  Polymer nanofibers with high thermal conductivities and outstanding thermal
stabilities are highly desirable in heat transfer-critical applications such as
thermal management, heat exchangers and energy storage. In this work, we unlock
the fundamental relations between the thermal conductivity and thermal
stability of polymer nanofibers and their molecular characteristics by studying
the temperature-induced phase transitions and thermal transport of a series of
polymer nanofibers. Ten different polymer nanofibers with systematically chosen
molecular structures are studied using large scale molecular dynamics
simulations. We found that high thermal conductivity and good thermal stability
can be achieved in polymers with rigid backbones, exemplified by
{\pi}-conjugated polymers, due to suppressed segmental rotations and large
phonon group velocities. The low probability of segmental rotation does not
only prevent temperature-induced phase transition but also enables long phonon
mean free paths due to reduced disorder scattering. Although stronger
inter-chain interactions can also improve the thermal stability, polymers with
such a feature usually have heavier atoms, weaker backbone bonds, and segments
vulnerable to random rotations, which lead to low thermal conductivities. This
work elucidates the underlying linkage between the molecular nature and
macroscopic thermal properties of polymer nanofibers, which is instrumental to
the design of thermally conductive polymer nanofibers with high temperature
stabilities.
