  We reveal that phononic thermal transport in graphene is not immune to grain
boundaries (GBs) aligned along the direction of the temperature gradient.
Non-equilibrium molecular dynamics simulations uncover a large reduction in the
phononic thermal conductivity ($\kappa_p$) along linear ultra-narrow GBs
comprising periodically-repeating pentagon-heptagon dislocations. Green's
function calculations and spectral energy density analysis indicate that
$\kappa_p$ is the complex manifestation of the periodic strain field, which
behaves as a reflective diffraction grating with both diffuse and specular
phonon reflections, and represents a source of anharmonic phonon-phonon
scattering. Our findings provide new insights into the integrity of the
phononic thermal transport in GB graphene.
