Flow-induced crystallization of isotactic polypropylene : modeling formation of multiple crystal phases and morphologies

A modeling framework is presented to describe flow-induced crystallization of isotactic polypropylene at elevated pressures in multiple crystal phases and morphologies. By combining two models for flow induced crystallization developed in previous work, all parameters but one are a priori fixed. In the present work only one additional parameter was introduced, determining the portion of spherulites nucleated by flow that form beta-crystals.Model calculations show good agreement with experimental data for crystal volume fractions of all phases over a very wide range of flow-conditions, with shear rates varying from 0-200 s−1, pressures varying from 100-1200 bar and shear temperatures from 130-180 ◦C. Moreover, the model provides a tool to investigate two open questions regarding crystallization of isotactic\u3cbr/\u3epolypropylene. First, it is shown that flow-induced formation of beta-phase can be accurately described by assigning a fixed portion of flow-induced spherulites to the beta-phase. Due to the high growth rate of beta-phase compared to the alpha- and \u3cbr/\u3egamma-phases, although over a relatively narrow temperature range, only a seemingly small portion of 0.2% of all flow-induced nuclei becoming beta-spherulites is enough to explain the experimentally observed volume fractions of up to 20%. Secondly, it is shown that experimentally found volume fractions of gamma-phase at high shear rates and pressures can only be matched if gamma-crystals can directly nucleate on highly oriented flow-induced crystallites (so-called shish)

Application of a multi-phase multi-morphology crystallization model to isotactic polypropylenes with different molecular weight distributions

\u3cp\u3eFlow-induced crystallization at elevated pressure of a set of metallocene isotactic polypropylenes (iPP) possessing different molecular weight distributions is studied using extended dilatometry experiments in an apparatus able to apply elevated pressure (up to 1200 bar) and strong shear flow (shear rates up to 180 s \u3csup\u3e−1\u3c/sup\u3e). The effect of flow on the crystallization temperature was quantified and the samples were analyzed using X-ray diffraction to measure the relative amounts of different crystal phases. The experimental results were used to test a flow induced crystallization model framework recently developed in our group which can describe the complex crystallization behavior of iPP and includes formation of multiple morphologies (spherulites, shish-kebab structure with lamellar branching) and crystal phases (α,β and γ). Almost all model parameters were left unchanged, except for the temperature description of the quiescent crystal growth rate and nucleation density (experimentally measured using optical microscopy) and an extra parameter which takes into account the fraction of high molecular weight for the creation of flow-induced nuclei. The model describes rather good the experimentally observed trends for what regards crystallization temperature and amount of α and γ-phase but substantial discrepancies were found in the amount of β-phase formed. This could be related to the presence of 2,1 insertion errors in metallocene samples which could inhibit the formation of this crystal phase. \u3c/p\u3

Modeling Flow-Induced Crystallization

A numerical model is presented that describes all aspects of flow-induced crystallization of isotactic polypropylene at high shear rates and elevated pressures. It incorporates nonlinear viscoelasticity, including viscosity change as a result of formation of oriented fibrillar crystals (shish), compressibility, and nonisothermal process conditions caused by shear heating and heat release as a result of crystallization. In the first part of this chapter, the model is validated with experimental data obtained in a channel flow geometry. Quantitative agreement between experimental results and the numerical model is observed in terms of pressure drop, apparent crystallinity, parent/daughter ratio, Hermans’ orientation, and shear layer thickness. In the second part, the focus is on flow-induced crystallization of isotactic polypropylene at elevated pressures, resulting in multiple crystal phases and morphologies. All parameters but one are fixed a priori from the first part of the chapter. One additional parameter, determining the portion of β-crystal spherulites nucleated by flow, is introduced. By doing so, an accurate description of the fraction of β-phase crystals is obtained. The model accurately captures experimental data for fractions of all crystal phases over a wide range of flow conditions (shear rates from 0 to 200 s−1, pressures from 100 to 1,200 bar, shear temperatures from 130°C to 180°C). Moreover, it is shown that, for high shear rates and pressures, the measured γ-phase fractions can only be matched if γ-crystals can nucleate directly on shish

Non-isothermal crystallization of semi-crystalline polymers : the influence of cooling rate and pressure

During industrial processing, polymer melts are exposed to local high cooling rates, strong deformation rates and high pressures. Nowadays, research in the field of semi-crystalline polymers still strives towards an accurate prediction of the evolution and final appearance of the crystalline morphology in polymer products. After all, the amount, number, phase and orientation of the crystallites act in a combined way and control the final optical and mechanical properties. This chapter discusses recent experimental and model developments concerning the influence of industrially relevant cooling rates and pressures on the non-isothermal crystallization of both an isotactic polypropylene and a linear low-density polyethylene grade. The influence of flow gradients is discussed in Chapter (Roozemond et al., Adv Polym Sci, 2016