Research program
Control and Modeling of Morphology of Semicrystalline Polymers under Realistic Processing Conditions
University Co-ordinator
Università degli Studi di NAPOLI "Federico II" -
INGEGNERIA CHIMICA - NAPOLI(NA)
Research Unit Leader
Nino GRIZZUTI
Description
Aim of the research is to investigate, both experimentally and theoretically, the interactions between flow and crystalline microstructure in thermoplastic polymers. The research activity of this Unit integrates within the National Project as specified in Model A of the proposal. For this reason the description of the research activities of this Unit will directly refer to the Research lines of the National Project.The experimental activity will be carried out on the specific reference polymer chosen for the whole project, i.e., an isotactic polypropylene (iPP).Research Line B. Interactions between molecular orientation, crystallization and rheological properties (months 1-18)This activity falls within Research Line B of the National Project. The early stages of crystallization are characterized by the formation and growth of crystalline micro-domains. The morphology (number, size, shape, orientation) of such domains is strongly dependent upon the molecular orientation of the amorphous phase as produced by processing conditions. Morphology, in turn, determines substantial changes on polymer processability, for example in terms of a major viscosity increase. Therefore, knowledge of the inter-relations between the early stage microstructure and the macroscopic polymer behaviour represents a fundamental information for process understanding and modeling.B.1 Experiments on the evolution of the melt orientation (months 1-12)This activity will be carried out by means of rheological and rheo-optical techniques for characterizing both the melt orientation and its effects on the early stages of crystallization Within this Project a new rheo-optical experiment will be designed and implemented. The set up is based on a transparent shear flow cell (already available), which will be completed by the construction of an optical line for birefringence measurements. The apparatus will be used for the quantitative determination of fluctuation density and birefringence in both cases of a non crystallizing melt and of a melt undergoing crystallization, under quiescent conditions and under shear flow. Rheological measurements will be performed under the same conditions used in the rheo-optical experiments, in order to directly compare the evolution of molecular orientation with the macroscopic rheological observables (shear stress, first normal stress difference). If necessary, and in order to establish an experimental reference, standard rheological characterization of the purely amorphous (melt) phase will be also performed.B.2 Modelling the effects of flow on the polymer melt (months 13-16)The experimental results obtained at point B.1 by this and other Reesearch Units will be interpreted by means of molecular models, based on the concept of "reptation", which describe the flow behaviour of the amorphous phase. Such model will provide quantitative predictions of the degree of orientation and stretching of the polymer chain as a function of the applied flow conditions. Model parameters are the polymer relaxation time and the molecular weight between entanglements, which will be determined by independent rheological measurements. The predictions of the model will be also compared with those obtained by the Research Unit 3, which will implement dumbbell-type models.B.3 Experimental study of the effects of crystallinity on the rheological behaviourIn this reasearch line a newly developed methodology, known as "inverse quenching" will be also applied. In an inverse quenching experiment the structure of the polymer is allowed to evolve up to point where, due to a sudden raise of temperature, the crystallization rate is essentially zeroed and the crystallization process is frozen. A pseudo-equilibrium structure results, where both melt and crystalline phase coexist and do not change with time. As a consequence, the above reported rheological and optical experiments can be performed at a fixed degree of crystallinity. Experiments will include measurements of viscosity and viscoelasticity of the polymer as a function of the extent of crystallization.B.4 Modelling the effects of crystallization on the rheological behaviourThe experimental results obtained by this and other Units at point B.3 will be used to elaborate and validate "ad hoc" rheological models. The objective is to formulate quantitative relations that predict the rheological properties (in particular viscosity) as a function of the degree of crystallinity. Two parlallel routes will be followed. On the one hand the crystallizing system will be described in terms of a suspension of hard spheres (the crystallites) in a viscoelastic medium. On the other hand the same system will be described as a colloidal structure deriving from the molecular interaction between the growing nuclei and the entangled melt phase. In this case the modelling will be based on the analogy between the nucleating polymer and a blend of linear chains (the melt) and hiper-branched stars (the nuclei). The comparison between the two different approaches with the experimental data will allow for the validation of one of the two models.Deliverables for Research Line B:First year (Phase 1)- Evolution of the rheo-optical apparatus to include measurements of birefringence.- Quantitative determination of density fluctuation and birefringence of the amorphous phase as a function of the thermal conditions and (if present) of the applied flow.- Comparison of the amorphous phase orientation measurements with rheological measurements under the same thermal and flow conditionsSecond year (Phase 2)- Quantitative prediction of the degree of orientation and stretching in the amorphous phase from reptation-based models. Comparison with experimental data and with dumbbell-type models.- Quantitative determination of the rheological properties under steady shear flow during the initial stages of crystallization, by means of the inverse quenching technique, as a function of the thermal and flow history.- A constitutive equation able to predict the relation between stress and deformation history during the initial crystallization stages. Such an equation must predict the viscosity of the system during crystallization while minimizing the number of adjustable parameters.Research Line C. Effects of flow on crystallization kinetics and morphology (months 1-18)This activity falls within Research Line C of the National Project. The term Flow-Induced Crystallization indicate all the effects that flow produces on both the kinetics and the morphology of the crystalline phase. Such effects can be extremely relevant, and can strongly affect the final morphology under processing conditions.C.1 Crystallization in rotational flows (moths 1-18)This Unit will perform experiments of flow induced crystallization under shear flow. Shear flow will be realized in two rotational apparatuses. The first is a standard rheometer, where the evolution of crystallinity can be followed by measuring rheological properties such as viscoelastic moduli (under quiescent conditions) and viscosity (under flow). The second is an evolution of the rheo-optical apparatus already described in the Research Line B, and constituted by a flow cell equipped with transparent plates. The instrument is currently equipped with an optical microscope. In this project a dedicated laser optical train will be designed and built. This will allow to monitor the crystallization kinetics through quantitative measurements of transmitted light intensity. The apparatus will be used also used for the quantitative determination of the SALS patterns of the crystallizing melt, under both quiescent and flow conditions. Such experiments will lead to quantitative information on the morphology (size, shape, orientation) of the growing crystallites. Rheological and rheo-optical experiments will be carried out under the same kinematics and thermal conditions (in the non isothermal case with cooling rates up to 30°C/min), to allow for a direct comparison of the morphological and rheological evolution. This aspect is directly related to Action 2 (see below).Deliverables for Research Line C:First year (Phase 1)- Realization of an experimental rheo-optical apparatus to carry out transmitted light intensity measurements to be related to the crystallization kinetics and morphology evolution.- Measurements of the characteristic crystallization times for iPP as a function of shear rate and, if possible, of molecular weight and molecular weight distribution.Second year (Phase 2)- Measurements of the characteristic crystallization times as a function of the thermal history.- Comparison and validation of the experimental measurements of flow-induced crystallization. Determination of scaling laws to correlate crystallization kinetics and morphology to: shear rate, temperature, cooling rate.- Quantitative determination of the morphology evolution during the early stages of crystallization, in the form of: growth rate, shape and orientation of the crystalline domains as a function of thermal and flow history.Research Line D. Molecular modeling of the early stages of crystallization (months 12-20)This activity runs within Research Line D of the National Project. The starting point of this activity is the molecular model that makes use of reptation-based arguments to obtain a quantitative estimate of the flow-induced change in free energy of the melt phase, and to predict the characteristic nucleation time as a function of the flow conditions. Coupled to classical quiescent crystallization models (e.g. Lauritzen and Hoffmann), this approach has already led to prediction of the nucleation characteristic time as a function of the applied flow conditions. It must be underlined that the model, at least in its original version, does not include adjustable parameters.During this Project the model will be profoundly modified and developed to account, at least, for the following aspects:- Thermal history. Coupled temperature effects on both the crystallization kinetics and the relaxation time spectrum of the polymer melt will be taken into account. In this way, predictions of the molecular model for the case of flow induced crystallization under slowly varying thermal histories will be obtained.- Transient flows. The model will be redefined for the case of non stationary flow histories. This will be considered in the expression of the crystalline nucleation rate, with the objective of determining simple crystallization kinetics expressions (Avrami type), where the effect of flow is accounted for in a quantitatively realistic way and without the use of adjustable parameters.The predictions of this part of modeling activity will be compared with the experimental results on flow enhanced crystallization kinetics obtained in Action 1 and with those obtained by other partners in the project.Deliverables for Research Line D:Second year (Phase 2)- A model based on the molecular description of the iPP melt, to allow for the prediction of the crystallization rate as a function of the applied flow history and of the thermal history (at least in the case of slowly varying cooling rates).Research Line F. Validation of the crystallization model by application to the injection molding process. (months 17-24).This activity falls within Research Line F of the National Project. The main objective of the Project is to elaborate an integrated mathematical model to predict the crystallization kinetics and morphology under process conditions, including the effects of thermal history (in particular high cooling rates), pressure history, and flow history. Such a model would constitute a relevant step forward in the understanding, design and simulation of semicrystalline polymer processes.F.2 Optimization of the crystallization models (months 21-24)In the final phase of the Project all Research Units will be involved in the validation of the global crystallization model. The results of numerical simulations (performed by a dedicated computer code that will include the crystallization model developed in this Project) will be compared with the experimental results from injection molding tests. At this stage the model will be optimized by performing the required adjustments to the sub-models, in order to obtain the best agreement between numerical simulation and experimental data. Particular attention will be given to the determination of the number of quantitative parameters necessary to implement the model, and to the independent experimental procedures necessary the determine such parameters. A parametric sensitivity analysis will be also performed, in order to individuate the parameters that more significantly affect the model predictions.Deliverables for Research Line F:- The fully optimized model to predict the evolution and the final crystallinity (kinetics and morphology of the crystalline phase) under process condition. Quantitative determination of the significant model parameters.
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