Challenges of making Performance Plastics from Polyolefins: In the Structure/Property Control Of Polyolefin Blends/Alloys
時間:2005-05-24
Han Charles C., Wang Z. G., and Zhang
Xiaohua State Key Laboratory of Polymer Physics and Chemistry Joint Laboratory of Polymer Science and Materials ICCAS, Beijing, China, 100080
Olefin polymers are the most widely used materials in the plastics industry today. To optimize their properties and processibility, blending/alloying is often used. This is particularly important for the metallocene-catalyst based polyolefin. On the other hand, in order to make low cost performance and recyclable plastics, polyolefin blends/alloys have become the primary choice in recent years. However, there are many challenges in catalysts development, polymerization chemistry, composition formulation, structure/morphology and processing control need to be solved before such low cost, high performance plastics can be produced.
We will specifically discuss one of the structure control problem in this talk. In the case of polyolefin blends, the mixtures can often undergo both liquid-liquid phase separation (LLPS) and crystallization, which produces very complicated morphology. This complication can be either advantageous or disadvantageous in terms of structural control/property tailoring, depending on whether or not complex structures yield desired properties of the final product needs. At the same time, due to the advancement of the metallocene, as well as the new generation of Ziegler-Natta catalysts technology, polyolefin’s with desired a-monomer fraction and narrower molecular mass distribution may be produced, and at the same time better control and improved properties can be expected from blending or alloying in the in-reactor polymerization processes. However, more detailed knowledge on relationships between the blend composition, MW’s, and a-monomer content to the final material structure properties, including the static and kinetic phase behaviors, such as the phase separation/crystallization kinetics, are needed in order to control the materials processing as well as final properties in use.
In this study, two blends systems, Poly(ethylene-co-hexene)/Poly(ethylene-co-butene) (PEH/PEB) and Poly(ethylene-co-hexene)/Poly(ethylene-co-octene) (PEH/PEOC), have been used. The dynamics of structure evolution in polyolefin blends undergoing simultaneous LLPS and crystallization or a single or multiple cycles of LLPS and crystallization have been investigated. Non-linear structural dynamics in polymer blends can be expected when dual or multiple phase ordering processes co-exist. In a typical case of LLPS and crystallization, linear dynamics of crystal growth and LLPS is observed at low and high temperatures respectively, where interferences from each other is negligible. In the crossover regime, interplay between LLPS and crystallization causes non-linear ordering because of the structural and rheological heterogeneity.
The important effect of composition fluctuation induced crystallization start to reveal as a new and important mechanism not only in the materials processing control but also as fundamental physical phenomena. The details on this fluctuation, which is caused by the early stages of spontaneous liquid-liquid phase separation in the spinodal region, on the nucleation and crystallization kinetics will be discussed in this talk.
