Polymer confinement created by electric fields
University of Cambridge
Thin liquid films exposed to an electric field break up and
form lateral patterns that exhibit a single characteristic length
scale (see Figure 1). This break-up mechanism can be modulated by
introducing a lateral variation in the strength of the electric
field. Examples of pattern formation in thin polymer films
exposed to homogeneous and laterally inhomogeneous films are
shown in the figures. In the framework of PolyFilm, we are
interested in these structures because of their small dimensions.
Since it is possible to manufacture structures with sizes of less
than 100 nm, small volumes are created, in which high molecular
polymer chains are confined.
Figure 1, above: A polymer-air double layer is destabilised in
an electric field by a capillary instability. This leads to the
formation of a wave pattern with a characteristic lateral length
scale. The final conformation is a hexagonal array of columns
that span the two capacitor plates. The inter-column spacing is
determined by the field strength and the polymer surface tension,
the column diameter is a function of the initial film thickness.
We shall perform two types of experiment related to pattern
formation using electric fields. The first of these relates to
copolymer morphologies in confinement. In figure 1 we schematised
the experiment for a single homopolymer layer; if we replace the
homopolymer with a block copolymer, pattern formation occurs on
two distinct length scales: alonger scale due to the formation of
polymer columns, similar to the homopolymer case, and a shorter
scale due to the microphase separation of the block-copolymer.
For small column diameters (< 100 nm), the microphase
morphology is confined. This experimental set-up is therefore
ideal to study the interplay of geometrical confinement and
electric fields on the microphase morphology of block copolymers
(Figure 2).
Figure 2, above: Schematic representation of the what happens when
the homopolymer in figure 1 is replaced with a diblock copolymer
film. This results in the interplay of two effects: (1) the
electric field aligns the interfaces between the two block
parallel to the electric field lines, and (2), the overall film
destabilises leading to the formation of columns (see Fig. 1).
Two possible conformation of the confined microphase morphology
are envisaged. A concentric shell structure minimises the surface
free energy of the polymer-air surface, at the expense of a high
interfacial curvature in the inner parts of the column. Planar
lamellae are, on the other hand the preferred morphology in the
absence of confinement, but have a surface energy penalty when
confined to a column. In the above, we assume the copolymer to be
a symmetric diblock.
A second class of experiments involves trading off van der
Waals forces using electric fields. In the experiments shown in
Figure 1, polymer-air interfaces that are perpendicular to the
electric field lines are unfavourable in terms of their
electrostatic free energy. The electric field therefore
suppresses thin films on the substrates that interconnect the
columns. However, very close to the substrate, unfavourable
electrostatic interactions can be overcome by the stabilising
effect of van der Waals forces,. The thickness of such a thin
films is determined by the interplay of these two forces
(electrostatic and van der Waals). It should therefore be
possible to quantitatively measure van der Waals disjoining
pressures in such an experimental set-up.
Figure 3, above. Interplay of electrostatic and van der Waals
interactions. In the absence of van der Waals force (or for
destabilising van der Waals forces where the Hamaker constant,
A>0), a thin film spanning the pattern generated by an
electric field is energetically unfavourable. A positive van der
Waals disjoining pressure (A<0), on the other hand, favours
the complete coverage of the substrate by a polymer film. In the
presence of an electric field, this leads to a thin film spanning
the electrostatically induced pattern. The equilibrium thickness
heq is determined by a force balance of these two effects.
