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

MODELLING STRUCTURE AND ADHESION AT POLYMER-POLYMER INTERFACES
D.N. Theodorou
School of Chemical Engineering, National Technical University of Athens,
GR-15780 Athens, Greece and ICE/HT-FORTH, GR-26500 Patras, Greece
email: doros@central.ntua.gr
ABSTRACT
Copolymers are remarkably effective in improving adhesion between immiscible polymers, or between
polymers and solid substrates on which the polymer chains do not adsorb strongly. Predicting the molecular
organisation and mechanical response upon deformation of polymer/polymer and polymer/solid interfaces
strengthened with copolymers can serve as a basis for the “molecular engineering” design of materials with
optimal interfacial properties.
Using the work of Scheutjens and Fleer (Scheutjens 1979) as a starting point, we have developed a
lattice-based self-consistent field (SCF) theory for capturing the molecular-level structure of interfaces formed
by polymer melts in contact with other polymer melts or solid substrates, in the absence or presence of
copolymers. Real polymer and copolymer chains are mapped onto the model representation invoked in the
theory in a manner that respects their mass density in the homogeneous bulk, their contour length, and their
conformational stiffness (characteristic ratio). The propagation of conformations is envisioned as a secondorder
Markov process, involving an energetic penalty for chain bending. Interaction (χ) parameters between
dissimilar segments, when needed, are obtained from fitting binary interfacial widths between the
corresponding polymer melts.
Application of the theory to interfaces between polystyrene (PS) and poly(methyl methacrylate) (PMMA)
(Fischel 1995) in the presence of a symmetric PS-PMMA diblock copolymer gave segment density profiles in
very favourable agreement with neutron reflectivity measurements (Russell 1991). The end-segment, mean
square junction-to-end distance normal to the interface, shape parameter, and bond-order parameter profiles
indicate that copolymer chains are significantly stretched under experimental conditions. For fixed surface
density of the copolymer, with increasing block length, a broad transition from reflected random coil to “brush”
behaviour occurs over the region where the block radii of gyration are 1.2 to 1.7 times the mean interchain
spacing. At very high surface densities, the copolymer can no longer reside as a monolayer at the surface.
Surface-phase equilibrium calculations predict that patches of trilayer will appear when the volume of
copolymer per unit surface is roughly three times the block unperturbed root mean square radius of gyration.
This predicted appearance of structures more complex than a monolayer at the surface is consistent with the
emergence of off-specular scattering in neutron reflectivity measurements. It suggests an upper limit to the
degree of mechanical strengthening of the interface by addition of larger amounts of block copolymer, as
seen experimentally.
Our SCF theory, in parallel with neutron reflectivity measurements, has been applied to probe interfaces
between PS of molar mass 186 kg/mol and polished silicon wafer substrates (SiO2), in the presence of
diblock copolymers of poly(2-vinyl pyridine) (PV2P) and PS as adhesion promoters (Retsos 2002). In these
systems, prepared from the melt, the PV2P (“anchor”) block of the copolymer adsorbs strongly on the
surface, while the “dangling” PS block mixes with the PS homopolymer. The molar mass of the dangling
block, which was deuterated and therefore visible in the experiments, was kept constant at 75 kg/mol, while
that of the anchor block was varied systematically between 3.4 and 102 kg/mol. The dangling block profile
was found to exhibit a maximum, which decreases in height and moves away from the surface as the length
of the anchor block is increased. Results from the SCF calculation are in very good agreement with the
experiment. The anchoring block is not flat (pancakelike) as often assumed, but forms a layer of thickness