Neutron reflectivity of polymer-plasticiser diffusion

Neutron reflectivity (NR) has been widely exploited to look at polymer thin films and in many ways is an ideal technique for studying polymer interfaces and surfaces, providing high-resolution concentration - depth profiles across the film thickness. Most NR studies to date have concentrated on thin films of amorphous polymers which possess T_g values well above room temperature. These polymers are ideally suited to NR measurements, firstly because they form homogeneously flat films and, secondly, heat-quench cycles can be used to study time-dependent processes. This has been used to great effect in NR studies of the initial stages of polymer - polymer interdiffusion or the kinetics of surface segregated layers for instance. One of the biggest drawbacks to this approach is that in polymer systems where one or more of the components has a T_g close to or less than room temperature, the polymers can still move during the measurement time of an NR profile, which typically takes 1-2 h for a full profile. Therefore, in order to study such systems, we have developed an approach to NR measurements that allows us to investigate diffusion processes in situ. Our new approach allows us to take NR profiles in only 20 s. This paper describes the method of real-time NR measurements in detail and illustrates the capabilities of the technique with highlights from some of our recent work on the early stages of polymer-plasticiser interdiffusion.