Regularities of Low-Temperature Deformation and Fracture of Polyimide Films of Kapton H Type of Different Thickness

Abstract

The mechanical characteristics (limit of forced elasticity σforc, fracture stress σfr, relative deformation to failure εfr) of polyimide films of kapton H type under uniaxial tension conditions along the direction of drawing in the temperature range (4.2-293 K), deformation rates (10-5 - 10-3 s-1) and film thicknesses (25, 75 and 125 μm) were investigated. It is discovered, that the forced-elastic state remains for all films up to 4.2 K of all strain rates - σfors<σfr. In this case, the reserve of elasticity significantly depends on the thickness of the film with a decrease in temperature. A sharp decrease in εfr occurs in films: 125 μm thick - at 77 K, 75 μm thick - at 4.2 K. Two variants of deformation curves are possible in a 25 μm thick film at 4.2 K: with a short nonlinear stage or with a long one proceeding jumpily. The working surface of the samples that have undergone jump deformation is covered with a deformation relief, partially representing a delayed highly elastic deformation. The σfors limit is most sensitive to the strain rate. The nature of the strain rate sensitivity σfors(έ) depends on the temperature and film thickness. The change to the opposite in the character of σfors(έ) and σfr(έ) with a decrease in temperature to 4.2 K in 75 and 125 thick films was found for a first time. Change in the character of σfors(έ) is not observed in 25 μm thick film which retains the maximum reserve of elasticity at 4.2 K