Molecular Mechanism Research into the Replication Capability of Nanostructures Based on Rapid Heat Cycle Molding

Aimed at the molding of polymer nanostructure parts, the interface model between long- and short-chain polycarbonates (PC) and nickel mold inserts was established by the molecular dynamics method. The molecular mechanism of the replication capability of polymer nanostructure part molding was discussed by analyzing the migration and diffusion of the molecular chain, concentration profile, filling morphology evolution, interface binding energy, and filling rate of conventional injection molding (CIM) and rapid heat cycle molding (RHCM). The results show that nanostructures are filled mainly during the packing stage. A short-chain PC system has a low glass transition temperature (Tg) and viscosity, good fluidity, and a high filling rate, so the replication capability of its nanostructures is good. A long-chain PC system has a fast cooling rate in CIM, its molecular chain motion is blocked, the filling rate is low, and the interface binding energy is small, and so its nanostructures have poor replication capability. However, the high temperature at the nanostructures can be maintained for a long time in RHCM, which promotes Brownian motion in the molecular chains. Under the action of packing pressure, molecular chains can overcome entanglement barriers and viscous resistance. Thus, the polymer concentration profile and filling rate increase with increasing packing pressure, which can produce more van der Waals energy. Furthermore, the evolution process of polymer filling morphology is realized by the Brownian motion of chain segments under packing pressure; that is, the diffusion motion of the molecular chain along the direction of a tube composed of other chains around it. With the increase of temperature or pressure, the migration and diffusion of the molecular chain can be promoted; thus, the replication capability of nanostructure parts for mold cavities can be enhanced.

Molecular Dynamics Study on the Deformation Behaviors of Nanostructures in the Demolding Process of Micro-Injection Molding

Polymer parts with nanostructures have broad applications, possessing excellent optical, electrochemical, biological, and other functions. Injection molding technology is one of the main methods for mass production of polymer parts with various shapes and sizes. The demolding process is vital to the replication quality of molded parts with nanostructures. For this study, molecular dynamics simulations of polypropylene (PP), polymethyl methacrylate (PMMA), and cycloolefin copolymer (COC) were conducted for the demolding process. The average velocity, density distribution, adhesion energy, and demolding resistance were introduced to analyze the deformation behaviors of polymer nanostructure from a nickel nano-cavity with an aspect ratio of 2:1. The shoulders of nanostructures were firstly separated from the nickel mold insert in the simulation. Under the external demolding force of 0.07 nN, PP and PMMA could be successfully demolded with some deformations, while COC could not be completely demolded due to the greater adhesion energy between COC and Ni. It was found that the maximum adhesion energy occurred in the separation process between the shoulder of the nanostructure and Ni and the huge adhesion energy was the main cause of demolding defects. The velocity difference of the whole polymer layer and polymer nanostructure was further analyzed to explain the nanostructure deformation. In order to improve the quality of demolding, the external force applied on polymers should be properly increased.