Craze microstructure from small-angle x-ray scattering (SAXS)

Abstract

The small-angle x-ray scattering from air crazes in polystyrene (PS) and polycarbonate (PC) as well as from methanol crazes in PS has been measured by using both pinhole-collimated and slit-collimated cameras. Under the normal conditions (x-ray beam in the plane of the craze, craze held under load), the SAXS pattern from all crazed samples consists of a cross through the origin, one very intense streak normal to the craze plane and a second, less intense, streak parallel to the craze plane (perpendicular to the craze fibrils). Tilting experiments reveal that the former streak, which we term the anomalous streak since it is not expected to arise from the known fibril geometry, is a rod in reciprocal space; the rocking curve has a half-width of about 1°. Filling polystyrene crazes with methanol produces a 40 fold decrease in intensity I at a scattering angle 20 of 3 mrad and an increase in slope of the I vs. 2Θ curve at small angles. Since the maximum decrease for any true craze diffraction is given by the ratio of the square of the electron density difference between PS and the fibril in the craze, i.e., by [(ρPS-ρair)/ (ρPS, -ρCH3 OH)]2 = 22, the larger decrease observed indicates that at least a portion of this streak is due to external reflection from the craze surfaces. The intensity streak parallel to the craze is a disk in reciprocal space and represents the small angle diffraction from cylindrical fibrils within the craze. A Porod-type analysis using the intensity in the high angle tail of this streak and the SAXS invarient yields values of the average fibril diameter [Dbar] = / of 6 nm for room temperature air crazes in PS, 33 nm for air crazes produced at 128° in PC, and of 11 nm for dried methanol crazes in PS. The 6 nm value for PS air crazes is in excellent agreement with recent TEM measurements in thin films. The PS and PC air craze samples exhibit interfibrillar interference maxima in the I vs. 2Θ curves. Methanol crazes in PS which have never been dried exhibit an interference peak at about the same angle as the air crazes, but, once dried, these crazes have an I vs. 2Θ curve which falls monotonically with 2Θ. Fibril correlation functions γ(r) and fibril axis radial distribution functions g(r) are computed from the intensity data. PS air crazes exhibit a wide distribution of fibril diameters and a g(r) ≃ 1 representing an almost random fibril axis distribution for r > 9 nm although there is a small broad maximum at r = 25 nm. For the PC high temperature crazes the fibril size distribution is more narrow and g(r) exhibits a stronger peak. The dried methanol craze g(r) shows a strong peak at r = 11 nm which corresponds to 11 nm diameter fibrils touching confirming TEM evidence that the changes in fibril microstructure on drying are due to a bundling together of fibrils under the action of the air-liquid interface. Irreversible changes in SAXS corresponding to permanent changes in fibril microstructure are also observed on unloading the crazed PS samples. The intensity in the fibril scattering streak shrinks inward to smaller angles, including the maximum, if present. These changes are thought to result from buckling of the fibrils, caused by the fact that unloading puts the craze into compression.