Microphase separation in semicrystalline block copolymers can be driven by two forces:  thermodynamic incompatibility between the blocks in the melt or crystallization of one or more blocks. The diblock copolymers investigated here are polyethylene-b-poly(3-methyl-1-butene), all containing ≈26 wt % ethylene, prepared by hydrogenating high-1,4-polybutadiene-b-high-3,4-polyisoprene. This block asymmetry leads to a hexagonally-packed cylindrical morphology when microphase separation is present in the melt. The molecular weights are varied to obtain differing degrees of incompatibility in the melt, ranging from a single-phase (disordered) melt to a strongly segregated melt. X-ray scattering and differential scanning calorimetry are used to investigate the morphology resulting from crystallization in these melts as a function of thermal history. Crystallization from strongly segregated melts is confined to the cylindrical microdomains and produces a morphology essentially independent of thermal history. In the most strongly segregated diblock crystallized at 10−20 °C/min, chain folding within these cylindrical microdomains is preferentially oriented with the chain axis tilted 18 ± 4° from normal to the cylinder axis. In contrast to strongly segregated melts, the morphology produced by crystallization from weakly segregated melts is highly dependent upon thermal history. Faster cooling kinetically confines crystallization to cylinders, while slower cooling results in complete disruption of the cylindrical melt mesophase upon crystallization, leading to a lamellar morphology with a larger domain spacing.

We investigate the development of mesophase order in compositionally asymmetric (13 wt % styrene) polystyrene−polyisoprene (SI) diblock and SIS triblock copolymers. The equilibrium ordered morphology of these polymers is a body-centered-cubic (bcc) lattice of spherical S microdomains. The ordering kinetics are measured by quenching the material from above the order−disorder transition temperature (T ODT) to a temperature where the ordered phase is stable and following the structure development by time-resolved small-angle X-ray scattering and dynamic oscillatory rheological measurements. The kinetics are significantly slower than those observed for nearly symmetric polymers of similar T ODT, resulting in a wider range of experimentally accessible quench depths. The temporal evolution of the storage modulus G‘ measured at constant frequency displays a two-step increase after a quench from above the ODT, for sufficiently low temperatures and/or high molecular weights. The long-time step corresponds to the development of a bcc lattice of spherical microdomains, while the first step results from the finite time required for large-amplitude composition fluctuations to develop fully.

The orientation of the crystal stems in diblock copolymers, where crystallization occurs within the cylindrical microdomains present in the melt mesophase, is investigated via X-ray scattering on flow-aligned specimens. A series of ethylene-b-(3-methyl-1-butene) diblocks (E/MB) is compared with an ethylene-b-(vinylcyclohexane) diblock (E/VCH), where MB is rubbery (Tg < Troom) and VCH is vitreous (Tg = 134 degreesC) at the freezing point of the E block. All of the diblocks contain 26-28 wt % E, leading to a morphology of hexagonally-packed E cylinders. Crystallization in all of these materials can be confined to the cylindrical microdomains. Not only do the crystals align preferentially within the semicrystalline cylinders, but this orientation varies depending on the ability of chains to diffuse during the crystallization process. When chain diffusion is most rapid, alignment is observed with the chain axis in the crystals perpendicular to the cylinder axis and the b axis (fast growth axis) coincident with the cylinder axis. However, when the chain mobility is limited, the crystal stems tilt with respect to a plane which is normal to the cylinder axis, allowing better accommodation of amorphous material at the crystallite surface.

We have discovered a synthetic process capable of producing olefin block copolymers (OBCs) with controlled block length polydispersity. Certain compositions of these OBCs self-assemble in the melt to form ordered mesophases. The morphologies and dielectric contrast of the semicrystalline and amorphous blocks produce transparent films exhibiting a partial photonic band gap for frequencies in the visible spectrum. The domain spacings are not only much larger than expected for monodisperse block copolymers of similar molecular weight, but they also exceed the predictions of recent theories for polydisperse block copolymers. An extension to Strong Segregation Theory demonstrates that many molecules have a weak preference for segregation to the interface versus the center of a domain. Minor perturbations can then produce highly swollen but relatively stable domains.