Structure Evolution and Deformation Behavior of Polyethylene Film during Biaxial Stretching

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 β-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.

Effect of biaxial stretching on the microstructure evolution, optical, mechanical and oxygen barrier properties of biodegradable poly(lactic acid) (PLA)/poly(butylene adipate-co-terephthalate) (PBAT) films

The poly(butylene adipate-co-terephthalate) (PBAT)/poly(lactic acid) (PLA) films have been widely used due to their biological degradability and excellent comprehensive properties. However, the reports regarding biodegradable PLA/PBAT films are rather scarce. In this work, systematical investigations of biaxially stretched PLA/PBAT films were performed. Compared with unstretched films, the PLA/PBAT 75/25 films with the stretching ratio of 5 × 1 exhibited an improvement on the crystallinity of PLA from 6 % to 58.6 %. According to 2D-WAXS results, the orientation of the α crystal in the MD increased with the increase of the stretching ratio. The stretched films showed favorable barrier properties. The oxygen permeability (OP) of 2 × 2 PLA/PBAT 75/25 films shows a decrement of 22 % compared with that of the unstretched films. Interestingly, the uniaxially stretched PLA/PBAT 75/25 films exhibits increased surface roughness (Ra) for 3 × 1 film whereas decreased Ra for the 5 × 1 film, which could be related to the phase separation under stretching. The tensile strength in the machine direction (MD) of the PLA/PBAT 75/25 films was improved up to 51.6 MPa for 5 × 1 film, which is 45 % higher than that of unstretched counterpart. The stretched films exhibit excellent mechanical and barrier properties, which could be utilized in packaging industry with high potential.

Ultrafast Photothermal Actuators with a Large Helical Curvature Based on Ultrathin GO and Biaxially Oriented PE Films

In nature, there are some amazing superfast actuations (Venus flytrap) and large-curvature helical deformations (the awn of Erodium). Although many bionic actuators have been made (electrothermal, hygroscopic, photoinduced), most of their actuations are slow and small, not comparable to the wonderful ones in nature. Here, we report an ultrafast photothermal actuator with large-curvature curling based on an ultrathin graphene oxide (GO) and biaxially oriented polyethylene (BOPE) bilayer film (thickness ∼11 μm). By virtue of the fast temperature changing rate (peak: 900 °C s^-1 during infrared heating and -1200 °C s^-1 during cooling) and the great difference in the coefficient of thermal expansion of GO and BOPE layers, the actuator deforms rapidly and greatly. The maximum bending speed and curvature can reach 5300° s^-1 and 22 cm^-1, respectively, which are comparable to those of wonderful natural actuators and far exceed the performances of the reported artificial actuators. Different from ordinary helical actuators made of uniaxial anisotropic materials, our actuator is based on a typical biaxial anisotropic material of BOPE. However, the morphing behaviors of this type of actuator have not been reported before. So for the first time, we systematically studied this problem through experiments and simulations using the GO-BOPE actuator as a prototype and have drawn clear conclusions. In addition, functional GO-BOPE actuators capable of winding around and manipulating tiny objects were also designed and developed. We think this ultrafast large-curvature photothermal actuator will have wide application prospects in bionic actuations and dexterous robots.

Initial Crystallization Effects in Coarse-Grained Polyethylene Systems After Uni- and Biaxial Stretching in Blow-Molding Cooling Scenarios

This study investigates the initial stage of the thermo-mechanical crystallization behavior for uni- and biaxially stretched polyethylene. The models are based on a mesoscale molecular dynamics approach. We take constraints that occur in real-life polymer processing into account, especially with respect to the blowing stage of the extrusion blow-molding process. For this purpose, we deform our systems using a wide range of stretching levels before they are quenched. We discuss the effects of the stretching procedures on the micro-mechanical state of the systems, characterized by entanglement behavior and nematic ordering of chain segments. For the cooling stage, we use two different approaches which allow for free or hindered shrinkage, respectively. During cooling, crystallization kinetics are monitored: We precisely evaluate how the interplay of chain length, temperature, local entanglements and orientation of chain segments influence crystallization behavior. Our models reveal that the main stretching direction dominates microscopic states of the different systems. We are able to show that crystallization mainly depends on the (dis-)entanglement behavior. Nematic ordering plays a secondary role.

Structural evolution and barrier properties in biaxially stretched polyethylene terephthalate/hydroxy-terminated polybutadiene films

This paper aims to investigate the crystallization and barrier properties of oxygen-scavenging polyethylene terephthalate films (OSP) at different stretching ratios and stretching rates. The results show that with the increase of the stretching ratio, more regular lamellar crystal was formed in the biaxially stretched OSP films, and the amorphous phase thickness between lamellae and the long period decreased. The presence of oxygen scavenger acted as heterogeneous nucleation, further promoting the crystallization of the OSP films. This was conducive to prolong the diffusion path of gas molecules through the film. Furthermore, the increase of the stretching ratio expanded the “active” oxygen barrier area of the oxygen scavengers. Thus, the barrier performance of the biaxially stretched OSP films was improved significantly. In addition, the variation of crystallinity and properties of OSP films with the stretching ratio was consistent with the variation with the stretching rate, but the stretching ratio had a greater impact. It was also found that the increase of the stretching ratio and the introduction of oxygen scavenger both increased the stretching strength of the OSP films, while the biaxially stretched OSP film maintained good optical properties.

Recent advances in post-stretching processing of polymer films with in situ synchrotron radiation X-ray scattering

The stretch-induced structural evolution mechanism is a long-standing scientific question in the post-stretching processing of polymer films. X-ray scattering, especially a combination of small- and wide-angle X-ray scattering (SAXS/WAXS), provides a powerful method to study the hierarchical structure of polymer films. Recent advances in synchrotron radiation (SR) light sources and detection techniques allow one to measure the structural evolution of polymer films during post-stretching processing in real time with ultrahigh time resolution, which benefits the understanding on this topic. This review summarizes some recent investigations on post-stretching processing of polymer films, which combine in situ X-ray scattering techniques with purposely designed tensile apparatus in terms of three aspects: uniaxial stretching, biaxial stretching and stretching with chemical reactions. Concerning the polymer bulk, traditional deformation mechanisms like stretch-induced crystallization (SIC), crystal slipping, phase transition and melting-recrystallization are discussed for the uniaxial and biaxial post-stretching of polymer films. New deformation models have been developed to focus on the structural evolution on the length scale of lamellar stacks, which consider the potential microphase separation of the interlamellar amorphous phase and microbuckling. For solution systems, the coupled effects of the mechanical work from external force and the chemical potential from possible chemical reactions are taken into account for the structural evolution during stretching in solution. Roadmaps of structural and morphological evolution in the processing parameter space (i.e., temperature, stress, strain and the concentration of additive in the bath solution) are eventually constructed for precursor films. The accumulation of a structural evolution database for post-stretching processing of polymer films can be expected to provide a helpful guide for industrial processing for high-performance polymers in the near future.