Mixed Polymer Brushes for “Smart” Surfaces

Mixed polymer brushes (MPBs) are composed of two or more disparate polymers covalently tethered to a substrate. The resulting phase segregated morphologies have been extensively studied as responsive “smart” materials, as they can be reversible tuned and switched by external stimuli. Both computational and experimental work has attempted to establish an understanding of the resulting nanostructures that vary as a function of many factors. This contribution highlights state-of-the-art MPBs studies, covering synthetic approaches, phase behavior, responsiveness to external stimuli as well as novel applications of MPBs. Current limitations are recognized and possible directions for future studies are identified

Oxygen Tolerance in Surface-Initiated Reversible Deactivation Radical Polymerizations: Are Polymer Brushes Turning into Technology?

Over the past three decades, the development of reversible deactivation radical polymerizations (RDRP), and advancements toward more user-friendly and accessible experimental setups have opened the door for nonexperts to design complex macromolecules with well-defined properties. External mediation, improved tolerance to oxygen, and increased reaction volumes for higher synthetic output are some of the many noteworthy technical improvements. The development of RDRPs in solution was paralleled by their application on solid substrates to synthesize surface-grafted "polymer brushes" via surface-initiated RDRP (SI-RDRP). This Viewpoint paper provides a current perspective on recent developments in SI-RDRP methods that are tolerant to oxygen, especially highlighting those that could potentially enable scaling up of the synthesis of brushes for the functionalization of technologically relevant materials

Photolabile SI-PET-RAFT Initiators for Wavelength-Selective Grafting and De-grafting of Polymer Brushes

Polymer brushes offer a wide array of applications in surface modification. While recent advances have made these chemistries more user-friendly, scientific questions about fundamental polymer properties often remain unanswered. For example, copolymer brush composition, chain end fidelity, and dispersity often remain prohibitively challenging to characterize. This conundrum produces a need for chemically precise pathways to evaluate polymer brushes. To this end, this contribution describes the synthesis of an o-nitrobenzyl-based photolabile initiator for surface-initiated reversible deactivation radical polymerization. The product can be immobilized on surfaces, enable growth of polymer brushes under visible light, and be cleaved under ultraviolet (UV) irradiation. Wavelength selectivity is confirmed using a combination of ellipsometry, tensiometry, and X-ray photoelectron spectroscopy, and patterning experiments indicate good spatial control over photocleaving. Finally, nuclear magnetic resonance spectroscopy indicates visibility of characteristic peaks for both chain ends after degrafting of the polymer brush

Neutral Hydrolysis of Post-Consumer Polyethylene Terephthalate Waste in Different Phases

Post-consumer polyethylene terephthalate (PET) was hydrolyzed in pure water over a wide range of temperatures (190–400 °C) and pressures (1–35 MPa) to produce terephthalic acid (TPA). Solid or molten PET was subjected to water as a saturated vapor, superheated vapor, saturated liquid, compressed liquid, and supercritical fluid. The highest TPA yields were observed for the hydrolysis of molten PET in saturated liquid water. Isothermal and non-isothermal hydrolysis of PET was also explored. Rapidly heating the reactor contents at about 5–10 °C/s (“fast” hydrolysis) led to high TPA yields, as did isothermal PET hydrolysis, but within 1 min instead of 30 min. Notably, these conditions resulted in the lowest environmental energy impact metric observed to date for uncatalyzed hydrolysis

Cross-Linking Density and Temperature Effects on the Self-Assembly of SiO2-PNIPAAm Core-Shell Particles at Interfaces

SiO2PNIPAAm coreshell microgels (PNIPAAm=poly(N-isopropylacrylamide)) with various internal cross-linking densities and different degrees of polymerization were prepared in order to investigate the effects of stability, packing, and temperature responsiveness at polarapolar interfaces. The effects were investigated using interfacial tensiometry, and the particles were visualized by cryo-scanning electron microscopy (SEM) and scanning force microscopy (SFM). The coreshell particles display different interfacial behaviors depending on the polymer shell thickness and degree of internal cross-linking. A thicker polymer shell and reduced internal cross-linking density are more favorable for the stabilization and packing of the particles at oilwater (o/w) interfaces. This was shown qualitatively by SFM of deposited, stabilized emulsion droplets and quantitatively by SFM of particles adsorbed onto a hydrophobic planar silicon dioxide surface, which acted as a model interface system. The temperature responsiveness, which also influences particleinterface interactions, was investigated by dynamic temperature protocols with varied heating rates. These measurements not only showed that the particles had an unusual but very regular and reversible interface stabilization behavior, but also made it possible to assess the nonlinear response of PNIPAAm microgels to external thermal stimuli

Orientation-Dependent Order–Disorder Transition of Block Copolymer Lamellae in Electric Fields

Electric fields have been shown to stabilize the disordered phase of near-critical block copolymer solutions. Here, we use in situ synchrotron small-angle X-ray scattering to examine how the initial orientation of lamellar domains with respect to the external field (φ) affects the shift in the order–disorder transition temperature (<i>T</i>_ODT) of lyotropic solutions of poly­(styrene-<i>b</i>-isoprene) in toluene. We find a downward shift of the transition temperature, which scales with lamellar orientation as Δ<i>T</i>_ODT ∼ cos² φ, in accordance with theory

Novel Strategy for Photopatterning Emissive Polymer Brushes for Organic Light Emitting Diode Applications.

A light-mediated methodology to grow patterned, emissive polymer brushes with micron feature resolution is reported and applied to organic light emitting diode (OLED) displays. Light is used for both initiator functionalization of indium tin oxide and subsequent atom transfer radical polymerization of methacrylate-based fluorescent and phosphorescent iridium monomers. The iridium centers play key roles in photocatalyzing and mediating polymer growth while also emitting light in the final OLED structure. The scope of the presented procedure enables the synthesis of a library of polymers with emissive colors spanning the visible spectrum where the dopant incorporation, position of brush growth, and brush thickness are readily controlled. The chain-ends of the polymer brushes remain intact, affording subsequent chain extension and formation of well-defined diblock architectures. This high level of structure and function control allows for the facile preparation of random ternary copolymers and red-green-blue arrays to yield white emission

Block Copolymer Nanocomposites in Electric Fields: Kinetics of Alignment

We investigate the kinetics of block copolymer/nanoparticle composite alignment in an electric field using in situ transmission small-angle X-ray scattering. As a model system, we employ a lamellae forming polystyrene-<i>block</i>-poly­(2-vinyl pyridine) block copolymer with different contents of gold nanoparticles in thick films under solvent vapor annealing. While the alignment improves with increasing nanoparticle fraction, the kinetics slows down. This is explained by changes in the degree of phase separation and viscosity. Our findings provide extended insights into the basics of nanocomposite alignment