Crystallization processes in quiescent and moving polymer melts under heat transfer conditions

In polymer solutions or blends, flow can strongly influence the degree of mixing of the components. In a shearing flow, droplets in a dispersion can be broken down to sizes comparable to the dimensions of the polymer molecules themselves, thereby inducing molecular-scale mixing. Demixing can also occur when the two components of the mixture differ greatly in viscoelastic properties. Shear or extensional flow can induce polymer migration in nonhomogeneous flows or in flows with curved streamlines, and can render turbid solutions or blends that are otherwise transparent. Flow can also induce polymer gelation, and can induce ordering transitions in liquid crystals or block copolymers. Here, we review these phenomena, discuss proposed mechanisms, and assess the degree to which recent theories can account for the observations. Because the phenomena are complex, multiple experimental probes and theoretical methods are required to study them. Successful theories must incorporate polymer/polymer or polymer/solvent thermodynamics, critical phenomena, and phase transitions, as well as polymer theology and the kinetics of diffusion or crystallization. The experimental techniques used to study these phenomena are equally wide ranging, and include turbidity measurements, light, x-ray, and neutron scattering, fluorescence quenching, microscopy, and theology.

Flow-induced mixing, demixing, and phase transitions in polymeric fluids

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.

Microrheological Modeling of Flow-Induced Crystallization

Three series of isotactic polypropylene characterized by different molecular weights and the same isotactic index have been studied during crystallization under static and shearing conditions. The shear is induced by the displacement of a glass fiber in the molten polymer. The monoclinic α-phase is here formed under shear with a columnar organization at the surface of the glass fiber, and does not appear under static condition. The growth-rate, constant during the shear-induced crystallization experiment, is compared with the result obtained from static crystallization. An important increase of the growth-rate due to the shear flow is observed. This increase depends on the molecular structure. The average molecular weights ―Mw and ―Mz seem to be the most important molecular parameters, for which an excellent correlation is obtained. The increase of these parameters ―Mw and ―Mz leads to a significant enhancement of the growth-rate, which can be multiplied by a factor of 10 in the present conditions.

Shear-induced crystallization of polypropylene : Influence of molecular weight

The present study is an attempt to formulate a new approach to the modeling of the injection moldings of semicrystalline polymers. A phenomenological approach is employed to develop the qualitative description of crystallization phenomena under nonisothermal flow occurring in various polymer processing operations. The unified crystallization model, which is suitable to describe the multilayer microstructure arising in the injection moldings of semicrystalline polymers, is proposed. The “competing mechanisms” of crystallization in terms of the induction time indices for introducing various microstructure layers are utilized to quantitatively distinguish between the highly oriented skin layer and spherulitic core in the moldings. The application of the equations of continuity, momentum and energy, along with the rheological model, the equation of state and the unified crystallization model, to the injection molding process leads to the modeling of crystallinity and microstructure development in the moldings. The material parameters to be used in the modeling of the crystallinity and microstructure development in the injection moldings of isotactic polypropylenes (i-PP's) with various molecular weights are determined. The melt viscosity as a function of shear rate and temperature is measured and fitted to the modified Cross model. Quiescent crystallization kinetics, including the induction time and the heat release due to crystallization, is characterized using differential scanning calorimetry. The spherulite growth rate of i-PP's under isothermal and nonisothermal quiescent crystallization conditions is measured by optical microscopy. The molecular deformation factor during the induction period of shear-in-duced crystallization is obtained from the extrusion-quenching and extrusion-relaxation-quenching experiments with a slit die. The variation of the induction time with shear rate obtained by Lagasse and Maxwell from the shearing-crystallization experiments is used to determine the shear enhancement coefficient of crystallization.

Crystallinity and microstructure in injection moldings of isotactic polypropylenes. Part 1: A new approach to modeling and model parameters

In the presented model elements of polymer melt rheology and polymer crystallization kinetics are combined. In particular, the proneness of the melt to the special type of crystallization which is characteristic for shear treatment is supposed to emerge only gradually during shear flow. Following Avrami's early ideas on crystal growth, an induction time is introduced. In principle, the model can be applied to any flow and temperature history. The special case of isothermal flow at constant shear rate is covered in greater detail: A favorable comparison is made with experimental results, as published by Lagasse and Maxwell [10].

G. Eder

Papers

Theory of shear-induced crystallization of polymer melts