Strain Stiffening Hydrogels through Self-Assembly and Covalent Fixation of Semi-Flexible Fibers

Biomimetic, strain-stiffening materials are reported, made through self-assembly and covalent fixation of small building blocks to form fibrous hydrogels that are able to stiffen by an order of magnitude in response to applied stress. The gels consist of semi-flexible rodlike micelles of bisurea bolaamphiphiles with oligo(ethylene oxide) (EO) outer blocks and a polydiacetylene (PDA) backbone. The micelles are fibers, composed of 9–10 ribbons. A gelation method based on Cu-catalyzed azide–alkyne cycloaddition (CuAAC), was developed and shown to lead to strain-stiffening hydrogels with unusual, yet universal, linear and nonlinear stress–strain response. Upon gelation, the X-ray scattering profile is unchanged, suggesting that crosslinks are formed at random positions along the fiber contour without fiber bundling. The work expands current knowledge about the design principles and chemistries needed to achieve fully synthetic, biomimetic soft matter with on-demand, targeted mechanical properties.     

Biomimetic Strain-Stiffening Self-Assembled Hydrogels

Supramolecular structures with strain-stiffening properties are ubiquitous in nature but remain rare in the lab. Herein, we report on strain-stiffening supramolecular hydrogels that are entirely produced through the self-assembly of synthetic molecular gelators. The involved gelators self-assemble into semi-flexible fibers, which thereby crosslink into hydrogels. Interestingly, these hydrogels are capable of stiffening in response to applied stress, resembling biological intermediate filaments system. Furthermore, strain-stiffening hydrogel networks embedded with liposomes are constructed through orthogonal self-assembly of gelators and phospholipids, mimicking biological tissues in both architecture and mechanical properties. This work furthers the development of biomimetic soft materials with mechanical responsiveness and presents potentially enticing applications in diverse fields, such as tissue engineering, artificial life, and strain sensors.     

Elastic Properties of Semi-Flexible Chains and Networks

Summary: Elastic properties of noncharged polymers of stiffness ranging from flexible to rigid chains are determined from Monte Carlo simulations. The discrete wormlike chain (WLC) model with self-interacting units is applied to chains of intermediate lengths. Elastic free energy and the force-extension profiles of chains of variable stiffness are computed in an isometric ensemble. Occurrence of a plateau on the force-extension curves at intermediate chain stiffness is noted. Qualitative differences are found between force profiles from simulations and from the standard (ideal) WLC model. The single-chain data on influence of bending stiffness were employed in the three-chain model of networks. Stress-strain relations for networks show a highly nonlinear behavior with the marked strain-stiffening effect.     

Dynamic swelling performance of hydrophobic hydrogels

Conventional gels manifest monotonous swelling or shrinking performance upon immersing in solvents until reaching an equilibrium state. Recently, we discovered that the “hydrophobic hydrogels” prepared from hydrophobic polymer networks demonstrated dynamic swelling performance without equilibrium states. Upon water immersion, the gels expanded tremendously at the first stage until reaching a swelling peak; subsequently, the gels shrunk at an extremely slow rate. While this phenomenon endows the ...     

Architecture of intracellular networks in plant matrices

Pectin is an integral component of plant cell walls. It is believed to form an interconnected network structure independent of the cellulose–xyloglucan network structure. Pectin gels are often used as a model for the pectin network structure within the plant cell wall. The middle lamella pectin can be extracted with chelating agents and is believed to be associated through cooperative binding of calcium ions in the so-called egg-box junction zones. Although a great deal is known about the nature of the junction zones in pectin gels, less is known about the long-range structure within calcium-set gels. Two plausible alternative models for long-range order in these gels are a pseudo rubber-like structure and a fibrous network structure. Atomic force microscopy studies of calcium-induced gel precursors, and fragments released from gels, suggest that association leads to a branched fibrous structure within the gels. Enzymatic de-esterification of high methoxy pectin in the presence of calcium ions can induce gelation of the pectin. Thus pectin gel networks may provide a model for a self-assembled network structure within the middle lamella region of the plant cell wall.     

A Simple and Versatile Method for the Construction of Nearly Ideal Polymer Networks

Polymer networks usually contain numerous inhomogeneities that deteriorate their physical properties and should be eliminated to create reliable, high‐performance materials. A simple method is introduced for the production of nearly ideal networks from various vinyl polymers through controlled polymerization and subsequent crosslinking. Monodisperse star polymers with bromide end groups were synthesized by atom‐transfer radical polymerization and end‐linked with dithiol linkers using thiol–bromide chemistry. This simple procedure formed nearly ideal polymer networks, as revealed from elasticity of the formed gel and model conjugation reactions involving linear polymers. The versatility of this method was demonstrated by preparing networks of common vinyl polymers, including polyacrylates, polymethacrylate, and polystyrene. This method can be used to prepare multiple functional nearly ideal gels and elastomers and to explore fundamental aspects of polymer networks.     

新型刺激响应性纤维素基含能凝胶的流变性能

研究了以不同基团含量的羧甲基纤维素硝酸酯(CMCN)为胶凝剂的含能凝胶细微结构与流变行为的关系.探讨了凝胶的形成机理,并采用线性的流变学方法研究了凝胶的屈服性、触变性、蠕变性及温敏性等动态黏弹性质,分别利用Herschel-Bulkley模型、Burger模型及Carreau-Yasuda模型对凝胶的流动曲线、蠕变曲线和频率曲线进行了数据拟合.研究发现,CMCN凝胶是由其分子结构上两亲性基团通过分子链间氢键及疏水键等非共价键相互作用形成的一种结构均匀的物理交联网络型凝胶.凝胶的非牛顿系数n均小于0.5.随着亲水性羧酸基团含量的增加,凝胶的屈服应力逐渐增大,触变恢复性逐渐增强,弹性与黏性柔量均减小,但其比值增加,蠕变的黏性响应性逐渐减弱而弹性响应性逐渐增强.凝胶的温敏性变化有一个力学松弛转变区,随着羧酸基团含量的增加,松弛转变区愈发明显,凝胶的温敏性也逐渐增强.     

Controlled Annealing in Adaptive Multicomponent Gels

We use a pH-driven annealing process to convert between co-assembled and self-sorted networks in multicomponent gels. The initially formed gels at low pH are co-assembled, with the two components coexisting within the same self-assembled structures. We use an enzymatic approach to increase the pH, resulting in a gel-to-sol transition, followed by a hydrolysis to lower the pH once again. As the pH decreases, a self-sorted network is formed by a two-stage gelation process determined by the pK_a of each component. This approach can be expanded to layered systems to generate many varied systems by changing composition and rates of pH change, adapting their microstructure and so allowing access to a far greater range of morphologies and complexity than can be achieved in single component systems.     

Linear rheology of water-soluble reversible neodymium(III) coordination polymers

The rheology of reversible coordination polymer networks in aqueous solution is studied. The polymers are formed by neodymium(III) ions and bifunctional ligands, consisting of two pyridine-2,6-dicarboxylate groups connected at the 4-positions by an ethylene oxide spacer. Neodymium(III) ions can bind three of these terdendate ligand groups. At high concentrations, the polymer networks yield viscoelastic materials, which can be described with the Maxwell model. The scaling of the elastic modulus, relaxation time, and zero-shear viscosity with concentration are in good agreement with the predictions of Cates' model that describes the dynamics of linear equilibrium polymers. This indicates that the networks have only few cross-links and can be described as linear equilibrium polymers. The gels are also thermo-reversible. At high temperatures, fast relaxation was found, resulting in liquidlike behavior. Upon cooling, the viscoelastic properties returned immediately. From the temperature dependence of the relaxation time, an activation energy of 49 kJ/mol was determined for the breaking and reptation of the polymers.     

Anisotropic ionogels of sodium laurate in a room-temperature ionic liquid

Anisotropic thermally reversible ionogels of sodium laurate (SL) were prepared in the first discovered room-temperature ionic liquid (RTIL), ethylammonium nitrate (EAN). Polarized optical microscope images indicate that the gels are birefringent, illuminating the presence of anisotropic structures. Small-angle X-ray scattering results reveal that SL and lauric acid (LA) molecules are arranged to form lamellar structures, but no SL crystallites were confirmed by the X-ray diffraction measurements. With an increase of the SL concentration, the interlayer distance decreases. Rheological measurements indicate that the anisotropic ionogels are highly viscoelastic and the storage modulus (G') increases with an increase of the SL concentration in EAN. Electrochemical measurements indicate that the anisotropic ionogels may have potential applications in electrochemical fields. The intermolecular hydrogen bond as well as the solvatophobic interaction of SL and LA formed by a chemical reaction, CH(3)(CH(2))(10)COONa + CH(3)CH(2)NH(3)NO(3) --> CH(3)CH(2)NH(2) upward arrow + NaNO(3) downward arrow + CH(3)(CH(2))(10)COOH, can play a role in the formation of three-dimensional networks having lamellar structures which are responsible for the anisotropic ionogels. The formation of anisotropic ionogels by surfactants in RTILs could be a new phenomenon, but this is not a very classic case of organogels.     

Anisotropic networks, elastomers and gels

Anisotropic networks, elastomers and gels exhibit piezoelectric, pyroelectric, ferroelectric and NLO properties of potential interest for use communication and processing technologies. The formation, properties and applications of such anisotropic, mainly liquid crystalline, networks are described. If some of the molecules in a liquid mixture contain at least two reactive groups which can be either photochemically or thermally polymerized, then crosslinked, anisotropic networks, elastomers and gels can be produced. Solid macroscopically aligned elastomers or networks can be formed as required beforehand or simultaneously by orientation of the sample. Anisotropic gels consist of a solid anisotropic network and non-covalently bonded, but strongly oriented domains of low molar mass liquid crystals. Anisotropic networks, elastomers preformed amorphous or liquid crystalline polymers incorporating additional reactive groups, which can be macroscopically oriented in the additional crosslinking reactions. Reversible networks, elastomers and gels can be prepared either non-covalently or covalently by thermally side group polymers and low molar mass molecules, liquid crystalline properties in the pure state. in many electro-optic devices for optical and gels can be prepared from liquid crystalline state and then fixed by reversible linkages between, for example, neither of which necessarily exhibit     

Distinct kinetic pathways generate organogel networks with contrasting fractality and thixotropic properties

The kinetics of the isothermal transformation of sols, comprised of a low molecular-mass organogelator (LMOG) and an organic liquid, to their organogel phases have been followed by circular dichroism (CD), fluorescence, small angle neutron scattering (SANS), and rheological methods. The thixotropic properties (in the sense that severe shearing followed by rest lead to reestablishment of viscoelasticity) of the gels have been examined as well by rheological measurements. The compositions of the samples were either 5alpha-cholestan-3beta-yl N-(2-naphthyl) carbamate (CNC) in an n-alkane (n-octane or n-dodecane) or 3beta-cholesteryl N-(2-naphthyl) carbamate (CeNC) in ethyl acetate. Values of Df, the mass fractal dimension of the microcrystalline self-assembled fibrillar networks (SAFINs) in the gels, have been extracted from the kinetic data using a model developed by Dickinson (J. Chem. Soc., Faraday Trans. 1997, 93, 111). The Df values, 1.1-1.3 for the CeNC gels and 1.3-1.4 or 1.6-1.8 (depending on the temperature of incubation of the sol phase) for CNC gels, are consistent with the gel network structures observed by optical microscopy. In addition, comparison of the temperature dependence of both n (the Avrami component) and K (the Avrami "rate constant") for CeNC/ethyl acetate gelation with those reported previously for gelation of CNC/n-alkane sols demonstrate that the very small change of a single bond in CNC to a double bond in CeNC causes significant differences in their gelation abilities and gel properties. The rheological measurements on CNC/n-alkane gels with spherulitic SAFIN units, formed by incubation of their sols at < or =28 degrees C, indicate that they are thixotropic. Gels with the same chemical composition but formed by incubation of their sols at > or =30 degrees C, leading to fiberlike SAFIN units, remain liquidlike after shearing regardless of the periods they are at rest. The time-dependent viscoelastic properties of the gel networks are treated according to a stretched exponential model. The observations from these studies provide detailed insights into the mechanisms of formation of molecular organogel phases and demonstrate the extreme sensitivity of the SAFINs and viscoelastic properties of such organogels to slight modifications in LMOG structure or sample history.     

Thermoreversible gelation of poly(vinylidene fluoride) in phthalates: the influence of aliphatic chain length of solvents

Thermoreversible gelation of poly(vinylidene fluoride) (PVDF) has been studied in a new series of solvents (phthalates), for example, dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DBP), and dihexyl phthalate (DHP) as a function of temperature and polymer concentration, both by test tube tilting and dynamic light scattering (DLS) method. The effect of aliphatic chain length (n) of diesters on the gelation kinetics, structure/microstructure and morphology of PVDF gels has been examined. Gelation rate was found to increase with increasing aliphatic chain length of diester. DLS results indicate that the sol-gel transformation proceeds via two-steps: first, microgel domains were formed, and then the infinite three-dimensional (3D) network is established by connecting microgels through polymer chains. The crystallites are responsible for 3D network for gelation in phthalates, and alpha-polymorph is formed during gelation producing higher amount of crystallinity with increasing aliphatic chain length of diester. Morphology of the networks of dried gels in different phthalates showed that fibril thickness and lateral dimensions decrease with higher homologues of phthalates. The scattering intensity is fitted with Debye-Bueche model in small-angle neutron scattering and suggested that both the correlation length and interlamellar spacing increases with n. A model has been proposed, based on electronic structure calculations, to explain the conformation of PVDF chain in presence of various phthalates and their complexes, which offer the cause of higher gelation rate for longer aliphatic chain length.     

新型含能纤维素基凝胶推进剂的流变性能研究

制备了以羧甲基纤维素硝酸酯(CMCN)为胶凝剂的硝基甲烷(NM)凝胶, 系统研究了CMCN 修饰基团比例变化所引起NM/CMCN凝胶流变性能差异性的规律. 探讨了凝胶的形成机理并采用线性的流变学方法研究了NM/CMCN凝胶的屈服性、触变性、蠕变性、温敏性等动态粘弹性质, 分别利用Herschel-Bulkley 模型、Burger 模型、Carreau-Yasuda模型、Andrade-Eyring 模型对凝胶的流动曲线、蠕变曲线、频率曲线和温敏曲线进行了数据拟合. 研究发现, NM/CMCN凝胶是由CMCN 大分子链上两亲性基团与有机小分子基团间通过分子链间疏水键、氢键等非共价键相互作用形成的一种物理交联网络型凝胶. 凝胶的非牛顿系数n均小于0.60. 随着硝酸酯基团含量的减少, NM/CMCN 凝胶的结构强度变弱, 表现出屈服应力逐渐减小、触变恢复性逐渐增强、粘性与弹性柔量均减小但粘性柔量比例增加的趋势; 凝胶的弹性响应性逐渐减弱而粘性响应性渐强, 内耗也逐渐增大;凝胶的活化能总体上增大, 温度稳定性变弱.     

NMR characterization of the formation kinetics and structure of di-O-benzylidene sorbitol gels self-assembled in organic solvents

The molecule 1,3:2,4-di-O-benzylidene sorbitol (DBS) is a common "gelator" that forms thermally reversible gels in diverse organic solvents. Solid-state (13)C and (1)H NMR techniques, along with electron microscopy, are utilized in an exploratory study of DBS in the gelled state where we consider both in situ and dried gels. The gels were formed in either acetone or benzene, with the former being a better solvent for DBS. We find the in situ or dried DBS gels to be composed of rigid twisted nanofibrils (～15 to 21 nm in diameter). The fibrils show local molecular ordering, but not crystalline order, and they contain no trapped solvent. The molecular mobility at the fibril surface is modestly enhanced, and all the free hydroxyl groups of the sorbitol moiety are involved in strong hydrogen bonding. We also attempted to find a truly crystalline form of DBS whose structure, as judged by the similarity of (13)C spectra, is close to that of the fibrils. We partially succeeded in this quest, employing melt crystallization followed by slow cooling. However, this sample was a mixed crystal having small domains, where only one type of domain was structurally similar to the fibrils. We also investigated the long-time evolution of the in situ DBS gel network. Specifically, high-resolution NMR kinetic studies were performed over periods of days where the residual concentration of DBS in acetone solution was monitored during and after gel formation. The DBS concentration on these long timescales evolved slowly, and we introduce a simple mathematical model and equation to describe this phenomenon.     

Nanocomposite Gels: New Advanced Functional Soft Materials

Summary: Nanocomposite gels (NC gels) consisting of poly(N-alkylacrylamides) and exfoliated inorganic clay were prepared by in-situ, free-radical polymerization at high yield under mild conditions (near ambient temperature, without stirring). Various shapes and different surface forms of NC gels were readily obtained using corresponding vessels and templates, since NC gels were always mechanically tough. The view that polymer/clay networks are formed in NC gels was supported by dynamic viscoelastic and swelling measurements. The entirely different mechanical properties of NC gels, compared with conventional, chemically-crosslinked hydrogels (OR gels), are discussed on the basis of their network structure. In addition to functions previously reported, NC gels exhibit further interesting characteristics, such as inherent incombustibility, good thermal conductivity, large heat capacity, they can be readily colored and fabricated as foams. NC gels can be utilized as environmentally-friendly, soft materials from the viewpoints of resources and waste, as their primary component is water.     

聚(醚-酐)凝胶性能的优化及用于包载难溶性药物的研究

在制备聚醚酐凝胶的基础上, 用两种不同的可光交联疏水性单体与聚醚酐大分子单体在紫外光引发下制备三维交联凝胶, 使得凝胶的溶胀性能和亲-疏水性能发生变化, 从而影响凝胶与难溶性药物的相容性; 选用吲哚美辛为模型药物, 通过后包合法将其包载于凝胶网络中, X射线衍射(XRD)检测结果表明, 药物能以分子或无定形态分散于其中, 优化后的凝胶可得到更高的载药量和包封率, 能有效地提高疏水性药物的溶出度, 且药物体外释放速率与单纯聚醚酐凝胶相比, 被有效延缓, 更适于临床应用.     

Gelation of a Model Globular Protein

Hen egg white lysozyme (HEWL) was exposed to various physical and chemical denaturing environments to encourage protein denaturation and consequent gelation. Its phase behavior was examined as a function of pH, temperature and also in the presence of the reductant dithiothreitol (DTT). Transparent viscoelastic gels form at low pH values while opaque gels form under alkaline conditions. No increase in viscosity was observed for systems in pure water unless 20 mM of DTT was added, which is known to break the disulfide bridges present in HEWL. The microstructure of the gel was studied using transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM). Gels formed at low pH contain fibrils ∼10 nm in diameter with various lengths while at high pH the gels are dominated by particulate aggregates. Thinner fibrils that are 4–6 nm in diameter are observed in the gels formed in the presence of DTT. In this case the distinct feature of the gels is they are thermoreversible and can be melted and reformed easily by varying the temperature.     

Rheological behavior and structural interpretation of waxy crude oil gels

A waxy crude oil which gels below a threshold temperature has been investigated under static and dynamic conditions, using a combination of rheological methods, optical microscopy, and DSC. Particular attention is given in this work to the influence of the mechanical history on gel strength and to describing the time-dependent rheological behavior. The gels display a strong dependence of the yield stress and moduli on the shear history, cooling rate, and stress loading rate. Of particular interest is the partial recovery of the gel structure after application of small stress or strain (much smaller than the critical values needed for flow onset) during cooling, which can be used to reduce the ultimate strength of the crude oil gel formed below the pour point. A second focus of this study is to further develop the physical interpretation of the mechanism by which wax crystallization produces gelation. Gelation of the waxy crude oil studied is suggested to be the result of the association between wax crystals, which produces an extended network structure, and it is shown that the system displays features common to attractive colloidal gels, for one of which, fumed silica (Aerosil 200) in paraffin oil, rheological data are reported. The colloidal gel model provides a simple and economical basis for explaining the response of the gelled oil to various mechanical perturbations and constitutes a fruitful basis from which to develop technologies for controlling the gelation phenomenon, as suggested by the rheological results reported.