and Atsushi Takahara

Fatigue failure mechanisms of short glass-fiber reinforced nylon 66 based on nonlinear dynamic viscoelastic measurement

Abstract The fatigue behavior of short glass-fiber reinforced nylon 66 under stress controlled fatigue tests was studied on the basis of the nonlinear dynamic viscoelasticity measurements. In order to analyze the effect of nonlinear viscoelasticity on the fatigue behavior, quantitative measurements of nonlinear viscoelasticity have been carried out based on Fourier analysis. It was found that the nonlinear viscoelastic behavior that was closely related to the irreversible structural change appeared markedly during fatigue process. The failure models in fatigue process were proposed based on the cross-section morphology under optical microscopic observation before final failure of the specimens. The fatigue behavior could be classified into the two failure mechanisms, depending on whether the fatigue test was carried out below or above glass transition temperature of the matrix nylon 66. The fatigue process proceeded with the following steps:(1) the damage started with void formation at fiber ends; (2) the microcracks propagated around the fiber ends ( T ≦ T g ) or the microcracks propagated being accompanied with debonding along the fiber sides and also, forming the crack walls( T > T g ); (3) the cracks propagated between the fiber ends( T ≦ T g ) in a brittle manner, or the crack walls dominantly remained being connected by bridges( T > T g ) in a ductile manner; (4) the fast crack propagation occurred, after the crack reached to a critical size, and finally, the specimen failed.

Microscopic lamellar organization in high-density polyethylene banded spherulites studied by scanning probe microscopy

Surface topography and lamellar aggregation structure of high-density polyethylene (HDPE) banded spherulites were investigated by scanning probe microscopy. HDPE films were prepared by isothermal crystallization at various crystallization temperatures from the melt. Polarizing near-field scanning optical microscopic (NSOM) observations for the HDPE films revealed submicron-scale correlation between surface topography and birefringence of banded spherulites. The height profile of the film surface along the spherulitic radius periodically changed corresponding to the intensity profile of transmitted light along the radius of the extinction ring. This correlation was more clearly observed in the topographic and NSOM images of permanganic etched PE films. Therefore, it was apparently suggested that the crystallographic c-axis of the orthorhombic unit cell was parallel and perpendicular to the film surface at the peak and the valley in the surface corrugation of the banded spherulite, respectively. The band spacing obtained by polarizing NSOM and atomic force microscopy (AFM) was comparable to that determined by polarizing far-field optical microscopic observation under crossed nicols. The band spacing and the peak-to-valley height difference in the corrugation increased with an increase in isothermal crystallization temperature. AFM observations directly indicated local lamellar orientation and stacking manner.

Analysis of surface composition of isotopic polymer blend based on time-of-flight secondary ion mass spectroscopy

Abstract Surface chemical composition of blend composed of monodisperse polystyrene (hPS) with the number-average molecular weight, Mn, of 19.7k and deuterated monodisperse polystyrene (dPS) with Mn of 847k was analyzed based on time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Although hPS possess higher surface free energy than dPS, ToF-SIMS revealed that hPS was preferentially segregated at the outermost surface of the blend films with various compositions. The surface segregation of hPS can be explained in terms of the molecular weight disparity for both components, i.e., an entropic effect.