Molecular Mechanism and Dynamics of Polymer Melt at Interface

Abstract

The slip phenomenon that fluids have a nonzero velocity on static wall is very common in confined polymeric systems and plays a crucial role in polymer processing, lubrication, adhesion, coating, and etc. In particular, the surface instability induced by the wall slip in polymer extrusion processes has been a most important issue in polymer industries, since it significantly restricts the production rate within a certain low range. In addition, it has been well known that the fluid slip at the wall can make a considerable influence on the rheological properties of a confined polymer system in comparison to the corresponding bulk properties. Although numerous researches appeared in the past regarding the slip phenomenon, there still remains a critical lack of molecular-level understanding on structural and dynamical behaviors of polymer chains at interface under an external flow field. In this work, we carried out a comprehensive and detailed study on the fundamental molecular mechanism and dynamics of polymer melt near the interface under shear flow using atomistic nonequilibrium molecular dynamics (NEMD) simulation of unentangled C30H62 and entangled C178H358 linear polyethylene (PE) melts confined to a nanometer scale system. The slip phenomena were analyzed in detail with a particular effort in revealing the underlying origins in the microscopic level. For all the polymer systems studied here, we found that there exist three distinctive regimes in terms of the degree of slip (ds). In the 1st regime where the flow strength is small, ds appears to keep increasing with increasing shear rate for both C30 and C178 PE melts. The behavior is associated with the chain rotation from the neutral direction to the flow direction, which effectively lowers the frictional force between polymer chains and the wall and thus promotes the fluid slip. In the 2nd regime with an intermediate flow strength, ds exhibits a rather plateau in the case of C30 PE melt and a decreasing behavior in the case of C178 PE melt. This regime is characterized as a dynamically stable range in chain rotation and tumbling mechanisms through a competition between the wall friction against the slip of adsorbed chains and the movement of adsorbed chains in the flow direction enhanced by the surrounding bulk chains promoting the slip. In addition, in the case of C178 PE melt, it is found that the degree of entanglement between adsorbed chains and the surrounding chains becomes weaker with increasing shear rate, leading to a decrease of momentum transfer from the surrounding chains to adsorbed chains and thus the decrease of ds. This effect is considered to be particularly important in interfacial slip for long, entangled polymer systems. With a further increase of shear rate (in the 3rd regime), ds turns out to increase for both C30 and C178 PE melts. In this regime, polymer chains at interface experience a large amount of strong collisions with each other and with the wall atoms, resulting in chains get out of interfacial region. This induces less energetically favorable wall-fluid interaction thus, slip increases. Also we derived a parameter expressed by microscopic conformation of interfacial chains which can demonstrate origin of slip at atomic-level. And trend of the parameter is well-fitted with degree of slip. The understanding confined fluid in microscopic level would help to give fundamental intuition with regard to application of confined fluid such as thin film or biological membranes etc. And one may control degree of slip depending on various application process based on origin of slip at atomic-level.