Plastics are excellent insulators, meaning they can efficiently trap heat — a quality that can be an advantage in something like a coffee cup sleeve. But this insulating property is less desirable in products such as plastic casings for laptops and mobile phones that overheat, in part, because the coverings trap the heat that the devices produce.
At the microscopic level, polymers are made from long chains of monomers, or molecular units, linked end to end. These chains are often tangled in a spaghetti-like ball. Heat carriers have a hard time moving through this disorderly mess and tend to get trapped within the polymeric snarls and knots. Previously developed insulators-turned-conductors could only dissipate heat in one direction, along the length of each polymer chain. Heat couldn’t travel between polymer chains due to weak Van der Waals forces — a phenomenon that essentially attracts two or more molecules close to each other.
A polymer thermal conductor — a plastic material that works as a heat conductor, dissipating heat rather than insulating it — was developed. The new polymer, which is lightweight and flexible, can conduct ten times as much heat as most commercially used polymers.
For electronics, polymers would offer a unique combination of properties, as they are lightweight, flexible, and chemically inert. Polymers are also electrically insulating, meaning they do not conduct electricity, and can therefore be used to prevent devices such as laptops and mobile phones from short-circuiting in their users’ hands.
Polymers with high thermal conductivity were created by simultaneously engineering intramolecular and inter-molecular forces — a method that could enable efficient heat transport along and between polymer chains.
A heat-conducting polymer known as polythiophene was developed — a type of conjugated polymer that is commonly used in many electronic devices.
A new way to engineer a polymer conductor using oxidative chemical vapor deposition (oCVD) was established, whereby two vapors are directed into a chamber and onto a substrate, where they interact and form a film. The oxidant was flowed into a chamber along with a vapor of monomers — individual molecular units that, when oxidized, form into the chains known as polymers. The polymers were grown on silicon/glass substrates, onto which the oxidant and monomers are adsorbed and reacted, leveraging the unique self-templated growth mechanism of CVD technology. Relatively large-scale samples, each measuring 2 square centimeters, were produced.
Each sample’s thermal conductivity was measured using time-domain thermal reflectance — a technique in which a laser is shot onto the material to heat up its surface, and then the drop in its surface temperature is monitored by measuring the material’s reflectance as the heat spreads into the material.
On average, the polymer samples were able to conduct heat at about 2 watts per meter per kelvin — about 10 times faster than what conventional polymers can achieve. The polymer samples appeared nearly isotropic, or uniform, suggesting that the material’s properties, such as its thermal conductivity, should also be nearly uniform. Following this reasoning, the team predicted that the material should conduct heat equally well in all directions, increasing its heat-dissipating potential.