Towards controlled polymer brushes via a self-assembly-assisted-grafting-to approach

Precise synthesis of polymer brushes to modify the surface of nanoparticles and nanodevices for targeted applications has been one of the major focuses in the community for decades. Here we report a self-assembly-assisted-grafting-to approach to synthesize polymer brushes on flat substrates. In this method, polymers are pre-assembled into two-dimensional polymer single crystals (PSCs) with functional groups on the surface. Chemically coupling the PSCs onto solid substrates leads to the formation of polymer brushes. Exquisite control of the chain folding in PSCs allows us to obtain polymer brushes with well-defined grafting density, tethering points and brush conformation. Extremely high grafting density (2.12 chains per nm2) has been achieved in the synthesized single-tethered polymer brushes. Moreover, polymer loop brushes have been successfully obtained using oddly folded PSCs from telechelic chains. Our approach combines some of the important advantages of conventional ‘grafting-to' and ‘grafting-from' methods, and is promising for tailored synthesis of polymer brushes.

For the synthesis of TEG-PCL30-OH, 1.545 g CL, 0.2 g initiator and 31 μL tin(II) 2ethylhexanoate were mixed in a Schlenk flask until homogeneous. The mixture was degassed and refilled with nitrogen 5 times before it was immersed into an oil bath at 110 oC. After 24 hr, the flask was quenched to room temperature to stop reaction. The mixture was then dissolved in dichloromethane and added dropwisely into cold methanol toG-OH, everything else was kept constant while 0.42 g initiator was added instead of 0.2 g.
In a typical reaction, 150 mg hydroxyl-teminated PCL was dissolved in 10 mL anhydrous dichloromethane. To this solution 0.7 mL (3-isocyanatopropyl)triethoxysilane and 0.85 μL dibutyltin dilaurate were added. The reaction proceeded under the protection of nitrogen for 12 hr before the functionalized PCL was collected by precipitating the solution into cold hexane. The functionalized polymer (TEG-PCL30-SiOR and TEG-
For the synthesis of diPCL-2SiOR, 100 mg of diPCL-2OH was dissolved in 10 mL anhydrous dichloromethane. To this solution 0.5 mL of 3-(triethoxysilyl)propyl isocyanate and 0.6 mg of dibutyltin dilaurate was added. The reaction was allowed to proceed at room temperature for 12 hr. After the reaction was stopped, the solution was concentrated using rotary evaporator and precipitated into cold methanol. DiPCL-2SiOR was collected via filtration and stored under vacuum at room temperature for characterizations and further reaction.
For the synthesis of diPCL-2SH, 100 mg of diPCL-2OH was dissolved in 3 mL 3mercaptopropionic acid. The solution was degassed and refilled with N 2 three times, and 20 mg of CALB was added into the solution, followed by two additional degas-refill N 2 cycles. The reaction was carried out at 60 o C for 48 hr before the solution was precipitated into cold methanol. DiPCL-2SH was collected by filtration and stored under vacuum at room temperature for characterizations and further reactions.

Crystallization of functionalized PCL with controlled lamellae thickness via self-seeding method
For all PCL samples, self-seeding method was used to grow single crystals in dilute 1-butanol solution with concentration around 0.03 wt.%. In a typical crystallization process, PCL was firstly dissolved in 1-butanol at 60 o C. The solution was then stored at -20 o C for 2 hr to allow fast crystallization of PCL. Immediately after that, the crystal suspension was kept at seeding temperature (T s =42 o C) for 10 min to form crystal seeds.
After that, PCL was allowed to crystallize at specific T c for at least 48 hr before the single crystals were separated from the solution via multiple centrifugation. The crystals were finally redispersed in 1-butanol for further reactions. Different seeding temperature (T s ) and crystallization temperature (T c ) were chosen for different PCL samples as summarized in Supplementary Table 1.

Preparation of PCL brushes on solid substrates using PSCs as templates
Single-tethered polymer brushes (STPBs) on flat glass surfaces were prepared using PSCs grown from TEG-PCL-SiOR as templates. Polymer loop brushes (PLBs) were prepared using PSCs grown from diPCL-SiOR as templates. Before all immobilization process, the glass substrates were cleaned using hot piranha solution for 30 mins to remove organic contaminants. PSCs were firstly deposited onto the substrates via spincoating method in order to form separated islands of PSCs for detailed atomic force microscopy (AFM) characterizations. After the deposition process, the coated glass slides were transferred to a sealed chamber to which 300 μL of ammonia was added. The reaction was allowed to proceed for 20 min before the ammonia was removed, and the coated glass slides were washed thoroughly using acetone by sonication bath for 10 min.
After that, the substrates were blown dry using nitrogen at ambient condition, and stored at room temperature for characterizations.
To prepare polymer brush coating with macroscopic coverage on glass substrate, dropcasting of PSCs instead of spincoating was adopted. After the solvent was completely evaporated, the polymer chains were allowed to reaction with glass surface via similar ammonia-catalyzed procedure described above. The coated substrate was also washed thoroughly using acetone in sonication bath for 10 min before further characterizations.
PCL loop brushes on flat gold substrate were prepared using PSCs grown from diPCL-2SH as templates. The Au substrates were prepared by thermal evaporation deposition of Cr followed by Au on a clean Si wafer, and cleaned using hot piranha solution prior to crystal immobilization. To prepare brushes, diPCL-2SH PSCs were dropcasted from crystal suspension in 1-butanol onto Au substrate. After the solvent evaporated, the PSCs were allowed to reaction with Au under vacuum for 2 days. The unbounded polymer was then washed away by acetone via sonication for 10 min and dried under nitrogen. The coated substrates were stored under vacuum until further characterizations. To oxidize either diPCL-2SH PSCs or PLBs on Au substrates, the coated substrates were left under ambient lab condition for 2 days. After that, these functionalized surfaces were used for X-ray photoelectron spectroscopy (XPS) measurements.