Semicrystalline block copolymers in rigid confining nanopores

TL;DR

This study explores how PLLA crystallizes within lamellae-forming PS-b-PLLA block copolymer confined in straight nanopores, revealing complex crystallization behaviors influenced by confinement, temperature, and domain structure.

Contribution

It provides new insights into PLLA crystallization mechanisms in confined block copolymer systems, including the effects of temperature and domain structure on crystallization pathways.

Findings

Concentric lamellae form along nanopores under weak confinement.

PLLA crystallization is influenced by vitrified PS domains and melt domain structure.

Helical motifs and preferential crystal growth directions are observed.

Abstract

We have investigated PLLA crystallization in lamellae-forming PS-b-PLLA confined to straight cylindrical nanopores under weak confinement (nanopore diameter D / equilibrium PS-b-PLLA period L0 larger than 4.8). Molten PS-b-PLLA predominantly forms concentric lamellae along the nanopores, but intertwined helices occur even for D/L0 = 7.3. Quenching PS-b-PLLA melts below TG(PS) results in PLLA cold crystallization strictly confined by the vitrified PS domains. Above TG(PS), PLLA crystallization is templated by the PS-b-PLLA melt domain structure in the nanopore centers, while adsorption on the nanopore walls stabilizes the outermost cylindrical PS-b-PLLA shell. In between, the nanoscopic PS-b-PLLA melt domain structure apparently ripens to reduce frustrations transmitted from the outermost immobilized PS-b-PLLA layer. The onset of PLLA crystallization catalyzes the ripening while transient ripening states are arrested by advancing PLLA crystallization. Certain helical structure motifs persist PLLA crystallization even if PS is soft. The direction of fastest PLLA crystal growth is preferentially aligned with the nanopore axes to the same degree as for PLLA homopolymer, independent of whether PS is vitreous or soft.