Stratified morphology of a polypropylene/elastomer blend following channel flow

Abstract

A thermoplastic olefin blend consisting of isotactic polypropylene (PP) and an ethylene-butene copolymer (EBR) impact modifier (25 wt % EBR) was subjected to a short, high-shear pulse within the flow channel of a pressure-driven microextruder following low-shear channel filling from a reservoir of the melt. The resulting morphology was examined by laser scanning confocal fluorescence microscopy (LSCFM), with contrast provided by a fluorescent tracer in the EBR minor phase. Shear experiments were performed under isothermal conditions with a known wall shear stress for a specified duration, providing a well-defined thermal and flow history. Low-shear channel filling produces small droplets across the central region of the channel and large droplets, consistent with steady-state shear, in the regions near the channel walls. After cooling the molten blend to a crystallization temperature of 153°C, a brief interval (5 s ϳ 1/2000 of the quiescent crystallization time) of high shear (wall shear stress: 0.1 MPa) induces rapid, highly oriented crystallization and a stratified morphology. Ex situ LSCFM reveals a "skin" at the channel walls (ϳ70 m) in which greatly elongated fiberlike droplets, oriented along the flow direction, are embedded in highly oriented crystalline PP. Further from the walls but directly beside the skin layers are surprising zones in which EBR domains show no deformation or orientation. Several zones of intermediate deformation and orientation at an angle to the flow direction are located closer to the center of the channel. At the center of the channel, EBR droplets are spherical, as expected for channel flow. The various strata are explained by the interplay of droplet deformation, breakup, and coalescence with the shear-induced crystallization kinetics of the matrix.

Characterisation of liquid film in a microstructured falling film reactor using laser scanning confocal microscopy

“…CLSM is a popular imaging and analysis technique that has been widely used in many areas, e.g. biology (mostly in fluorescent mode [32][33][34][35]), semiconductor materials science (mostly in reflection mode [33]) and polymer science [36][37][38][39]. The key feature of CLSM is that it has a very short depth-of-field.…”

Quantitative probing the interfacial structure of TPO/CPO blends by transmission electron microscopy via EDX

“…Some of our ideas about the stretching of the EB droplets in the PP matrix and the formation of fiber-like structures are based on finding of Moffitt et al [11], who studied the influence of laminar flow on the morphology of iPP/EB blends using LCFM in conjunction with dye-labeled EB. This model TPO blend had the same iPP used here and a similar EB content, but employed a nearly amorphous EB (EB28) containing 28 wt% butane.…”

“…In previous publications, we examined the ability of this CPO to promote adhesion between slabs of TPO or its constituents, as measured through lap shear tests [10]. Through a combination of covalent dye labeling and laser confocal fluorescence microscopy (LCFM), we investigated aspects of the morphology of CPO-coated injection-molded plaques [11], as well as simple blends of CPO with the TPO components [12]. These studies support the idea that CPO interacts more strongly with the impact modifier (here semicrystalline EB9) than with iPP.…”

Interfacial strengthening of high-impact polypropylene compounds by reactive modification

“…Studies [53,54] have shown that when a polypropylene/rubber system is injection moulded, the surface morphology of formed parts, due to the shear induced crystallisation, could be described as a surface layer of a polypropylene rich region, beneath which lies a layer rich in rubber particles. Surface stability of such multiphase polymer systems depends very strongly on the phase separation behaviour in the surface region.…”