Flower‐Like Colloidal Particles through Precipitation Polymerization of Redox‐Responsive Liquid Crystals

Abstract We report on the synthesis of monodisperse, flower‐like, liquid crystalline (LC) polymer particles by precipitation polymerization of a LC mixture consisting of benzoic acid‐functionalized acrylates and disulfide‐functionalized diacrylates. Introduction of a minor amount of redox‐responsive disulfide‐functionalized diacrylates (≤10 wt %) induced the formation of flower‐like shapes. The shape of the particles can be tuned from flower‐ to disk‐like to spherical by elevating the polymerization temperature. The solvent environment also has a pronounced effect on the particle size. Time‐resolved TEM reveals that the final particle morphology was formed in the early stages of the polymerization and that subsequent polymerization resulted in continued particle growth without affecting the morphology. Finally, the degradation of the particles under reducing conditions was much faster for flower‐like particles than for spherical particles, likely a result of their higher surface‐to‐volume ratio.


Characterization
Nuclear magnetic resonance (NMR) spectra were recorded on a 400 MHz Bruker Avance III HD spectrometer with tetramethyl silane used as an internal standard. Differential scanning calorimetry (DSC) curves were measured with a DSC Q2000 from TA Instruments. Scanning electron microscopy (SEM) images were taken with a JEOL TM 220 A. The average diameter, coefficient of variation (CV), and circularity were calculated using ImageJ (CV = standard deviation/average diameter; circularity = 4π × area / perimeter 2 ). Transmission electron microscopy (TEM) images were taken with a Tecnai 20 (type Sphera) by FEI operating with a LaB 6 filament at 200 kV. Tomographic tilt series are manually collected from minimum -65° to maximum +65° at 5° steps with Inspect3D software (Thermo Fisher Scientific). Alignment and reconstruction of the series were performed using IMOD. The dry LC particle powders were filled in a 1 mm diameter glass capillary for X-ray diffraction (XRD), which was performed on a Ganesha lab instrument with a GeniX-Cu ultralow divergence source producing X-ray photons with a wavelength of 1.54 Å and a flux of 1 × 10 8 ph/s. Scattering patterns were collected with a Pilatus 300 K silicon pixel detector with 478 × 619 pixels, each 172 μm 2 in size. Dynamic light scattering (DLS) was measured with Anton Paar Litesizer 500 in backscattering mode at 25 °C. Matric-assisted laser desorption/ionization time-of-flight (MALDI-TOF) measurements were carried out on a Perspective Biosystem Voyager-DE PRO spectrometer by using α-Cyano-4-hydroxycinnamic acid as matrix material.

Synthesis of disulfanediylbis(4,1-phenylene) bis(4-(3-(acryloyloxy)propoxy)benzoate) (1)
4-[(4-Hydroxyphenyl)disulfanyl]phenol was synthesized via a published procedure. 1 Briefly, 2 g of 4-hydroxybenzenethiol was added to 8 mL DMSO and the solution was heated to 60 °C for 2 hours. After the reaction, ice water was added to the solution and the mixture was filtered. The pale yellow solid was collected and dried in a vacuum oven overnight (3.91 g, 15.6 mmol, 99%). 4-[(4-Hydroxyphenyl)disulfanyl]phenol (2 g, 8 mmol), 4-(3-acryloxy-propyl-1-oxy)benzoic acid (4.4 g, 17.6 mmol), DMAP (0.391 g, 3.2 mmol), and 30 mL dry dichloromethane (DCM) were added to a round-bottom flask equipped with a magnetic stir bar. The mixture was cooled to 0 °C and EDC (3.8 g, 20 mmol) dissolved in 10 mL DCM was added dropwise. The mixture was stirred at room temperature overnight. After the reaction, the solution was washed with 1 M HCl, saturated NaHCO 3 , and deionized water. DCM was removed under reduced pressure and the solid recrystallized in ethanol twice to yield the off-white powder product.   Figure S3) It should be noted that monomer 1 can also be synthesized with pyridine instead of DMAP as the catalyst using the same procedure as described above. The H and C NMR spectra of 1 are the same but in the MS spectrum the DMAP adduct no longer present: MS (MALDI-TOF): [M + H] + calculated for C 38 H 35 O 10 S 2 : 715.17; found [M + Na] + 737.17. (Figure S4) Synthesis of the LC particles 90 mg of 1 and 10 mg of 2 were added to a 50 mL round-bottom flask; the flask was pumped and backfilled with N 2 3 times to remove O 2 . The initiator (AIBN (65 °C), BPO (75 °C), ACCN (85 °C and 90 °C), Luperox P80 (95 °C), or tert-butyl peroxybenzoate (105 °C), 4 wt% vs. monomers)) was dissolved in 5 mL solvent and added to the flask. The flask was put into an oil bath preheated to the polymerization temperature for 24 hours. After polymerization, ethanol was added to the suspension, which was then centrifuged at 6500 rpm for 5 minutes. The solvent was removed, and the particles were washed with ethanol 3 times and centrifuged to yield the particles.

Degradation of the LC particles
0.1 mg LC particles were sonicated and dispersed in 1 mL 10 mM KOH solution. A DLS measurement was performed at 25 °C (t = 0 min). Then, 10 µL 0.1 M 2-mercaptoethanol aqueous solution was added to the suspension. A series of DLS measurements were performed at 25 °C with a time interval of 2 mins.

Monomer conversion measurement
The polymerizations were stopped at specific times (2 hours, 4 hours, or overnight (approximately 20 hours)). The particles were separated from the solution by centrifugation (6500 rpm for 5 minutes) and the solution retained. The solutions were carefully concentrated below 40 °C to avoid further polymerization, and 1 H-NMR spectra were taken ( Figure S8). The integrals of the acrylate peaks (I 1 , the sum of three groups at 5.7 to 6.5 ppm, labelled as "a" in Figure S8,) and reference peaks (I 2 , at 4.1 to 4.2 ppm, labelled as "b+b'" in Figure S8) were measured, and the monomer conversions in the solutions were calculated by con. = 1 -2/3 × I 1 /I 2 ( Figure S9 and Table S1).

Absorption and release of Rhodamine B
50 μL 1 mg/mL Rhodamine B aqueous solution, 1 mL 1 mg/mL LC particles (polymerized at 65 °C in phenyl acetate) dispersed in 10 mM KOH aqueous solution were added to 8.95 mL 10 mM KOH aqueous solution. The suspension was left for an hour prior to centrifugation, and the UV-Vis spectrum of the supernatant was measured (red line in Figure S10). The supernatant was returned to the bulk solution, and the particles were redispersed by sonication. 10 µL 1 M 2-mercaptoethanol aqueous solution was added to the suspension, which was centrifuged again after 15 minutes and the UV-Vis spectrum of the supernatant was measured (blue line in Figure S10)