Methodology and Equipment for Optical Studies of Fast Crystallizing Polymers (original) (raw)
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Heat Transfer and Crystallization During Fast Cooling of Thin Polymer Film
Almost all the polymer transformation processes involve a step in which the melt undergoes a cooling that causes solidification. During the solidification, the semi-crystalline polymers can develop structures partially ordered, often typified by threedimensional objects known as spherulites (mainly if solidification takes place in absence of flow, i.e. in quiescent conditions). The volumetric crystallinity degree of filling, the number and the dimensions of the spherulites in turn strongly affect the final object properties. Therefore, detailed knowledge of crystallization kinetics is highly desirable to manage polymer transformation processes. To investigate the crystallization kinetics, recently we developed an apparatus able to quench thin polymer films, simultaneously recording the sample temperature and the intensity of a laser beam passing through the solidifying polymer slab . The interaction between light and polymer was also modelled for a sample homogeneous, i.e. during a cooling not fast enough to produce crystallinity distribution within the sample. Aim of this work is to model the interaction between light and crystallizing polymer, even if the solidifying sample is not homogeneous, and to compare the predicted light intensity with the experimentally measured.
MONITORING CRYSTALLIZATION KINETICS UNDER HIGH COOLING RATE
An experiment useful to investigate crystallinity evolution during fast cooling, comparable with cooling rates attained in industrial processes, is extremely attractive. In this work, a setup able to quench thin polymer films while recording the sample thermal history and light intensity of a laser beam transmitted by the sample is described. The experimental results are analyzed on the basis of a new model, which relates the transmitted light intensity to the crystallinity evolutions during the cooling runs.
Chemical Engineering Science, 2002
A new experimental route for investigating polymer crystallization under very high cooling rates (up to 2000 • C=s) is described. A complete and exhaustive description of the apparatus employed for preparing thin quenched samples (100 -200 m thick) is reported, the cooling mechanism and the temperature distribution across sample thickness is also analysed, showing that the ÿnal structure is determined only by the thermal history imposed by the fast quench apparatus.
Advances in Polymer Technology, 2009
An experimental route for investigating polymer crystallization over a wide range of cooling rates (from 0.01 to 1000 • C/s) and pressures (from 0.1 to 40 MPa) is illustrated, using a method that recalls the approach adopted in metallurgy for studying structure development in metals. Two types of experimental setup were used, namely an apparatus for fast cooling of thin films (100-200 μm thick) at various cooling rates under atmospheric pressure and a device (based on a on-purpose modified injection molding machine) for quenching massive samples (about 1-2 cm 3 ) under hydrostatic pressure fields. In both cases, ex situ characterization experiments were carried out to probe the resulting structure, using techniques such as density measurements and wide-angle x-ray diffraction (WAXD) patterns. The cooling mechanism and temperature distribution across the sample thickness were analyzed. Results
Heat transfer and crystallization kinetics during fast cooling of thin polymer films
Heat and Mass Transfer, 2007
In this work, the heat transfer phenomena taking place during the cooling of thin films of crystallizable polymers were analyzed. The thermal histories, as recorded during experimental cooling runs carried out at various cooling rates, were compared with the predictions of a general purpose numerical code, which was resulted able to capture all the main features of the process. Thus, the conditions which allow homogeneous cooling (negligible temperature gradient within the sample) or homogeneous cooling history (the same cooling history for all the positions within the sample) were predicted by the simulation code.
Crystallization of polymers at constant and high cooling rates: A new hot-stage microscopy set-up
Polymer Testing, 2010
A prototype hot-stage termed «Polymer High Cooling -Optics» was especially designed to follow on-line the nucleation and growth of crystallizing entities at constant cooling rates. The optically transparent hot-stage receives a gaseous flow at a constant temperature, which passes over the sample. Quiescent crystallization is monitored in real-time at relatively moderate (from 30 to 500 C/min) and even at high (from 500 to 2000 C/min) constant cooling rates. As a first application of this new set-up, two relevant parameters of the a-modification of isotactic polypropylene, i.e., the crystallization temperature and the spherulite growth rate, were successfully captured for different constant cooling rates in a reproducible way. The superposition of these newly-collected growth rate data with those compiled by demonstrates the reliability of our experimental determination.
Modeling the interactions between light and crystallizing polymer during fast cooling
Applied Physics A-materials Science & Processing, 2004
In this work, an experimental set-up able to quench thin polymer films whilst recording the sample thermal history as well as the overall and depolarized light intensities of a laser beam emerging from the sample is described. The interactions between the light beam and the crystallizing material have been modeled accounting for absorption and scattering phenomena. The proposed model was found to be able to reproduce the experimentally observed behavior of light intensities and it was validated by comparison with conventional DSC analysis. On the basis of this model, a method to obtain crystallinity evolution is proposed and applied to some fast cooling runs. The method was applied to quenching runs of an iPP carried out under cooling conditions more than one order of magnitude faster than those allowed for the DSC technique. The crystallization temperature dependence upon cooling rate was obtained from these runs in a cooling rate range never before explored.
Crystallization of polymer melts under fast cooling. II. High-purity iPP
Journal of Applied Polymer Science, 1992
Samples of a high-purity isotactic polypropylene (iPP) were quenched from the melt so as to monitor cooling history. A continuous variation of morphology and crystal structure was obtained with cooling rate. This is discussed in relation to sample thermal history evidencing that cooling history relevant to quenched samples is in the neighborhood of 90°C. In particular the samples are essentially mesomorphic when at this temperature cooling rates larger than 80°C/s were adopted, while below a few tens of °C/s only α-monocline form is obtained. Densities of quenched samples were compared with predictions of an isokinetic extrapolation of Avrami model of polymer crystallization kinetics.