We investigated the effects of lithium bromide (LiBr) on the structure and properties of polyamide 6 (PA6). The strong ion-dipole interactions between lithium cations and the amide groups in PA6 greatly increased the glass transition temperature (T-g) and retarded the crystallization rate of PA6. As a result, compression-molded PA6 blends were highly transparent and had high T-g values. The rheological terminal region was obvious in the blends because the ion-dipole interactions weakened at high temperatures. This indicates that the melt processability was barely affected by LiBr. We also evaluated the optical anisotropy of the polymer to determine its suitability as a functional optical film. We found that hot-stretched blend films had large positive orientation birefringence with significantly weak wavelength dispersion, which can be attributed to the enhanced anisotropic polarizability of PA6. We also found that the stress-optical coefficient in the glassy region decreased with increasing LiBr content. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1513-1520