Stable polymer brushes with effectively varied grafting density synthesized from highly crosslinked random copolymer thin films

Threshold of Surface Initiator Concentration for Polymer Brush Growth by Surface-Initiated Atom Transfer Radical Polymerization

The surface modification of various materials by grafting functional molecules has attracted much attention from fundamental research to practical applications because of its ability to impart various physical and chemical properties to the surfaces. One promising approach is the use of polymer brushes synthesized by atom transfer radical polymerization (ATRP) from surface-tethered initiators (SIs). In this study, for the purpose of controlling the grafting amounts/densities of polymer brushes, we developed a facile method to precisely regulate SI concentrations of SI layers (SILs) by serial dilution based on a sol-gel method. By simply mixing organosilanes terminated with and without an initiator group ((p-chloromethyl) phenyltrimethoxysilane (CMPTMS) and phenyltrimethoxysilane (PTMS), respectively) with tetraethoxysilane (TEOS), SI concentrations of SILs could be arbitrarily tuned precisely by varying dilution factors of (CMPTMS + PTMS)/CMPTMS (DFs, 1-107). The resulting SILs prepared at different DFs were highly smooth and transparent. X-ray photoelectron spectroscopy (XPS) also confirmed that the SIs were homogeneously distributed at the topmost surface of the SILs and their concentrations were proven to be accurately and precisely controlled from high to extremely low, comparable to theoretical values. Subsequent SI-ATRP in air ("paint-on" SI-ATRP) of two different types of monomers (hydrophobic/nonionic (2,3,4,5,6-pentafluorostyrene) and hydrophilic/ionic (sodium 4-styrenesulfonate)) demonstrated that polymer brushes with different grafting amounts/densities were successfully grafted only from SILs with DFs of 1-104 (theoretical SI concentrations: 3.9 × 10-4 ∼ 3.5 units/nm2), while at DFs of 105 and above (theoretical SI concentrations: <3.9 × 10-5 units/nm2), no sign of polymer brush growth was confirmed by thickness, XPS, and water contact angle data. Therefore, we are the first to gather evidence that the approximate threshold of SI concentration required for "paint-on" SI-ATRP might be on the order of 10-4 ∼ 10-5 units/nm2.

Synthetic strategies to enhance the long-term stability of polymer brush coatings

High-density, end-anchored macromolecules that form so-called polymer brushes are popular components of bio-inspired surface coatings. In a bio-memetic approach, they have been utilized to reduce friction, repel contamination and control...

Multistimuli-Responsive Optical Hydrogel Nanomembranes to Construct Planar Color Display Boards for Detecting Local Environmental Changes

Planar metal-insulator-metal (MIM) optical cavities are attractive for biochemical and environmental sensing applications, as they offer a cost-effective cavity platform with acceptable performances. However, localized detection and scope of expansion of applicable analytes are still challenging. Here, we report a stimuli-responsive color display board that can exhibit local spectral footprints, for locally applied heat and alcohol presence. A thermoresponsive, optically applicable, and patternable copolymer, poly(N-isopropylacrylamide-r-glycidyl methacrylate), is synthesized and used with a photosensitive cross-linker to produce a responsive insulating layer. This layer is then sandwiched between two nanoporous silver membranes to yield a thermoresponsive MIM cavity. The resonant spectral peak is blue-shifted as the environmental temperature increases, and the dynamic range of the resonant peak is largely affected by the composition and structure of the cross-linker and the copolymer. The localized temperature increase of silk particles with gold nanoparticles by laser heating can be measured by reading the spectral shift. In addition, a free-standing color board can be transferred onto a curved biological tissue sample, allowing us to simultaneously read the temperature of the tissue sample and the concentration of ethanol. The stimuli-responsive MIM provides a new way to optically sense localized environmental temperature and ethanol concentration fluctuations.

Photoinduced Modulation of Polymeric Interfacial Behavior Controlling Thin-Film Block Copolymer Wetting

The tunable surface-wetting properties of photosensitive random copolymer mats were used to spatially control the orientations of thin-film block copolymer (BCP) structures. A photosensitive mat was produced via thermal treatment on spin-coated random copolymers of poly(styrene-ran-2-nitrobenzyl methacrylate-ran-glycidyl methacrylate), synthesized via reversible-deactivation radical polymerization. The degree of UV-induced deprotection of the nitrobenzyl esters in the mat was precisely controlled through the amount of UV-irradiation energy imparted to the mat. The resulting polarity switching of the constituents collectively altered the interfacial wetting properties of the mat, and the tunability allowed lamellar or cylinder-forming poly(styrene-b-methyl methacrylate) BCP thin films, applied over the mat, to change the domain orientation from perpendicular to parallel at proper UV exposures. UV irradiation passing through a photomask was capable of generating defined regions of BCP domains with targeted orientations.