Large-Grained Cylindrical Block Copolymer Morphologies by One-Step Room-Temperature Casting

We report a facile method of ordering block copolymer (BCP) morphologies in which the conventional two-step casting and annealing steps are replaced by a single-step process where microphase separation and grain coarsening are seamlessly integrated within the casting protocol. This is achieved by slowing down solvent evaporation during casting by introducing a nonvolatile solvent into the BCP casting solution that effectively prolongs the duration of the grain-growth phase. We demonstrate the utility of this solvent evaporation annealing (SEA) method by producing well-ordered large-molecular-weight BCP thin films in a total processing time shorter than 3 min without resorting to any extra laboratory equipment other than a basic casting device, i.e., spin- or blade-coater. By analyzing the morphologies of the quenched samples, we identify a relatively narrow range of polymer concentration in the wet film, just above the order–disorder concentration, to be critical for obtaining large-grained morphologies. This finding is corroborated by the analysis of the grain-growth kinetics of horizontally oriented cylindrical domains where relatively large growth exponents (1/2) are observed, indicative of a more rapid defect-annihilation mechanism in the concentrated BCP solution than in thermally annealed BCP melts. Furthermore, the analysis of temperature-resolved kinetics data allows us to calculate the Arrhenius activation energy of the grain coarsening in this one-step BCP ordering process.

shows the wet film drying rates calculated from the in situ thickness measurements during spin-coating of 0.8 wt.% solutions of S2VP C116 at 2000 rpm at 23 °C in the absence of additional convection other than that caused by the spinning motion and gentle flow of inert gas purging the spincoater motor assembly. The rates for pure toluene and mesitylene solutions are approximate extrapolations based on a few recorded data points. Table S1. Film drying rates of pure solvents and solvent mixtures at room temperature measured in situ during spin-coating at 2000 rpm at 23 °C. The initial concentration of C116 was 0.8 wt.%. Data obtained by fitting spectral reflectance curves recorded at normal incidence in 300-800 nm range with Filmetrics F20 spectral reflectometer.

Mes
In the case of high TMOT concentration in the casting mixture, the resulting morphology is a combination of horizontal cylinders and reticular perforated lamellae ( Figure S3). Figure S3. SEM image of the as-cast S2VP C116 morphology obtained by spin-casting from 0.8% TMOT/Tol 1:4 mixture at room temperature. Poly(2-vinylpyridine) blocks were converted to Al2O3 replica prior to SEM imaging.
The drying rate of the DMOT/Tol 1:4 mixture is strongly affected by the substrate's temperature as depicted in Figure S3. For the lowest tested temperature (15 °C), the drying rate can be as low as 0.2 nm/s. We observed, however, that under these conditions the film is drying non-uniformly, with a visible motion of the evaporation front, and the exact drying rate and thus the processing history is likely position-dependent. Figure S4. Film-thickness vs. time for S2VP C116 wet films cast from 0.8% DMOT/Tol 1:4 mixture evaporating on a thermostated hot-plate. The inset shows the calculated evaporation rates approximated by linear fits for d = 50-250 nm (dry film thickness ≈ 50 nm). The insets show SEM images of the resulting morphologies and calculated grain-size values.
"Evaporation caps" shown in Figure S4 reduce the evaporation rate, offer spatially-uniform evaporation, and allow high-temperature long-time evaporation experiments. The opening in the cylindrical evaporation cap acts a dual role of the solvent evaporation route and the thickness monitoring window. The diameters of the orifice in each cap (5 mm, 7 mm, 10 mm) are incremented to step-up the aperture areas in the 1:2:4 sequence. The corresponding evaporation rates ratio was observed only at the highest temperature tested (45 °C). The drying rate can be accelerated by introducing a flow of inert gas (N2) over the sample surface. Special care was taken to homogenize the flow of nitrogen directed onto the film by a sintered-metal gas diffuser (φ = 25 mm) directed from a ~ 60 mm distance at a ~ 60° angle at the drying sample. The gas flow-rates were adjusted by a needle valve and recorded by a variable-area flow meter (Dwyer). The evaporation rates are listed in Table S2. Figure S5. A photograph displaying a set of "evaporation caps"low-profile hollow cylinders with circular openings used to slow down the evaporation of solvents while permitting the in situ thickness monitoring. The orifice diameter in each cap (5 mm, 7 mm, 10 mm) is incremented to step up the aperture area in the 1:2:4 sequence.

Blade-coating
The custom-built blade-coating apparatus following the design by Stafford et al. 1 was used to deposit thin polymer films on Si substrates. A 25 mm-wide stationary microscope glass slide oriented at an 8° angle positioned 200 µm above a substrate was used to spread the BCP solution. The motion of the substrate relative to the blade was realized by the motorized linear stage (Newport, ILS200CC). Before the coating, 16 μl of 0.8% C116 in DMOT/Tol 1:4 mixture was deposited in the gap between the blade and substrate and spread over the 70 mm distance by the computer-controlled motorized stage at constant acceleration (18 mm s −2 ) producing a film with a thickness gradient.

Transfer of Ordered Films
We demonstrate a possibility of the single-step cast S2VP C116 film transfer onto a curved substrate i.e. pencil graphite utilizing a transfer method described in detail elsewhere. 2 In short, a thin layer of 10% of poly(4-styrene sodium sulfonate) was spin-cast at 3 krpm on a silicon substrate and then baked at 120 °C in air for 1 min. Then, the spin-casting of 0.8% DMOT/Tol 1:4 mixture at 2 krpm was performed and the film was further vacuum baked at 150 °C for 10 min to remove residual solvent. The C116 film was floated onto the water surface by gradual immersion of the substrate using a dip-coater motorized arm (L2006A1, Ossila Ltd.)the layer exhibited no visible mechanical damage. A pencil graphite surface was activated in the plasma etcher (120 mTorr O2, 120 W RF power, 35 kHz, PE-25, Plasma Etch) for 10 seconds before the deposition process. The film was gently picked up on the graphite and dried in air.

Dip-coating
Dip-coating compatibility tests were carried out using a commercial dip-coater (Ossila L2006A1). Plasma-cleaned rectangular silicon substrates (5 mm × 50 mm) were immersed in a 0.8% DMOT/Tol 1:4 mixture for 30 seconds. Several withdrawal speeds in the capillary or drainage regime (from 0.01 mm s −1 to 5 mm s −1 ) were tested to find the optimal dip-coating conditions. Poly(2-vinylpyridine) blocks were converted to Al2O3 replica prior to SEM imaging. Vapor pressure of 3,4,5-trimethoxytoluene (TMOT) is lower than 3,4-dimethoxytoluene (DMOT) and its relative concentration in the mixture with toluene is expected to increase faster than that of DMOT upon evaporation (when comparing mixtures at the same evaporation progress i.e., mass loss).

1 H NMR analysis and Hansen solubility parameters calculation
To track the composition evolution of the solvent mixture (in the liquid phase) during evaporation, a controlled evaporation experiment was performed. The progress of evaporation was monitored by placing a known amount (≈ 2 g) of the solvent mixture (1:4 DMOT/Tol or TMOT/Tol) on an aluminum weighing pan inside a digital laboratory balance. The protective sash of the balance was opened to facilitate the evaporation. When 10%, 30%, 50%, and 70% loss of the initial mass was reached, a small aliquot of the mixture was gently aspirated from the pan with a pipette and transferred into an NMR test tube filled with CDCl3. The mixture composition was analyzed with an NMR spectrometer (Agilent 400 MHz). Quantification of the area under the peaks originating from methyl and methoxy group protons was performed with MestReNova NMR analysis software to determine the molar concentration ratio of DMOT/Tol and TMOT/Tol in each sample. The peak designated as A, located at 2.36 ppm, originates from the overlapping peaks of the methyl groups (S, 3H) from toluene and DMOT or TMOT. The peak B, at 3.85 ppm, originates from the methoxy group present only in DMOT or TMOT (S, 6H and S, 9H for DMOT and TMOT, respectively). After accounting for the number of methoxy groups in DMOT and TMOT, the area under peak B was used, to estimate the relative inputs of toluene and DMOT or TMOT into the methyl peak area (peak A), enabling the calculation of their molar ratio.

= (2 ) cos
where K is a dimensionless shape factor, λ is the X-ray wavelength [nm], ∆ is a peak full-width at halfmaximum parameter -FWHM [rad], and Θ is the Bragg diffraction angle [rad]. Using the momentum transfer vector formula to convert between the real-and reciprocal-space peak widths: = 4 sin 2 /2 sin 2 /2 = 4 Assuming sin(2Θ/2) ≈ 2Θ/2, valid for small scattering angles: (2 The instrumental broadening was estimated from the scattering pattern of a high-resolution sample (diffraction grating with 80 nm pitch). It was estimated to be 0.0002 Å −1 (FWHM) and accounted for in grain-size calculations: The SAXS patterns analysis was performed using the SciAnalysis package. 14 The qy profiles of the primary scattering peak at q* = 0.0125 Å −1 were fitted to the Gaussian function. The peak FWHM parameters were calculated from the Gaussian σ, using a formula: FWHM = 2.355 × σ. We note that in the scattering configuration used in this study (λ = 0. 9184 Å, sample-to-detector distance = 5 m), the resolution of the detector i.e. detector pixel size is 0.00024 Å −1 in the reciprocal-space units. Obtaining a reliable Gaussian peak is therefore limited by this resolution and, assuming the FWHMMEASURED = 2 × pixel-size, the maximum resolvable grain size is ≈ 1.5 µm.