Abstract
Flow induced crystallization of semi-crystalline polymers is an important issue in polymer science and engineering because the changes in morphology strongly affect the properties of polymer materials. In this study, a phase field technique considering polymer characteristics was established for modeling and predicting the resulting morphologies. The considered crystallization process can be divided into two stages, which are nucleation upon the flow induced structures and subsequent crystal growth after the cessation of flow. Accordingly, the proposed technique consists of two parts which are a flow induced nucleation model based on the calculated information of molecular orientation and stretch, and a phase field crystal growth model upon the oriented nuclei. Two-dimensional simulations are carried out to predict the crystallization morphology of isotactic polystyrene under an injection molding process. The results of these simulations demonstrate that flow affects crystallization morphology mainly by producing oriented nuclei. Specifically, the typical skin-core structures along the thickness direction can be successfully predicted. More importantly, the results reveal that flow plays a dominant part in generating oriented crystal morphologies compared to other parameters, such as anisotropy strength, crystallization temperature, and physical noise.
Highlights
For semi-crystalline polymers, the ultimate properties of the products fabricated by freezing liquids into solids strongly depend on the features of crystalline structures formed during solidification [1]
The first step is to predict the molecular conformation by simulating the viscoelastic flow in the framework of the dumbbell model; the second step is to generate oriented nuclei dependent on the flow induced structures; and the last step is to simulate the crystal growth via the phase field method
It is evident that physical noise has a great influence on the flow induced crystallization morphology compared to the results shown in Figure 5, where both the anisotropy and noise are not involved
Summary
For semi-crystalline polymers, the ultimate properties of the products fabricated by freezing liquids into solids strongly depend on the features of crystalline structures formed during solidification [1]. These methods directly use a group of nonlinear partial differential equations to describe the complex crystal growth of a polymer This makes the phase field methods able to simulate very rich crystallization morphologies and microstructures with moderate computing cost [28]. Morphologies produced by flow, this paper will first establish a flow induced nucleation model with the aid of calculable flow induced molecular orientation and stretch, predict the subsequent crystal growth via a modified phase field method.
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