Crystallization within lamellar block copolymer microdomains, at temperatures above the glass transition of the amorphous block - a form of "soft", one-dimensional confinement - is investigated in a series of hydrogenated diblocks of norbornene and hexafluoroisopropanol-substituted norbornene, abbreviated hPN-hPHFAN. Crystallization results in a large (up to 23%) increase in the lamellar period (d), but in contrast to a multitude of previous reports - where large changes in d signify breakout of the crystallites from the microdomains, such that the initial and final domain structures and orientations bear no coherent relationship to each other - the hPN crystallites in hPN-hPHFAN remain confined within the microphase-separated lamellae, as revealed by small-angle X-ray scattering on highly oriented flow-aligned specimens. As the d-spacing dilates, the lamellae contract affinely in the lateral directions (as measured by confocal optical microscopy on a millimeter-scale flow-aligned specimen); dimensional changes in the macroscopic specimen (millimeter scale) precisely track the changes in d (≈50 nm scale). The increase in d, and concomitant changes in the macroscopic specimen dimensions, are fully reversible on melting, over multiple melting-recrystallization cycles. The crystallites are found to be oriented with the b-axis parallel to the microdomain interfaces and the crystal stems tilted with respect to the lamellar normal. In this orientation, the crystal thickness is limited by the thickness of the hPN domains, such that the domain spacing increase is driven by the enhanced thermodynamic stability of thicker hPN crystals.
All Science Journal Classification (ASJC) codes
- Organic Chemistry
- Polymers and Plastics
- Inorganic Chemistry
- Materials Chemistry