A molecular dynamics study of epitaxy-induced chain orientation and ordering of isotactic polypropylene

• Published in: Polymer (Guilford) 2020-12, Vol.210, p.122987, Article 122987
• Main Authors: Raghuram, E., Panwar, Ajay Singh
• Format: Article
• Language: English
• Subjects: Chains; Chirality; Coils; Crystal structure; Crystallinity; Crystallization; Epitaxy; Evolution; Heterogeneous nucleation; Isotactic polypropylene crystallization; Isotacticity; Mesophase; Molecular conformation; Molecular dynamics; Nucleation; Order parameters; Orientation; Polypropylene; Segments
• ISSN: 0032-3861; 1873-2291
• DOI: 10.1016/j.polymer.2020.122987

Phase selective nucleation of single chain isotactic polypropylene (iPP) into specific crystalline polymorphs is enabled by heterogeneous nucleating agents. Molecular dynamics simulations were used to investigate the ordering of isolated iPP chains over ideal (α-iPP and β-iPP) and commercial (triphenodithiazine, TPDT) heterogeneous nucleating surfaces during the early stages of nucleation. The ordering of an iPP chain during nucleation is analyzed by following the evolution of helix conformations, chiral order parameter, chain orientation and radius of gyration. The results show that α-iPP and β-iPP have a greater tendency to select chirality. However, TPDT is the most efficient in orienting the chain because of bulky phenyl groups on its surface. Based on helix evolution and segment orientation, early-stage ordering of iPP on crystalline surfaces is proposed to proceed by two different routes; (i) CTEH (coil-to-extended helix) where helix formation and segment orientation occur simultaneously, and (ii) CTEC (coil-to-extended coil) where segment orientation occurs without helix formation, thus indicating multistage nucleation through an intermediate mesomorphic phase. Whereas, CTEC is the predominant ordering route on TPDT, both CTEC and CTEH occur simultaneously on α-iPP and β-iPP, emphasizing the important role of epitaxy in early-stage ordering during crystallization. Modes of iPP ordering and helix evolution in presence of -iPP, -iPP and TPDT surfaces. [Display omitted] •Multi-stage nucleation plays a partial role in iPP crystallization.•The formation of an intermediate mesophase was observed.•The intermediate mesophase exerts more influence during nucleation than growth.