Control of crystal orientation of spatially confined PCL homopolymers by cleaving chain-ends of PCL blocks tethered to nanolamella interfaces

We have examined the crystalline nanomorphology (i.e., crystallinity, melting temperature (reflecting crystal thickness), and crystal orientation) of poly(ε-caprolactone) (PCL) blocks and PCL homopolymers spatially confined in lamellar microdomain structures (nanolamellae) using two polystyrene (PS)-b-PCL diblock copolymers (with either PCL chain-end anchored to nanolamella interfaces) and one PS-b-PCL-b-PS triblock copolymer (with both chain-ends anchored). These copolymers had photocleavable o-nitrobenzyl groups at all the block junctions, so the PCL blocks could be converted into PCL homopolymers by UV irradiation with keeping the nanolamellar geometry. The crystallization of photocleaved PCL homopolymers was achieved by two procedures; (1) the amorphous PCL blocks were first converted into PCL homopolymers and they were crystallized at room temperature (RT) (one-step crystallization) or (2) the PCL blocks were first crystallized at selected temperatures to form some crystalline nanomorphology and then converted into PCL homopolymers to further crystallize and/or anneal at RT (two-step crystallization). The crystallinity and melting temperature of PCL homopolymers were similar for both procedures, suggesting that a certain amount of PCL homopolymers formed lamellar crystals with a comparable thickness. However, the lamellar crystals showed significantly different orientations between two procedures, because some crystal orientation was made by the advance crystallization of PCL blocks and remained unchanged after UV irradiation/subsequent crystallization in two-step crystallization, yielding a substantial difference in the crystal orientation of PCL homopolymers in nanolamellae.