Polystyrene-block-poly(ethylene oxide) Bottlebrush Block Copolymer Morphology Transitions: Influence of Side Chain Length and Volume Fraction

A systematic study was conducted to investigate the morphology transitions that occur in polystyrene-block-poly(ethylene oxide) (PS-b-PEO) bottlebrush block copolymers (BBCP) upon varying PEO volume fraction (f(PEO)) from 22% to 81%. A series of PS-b-PEO BBCPs with different PEO side chain lengths were prepared using ring-opening metathesis polymerization (ROMP) of PEO-norbornene (PEO-NB) (M-n similar to 0.75, 2.0, or 5.0 kg/mol) and PS-norbornene (PS-NB) (M-n similar to 3.5 kg/mol) macromonomers (MM). A map of f(PEO) versus side chain asymmetry (M-n(PEO-NB)/M-n(PS-NB)) was constructed to describe the BBCP phase behavior. Symmetric and asymmetric lamellar morphologies were observed in the BBCPs over an exceptionally wide range of fPEo from 28% to 72%. At high f(PEO), crystallization of PEO was evident. Temperature controlled SAXS and WAXS revealed the presence of high order reflections arising from phase segregation above the PEO melting point. A microphase transition temperature T-MST was observed over a temperature range of 150-180 degrees C. This temperature was relatively insensitive to both side chain length and volume fraction variations. The findings in this study provide insight into the rich phase behavior of this relatively new class of macromolecules and may lay the groundwork for their use as templates directing the fabrication of functional materials.