Mechanical, hygroscopic, and thermal properties of improved ultrathin polymeric films for magnetic tapes are presented. These films include poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN), and aromatic polyamide (ARAMID). PET films are currently the most commonly-used polymeric substrate material for magnetic tapes, followed by PEN and ARAMID. The thickness of the films ranges from 6.2 to 4.8 gm. Tensile tests were run to obtain the Young's modulus, F5 value, strain at yield, breaking strength, and strain at break. The storage modulus, E', and the loss tangent, tan 5, were measured using a dynamic mechanical analyzer (DMA) at temperature ranges of -50 to 150degreesC (for PET) and -50 to 210degreesC (for PEN and ARAMID) and at a frequency range of 0.016-28 Hz. Frequency-temperature superposition was used to predict the dynamic mechanical behavior of the films over a 28-decade frequency range. Short-term longitudinal creep behavior of the films during 10, 30, 60, and 300 s, 7 MPa, were measured at 25 and 55degreesC. Long-term longitudinal creep measurements were performed at 25, 40, and 55degreesC for 100 h. The Poisson's ratio and 50-h long-term lateral creep were measured at 25degreesC/15% RH, 25degreesC/50% RH, 25degreesC/80% RH, and 40degreesC/50% RH. The in-plane coefficient of hygroscopic expansion (CHE) at 25degreesC/20-80% RH and the coefficient of thermal expansion (CTE) at 30-70degreesC were measured for all the samples. The properties for all films are summarized. The relationship between the polymeric structure and the mechanical and physical properties are discussed, based on the molecular structure, crystallinity, and molecular orientation. (C) 2003 Wiley Periodicals, Inc.