by josh perry, editor
in a report released by science china press, researchers detail their work on amorphous polymer chains to gain a greater understanding for how heat carriers move in the polymers and its effect on thermal conductivity.
the molecular chains are randomly oriented and entangled with each other. heat carriers could 'jump' to another chain as the red arrow denoted. (science china press)
amorphous polymers are intrinsically disorder, according to the report, with long, entangled molecular chains and they are typically used as insulators because of a low thermal conductivity. researchers use electrospinning to control how the chains are aligned; high voltage and stretching force during the chain formation aligns the molecules along the axis of the fiber and avoids entangling.
previous research showed that thinner nanofibers, smaller than 50 nanometers, have a much greater thermal conductivity thank bulk polymers.
the researchers examined how heat carriers moved within and between polymer chains with the assumption that intra-chain heat transport would be greater than inter-chain because of the stronger covalent bonds between the chains.
“supposing the heat carries undergo a random walk in the molecular chains, the thermal diffusivity is corresponded to different chain conformations,” the report explained. “in ultra-thin polymer nanofibers, all the molecular chains are oriented along the fiber axis, thus the heat carriers could only move forward or backward within the molecular chain along the fiber axis, leading to the upper limit of thermal conductivity.”
in bulk-like polymer, the molecular chains are moving to allow the heat carriers to move in directions other than just along the fiber’s axis and the heat carriers can jump from one chain to another when chains interact.
“in other words, heat carriers in bulk polymers undergo many invalid 'steps' for thermal diffusivity along the desired direction,” the report continued. “in order to describe the diameter dependence of chain conformation, an empirical function is proposed and explains well the experiment results.”
the research was recently published in national science review. the abstract read:
“the 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 (3d) to quasi-one dimension (1d) in polyimide(pi) 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 the upper limit of thermal conductivity of amorphous polymers in the quasi-1d limit.”