Hierarchical Superstructures by Combining Crystallization‐Driven and Molecular Self‐Assembly

Abstract Combining the unique corona structure of worm‐like patchy micelles immobilized on a polymer fiber with the molecular self‐assembly of 1,3,5‐benzenetricarboxamides (BTAs) leads to hierarchical superstructures with a fir‐tree‐like morphology. For this purpose, worm‐like patchy micelles bearing pendant, functional tertiary amino groups in one of the corona patches were prepared by crystallization‐driven self‐assembly and immobilized on a supporting polystyrene fiber by coaxial electrospinning. The obtained patchy fibers were then immersed in an aqueous solution of a tertiary amino‐functionalized BTA to induce patch‐mediated molecular self‐assembly to well‐defined fir‐tree‐like superstructures upon solvent evaporation. Interestingly, defined superstructures are obtained only if the pendant functional groups in the surface patches match with the peripheral substituents of the BTA, which is attributed to a local increase in BTA concentration at the polymer fibers’ surface.


Synthesis of N 1 ,N 3 ,N 5 -tris[2-(dimethylamino)-ethyl]-1,3,5-benzenetricarboxamide (BTA-Methyl).
BTA-Methyl was synthesized as described previously. [1] Briefly, trimesic acid trimethyl ester was dispersed in N,N-dimethylethylenediamine under an argon atmosphere. The mixture was heated to 125 °C, stirred overnight and subsequently allowed to cool down to room temperature. The resulting mixture was dispersed in acetone and heated until an almost clear solution was obtained. The hot solution was filtrated using a sintered glass funnel filter. The solvent was removed and the product was dried in a vacuum oven at 50 °C over night, yielding a white powder.

Synthesis of triblock terpolymers.
Polystyrene-block-polyethylene-block-poly(methyl methacrylate) (S40E21M39 108 ) was synthesized by a combination of living anionic polymerization and catalytic hydrogenation, as described elsewhere. [2] Polystyrene-blockpolyethylene-block-poly(N,N-dimethylaminoethyl methacrylamide) (S33E17DMA50 132 ) was prepared via post-polymerization functionalization of the poly(methyl methacrylate) (PMMA) block of S40E21M39 108 . [3,4] In the used triblock terpolymer notation the subscripts describe the mass fraction of the corresponding block in wt.% and the superscript denotes the overall number average molecular weight (Mn) in kg·mol −1 . Mn was determined by a combination of MALDI-ToF MS (matrix-assisted laser desorption/ionization time of flight mass spectrometry) and 1 H NMR (nuclear magnetic resonance) spectroscopy, employing the absolute Mn of the polystyrene precursor from MALDI-ToF for 1 H NMR signal calibration.

Formation of patchy worm-like triblock terpolymer micelles.
The patchy worm-like micelles were prepared by crystallization-driven self-assembly (CDSA) of the triblock terpolymers S40E21M39 108 and S33E17DMA50 132 in THF according to our previous work. [2,3] The polymers were dissolved in THF (c = 10 g·L -1 ) at 65 °C for 0.5 h using a thermostated shaker unit (HCL-MKR 13, Ditabis). The selfassembly process occurred by subsequently cooling to the crystallization temperature (Tc) of the polyethylene middle block ( Table S1).
The process was allowed to proceed for 24 h with 200 rpm resulting in in the respective patchy worm-like micelle dispersions.  [2,5] S33E17DMA50 132 21 18 ± 5 17 ± 5 510 ± 310 [3] [a] Subscripts describe the mass fraction of the corresponding block in wt.% and the superscript denotes the overall molecular weight in kg·mol −1 .
[b] crystallization temperature of the PE block, c = 10 g·L -1 in THF.
[c] Average sizes ± standard deviation as determined by TEM image analysis of at least 100 micelles/patches.

Electrospinning.
Preparation of patchy polymer fibres PScore / SEM and PScore / SEDMA. Patchy polymer fibres were produced by coaxial electrospinning, according to our previous work. [3,6] To this end, a 7 wt.% polystyrene (PScore) (Mn = 1.8·10 6 g·mol -1 ) solution in DMF was used as core and for the shell dispersions of patchy worm-like SEM or SEDMA micelles in THF (c = 10 g·L -1 ) were employed. The fibres were spun on a collector placed at a distance of 20 cm from the coaxial needle (COAX_2DISP sealed coaxial needles, LINARI NanoTech, dcore = 0.51 mm and dshell = 1.37 mm) at a temperature of 20.8 °C and a relative humidity of ca. 30%. For electrospinning, a high voltage of 11.4 kV at the needle and -1.0 kV at the collector were applied. The feed rate of the PScore solution was 1.2 mL·h -1 and for the micellar dispersions 1.0 mL·h -1 . Neat polystyrene fibres were prepared as reference material in the same manner but without using the micellar dispersions.

Self-assembly of aqueous BTA-Methyl solutions upon solvent evaporation onto aluminium foil.
25 µL of an aqueous BTA-Methyl solution with concentrations ranging from 0.025 to 1.000 wt.% were dropped onto aluminium foil and the solvent was allowed to evaporate at ambient conditions. After solvent evaporation, turbid films were obtained and investigated by scanning electron microscopy.

Self-assembly of aqueous BTA-Methyl solutions onto patchy polymer fibres.
The PScore / SEDMA and PScore / SEM fibres as well as the neat PS fibres on aluminium foil were immersed into BTA-Methyl solutions (varying in concentration ranging from 0.025 wt.% to 0.250 wt.% with a pH value from 7 to 11, respectively) for a fixed time of 30 s and allowed to dry at ambient conditions for complete solvent evaporation.

Methods
Scanning electron microscopy. For scanning electron microscopy measurements, a FEI Quanta FEG 250 scanning electron microscope (Thermo Fisher Scientific) equipped with a field emission gun was used. The untreated samples, i.e. without applying a sputter coating, were measured in the beam deceleration mode (only Figure 1A, 1C and 3B) or in the low vacuum mode. Measurements in the beam deceleration mode were conducted under high vacuum at an acceleration voltage of 6 kV. This mode was used to image surfaces at high magnification with a concentric back scattered (CBS) electron detector, which is insensitive to sample charging. Here, an additional negative voltage (bias, -4 kV) was applied to the stage. In this way, the primary electrons were decelerated to 2 kV when reaching the sample and interacted electrons were accelerated toward the CBS detector. The samples measured in the low vacuum mode (water pressure of 40 Pa in the sample chamber) were mounted on a sample holder using an adhesive graphite pad and were investigated with an acceleration voltage of 3 kV with a large-field (gaseous secondary electron) detector (LFD) for topographical details.
Transmission electron microscopy. The morphology of triblock terpolymer worm-like micelles were analysed by elastic bright-field transmission electron microscopy (TEM) on a Zeiss 922 Omega EFTEM (Zeiss NTS GmbH, Oberkochen, Germany). Zero-loss filtered images were recorded digitally on a bottom mounted CCD camera system (Ultrascan 1000, Gatan) at an acceleration voltage of 200 kV. The micrographs were processed with the digital imaging processing system of Gatan (Digital Micrograph 3.9 for GMS 1.4). The samples were diluted to c = 0.1 g·L -1 and a droplet was placed onto a carbon coated copper grid. The residual solvent was immediately blotted by filter paper, dried in a vacuum oven (20 mbar, room temperature) and stained with RuO4 vapor (selective staining of PS). The average length and patch size were determined by measuring at least 100 micelles/patches using the software ImageJ. [7] Raman imaging. A WITec alpha 300 RA+ imaging system, equipped with a UHTS 300 spectrometer and a back-illuminated Andor Newton 970 EMCCD camera, was employed for confocal Raman imaging. The measurements were conducted at an excitation wavelength of λ = 532 nm, using a laser power of 4 mW and an integration time of 0.6 s·pixel -1 (100x objective, NA = 0.9, step size 100 nm·pixel -1 , software WITec Control FIVE 5.3). All spectra were subjected to a cosmic ray removal routine and baseline correction. The spatial distribution of PS and BTA-Methyl was determined using the tool "true component analysis" in the WITec Project FIVE 5.3 software.