Controlled Synthesis and Crystallization-Driven Self-Assembly of Poly(ε-caprolactone)-b-polysarcosine Block Copolymers

TL;DR

Researchers developed a method to control the self-assembly of a biocompatible polymer for biomedical uses by adjusting polymer lengths and solvents.

Contribution

A modified two-step crystallization strategy enables uniform self-assembly of PCL-b-PSar block copolymers in alcohol solvents.

Findings

Short PSar blocks in PCL-b-PSar form two-dimensional lamellar crystals.

Long PSar blocks induce hierarchical structures or surface aggregation depending on PCL length.

Solvent choice significantly affects self-assembly pathways and final structures.

Abstract

Poly(ε-caprolactone)-b-polysarcosine (PCL-b-PSar) block copolymers (BCPs) emerge as a promising alternative to conventional poly(ε-caprolactone)-b-poly(ethylene oxide) BCPs for biomedical applications, leveraging superior biocompatibility and biodegradability. In this study, we synthesized two series of PCL-b-PSar BCPs with controlled polymerization degrees (DP of PCL: 45/67; DP of PSar: 28–99) and low polydispersity indexes (Đ ≤ 1.1) and systematically investigated their crystallization-driven self-assembly (CDSA) in alcohol solvents (ethanol, n-butanol, and n-hexanol). It was found that the limited solubility of PSar in alcohols resulted in competition between micellization and crystallization during self-assembly of PCL-b-PSar, and thus coexistence of lamellae and spherical micelles. To overcome this morphological heterogeneity, we developed a modified self-seeding method by employing a two-step crystallization strategy (i.e., Tc1 = 33 °C and Tc2 = 8 °C), achieving conversion of micelles into crystals and yielding uniform self-assembled structures. PCL-b-PSar BCPs with short PSar blocks tended to form well-defined two-dimensional lamellar crystals, while those with long PSar blocks induced formation of hierarchical structures in the PCL45 series and polymer aggregation on crystal surfaces in the PCL67 series. Solvent quality notably influenced the self-assembly pathways of PCL45-b-PSar28. Lamellar crystals were formed in ethanol and n-butanol, but micrometer-scale dendritic aggregates were generated in n-hexanol, primarily due to a significant Hansen solubility parameter mismatch. This study elucidated the CDSA mechanism of PCL-b-PSar in alcohols, enabling precise structural control for biomedical applications.

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