posted on 2024-06-19, 16:34authored byTae Yong Ha, Seung Heum Jeong, Chunggi Baig
We present a detailed analysis of the rheological behavior
of entangled
short-chain-branched (SCB) ring polymers at interfaces via direct
comparison with the corresponding pure (unbranched) ring polymers
using atomistic nonequilibrium molecular dynamics simulations of confined
polyethylene 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 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.