dimensions to quasi-one dimension in polyimide nanofibers at a given temperature. A theoretical model based on the random walk theory has been proposed to quantitatively describe the interplay between the inter-chain hopping and the intra-chain hopping in nanofibers. This model explains well the diameter dependence of thermal conductivity and also speculates on the upper limit of thermal conductivity of amorphous polymers in the quasi-1D limit.

An empirical model to predict temperature‐dependent thermal conductivity of amorphous polymers

ABSTRACTThermal conductivity in polymers has been theoretically and experimentally studied in good detail, but there is a need for more accurate models. Polymeric thermal conductivity exhibits a plateau‐like transition at temperatures around 10 K, which is not well accounted for by existing models. In this work, an empirical model that can predict temperature‐dependent thermal conductivity of amorphous polymers is developed. The model is based on kinetic theory and accounts for three sets of vibrational modes in polymers, and is developed using classical expressions, results of previous simulations, and experimental data. Fundamental material properties like density, monomer molecular weight, and speed of sound are the only input parameters. The model provides estimates for the locations of transitions between different sets of vibrational modes, an upper limit for the thermal conductivity, and temperature‐dependent thermal conductivity, which are in good agreement with experimental data. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1160–1170

Role of Chain Morphology and Stiffness in Thermal Conductivity of Amorphous Polymers.

Designing thermally conductive polymer is of scientific interest and practical importance for applications like thermal interface materials, electronics packing, and plastic heat exchangers. In this work, we study the fundamental relationship between the molecular morphology and thermal conductivity in bulk amorphous polymers. We use polyethylene as a model system and performed systematic parametric study in molecular dynamics simulations. We find that the thermal conductivity is a strong function of the radius of gyration of the molecular chains, which is further correlated to persistence length, an intrinsic property of the molecule that characterizes molecular stiffness. Larger persistence length can lead to more extended chain morphology and thus higher thermal conductivity. Further thermal conductivity decomposition analysis shows that thermal transport through covalent bonds dominates the effective thermal conductivity over other contributions from nonbonded interactions (van der Waals) and translation of molecules disregarding the morphology. As a result, the more extended chains due to larger persistence length provide longer spatial paths for heat to transfer efficiently and thus lead to higher thermal conductivity. In addition, rigid rod-like polymers with very large persistence length tend to spontaneously crystallize and form orientated chains, leading to a thermal conductivity increase by more than 1 order of magnitude. Our results will provide important insights into the design of thermally conductive amorphous polymers.

THERMAL TRANSPORT IN POLYMERS

An introduction to polymers and their thermal conductivity is provided, with particular attention paid to recent work that has highlighted the potential to make high thermal conductivity polymers. The thermal conductivity of amorphous polymers is generally low, on the order of 0.1–1.0 W m−1 K−1; however, polymers can be inexpensive to manufacture and they are corrosion resistant and lightweight, which makes them attractive for heat transfer applications. To realize their potential, higher thermal conductivity and higher strength is needed, which can be achieved to some extent by adding fillers to a polymer matrix. A review of the strategies employed to raise the thermal conductivity of polymers is provided along with an introductory review of the physics that intrinsically allows individual polymer molecules to serve as good heat conductors.

Thermal conductivity behaviour of polymers around glass transition and crystalline melting temperatures

The thermal conductivity of five semi-crystalline and four amorphous polymers was determined within a wide range of temperature, starting at room temperature and going up to temperatures above the polymer melting point (Tm) for semi-crystalline polymers or above the glass transition temperature (Tg) for amorphous polymers. Two transient techniques were employed in the experimental investigation: the hot wire technique for the group of amorphous polymers, and the laser flash technique for the semicrystalline polymers. As expected, the experimental results show that Tg exerts a measureable influence on the thermal conductivity of amorphous polymers. In the case of semi-crystalline polymers, a singular behaviour of the thermal conductivity is observed within the Tm range. In order to explain the anomalous behaviour, the influence of these transition temperatures on the thermal conductivity behaviour with temperature has been analysed in terms of a phonon conduction process and temperature variations of specific heat and modulus of elasticity of the analyzed polymers.

Interfacial thermal conductance in spun-cast polymer films and polymer brushes

Interfaces between inorganic materials and anharmonic polymers have potentially intriguing thermal transport behavior. The low thermal conductivity of amorphous polymers limits significant interfacial effects to polymer film thicknesses of only a few nanometers. We use time-domain thermoreflectance to directly measure interfacial effects in the thermal conductance of spun-cast poly(methyl methacrylate) (PMMA) thin films and PMMA brushes “grafted-from” the substrate. PMMA brushes are expected to have polymer chains partially aligned perpendicular to the substrate, yet only a modest increase (13%) in thermal conductivity is observed over spun-cast layers.

Correlation and prediction of the thermal conductivity of amorphous polymers

A group contribution model was developed for the correlation and prediction of the thermal conductivity of glassy and liquid amorphous polymers. The model is predictive and only requires the structure of the polymers and the existing group parameters in the calculation. A total of 28 polymer systems with 700 data points were used to test the model. Its predictive accuracy is normally better than 15%. The new model is simple to apply, which is very convenient for engineering use.

双子プローブによる溶融ポリマーの熱伝導率と装置の開発

Determining the thermal conductivity of molten polymers is very important in flow and thermal analyses using computer aided engineering (CAE) programs. The present study introduces a thermal conductivity device that consists of two probes that are inserted into the experimental specimen to be measured and a reference specimen. Using this device, the effect of temperature on thermal conductivity and thermal diffusivity within the range of 50°C to 280°C is determined. The accuracy of the proposed device was determined to be within ±4∼6%. Instrument calibration can be performed easily for each measurement because the instrument incorporates both a reference and measurement probe. The thermal conductivity of crystalline polymers gradually decreased as the temperature was increased, and thermal conductivity shows a minimal value at the melting point. In contrast, the thermal conductivity of amorphous polymers was remarkably independent of temperature. The thermal conductivity of the molten state is 10∼40% smaller than that of the solid crystalline state. In the case of amorphous polymers, thermal conductivity remains relatively constant or increases slightly with increasing temperature. The thermal diffusivity of crystalline polymers showed a similar trend as that of the thermal conductivity, whereas in amorphous polymers thermal diffusivity gradually decreased with increasing temperature.

Thermal conductivity of amorphous polymers

The dependence of the thermal conductivity and thermal diffusivity of amorphous polymers on pressure and temperature has been experimentally determined.

A generalized thermal conductivity dependence for amorphous polymers

A method of treating the experimental data for the thermal conductivity of amorphous polymers is presented. By means of the proposed method of treatment an invariant thermal conductivity curve for a wide range of temperatures is derived.

Universal relation for the thermal conductivity of amorphous polymers

An invariant relation is derived for the thermal conductivity of amorphous polymers above the vitrification temperature.

Effects of Radiation-Induced Cross-Links on the Low-Temperature Thermal Properties of Polystyrene

Measurements of the thermal conductivity and heat capacity in the liquid-helium temperature range of polystyrene which was cross linked by exposure to an 80-MeV electron beam are reported. The measurements were performed over a range of cross-linking coefficient from 0 to 12. No significant change in the thermal conductivity was observed, but about a 25% reduction in the heat capacity occurred. The thermal conductivity results are compatible with the existing theory of the thermal conductivity of amorphous polymers, while the heat capacity result is taken to indicate a significant reduction in the ``excess heat capacity'' associated with nonacoustic, low-frequency modes.

Temperature Dependence of the Thermal Conductivity of Amorphous Polymers: Polymethyl Methacrylate

An analysis of the thermal conductivity of polymethyl methacrylate is presented which fits the experimental data to better than 10% from 1°K to the glass temperature. The model employed is a slight modification of Klemens' theory of the thermal conductivity of amorphous substances in which explicit account of the fibrous nature of polymers is taken by assigning a portion of the specific heat to one-dimensional vibrations and assigning to these vibrations a mean-free path which is different from that for three-dimensional vibrations. It is found that the resulting mean-free path for vibrations along the polymer chains is of the order of 7 Å.

Low-Temperature Thermal Conductivity of Amorphous Polymers: Polystyrene and Polymethylmethacrylate

Measurements of the thermal conductivity between 4.5° and 1°K are reported for four commercial samples of polymethylmethacrylate and two commercial samples of polystyrene. It is shown that the thermal conductivity in this temperature range can be understood in terms of Klemens' theory of the thermal conductivity of amorphous substances in which the role of transverse vibrations is unimportant at low temperatures. Further, it is shown that the thermal conductivity of a partially oriented sample of polymethylmethacrylate is but slightly higher along the direction of stretch than for an unoriented sample; hence that vibrations along the polymer chains do not play a dominant role in the transport of heat at liquid-helium temperatures. Indirect measurements of the specific heat of these substances have also been performed and yield C/T3=735±50 erg/cm3-(K°)4 for polystyrene and C/T3=446±38 erg/cm3-(K°)4 for polymethylmethacrylate.