The theories of thermal transport in polymers, which are mainly qualitative, are reviewed. The behaviour of amorphous polymers is better understood than that of partially crystalline ones. An apparatus for measuring the thermal conductivity of solid and molten polymers from room temperature to 250 degrees C is described. 'Edge effects' are eliminated by enclosing the heat source and samples inside the heat sink; hence guard rings are not used. The heat flow is three dimensional and the conductivity is obtained by a numerical calculation. Two disc shaped samples 48mm diameter and 2mm thick are required. An apparatus for measuring the thermal diffusivity of solid and molten polymers from room temperature to 250 degrees C is also described. Disc samples the same size as for the conductivity apparatus are used and the heat equation is again solved numerically. Conductivity and diffusivity results are reported for high density polyethylene, polypropylene, three grades of polymethyl methacrylate, three grades of polystyrene, and a series of carbon black filled natural rubber compounds. To show the effect of molecular weight, conductivity results for a range of polyethylene glycols and a range of silicone fluids are also presented.

Understanding thermal transport processes can guide the rational design of devices and systems for thermal energy conversion and management. Despite the significant progress in thermal transport of inorganic crystals, thermal transport in complicated materials, such as polymers and hybrid materials, remains largely unexplored. This thesis first presents our discovery of the large thermal rectification effects in the novel tapered bottlebrush polymers using nonequilibrium molecular dynamic simulations. In sharp contrast to all other reported asymmetric nanostructures, we observed that the heat current from the wide end to the narrow end in tapered bottlebrush polymers is smaller than that in the opposite direction. It was demonstrated that a more disordered to less disordered structural transition within tapered bottlebrush polymers is essential for generating non-linearity in heat conduction for thermal rectification. Moreover, the thermal rectification factor increases with device length, reaching as high as ~70% with a device length of 28.5nm. This large thermal rectification with strong length dependence uncovers an unprecedented phenomenon - diffusive thermal transport in the forward direction and ballistic thermal transport in the backward direction. This thesis then focuses on thermal transport properties in hybrid materials: graphene-C60 heterostructures and hybrid organic-inorganic (CH3NH3)3Bi2I9 crystals. Graphene-C60 heterostructures assembled by van der Waals interactions between graphene and C60 have shown exciting potential for multifunctional devices. Understanding thermal transport in graphene-C60 heterostructures is the key to guiding the design of vdW heterostructures with desired thermal transport properties. Our equilibrium molecular dynamics simulations found that the in-plane thermal conductivity of the graphene-C60 heterostructure is as high as about 234 W/(mK) at room temperature, exceeding those of most pure metals. On the other hand, vdW interactions enhance the interfacial thermal conductance between graphene and C60 by strengthening out-of-plane phonon couplings between graphene and C60 and increasing in-plane and out-of-plane phonon couplings of the graphene layer. Our study demonstrates that the interfacial thermal conductance of graphene-C60 heterostructure is comparable to that of graphene-hexagonal boron-nitride (hBN) heterostructure. Hybrid perovskite analogues, such as methylammonium bismuth iodide (CH3NH3)3Bi2I9, have emerged as candidate photovoltaic and thermoelectric materials due to their low toxicity and high stability. Thermal transport and phonon properties of (CH3NH3)3Bi2I9 were studied neither experimentally nor theoretically, which hinders the optimal selection and design of stable, non-toxic hybrid perovskite material for photovoltaic and thermoelectric applications. We mapped out the phonon dispersion of (CH3NH3)3Bi2I9 single crystals at 300 K using inelastic x-ray scattering. The frequencies of acoustic phonons are among the lowest of crystals. Nanoindentation measurements verified that these crystals are very compliant and considerably soft. The frequency overlap between acoustic and optical phonons results in strong acoustic-optical scattering. All these features lead to an ultralow thermal conductivity.

Among various branches of polymer physics an important position is occupied by that vast area, which deals with the thermal behav ior and thermal properties of polymers and which is normally called the thermal physics of polymers. Historically it began when the un usual thermo-mechanical behavior of natural rubber under stretch ing, which had been discovered by Gough at the very beginning of the last century, was studied 50 years later experimentally by Joule and theoretically by Lord Kelvin. This made it possible even at that time to distinguish polymers from other subjects of physical investigations. These investigation laid down the basic principles of solving the key problem of polymer physics - rubberlike elasticity - which was solved in the middle of our century by means of the statistical thermodynamics applied to chain molecules. At approx imately the same time it was demonstrated, by using the methods of solid state physics, that the low temperature dependence of heat capacity and thermal expansivity of linear polymers should fol low dependencies different from that characteristic of nonpolymeric solids. Finally, new ideas about the structure and morphology of polymers arised at the end of the 1950s stimulated the development of new thermal methods (differential scanning calorimetry, defor mation calorimetry), which have become very powerful instruments for studying the nature of various states of polymers and the struc tural heterogeneity.

The broad objective of the work was to relate charge, heat, and mass transport in the Air Force's extended chain polymers (especially poly-para-phenylene benzobisthiazole PBT) to other electrical, thermal, mechanical nad microstructural properties of the polymers and also to compare these very unusual, highly anisotropic Air Force materials with other materials when it is scientifically relevant or when potential applications may be involved. Special techniques were developed for making transport-property determinations on samples in the form of thin fibers and small area films. Two types of miniature cells were developed in the study of PBT transport properties. A very promising area of the research relates to an anisotropic version of the Barker-Sharbaugh weak electrolyte model for ionic conduction in polymers. A new technique which has been termed the diffusion controlled-differential current (DCDC) method evolved from experiments related to the weak electrolyte model. This DCDC-technique looks promising as a new analytical tool. The results for PBT turned out to be especially interesting because the ratio of ionic conductivities parallel and perpendicular to the chain axis was very large (100,000 at 300 K) and temperature dependent (smaller ratio at a higher temperature). Special techniques for the thermal conductivity allowed the axial and perpendicular thermal conductivities to be determined.