Temperature−Composition Phase Diagrams for Binary Blends Consisting of Chemically Dissimilar Diblock Copolymers

Alternating Gyroid in Block Polymer Blends

Alternating gyroid is a lower symmetry variant of the double gyroid morphology, where the left-handed and right-handed chiral networks are physically distinct. This structure is of particular interest for photonic applications owing to predictions of a complete photonic band gap subject to the requirement of a large dielectric contrast between the individual networks and sufficient optical matching between one of the networks and the matrix. We provide evidence, via self-consistent field theory (SCFT), that stoichiometric blends of double-gyroid-forming AB and BC diblock copolymers with relatively immiscible A and C blocks should form an alternating gyroid morphology with complementary three-dimensional A and C networks that have a free energy that is nearly degenerate with two phase-separated double gyroid states. Solvent casting offers the potential for trapping this binary mixture of diblock copolymers in this metastable alternating gyroid phase. Theory further predicts that the addition of a minuscule amount (<1%) of ABC triblock terpolymer will open an alternating gyroid stability window in the resulting ternary-phase diagram. The surfactant-like stabilization produced by the triblock is relatively insensitive to its exact composition provided the B-block forms a sufficiently long bridge between the A-rich and C-rich networks. This blending strategy provides significant synthetic and material processing advantages compared to prevailing methods to produce an alternating gyroid phase in block polymers.

Versatile Controls of Microdomain Morphologies and Temperature Dependencies in Lamellar Spacing by Blending Diblock Copolymers Bearing Antisymmetric Compositions

The morphologies of the microphase-separated structures in the binary blends of diblock copolymers (AB/AB) have been studied intensively for the case of diblock copolymers bearing antisymmetric compositions with similar molecular weights. Here, the two diblock copolymers 1 and 2, of which compositions are 0.5 - x and 0.5 + x (0 < x < 0.5), respectively, were blended, and the morphology diagram was constructed in the plot of χZ vs the average composition of the A component, where χ is the interaction parameter between A and B segments and Z is the average degree of polymerization of the two AB diblock copolymers. The temperature-dependent morphologies were analyzed by synchrotron small-angle X-ray scattering (SAXS) measurements. It was found that the morphology diagram agrees in principle with the theoretical one for the neat AB diblocks by Matsen and Bates (Macromolecules 1996, 29, 1091-1098), although the disordered phase was a bit expanded in the experimentally determined morphology diagram. Anomalous temperature dependencies in the lamellar spacing have been also comprehensively studied for the binary blends of antisymmetric diblock copolymers as a function of the degree of compositional asymmetry by closely adjusting the average composition in the blend specimen at 0.50. For this purpose, more than 20 neat diblock copolymers have been synthesized with a wide range of compositions from 0.20 to 0.87 and a range of molecular weight of 12 000-33 800. The temperature dependencies of the lamellar spacing were also analyzed by synchrotron SAXS measurements. As a result, the following things were found. The scaling exponent α in D ∼ T α was still negative but slightly larger than the usual value (i.e., α = -0.33) for the smaller degree of asymmetry in the composition (i.e., x is small), while α became positive for the higher degree of asymmetry. The latter result is very anomalous because the temperature dependence is opposite (i.e., the lamellar spacing increases with an increase of temperature). The value of α was found to be linearly rationalized with the degree of asymmetry τ (which is especially introduced in the current paper for this purpose), for the binary blends with the average composition of 0.50. Based on this result, one can prepare lamellar microdomains, of which spacing does not change with temperature, by blending two diblock copolymers with τ = 1.33 (corresponding to 0.3 and 0.7 of compositions) having similar molecular weights. This would be important for manufacturing materials with properties (for instance, the optical property) independent of temperature. From the current study, the binary blends of the antisymmetric diblock copolymers are concluded to be versatile such that the precise controls of the morphologies and the temperature dependencies of the lamellar microdomains are plausible.

Host-guest self-assembly in block copolymer blends

Ultrafine, uniform nanostructures with excellent functionalities can be formed by self-assembly of block copolymer (BCP) thin films. However, extension of their geometric variability is not straightforward due to their limited thin film morphologies. Here, we report that unusual and spontaneous positioning between host and guest BCP microdomains, even in the absence of H-bond linkages, can create hybridized morphologies that cannot be formed from a neat BCP. Our self-consistent field theory (SCFT) simulation results theoretically support that the precise registration of a spherical BCP microdomain (guest, B-b-C) at the center of a perforated lamellar BCP nanostructure (host, A-b-B) can energetically stabilize the blended morphology. As an exemplary application of the hybrid nanotemplate, a nanoring-type Ge2Sb2Te5 (GST) phase-change memory device with an extremely low switching current is demonstrated. These results suggest the possibility of a new pathway to construct more diverse and complex nanostructures using controlled blending of various BCPs.