Control of Nanostructural Dimension by Crystallization in a Double-Crystalline Syndiotactic Poly(4-methyl-1-pentene)-block-poly(l-lactide) Block Copolymer

The control of nanostructural dimension by crystallization-induced chain stretching was investigated in a novel double-crystalline block copolymer, syndiotactic poly(4-methyl-1-pentene)- block -poly( l -lactide) ( s PMP–PLLA), featuring a lamellar phase. Because of the similar glass transition temperatures of s PMP and PLLA, their blocks could crystallize under soft confinement (i.e., a crystallization temperature higher than the glass transition temperatures of the constituent blocks) in s PMP–PLLA. With the strong segregation of s PMP–PLLA, the first-crystallized s PMP block was templated by microphase separation to form confined crystalline s PMP lamellae within the microphase-separated lamellar texture. Most interestingly, the first-crystallized s PMP block may also induce significant stretching of the PLLA chains from the lamellar interface, resulting in the increase of microdomain thickness of the PLLA block. With the increase of crystallization temperature, this chain stretching may become more significant, resulting in a large increase (∼34%) of the lamellar long period. The double-crystalline lamellar morphologies having homeotropic orientation for both s PMP and PLLA crystals can be acquired in the shear-aligned s PMP–PLLA as evidenced by simultaneous 2D small-angle X-ray scattering and wide-angle X-ray diffraction, giving uniform birefringence under polarized light microscope with thermal reversibility. As a result, the switchable lamellar nanostructures having significant dimensional change can be carried out by simply controlling crystallization or melting of the crystallizable blocks.