AbstractThe morphology of nonperiodic layer (NPL) crystallites appearing in three thermotropicrandomcopolyesters composed of 30/70, 58/42, and 75/25 monomer ratios ofp‐oxybenzoate (B) and 2,6‐oxynaphthoate (N) is investigated with transmission electron microscopy. Copolymer films, produced by shearing the materialsabovetheir melting points and subsequently annealing them above their glass transition temperature, exhibit ordered entities averaging no more than 20 nm thick and 40–60 nm long, as observed with dark‐field imaging. Selected‐area electron diffractograms exhibit sharp equatorial reflections, indicating ordered packing of random intramolecular sequences of the molecules. Microdensitometry of these diffraction patterns reveal that both the equatorial and first meridional reflections are radiation sensitive, decaying exponentially with dose. Saturation doses (v) for the crystallites depend on the copolymer composition and degree of polymerization (DP) and are estimated to be approximately 130–150C/m2for the high‐molecular‐weight copolymers (DP ≈ 150), with the N‐rich copolymer possessing greater radiation resistance, and about 430 C/m2for a low‐molecular‐wei