Interfacial Diffusion and Bonding in Multilayer Polymer Films: A Molecular Dynamics Simulation

Two-step heat fusion kinetics and mechanical performance of thermoplastic interfaces

Thermoplastic polymers and composites are ubiquitous in the industry for their reshaping and fusing capabilities at elevated temperatures. The quality of heat-fused thermoplastic interfaces is of great concern for adhesion, coating, and welding applications, especially those between dissimilar materials. Kinetic evolution of the microstructures defines the mechanical performance of heat-fusion thermoplastic interfaces, which is studied here using polyethylene and polypropylene as an example. Key factors such as the viscosity and compatibility of polymers and the time and temperature of fusion are discussed by combining molecular-level simulations and structural-level hot-compression experiments. Inter-diffusion and entanglement of polymer chains are identified as the two elementary kinetic steps of the fusion, which dominate the control on the stiffness and strength of the interfaces, respectively. Experimental data shows that the quality of fused interfaces can be improved by reducing the viscosity and the interaction parameter. Following the same set of time-scaling relations as identified in the simulations, the two-step characteristics and their effects on the stiffness and strength are experimentally validated. Both simulation and the experiment results show that Young’s modulus of fused interfaces recovers faster than the strength that is controlled by polymer entanglement to a large extent, rather than diffusion. These findings add insights into the design of fusion processes, laying the ground for the applications of thermoplastic polymers and composites.

Interfacial Phenomena in Multi-Micro-/Nanolayered Polymer Coextrusion: A Review of Fundamental and Engineering Aspects

The multilayer coextrusion process is known to be a reliable technique for the continuous fabrication of high-performance micro-/nanolayered polymeric products. Using laminar flow conditions to combine polymer pairs, one can produce multilayer films and composites with a large number of interfaces at the polymer-polymer boundary. Interfacial phenomena, including interlayer diffusion, interlayer reaction, interfacial instabilities, and interfacial geometrical confinement, are always present during multilayer coextrusion depending on the processed polymers. They are critical in defining the microstructural development and resulting macroscopic properties of multilayered products. This paper, therefore, presents a comprehensive review of these interfacial phenomena and illustrates systematically how these phenomena develop and influence the resulting physicochemical properties. This review will promote the understanding of interfacial evolution in the micro-/nanolayer coextrusion process while enabling the better control of the microstructure and end use properties.

EG/TPU composites with enhanced flame retardancy and mechanical properties prepared by microlayer coextrusion technology

In this work, expandable graphite (EG)/thermoplastic polyurethane (TPU) composites with excellent exfoliation, dispersion and two-dimensional plane orientation of the EG fillers were manufactured by microlayer coextrusion technology. The influence of microlayer coextrusion technology on flame retardancy and mechanical properties of microlayer coextruded composites was investigated. The exfoliation, dispersion and orientation of the EG fillers in TPU matrix were characterized by SEM and XRD. The flame retardancy and thermal stability of the composites were characterized by UL 94, LOI, TGA and CCT. The mechanical properties of the composites were characterized by tensile tests. SEM and XRD showed that microlayer coextrusion technology could not only greatly promote exfoliation and dispersion of the EG fillers in TPU matrix, but also could enhance the two-dimensional plane orientation of the EG fillers in TPU matrix. As compared with the conventional blended composites, the microlayer coextruded composites showed enhanced flame retardancy and mechanical properties, with 15 wt% of EG, the as prepared EG/TPU composites showed a V-0 flame retardance level, whereas EG/TPU composite prepared by conventional blending only showed a V-2 flame retardance level. The exfoliation, dispersion and two-dimensional plane orientation of the EG fillers in TPU matrix were believed to play a critical role in the improvement of flame retardancy. The significance of this research was providing a new feasible idea to fabricate flame retardant composites with excellent mechanical properties.

In this work, expandable graphite (EG)/thermoplastic polyurethane (TPU) composites with excellent exfoliation, dispersion and two-dimensional plane orientation of the EG fillers were manufactured by microlayer coextrusion technology.