Research program
Control and Modeling of Morphology of Semicrystalline Polymers under Realistic Processing Conditions
University Co-ordinator
Università degli Studi di SALERNO -
INGEGNERIA CHIMICA E ALIMENTARE - FISCIANO - SALERNO(SA)
Research Unit Leader
Giuseppe TITOMANLIO
Description
The project is rather structured, and develops along all the topics denoted as A, B, C, D, E e F, in the scheme adopted in the research program description section of "model A". The most part of the activity will be carried on by adopting a single material, an isotactic polypropylene, already adopted in previous research activities carried out with the same research units joining this project.As far as topic A is concerned (activities on the evolution of quiescent crystallization morphology), main research actions are reported belowA.2.1 Experiments of crystallization under high cooling rates (months 1-12)A consistent effort in the project will be spent on the construction and development of an apparatus for the monitoring of crystallization kinetics during quenches from the melt to room temperature of thin polymeric films. This goal will be pursued through the analysis of transmitted light intensity. The temperature evolution will be measured by means of thin thermocouples placed inside the sample. Results will be compared with standard calorimetric tests. The apparatus should be able to measure on-line quiescent crystallinity, for the first time under high cooling rates (hundreds of °C/s).A prototype of this apparatus, able to reach some tens of °C/s, has already been built by this Research Unit. During this project, this prototype will be improved to allow monitoring of crystallization evolution at cooling rates of hundreds of °C/s. In particular, sufficiently fast detectors should be selected to increase the sampling speed (very thin thermocouples and fast photoresistors and/or photodiodes) and, in order to measure the small angle light scattering (SALS) during crystallization a fast video camera will be purchased. The analysis of optical data will give crystallinity evolution (from transmitted light intensity) and size of spherulites (from SALS pattern analysis), which will be related to temperature evolution data of the same experiment. To this purpose, a calibration procedure of the optical data will be identified on the basis of standard data (isothermal calorimetry, DSC, and results of the following analysis performed on final samples: density, WAXS, FT-IR, optical microscopy, SEM and AFM) which will be coupled to optical signals on the same sample. Crystalline structure and morphology will be further analysed by SEM and AFM.A.2.2 Experiments of crystallization under high pressure (months 1-12)An apparatus will be developed to solidify thin polymeric films (carefully shielded from the confining fluid) under hydrostatic pressures up to some thousands of bars and under cooling rates of the order of 10°C/s. The samples solidified inside this apparatus will be analysed by several techniques (density, WAXS, FT-IR, optical microscopy, SEM and AFM) in order to correlate pressure and temperature history experienced during solidification to the resulting morphology.A.3 Modeling of morphology evolutions in quiescent conditions (months 5-16)The analysis of experiments of Action A.2 and of data obtained by other R.U. will provide quantitative information of crystallization kinetics in an extremely wide range of solidification conditions. Models already available in the literature will be initially tested against those data. All data will be simultaneously considered to identify a model for the evolution of morphology during crystallization. The modelling will target first the volume fractions of the different crystalline phases. Later on, also information regarding nucleation (also focusing on possible athermal effects) and spherulitic growth rate will be considered.To this purpose, Kolmogoroff's model will be adopted, since this model allows to specify the contributes due to nucleation and growth. Data regarding both phenomena will be provided by other R.U., in the widest possible range of temperature. A model describing the evolution of morphology in a very wide range of cooling conditions, although quiescent, will be identified on the basis of all available data. The presence of different crystalline phases will be described by means of a parallel of kinetics, competing for the same available amorphous volume. The model will be the reference for the description of the effect of flow on crystallization kinetics.Expected results for activity A:1st year: - set-up of an apparatus for crystallinity and spherulite dimensions on-line measurements under fast cooling rates- set-up of an apparatus for solidification under high pressure2nd year: - quiescent kinetic model for the description of the evolution of volume fractions of each crystalline phase and of the dimensions of spherulites in a wide range of solidification conditions.As far as topic B is concerned (activity on melt rheology and effect of crystallinity on rheology), main research actions are reported belowB.1 Experiments on melt orientation evolution (months 1-12)Optical measurements will be performed during the crystallization process by an optical analysis module (OAM-II, Rheometrics), already available. By means of this apparatus, in-situ birefringence and dichroism measurements (during the oscillations) will be performed and, after calibration, will be related to orientation evolution in the sheared melt during both isothermal and slow-decreasing experiments (up to 1°C/s). The results will be the basis for the modelling step, action B.2. In the frame of this experimental activity, iPPs with different molecular masses and molecular mass distributions will be adopted to test the importance of long tail molecules on flow-induced molecular orientation.B.2 Modelling of flow effect on molten polymers (months 13-16)During this action, a model will be identified to describe the experimental results of the evolution of orientation by effect of shear flow, gathered during activity B.1 by means of birefringence measurements performed by this R.U. and by light scattering data obtained by other R.U.As far as the description of the viscoelastic nature of the polymer, the elastic Dumbbell model will be adopted as a starting point. This model, indeed, satisfactorily describes many rheological features of viscoelastic fluids, accounting of both molecular orientation and stretching. The elastic Dumbbell model will be first applied in its simplest formulation, i.e. assuming a linear elastic behaviour. Main advantage of this model is the presence of only one parameter (the relaxation time, which is related to material properties) linking the history of deformation rate to molecular orientation. If this model would not give sufficient description of main features of experimental data, a more detailed description of the molecule chain will be adopted, constraining its extension and/or assuming that relaxation time, similarly to viscosity, is related to deformation rate.The results of the modelling step will be compared with the results obtained by more detailed, although more complex, models adopted by the R.U.2B.3 Experiments on the effect of crystallinity on rheology (months 13-20)The moduli G' and G" evolutions will be monitored by rotational rheometers, looking also at the crystallization and its impact on rheology, both during isothermal runs and during cooling runs, carried out at the cooling rates allowed to the rheometers. Results of this experimental activity will constitute the source for the subsequent modeling step B.4.B.4 Modelling of rheology with quiescent crystallinity (months 13-20)Based on the experimental results obtained during activity B.3 by this and the R.U. 2, together with the R.U.2 the modelling of the effect of crystallinity on melt rheology will be attempted. On the basis of results of activity B.3, together with the evolution of crystalline degree (measured by turbidity data, by DSC tests and evaluated by a suitable model of crystallization kinetics) a model able to provide the evolution of rheological parameters during isothermal and non-isothermal crystallization will be identified.As far as the effect of small crystalline fractions ("early stages" of crystallization), each crystalline unit will be considered as a physical crosslink, thus contributing to the increase of G' and G''. The number of crystalline units is on its turn due to the previous nucleation, and thus to the variables which determine nucleation itself.Expected results for activity B:1st year: - results of experimental tests on melt orientation evolution in different flow conditions2nd year: - model of melt orientation evolution in different flow conditions- model of the effect of crystallinity on rheologyAs far as topic C is concerned (experiments on effects of crystallization during flow), main research actions are reported belowC.1 Crystallization in rotational devices (months 1-20)The effect of shear flow on flow-induced crystallization will be analysed by rotational rheometry, combining time and shear rate in order to achieve a given orientation degree. This procedure will be repeated for several orientation degrees, in experiments both of orientation relaxation and steady shear flow. These tests will be carried out by the optical analysis module which allows to monitor orientation and at the same time provides data regarding crystallinity (through turbidity).Steady shear flow tests will also be carried out in a stress controlled rheometer: in particular oscillations will be superposed on a steady shear flow, thus monitoring material crystallization through the evolution of G' and G". Like the activity B.1, the experimental work of the activity C.1 will involve other iPPs, to investigate the role played by the molecular mass in the flow-induced crystallization.All the results obtained by rheological and rheo-optical measurements will be coupled with the results of optical observation during flow carried out by other R.U. by means of a Linkam shearing cell. Experiments will be carried out under isothermal and slow-varying thermal conditions.Expected results for activity C:2nd year: - results of experimental tests on crystallinity evolution under different orientation levels.As far as topic D is concerned (modelling of morphology evolution in crystallization during flow, months 13-20), main research actions are reported belowOn the basis of the description of quiescent morphology evolution, resulting from activity A.3, and of the modelling of flow effects on the melt, resulting from activity B.2, the effect of flow on both the driving force for crystallization, evaluated through the increase of the melting temperature, and the mobility of the macromolecule, will be analysed. The latter is proportional to the inverse of relaxation time (which, on its turn, is also a function of melt orientation).The effect of orientation will be also kept into account with reference to the morphology of crystallization, which will be considered either spherulitic or fibre-like depending on orientation degree, and in agreement with the results of activity C.3.Expected results for activity D:2nd year: - crystallization kinetic model able to describe crystalline morphology (spherulitic or fibre-like) as a function of orientation level.As far as topic E is concerned (molding and morphology of samples, months 13-20), main research actions are reported belowRectangular samples of iPP will be injection moulded. Screw displacement, thermal history at mould surface and pressure evolution in several positions along the flow-path will be recorded during each moulding test. The injection moulding machine currently adopted by this R.U. is outdated, and it cannot assure a sufficient control precision of the main process parameters: injection flow rate, injection time, holding pressure. The purchase of a new, low capacity injection moulding machine, able to accurately control each processing variable, is planned. The morphology of moulded samples obtained under different moulding conditions will be analysed by means of several microscopic and diffractometric techniques. By means of these techniques, it will be possible to characterise the parameter of the typical skin-core morphology of moulded samples. In particular, not only the thickness of oriented and spherulitic layers inside the samples, but also the dimensions of structures (both the radius of spherulites and the distance between crystalline entities) will be measured. By means of WAXS and FT-IR measurements, the orientation distribution inside the samples will also be obtained.Expected results for activity E:2nd year: - relationship between processing conditions and morphology distribution inside the mouldingsAs far as topic F is concerned (model validation in the injection molding process), main research actions are reported belowF.1.1 Implementation of modeling results (D) into the software (months 17-24)The model for crystallization kinetics identified during action D will be implemented in a software for injection moulding simulation developed by this R.U. This software will be improved, keeping into account the models, identified also by other R.U. joining this project, of the evolution of rheology during crystallization, of the evolution of orientation and of the effect of flow on crystallization kinetics and morphologyF.1.2 Simulation of morphology and orientation distribution (months 17-24)The injection moulding tests carried out during action E will be simulated, and the results of calculations will be compared with experimental results of both the pressure evolution during the moulding cycle and the final morphology and orientation distributions inside the samples, obtained during action E.F.2 Optimization of models for crystallization kinetic and morphology (months 21-24)The overall model obtained by the submodels joining will be validated by comparison between injection moulding simulation runs and experimental data of injection moulded samples morphology, obtained respectively in the frame of activities F.1.2 and E. On the basis of this comparison, working together with other R.U., the values of the submodels parameters will be re-evaluated; if a full submodels re-analysis will need, this revision will be carried out with the aim of obtaining a satisfactory description for all the experimental results, including the process simulation.Expected results for activity F:2nd year: - a model able to full describe the crystallization kinetics and the morphology for the material- final version of injection moulding simulation code able to predict distributions of orientation, crystalline degree of each phase and morphology in injection mouldings.
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