Photoinduced Morphology Change in Ionic Supramolecular Block Copolymer

Physically mixing two dissimilar polymers often results in a macroscopically segregated blend with poor optical clarity and mechanical properties. Previously, we demonstrated that chain end functionalization of immiscible polymer blends with oppositely paired acid and base groups leads to an ionic supramolecular block copolymer with electrostatically stabilized microdomains that suppress macroscopic phase separation. In this work, a functionalized polydimethylsiloxane with a photochromic diarylethene (DAE) end-group (PDMS-ω-DAE) is blended with a sulfonic acid end-functionalized polystyrene (PS-ω-SO3H), forming an ionic supramolecular block copolymer with tunable morphology by light. When the DAE is irradiated with UV light, it triggers an isomerization from the ring-opened (DAE–O) to the ring-closed (DAE–C) form. The light-induced conformational change in the chain-end group chemistry substantially alters the ionic junctions, leading to a phase structure difference in the solid-state from hexagonally packed cylinders (HEX) to lamellae (LAM). After UV radiation, the more localized positive charge on DAE–C led to stronger ionic bonds between the two dissimilar blocks, while the repulsion between adjacent DAE–C groups increased. The stronger electrostatic repulsion along the interface resulted in increased interfacial area per polymer chain and thus induced such phase transition. The light-induced phase transition of this system demonstrates that the ionic interactions can be tuned on-demand to create different morphologies from a single polymer blend.

Methods
Methods
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Solution-State Characterization. 1H and 13C NMR spectra were recorded on a Bruker 500 MHz NMR spectrometer using CDCl3 as the solvent. 
Chemical shifts are reported relative to residual solvent peaks (δ 7.26 ppm for CDCl3 in 1H NMR and δ 77.05 for CDCl3 in 13C NMR). 

Size-exclusion chromatography (SEC) was obtained with a Waters SEC system equipped with a refractive index detector and 
Agilent PLgel 5 μm MiniMIX-D column at 35 °C with THF as the eluent. Molecular weight distribution was estimated by a series of polystyrene standards. 

Mass spectral data was collected on the Shimadzu LCMS-9030 Q-TOF Mass Spectrometer with an electrospray ionization (ESI) source. 

Matrix-assisted laser desorption ionization (MALDI-TOF) mass spectrometry. MALDI-TOF experiments were performed using a 
Bruker Microflex MALDI-TOF mass spectrometer (Bruker Daltonics). Samples were prepared in tetrahydrofuran at 5 mg/ml. 
Trans-2-[3-(4-tert-Butyl-phenyl)-2-methyl-2-propenylidene] malononitrile (DCTB) (10 mg/mL) and potassium trifluoroacetic acid (KTFA) (10 mg/ml)
were used as a matrix and cationization agent respectively. One microliter of sample, 1 µl of KTFA, and 20 µl of DCTB were mixed thoroughly. 
About 0.5 µl of this mixture was deposited on a stainless-steel sample holder.

Fourier-Transform Infrared Spectroscopy (FTIR). FTIR measurements were performed on a Thermo Nicolet iS10 spectrometer equipped with a Smart Diamond 
attenuated total reflectance (ATR) accessory. A background spectrum was obtained every 30 minutes, and sample spectra were taken using 64 scans in absorbance mode.

UV-Vis Spectroscopy and UV-Vis Kinetic Measurements. UV-Vis absorption spectra were recorded on Agilent 8453 UV-Vis spectrometer. 
The switching kinetics of PS-ω-SO3−/ PDMS-ω-DAE-H+–O ionic blend was measured on a home-built pump-probe setup, as previously reported by our group.
The UV pump source was generated by a Deep UV LED (Thorlabs M300F2, 300 nm), positioned to illuminate the sample directly without coupling into a multimode 
optical fiber (≈1.32 mW/cm2). The probe beam was generated by a high-power MINI Deuterium Tungsten Halogen Source w/shutter 200–2000 nm (Ocean Optics DH-MINI) 
coupled into a multimode fiber with an output collimator for the light delivery. The probe light was modulated by a shutter (Uniblitz CS25), which could be 
controlled manually or through a digital output port (National Instruments USB-6009) using LabVIEW. Pump and probe beams overlapped using steering and focusing optics 
at a 90° angle inside a sample holder, allowing 10×10 mm2 rectangular spectrophotometer cells to be held within or cast film samples to be held to the front using metal 
spring clips. The solutions were continuously stirred during the measurements by a miniature stirring plate inserted into the sample holder (Starna Cells SCS 1.11). 
Both pump and probe beams were nearly collimated inside the cell with a diameter of about 2 mm. The pump beam was blocked after passing through the sample, and the probe 
beam was directed by a system of lenses into the detector (Ocean Optics Flame-S1-XR spectrometer), which acquired spectra of the probe light. The detector was connected 
to a PC via a USB port. The experimentwas controlled by a National Instrument LabVIEW program, which collected the probe light spectra, determined sample optical absorption
spectra, controlled pump, and probe light sources.

Small-angle X-ray Scattering (SAXS). Samples were loaded into aluminum washers (1 mm thickness) and sealed with Kapton tape. All experiments were performed at the National 
Synchrotron Light Source II (NSLS-II, beamline 11-BM, Brookhaven National Laboratory), with an X-ray energy of 13.5 keV. The sample to detector distance (3m) was calibrated 
using a silver behenate standard, and the isotropic 2D scattering patterns were reduced into 1D scattering intensity as a function of the wavevector q = 4π sin(θ/2)/λ, where θ 
is the scattering angle and λ is the X-ray wavelength. Temperature-dependent SAXS experiments were conducted on selected ionic polymer blends, heated incrementally to 65, 85,
95, 105, 120, and 140 °C, followed by an equilibration time of 15 min at each temperature.

Transmission Electron Microscopy (TEM). Thin sections (ca. 120 nm) for electron microscopy were obtained by cryo-microtoming at a Leica EM UC 7 at −100 °C to −80 °C and 
collected on 200-mesh copper grids. TEM experiments were performed on Talos™ F200X G2 TEM at an acceleration voltage of 200 keV.