Ordered Nanostructures in Thin Films of Precise Ion-Containing Multiblock Copolymers.

We demonstrate that ionic functionality in a multiblock architecture produces highly ordered and sub-3 nm nanostructures in thin films, including bicontinuous double gyroids. At 40 °C, precise ion-containing multiblock copolymers of poly(ethylene-b-lithium sulfosuccinate ester) n (PESxLi, x = 12 or 18) exhibit layered ionic assemblies parallel to the substrate. These ionic layers are separated by crystalline polyethylene blocks with the polymer backbones perpendicular to the substrate. Notably, above the melting temperature (T m) of the polyethylene blocks, layered PES18Li thin films transform into a highly oriented double-gyroid morphology with the (211) plane (d 211 = 2.5 nm) aligned parallel to the substrate. The cubic lattice parameter (a gyr) of the double gyroid is 6.1 nm. Upon heating further above T m, the double-gyroid morphology in PES18Li transitions into hexagonally packed cylinders with cylinders parallel to the substrate. These layered, double-gyroid, and cylinder nanostructures form epitaxially and spontaneously without secondary treatment, such as interfacial layers and solvent vapor annealing. When the film thickness is less than ∼3a gyr, double gyroids and cylinders coexist due to the increased confinement. For PES12Li above T m, the layered ionic assemblies simply transform into disordered morphology. Given the chemical tunability of ion-functionalized multiblock copolymers, this study reveals a versatile pathway to fabricating ordered nanostructures in thin films.

Introduction

Understanding the structural characteristics of polymer thin films is critical to numerous emerging technologies.[1,2] For example, self-assembly of block copolymer (e.g., layer and cylinder) in thin films is essential to using these materials for nanolithography because the phase behavior can be significantly perturbed relative to the bulk behavior due to interfacial and confinement effects.[3−5] Also, conjugated polymers are widely studied in thin-film geometries because the alkyl chain crystallinity and π–π stacked assemblies impact the electron transport properties,[6,7] which are valuable for transistors, light-emitting diodes, and photovoltaics.[8−11] Similarly, structure–property relationships of ion-containing polymer thin films have received significant interest for their ion transport abilities.[12−15] The perfluorinated sulfonic acid polymers (e.g., Nafion) are among the most extensively studied ionomers for their applications in the catalyst layer of fuel cells and solar-fuel generators.[16,17] Specifically, the studies of Nafion have indicated that producing aligned morphologies of phase-separated sulfonic acid aggregates in a thin film can improve the proton transport properties or catalytic performance.[16,18−20]

Recent advances in polymer chemistry have enabled the precise segmentation of functional groups in polyethylene-based copolymers in contrast to the randomly distributed sulfonic acid groups of Nafion.[21−23] Notably, sulfonic acid groups placed at every 21st carbon of polyethylene blocks form well-defined ionic layers that achieve comparable proton conductivity to commercial Nafion.[24] In addition, strictly alternating multiblock copolymers composed of polyethylene and sulfosuccinate ester blocks with metal counterions (Li+, Na+, and Cs+) have shown various ordered ionic aggregate morphologies, including layered, double-gyroid, and hexagonally packed cylinders.[25−28] Nanofabrication using the bicontinuous double-gyroid morphology is particularly important for applications in metamaterials and electrochemical devices.[29−33] While polyethylene-based multiblock copolymer ionomers are attractive platforms to explore as ion-conducting membranes and nanostructured templates, the ordered ionic assemblies and polyethylene backbones in these precise ionomers have not been studied in thin films. More broadly, relative to the diblock and triblock copolymer systems, thin-film studies are noticeably lacking for (AB) alternating multiblock copolymers presumably due to the difficulties in achieving the strong segregation strength required for microphase separation.[34,35]

We now report precise ion-containing multiblock copolymers that produce desirable double gyroids and other ordered nanostructures in thin films with exceptional fidelity. The temperature-dependent thin-film morphologies are investigated by in situ grazing-incidence wide- and small-angle X-ray scattering (GIWAXS and GISAXS). Notably, within the ∼25–50 nm thick films, we observe highly oriented double-gyroid morphologies in a poly(ethylene-b-lithium sulfosuccinate ester), where the cubic lattice parameter is only 6.1 nm. This gyroid morphology exhibits a thermotropic order-to-order transition into hexagonally packed cylinders with a lattice parameter of ∼2.7 nm. These ordered nanostructures with sub-3 nm domain spacings form spontaneously without any secondary treatment, such as neutral layers, top coats, and solvent vapor annealing.

Results and Discussion

Precise ion-containing multiblock copolymers, poly(ethylene-b-lithium sulfosuccinate ester) (PESxLi, x = 12 and 18), contain exactly x-methylene carbons strictly alternating with short polar blocks of lithium sulfosuccinate esters (Scheme ). Note that these polymers are synthesized via step-growth polymerization of two monomers of fixed lengths, thereby achieving molecular weight monodispersity of the repeating blocks. The synthesis of materials, molecular weights, and bulk morphologies were previously reported.[28] End-group analysis using 1H nuclear magnetic resonance spectroscopy determines that the molecular weights of PES12Li and PES18Li are 20 and 7.7 kg/mol, respectively.

Scheme 1

Chemical Structure of Precise Ion-Containing Multiblock Copolymers: (a) PES12Li (n = 55) and (b) PES18Li (n = 17)

In situ GIWAXS and GISAXS provide the structural characteristics of PES12Li in a 25 nm thin film. At 40 °C, in-plane scattering intensities at q ≈ 1.5 Å–1 indicate that the polyethylene blocks crystallize into a hexagonal crystal with the chain axis perpendicular to the substrate (Figure a).[36,37] Above the melting point (Tm) at 140 °C, amorphous polyethylene blocks exhibit no preferential orientation as indicated by the broad and isotropic intensity ring at q ≈ 1.4 Å–1 (Figure b). The melting transition of polyethylene coincides with changes in the small-angle scattering features originating from the polar blocks of lithium sulfosuccinate esters, namely, the ionic aggregates. In situ GISAXS experiments are performed to provide better resolution at q < 0.6 Å–1. In Figure c, intensities along the q axis with a peak ratio (q/q*) of 1:2 correspond to the ionic layers parallel to the substrate. The polar blocks of the PES12Li are expected to contact the −OH groups on the silicon substrate, while polyethylene blocks crystallize (see inset schematic in Figure a). In precisely functionalized polyethylenes synthesized by acyclic diene metathesis, chain folding accommodates both crystalline polyethylene and ionic layers.[37] Thus, the layer spacing of 2π/q* = 2.5 nm in PES12Li is the combined length of a polyethylene block (12 all-trans carbons, 1.4 nm) and two polar blocks. At 140 °C, GISAXS shows a disordered ionic aggregate morphology accompanying the amorphous polyethylene, as evidenced by the isotropic intensity ring (q ≈ 0.3 Å–1), Figure d. Thus, grazing-incidence X-ray scattering highlights the preferentially oriented ionic layers and polyethylene chain of PES12Li in thin films below Tm.

Figure 1

In situ 2D (a, b) GIWAXS and (c, d) GISAXS patterns of PES12Li thin films (∼25 nm thick) at 40 and 140 °C showing a transition from parallel layers to a disordered morphology. Exposure times are (a, b) 20 min, (c) 2 h, and (d) 1 h. Inset schematic in (a) represents the layered polar blocks (blue) separated by the crystalline polyethylene blocks (red).

While PES12Li and PES18Li are similar at 40 °C, PES18Li exhibits order to order transitions. The scattering intensities for PES18Li at q ≈ 1.5 Å–1 arise from the vertically aligned polyethylene chains, and the out-of-plane scattering intensities with a peak ratio (q/q*) of 1:2 and q* ≈ 0.20 Å–1 indicate parallel ionic layers with d ≈ 3.1 nm (Figure a and 2d). This lattice parameter is 0.6 nm larger than that in PES12Li and consistent with the additional length of the alkyl block (18 all-trans carbons, 2.1 nm). The PES18Li thin film at 140 and 180 °C exhibits highly oriented double gyroids and hexagonally packed cylinders, respectively (Figure b and 2c). In Figure e, the GISAXS intensities are indexed for the cubic double-gyroid (Ia3̅d) morphology where the (211) plane is preferentially oriented parallel to the substrate.[38,39] These peak assignments were made by calculating the scattered intensities for a double gyroid using GIXSGUI, see Figure S1.[40] The cubic lattice parameter of the double-gyroid morphology is agyr = 6.1 nm, corresponding to a film thickness of ∼7agyr (∼44 nm). At 180 °C, the gyroid morphology transitions into hexagonally packed cylinders oriented parallel to the substrate, as evidenced by the assigned peaks for the 2-dimensional orthorhombic lattice (Figure f). The calculated GISAXS profile, including both reflected and transmitted beams, is provided in Figure S2. The orthorhombic lattice parameters are aortho = 2.7 nm and bortho = 4.7 nm, and the film thickness (44 nm) is ∼9bortho. The orthorhombic lattice is equivalent to a hexagonal lattice as shown in Figure S3.[41,42] Using the polar volume fraction (0.31) of PES18Li, we compute the cylinder diameter to be ∼1.1 nm. Note that the transitions between the double-gyroid and hexagonally packed cylinder morphologies are reversible, and the double-gyroid morphology reforms upon cooling. This precise ion-containing multiblock copolymer exhibits an exceptional ability to produce highly oriented and sub-3 nm ordered nanostructures in thin films. Further, these ordered nanostructures are obtained spontaneously upon heating without any secondary treatment, such as interfacial layers and solvent vapor annealing.

Figure 2

In situ 2D (a–c) GIWAXS and (d–f) GISAXS patterns of PES18Li thin films (∼44 nm thick) at 40, 140, and 180 °C showing parallel layers, double gyroids, and hexagonal cylinders. Miller indexes in (e) and (f) are shown for the double gyroid and 2D orthorhombic lattice, respectively. Exposure times are (a–d) 20 min, (e) 3 h, and (f) 2 h.

We further investigate the stability of double gyroids in PES18Li thin films with increasing 2-dimensional confinement, i.e., decreasing film thickness. Temperature-dependent ionic aggregate morphologies are determined from in situ GISAXS experiments for 17, 26, and 44 nm thick PES18Li films and summarized in Figure . The 2D scattering patterns collected in 10 °C increments are provided in Figures S4–S6. At all film thicknesses below 120 °C, ionic layers are aligned parallel to the substrate due to the crystalline polyethylene. For the 44 nm thick film, double gyroids exist at 130–160 °C and then transition into cylinders upon heating, similar to the bulk behavior.[28] These order to order transitions are epitaxial with the differences of characteristics lengths < 0.2 nm at the transition temperatures (Figure S7). Highly oriented double gyroids are also identified within the 26 nm thick film, where the film thickness coincides with ∼4agyr or ∼10d211. In a further confined geometry (17 nm thick film), the double-gyroid and hexagonally packed cylinder morphologies coexist at 130–150 °C. The thin-film confinement significantly impacts the morphologies when the film thickness is on the order of a few lattice parameters, as widely observed in other block copolymer systems.[43,44] Here, we show that PES18Li spontaneously self-assembles into highly aligned double gyroids in thin films as thin as 26 nm, ∼4agyr.

Figure 3

Temperature-dependent morphology diagram of PES18Li thin films with thicknesses of 17, 26, and 44 nm. Bulk morphologies are obtained from ref (28). Layered (L), double-gyroid (G), and hexagonally packed cylinders (H) morphologies of ionic assemblies are determined from the in situ GISAXS with temperature increments of 10 °C upon heating. See Figures S4–S6 for the corresponding 2D scattering patterns.

In previously studied diblock copolymers and giant surfactants, double gyroids exist in relatively thick films (>150 nm) to meet the commensurability of their large lattice parameters.[43,45] Alternatively, controlling interfacial layers with top coats and solvent vapor annealing can facilitate the formation of double gyroids in thinner films.[46−48] Recently, a disaccharide–atactic polypropylene conjugate showed double gyroids (agyr = 13.1 nm) in thin films (15–150 nm) on carbon-coated silicon substrates, suggesting hydrogen-bonding interaction of saccharides is a crucial factor in determining the morphology.[49] Here, we demonstrate that a precise ion-containing multiblock copolymer self-assembles into highly oriented double gyroids with a smaller lattice parameter of 6.1 nm in thin films (≥26 nm). We attribute these double-gyroid morphologies in thin films to (1) the precisely alternating (AB) multiblock architecture with perfectly monodisperse block length and (2) the large interaction parameter between the blocks that drive microphase separation via ionic functionalities.

Conclusions

We investigated the structural characteristics of two precise ion-containing multiblock copolymers, poly(ethylene-b-lithium sulfosuccinate ester). As-prepared thin films form layered ionic assemblies separated by the crystalline polyethylene blocks. The ionic layers are aligned parallel to the substrate, and the domain spacings are 2.5 and 3.1 nm for PES12Li and PES18Li, respectively. Importantly, PES18Li produces highly oriented double gyroids with a cubic lattice parameter (agyr) of just 6.1 nm in confined thin-film geometries with thicknesses of ∼4agyr and 7agyr. A thinner film (<3agyr) resulted in a mixed morphology of double gyroids and hexagonally packed cylinders. In these polymer thin films under confinement, the formation of double gyroids is attributed to the strictly alternating lithium sulfosuccinate ester blocks that strongly interact to produce the double gyroids with the (211) plane (d211 = 2.5 nm) parallel to the substrate. In addition, we reveal that the multiblock copolymers with monodisperse block lengths can self-assemble into ordered nanostructures in thin films, which will expand their utility. Future studies may explore double gyroids in thin films by modulating the polymer chemistries, as well as develop the potential applications of these ion-containing multiblock copolymer thin films into ion transport membranes and nanopatterning templates.

Experimental Section

No unexpected and significant hazards or risks were encountered during the experimental procedures.

Preparation of Polymer Thin Films

Details of materials synthesis, 1H NMR characterization, and gel-permeation chromatography have been reported previously.[28] Thin films were prepared by spin coating PES12Li (0.5 wt %) and PES18Li (0.3, 0.5, and 1.0 wt %) polymer solutions in methanol on a bare silicon wafer for 1 min at 6000 rpm under ambient conditions. Silicon wafers were UV–ozone cleaned and directly used without any post-treatment. The film thicknesses were measured using a F3–UV reflectometer equipped with a LS-DT2 light source and SS-5 stage (Filmetrics). All samples were dried at 50 °C for 24 h under vacuum prior to X-ray scattering experiments. The melting temperatures of bulk PES12Li and PES18Li are 73 and 117 °C, respectively, according to differential scanning calorimetry.

Grazing-Incidence X-ray Scattering

Grazing-incidence X-ray scattering experiments were performed using the Dual-source and Environmental X-ray Scattering (DEXS) facility at the University of Pennsylvania. The DEXS facility is equipped with a GeniX3D beam source (8.05 keV, Cu Kα, λ = 1.54 Å) and a PILATUS 1 M detector. The 190 and 375 mm sample-to-detector distances were used for the in situ grazing-incidence wide-angle (GIWAXS) and small-angle (GISAXS) experiments. The sample to detector distances were calibrated using AgBeh. The incident beam angle was 0.20° for all measurements.

For PES12Li (25 nm thick) and PES18Li (44 nm thick) samples, in situ GIWAXS and GISAXS experiments were performed at specific temperatures of 40, 140, and 180 °C with a heating rate of 10 °C/min and an equilibration time of 10 min. Exposure times are specified in the figure captions.

For PES18Li (17, 26, and 44 nm) samples, in situ GISAXS experiments were performed at 40–180 °C with a temperature interval of 10 °C upon heating. The heating rate was 10 °C/min with an equilibration time of 5 min at each temperature, and exposure times were 20 min for all measurements. The assignments of the Miller indexes for 2D scattering patterns were made using GIXSGUI.[40]