Microrheological Modeling of Flow-Induced Crystallization

Abstract

The problem of flow-induced crystallization (FIC) of polymer melts is addressed via a microrheological approach. In particular, the Doi−Edwards model with the so-called independent alignment approximation (DE−IAA) is used to calculate the flow-induced change of free energy. Subsequently, the crystallization induction time, i.e., the nucleation characteristic time, is calculated in isothermal steady shear and uniaxial elongational flows. Asymptotic, analytical expressions for the induction time are also derived in the limit of low and high Deborah number (the product of the deformation rate and the polymer relaxation time). The DE−IAA model is found to give more realistic predictions than those of simpler, dumbbell-like models already proposed in the literature. When compared to existing FIC experimental data in shear flow, good quantitative agreement is found with the polymer relaxation time as the only adjustable parameter of the model.

Modeling Flow-Induced Crystallization

“…Their model was validated to capture accurately the shear layer thickness in a number of experiments [63,66]. Several other authors have adapted an expression for free energy to incorporate deformation and derive nucleation rate [38,[70][71][72][73][74][75][76]. Crystallization rate can be calculated using these models, but no information about the final morphology is obtained.…”

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

“…It is important to note that the whole viscoelastic behavior of the melt affects the capability of the molecules to be oriented by the imposed flow and their relaxation time. 23,[98][99][100][101] The chain-chain overlap and entanglements affect the relaxation behavior of both short and long chains. The short chains may be oriented by the long ones and take longer time to relax; on the other hand, short chains will act as diluents for the long ones and reduce their relaxation time.…”

“…In most processing operations, such as fiber spinning, film blowing, extrusion, injection molding, etc., it takes place in the presence of or just after flow, which is referred to as flow-induced crystallization. [14][15][16][17][18][19][20][21][22][23][24] The imposed flow field conditions (i.e., elongational, shear, or mixed), temperature, and mechanical strain all affect the molecular conformations of polymer chains. The deformed melt becomes an ensemble of random, fully, and partly extended/oriented entangled chains.…”

“…Consequently, the crystallization kinetics can increase by several orders of magnitude and the final solid-state morphologies are usually an overlap of spherulities and flow-induced oriented crystalline structures, such as cylindrites, shish-kebabs. [14][15][16][17][18][19][20][21][22][23][24] Clearly, the size, shape or form, orientation, and perfection of the crystals, as well as the fraction (i.e., crystallinity) of the polymer bulk they constitute will affect the polymer solid-state properties. For example, it is well-established that the inhomogeneous and anisotropic structures of skin and core of injection-molded samples [25][26][27][28][29][30][31][32] can significantly influence the physical properties such as dimension stability, 33 Young's modulus, and tensile strength.…”

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

“…It is generally considered that the shear rate needs to surpass a critical value, _ c min , for the shish nuclei formation. 2 Although a number of studies have focused on the issue that the _ c min relates to a specific relaxation time, the reptation time s d or the Rouse time s R , [6][7][8] it was recently confirmed experimentally in two studies that the _ c min correlates the inverse of s R . 9,10 Thus, stretching of the molecules, and not just orientation of the primitive path of the molecules, is required for the formation of oriented morphology under flow conditions.…”