Double-crystalline diblock copolymers of linear polyethylene (LPE) and hydrogenated polynorbornene (hPN) are synthesized, and their crystallization is examined. LPE and hPN homopolymers are both highly crystalline, with similar melting points near 150 °C and χhPN/LPE = 0.02. For symmetric diblocks, even when moderately segregated, crystallization of hPN breaks out of the melt microdomains to create volume-filling spherulites. The LPE block subsequently crystallizes between the hPN crystallites, with the hPN and LPE crystal stems oriented orthogonally. At shallow undercoolings, crystallization proceeds in two distinct steps, but at deeper undercoolings, only a single process is observed, following the temperature dependence of hPN crystallization. At these deeper undercoolings, crystallization of hPN initiates LPE crystallization so that the two processes are nearly simultaneous. By changing the block ratio, the crystallization behavior can be broadly tuned: in hPN-rich diblocks, the two crystallization processes are well-separated at any temperature, while in LPE-rich diblocks, the LPE block crystallizes first.

No abstract

No abstract

We report the morphology and orientation of the crystals which form within the microdomain structures of diblock copolymers of linear polyethylene and glassy poly(vinylcyclohexane), LPE/ PVCH, across the spherical, cylindrical, and lamellar morphologies. Compared with short-branched polyethylene (such as hydrogenated polybutadiene), confinement of LPE within spheres, within cylinders, or between PVCH cylinders directly limits the crystal thickness and thereby the crystal melting point. Conversely, crystals formed within LPE lamellae are stacked orthogonally to the LPE/PVCH microdomain layering, so there is no direct limitation imposed on crystal thickness by confinement. As with LPE homopolymer, LPE crystals within lamellae thicken when annealed below the melting point, ultimately forming crystals whose thickness is significantly larger than their lateral extent, set by the bounding PVCH layers. The ribbon-like crystals which form within LPE cylinders or lamellae have a strong orientational coupling to the microdomains; prealignment of the cylindrical or lamellar mesophase by extensional flow yields macroscopic specimens with pronounced b-axial and a-axial orientations, respectively, after subsequent quiescent crystallization.

Double-crystalline diblock copolymers of linear polyethylene (LPE) and hydrogenated polynorbornene (hPN) are synthesized, and their crystallization behavior and morphology are examined using small-angle (SAXS) and wide-angle X-ray scattering (WAXS). In symmetric hPN/LPE diblocks with molecular weights above 50 kg/mol, the hPN block has previously been shown to crystallize first and set the solid-state microstructure. Two-dimensional WAXS on hand-drawn fiber specimens reveals that the LPE crystals formed in confinement stack orthogonally to the hPN crystals. By adjusting total molecular weight, the order of block crystallization may be reversed, even while holding the block length ratio fixed. At a diblock molecular weight of 20 kg/mol, simultaneous time-resolved SAXS/WAXS reveals that the LPE block crystallizes first, even when LPE is the minority component, and restricts hPN to crystallize between the LPE lamellae. The relative orientation of the LPE and hPN crystals in the lower molecular weight diblocks is examined by modeling changes in the SAXS primary peak intensity on cooling two diblocks through the hPN crystal–crystal transition, where hPN densifies as it adopts a rotationally ordered crystal structure. Only a perpendicular stacking of hPN and LPE crystals consistently yields the large reduction in primary SAXS peak intensity observed for both diblocks. Thus, even though the templating block switches from hPN to LPE as the diblock molecular weight is reduced, the orthogonal stacking motif is retained for both high- and low-molecular-weight copolymers.