The high melt strength polypropylene (HMSPP) is fabricated by direct polymerization through controlling the species and ratios of the external electron donors in the Ziegler−Natta catalyst system at different reaction stages. The polypropylene exhibits wide molecular weight distribution and higher melt strength. The expanded polypropylene (EPP) beads are prepared by the above HMSPP through an autoclave-based batch process. The effects of melt strength on the cellular morphology of EPP are investigated by scanning electron microscope and melt strength meter, respectively. Results indicate higher melt strength facilitates the control of the foaming process and the cellular structure compared with a commercial propylene homopolymer such as T03. The bimodal cell structure is obtained with the single foaming agent CO 2 in both of the EPP beads from HMSPP and commercial polypropylene.
A molecular level understanding of the properties of polymer electrolytes can only be obtained through an identification of the types of ionic structures formed in these materials and their relationship to the motion of ions in these systems. Vibrational spectroscopy has proved to be a powerful tool in characterizing such local structures, and a detailed study of the infrared spectrum of a set of sulfonated PEO/aromatic polyesters is presented here. An analysis of the symmetric SO 3 stretching mode revealed no detectable amounts of "free" SO 3 ions in any of the ionomers, but bands due to ion pairs and aggregates were identified. The band due to aggregates increased in intensity relative to the band due to ion pairs as the temperature increased. Vibrational modes due to the poly(ethylene oxide) segments of the copolymers were characteristic of chains in the amorphous state. However, bands due to sequences of trans and gauche O-C-C-O-C-C-O conformations characteristic of the ordered state appeared to be favored upon complexation with ions. The interaction between cations and PEO portions of the chain "locks" the segments in their preferred structure to a large degree, so that compared to non-sulfonated copolymers there are relatively small changes in the relative intensities of conformationally sensitive bands with temperature.
Polypropylene (PP) is an outstanding material for polymeric foams due to its favorable mechanical and chemical properties. However, its low melt strength and fast crystallization result in unfavorable foaming properties. Long-chain branching of PP is regarded as a game changer in foaming due to the introduction of strain hardening, which stabilizes the foam morphology. In this work, a thorough characterization with respect to rheology and crystallization characteristics of a linear PP, a PP/PE-block co-polymer, and a long-chain branched PP are conducted. Using these results, the processing window in foam-extrusion trials with CO2 and finally the foam properties are explained. Although only LCB-PP exhibits strain hardening, it neither provide the broadest foaming window nor the best foam quality. Therefore, multiwave experiments were conducted to study the gelation due to crystallization and its influence on foaming. Here, linear PP exhibited a gel-like behavior over a broad time frame, whereas the other two froze quickly. Thus, apart from strain hardening, the crystallization behavior/crystallization kinetics is of utmost importance for foaming in terms of a broad processing window, low-density, and good morphology. Therefore, the question arises, whether strain hardening is really essential for low density foams with a good cellular morphology.
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