Collapse of polymer brushes grafted onto planar ... - Wageningen UR

DETAILED ATOMISTIC MONTE CARLO SIMULATION OF GRAFTED POLYMER MELTS:
THERMODYNAMIC, CONFORMATIONAL AND STRUCTURAL PROPERTIES
K.Ch. Daoulas, V.G. Mavrantzas
Institute of Chemical Engineering and High-Temperature Chemical Processes,
ICE/HT-FORTH, GR 26500, Patras, Greece
email: daoulas@physics.upatras.gr
ABSTRACT
The thermodynamic and conformational properties of polymer melts grafted on a solid substrate as obtained
from detailed, atomistic Monte Carlo simulations with the end-bridging algorithm are presented. The interface
between a basal graphite plane (as well as a non-interacting hard surface) and a bulk polyethylene (PE) melt,
a few or all chains of which are grafted on the plane, has been studied. Three different PE melts, of mean
molecular length C78, C156 and C250, have been investigated, at grafting densities ranging from 0.54 nm –2 to
2.62 nm –2 . For melts composed of grafted and free chains, it is observed that, at moderate to high surface
densities (σ ≥ 1 nm –2 ), the region close to the substrate is fully occupied by segments belonging to grafted
chains, which are forced by their chemical grafting to have their first segment on the interface. As the grafting
density increases, free chains are progressively expelled from the surface region, in agreement with scaling
arguments and the predictions of a lattice-based self-consistent mean-field (SCF) theory. For melts grafted
on a graphite plane, it is also seen that the local melt density in the region closest to the interface is
systematically higher than in the bulk, exhibiting distinct local maxima due to polymer adsorption. Results for
the chain conformation tensor demonstrate that chains are significantly stretched in the direction
perpendicular to the surface, even for moderate surface densities. For the C250 PE melt at a grafting density
σ = 1.31 nm –2 , for example, the average chain dimension perpendicular to the interface is 1.9 times larger
than its equilibrium value in the bulk. The profile of the chain end density is also seen to exhibit universal
behavior in agreement with the predictions of the SCF theory. Additional results concerning the mean chain
conformational path, the structure of the interfacial area for both systems studied (fully grafted and mixtures
of grafted and free chain systems) and their 2 H NMR spectra are also presented. Further, the dependence of
all these properties on chain polydispersity will be assessed.