Effect of substrate surface energy on transcrystalline growth and its effect on interfacial adhesion of semicrystalline polymers

The effect of substrate surface energy on transcrystalline growth at the interface of a semicrystalline polymer and its effect on interfacial adhesion were investigated for substrates treated with various silane coupling agents. A thin film of isotactic polypropylene (iPP) crystallized on a high surface energy substrate (treated with gamma-(aminopropyl)triethoxysilane) was composed entirely of trans-crystallites. On the other hand, when the iPP film was crystallized on a low surface energy substrate (treated with perfluorodecyltrichlorosilane), the interface was dominated by spherulites, and only a very thin transcrystalline region (thickness similar to1 mum) was observed. The substrate surface energy was found to exert a significant influence on the crystallinity, density of nuclei, crystal microstructure (e.g., lamellar thickness and crystal orientation), and thickness of the transcrystalline region near the interface. The adhesion energy measured by the asymmetric double cantilever beam (ADCB) test increased strongly (from 1 to 100 J/m(2)) with surface energy. Examination of the fractured specimens using atomic force microscopy and scanning electron microscopy revealed fibrillation of the iPP induced by the strong interfacial adhesion; this was found to be associated with the breakdown of the fibrils, which is the characteristic fracture mechanism of transcrystallites at the interface in high surface energy samples. At lower surface energies, however, cracking occurs at the boundary between the transcrystallites and the spherulites because of the weak boundary layer near the interface.