Molecular characteristics of ring and short-chain branched polymers at bulk and confined systems

Abstract

One of the main goals in polymer rheology is to interpret the structural and rheological properties of polymeric materials based on their molecular features (e.g., molecular architecture, molecular weight, and chemical constituents). Over the past decades, numerous experiments and simulations have shown that the molecular features of polymeric materials strongly affect their dynamic and rheological responses to applied flow fields. The physical origins of these responses are tied to the fundamental molecular dynamics, which are based on the molecular architecture. Therefore, numerous studies on the influence of molecular architecture have been conducted based on the well-established theory and models for linear and long-chain branched polymer architectures. Recently, extensive research focused on the closed-loop ring geometry and short-chain branches has been carried out. Compared to linear and long-chain branched polymer architectures, ring and short-chain branched (SCB) polymers exhibit different structures and dynamics, leading to distinct rheological properties. In chapter 1, we present the synergistic effect of ring geometry and short branches on the structural and rheological properties of a polymeric system. We analyzed the short-chain branched (SCB) ring polyethylene (PE) melt system using atomistic nonequilibrium molecular dynamics (NEMD) simulations under shear over a wide range of flow strengths and compared the results to those of the linear, ring, and SCB linear PE melt systems. Specifically, a comprehensive analysis covering the macroscopic rheological behaviors to the microscopic molecular dynamics was carried out. Compared to the linear system, which exhibits typical shear thinning behavior of viscosity, the ring system generally exhibits slightly weaker shear thinning on the overall flow regimes. This behavior is closely related to the closed-loop geometry of the ring, which results in a more compact structure compared to the linear analogue and leads to a new topological interaction termed penetration. The short-chain branching generally leads to a more compact chain structure and a lower degree of chain orientation toward the flow (x-)direction due to the fast random dynamics of the short chains. Therefore, the SCB linear and SCB ring systems exhibit weaker shear thinning behavior of viscosity compared to their pure system analogues, particularly in the intermediate flow regime. Importantly, whereas the ring and linear systems have only slight differences in viscosity, the synergistic effect of ring geometry and short branches causes the SCB ring system to have remarkably weaker shear thinning than the SCB linear system. When considering the mesoscopic structure, the characteristics of the SCB ring system become more noticeable. The fast random dynamics of the short branches, which can be attributed to their short characteristic relaxation time, cause the overall chain dimension to contract. In contrast, in the SCB ring system, some SCB ring molecules exhibit more stretched structures than the largest chain of the pure ring system. This is attributed to the synergistic effect of penetration caused by the ring geometry and the short branches even though the penetration conformation appears for both the pure ring and SCB ring systems. The contracted surface of the SCB ring polymer tends to enhance the friction between the threaded and threading ring chains involved in the penetration structure. Accordingly, the threaded ring molecules contribute more to the stress of the system than other free ring chains. This synergistic effect of ring geometry and short branches results in more penetration conformations in the SCB ring system than in the pure ring system, leading to anomalous structural and rheological properties. In chapter 2, We present a detailed analysis of the rheological behavior of entangled SCB ring polymers at interfaces via a direct comparison with the corresponding pure (unbranched) ring polymers using atomistic nonequilibrium molecular dynamics simulations of confined PE melt systems under shear flow. To elucidate the general structural and dynamical characteristics of interfacial polymer chains, we analyze various physical properties of the chains in the bulk and interfacial regions separately within the confined systems, such as the chain radius of gyration and its distribution, the average streaming velocity profile, and the degree of interfacial slip with respect to the applied flow strength. The pure ring polymer melt has a highly extended and aligned chain structure along the flow (x-)direction at the interface even under weak flow fields, indicative of the strong wall effects via the attractive polymer–wall interactions. In contrast, the interfacial SCB ring chains generally form a compact structure like that of the corresponding bulk chains in the weak flow regime, representing a significant role of the short branches to effectively diminish the wall effect. In conjunction with these structural characteristics, the entangled SCB ring polymer melt displays a markedly smaller degree of interfacial slip than the corresponding pure ring analogue in the weak-to-intermediate flow regimes. Furthermore, while both the pure ring and the SCB ring polymer melt systems reveal similar fundamental molecular mechanisms at the interface with respect to the flow strength (i.e., z-to-x rotation, loop wagging, loop migration, and loop tumbling mechanisms), the SCB ring polymer melt displays relatively weaker loop migration and loop wagging dynamics with highly curvy backbone structures in the intermediate flow regime. In the strong flow regime, both the pure ring and the SCB ring systems exhibit the loop tumbling mechanism together with intensive collisions between the interfacial chains and the wall. However, the interfacial SCB ring chains execute substantial loop migration dynamics even at high flow fields, which facilitates interfacial slip.