2007-06-07
10:15 at HCI J 574

Crystallization of isotactic polypropylene and effect of stereotacticity defects on the formation of order

Rahmi Ozisik

Rensselaer Polytechnic Institute, Materials Science and Engineering, Troy, NY 12180

Because of recent developments in metallocene catalysts, a fine-tuning of material properties becomes possible by controlling the tacticity of polypropylene chain so that polypropylenes with properties ranging from semi-crystalline thermoplastic to thermoplastic elastic can be achieved. As a result, the effect of defects on the properties of polypropylene, especially on its crystallization behavior, has received an increasing interest. The properties of these new polypropylenes depend significantly on the concentration and distribution of defects along the polymer chain. In general, the isotactic segments can afford a high degree of crystallinity if they are long enough so that the material would display the behavior of tough thermoplastics. However, the introduction of stereodefects would reduce the isotacticity and lead to some extent of elastic behavior. Coarse-grained, on-lattice Monte Carlo simulations are performed to investigate the role of stereotacticity defects along an isotactic polypropylene chain on the formation of helices, which form the basic crystalline order within the chain. For this reason, systems with various stereodefect configurations are studied and are compared to neat isotactic polypropylene. All systems are equilibrated above the melting temperature and are cooled to lower temperatures in a stepwise manner, making sure each system is equilibrated at every temperature. Results indicate that chain ends have the lowest probability of being in helices. Addition of a single stereoerror (mrm) decreases the probability of five repeat units’ participation in helices (the repeat unit that contains the stereodefect and two nearest repeat units on both sides). The probability profile becomes more complicated when the number of stereodefects increases, however, the results indicate that the effects of many stereodefects can be explained by a simple addition of the effect of each stereodefect considered individually. The results also indicate that the presence of even a single stereodefect eliminates (within the temperature range studied) the transition to longer, more stable helices observed in neat isotactic polypropylene.