Materials often exhibit a trade-off between stiffness and extensibility; for example, strengthening elastomers by increasing their cross-link density leads to embrittlement and decreased toughness. Inspired by cuticles of marine mussel byssi, we circumvent this inherent trade-off by incorporating sacrificial, reversible iron-catechol cross-links into a dry, loosely cross-linked epoxy network. The iron-containing network exhibits two to three orders of magnitude increases in stiffness, tensile strength, and tensile toughness compared to its iron-free precursor while gaining recoverable hysteretic energy dissipation and maintaining its original extensibility. Compared to previous realizations of this chemistry in hydrogels, the dry nature of the network enables larger property enhancement owing to the cooperative effects of both the increased cross-link density given by the reversible iron-catecholate complexes and the chain-restricting ionomeric nanodomains that they form.

Phase separation and coacervate complex formation of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) were investigated as model pair of oppositely charged, weak polyelectrolytes in aqueous solution. Both fully or partially neutralized PAA (sodium polyacrylate) and PAH were employed. Important factors affecting the complexation were systematically varied including the polyacid/polybase mixing ratio (10−90 wt %), ionic strength as salt concentration (0−4700 mM), polymer concentration (0.02−2.0 wt %), pH (5 and 7), and temperature (30−75 °C). Sample turbidity was utilized as an indicator of polyelectrolyte complex formation. Phase separation in the solution was also observed by optical microscopy in the distinguishable forms of either precipitate or coacervate. In the absence of salt, polyelectrolyte complexation always resulted in the formation of a precipitate. In the presence of sodium chloride, complex formation does not take place (neither precipitate nor coacervate) when either polyelectrolyte is present in large excess. Increasing salt concentration causes a change from solid precipitate to fluid coacervate phase, and finally a one-phase polyelectrolyte solution is obtained. Temperature affected the precipitate-to-solution transition only in the case of samples with low concentrations of either PAA or PAH. The data generated led to the construction of phase diagrams that illustrate how the various parameters control the demixing and the precipitate−coacervate−solution phase transitions. We find such phase diagrams for simple, flexible synthetic macromolecular systems to be rare in the polymer science literature. Ternary phase diagrams were prepared, which showed the influence of relative polymer and salt concentration on the phase behavior of the aqueous PAA/PAH system. We believe data such as these will both improve both the reliable applications of polymer coacervates and the development of new macromolecular assemblies based on charge complexation.

Phase separation of polyelectrolyte complexes (PECs) between the polyacid (sodium salt) and polybase (hydrochloride) of poly(acrylic acid) (PAA) and poly-(allylamine) (PAH), respectively, has been investigated in aqueous solution. Chain length of the PAA was varied (25 < P w < 700) holding P w of the PAH constant at 765. The polyacid/ polybase mixing ratio (10−90 wt %) and the ionic strength as salt concentration (0−3,000 mM) were systematically varied. Sample turbidity was utilized as an indicator of PEC formation, complemented by optical microscopy for discrimination between precipitate and coacervate. Salt-free systems always resulted in PEC precipitates; however, coacervates or polyelectrolyte solutions, respectively, were formed upon exceeding critical salt concentrations, the PEC formation also depending on the employed PAA/PAH ratio. The lower the PAA molecular weight, the lower were the critical salt concentrations required for both the precipitate/coacervate and coacervate/solution transitions. The experimental phase behavior established here is explained by molecular models of coacervate complexation, addressing effects of polyelectrolyte molecular weight and salt screening.

Numerous attempts have been made to translate mussel adhesion to diverse synthetic platforms. However, the translation remains largely limited to the Dopa (3,4-dihydroxyphenylalanine) or catechol functionality, which continues to raise concerns about Dopa's inherent susceptibility to oxidation. Mussels have evolved adaptations to stabilize Dopa against oxidation. For example, in mussel foot protein 3 slow (mfp-3s, one of two electrophoretically distinct interfacial adhesive proteins in mussel plaques), the high proportion of hydrophobic amino acid residues in the flanking sequence around Dopa increases Dopa's oxidation potential. In this study, copolyampholytes, which combine the catechol functionality with amphiphilic and ionic features of mfp-3s, were synthesized and formulated as coacervates for adhesive deposition on surfaces. The ratio of hydrophilic/hydrophobic as well as cationic/anionic units was varied in order to enhance coacervate formation and wet adhesion properties. Aqueous solutions of two of the four mfp-3s-inspired copolymers showed coacervate-like spherical microdroplets (ϕ ≈ 1−5 μm at pH ∼4 (salt concentration ∼15 mM). The mfp-3s-mimetic copolymer was stable to oxidation, formed coacervates that spread evenly over mica, and strongly bonded to mica surfaces (pull-off strength: ∼17.0 mJ/m 2 ). Increasing pH to 7 after coacervate deposition at pH 4 doubled the bonding strength to ∼32.9 mJ/m 2 without oxidative cross-linking and is about 9 times higher than native mfp-3s cohesion. This study expands the scope of translating mussel adhesion from simple Dopa-functionalization to mimicking the context of the local environment around Dopa. M arine mussels (Figure 1a) attach to hard surfaces, e.g., mineral and metal, in the intertidal zone where waves with and without suspended sand often exceed 25 m/sec velocities. 3,4-Dihydroxyphenylalanine (Dopa), a main constituent in mussel foot proteins (mfps) and substantially contributing to wet adhesion, has been incorporated in synthetic polymers to mimic the bio wet-adhesion. 1−5 However, other constitutional features of mfps, e.g., cationic residues (lysine, K), anionic residues (aspartic acid, D), nonionic polar residues (asparagine, N), and nonpolar residues (alanine, A), have not typically been included in mussel-inspired synthetic wet-adhesion systems. 1,2Here, we studied the microphase behavior and wet-adhesion of copolyampholytes with fixed catechol content and varied other key functionalities. Potential effects of aromatic moieties (Tyr, Trp) besides Dopa in mfp-3s have not been specifically tested in the present structural design of the model copolyampholytes. Conditions for the experiments were adjusted according to the microenvironmental conditions of adhesive protein deposition under the mussel's foot including acidic to neutral pH and ionic strength of ≤100 mM. 3,4 In mussel adhesion, polyelectrolyte adhesive proteins or mfps are presented to target surfaces after being condensed as a dense fluid by complex coacervation, a critical ste...

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Several acrylates and methacrylates were copolymerized with two 4-aminoazobenzene methacrylic derivatives. The photochemical and thermal cis-trans isomerization about the azolink was investigated in the bulk polymers. All the copolymers show photochromic behaviour without fading; the photostationary states upon irradiation with UV-light of the appropriate wavelengths were similar in bulk and in solution. The thermal cis-trans isomerization follows simple first order kinetics in the rubbery specimens as in solution. Below the glass transition temperature T8, the isomerization can only be described by two simultaneous first order reactions. Depending on the conditions of irradiation and on the reaction temperature, a portion of the azoaromatic compound reacts anomalously fast. The temperature dependence of the relaxation time of the isomerization reaction in all the polymer films investigated can be represented by a single WLF-type graph; the corresponding Arrhenius plot is curved above Tg. The findings are interpreted by considering the effects of the free volume and of the chain segmental mobility on the cis-trans isomerization. A rotational and a translational relaxation mechanism is discussed for the normal and the anomalously fast reaction below TB. *) In the following the same abbreviations are used for the other copolymers, too, i.e. CoP(EMA-la) means copolymer of ethyl methacrylate and la, etc.

We determined the self part of the intermediate scattering function in liquid polyethyleneoxide ͑PEO͒ and PEO-alkali iodide complexes by means of neutron spin-echo spectroscopy and molecular dynamics ͑MD͒ computer simulations. We present the first accurate quantitative results on the segmental dynamics in the time range up to 1 ns and the wave-vector range from a few nm Ϫ1to approximately 20 nm Ϫ1 . We investigate the influence of polymer chain length, salt concentration, and cation type. We find that the neutron data and MD data for pure PEO agree very well. A relatively small concentration of dissolved salt ͑1 metal ion per 15 monomers͒ leads to a slowing down of the segmental motions by an order of magnitude. Here, the MD simulations agree qualitatively. Increasing the chain length from 23 to 182 monomers has no significant effect except at the highest salt concentration. Similarly, changing the cation from Li to Na hardly makes any difference. The Rouse model does not adequately describe our data. © 2000 American Institute of Physics. ͓S0021-9606͑00͒51525-6͔ Amorphous polymer electrolytes provide an environment-friendly alternative for liquid electrolytes used in batteries, fuel cells, electrochemical displays, and chemical sensors.1 A polymer electrolyte is a complex of a polar polymer with a metal salt. In order to optimize performance of applications, it is of fundamental importance to understand the mechanism of ion transport, which is closely coupled to the segmental motions of the polymer chain. The systems most studied are poly͑ethyleneoxide͒ ͑PEO͒ and poly͑propyleneoxide͒ ͑PPO͒ salt complexes.From Brillouin light scattering of PPO-salt systems 2 and MD simulations of PEO-NaI systems 3 it appears that the Na ϩ ions form crosslinks between different oxygen atoms within a polymer chain, which causes slowing down of movement of polymer segments. Quasielastic neutron scattering measurements on the PPO-LiClO 4 complex have confirmed this effect, but because of the limited energy resolution it was impossible to obtain quantitative results for the effect of solvated salt on the structural relaxation.4 Londono et al. 5 have performed neutron diffraction with isotopic Li substitution in combination with MD simulations in order to determine the partial pair distribution function g Li,O (r). They obtained a Li-O coordination number of about 3.5 for PEOLiI ͑O:Mϭ5, which is the number of ether oxygens of the polymer chain per metal ion͒, confirming crosslinking between cations and ether oxygens. It has been shown that the conductivity characteristics for PPO-Li salt and PPO-Na salt are very similar.6 Therefore, we expect that the influence of Li and Na on the polymer dynamics in PEO is similar.Until today, no quantitative results were available on the local dynamics of the backbone segments of the polymer nor on the influence of various parameters such as salt concentration, polymer chain length, and different ions. Neutron spin echo ͑NSE͒ is the technique of choice regarding energy resolution and wave-vector rang...