LIQUID NITROGEN EFFECT ON THE MORPHOLOGY OF PMMA THIN FILMS ON YBCO. Amita Malik, M. Atreyi, Department of Chemistry, University of Delhi, Delhi, India, G. L. Bhalla, G. C. Trigunayat, Department of Physics & Astro-physics, University of Delhi, Delhi, India

High temperature superconducting YBCO was encapsulated with polymethylmethacrylate (PMMA) film by plasma polymerisation of methylmethacrylate (MMA). YBCO samples with encapsulating films of varying thickness (5-14 microns), obtained by varying the length of plasma polymerisation, were subjected to 50 cryo-thermal cycles, each consisting of keeping the sample in liquid nitrogen for 1 minute and then at ambient environment for 30 minutes. The changes in the morphology of PMMA after every 10 cryo-thermal cycles were examined by scanning electron microscope. It was generally observed that, the PMMA encapsulating film first develops inhomogeneities and then shrinks, with the extent of transformation depending on the number of cryo-thermal cycles and thickness of the film. Such morphological changes were not observed on the PMMA film when the samples were continuously kept in the liquid nitrogen for a few hours. The changes in the morphology could be attributed to the release of thermal stresses developed during the plasma polymerisation of MMA, coupled with entrapping of gases, which cause thermal strains during the repeated cryo-thermal cycles.