Description of the tension kinetics of amorphous and amorphous-crystalline polymers under conditions of moderate and deep cooling by the common nonlinear rheological equation

The comparative analysis of the results of low-temperature mechanical tests of the samples of amorphous and amorphous-crystalline polyimide of the Kapton H type was carried out. In the experiments [East Eur. J. Phys., No. 4, 144 (2020)], tensile diagrams σ(ɛ;T,ε˙) of such samples were recorded, namely, the dependences of the deforming stress on the strain ɛ = ε˙t at constant values of the strain rate ε˙ = 7⋅10–5, 7⋅10–4, 6⋅10–3 s–1, and temperature T = 293, 77, and 4.2 K. The initial aim of these experiments was to study the effect of moderate (77 K) and deep (4.2 K) cooling on the structure and some mechanical characteristics of polyimide, important for its use in cryogenic and aerospace engineering. Later (Low Temp. Phys. 49, 521 (2023) [Fiz. Nyzk. Temp. 49, 569 (2023)]), there was a need and opportunity to supplement the experimental results with additional analysis in order to use them to test new aspects of the theory of low-temperature elastic-viscous deformation of polymers, in particular, the description of the effects of forced elasticity and their competition with brittle fracture processes. A detailed comparison of the tensile diagrams of the polyimide samples with amorphous and amorphous-crystalline mole­cular structures performed in this study showed that at T = 293 K both structures have clearly pronounced pro­perties of the elastomers, namely, the rubber-like materials with high elasticity and the ability to reversible deformation. It has been established that amorphous samples retain these properties up to deep cooling at T = 4.2 K, and amorphous-crystalline ones only to a state of moderate cooling: at T < 77 K they acquire the properties of glassy materials with brittle fracture at the initial stage of elastic deformation. It is also shown that the kinetics of highly elastic deformation of polyimide with molecular structures of both types is due to the thermomechanical activation of soliton-like elaston excitations on molecular chains in the amorphous component of the material and is described by a nonlinear rheological equation derived earlier for the molecular model of an amorphous polymer: Low Temp. Phys. 48, 253 (2022) [Fiz. Nyzk. Temp. 48, 281 (2022)], Low Temp. Phys. 49, 228 (2023) [Fiz. Nyzk. Temp. 49, 246 (2023)]. By comparing the results of experiments and theory, an analytical description of the tension diagrams σ(ɛ;T,ε˙) of polyimide samples with molecular structures of both types was obtained, as well as empirical estimates of their rheological characteristics and microscopic parameters of elaston excitations. During low-temperature deformation of a polymer with a mixed structure, rigid crystalline fibrils immersed in the softer amorphous medium undergo only minor elastic deformations, but significantly increase the intensity of elaston activation and fracture processes in the amorphous component. Upon cooling, this leads to the convergence of critical stresses of highly elastic relaxation and fracture and to the transformation of an elastomer with such a structure into a glassy brittle material.