Terminal-Group Compatibility in Block Copolymers: Tuning Chain Stretching and Self-Assembled Morphologies

Abstract

We investigate polyisoprene-b-poly(ethylene oxide) (PI-b-PEO) block copolymers bearing distinct terminal functionalities across two molecular weights and three α/ω end-group combinations (s-Bu/OH, s-Bu/OMe, and OH/OMe). In the PI-b-PEO (6.3–6.0 kg mol–1) series, s-Bu-PI-b-PEO-OH and s-Bu-PI-b-PEO-OMe form lamellae (LAM), whereas OH-PI-b-PEO-OMe forms hexagonally packed cylinders (HEX). This transition is accompanied by a ∼30% reduction in domain spacing, suppressed PEO crystallization, and orders-of-magnitude higher melt moduli. Rheology indicates defect-mediated relaxation in s-Bu-PI-b-PEO-OH, but highly constrained, network-like dynamics in OH-PI-b-PEO-OMe. These observations are attributed to clustering of the –OH termini within the PI domains, reducing chain extension and altering interfacial curvature. In the PI-b-PEO (10.2–6.3 kg mol–1) series, all samples adopt HEX morphologies, yet OH-PI-b-PEO-OMe again shows ∼40% reduced domain spacing and suppressed PEO crystallization. This work identifies terminal functionality as a powerful molecular design parameter for controlling chain stretching, morphology, thermal behavior, and viscoelasticity in block copolymers with broad molecular-level precision.