Kavli Affiliate: Austin J. Minnich
| First 5 Authors: Taeyong Kim, Stavros X. Drakopoulos, Sara Ronca, Austin J. Minnich,
| Summary:
The thermal transport properties of oriented polymers are of fundamental and
practical interest. High thermal conductivities ($gtrsim 50$
Wm$^{-1}$K$^{-1}$) have recently been reported in disentangled ultra-high
molecular weight polyethylene (UHMWPE) films, considerably exceeding prior
reported values for oriented films. However, conflicting explanations have been
proposed for the microscopic origin of the high thermal conductivity. Here, we
report a characterization of the thermal conductivity and mean free path
accumulation function of disentangled UHMWPE films (draw ratio $sim 200$)
using cryogenic steady-state thermal conductivity measurements and transient
grating spectroscopy. We observe a marked dependence of the thermal
conductivity on grating period over temperatures from 30 – 300 K. Considering
this observation, cryogenic bulk thermal conductivity measurements, and
analysis using an anisotropic Debye model, we conclude that longitudinal atomic
vibrations with mean free paths around 400 nanometers are the primary heat
carriers and that the high thermal conductivity for draw ratio $gtrsim 150$
arises from the enlargement of extended crystals with drawing. The mean free
paths appear to remain limited by the extended crystal dimensions, suggesting
that the upper limit of thermal conductivity of disentangled UHMWPE films has
not yet been realized.
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