Correlative characterization of stereocomplex formation in blends of aliphatic polyester P(PCLm-b-PLLAn) multiblock-copolymers and PDLA

Abstract

In phase-separating multiblock copolymers it is a challenge to quantify the relationship between molecular structure and functional properties, yet this quantification is crucial for processing and applications. Here, we describe the molecular structure and phase behavior-properties relationships for a modular system of poly[(ε-caprolactone)-b-poly(L-lactide)] multiblock copolymers with well-defined long/short block lengths and their blends with poly(D-lactide) (PDLA) of varying lengths. The formation of crystallite types and sizes as well as absolute and relative crystallinities of PCL, PLA homocrystallites (HC), and PLA stereocrystallites (SC) were studied by DSC and WAXS, and visualized by TEM, POM, and AFM. We reveal that SC formation occurs in blends containing a ratio between 1 : 1 and 1 : 4 ratio of PDLA and PLLA. In systems with much longer PCL than PLLA sequence length (113 : 18), SC formation is inhibited. Blend crystallinity was highest for a medium PDLA length. SC formation is preferred over HC formation, and SCs act as nucleation points for PCL crystallization. In our work the segment length had a trend to correlate with crystallite sizes. Tensile strength (from 0.5 to 8 MPa) and elongation at break (from 10% to >750% at room temperature) could be increased simultaneously by allowing SC formation, which in the studied blends correlated with low overall crystallinity. Our study shows strategic polymer synthesis and blending for the precise control of stereocomplex formation and fine-tuning in high-performance PLA-based materials. These findings support the knowledge-based choice of blend composition and segment length to tailor versatile materials with tunable mechanical and thermal properties.