Molecular Dynamics Simulation Of Melting Behaviors Of α-and β-IPP Crystals

Crystallization and melting are basic problems in phase behaviors studies ofcrystals. We studied the melting behaviors of α-and β-isotactic polypropylenecrystals through molecular dynamic simulation and give some results.We constructed two common crystal forms of isotactic polypropylene: α-iPP andβ-iPP. The calculated result of static structure factors of our models illustrated that weconstructed the two crystal types of iPP successfully. We adopted the all atom forcefield developed by Theodorou. The apparent melting point of our system wasdetermined through changes of density. The system structures of crystal were furtherquantitatively analyzed through changes of static structure factors, ratio of left-handedhelix and so on, which combine the computer simulation and real experiment.The results showed that, both α-iPP and β-iPP crystals melting underwent aprocess of thermal expansion and collapse of ordered structures: the crystal systemhad an expansion in volume as temperature increasing to give bigger space for atomvibration. Comparisons of RMSD values of different parts of polymer chains showedthat melting was started from edges of crystal and spread into inner parts astemperature increased, which corresponded with configuration evolutions. α-iPP hadhigher potential energy and lower density than β-iPP before melting and showed atrend of slower decrease of static structure factor. These results indicated that α-iPP isin a more stable state and has better thermal stability than β-iPP. Densities andpotentials showed a sudden drop and ratio of left-handed helical chains in α-iPP is50%while almost0in β-iPP before melting and finally both fell to50%just at thetime when melting occurs totally. Apparent melting points of α-iPP were higher thanβ-iPP in the same degree of polymerization and heating rate.The influences of heating rate and chains length on melting are analyzedaccording to “melting kinetics” model of Toda and Gibbs-Thomson equation. Theapparent point with a certain chain length increases with heating rates which lead to asuperheating phenomenon in melting process. Following a deformation of equations,shifted “melting point” was plotted with corresponding difference value of R=0.25and showed a linear relationship. On the other hand, melting temperature aftereliminating the effects of “melting kinetics” are also affected by crystal thickness. Treat the length of iPP molecules as crystal thickness in our simulation system, the“shifted melting points” showed a linear relationship with reciprocal of chain length.The formula fitting reuslts showed that the equilibrium melting points of perfect α-iPPand β-iPP crystals with infinte chain length are577.18K and522.41K, respectively.The fitting values of σe/△h (ratio between interface free energy and fusion heat perunit volume) showed that β-iPP is easied to nucleate when melting.