RICERCA ITALIANA     

Control and Modeling of Morphology of Semicrystalline Polymers under Realistic Processing Conditions

Università degli Studi di Salerno

Abstract

Aim of this project is to gain a deeper understanding and to build a sufficiently accurate modelling of microstructure evolution during polymer crystallization under processing conditions, i.e. under high pressures and cooling rates and under flow.
The proposed action encompasses many aspects on both the experimental and the theoretical side. In spite of this complexity, the objectives of the project are sharply defined. The Research Units (RU) participating to the project are complementary. Their skills cover all different aspects of the research work plan.
Most of the activity will be carried out with reference to an isotactic polypropylene (iPP) already adopted by the RUs involved in the project in previous research works. This choice is expected to minimize the initial characterization phases.
In the initials stages of the project the evolution of crystallinity and morphology under conditions much closer to processing than those normally available will be studied. To this purpose experimental apparatuses to allow for solidification in quiescent conditions under high cooling rates (up to hundreds of °C/s) and elevated pressure (up to thousands of bars) will be set up. Additionally, crystallization and morphology evolution during solidification will be monitored by recording signals of transmitted and scattered light intensity. Information concerning the morphology of solidified samples, the analysis of the transmitted light intensity during >>>

Principal Investigator

Giuseppe TITOMANLIO Università degli Studi di SALERNO

Research Objectives

It is an obvious fact that the morphology distribution in polymeric manufacture is determined by the coupling between the thermo-mechanical history of each volume element and the specific characteristics of the polymer considered. Such a morphology is the final stage of a complex process of micro- and macro-structural evolution starting from the molten state. In more details, the evolution of morphology is determined by temperature, pressure and deformation histories applied to the material before, during and after its solidification. Such factors are expected to affect the crystallisation temperature, the rate and the modes of structural evolution at different length scales.
The morphological evolution during polymer crystallisation has been extensively studied in the past both experimentally and theoretically. In most cases, however, only specific aspects of the global problem have been considered. Furthermore, all models rely on experimental information obtained under conditions that are typically far away from those actually realized during the industrial transformation processes.
An integrated experimental activity between the partner laboratories will be developed within this research project on a single polymer, namely, an isotactic polypropylene (iPP). The aim will be to cover, within the same experimental activity, a set of experimental conditions as wide as possible and significantly closer than any other set already available to real processing >>>

First Results

Work-package A
- Analytical expressions for the dependence of nucleation density, crystal growth rate and overall kinetic constant on isothermal crystallisation temperature (RU1)
- Assessment of athermal nucleation effects (RU1)
- Setting up an innovative light scattering apparatus for monitoring crystallinity and morphology evolution during crystallisation under cooling rates as high as hundreds °C/sec (RU3)
- Setting up an apparatus for sample solidification under high pressure at cooling rates of decades of °C (RU3 )
- Optimised procedures for morphological investigation by means of Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (RU1, RU3)
- Quantitative information on morphological features of samples prepared in quiescent conditions at high cooling rates and under pressure (RU1, RU3)
- Correlation between solidification conditions (cooling rate and solidification pressure) and phase distribution, spherulitic dimensions, lamellae thickness, long period (i.e. average size of a domain constituted by a crystalline region and a contiguous amorphous region) in the aforementioned samples (RU1)
- Model for the development of crystallinity and morphology validated over a very wide range of cooling histories and pressures. The model will also account for the competing evolution of different ordered phases under quiescent processing conditions (RU1, RU3)

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Timescale

24 months

National and international background

Thermoplastic polymers are usually processed in the molten state and solidification takes place under fast cooling during or immediately after flow. Often the polymer is exposed to simultaneous flow and cooling, where the cooling rate can be very high, up to several hundreds of degrees per second. Sometimes, for example during injection moulding, both flow and solidification take place at high pressure (several hundreds of atmospheres). In all cases, process conditions do have profound influence upon the resulting microstructure; in fact, both crystallization kinetics and morphology (size, shape, orientation of crystallites) can be dramatically different with respect to reference isothermal, quiescent, and low pressure conditions. The crystallinity evolution, in turn, affects the polymer rheological response, thus implying further changes to the material solidification behaviour.
For the above reasons, a detailed understanding of transformation processes, and particularly of the injection moulding process, requires a correspondingly detailed description of kinetics and of morphology evolution during crystallisation in a wide range of cooling rates, flow and pressure conditions. The understanding also of the rheological behaviour during solidification is a also a crucial point.
Prediction and control of microstructure during processing represent an important research field in industrial engineering. The recent implementation of the European Union program COST >>>