Domain Spacing and Composition Profile Behavior in Salt-Doped Cyclic vs Linear Block Polymer Thin Films: A Joint Experimental and Simulation Study

Abstract

Herein, we leveraged lithium salt doping of linear and cyclic block polymers (BPs) in thin film geometries to demonstrate how BP architecture influences self-assembled nanofeature sizes and interfacial widths in nanostructured thin films as a function of segregation strength. To mitigate potential issues with sample-tosample variability and time-consuming synthesis, we used a single linear BP specimen that was ring-closed to generate an analogous cyclic BP, and we mixed the cyclic and/or parent linear BP with a lithium salt to modulate effective segregation strength (χ eff N). Based on X-ray reflectivity analyses, cyclic polystyrene-block-poly[oligo-(ethylene glycol) methacrylate] BP thin film specimens had ∼20% smaller domain spacings than their linear counterparts at all χ eff Ns and lower absolute sensitivities to changes in χ eff at identical molecular weights. We also report the first direct measurements of interfacial widths in cyclic BP assemblies, which quantitatively demonstrated that interfacial mixing in cyclic BPs was greater relative to linear BPs. Furthermore, the trends in domain characteristics with increasing salt concentration qualitatively agreed with results from molecular dynamics (MD) simulations with increasing χ, despite the fact that salt species were not explicitly included in the MD simulations. Our results underscore the utility of lithium salt doping to explore the BP phase behavior of synthetically challenging macromolecules and demonstrate key architecture/segregation strength relationships in cyclic BP thin films, which provides useful information to further evaluate cyclic BP suitability for nanoscale patterning and templating applications.

Cyclic polymers: Advances in their synthesis, properties, and biomedical applications

“…As discussed previously, the presence of impurities in a cyclic sample is almost inescapable. For the RC techniques, the most common ones are uncyclized chains due Epoxides [39,77] Lactones [76,134] CuAAC Acrylics [52][53][54]63,192,193,198,210,218] Epoxides [65] Lactones [57,59,60,65,66,197] Oxazolines [67,137] Vinyls [47,55,56,61,62,64,192] Diels to an inefficient cyclization reaction or an incomplete functionalization of the chain ends of the linear precursor, and cyclic or linear oligomers generated by an intermolecular coupling rather than intramolecular due to inappropriate experimental conditions (i.e., when the cyclization is performed under a too high concentration). [22] The RE methods are usually less concerned by this issue, because the cyclic structure is maintained throughout the whole process and there is no linear intermediate.…”

“…[188] Since then, many other groups reported a similar behavior for other polymer systems. [189][190][191][192] Zhu et al reported on the comparison of cyclic and linear triblock copolymers of PS and poly(butadiene) (PBD). By gradually increasing the percentage of PS in the copolymer, they could not only study the lamellar morphology, but also random PS spheres and cylinders in a PBD matrix and the gyroid morphology.…”

“…[191] A last example was recently provided by Gartner et al who measured a 20% reduction of the domain spacing for c-[poly(styrene)-bpoly(oligo(ethylene glycol)methacrylate)] in comparison with its linear analogue. [192] In solution, ring polymers also exhibit interesting properties. Different groups synthesized cyclic polymers bearing fluorescent functional groups, either as pendant groups [61,193] or included in the polymer backbone, [194] and they all observed a significant increase of the fluorescence with respect to the linear analogues, whatever the polymer system that is employed.…”

“…[ 191 ] A last example was recently provided by Gartner et al who measured a 20% reduction of the domain spacing for c ‐[poly(styrene)‐ b ‐poly(oligo(ethylene glycol)methacrylate)] in comparison with its linear analogue. [ 192 ]…”

Blending Linear and Cyclic Block Copolymers to Manipulate Nanolithographic Feature Dimensions

“…Thus, even though the absolute values found for the interfacial roughness of pure linear BCPs and pure cyclic BCPs are similar, taken as a fraction of the domain spacing, the cyclic BCP has wider interfaces; these results are consistent with the experimental findings of Gartner et al . 24 …”