Copolymer Brush Particle Hybrid Materials with “Recall-and-Repair” Capability

The effect of sequence structure on the self-healing and shape-memory properties of copolymer-tethered brush particle films was investigated and compared to linear copolymer analogs. Poly(n-butyl acrylate-co-methyl methacrylate), P(BA-co-MMA), and linear and brush analogs with controlled gradient and statistical sequence were synthesized by atom transfer radical polymerization (ATRP). The effect of sequence on self-healing in BA/MMA copolymer brush particle hybrids followed similar trends as for linear analogs. Most rapid restoration of mechanical properties was found for statistical copolymer sequence; an increase of the high Tg (MMA) component provided a path to raise the material’s modulus while retaining self-heal ability. Creep testing revealed profound differences between linear and brush systems. While linear copolymers featured substantial viscous deformation when exposed to constant stress in the linear regime, brush analogs displayed minimal permanent deformation and featured shape restoration. The reduction of flow was interpreted to be a consequence of slow cooperative relaxation due to the complex microstructure of brush particle hybrids in which long-range motions are constrained through entanglements and slow-diffusing particle cores. The rubbery-like response imparts BA/MMA copolymer brush material systems concurrent “shape-memory” and “self-heal” capability. This ability to “recall-and-repair” could find application in the design of functional hybrid materials, for example, for soft robotics.


Synthesis of linear PBA-statistical-PMMA copolymer and SiO 2 -g-PBA-statistical-PMMA particle brushes.
Initiator (EBiB/SiO2-Br), monomer: BA and MMA, solvents (anisole), CuBr2, and Me6TREN, molar ratios shown in supporting information, were mixed thoroughly in a sealed Schlenk flask, followed by degassing by bubbling with nitrogen.Then the Sn(EH)2 was injected into the Schlenk flask to activate the catalyst complex, and the flask was immediately put into an oil bath set at the desired temperature.The conversion was monitored and controlled under 10% by 1 H-NMR.The final products were precipitated in cold methanol and then dissolved and stored in THF.

Synthesis of SiO 2 -g-PBA-grad-PMMA/SiO 2 -g-PMMA-grad-PBA particle brushes.
Initiator (SiO2-Br), monomer (BA/MMA), solvents (anisole), CuBr2, and Me6TREN, molar ratios shown in supporting information, were mixed thoroughly in a sealed Schlenk flask, followed by degassing by bubbling with nitrogen.Then the Sn(EH)2 was injected into the Schlenk flask to activate the catalyst complex, and the flask was immediately put into an oil bath set at the desired temperature.Meanwhile, nitrogen pre-degassed MMA/BA monomer was injected to the reaction by syringe pump at the specific feeding rate.The conversion was monitored and controlled under 10% by 1 H-NMR.The final products were precipitated in cold methanol and then dissolved and stored in THF.

Nuclear Magnetic Resonance Spectroscopy (NMR)
Conversion of polymerization was monitored by 1 H NMR on a Bruker Advance 500 MHz NMR instrument in CDCl3 at room temperature.

Fabrication of a bulk film
THF dissolved linear copolymers and particle brushes dispersion solutions were transferred into 15 mm × 5 mm rectangular Teflon molds.The solvent was slowly evaporated over 48 h at room temperature generating transparent nanocomposite films with a thickness of 0.1-0.2mm.
The residual solvent was removed from the bulk films by transferring them to a vacuum oven at 120 °C for 24 h.

Size Exclusion Chromatography (SEC)
Number-average molecular weights (Mn) and molecular weight distributions (MWD) of samples were determined by size exclusion chromatography (SEC).The SEC was conducted with an Agilent 1260 Iso pump and Waters 410 differential refractometer using PSS columns (Styragel 105, 103, 102 Ǻ) with THF as an eluent at 35 ºC and at a flow rate of 1 mL min -1 .
Linear PMMA standards were used for calibration.Toluene was used as internal standards for the system.

Creep test:
Creep experiments were performed on pristine specimens with applied stress of 10 kPa for 90 seconds at room temperature (TA RSA-G2), followed by a recovery time of 180 seconds in which stress was removed.

Self-healing Test
A bulk film was severed, then physically reattached within 1 min at 50 o C, and allowed to selfhealing for a specific time under 100 o C.After that, the same film after healing was characterized by a tensile test as described above.Then, compared the pristine samples and calculated the recovery ratios.

Shape Memory Test
For the U-bending test 26 , a 15 mm × 5 mm rectangular bulk film with flat permanent shape was bent to U-shape and fixed by a drawing pin at 80 o C for 30 minutes subsequently quenching to room temperature.Then the fixed U-shape film was reheated at 80 o C for specific time for shape recovery.For 3D shape memory test, the details were described in results and discussion section.

Transmission Electron Microscopy (TEM).
TEM was carried out using a Thermo Fisher Themis 200 G3 electron microscope.The diameters and inter-particle distances of the SiO2 nanoparticles were determined from statistical analysis of the TEM micrographs using MatLab software.

Figure S1 .
Figure S1.Normalized derivative DSC heat flow curves.The onset and offset points of the glass transitions are highlighted in the figures.Measurements were performed at the third cooling cycle at a heating rate of 20 o C/min