Self-assembly of semicrystalline bottlebrush statistical copolymers with various grafting density

We synthesized bottlebrush statistical copolymers with poly(ethylene oxide) (PEO) and poly(dimethylsiloxane) (PDMS) side chains grafted to a polynorbornene backbone. The grafting density was tuned through copolymerization with a phenyl derivative of norbornene. An analysis of the one-dimensional scattering length density correlation function revealed that the backbone acted as a transition zone between the PEO and PDMS domains at high grafting density, while forming a distinct third domain with a smaller density than both PEO and PDMS at low grafting density. Self-consistent field theory calculations showed that zigzagging of the backbone, driven by the microphase-separation of PEO and PDMS, was essential for the formation of a distinct backbone domain at low grafting densities. The PEO crystallites were randomly oriented within the amorphous PEO domains. Ordering PEO chains into crystal unit cells inevitably disrupted the nematic order of PEO side chains in the melt and the lateral ordering of the microphase-separated lamellar morphologies. As a result, both the static intrinsic birefringence and form birefringence decreased when PEO crystallized upon cooling.