Gelation, Phase Separation, and Fibril Formation in Aqueous Hydroxypropylmethylcellulose Solutions.

The thermoresponsive behavior of a hydroxypropylmethylcellulose (HPMC) sample in aqueous solutions has been studied by a powerful combination of characterization tools, including rheology, turbidimetry, cryogenic transmission electron microscopy (cryoTEM), light scattering, small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS). Consistent with prior literature, solutions with concentrations ranging from 0.3 to 3 wt % exhibit a sharp drop in the dynamic viscoelastic moduli G' and G″ upon heating near 57 °C. The drop in moduli is accompanied by an abrupt increase in turbidity. All the evidence is consistent with this corresponding to liquid-liquid phase separation, leading to polymer-rich droplets in a polymer-depleted matrix. Upon further heating, the moduli increase, and G' exceeds G″, corresponding to gelation. CryoTEM in dilute solutions reveals that HPMC forms fibrils at the same temperature range where the moduli increase. SANS and SAXS confirm the appearance of fibrils over a range of concentration, and that their average diameter is ca. 18 nm; thus gelation is attributable to formation of a sample-spanning network of fibrils. These results are compared in detail with the closely related and well-studied methylcellulose (MC). The HPMC fibrils are generally shorter, more flexible, and contain more water than with MC, and the resulting gel at high temperatures has a much lower modulus. In addition to the differences in fibril structure, the key distinction between HPMC and MC is that the former undergoes liquid-liquid phase separation prior to forming fibrils and associated gelation, whereas the latter forms fibrils first. These results and their interpretation are compared with the prior literature, in light of the relatively recent discovery of the propensity of MC and HPMC to self-assemble into fibrils on heating.

Extensional Flow Behavior of Methylcellulose Solutions Containing Fibrils.

The extensional properties of semidilute aqueous methylcellulose (MC) solutions have been characterized. Pure aqueous MC solutions are shear-thinning liquids at room temperature. With the addition of 8 wt % NaCl, a fraction of MC self-assembles into long fibrils, which modify the rheological properties of the original MC solution. Capillary Breakup Extensional Rheometry (CaBER) was used to characterize salt-free and 8 wt % NaCl solutions of MC at room temperature. The salt-free solutions exhibit only power-law behavior whereas solutions with NaCl exhibit both power-law and elastic regimes. As MC concentration increases, the extensional relaxation time also increases strongly, from 0.04 s at 0.5 wt % to 4 s at 1 wt %. In addition, the apparent extensional viscosity rapidly increases as a function of increasing MC concentration, from 40 Pa·s at 0.5 wt % to 1300 Pa·s at 1 wt %. This behavior is attributed to the presence of fibrils in the MC solutions containing NaCl.

Robust Polymer Electrolyte Membranes with High Ambient-Temperature Lithium-Ion Conductivity via Polymerization-Induced Microphase Separation.

Mechanically robust polymer electrolyte membranes (PEMs) exhibiting high ionic conductivity at ambient temperature are a prerequisite for next-generation electrochemical devices. We utilized a polymerization-induced microphase separation (PIMS) strategy to prepare nanostructured materials comprising continuous conducting nanochannels intertwined with a mechanically and thermally robust cross-linked polymeric framework. Addition of succinonitrile (SN) rendered the poly(ethylene oxide)/lithium (Li) salt conducting domains completely amorphous, resulting in outstanding conductivities (∼0.35 mS/cm) at 30 °C. Concurrently, a densely cross-linked polystyrene framework provided mechanical robustness (modulus E' ≈ 0.3 GPa at 30 °C) to the hybrid material. This work highlights a facile, single-pot strategy involving a homogeneous liquid reaction precursor that yields a high-performance ion-conducting membrane attractive for lithium-battery applications.

Self-Supporting, Hydrophobic, Ionic Liquid-Based Reference Electrodes Prepared by Polymerization-Induced Microphase Separation.

Interfaces of ionic liquids and aqueous solutions exhibit stable electrical potentials over a wide range of aqueous electrolyte concentrations. This makes ionic liquids suitable as bridge materials that separate in electroanalytical measurements the reference electrode from samples with low and/or unknown ionic strengths. However, methods for the preparation of ionic liquid-based reference electrodes have not been explored widely. We have designed a convenient and reliable synthesis of ionic liquid-based reference electrodes by polymerization-induced microphase separation. This technique allows for a facile, single-pot synthesis of ready-to-use reference electrodes that incorporate ion conducting nanochannels filled with either 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide or 1-dodecyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide as ionic liquid, supported by a mechanically robust cross-linked polystyrene phase. This synthesis procedure allows for the straightforward design of various reference electrode geometries. These reference electrodes exhibit a low resistance as well as good reference potential stability and reproducibility when immersed into aqueous solutions varying from deionized, purified water to 100 mM KCl, while requiring no correction for liquid junction potentials.

2-Hydroxyethylcellulose and Amphiphilic Block Polymer Conjugates Form Mechanically Tunable and Nonswellable Hydrogels.

Herein, we report a family of mechanically tunable, nonswellable hydrogels that are based on a 2-hydroxyethylcellulose (HEC) scaffold grafted with amphiphilic diblock copolymers. Poly[(oligo(ethylene glycol)methyl ether methacrylate]-b-poly(methyl methacrylate) (POEGMA-b-PMMA) diblock copolymers of different compositions were created via RAFT polymerization using an alkyne terminated macro chain transfer agent (CTA). 2-Hydroxyethylcellulose (HEC) was modified with azide groups and the diblock copolymers were attached to the backbone via the copper-catalyzed click reaction to yield HEC-g-(POEGMA-b-PMMA) graft terpolymers. The resulting conjugates were soluble in DMF and able to form hydrogels upon simple solvent exchange in water. By increasing the concentration of the conjugates in DMF, the storage moduli of the hydrogels increased and the pore size in the gel decreased. After hydrogel formation, the structures were also found to be nonswellable (no macroscopic volume change upon incubation in water), which is an important feature for retaining size and mechanical integrity of the gels over time. Moreover, these materials were able to be electrospun into fibers that, upon hydration, formed fibrous hydrogel structures. The nonswellable and tunable mechanical properties of these materials imply great potential for a variety of applications such as personal care, active delivery, and tissue engineering.

Linear and Nonlinear Rheological Behavior of Fibrillar Methylcellulose Hydrogels.

Cryogenic transmission electron microscopy and small-angle neutron scattering recently have revealed that the well-known thermoreversible gelation of methylcellulose (MC) in water is due to the formation of fibrils, with a diameter of 15 ± 2 nm. Here we report that both the linear and nonlinear viscoelastic response of MC solutions and gels can be described by a filament-based mechanical model. In particular, large-amplitude oscillatory shear experiments show that aqueous MC materials transition from shear thinning to shear thickening behavior at the gelation temperature. The critical stress at which MC gels depart from the linear viscoelastic regime and begin to stiffen is well predicted from the filament model over a concentration range of 0.18-2.0 wt %. These predictions are based on fibril densities and persistence lengths obtained experimentally from neutron scattering, combined with cross-link spacings inferred from the gel modulus via the same model.