(597d) Slip-Link Model for Polydisperse Melts in Industrial Processes

The deformation and flow of polymers are important for industrial processing and product fabrication. Significant efforts have been invested in developing rheological models for entangled polymer melts. Currently, these models can predict the mechanical response from knowledge of the molecular weight distribution (MWD), long-chain branching distribution, and/or other molecular-level material characterization information. However, transitioning a model from idealized flows in the lab to engineering applications requires additional effort. For example, the discrete slip-link model (DSM) is an established computational single-chain model for the rheology of entangled polymer chains. Recently, it has been applied to the study of the industrial processing of linear low-density polyethylenes (LLDPEs). These applications revealed several challenges due to the broad polydispersity of the LLDPEs and the control variables relevant to industrial processing.

Firstly, flow-induced crystallization studies require simulations of polydisperse polymers in flows with high strain rates and high strains. Our study investigates universal behavior in shear and extensional flows, probes the range of applicable strain rates, and extends to polydisperse polymers. These calculations are challenging due to the computational cost of the model, especially for long chains with large numbers of entanglements. We seek to overcome this challenge by exploiting the universal behavior of polymer melts with different numbers of entanglements. We propose a surrogate flow simulation with a reduced number of entanglements to reduce the computational cost, followed by re-scaling. The proposed simulation strategy reduces the computational cost by more than two orders of magnitude. Secondly, in applications like polymer film welding, the flow is defined by pressure or stresses, not by strain rates and deformation fields. For DSM, this scenario is challenging since the model is formulated for the input of the deformation tensor and output of the extra stress tensor. Therefore, we implement a stress-controlled scheme based on a PI (proportional-integral) controller for slip-link calculations. The scheme is developed for polydisperse systems and is compatible with the cost-reducing strategy above. We present its application to flows of welding LLDPE film.