Molecular-dynamics simulation of crystallization in helical polymers.

Area of Science:

• Polymer Science
• Materials Science
• Computational Chemistry

Background:

• Crystallization mechanisms in helical polymers are complex and poorly understood.
• Molecular structure, including side groups, significantly influences polymer chain dynamics and crystallization.
• Previous studies often simplify polymer models, limiting insights into realistic crystallization processes.

Purpose of the Study:

• To elucidate the molecular mechanisms governing crystallization in helical polymers using molecular dynamics (MD) simulations.
• To differentiate and compare the crystallization behavior of simple bare helices versus complex helical polymers with side groups.
• To investigate the effect of external factors, such as growth surfaces, on polymer crystallization kinetics.

Main Methods:

• Development and utilization of the united atom model to construct two types of helical polymers: bare helices and those with side groups.
• Execution of molecular dynamics simulations under various conditions, including rapid cooling, long-time annealing, and crystallization on a growth surface.
• Analysis of intramolecular and intermolecular ordering, crystallite growth, and chiral recognition during the crystallization process.

Main Results:

• Bare helices exhibit faster crystallization, forming partially ordered states upon cooling, which improve significantly with annealing.
• Helical polymers with bulky side groups show extremely slow crystallization kinetics.
• Crystallization on a growth surface accelerates for bare helices via cooperative intermolecular and intramolecular dynamics.
• Realistic models of isotactic polypropylene (iPP) demonstrate sluggish crystallization, but rapid ordering occurs in confined dimensions.
• iPP molecules exhibit precise helical sense recognition and chiral selection when crystallizing onto a substrate of the same polymer.

Conclusions:

• The presence and nature of side groups critically dictate the crystallization rate and mechanism of helical polymers.
• Annealing and surface interactions can significantly enhance the ordering and crystallization of helical polymers.
• Molecular dynamics simulations provide valuable insights into the stereospecific crystallization of complex polymers like iPP, revealing chiral recognition phenomena.