Utilization of Melt Fracture Phenomenon for the Preparation of Shark Skin Structured Hydrophobic Film

With the application of biomimetic shark skin microstructures with hydrophobicity in microfluidics, sensors and self-cleaning materials, microstructure processing methods are increasing. The preparation process has higher requirements for processing cost and efficiency. In this paper, linear low-density polyethylene (LLDPE) hydrophobic films were prepared with the help of melt fracture phenomenon. The equipment is a self-made single screw extruder. By adjusting the process parameters, the biomimetic shark skin structured LLDPE films with good hydrophobic property can be obtained. The surface microstructure shape of the product is related to kinds of additive, die temperature and screw speed. When AC5 was selected as an additive, the optimal processing parameter was found to be 160 °C die temperature and 80 r/min screw speed. A contact angle of 133° was obtained in this situation. In addition, the influences of die temperature and screw speed on the size of shark skin structure were also systematically investigated in this paper. It was found that the microstructure surface with hierarchical roughness had a better hydrophobic property.

Analysis of Microwave Thermal Stress Fracture Characteristics and Size Effect of Sandstone under Microwave Heating

Microwave-induced rock fracture is one of the promising approaches of achieving non-blasting continuous mining and assisted mechanical rock breaking. It is of great theoretical and practical significance to study the temperature effect and fracture characteristics of rocks of different sizes under microwave heating; however, there are few studies in this field. Microwave heating of φ 50 × 100 mm, φ 50 × 50 mm, and φ 50 × 25 mm sandstone samples with different heating powers and times was performed to measure the temperature of the sample, the microwave energy absorbed, the mass, and the P-wave velocity before and after heating. The results show suppress that (i) under the same heating conditions, the mass difference and the temperature increase range of φ 50 × 100 mm and φ 50 × 50 mm samples are larger than that of the φ 50 × 25 mm samples; (ii) the wave velocity change rate and the damage factor of samples increase with the increase of heating power and time; (iii) different size specimens have different crack- propagation modes. The main crack of φ 50 × 100 mm specimens usually starts from the middle of the height of the specimen; for the φ 50 × 50 mm specimens, it usually starts from the middle or bottom-end surface of the specimen height; the main crack of φ 50 × 25 mm specimens starts from the vertical surface of the specimen. With an increase in the heating time, the length and width of the main crack continuously increase and secondary cracks are generated. The fracture mode of the sample is also related to the size of the sample. The fracture mode can be divided into three parts: melt fracture, thermal-expansion fracture, and secondary thermal-expansion fracture. The relationship between the sample temperature and the absorbed microwave energy is approximately linear.

Linear Viscoelasticity and Flow of Self-Assembled Vitrimers: The Case of a Polyethylene/dioxaborolane System

For vitrimer systems obtained by dynamic cross-linking of polymer chains, incompatibility effects between the cross-links and polymer backbone can lead to microphase separation, resulting in a network made of cross-linked aggregates. Additionally, when there is a wide distribution of the number of cross-links per chain, macrophase separation can occur. Here, we investigate the linear viscoelasticity and flow of a polyethylene (PE) vitrimer that has cross-linkable dioxaborolane maleimide grafts, which aggregate into a hierarchical nanostructure. To elucidate the role of self-assembly, noncross-linked graft functionalized PE was first studied. It had a terminal relaxation time that was orders of magnitude larger than both neat PE and partially peroxide cross-linked PE. When dioxaborolane cross-linker was added to form the vitrimer, the resulting material could not achieve terminal relaxation within 8 hr. The graft-poor soluble and graft-rich insoluble portions of the PE vitrimer were then isolated and characterized. The soluble portion expressed similar flow behavior as neat PE, while the insoluble portion – which is a network of cross-linked aggregates – relaxed very little over 8 hr. When the insoluble and soluble portions were blended, the rheological behavior of the original vitrimer was basically recovered, showing that the soluble portion acts as a lubricant. When the insoluble portion was blended with neat PE, the material relaxed much more stress, but still did not reach steady-state flow within 8 hr. When high stresses were applied, however, PE vitrimer flowed. Nonlinear rheology experiments revealed melt fracture at high strains and suggested that flow is enabled by rapid healing, which follows fracture events. The presence of macroscopic phase separation facilitated flow.

Linear Viscoelasticity and Flow of Self-Assembled Vitrimers: The Case of a Polyethylene/dioxaborolane System

For vitrimer systems obtained by dynamic cross-linking of polymer chains, incompatibility effects between the cross-links and polymer backbone can lead to microphase separation, resulting in a network made of cross-linked aggregates. Additionally, when there is a wide distribution of the number of cross-links per chain, macrophase separation can occur. Here, we investigate the linear viscoelasticity and flow of a polyethylene (PE) vitrimer that has cross-linkable dioxaborolane maleimide grafts, which aggregate into a hierarchical nanostructure. To elucidate the role of self-assembly, noncross-linked graft functionalized PE was first studied. It had a terminal relaxation time that was orders of magnitude larger than both neat PE and partially peroxide cross-linked PE. When dioxaborolane cross-linker was added to form the vitrimer, the resulting material could not achieve terminal relaxation within 8 hr. The graft-poor soluble and graft-rich insoluble portions of the PE vitrimer were then isolated and characterized. The soluble portion expressed similar flow behavior as neat PE, while the insoluble portion – which is a network of cross-linked aggregates – relaxed very little over 8 hr. When the insoluble and soluble portions were blended, the rheological behavior of the original vitrimer was basically recovered, showing that the soluble portion acts as a lubricant. When the insoluble portion was blended with neat PE, the material relaxed much more stress, but still did not reach steady-state flow within 8 hr. When high stresses were applied, however, PE vitrimer flowed. Nonlinear rheology experiments revealed melt fracture at high strains and suggested that flow is enabled by rapid healing, which follows fracture events. The presence of macroscopic phase separation facilitated flow.

Linear Viscoelasticity and Flow of Self-Assembled Vitrimers: The Case of a Polyethylene/dioxaborolane System

For vitrimer systems obtained by dynamic cross-linking of polymer chains, incompatibility effects between the cross-links and polymer backbone can lead to microphase separation, resulting in a network made of cross-linked aggregates. Additionally, when there is a wide distribution of the number of cross-links per chain, macrophase separation can occur. Here, we investigate the linear viscoelasticity and flow of a polyethylene (PE) vitrimer that has cross-linkable dioxaborolane maleimide grafts, which aggregate into a hierarchical nanostructure. To elucidate the role of self-assembly, noncross-linked graft functionalized PE was first studied. It had a terminal relaxation time that was orders of magnitude larger than both neat PE and partially peroxide cross-linked PE. When dioxaborolane cross-linker was added to form the vitrimer, the resulting material could not achieve terminal relaxation within 8 hr. The graft-poor soluble and graft-rich insoluble portions of the PE vitrimer were then isolated and characterized. The soluble portion expressed similar flow behavior as neat PE, while the insoluble portion – which is a network of cross-linked aggregates – relaxed very little over 8 hr. When the insoluble and soluble portions were blended, the rheological behavior of the original vitrimer was basically recovered, showing that the soluble portion acts as a lubricant. When the insoluble portion was blended with neat PE, the material relaxed much more stress, but still did not reach steady-state flow within 8 hr. When high stresses were applied, however, PE vitrimer flowed. Nonlinear rheology experiments revealed melt fracture at high strains and suggested that flow is enabled by rapid healing, which follows fracture events. The presence of macroscopic phase separation facilitated flow.

Linear Viscoelasticity and Flow of Self-Assembled Vitrimers: The Case of a Polyethylene/dioxaborolane System

For vitrimer systems obtained by dynamic cross-linking of polymer chains, incompatibility effects between the cross-links and polymer backbone can lead to microphase separation, resulting in a network made of cross-linked aggregates. Additionally, when there is a wide distribution of the number of cross-links per chain, macrophase separation can occur. Here, we investigate the linear viscoelasticity and flow of a polyethylene (PE) vitrimer that has cross-linkable dioxaborolane maleimide grafts, which aggregate into a hierarchical nanostructure. To elucidate the role of self-assembly, noncross-linked graft functionalized PE was first studied. It had a terminal relaxation time that was orders of magnitude larger than both neat PE and partially peroxide cross-linked PE. When dioxaborolane cross-linker was added to form the vitrimer, the resulting material could not achieve terminal relaxation within 8 hr. The graft-poor soluble and graft-rich insoluble portions of the PE vitrimer were then isolated and characterized. The soluble portion expressed similar flow behavior as neat PE, while the insoluble portion – which is a network of cross-linked aggregates – relaxed very little over 8 hr. When the insoluble and soluble portions were blended, the rheological behavior of the original vitrimer was basically recovered, showing that the soluble portion acts as a lubricant. When the insoluble portion was blended with neat PE, the material relaxed much more stress, but still did not reach steady-state flow within 8 hr. When high stresses were applied, however, PE vitrimer flowed. Nonlinear rheology experiments revealed melt fracture at high strains and suggested that flow is enabled by rapid healing, which follows fracture events. The presence of macroscopic phase separation facilitated flow.

Numerical modelling of two-dimensional melt fracture instability in viscoelastic flow

Computationally modelling the two-dimensional (2-D) Poiseuille flow along and outside a straight channel with a differential viscoelastic constitutive equation, we demonstrate unstable dynamics involving bifurcations from steady flow to periodic melt fracture (sharkskin instability) and its further transition regime to a chaotic state. The numerical simulation first exposes transition from steady flow to a weak instability of periodic fluctuation, and in the middle of this periodic limit cycle (in the course of increasing flow intensity) a unique bifurcation into the second steady state is manifested. Then, a subcritical (Hopf) transition restoring this stable flow to stronger periodic instability follows, which results from the high stress along the streamlines of finite curvature with small vortices near the die lip. Its succeeding chaotic transition at higher levels of flow elasticity that induces gross melt fracture, seems to take a period doubling as well as quasiperiodic route. By simple geometrical modification of the die exit, we, as well, illustrate reduction or complete removal of sharkskin and melt fractures. The result as a matter of fact suggests convincing evidence of the possible cause of the sharkskin instability and it is thought that this fluid dynamic transition has to be taken into account for the complete description of melt fracture. The competition between nonlinear dynamic transition and other possible origins such as wall slip will ultimately determine the onset of the sharkskin and melt fractures. Therefore, the current study conceivably provides a robust methodology to portray every possible type of melt fracture if combined with an appropriate mechanism that also results in flow instability.