Implications of Grain Size Variation in Magnetic Field Alignment of Block Copolymer Blends

Abstract

Recent experiments have highlighted the intrinsic magnetic anisotropy in coil-coil diblock copolymers, specifically in poly(styrene-block -4-vinylpyridine) (PS-b-P4VP), that enables magnetic field alignment at field strengths of a few tesla. We consider here the alignment response of two low molecular weight (MW) lamallae-forming PS-b-P4VP systems. Cooling across the disorder-order transition temperature (T odt ) results in strong alignment for the higher MW sample (5.5K), whereas little alignment is discernible for the lower MW system (3.6K). This disparity under otherwise identical conditions of field strength and cooling rate suggests that different average grain sizes are produced during slow cooling of these materials, with larger grains formed in the higher MW material. Blending the block copolymers results in homogeneous samples which display T odt , d-spacings and grain sizes that are intermediate between the two neat diblocks. Similarly, the alignment quality displays a smooth variation with the concentration of the higher MW diblock in the blends and the size of grains likewise interpolates between limits set by the neat diblocks, with a factor of 3.5X difference in the grain size observed in high vs low MW neat diblocks. These results highlight the importance of grain growth kinetics in dictating the field response in block copolymers and suggests an unconventional route for the manipulation of such kinetics.

Controlling orientational order in block copolymers using low-intensity magnetic fields

“…(7)(8)(9)(10)(11)(12)(13)(14)(15) The LC mesophase within the BCP superstructure provides a ≈100-to 1,000-fold increase in ∆χ relative to non-LC BCPs. Conversely, recent work has demonstrated that the intrinsic susceptibility anisotropy (∆χ ≈ 10 −8 ) of non-LC BCPs can be sufficient to provide strong alignment at field strengths of 5-6 T, due to the large grain sizes produced in certain low-molecular-weight BCPs, with ξ > 1 µm (16,17). The aforementioned cases contrast sharply with smallmolecule LCs where alignment can be driven readily at fields below 1 T (18)(19)(20).…”

Developing Anisotropy in Self‐Assembled Block Copolymers: Methods, Properties, and Applications

“…This can lead to nearly two orders of magnitude enhancement in magnetic property contrast compared with conventional coil-coil BCPs (from ≈10 −8 to ≈10 −6 , dimensionless units). [216,225,226] In general, magnetic fields can be applied to samples with arbitrary shape since there is no requirement for direct contact between sample and instrument. A representative example apparatus that applies magnetic fields to bulk BCP samples is shown in Figure 6.…”

Macroscopic Alignment of Block Copolymers on Silicon Substrates by Laser Annealing

“…The introduction of directing biases during the microphase separation, known as directed self-assembly (DSA), induces long-range order and, frequently, shortens the time of annealing . Currently, the arsenal of DSA techniques is very extensive and includes methods based on physical and chemical interactions. − The former include utilization of electric − and magnetic fields, − mechanical shearing, , and graphoepitaxy. − The latter rely on altering chemical interactions between block copolymers and their environment. Notably, chemical epitaxy DSA involves patterned substrates, which induce preferential or neutral wetting of BCP blocks. ,− Alternatively, self-assembly can be accelerated by lowering BCPs’ viscosity by blending them with low molecular weight polymers or other plasticizers such as small molecules during exposure to solvent vapor − or immersion in a poor solvent. , …”