Skip to main content
European Commission logo print header

Confined Polymer Films: Deviations from bulk behaviour

Final Activity Report Summary - POLYFILM (Confined polymer films: deviations from bulk behaviour)

POLYFILM is concerned with five themes where thin polymer films are relevant.

Conformation: In the absence of any other interactions, a polymer can be considered a long chain-like molecule, which wants to conform itself in a random walk, because this maximises its conformation entropy. When this polymer is confined it can still be squeezed and have a random walk conformation, but this is not necessarily what happens. This example is not a guide to the work that we have performed, but rather an indication of why there is rich physics in studying the conformation of confined polymers. It has, for example, implications for all of the other themes of this network. With POLYFILM support, we were able to create the world's smallest polymer crystals which has great importance in the creation of organic nanoparticles.

Morphology: Understanding and controlling the structure of materials is important in, for example, polymeric photovoltaic devices. Here, light is converted to an exciton (bound electron-hole pair) which must dissociate so that the negative and positive charges travel unimpeded to electrodes to generate a current. If the exciton decays radiatively, or if the path to electrodes is impeded, then device efficiency is limited. In POLYFILM we have succeeded in fabricating the first polymeric photovoltaic block copolymer. Block copolymers are polymers chemically joined together, and these are important because their size is similar to the length an exciton can travel before it decays.

Glass Transition: The glass transition is not thermodynamic, but rather a kinetic transition with a step change in volume expansivity around the transition. The dynamics of polymers is strongly related to the glass transition. This is not because polymers flow; it is a myth that if you wait long enough glasses are liquid-like, but rather because their local dynamics provides information on the space that the molecule has in which to move. Confined polymer films are key because the glass transition of a thin polymer film is markedly different from the bulk. Because of the multiple length scales involved, there are theoretical difficulties in studying the glass transition, especially involving computer simulations. We have been able to include the (industrially important) effect of solvent in computer simulations of the glass transition.

Diffusion and dynamics: Diffusion and dynamics have played a major role in the history of polymer science. Polymers are viscoelastic materials, and this is because they have multiple relaxation times. A molten polymer (i.e. one surrounded by polymers of its own kind and no solvent in a fluid state) is confined within a "tube" defined by its neighbours. This allows only limited lateral motion, and bulk polymeric flow can only be along its contour path, an insight winning the late Pierre-Gilles de Gennes, named on the original proposal, the Nobel Physics Prize. How such motions play a role in confined systems is not at all clear. We have been able to distinguish between the interfacial causes for mobility reduction and the glass transition, which is great importance in understanding polymer crystallisation, for example.

Switchability: If molecular nanotechnology is to influence our lives we shall need to be able to control our nano-machines. Switching with the use of electric fields has been described elsewhere in this report, but there are times when the material must respond to its environment. As an example, one might consider drugs to counter diabetes, which must act depending on sugar concentrations. This responsive behaviour built into the system is something that polymer materials are very good at. Polymers such as polyelectrolytes respond to environmental cues. A polyacid will collapse in an acidic environment, whereas it will be swollen in a basic environment. Using POLYFILM support, we were able to create a water-based reversible adhesive that is both inexpensive and environmentally friendly.