Tailored Gradient Morphologies and Anisotropic Surface Patterns in Polymer Blends

Abstract

Gradient morphologies and surface patterns can provide a template for smart materials with tailored local properties. Fabricating such structures in a onestep melt-extrusion process will be most beneficial as they usually require several manipulation steps. In this study, gradient morphologies are developed by melt-spinning matrix-dispersed polypropylene/polystyrene blends as a template system and dynamic control of their formation is achieved, which further leads to anisotropic surface patterns. Cross and longitudinal sections and the surface of extruded fibers are evaluated by scanning electron microscopy. Distinct microstructure gradients regarding size and shape of the dispersed phase are formed in the bulk. After dissolving the dispersed phase, ordered (sub)micron channels appear on the surface, which are further refined by hot-drawing. The resulting textures form anisotropic grooves along the fiber axis on the surface, as verified by atomic force microscopy. Capillary theory is applied to elucidate the formation mechanisms of gradient microstructures in polymer blends during processing. It is shown that all these features can be tailor-made by using the extrusion die as a design tool. By transferring the knowledge to suitable polymer blends and processes, the designed gradient and surface structures can find applications, for instance, in materials with adaptive mechanical or interfacial properties.

Tuning gradient microstructures in immiscible polymer blends by viscosity ratio

“…Cox and Grace predicted the droplet size during simple melt processing and proposed the deformation of a droplet to be for all viscosity ratios under shear flow. Theoretically, a high viscosity ratio will result in large deformed droplets and a coarse morphology, whereas a low or matching viscosity ratio could result in fine fibrils and uniform morphology . In most experimental cases, the dispersed phase was found to be finer for viscosity ratios less than one ( p ≤ 1) than that for viscosity ratios larger than one ( p > 1) .…”

“…By applying similar ideas to synthetic immiscible polymers, desirable morphologies and properties can be achieved by controlling the external field (e.g., gradient temperature field and electric fields), polymer diffusion in interpenetrating polymer networks, and phase separation in multicomponent blends . Recently, we produced polymer fibers with gradient microstructures by tuning melt processing parameters and capillary die design . We showed that gradient fibers with fine thin fibrils in the surface region and large slightly‐deformed droplets in the central region can be achieved by special capillary design .…”

“…Recently, we produced polymer fibers with gradient microstructures by tuning melt processing parameters and capillary die design . We showed that gradient fibers with fine thin fibrils in the surface region and large slightly‐deformed droplets in the central region can be achieved by special capillary design . At the surface of such fibers, submicron channels aligned along the fiber axis were obtained by solvent etching, resulting in significantly altered surface properties.…”

“…At the surface of such fibers, submicron channels aligned along the fiber axis were obtained by solvent etching, resulting in significantly altered surface properties. The formation of gradient structures has been shown to be triggered either by temperature gradients or by shear field gradients . As for the latter mechanism, we aim to show here how the direction of gradient morphologies can be smartly reversed by a proper material selection based on their viscosity ratio.…”

“…The gradient structures in polymer blends originate mainly from the structural reformations of the processed material due to the gradient of the flow field in the capillary die . The development of radial and axial flow fields in a capillary are complex and largely determined by capillary length, diameter, and entrance angle .…”