The thermal conductivity of poly(L-lactic acid) specimens subjected to uniaxial elongational deformations in the rubbery state followed by quenching is investigated experimentally. A novel optical technique known as forced Rayleigh scattering is used to measure two components of the thermal diffusivity tensor as a function of elongation. The component along the direction of elongation increases, while the component in the direction perpendicular to elongation decreases, relative to the equilibrium value. Measurements of the stress at the point of quenching, as well as the density and specific heat capacity, are also reported as a function of elongation. Anisotropy of the thermal conductivity tensor is more than 50% at moderate elongations, and it is found to be a nonlinear function of stress. The latter is in contrast to results found in previous studies where a linear relationship between thermal conductivity anisotropy and stress, or the stress-thermal rule, has been observed for several amorphous polymer systems. Failure of the stress-thermal rule is attributed to the presence of semicrystalline domains in the deformed samples. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 547-553