Shear-Enhanced Crystallization in Isotactic Polypropylene. 3. Evidence for a Kinetic Pathway to Nucleation

Abstract

The exact value of the wall shear strain is not known. The known quantities are the wall shear stress, the duration for which it is applied and the mass extruded during that time. The flow curve of the polymer at a temperature above the nominal melting temperature is also known, and the time-temperature shift behavior of polypropylene can be applied to the melt. Some of the unknowns that prevent quantitative determination of the wall shear strain are the transient behavior upon inception of non-linear shearing and the development of structures that progressively modify the rheology of the sample due to flow-induced acceleration of crystallization.Here, we give three different estimates of the wall shear strain based on the mass of polymer extruded: a geometical estimate, an estimate based on a Newtonian fluid, and an estimate based on a shear-thinning power-law fluid. All of them assume that steady-state is reached instantaneously and make no attempt to account for the rheology changing with time during shear. The values of the strain, γ ge , reported in our paper are "pseudo"-strains based solely on the geometry of the flow cell. We estimate the strain limit of our instrument as a ratio of the usable length (≈ 5 cm) of the rectangular flow channel to its thickness (0.5 mm) which yields γ ge = 100. Thus, when a mass equal to all the polymer initially in the flow channel [corresponding to a volume of 5cm x 0.635cm

Thermal stability of shear‐induced precursor structures in isotactic polypropylene by rheo‐X‐ray techniques with couette flow geometry

“…) corroborated the earlier observations, not only in PE but also in a variety of other polymers, such as iPP, 43,44,50,[52][53][54][55][56][57][70][71][72][73][75][76][77][78] isotactic poly-1-butene (iPB), 90 polyethylene terephthalate (PET), 91 polydiethylsiloxane (PEDS), 91 etc. In some report, the quasicrystallographic flowinduced structures are referred to as ''mesophase.''…”

“…1,[59][60][61][62][63] Under flow, there are substantial evidences of the formation of long-lived metastable oriented structures, even at temperatures above the normal melting point of the polymer (T mp ). 44,52,[64][65][66][67][68][69][70][71][72][73] The first direct experimental evidence was obtained from rheoRaman 66 studies of polyethylene (PE) melt at 140 8C, a temperature significantly above its melting point. It was shown that a fraction of PE molecules adopted all-trans sequence configurations upon shearing, which remained stable for several hours even after cessation of shear.…”

“…It was shown that a fraction of PE molecules adopted all-trans sequence configurations upon shearing, which remained stable for several hours even after cessation of shear. The recent results of the techniques that couple flow (parallel plate, [42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57]66,68,[74][75][76][77][78] fiber pullout, 14,79,80 die extrusion 15,[70][71][72][73][81][82][83] ) with a physical probe of the structure development (transmitted light, [70][71][72][73][81][82][83] optical microscopy, 14,[75][76][77][78]79,80 X-ray diffraction, 42-57 atomic force microscopy, …”

Effect of processing parameters on the morphology development during extrusion of polyethylene tape: An in-line small-angle X-ray scattering (SAXS) study

“…The route to crystallization during flow is generally accepted to be via the formation of 'shishkebab' morphology [6][7][8][9]. Proposed mechanisms and kinetics for the formation of shish-kebab morphology usually relate to data from shear flow conditions using custom made shearing devices [10][11][12][13][14][15][16].…”

Using multimodal blends to elucidate the mechanism of flow‐induced crystallization in polymers

“…9 Thus, the amount of mechanical work performed by flow on the polymer melt controls the formation of oriented shish nuclei via the shear-induced nucleation of stretched molecules. In this respect, it needs to be pointed out that neither a critical stress 15 nor a critical strain 16 should be used to describe the onset of oriented crystal morphology as these parameters relate to the later stage-transformation of shish nuclei rows into a fibrillar structure (shish). In this study, boundary specific work, involving both the shear rate and the time of shearing, is used as a term to describe the threshold flow conditions for the onset of an oriented morphology.…”