Pc Peter Roozemond

Flow-enhanced nucleation of poly(1-butene): Model application to short-term and continuous shear and extensional flow

A modeling framework for flow-enhanced nucleation of polymers is applied to a broad set of data from literature. Creation of flow-induced pointlike nuclei is coupled to chain stretch of the high-molecular weight tail of the material, calculated with a rheological constitutive model. As the flow-induced nuclei grow, the crystalline volume fraction increases and with it the viscosity of the material. This is accounted for by describing the material as a suspension of spheres in a viscoelastic matrix. Calculations are compared with a broad set of experimental data from literature on three grades of poly(1-butene). First, a parameter set is determined by fitting model results to flow-induced nucleation densities from short-term shear experiments. Next, this parameter set is used to validate the framework in continuous flow experiments in which viscosity is monitored during a constant flow rate. In this way, we demonstrate the approach is applicable to not only short-term shear but also continuous flow. It was observed in experiments that for continuous extensional flow, the viscosity shows an upturn at a constant strain, the value of which is independent of strain rate. We hypothesize that this upturn is related to long chains entering the chain stretch regime, as a result of the extension rate exceeding the inverse of the Rouse time of the longest chains.

Suspension-like hardening behavior of HDPE and time-hardening superposition

The rheology of solidifying high-density polyethylene (HDPE) is investigated. Experiments on an HDPE were performed with a novel RheoDSC device. Results agree quantitatively with simulations for a suspension of elastic spheres in a viscoelastic matrix except for very low values of space filling (<5%), indicating that the rheological behavior of the crystallizing melt in the frequency range investigated is purely suspension like. The hardening behavior of the material is characterized in two different ways; a normalized rheological function and a time-hardening superposition (THS) master curve of rheological properties. An improvement is proposed to the procedure for performing THS that was previously used in the literature. Based on this procedure, a novel method for predicting the rheological properties of crystallizing melts is presented.

Flow-induced crystallization studied in the RheoDSC device : quantifying the importance of edge effects

Flow-induced crystallization is investigated through short-term shear flow experiments on poly(1-butene) in the RheoDSC device. We demonstrate that the DSC signal shows contributions from spherulitic morphology in the center of the sample and oriented structures at the edge of the sample, the latter being induced by edge effects at the free surface. It is shown that, although small in terms of volume, the crystallization at the edge has a dominating influence on the measured rheology. We show how these kinds of effects can be recognized in stand-alone rheometric studies of flow-induced crystallization.

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.

Quantification of isothermal crystallization of polyamide 12: Modelling of crystallization kinetics and phase composition

Abstract The crystallization of Polyamide 12 (PA12) has been investigated using a new experimental setup which allows in-situ synchrotron Wide Angle X-ray Diffraction (WAXD) during Flash-DSC measurements. The experimental results are used to parameterize and validate a new numerical model to quantify the quiescent crystallization kinetics, under isothermal conditions, of the three important crystal structures of PA12, i.e. the, γ − , α ′ − and mesomorphic phase. The experimental approach is based on nucleation and growth after quenching the material from the melt to an isothermal temperature, described by the Schneider rate equations and the Kolmogorov-Avrami expression for the space filling. The experimental overall crystallization rate, expressed in terms of the crystallization half-time, as well as the phase composition, are well captured by the model over a wide range of temperatures, i.e. between the glass transition and the melting temperature. It is shown that at temperature below and above 100 ° C different nucleation mechanisms are dominant causing the bimodal dependence of the crystallization rate. This work forms the basis for a full model for non-isothermal conditions for which transitions between different phase have to be taken into account.

Flow-Induced Crystallization of Polyamide-6

Abstract Flow-induced crystallization has been widely studied for a variety of polymers with the main focus on polyolefins. In this work, research has been conducted on the flow-induced crystallization of another important class of polymers, namely polyamides. Different polyamides-6 with varying molecular weight have been studied. At relatively modest values of shear rate, rheology has been used to study the kinetics of flow-induced crystallization. Typical scaling relations based on the longest relaxation time and the Rouse time – usually obtained for polyolefins – are tested for the polyamides under investigation in order to identify the different regimes of flow-induced crystallization. At high shear rates, a correct rheological signal was impossible to collect. However, the rheometer was used in this case to prepare the sample to be studied ex-situ by Wide Angle X-ray Scattering experiments to determine the onset shear rate for the formation of highly oriented shish-kebab structures.

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).