Investigation of rheology and morphology to follow physical fibrillar network evolution through fiber spinning of PP/PA6 blend fiber

Abstract

This work is aimed at investigating the influence of fibrillar morphology of deformed Polyamide 6 (PA6) droplets dispersed in Polypropylene (PP) matrix on the melt viscoelastic behavior of their blends. The blends of PP with various amounts of PA6 (1%, 6%, 10%, and 20%) were prepared by melt mixing in a co‐rotating twin screw extruder and fibrillated by fiber spinning process. Scanning Electron Microscopy revealed that the PA6 spherical droplets form fibrillar inclusions after fiber spinning. The steady and transient shear rheological responses of samples were evaluated in both linear and nonlinear ranges of deformation. Non‐terminal behavior of storage modulus at low frequency appeared as a typical characteristic of fibrillar morphology whose width and value depend on fibril growth. Storage modulus and complex viscosity of the blends containing PA6 fibrillated structure were remarkably enhanced compared to as‐extruded samples. The fibrillar‐induced elasticity of the fibers is a distinguishable behavior which was revealed by conducting transient stress and creep‐recovery measurements and upon appearing mature fibrils, elasticity of the polymer blend fibers increased significantly. POLYM. ENG. SCI., 58:1251–1260, 2018. © 2017 Society of Plastics Engineers

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“…Startup of Steady Shear Flow : Transient stress measurements equip a researcher to follow structural changes by using different amounts and types of stresses. It seems that startup of steady shear measurement is one of the efficient techniques by which it is possible to analyze structural changes of fibrils 30,39,47…”

“…As was discussed, the droplet–droplet collision in blend system of PP/PA6 fiber was considered as the most probable assumption in blend system containing 20 wt% of PA6 to prevent the droplets to be fully elongated, which leads to less storage modulus and higher frequency dependency (less elongated fibrils than the fiber with 10 wt% of PA6) 30. At high level of shear rate, the fiber containing 10 wt% of PTT and spun at 240 °C shows less transient stress response than the fiber containing 6 wt% of PTT at the beginning of the shear flow, but then the transient stress response of the fiber with 10 wt% of PTT exceeds showing a peak and then reaching to plateau.…”

“…In addition, according to spectacularly increasing behavior of transient stress response of blend system of PP/PA6 with 6 wt%, the fibrillation threshold was considered at 6 wt% 30. It is expected to observe significant difference between fibers' moduli, since they have considerable difference (≈400 nm) in fibril diameter; however, the fibers containing 6 and 10 wt% of PTT spun at 195 °C and the fibers containing 10 and 20 wt% of PTT spun at 240 °C show close values of storage moduli.…”

“…In addition to traditional microscopic techniques which are used to study fibrils structurally, rheology has also represented itself as a powerful method to follow droplet deformation as the most important mechanism during processing. Mainly three test methods have been used to analyze the influence of fibrillation on melt viscoelastic properties of the blend systems: melt flow rate (MFR),25 extensional rheometry,26–28 and oscillatory rheometry 8,13,29–34. To the best of our collection, most of the works have concentrated on reporting the typical characteristics of fibrillar systems observed as a secondary plateau in a low‐frequency region, which is shared with nanocomposite polymers containing nano‐fillers35 and there is a lack of information about specific steps of fibrillation and its influence on elasticity of the blend system which is under control of fibril polymorphism.…”

“…Melt viscoelastic properties of the reported polymer blend systems have been mainly extracted via oscillatory rheometry. Although several works reported the appearance of a non‐terminal trend in the low‐frequency region13,28–30,36 by oscillatory tests performed in linear region, the growth of the physical fibrillar network (PFN) was observed as a secondary plateau in the low‐frequency region 23,25–27,29,30,37a,b. In addition, extensional viscometry test has also been utilized to observe the effect of in situ generation of elongated dispersed phase, and it was reported that strain hardening is the consequence of forming fibrils in situ whose growth depends on shear rate and time 33,28…”