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 γ-(aminopropyl)triethoxysilane) was composed entirely of transcrystallites. 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 ∼1 µm) 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.

ABSTRACT:The toughening behavior of polycarbonate modified with core-shell type particles was investigated. The alloys were found to exhibit maximum impact strength upon addition of a modifier with a poly(butyl acrylate) rubbery core of 0.25 m diameter. The incorporation of particles with diameter greater than 0.25 m resulted in decreased impact strength. The influence of rubber phase contents on toughness was also studied. It was observed that the alloys exhibited maximum impact strength upon addition of 4 wt % rubber phase. Further increase in the rubber phase content resulted in reduced impact strength. Fractography of the samples showed that, below 4 wt % rubber phase content, the fracture occurs mainly by internal crazing and, from 4 wt % onward, only by shear deformation. When the effect of dual particle size distribution was analyzed, it was found that there was only a moderate increase in toughness compared with alloys containing monosized particles.

With the aim of developing a material with switchable tack properties, a side-chain liquid crystalline polymer containing a poly(oxyethylene) backbone and n-heptylsulfonylmethyl side chains was synthesized. The inner structure, surface properties, and tack behavior of this polymer film were investigated. An X-ray reflectivity study of a film of the polymer coated onto a silicon wafer showed that, upon annealing above the glass transition temperature, the film has a lamellar structure in which the ordered layers are parallel to the substrate surface. The layer thickness is roughly twice the length of the fully extended side chain, indicating a double-layered structure of the polymer with side chains normal to the polymer backbone having an almost all-trans conformation. The contact angle decreases abruptly at the isotropic transition temperature, which results from the disappearance of the smectic order. The coiling of the polymer backbone and disordering of side chains occur above the isotropic transition temperature. NEXAFS spectroscopy and AFM results confirmed that at temperatures below the isotropic transition temperature the side chains in the top layer are oriented such that they are almost perpendicular to the surface, which results in the packing of the CH3 groups at the surface and hence a very low surface energy. A remarkable change in tack properties is observed at the smectic-to-isotropic transition temperature. The dramatic change in tack properties, which occurs within a narrow temperature range, is due to the change in the surface energy and viscoelastic behavior of the film. This reversible and dramatic change in the tack force over a narrow temperature range could potentially be exploited in applications that require materials with switchable tackiness.