Permeable nanoconfinement of hierarchical block copolymer volume gratings

Abstract

A hierarchical structure of poly(ethylene oxide)-b-poly(ε-caprolactone) (PEO-b-PCL) block copolymer (BCP) confined between crosslinked resin was patterned into Bragg volume gratings using a holographic polymerization (HP) process. The BCP formed a lamellar structure confined between the layers of the grating created by HP. The periods of the volume grating and the BCP were controlled to be ∼200 nm and 20 nm, respectively. These two different length scale layers were aligned parallel to one another yielding a polymeric film which exhibits distinct diffraction behavior due to a periodic refractive index variation. This system exhibits complex thermo-optical behavior during heating and cooling cycles with reversible changes in both the diffraction wavelength and efficiency induced by BCP melting and crystallization in the confined region. Transmission electron microscopy studies show reversible diffusion of PEO-b-PCL into and out of the crosslinked resin, indicating that the nanoconfinement imposed by the resin is soft and permeable for the BCP. The morphological changes in nanoconfinement with temperature account for the complex thermo-optical behavior of the grating and the system provides an interesting platform to investigate soft nanoconfinement of BCP materials.