Block polymers incorporating highly incompatible segments are termed "high chi" polymers, where chi is the Flory-Huggins interaction parameter. These materials have attracted a great deal of interest because low molar mass versions allow for the formation of rnicrophase-separated domains with very small (<10 nm) feature sizes useful for nanopatterning at these extreme dimensions. Given that well-established photolithographic techniques now face difficult challenges of implementation at scales of 10 nm and below, the drive to further develop high chi block polymers is motivated by trends in the microelectronics industry. This Viewpoint highlights our perspective on this niche of block polymer self-assembly. We first briefly review the relevant recent literature, exploring the various block polymer compositions that have been specifically designed for small feature size patterning. We then overview the now standard method for the benchmarking chi values between different pairs of polymers and the consequences of low N and high chi on the thermodynamic aspects of microphase separation. Finally, we comment on restrictions going forward and offer our perspective on the future of this exciting area of block polymer self-assembly.
Herein we report an aqueous photoinitiated polymerization-induced self-assembly (photo-PISA) for the preparation of a remarkably diverse set of complex polymer nanoparticle morphologies (e.g., spheres, worms, and vesicles) at room temperature. Ultrafast polymerization rates were achieved, with near quantitative monomer conversion within 15 min of visible light irradiation. An important feature of the photo-PISA is that diblock copolymer vesicles can be prepared under mild conditions (room temperature, aqueous medium, visible light), which will be important for the preparation of functional vesicles loaded with biorelated species (e.g., proteins). As a proof of concept, silica nanoparticles and bovine serum albumin (BSA) were encapsulated in situ within vesicles via the photo-PISA process.
Photochemically induced ATRP was performed with visible light and sunlight in the presence of parts per million (ppm) copper catalysts. Illumination of the reaction mixture yielded polymerization in case of 392 and 450 nm light but not for 631 nm light. Sunlight was also a viable source for the photoinduced ATRP. Control experiments suggest photoreduction of the Cu-II complex (ligand to metal charge transfer in the excited state), yielding a Cu-I complex, and a bromine radical that can initiate polymerization. No photo-activation of Cu-I complex was detected. This implies that the mechanism of ATRP in the presence of light is a hybrid of ICAR and ARGET ATRP. The method was also used to synthesize block copolymers and polymerizations in water.
The ruthenium-based photoredox catalyst, Ru(bpy)(3)Cl-2, was employed to activate reversible addition fragmentation chain transfer (RAFT) dispersion polymerization via a photoinduced electron transfer (PET) process under visible light (lambda = 460 nm, 0.7 mW/cm(2)). Poly(oligo(ethylene glycol) methyl ether methacrylate) was chain extended with benzyl methacrylate to afford in situ self-assembled polymeric nanoparticles with various morphologies. The effect of different intrinsic reaction parameters, such as catalyst concentration, total solids content, and cosolvent addition was investigated with respect to the formation of different nanoparticle morphologies, including spherical micelles, worm-like micelles, and vesicles. Importantly, highly pure worm-like micelles were readily isolated due to the in situ formation of highly viscous gels. Finally, "ON/OFF" control over the dispersion polymerization was demonstrated by online Fourier transform near-infrared (FTNIR) spectroscopy, allowing for temporal control over the nanoparticle morphology.
This report describes the synthesis and characterization of boronate ester-cross-linked hydrogels capable of self-healing behavior at neutral and acidic pH. This atypically wide pH range over which healing behavior is observed was achieved through the use of an intramolecular coordinating boronic acid monomer, 2-acrylamidophenylboronic acid (2APBA), where the internal coordination helped to stabilize cross-links formed at acidic and neutral pH. Two different hydrogels were formed from a 2APBA copolymer cross-linked with either poly(vinyl alcohol) or a catechol-functionalized copolymer. The self-healing ability of these hydrogels was characterized through physical testing and rheological studies. Furthermore, the catechol cross-linked hydrogel was shown to be oxygen sensitive, demonstrating reduced self-healing and stress relaxation after partial oxidation. The synthesis of these hydrogels demonstrates a new strategy to produce boronic acid materials capable of self-healing at physiological pH.
Organic catalysis in ring-opening polymerization (ROP) has become a powerful alternative to more traditional metal-based catalysts. The field has developed to a point at which there are not only excellent low cost and easy to use organocatalysts for day-to-day polymerizations, but the ability to precisely control the synthesis of advanced polymer architectures and ROP monomers that are extremely challenging to polymerize with other catalysts now exists. This viewpoint article will highlight the key advances in organocatalyst design with the aim of encouraging the wider application of organic catalysts in ROP.
The field of sustainable polymers is growing and evolving at unprecedented rates. Researchers are increasingly concerned with the feedstock origins and the degradation behavior of, especially, large-scale commodity packaging plastics. A perspective is offered here for the design of sustainable polymers, specifically addressing opportunities for monomer development and polymer degradation. Key concepts include: water degradability instead of biodegradability; incorporation of novel main-chain functionality, such as acetals; utilization of lignin-based aromatics; and direct polymerization of biogenic C1 feedstocks.
The development of hydrogels for cartilage replacement and soft robotics has highlighted a challenge: load-bearing hydrogels need to be both stiff and tough. Several approaches have been reported to improve the toughness of hydrogels, but simultaneously achieving high stiffness and toughness remains difficult. Here we report that alginate-polyacrylamide hydrogels can simultaneously achieve high stiffiiess and toughness. We combine short-and long-chain alginates to reduce the viscosity of pregel solutions and synthesize homogeneous hydrogels of high ionic cross-link density. The resulting hydrogels can have elastic moduli of similar to 1 MPa and fracture energies of similar to 4 kJ m(-2). Furthermore, this approach breaks the inverse relation between stiffness and toughness: while maintaining constant elastic moduli, these hydrogels can achieve fracture energies up to similar to 16 kJ m(-2). These stiff and tough hydrogels hold promise for further development as load-bearing materials.
Dynamic polymer hydrogels with an environmental adaptive self-healing ability and dual responsive sol-gel transitions were prepared by combining acylhydrazone and disulfide bonds together in the same system. The hydrogel can automatically repair damage to it under both acidic (pH 3 and 6) and basic (pH 9) conditions through acylhydrazone exchange or disulfide exchange reactions. However, the hydrogel is not self-healable at pH 7 because both bonds are kinetically locked, whereas the hydrogel gains self-healing ability by accelerating acylhydrazone exchange with the help of catalytic aniline. All of the self-healing processes are demonstrated to be effective without an external stimulus at room temperature in air. The hydrogel also displays unique reversible sol-gel transitions in response to both pH (HCl/triethylamine) and redox (DTT/H2O2) triggers.
We report the use of visible light to mediate a RAFT dispersion polymerization in the absence of external catalyst or initiator to yield nanoparticles of different morphologies according to a polymerization-induced self-assembly (PISA) mechanism. A POEGMA macro-chain transfer agent (macro-CTA) derived from a 4-cyano-4-((dodecylsulfanylthiocarbonyl)sulfanyl)pentanoic acid (CDTPA) RAFT agent can be activated under blue (460 nm, 0.7 mW/cm(2)) or green (530 nm, 0.7 mW/cm(2)) light and act simultaneously as a radical initiator, chain transfer agent, and particle stabilizer under ethanolic dispersion conditions. In particular, the formation of worm-like micelles was readily monitored by the increase of reaction viscosity during the polymerization; this method was shown to be particularly robust to different reaction parameters such as macro-CTAs of varying molecular weight. Interestingly, at high monomer conversion, different morphologies were formed depending on the wavelength of light employed, which may be due to differing degrees of polymerization control. Finally, the in situ encapsulation of the model hydrophobic drug, Nile Red, was demonstrated, suggesting applications of this facile process for the synthesis of nanoparticles for drug delivery applications.
Living radical polymerization of acrylates and acrylamides from trithiocarbonate iniferters using a compact fluorescent lamp (CFL) bulb and 10-phenylphenothiazine as an organic photoredox catalyst is reported. With this system, chain growth can be efficiently switched between "on" and "off" in response to visible light. Polymer molar masses increase linearly with conversion, and narrow molar mass distributions are obtained. The excellent fidelity of the trithiocarbonate-iniferter enables the preparation of triblock copolymers from macro-iniferters under the same visible-light mediated protocol, using UV light without a photoredox catalyst or under traditional thermally induced RAFT conditions. We expect that the simplicity and efficiency of this metal-free, visible-light-mediated polymerization will enable the synthesis and modification of a range of materials under mild conditions.
The development of an operationally simple, metal-free surface-initiated atom transfer radical polymerization (SI-ATRP) based on visible-light mediation is reported. The facile nature of this process enables the fabrication of well-defined polymer brushes from flat and curved surfaces using a "benchtop" setup that can be easily scaled to four-inch wafers. This circumvents the requirement of stringent air-free environments (i.e., glovebox), and mediation by visible light allows for spatial control on the micron scale, with complex three-dimensional patterns achieved in a single step. This robust approach leads to unprecedented access to brush architectures for nonexperts.
In this communication, we report the preparation and characterization of new shape memory assisted self-healing (SMASH) coatings. The coatings feature a phase-separated morphology with electrospun thermoplastic poly(epsilon-caprolactone) (PCL) fibers randomly distributed in a shape memory epoxy matrix. Mechanical damage to the coating can be self-healed via heating, which simultaneously triggers two events: (1) the shape recovery of the matrix to bring the crack surfaces in spatial proximity, and (2) the melting and flow of the PCL fibers to rebond the crack. In controlled healing experiments, damaged coatings not only heal structurally, but also functionally by almost completely restoring the corrosion resistance. We envision the wide applicability of the SMASH concept in designing the next-generation self-healing materials.
Vitrimers, strong organic glass formers, are covalent networks that are able to change their topology through thermoactivated bond exchange reactions. At high temperatures, vitrimers can flow and behave like viscoelastic liquids. At low temperatures, exchange reactions are very long and vitrimers behave like classical thermosets. The transition from the liquid to the solid is reversible and is, in fact, a glass transition. By changing the content and nature of the catalyst, we can tune the transesterification reaction rate and show that the vitrimer glass transition temperature and the broadness of the transition can be controlled at will in epoxy-based vitrimers. This opens new possibilities in practical applications of thermosets such as healing or convenient processability in a wide temperature range.
While much research has been aimed at new approaches to the healing of cross-linked polymers, there is a dearth of effort in the healing of renewable cross-linked polymers. In this study, the healing ability of renewable polylactide-based vitrimers containing Sn(Oct)(2) was investigated. The vitrimers were prepared from hydroxyl-terminated star-shaped poly((+/-)-lactide) samples that were cross-linked with methylenediphenyl diisocyanate. Cross-linked samples were prepared with isocyanate:hydroxyl (IC:OH) ratios of 0.6:1 to 1.1:1 and initial catalyst:hydroxyl (Sn(Oct)(2):OH) ratios ranging from 0.013:1 to 0.050:1. The plateau modulus and T-g values for the cross-linked samples decreased with decreasing IC:OH ratio. The materials were subjected to stress relaxation analyses, which confirmed that the materials not only were vitrimeric in nature but also exhibited remarkably short characteristic relaxation times of less than 50 s at 140 degrees C. Finally, vitrimers fractured in uniaxial tensile testing were healed by compression molding and exhibited up to 67% recovery of ultimate elongation, up to 102% recovery of tensile strength, and up to 133% recovery of tensile modulus values.