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The thermal transport properties of oriented polymers are of fundamental andpractical interest. High thermal conductivities ($\gtrsim 50$Wm$^{-1}$K$^{-1}$) have recently been reported in disentangled ultra-highmolecular weight polyethylene (UHMWPE) films, considerably exceeding priorreported values for oriented films. However, conflicting explanations have beenproposed for the microscopic origin of the high thermal conductivity. Here, wereport a characterization of the thermal conductivity and mean free pathaccumulation function of disentangled UHMWPE films (draw ratio $\sim 200$)using cryogenic steady-state thermal conductivity measurements and transientgrating spectroscopy. We observe a marked dependence of the thermalconductivity on grating period over temperatures from 30 - 300 K. Consideringthis observation, cryogenic bulk thermal conductivity measurements, andanalysis using an anisotropic Debye model, we conclude that longitudinal atomicvibrations with mean free paths around 400 nanometers are the primary heatcarriers and that the high thermal conductivity for draw ratio $\gtrsim 150$arises from the enlargement of extended crystals with drawing. The mean freepaths appear to remain limited by the extended crystal dimensions, suggestingthat the upper limit of thermal conductivity of disentangled UHMWPE films hasnot yet been realized.

Origin of high thermal conductivity in disentangled ultra-high molecular weight polyethylene films: ballistic phonons within enlarged crystals