Thin liquid-crystalline polymer films : nucleation, crystallisation, instabilities and growth

Liquid-crystalline polymers, LCPs, possess an ordered liquid state between their crystalline solid state and their isotropic state. It is not only their ordering that makes this material interesting but actually their hybrid character, i.e., they behave both like liquid crystals and like polymers. In the bulk this class of material has been studied extensively and it is still under further study. However, studies of the surface phenomena and thin films of LCPs are very scarce.

What started as a study with a high practical impact, namely the use of side-chain liquid-crystalline polymers as a primer layer for coatings, evaluated into a thorough investigation on surface phenomena of thin films of ordered fluids.

The thin films were prepared by spin-coating solutions of LCP material, based on alternating copolymers of maleic acid anhydride and a-olefins carrying terminal mesogenic methoxybiphenyl groups, onto silicon wafers. This resulted in smooth isotropic thin (nanometer-scale) films. The main topics investigated on these films are wetting, ordering and dewetting. These phenomena could clearly be made visible by the following three main techniques. By atomic force microscopy (AFM) the surface topography and topology can be imaged and structures of only less than a few nanometers in depth can be made visible. This was done for areas in the range of several mm2 to 0.02 mm2. By optical reflection microscopy this area could be extended to larger areas up to 0.5 mm2, however, without information on the topography. More information on the inner structures in the film was obtained by X-ray reflectometry.

The combination of AFM and X-ray reflectometry turned out to be very powerful in describing the morphology of and structures in thin films. The initial isotropic spin-coated films changed upon annealing above the glass transition temperature, Tg. First randomly oriented micro-crystalline domains are formed which corrugate the polymer-air interface. The size of these corrugations depends on the initial film thickness: it increases with film thickness. At the same time, however, a laterally macroscopic crystal starts to grow from the substrate surface in the direction of the polymer-air interface, at the expense of these domain structures. Finally, a nicely ordered single crystal with parallel ordered bilayers is formed in the film as well as at the polymer-air interface. This ordering was visible in the AFM-images as terrace-like structures with a height corresponding to a bilayer, and in the X-ray spectra as a Bragg peak.

This one-dimensional crystallisation, actually recrystallisation, depends strongly on the temperature due to viscosity effects: the ordering was completed much faster at higher temperatures. An Arrhenius-type plot gives an activation energy of 122 kJ/mol, which we ascribe to the expected reorientations of the mesogenic groups during the recrystallisation process.

When the films are heated for a long period they break up. Holes, which are also visible by optical microscopy, appear which then grow in time. The growth rate of the holes depends on the temperature and film thickness. At the final stage of dewetting only droplets remain on top of a rather stable bilayer. The bilayer is even present above the isotropisation temperature.

This indicates autophobic behaviour which is even more pronounced in the mesophase where a layered film is present, and where dewetting may occur over several ordered layers.

In all cases the dewetting is not linear in time and polymer slippage seems to take place. In the case of polymer slippage, a t2/3 dependence is expected for the growth rate, which is indeed found above the isotropisation temperature. In the mesophase the dewetting differs from the "normal" slippage behaviour and a weaker time dependence is observed. We ascribe this deviation to the mesogen-mesogen interactions which have to be broken-up upon dewetting.

Around the isotropisation temperature there is a strong increase in the (initial) dewetting velocity of over more than 2 orders of magnitude. This acceleration is most probably due to the strongly decreasing viscosity because of the breaking up of the mesogen-mesogen interactions.

Also the effect of chemically and physically modified wafers on the ordering and stability against break-up is investigated. In the case of unmodified silicon wafers, we found that the polymer-substrate interaction induces parallel layering in the film, resulting in stable films. By changing the surface groups by surface modification with several silane compounds with different end groups, the interaction between the polymer and the surface can be tuned. In this way we obtained very hydrophobic surfaces for silanes with methyl or allyl end groups, and more intermediate surface properties for silanes with phenyl, chloride, carbomethoxy or cyano end groups. After surface modification film formation was only possible on the intermediate surfaces.

Above Tg these films show a characteristic behaviour which is strongly temperature dependent. At low temperatures in the mesophase holes nucleate which are encircled by unstable rims. Upon further annealing the rim instability decreases and the dewetting velocity increases. This feature also occurs for another completely different side-chain liquid-crystalline polymer with a methacrylate backbone and cyanobiphenyl groups in the side chains. We ascribe the peculiar dewetting behaviour to the presence of polycrystalline domains in the thin films. Especially their size and orientation and their ability to deform under shear are held responsible for the rim instabilities and, consequently, for the droplets remaining behind in the dry patches.

By adsorbing a monolayer of negatively charged nano-sized silica particles onto a pre-adsorbed positively charged polyelectrolyte a controllable way to roughen a silicon surface is introduced. By sintering at 1000 °C, the particles partly fuse with the substrate thus forming a rough, yet pure silica surface. By changing the radius of the adsorbing silica particles the degree of roughness can be tuned.

When the polymer film thickness exceeds the size of the substrate textures, upon annealing initially smooth films get rough by the formation of ordered domains, i.e., random polycrystalline domains. In time, these ordered domains grow in size and decrease in number. Apparently, the surface structures act as nucleation points for the formation of crystal domains.Big domains grow at the expense of smaller ones (Ostwald ripening).

In the isotropic phase the domains coalesce as an effect of the surface tension. The liquid film then remains stable, which is not the case on smooth substrates. Cooling down from the isotropic phase to the mesophase results in the formation of needle-like structures, so called batonnets, which is a typical texture for smectic A phases as found in bulk experiments.