Investigation on the creep mechanism of PA6 films based on quasi point defect theory.

Polyamide 6 (PA6) films with significant α relaxation process was selected as the model system. The creep behavior and rheological mechanism during deformation in the amorphous regions of semi-crystalline polymers are systematically investigated by carrying out creep experiments. Based on the quasi point defect (QPD) theory, the complete physical process of PA6 film creep behavior from elasticity to viscoelasticity and viscoplasticity was analyzed and modeled from the perspective of structural heterogeneity. The results demonstrate that the creep deformation of PA6 film is a typical thermo-mechanical coupling and nonlinear mechanics process, and potential creep mechanisms corresponds to stress-induced local shear deformation enhancement and thermal activation-induced particle flow diffusion. The elastic-plastic transition involved in the creep deformation process of semi-crystalline polymer originates from the activation of quasi-point defective sites in the amorphous region, the expansion of sheared micro-domains and irreversible fusion. The generalized fractional Kelvin (GFK) model is proposed, and the physical meaning of parameters is explained by combining the quasi point defect theory and creep delay spectrum(L(τ)). Finally, the effectiveness of the GFK model and the QPD theory in studying the deformation behavior of PA6 films was validated by comparing experimental data with theoretical results, which theoretically reveals the structural evolution of PA6 film during creep process.

Investigation of Dynamic Viscoelastic Asymmetric Response of PA6 Film Based on Fractional Rheological Model.

Polyamide 6 (PA6) film as a typical viscoelastic material, satisfies the time-temperature superposition (TTS), and demonstrates obvious dynamic strain amplitude and frequency correlation under dynamic load. The investigation of the dynamic mechanical behavior of PA6 film is essential to ensure the safety of these materials in practical applications. In addition, dynamic mechanical property testing under conventional experimental conditions generally focuses on the short-term mechanical performance of materials. Therefore, the dynamic viscoelasticity of PA6 film was tested using a dynamic thermo-mechanical analyzer (DMA) in this study, and the complex modulus master curve was constructed based on time-temperature superposition (TTS) to realize the accelerated characterization of long-term mechanical properties. Furthermore, according to experimentally obtained asymmetric characteristics of the Cole-Cole diagram and the loss modulus master curve of the PA6 film, the parameter distribution of the fractional Zener model and the modified fractional Zener model were compared, and the asymmetric dynamic viscoelastic response of PA6 film under different conditions was systematically investigated using these models. The results indicate that the modified fractional Zener model can truly describe the dynamic asymmetric characteristics of PA6 film, verify the feasibility and advantages of the modified fractional rheological model, and provide some theoretical guidance for exploring the tensile rheological mechanism of PA6 film.

Investigation on the Correlation between Biaxial Stretching Process and Macroscopic Properties of BOPA6 Film.

Biaxially oriented polyamide 6 (BOPA6) films were prepared by extrusion casting and biaxial stretching with polyamide 6. The effects of different biaxially oriented on the macroscopic properties of BOPA6 were investigated by characterizing the rheological, crystallization, optical, barrier and mechanical properties. The results show that the increase of stretching temperature leads to the diffusion and regular stacking rate of BOPA6 chain segments towards crystal nuclei increases, the relative crystallinity increases, reaching 27.87% at 180 °C, and the mechanical strength and optical performance decrease. Heat-induced crystallization promotes the transformation of ß-crystals to α-crystals in BOPA6, resulting in a more perfect crystalline structure and enhancing oxygen barrier properties. BOPA6 chains are oriented, and strain-induced crystallization (SIC) occurs during the biaxial stretching. Further increasing the stretch ratio, the relative crystallinity increased to 30.34%. The machine direction (MD) and transverse direction (TD) tensile strength of BOPA6 (B-33) are nearly two times higher than the unstretched film, reaching 134.33 MPa and 155.28 MPa, respectively. In addition, the permeation decreases to 57.61 cc·mil/(m2 day), and the oxygen barrier performance has improved by nearly 30% compared to the sample B-22. BOPA6 has a high storage modulus at a high stretching rate (300%/s). Rapid chain relaxation would promote the molecular chain disorientation, destroy the entangled network of the molecular chain, and lead to a decrease in tensile strength, reducing to about 110 MPa.

Hydrologic gradient changes of soil respiration in typical steppes of Eurasia.

Soil respiration (RS) is affected by many factors and shows significant diurnal and seasonal changes at different spatial and temporal scales. However, in a semi-arid steppe, the mechanism of the dynamic influence of environmental factors on RS is not clear, and the effect of subtle changes of soil water on RS under drought stress is yet to be explored. Therefore, Xilin River Basin, was the study area and a hydrological gradient on the four ecosystems for RS and hydrometeorological monitoring was selected. We proposed the use of dynamic sunrise and sunset time to distinguish day from night and determine related statistics. Additionally, we analyzed the temporal variation of RS and its response process and mechanism for hydrometeorological factors during the growing season and at daily scales. Further, we quantitatively simulated the RS of 594 scenarios in different growing season stages, ecosystems, and precipitation patterns. Results showed that: (1) in the hydrological gradient belt, different ecosystems exhibited the same trend but different characteristics of RS regulation. From May to November 2020, RS was 2.34-3.89, 1.89-5.97, 1.90-5.27 and 2.29-3.45 gC m-2 day-1 for the four ecosystems. (2) The use of dynamic sunrise and sunset time to distinguish day and night can more accurately describe the statistical value of each variable, which exhibits remarkable feasibility in daily scale research. (3) The changes in RS were adequately explained by temperature at various time scales, and the photosynthetically active radiation was positively correlated with RS at the daily scale. The special soil water content (MS) condition in the study area was not sufficient to explain RS. (4) Precipitation can affect RS by changing soil and air; however, only when precipitation exceeds the effective precipitation threshold of 0.6 ± 0.3 mm, it significantly affects RS.