Thermoplastic elastomer hydrogel networks, based on swelling of nanostructured blends of amphiphilic, sphere-forming AB diblock and ABA triblock copolymers, provide direct access to thermally processable plastics that exhibit exceptional elastic recovery and fatigue resistance even after hydration. In such two-component systems, the ratio of ABA triblock copolymer to AB diblock copolymer is used to control the resultant swelling ratio, system modulus, and overall mechanical response. In this report, we introduce a simplified one-component alternative which exploits a single-component, photoreactive AB diblock copolymer precursor to controllably generate ABA triblock copolymer in situ during melt-processing. This was accomplished using efficient photoinduced [4 + 4] cycloaddition (λ = 365 nm) between terminal anthracene units on a ω-anthracenylpolystyrene-b-poly(ethylene oxide) diblock copolymer precursor (SO-anth, fPS = 0.13, Mn = 70 100 g mol–1) to produce the desired amount of polystyrene-b-poly(ethylene oxide)-b-polystyrene (SOS) triblock copolymer. The amount of SOS triblock copolymer formed was tunable (from 11.7 to 45 mol %) using UV exposure time (2 to 20 min, ∼30 mW cm–2), giving direct control over swelling and mechanical properties in the resultant hydrogels produced upon subsequent vitrification of the melt sample followed by addition of water. Hydrogels produced in this manner were found to exhibit dynamic shear moduli and shape preservation characteristics typical of preblended, two-component SO/SOS TPE hydrogels of similar SOS concentrations.

This report introduces a unique method of significantly improving toughness in highly swollen block copolymer-based thermoplastic elastomer (TPE) hydrogels by converting an intrinsically large population of dangling chain ends into a mechanically active second network. In one form, the TPE hydrogels developed by our group are based on swelling of a vitrified melt-blend of two amphiphilic block copolymer species, sphere-forming polystyrene-poly(ethylene oxide) (SO-H) diblock and triblock (SOS) copolymers. Here, the PEO midblock in the SOS triblock copolymer serves to tether adjacent PS spherical aggregates, producing hydrogel networks that are incredibly elastic and mechanically robust, preserving their shape even at the very high intrinsic swelling ratios produced at low SOS concentrations (e.g, 37 g of H2O/(g of polymer) at 3.3 mol % SOS). In this report, we advance the utility of this framework by exploiting the hundreds of dangling PEO chain ends per spherical aggregate to form a second, mechanically active network. The approach is based on a stepwise installation of two tethering SOS triblock copolymer populations. The first is present directly during melt-state self-assembly of the original diblock/triblock copolymer blend and inherently determines the equilibrium swelling ratio of resulting hydrogel. The second population is then introduced postswelling, by simply coupling the dangling SO diblock copolymer chain ends under conditions largely free of the mechanical stress osmotically imposed on the primary network. Notably, this action simply shifts the ratio of diblock and triblock copolymer without compromising the thermoplasticity of the network. Here, we use the facile water-based coupling of PEO-terminal azide and alkyne groups to demonstrate the scale of toughness enhancements possible through conversion of dangling ends into a second network. The dangling-end double networks produced exhibit remarkable improvements in tensile properties (tensile modulus, toughness, strain at break, and stress at break), including a 58-fold increase in mean toughness (to 361 kJ/m3) and a 19-fold increase in mean stress to break (to 169 kPa) in highly swollen samples containing up to 95% (g/g) water. Importantly, these improvements could be realized without altering water content, shape, small-strain dynamic shear, and unconfined compressive properties of the original TPE hydrogels.

Using a unique one-pot convergent anionic polymerization strategy, 18 (polystyrene)star-b-(polyisoprene)linear-b-(polystyrene)star (SnISn) pom-pom triblock copolymers were synthesized varying a range of architectural parameters including PS arm molecular weight (Mn,star), the number of arms contained in the star (n), and the PI midblock molecular weight (Mn,PI). A selected series of five of these 18, in which Mn,star was held approximately constant between 14.3 and 16.5 kDa, but with the numbers of arms in the star and PI midblock molecular weight varied, were selected for detailed characterization using rheology, AFM, and SAXS. The five selected all shared PS as the minority component, with star volume fractions (fPS) varying between 0.11 and 0.22. All samples showed clear phase separation, with three of the five adopting a highly ordered hexagonal packing of cylinders (HPC) confirmed through SAXS and AFM. The remaining two systems were limited to liquid-like packing of cylindrical domains (LLP). Longer midblock molecular weights and increased numbers of arms in the star both showed a propensity to hinder formation of a highly ordered hexagonal lattice. Increasing the number of arms in the star also favored transitions to a disordered phase at lower temperatures when overall SnISn molecular weight was held constant. The behavioral trends identified suggest interfacial packing frustration plays a prominent role in determining the ability of the system to develop highly ordered periodic structures. The chain crowding produced by the PS star architecture intrinsically favors interfacial curvature toward the majority PI component, contrary to that intrinsically favored by the block composition alone. In the two systems in which the frustration was architecturally most severe (largest n of 7.1, highest Mn,PI of 191 kDa), evolution of a hexagonal lattice could not be induced, even after significant thermal annealing. The pom-pom architecture itself also appears to have a significant impact on entanglement relaxation dynamics, with development of HPC morphologies only possible at elevated temperatures.

Tethered micelle hydrogel networks based on the solution assembly of amphiphilic ABA-type block copolymers are prevalent throughout the hydrogel literature. However, the mechanical response of such systems is often determined largely by the integrity of the micellar core produced during solution assembly, not by the elements of the network structure upon which it is based. Using a solvent-free fabrication method based on the melt-state self-assembly of sphere-forming polystyrene-b-poly(ethylene oxide) (SO) diblock and SOS triblock copolymers blends, we have been able to produce tethered micelle hydrogel networks with fully vitrified cores that enable the elements of the network structure to determine the mechanical response. Here, we explore the impact of using PEO midblocks of different lengths within the SOS tethers, in an effort to elucidate the role played by water content, tether concentration, and tether length in mechanical property determination. In doing so, we were able to establish coronal layer overlap as the primary contributing factor in regulating the dynamic elastic moduli exhibited by tethered micelle systems. Variation of either tether concentration or tether length could be used to tune the degree of coronal layer overlap, enabling direct and accurate control over hydrogel mechanical response. While such control is likely a unique feature of the melt-state fabrication approach applied here, the conclusions with respect to the role of coronal layer overlap and tether (bridging) concentration in determining the mechanical potential of the network should be applicable to all ABA-type tethered micelle systems, regardless of fabrication methodology.

•Diblock copolymers were formed by ATRP of styrene and imidazolium-styrene monomers.•SAXS was used to detect the formation of ordered, phase-separated nanostructures.•Nanostructure formation depends on block ratio and imidazolium alkyl side-chain size.A series of imidazolium-based noncharged-charged diblock copolymers (1) was synthesized by the direct, sequential ATRP of styrene and styrenic imidazolium bis(trifluoromethyl)sulfonamide monomers with methyl, n-butyl, and n-decyl side-chains. Small-angle X-ray scattering studies on initial examples of 1 with a total of 50 repeat units and styrene:imidazolium-styrene repeat unit ratios of 25:25, 20:30, and 15:35 showed that their ability to form ordered nanostructures (i.e., sphere and cylinder phases) in their neat states depends on both the block ratio and the length of the alkyl side-chain on the imidazolium monomer. To our knowledge, the synthesis of imidazolium-based BCPs that form ordered, phase-separated nanostructures via direct ATRP of immiscible co-monomers is unprecedented.

The development of nanostructured polymeric systems containing directionally continuous poly(ionic liquid) (poly(IL)) domains has considerable implications toward a range of transport-dependent, energy-based technology applications. The controlled, synthetic integration of poly(IL)s into block copolymer (BCP) architectures provides a promising means to this end, based on their inherent ability to self-assemble into a range of defined, periodic morphologies. In this work, we report the melt-state phase behavior of an imidazolium-containing alkyl–ionic BCP system, derived from the sequential ring-opening metathesis polymerization (ROMP) of imidazolium- and alkyl-substituted norbornene monomer derivatives. A series of 16 BCP samples were synthesized, varying both the relative volume fraction of the poly(norbornene dodecyl ester) block (fDOD = 0.42–0.96) and the overall molecular weights of the block copolymers (Mn values from 5000–20 100 g mol–1). Through a combination of small-angle X-ray scattering (SAXS) and dynamic rheology, we were able to delineate clear compositional phase boundaries for each of the classic BCP phases, including lamellae (Lam), hexagonally packed cylinders (Hex), and spheres on a body-centered-cubic lattice (SBCC). Additionally, a liquid-like packing (LLP) of spheres was found for samples located in the extreme asymmetric region of the phase diagram, and a persistent coexistence of Lam and Hex domains was found in lieu of the bicontinuous cubic gyroid phase for samples located at the intersection of Hex and Lam regions. Thermal disordering was opposed even in very low molecular weight samples, detected only when the composition was highly asymmetric (fDOD = 0.96). Annealing experiments on samples exhibiting Lam and Hex coexistence revealed the presence of extremely slow transition kinetics, ultimately selective for one or the other but not the more complex gyroid phase. In fact, no evidence of the bicontinuous network was detected over a 2 month annealing period. The ramifications of these results for transport-dependent applications targeting the use of highly segregated poly(IL)-containing BCP systems are carefully considered.

Direct access to nanostructured hydrogel networks through high fidelity photocuring of sphere-forming block copolymer melts is demonstrated. Hydrophobic junction points within the hydrogel network are based on an underlying lattice of body-centered cubic spheres (SBCC), produced via melt-state self-assembly of blended AB diblock and ABA triblock copolymer amphiphiles. Integrated thermally stable photocuring chemistry allows for in situ trapping of these spherical domains, independent from the required melt processing necessary to achieve the highly ordered BCC lattice. Swelling of the photocured solids in aqueous (and organic) media afforded highly elastic gels exhibiting excellent mechanical properties (G′ ∼ 103 Pa) and complete preservation of the cured solid shape. The hydrogels fabricated in this study were produced from partially epoxidized (19.6%, relative to diene repeat units) blends of polybutadiene-b-poly(ethylene oxide) diblock (PB–PEO, fPB = 0.13, Mn = 29 500 g mol–1, 88.5 mol %) and PB–PEO–PB triblock (fPB = 0.13, Mn = 59 000 g mol–1, 11.5 mol %) copolymers synthesized via anionic polymerization. Addition of UV-activated cationic photoinitiator (4-iodophenyl)diphenylsulfonium triflate (0.5 mol %) produced composite samples exhibiting a highly ordered SBCC morphology after annealing at moderate temperatures (4 h at 80 °C or 60 s at 140 °C) above the PEO melting transition. Composite films (0.33 mm thickness) were then photocured directly from the melt state, postanneal. Cured samples retained the SBCC structure with extremely high fidelity, effectively prestructuring the network of junction points prior to swelling. The photopatterning potential of these uniquely designed hydrogels is also demonstrated.

In this work, we report the novel application of thermally stable photocuring chemistry toward high fidelity translation of block copolymer based melt-state morphologies into their equivalent solid analogues. The thermal stability of the cationic photcuring chemistry allows for both extended thermal processing prior to cure, as well as precise trapping of selected morphologies afforded by the temperature independent initiation mechanism. We demonstrate this powerful approach using a model polyisoprene-b-poly(ethylene oxide) (PI−PEO, fPEO = 0.39, Mn = 10 120 g mol−1) block copolymer, prepared by two step anionic polymerization and subsequent partial epoxidation (7.3−16.8 mol % relative to diene repeat units) with 3-chloroperoxybenzoic acid. Small angle X-ray scattering (SAXS) and dynamic rheology were used to determine morphological behavior of the block copolymers synthesized. The targeted PI−PEO parent block copolymer exhibited multiple melt-state morphologies including crystalline lamellae (Lc), hexagonally packed cylinders (C), bicontinuous gyroid (G), and a final isotropic disordered state (Dis). The partial epoxidation and the addition of the UV activated cationic photoinitiator (4-iodophenyl)diphenylsulfonium triflate (1.0−1.25 mol %) acted only to shift transition temperatures between phases, without disturbing the overall morphological sequence present in the neat, unmodified PI−PEO block copolymer. Exposure of the photacid/block copolymer blends to UV radiation at selected temperatures permitted successful permanent trapping of both the cylinder and gyroid morphologies from a single block copolymer sample, as verified by pre- and postcure SAXS measurements. Importantly, this approach should be applicable to any block copolymer in which cationically polymerizable functional groups can be incorporated.

A new structural motif for the generation of highly distensible, highly elastic, nanostructured hydrogels is presented. Based on the swelling of vitrified melt-phase blends of sphere-forming polystyrene–poly(ethylene oxide) diblock and polystyrene–poly(ethylene oxide)–polystyrene triblock copolymers, the equilibrium swelling ratio (3.8–36.9 g H2O per g polymer) and dynamic elastic modulus (G′ = 1700–160000 Pa) of these novel hydrogel systems were found to be remarkably tunable through simple manipulation of temperature (10–50 °C) and triblock copolymer content (3.3–72.0 mol%). Mechanical properties were found to be almost exclusively a function of triblock copolymer content, independent of temperature induced changes in swelling ratio. The resulting hydrogels were highly elastic at all swelling ratios with G′/G″ ≈ 102 for the range of triblock copolymer concentrations examined. Hydrogel samples exhibited excellent preservation of dry polymer shape upon swelling, with complete recovery of both shape and mechanical performance following repeated compression–decompression cycles.