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 σ_forc, fracture stress σ_fr, relative deformation to failure ε_fr) 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 - σ_fors<σ_fr. In this case, the reserve of elasticity significantly depends on the thickness of the film with a decrease in temperature. A sharp decrease in ε_fr 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 σ_fors limit is most sensitive to the strain rate. The nature of the strain rate sensitivity σ_fors(έ) depends on the temperature and film thickness. The change to the opposite in the character of σ_fors(έ) and σ_fr(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 σ_fors(έ) is not observed in 25 μm thick film  which retains the maximum reserve of elasticity at 4.2 K