RESUMO
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.
RESUMO
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.
