Research program
Control and Modeling of Morphology of Semicrystalline Polymers under Realistic Processing Conditions
University Co-ordinator
Università degli Studi di GENOVA -
CHIMICA E CHIMICA INDUSTRIALE - GENOVA(GE)
Research Unit Leader
Giovanni Carlo ALFONSO
Description
The research programme is illustrated with reference to the Table reported in Model A by the main coordinator. Experiments will be performed using an isotactic polypropylene whose molecular characteristics and, in part, its crystallization behaviour in standard conditions are already known since it has been used in a previous investigation involving all partners of this project.A: ACTIVITIES ON THE EVOLUTION OF QUIESCENT CRYSTALLIZATION MORPHOLOGY (Months 1-12)The first target that must be achieved to model the progress of crystallization under realistic processing conditions is determination of material's properties relevant to crystallization kinetics in ideal laboratory conditions: constant temperature and pressure and absence of flow. This information must be integrated with detailed examination of crystalline structure and morphology over the whole length scale range, from spherulitic to lamellar. In this project, the characterization of morphology will be performed in quantitative terms by identifying and measuring well defined morphological features, e.g. spherulitic size, width of fibrils and lamellar thickness. The same kind of information must be collected on samples obtained from crystallization runs in stronglynon-isothermal conditions, under high hydrostatic pressure and also in non-isothermal conditions under pressure. The role of this Research Unit (RU) in activity A is to contribute to the above characterization using microscopies and thermal analysis. A further contribution will be provided in the critical evaluation of the experimentally determined parameters and their optimization for the description of crystallization under severe dT/dt and P conditions.A.1: Characterization of quiescent kinetics and morphology in standard conditions (Months 1-12)- NucleationEvaluation of nucleation density as a function of isothermal crystallization temperature and identification of possible contributions due to athermal nuclei by performing cooling experiments starting from different melt temperatures. To extend the investigated temperature and cooling rate ranges, the comparative procedure recently invented by Janischitz-Kriegl and Eder will be adopted whenever dimensions of target objects will fall below the instrumental resolution limits.- Growth kineticsMeasurement of spherulitic radial growth rate in isothermal conditions as a function of crystallization temperature. To extend the temperature range over which data will be collected, samples with very low thermal capacity will be used and an efficient quenching apparatus will be designed. The radius of spherulites will also be measured during cooling at constant rate to verify that growth rate depends only on actual temperature and it is independent of cooling rate.- Overall crystallization kineticsInvestigation of overall crystallization rate in isothermal conditions and in slowly changing temperature regimes, (cooling rate up to 1°C/s) by means of thermal analysis. Data obtained in the form of x(t), in isothermal experiments, or in the form x[T(t)], in experiments at constant cooling rate, will be elaborated according to Avrami and Nakamura equations, respectively.- Spherulitic and interspherulitic morphologyMorphology of samples crystallized in isothermal and weakly non-isothermal conditions will be investigated at various length scales. Polarized optical microscopy will be used to investigate the spherulitic size and gross texture. SEM on etched fracture surfaces will be adopted to measure spherulitic size in microspherulitic samples and fibrillar diameter in large spherulites. TEM of RuO4 stained thin cryo-sectioned samples will be used to investigate morphology at lamellar level.A.2: Characterization of quiescent kinetics and morphology evolution in processing conditions. (Month 5-12) A.2.1: Experiments of crystallization under high cooling rates Micro-thermocouples will be embedded in 2 mm thick samples at various depths from their wider surfaces and their signal will be recorded by a fast acquisition device (100 readings/s) to monitor the time and space temperature distribution during fast cooling (up to 20°C/s) obtained by dipping the molten polymer in fluid environments at different temperatures. The obtained temperatureprofiles and the examination of the morphology in close proximity of the sensors will enable us to evaluate the kinetics of latent heat release. Comparison of this information with the model for the kinetics of isothermal crystallization obtained in A.1 will highlight possible deviations from isokinetic models already at cooling rates of the order of few tens of °C/s. Morphology will also beinvestigated on samples prepared by RU3 under conditions of very fast cooling rates to establish the relevance of athermal nucleation mechanism in the solidification process that takes place in typical injections molded products.A.2.2: Experiments of crystallization under high pressure This RU will investigate, by SEM and TEM, the morphology of representative samples prepared by RU3 under high hydrostatic pressure. The acquired morphological information will integrate the results obtained by RU3 by means of different techniques.A.3: Modelling of morphology evolution in quiescent conditions (Months 9-12)In cooperation with RU3, nucleation and growth rate data will be implemented to various crystallization models for describing the progress of crystallinity in isothermal conditions and to predict the behaviour of the material under hydrostatic pressure and in rapidly changing thermal regimes. The results obtained from modelling will be compared with those obtained under slowly changing temperature conditions and with those obtained in thermal and pressure conditions similar to those encountered in real processing. Models' validation will be based also on their performance in predicting the morphological features established as indicated in A.2.Deliverables for Activity AFirst year- Analytical expressions for the temperature dependence of nucleation density, spherulitic growth rate, overall crystallization rate constant.- Evidences for meaningful or negligible contribution of athermal nucleation to nucleation density- Optimized procedures for morphological investigation: etching procedure for SEM observation and etching + surface replica or cryo-sectioning and staining for TEM.- Quantitative characterization of morphology in samples crystallized at high cooling rates and under pressure- Model for the development of crystallinity and morphology under quiescent processing conditionsC: EXPERIMENTS ON EFFECT OF FLOW ON CRYSTALLIZATION (Month 1-20)It is known that polymer crystallization is strongly affected by segmental orientation and molecular deformation of the constituent chains. Major effects are observed in the crystallization rate, that is increased by orders of magnitude, and in the resulting morphology, undergoing a clear transition, from spherulitic to fibrillar, on increasing the intensity of the flow field. Recent results appeared in the literature indicate that melt flow induces the formation of long living bundles of parallel chain segments that are precursors of nuclei in the subsequent cooling to below melting point. In line with this hypothesis, this RU will perform experiments aimed at finding quantitative relations between parameters characterizing the flow history and the resulting nucleation density at various crystallization temperatures. In addition, the life time of these nucleation precursors after cessation of flow will be evaluated. Experiments targeted at identifying the nature of flow-induced nucleation precursors and at capturing the structural features in the early stages of crystallization will be performed at the synchrotron facilities of Hamburg and Grenoble by simultaneous WAXD/SAXS. Applications have already been submitted. The beam-line at ESRF has a spatial resolution of about 5 micron and will enable us to obtain information from volume elements submitted to the very intense shear flow around a fibre pulled through the molten polymer (see C.2). All together, the above quantitative information will be used to elaborate a comprehensive crystallization model capable to account for the whole thermo-mechanical history experienced by the sample prior to crystallization.C.1: Crystallization in rotational devicesEffect of flow on nucleation density and growth rate. (Months 1-20)Experiments specifically addressed to investigate the role of flow on nucleation density will be performed by means of an already available shearing stage that enables one to apply flows of controlled intensities during variable time periods in strictly controlled temperature conditions. The planned experimental approach has already been adopted by this RU to obtain reproducible andinteresting data on flow enhanced nucleation of isotactic poly(1-butene).Initially experiments will be performed under constant shearing and crystallization temperatures (T) and measuring the flow induced nucleation density (Ns) in samples submitted to the same shear rate (g) during different shearing times (ts). Afterwards, always in the same thermal conditions, experiments will be carried out using different shear rates in the range from 1/100 to 100 1/s.Further experiments will be performed by repeating the above experiments at several temperatures at which crystallization takes place in a reasonable time interval. In analogy with the results already obtained on poly(1-butene) it is expected to find an empirical equation of the formNs(M, T, g, ts) = C Ma exp[E/RT] g^n tswhere C is a material constant, M is the weight average molar mass and E is an apparent activation energy of the flow induced formation of nuclei. Additional experiments will be performed in similar conditions, keeping constant only the crystallization temperature and applying various combination of flow variables, including application of shear at temperatures well above the melting temperature of the polymer. It is expected that, all together, the acquired information will enable us to predict, with reasonable accuracy, the number of active crystallization nuclei produced by any arbitrary thermal and flow history. The above experimental approach also enable us to measure the crystal growth rate and to ascertain, as it emerges from the recent literature, that previous flow history has no effect on this additional kinetic parameter.Experiments on the role of crystallinity and spherulitic size on rheological and crystallization behaviour of undercooled melts. (Month 13-16)The original experimental approach of the "inverse quenching" proposed by RU2 will be adopted by this RU to clarify if the crystallization process taking place after having submitted to shear flow a semicrystalline sample depends on the size of the sheared dispersed phases. The experiments will consist in the evaluation of nucleation density and morphological development obtained bysubmitting a slightly undercooled polymer melt containing dispersed spherulites to controlled shear flow histories. The understanding of interactions between flow of biphasic systems and subsequent crystallization will provide valuable information for the description of crystallization in various positions inside the molds.C.2: Crystallization induced by fiber pulling (Months 1-20)Development of cylindritic morphology around a solid fiber quickly pulled through the molten polymer. This extremely sensitive morphological marker will be used to evaluate how long do survive, after cessation of flow, the nucleation precursors generated by the flow field. By performing experiments at various "relaxation" temperatures, the temperature dependence of the relevant characteristic time will be evaluated. If available, samples with different molecular weights and molecular weight distributions will also be investigated.The above evidences will be integrated by experiments performed at the ERSF of Grenoble, using an X-ray micro-beam with diameter of few microns, in the framework of a research project recently submitted that is aimed at gaining direct evidences of precrystalline order in molten polymers submitted to intense flow fields.C.3: Experiments on the effect of flow on morphology (Months 12-20)Optical and electron microscopy will be used to study the morphologies developing in the semicrystalline polymeric material when it is solidified, at constant temperature, after it has undergone known flow histories. Within this approach, duration and intensity of the flow field, as well as flow temperature and crystallization temperature, will be independently varied to precisely match the conditions corresponding to incipient morphological transitions produced by flow. Initially, polarized optical microscopy will be used to find the critical flow conditions originating morphologies of fibrillar type. Samples solidified in a range of conditions close to those just mentioned will be investigated by scanning electron microscopy in the attempt to evidence the changes in the intraspherulitic organization associated to the gross morphological transition. Samples showing evidences of incipient formation of oriented superstructures will be further investigated by transmission electron microscopy with the aim to identify the basic morphological entities originating oriented superstructures.Deliverables for Activity CFirst year- Temperature dependence of the lifetime of flow induced nucleation precursors- Optimized procedure for characterization of morphology in samples crystallized under flow conditionsSecond year- Quantitative correlation between nucleation density and flow conditions- Quantitative morphological characterization of samples crystallized under flow conditions- Morphological evidences of the interaction between flow applied to partially crystallized systems and nucleation densityD: MODELING OF MORPHOLOGICAL EVOLUTION IN CRYSTALLIZATION DURING FLOW (Month 13-20)Information acquired by this RU from the crystallization experiments performed on flowing systems or just after interruption of flow and from the characterization of morphological changes induced by flow will be sheared with collaborating teams. This will enhance our capabilities to formulate models capable of describing the resulting morphology and of predicting the flow conditions (T, shear rate, shear deformation, shear stress, etc.) under which detectable changes in the morphology are generated.Deliverables for Activity DSecond yearModel for the development of morphology under flow conditionsE: INJECTION MOLDING OF SAMPLES AND THEIR CHARACTERIZATION (Month 13-20) This RU will cooperate with RU3 to perform a comprehensive and quantitative detailed characterization of injection moulded specimens obtained by RU3 under controlled conditions. To this aim the optimized procedures for optical end electron microscopy investigation established in C.3 will be usedDeliverables for Activity ESecond yearOptimized procedure for quantitative morphological investigation of injection moulded samplesMorphological map of injection moulded samplesF: VALIDATION OF THE CRYSTALLIZATION MODEL BY THE INJECTION MOLDING PROCESS (Months 20-24) F.2: Optimization of models for crystallization kinetics and morphology (Months 20-24)Together with cooperating RUs, the predicting capabilities of crystallization models in complex conditions, both in terms ofcrystallization kinetics and of structural and morphological features, will be tested. Validation will be performed by comparing the results calculated from the most promising models, with optimized parameters, to the results acquired from flow-induced crystallization experiments, from injection molding runs and from the detailed morphological characterization of products.
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