Fractionated crystallization of isotactic polypropylene in water-borne dispersions.
Year: 2010
Type: dissertation
Abstract: Though polymers exist for many years, crystallization by homogeneous nucleation has seldom been studied. To explore this kind of nucleation, polyolefin-water matrix systems have been developed. Low-viscosity, not-vitrified water is not expected to influence the crystallization of the polyolefin; it is safer to work with than with any organic solvent; it is completely inert from an environmental point of view, and therefore potentially of interest for industrial applications.Usually, polymer crystallization occurs by way of heterogeneous nucleation because in a polymer a large number of heterogeneous nuclei is present, facilitating the crystallization process. However, in case these nuclei are absent or when somehow they are removed, homogeneous crystallization can occur. Hence, if the polymer is dispersed into a large enough number of particles the vast majority of them will not contain any heterogeneous nucleus and thus they must undergo homogenous crystallization.In this work water-borne dispersions of a maleic anhydride-grafted, isotactic polypropylene (MA-g-PP) covering a very wide range of particle size distributions were successfully prepared. The dispersions proved to be stable over very long periods of time and over a broad range of temperatures. Thus, by varying the nucleation density in droplets by changing their dimensions, the different modes of crystallization sought - heterogeneous, fractionated and homogenous crystallization - have indeed been found, enabling the study of the various modes.The particle size distributions of these waterborne MA-g-PP dispersions have been determined and compared to the crystallization (from the melt) and subsequent melting behavior. The larger-particle dispersions showed crystallization at the usual (bulk) crystallization temperatures; dispersions having intermediate-size particles showed several ways of fractionated crystallization, and the smaller-particle dispersions showed crystallization exclusively at a very high, extra supercooling resulting from homogeneous nucleation. The obtained lowest crystallization temperature - 34 ºC - was the lowest ever reported for isotactic polypropylene (iPP), and has only recently been realized by two other groups. The obtained particle size distributions cover approximately three orders of magnitude, with size averages ranging from less than 100 nm up to a few tens of micrometers. Such a complete set of samples is seldom available and it constitutes a solid base for the research work carried out.Next, a deeper analysis of the process of crystal formation was undertaken. Self-nucleation was successfully used in order to confirm that fractionated and homogenous crystallization were due to the increased number of droplets as compared to the number of active impurities. However, due to the vast number of droplets in the dispersions having the smallest particles it was necessary to use extremely low self-nucleation temperatures in order to change all different crystallization modes into crystallization in the bulk, heterogeneous crystallization temperature range. Most remarkably, in this case the self-nucleation regimes II and III - as defined and used in literature - overlap.Concerning the density of nucleating seeds, a novel procedure that takes into account the entire size distribution was proposed for the estimation of that density. The results show that the nucleation density in the dispersions is always larger than in bulk, even when polymer dispersions that crystallize exclusively in the bulk crystallization temperature range are considered. Besides, the number of less active nucleating seeds was also estimated, as well as the required particle size in order not to observe any crystallization until the homogenous crystallization temperature has been reached. It turns out that the size of the dispersed droplets has to be drastically reduced to sub-micrometer dimensions in order to obtain homogenous crystallization exclusively.Then, an analysis of the kinetics of the crystallization process was performed in order to establish which type of primary nucleation takes place at each crystallization temperature. The originality of the proposed and performed analysis consists in taking into consideration the particle size distribution of the samples, and not just an average particle size, as it is commonly done. It was possible to conclude that no impurity is involved in the primary nucleation observed at the maximum supercooling. Moreover, the results of the analysis favor the possibility of volume nucleation over surface nucleation.Finally, the mesomorphic phase of isotactic polypropylene has been studied. The enthalpy of the mesophase was determined over a wide temperature range, on the basis of a number of reasonable assumptions. Next, the enthalpy of crystallization/fusion obtained has been used to fully characterize the samples regarding the crystallinity and polymorph evolution as function of temperature. The morphology of the different crystal phases observed was analyzed too, showing interesting but sometimes contradicting results.
Source: Lirias
Author: Julen Ibarretxe
Language: English
Subfield: Polymers and Plastics
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2529849520