Thermally-induced transition of lamellae orientation in block-copolymer films on ‘neutral’ nanoparticle-coated substrates

Abstract

Block-copolymer orientation in thin films is controlled by the complex balance between interfacial free energies, including the inter-block segregation strength, the surface tensions of the blocks, and the relative substrate interactions. While block-copolymer lamellae orient horizontally when there is any preferential affinity of one block for the substrate, we recently described how nanoparticle-roughened substrates can be used to modify substrate interactions. We demonstrate how such 'neutral' substrates can be combined with control of annealing temperature to generate vertical lamellae orientations throughout a sample, at all thicknesses. We observe an orientational transition from vertical to horizontal lamellae upon heating, as confirmed using a combination of atomic force microscopy (AFM), neutron reflectometry (NR) and rotational small-angle neutron scattering (RSANS). Using molecular dynamics (MD) simulations, we identify substrate-localized distortions to the lamellar morphology as the physical basis of the novel behavior. In particular, under strong segregation conditions, bending of horizontal lamellae induce a large energetic cost. At higher temperatures, the energetic cost of conformal deformations of lamellae over the rough substrate is reduced, returning lamellae to the typical horizontal orientation. Thus, we find that both surface interactions and temperature play a crucial role in dictating block-copolymer lamellae orientation. Our combined experimental and simulation findings suggest that controlling substrate roughness should provide a useful and robust platform for controlling block-copolymer orientation in applications of these materials.

Thickness-dependence of block copolymer coarsening kinetics

“…In the simulations, block copolymer chains are simulated as freely-jointed linear chains of beads, with chemical incompatibility between blocks enforced by adjusting the relative interaction potentials between beads. 56,57 The number ratio between the distinct bead types is selected so that the system spontaneously forms a cylinder phase. An inplane BCP orientation is similarly enforced using the substrate-interaction parameters, reproducing the experimentally-observed substrate wetting behavior.…”

“…We simulated the behavior of block copolymer thin films using our previously-described molecular dynamics (MD) methods and parameters. 56,57 Briefly, polymers chains are coarse-grained as linear chains of beads of two distinct types, segregated lengthwise along the chain into two blocks (using a 3:7 ratio to give rise to a cylinder morphology). The two bead types have distinct self-cohesive interaction strengths, such as the two phases spontaneously phase-separate as the molecular dynamics simulation proceeds.…”

Rapid ordering of block copolymer thin films

“…As will be discussed at length in this review article, the order of BCP materials can in fact be influenced in a number of different ways [48][49][50]. BCPs are responsive to temperature [51][52][53] and solvent [54,55] conditions; to thermal [56,57], shear [58][59][60][61][62][63], electric [64][65][66][67], and magnetic [68][69][70][71][72] fields; and to substrate topography [73,74] and chemical makeup [75]. One can further broaden the range of possible structures by considering blends of BCPs with other materials: especially nanoparticles [76][77][78][79][80][81], homopolymers [82][83][84][85][86], and other BCPs [87][88][89][90][91][92][93]…”

Cooperative Ordering and Kinetics of Cellulose Nanocrystal Alignment in a Magnetic Field

“…Two dimensional SAXS data was quantified in two main ways (η and S parameters); the I(χ) curve was fit phenomenologically to yield an ad-hoc order parameter (η) by fitting the curve to: [41][42][43] ( Equation 1) where χ 0 is the alignment direction, and C is a scaling constant, with η varying from 0 (isotropic) to 1 (fully anisotropic). Furthermore, an assessment of the orientation of the CNCs themselves, where the average occurs over the orientations of all entities with respect to the desired director (χ) was performed.…”