Tuning the thermal conductivity of polymers with mechanical strains

The low thermal conductivity of polymers limits their heat spreading capability, which is one of the major technical barriers for the polymer-based products, especially electronics, such as organic light emitting diodes. It is highly desirable to enhance the thermal conductivity of polymer materials including polymer composites. Mechanical stretching could align polymer chains which are intrinsically low-dimensional material that could have very high thermal conductivity and thus enhancing the thermal conductivity of polymers. In this work, the all-atom model molecular-dynamics simulation is conducted to investigate the tuning of polymer thermal conductivity using mechanical strains. The simulation results show that the thermal conductivity of polymers increases with the increasing strain and the enhancement is larger when the polymer is stretched slower. Molecular weight also affects the thermal conductivity under the same stretching condition. More importantly, the thermal-conductivity enhancement could be exponentially fitted with the orientational order parameter which describes the chain conformation change. This study could guide the development of advanced reconfigurable and tunable thermal management technologies.

Figure 1

(Color online) Numerical simulation procedure for building structure-property relationships of polymers when they are under mechanical strains, which involves three major steps: sample preparation, deformation, and thermal-conductivity calculation simulations. After amorphous polymer sample is prepared, the deformation process generates a series of strained samples at different strains, then the thermal conductivity of these strained samples are calculated.

Figure 2

(a) A schematic representation of the simulation cell used to compute the thermal conductivity and (b) typical temperature profile and linear fitting of nonequilibrium molecular-dynamics simulation for thermal conduction.

Figure 3

(Color online) The change in the thermal conductivity of polymers in the stretching direction and perpendicular to the stretching direction as a function of tensile strains. It also shows that the slower the strain rate, the higher the enhancement of the thermal conductivity in the stretching direction.

Figure 4

(Color online) The dependence of the thermal-conductivity enhancement on molecular weight. The larger the molecular weight, the higher the thermal conductivity when the polymer is stretched at the same strain rate of ${10}^9{\text{s}}^{-1}$ to the same strain.

Figure 5

(Color online) (a) 3D unit cell of the polymer sample under zero strain, (b) projection to XY plane of the samples before deformation, (c) when stretched at $\epsilon =1$ with a strain rate of ${10}^9{\text{s}}^{-1}$, and (d) when stretched at $\epsilon =2$ with a strain rate ${10}^9{\text{s}}^{-1}$.

Figure 6

(Color online) The change in orientational order parameter $P_2$ as a function of the mechanical strain after relaxation. The orientation will increase mostly at small strains while slow down the increasing rate at relatively larger strains.

Figure 7

(Color online) The relationship between the logarithm of the enhanced thermal conductivity and the orientational order parameter $P_2$ using linear fit.