Dynamic thermal gradient-based processes for directed self-assembly of block copolymer (BCP) thin films such as Cold Zone Annealing (CZA) have demonstrated much potential for rapidly fabricating highly ordered patterns of BCP domains with facile orientation control. As a demonstration, hexagonally packed predominantly vertical cylindrical morphology, technologically relevant for applications such as membranes and lithography, was achieved in 1µm thick cylinder-forming PS-b-PMMA (cBCP) films by applying sharp thermal gradients (CZA-Sharp) at optimum sample sweep rates. A thorough understanding of the molecular level mechanisms and pathways of the BCP ordering that occur during this CZA-S process is presented, useful to fully exploit the potential of CZA-S for large-scale BCP-based device fabrication. To that end, we developed a customized CZA-S assembly to probe the dynamic structure evolution and ordering of the PS-b-PMMA cBCP film in-situ as it undergoes the CZA-S process using the Grazing Incidence Small Angle X-ray Scattering (GISAXS) technique. Four distinct regimes of BCP ordering were observed within the gradient that include microphase separation from an 'as cast' unordered state (Regime I), evolution of vertical cylinders under a thermally imposed strain gradient (Regime II), reorientation of a fraction of cylinders due to preferential substrate interactions (Regime III) and finally grain-coarsening on the cooling edge (Regime IV). The ordering pathway in the different regimes is further described within the framework of an energy landscape. A novel aspect of this study is the identification of a graincoarsening regime on the cooling edge of the gradient, previously obscure in zone annealing studies of BCPs. Such insights into the development of highly ordered BCP nanostructures under template-free thermal gradient fields can potentially have important ramifications in the field of BCP directed self-assembly and self-assembling polymer systems more broadly. Graph showing variation of χ within CZA-S gradient, indexing of peaks to hexagonal lattice showing parallel cylinders, surface GISAXS image of post CZA-S sample, example of curve fitting to the 1-dimensional GISAXS integrated intensity profiles to obtain peak widths (fwhm) and orientation correlation lengths, plot of peak widths with respect to CZA-S temperature and time (PDF) AUTHOR INFORMATION

Designing next-generation lightweight pulsed power devices hinges on understanding the factors influencing the energy storage performance of dielectric materials. Polymer dielectric films have a quadratic dependence of energy storage on the voltage breakdown strength, and strategies to enhance the breakdown strength are expected to yield a path toward high energy storage densities. Highly stratified lamellar block copolymer (L-BCP) films of model polystyrene-b-polymethylmethacrylate (PS-b-PMMA) exhibited as much as ∼50% enhancement in breakdown voltage (E BD) (225% increase in stored energy density, U ∼ E BD 2) compared to unordered as-cast L-BCP films. Such an energy density using amorphous polymer is on par with industry-standard semicrystalline biaxially oriented polypropylene (BOPP) and as such a notable development in the field. This work develops a deeper understanding of the molecular mechanisms of E BD enhancement in L-BCP films, relating E BD directly to molecular weight (M n), with interpretation to effects of chain-end density and distribution, interface formation, layer thickness, and their relative contributions. As-cast disordered L-BCP films show decreasing E BD with decreasing M n similar to homopolymer studies because of the increase of homogeneously distributed chain ends in the film. E BD increases significantly in parallel ordered L-BCP films because of the combination of interface formation and spatial isolation of the chain ends into segregated zones. We further confirm the role of chain ends in the breakdown process blending a low M n L-BCP with matched M n homopolymers to attain the same layer spacing as neat L-BCP of higher M n. E BD shows a significant decrease at low homopolymer fractions because of increased net chain-end density within swollen ordered L-BCP domains in wet-brush regime, followed by increased E BD because of layer thickness increase via segregated “interphase layer” formation by excess homopolymers. Notably, E BD of homopolymer swollen L-BCPs is always lower than that of neat L-BCPs of the same domain spacing because of overall adverse chain-end contribution from homopolymers. These findings provide important selection rules for L-BCPs for designing next-generation flexible electronics with high energy density solid-state BCP film capacitors.

Template-free directed self-assembly of ultrathin (approximately tens of nanometers) lamellar block copolymer (l-BCP) films into vertically oriented nanodomains holds much technological relevance for the fabrication of next-generation devices from nanoelectronics to nanomembranes due to domain interconnectivity and high interfacial area. We report for the first time the formation of full through-thickness vertically oriented lamellar domains in 100 nm thin polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) films on quartz substrate, achieved without any PMMA-block wetting layer formation, quartz surface modification (templating chemical, topographical) or system modifications (added surfactant, top-layer coat). Vertical ordering of l-BCPs results from the coupling between a molecular and a macroscopic phenomenon. A molecular relaxation induced vertical l-BCP ordering occurs under a transient macroscopic vertical strain field, imposed by a high film thermal expansion rate under sharp thermal gradient cold zone annealing (CZA-S). The parametric window for vertical ordering is quantified via a coupling constant, C (= v∇T), whose range is established in terms of a thermal gradient (∇T) above a threshold value, and an optimal dynamic sample sweep rate (v ∼ d/τ), where τ is the l-BCP's longest molecular relaxation time and d is the T - T distance. Real-time CZA-S morphology evolution of vertically oriented l-BCP tracked along ∇T using in situ grazing incidence small angle X-ray scattering (GISAXS) exhibited an initial formation phase of vertical lamellae, a polygrain structure formation stage, and a grain coarsening phase to fully vertically ordered l-BCP morphology development. CZA-S is a roll-to-roll manufacturing method, rendering this template-free through-thickness vertical ordering of l-BCP films highly attractive and industrially relevant.

We report a generic phenomenon in the ordering of block copolymer thin films, wherein the morphology spontaneously antialigns with respect to film discontinuities during thermal annealing. The aligned near-edge region propagates further from the edge with increased annealing time and temperature, while also being responsive to heating rate and applied stress fields. This effect is ascribed to a combination energetic preference for morphological orientation at boundaries as well as stress relaxation of the polymer film as it creeps orthogonal to film edges.

To utilize the full potential of block copolymer (BCP) thin films for use in technological devices ranging from ion conduction membranes and transistors to nanowire array antennas, rapid self-assembly of lamellar block copolymers (l-BCPs) with vertically oriented lamellar domains on a variety of unmodified substrates is needed. l-BCPs have an inherently larger interfacial area for transport compared to their cylindrical counterpart. Our observations demonstrate that the as-cast weakly ordered vertically oriented state of l-BCP films of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) from the directional evaporation of select solvents act as "seed templates" for their ultra-fast evolution (≈30 s) into well-ordered vertically oriented nanostructures, using a thermal gradient-based cold zone annealing (CZA) technique. Vertical lamellae are obtained on unmodified substrates, Quartz and Kapton, and the kinetics of l-BCP ordering is much faster by CZA as compared to the isotropic oven annealing. The rapid ordering kinetics of vertical l-BCPs is tested and found applicable to different molecular masses and film thicknesses ranging from 20 nm to 480 nm, which ultimately flip over to their equilibrium parallel morphology at the upper limits of annealing times. This rapid ordering strategy for the vertical orientation of l-BCPs using roll-to-roll compatible CZA would be highly relevant for fundamental studies of interfacial transport as well as for industrial applications from membranes to nanowires.