Controlling the morphology of spin coated polymer blend films.

Thin films of polymer mixtures made by spin-coating can phase separate in two
ways - by forming lateral domains, or by separating into two distinct layers. The
latter situation - self-stratification or vertical phase separation - could be
advantageous in a number of practical applications, such as polymer
photovoltaics. In our experiments, we have used time-resolved small-angle light
scattering and light reflectivity during spin coating to study the structure
development in PS/PMMA and PFB/F8BT blends, solution cast in toluene. A
sample cell was designed, made and mounted on the apparatus to manipulate the
evaporation rate. Having solved the Meyerhofer equation for thinning rate and by
fitting the model to the experimental data, we are able to extract the evaporation
rate of toluene during spin coating. We demonstrate that, by controlling the
evaporation rate during the spin-coating process, we can obtain either selfstratification
or lateral phase separation in the same system. We relate this to a
previously hypothesised mechanism for phase separation during spin coating in
thin films, according to which a transient wetting layer breaks up due to a
Marangoni-type instability driven by a concentration gradient of solvent within
the drying film. Our results show that a high evaporation rate leads to a laterally
phase separated structure, while reducing the evaporation rate suppresses the
interfacial instability and leads to a self-stratified final film. Using the set up we
developed to control the morphology through evaporation rate, we made
preliminary photovoltaic devices. It is possible to control the efficiency of the
polymer photovoltaics by means of process parameters such as evaporation rate.