A study of the porosity of water-plasticised polyacrylonitrile films by thermal analysis and microscopy

In a previous study, the morphology of water-plasticised polyacrylonitrile (PAN) film, made by compression moulding moist polymer powder, was discussed. The X-ray diffraction pattern of the plasticised film was different from that of dry PAN and this led to the suggestion that hydrated polymer crystallites had been formed by a portion of the water. The location of the remainder of the water was unknown. In this work, the thermal behaviour of the water-plasticised PAN at sub-ambient temperatures has been examined. When the film was cooled below room temperature, two distinct exothermic peaks were observed at 17°C and 38°C due to the formation of ice. By analysing the data in terms of thermoporometry, which has been more widely used to study the freezing of condensates in porous ceramic materials, it was concluded that the water-plasticised films had a micro-porous morphology. The DSC results suggested a bimodal distribution of pore sizes, but scanning electron microscopy revealed a third category of pores with larger size. These large pores were not detected by calorimetry because they were not fully filled with water.