Flexible Crosslinking Research Articles

Thermal and mechanical properties of new Main-Chain Liquid-Crystalline Elastomers

New Main-Chain Liquid-Crystalline Elastomers (MCLCEs) were synthesized based on reacting vinyloxy-terminated mesogens under hydrosilylation conditions with a flexible crosslinker. These main-chain systems showed smectic and nematic mesophases and their anisotropic properties were mechanically and thermally analysed as function of the crosslinking density. Due to the suitable chemistry used in this work low crosslinking densities have been achieved (2.5 mol-%) with low soluble content (5%). For the first time, the degree of crosslinking could be adjusted and nematic or smectic MCLCEs with tuneable thermal and mechanical properties were obtained.

Actin Crosslinking Proteins Recognize Distinct Arrangements of Actin Filaments

Self-assembly of complex structures is commonplace in biology but often poorly understood. In the case of the actin cytoskeleton, a great deal is known about the components that comprise higher order structures, such as lamellar meshes, filopodial bundles and stress fibers. Each of these cytoskeletal structures contains actin filaments and crosslinking proteins, but the role of crosslinking proteins in the initial steps of structure formation has not been clearly elucidated. We employ a novel optical trapping assay to investigate the behaviors of fascin and alpha-actinin during the first steps of structure assembly. Here we show that these proteins have distinct binding characteristics that cause them to recognize and crosslink filaments that are arranged with specific geometries. Alpha-actinin is a promiscuous crosslinker, linking filaments over a range of angles. These angles include parallel (00), anti-parallel (1800), and series of intermediate crossing angles (15-1650). It is also an extremely flexible crosslinker, maintaining connection even when the link is rotated. Conversely, fascin is extremely selective, only crosslinking filaments in a parallel orientation. This means that for fascin bundles to form the actin filaments must occupy a parallel orientation before the structure can be stabilized. In the case of alpha-actinin structures, crosslinks could form at any point during the process of structure assembly. We have shown that crosslinking proteins recognize specific orientations of actin filaments, which places constraints on how cytoskeletal structures assemble and organize.

Flexible cross-linking of shape memory polyurethane by a long polyethyleneglycol spacer

Flexible linking of shape memory polyurethane (SMPU) was tried with a long polyethyleneglycol (PEG) spacer (PEG-600 or PEG-1000) longer than the one tested before (PEG-200). The SMPU was composed of MDI, polytetramethyleneglycol (PTMG), 1,4-butanediol (BD), glycerol, and PEG as a spacer. PEG connected the glycerol hydroxyl group of polyurethane chain via MDI as a connecting agent. Significant increase in both maximum stress and strain at break was attained simultaneously, because of the flexible structure of PEG. Especially, it was shown in the stress-strain curve that the PEG-linked SMPU showed similar behavior and superior tensile mechanical properties to natural rubber. Shape recovery of the PEG-linked SMPU went up to 96 %, and shape retention remained low compared to linear one. PEG-1000 was better than PEG-600 in enhancing the overall tensile strength and shape memory of SMPU.

Thermal and Mechanical Properties of New Main-Chain Liquid-Crystalline Elastomers

New Main-Chain Liquid-Crystalline Elastomers (MCLCEs) were synthesised based on reacting vinyloxy-terminated mesogens under hydrosilylation conditions with a flexible crosslinker. These main-chain systems showed smectic and nematic mesophases and their anisotropic properties were mechanically and thermally analysed as function of the crosslinking density. Due to the suitable chemistry used in this work low crosslinking densities have been achieved (2.5 mol-%) with low soluble content (5%). For the first time, the degree of crosslinking could be adjusted and nematic or smectic MCLCEs with tuneable thermal and mechanical properties were obtained.

Evaluation of the mechanical properties of lyocell textile materials crosslinked with 2,4‐diacrylamidobenzenesulfonic acid under swollen and nonswollen conditions

AbstractLyocell fabrics were treated with 2,4‐diacrylbenzenesulfonic acid (DABS) crosslinking agent under wet and dry reaction conditions, and also for comparison with a conventional dimethyloldihydroxyethylene urea (DMDHEU) agent, reacted under dry conditions. All treatments with DABS led to an improvement in wet and also dry fabric abrasion resistance, measured using the Martindale test, whereas treatment with DMDHEU reduced dry abrasion resistance. A model based on the kinetic strength theory of Zhurkov was used to interpret results, where dry‐state abrasion is reduced through stress concentration at the rigid DMDHEU crosslinks, which does not occur with the larger flexible DABS crosslinks. Tensile testing of treated lyocell fabrics revealed that breaking strength initially increased at low levels of DABS fixation, but fell at similar levels of DMDHEU fixation. The effect with DABS is due to an increase in the effective molecular weight, which can outweigh the development of stress concentration due to crosslinking. The Zhurkov model was also used to interpret the wet‐state abrasion resistance of lyocell fabrics. From this it is concluded that water swelling leads to an increase in stress concentration within fiber structure because of the loss of lateral interfibrillar connectivity, which is reestablished by both DMDHEU and DABS crosslinking. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Effective-medium approach for stiff polymer networks with flexible cross-links

Recent experiments have demonstrated that the nonlinear elasticity of in vitro networks of the biopolymer actin is dramatically altered in the presence of a flexible cross-linker such as the abundant cytoskeletal protein filamin. The basic principles of such networks remain poorly understood. Here we describe an effective-medium theory of flexibly cross-linked stiff polymer networks. We argue that the response of the cross-links can be fully attributed to entropic stiffening, while softening due to domain unfolding can be ignored. The network is modeled as a collection of randomly oriented rods connected by flexible cross-links to an elastic continuum. This effective medium is treated in a linear elastic limit as well as in a more general framework, in which the medium self-consistently represents the nonlinear network behavior. This model predicts that the nonlinear elastic response sets in at strains proportional to cross-linker length and inversely proportional to filament length. Furthermore, we find that the differential modulus scales linearly with the stress in the stiffening regime. These results are in excellent agreement with bulk rheology data.

Mechanics of Biophysical Networks with Flexible Cross-links

Various mechanical properties and functions of euchariotic cells largely originate from the cytoskeleton. The predominant cytoskeletal constituent is the biopolymer filamentous actin (F-actin). In the presence of various cross-linking proteins, F-actins can comprise two rather different structures: isotropic orthogonal networks or bundled fibers. Actin bundles are formed mostly by short and stiff cross-linking proteins (like α-actinin and scruin), while large and flexible cross-linkers, such as filamin, lead to an orthogonal network. Orthogonal networks can also be formed at lower concentrations of short cross-linking proteins, but rheological experiments of in vitro F-actin networks showed that the mechanical response of such networks is different from that of networks cross-linked with filamin. Moreover, atomic force microscope stretching experiments on filamin demonstrated the possibility of force-induced domain unfolding, characterized by a sawtooth-like pattern in the force-displacement curve.Here we present a 3D discrete model of F-actin networks that extends our previous, rigidly cross-linked network model by incorporating a flexible cross-linking model for human filamin A (hsFLNa). The implemented hsFLNa element has a highly nonlinear response to stretching, incorporating the transition to a softer response that characterizes filamin domain unfolding. Simple shear simulations of F-actin/hsFLNa networks show that the response of such networks is dominated by the behavior of the hsFLNa cross-linkers, while F-actin behaves almost rigid. We observe that force-induced unfolding of the hsFLNa relaxes the stresses in actin filaments, thus allowing for large network strains. By contrast, the shearing of F-actin networks with rigid cross-links leads to a large number of actin filaments stressed well beyond their breaking force. An increase in actin concentration increases the initial shear modulus, while the maximum network stiffening depends on the hsFLNa axial stiffness. The calculated initial modulus of F-actin/hsFLNa networks is found to be comparable with experimental measurements.

Microvoid formation and strain hardening in highly cross-linked polymer networks

Using molecular dynamics simulations of a generic model, we observe strain hardening in highly cross-linked polymer glasses under tensile deformation. We show that formation of microvoids, without bond breaking, constitutes the microscopic origins of strain hardening. A well-defined functional form is observed for the void size distribution that is consistent with voids in dense equilibrium Lennard-Jones particle packings, independent of strain. Microvoid-based strain hardening is not observed in a separate model with tetrahedral bond angle constraints, indicating that flexible cross-linkers are the key factor in the development of strain hardening behavior.

Homogeneous Poly(acrylamide) Hydrogels Made by Large Size, Flexible Dimethacrylate Cross-Linkers

Figure 1. (A) G′(filled symbols) and G′′(open symbols) of the reaction system in the presence of DMA-4 shown as a function of the reaction time. DMA-4 ) 0.3 (9), 2.0 (b), 3.3 ((), and 5 mol % (.). (B) The effective cross-link density νe of the hydrogels shown as a function of the cross-linker concentration. The solid curves are a guide to the eye. x ) 1( ∆ ), 2( 2 ), 4( b ), 9( O), and 14 (1).

Novel Structures for α-Actinin:F-Actin Interactions and their Implications for Actin–Membrane Attachment and Tension Sensing in the Cytoskeleton

We have applied correspondence analysis to electron micrographs of 2-D rafts of F-actin cross-linked with α-actinin on a lipid monolayer to investigate α-actinin:F-actin binding and cross-linking. More than 8000 actin crossover repeats, each with one to five α-actinin molecules bound, were selected, aligned, and grouped to produce class averages of α-actinin cross-links with ∼9-fold improvement in the stochastic signal-to-noise ratio. Measurements and comparative molecular models show variation in the distance separating actin-binding domains and the angle of the α-actinin cross-links. Rafts of F-actin and α-actinin formed predominantly polar 2-D arrays of actin filaments, with occasional insertion of filaments of opposite polarity. Unique to this study are the numbers of α-actinin molecules bound to successive crossovers on the same actin filament. These “monofilament”-bound α-actinin molecules may reflect a new mode of interaction for α-actinin, particularly in protein-dense actin–membrane attachments in focal adhesions. These results suggest that α-actinin is not simply a rigid spacer between actin filaments, but rather a flexible cross-linking, scaffolding, and anchoring protein. We suggest these properties of α-actinin may contribute to tension sensing in actin bundles.

Adhesion of membranes via switchable molecules

Biomimetic membranes that adhere to a solid substrate or another interface via switchable crosslinker molecules are studied theoretically using analytical methods and Monte Carlo simulations. The flexible crosslinkers exhibit two conformations which have a different end-to-end distance and, thus, lead to different local separations of the membrane from the substrate surface. Transitions between the molecular conformations can be induced by light, electric potential, or changes in pH and lead to active shape fluctuations of the membrane and, thus, to an increased membrane roughness. The forward and backward transitions are characterized by two transition rates, omega(+) and omega(-), respectively, which define the average fraction X=omega(+) /(omega(+) + omega(-)) of + (or on) states and the mean switching rate omega = (omega(+) + omega(-)) / 2. The membrane roughness is explicitly calculated as a function of X and omega. It is shown that the interplay of active and thermal fluctuations is subtle and that it is, in general, not possible to describe the active fluctuations in terms of an effective temperature.

Thermo-mechanical properties of tri-functionally crosslinked liquid single crystal elastomers

We have investigated thermo-mechanical properties and optical birefringence of the V3 cross-linker liquid single crystal elastomers (V3LSCEs) with different concentrations of the flexible tri-functional cross-linker as a function of temperature. The length of V3LSCEs along the director n ˆ shrinks on heating while it expands perpendicular to n ˆ . Drastic length changes as well as large variations in the optical birefringence, Δ n, are observed near T c. The maximum contraction increases with increasing cross-linking concentration. When the anisotropy of the shape change is plotted as a function of Δ n, a linear dependence is observed only deep in the nematic phase for larger cross-linking concentrations.

Single Molecule Recognition between Cytochrome C 551 and Gold-Immobilized Azurin by Force Spectroscopy

Recent developments in single molecule force spectroscopy have allowed investigating the interaction between two redox partners, Azurin and Cytochrome C 551. Azurin has been directly chemisorbed on a gold electrode whereas cytochrome c has been linked to the atomic force microscopy tip by means of a heterobifunctional flexible cross-linker. When recording force-distance cycles, molecular recognition events could be observed, displaying unbinding forces of ∼95 pN for an applied loading rate of 10 nN/s. The specificity of molecular recognition was confirmed by the significant decrease of unbinding probability observed in control block experiments performed adding free azurin solution in the fluid cell. In addition, the complex dissociation kinetics has been here investigated by monitoring the unbinding forces as a function of the loading rate: the thermal off-rate was estimated to be ∼14 s −1, much higher than values commonly estimated for complexes more stable than electron transfer complexes. Results here discussed represent the first studies on molecular recognition between two redox partners by atomic force microscopy.

Polymer-Supported Bis(oxazolines) and Related Systems: Toward New Heterogeneous Enantioselective Catalysts

Polymer-supported bis(oxazoline) and related ligands have been prepared by grafting and by polymerization methodologies under a variety of conditions. The corresponding Cu complexes have been assayed as catalysts for the cyclopropanation of styrene in the presence of diethyl azoacetate. Data reveal that insoluble systems prepared by polymerization are much more effective that those prepared by grafting. The best performances, in particular in terms of selectivity, were obtained for tert-butylglycine-derived systems. In some cases, functional resins that are more active, chemoselective, and enantioselective than the corresponding homogeneous analogue systems have been obtained. The presence of additional functionalities in the polymeric matrix is, in general, detrimental for the performance of the supported catalyst, even if the presence of more flexible cross-linkers enhances the accessibility of the functional sites. The Copper bis(oxazoline) polymeric systems are easily recycled and reused and provide materials of high utility for their use in facilitated chemical synthesis.

Molecular modeling of cellulose in amorphous state part II: effects of rigid and flexible crosslinks on cellulose

It is essential to understand the molecular level response of crosslinked cellulose chain segments upon deformation, in order to develop new agents which convey high durable-press (DP) rating to cellulose fibers with minimal strength loss. In this work models of amorphous cellulose crosslinked with both rigid and flexible crosslinks were constructed computationally for this purpose. Rigid crosslinks bound cellulose molecular segments together and blocked the chain slippage, providing cellulose models with a higher initial modulus and better elastic response. However, the loss of the chain slippage led to stress being distributed unevenly among cellulose chains. Chains in some regions were subjected higher stress and these regions were opened up much more than the rest of the cellulose, which presumably caused models to fail. When conformationally flexible crosslinks were used, breaking strain of cellulose was not significantly reduced but deformation recovery was not improved either, in comparison with the models of untreated cellulose. Conformational transitions were observed in the flexible crosslinks during extension. These results help to explain how and why rigid crosslinks work to provide wrinkle resistant properties and why they also lower tensile strength, and that just using a conformationally flexible crosslinking will not provide any recovery.

Mechanical properties of mono-domain side chain nematic elastomers.

We investigate the behavior of the shear rigidity modulus G = G' + iG" of three mono-domain side chain liquid-crystalline elastomers composed of side chain polysiloxanes cross-linked by either flexible or rigid cross-linkers. The measurements were taken in a frequency domain ranging from approximately 0.02 Hz to approximately 10(4) Hz applying the shear in a plane perpendicular to or containing the director. The measurements as a function of temperature show an anisotropy of G' which appears around T(NI), when decreasing the temperature, and which is due to the expected lowering of G'(//) coming from the coupling between the shear and the director. The measurements as a function of frequency show that G has two components for both geometries, in both the isotropic phase and in the nematic phase around the phase transition. One reflects the network behavior in its hydrodynamic regime ( G' is constant and G'' is approximately f, where f is the frequency), the other which appears at higher frequencies is characterized by a scaling law behavior (G' is approximately G" is approximately f(0.5)) of the Rouse type. We discuss the results in the framework of available theories and show that the three elastomers present a non-soft behavior, even for the elastomer for which the contrary was claimed, and that there is no separation of time scales between the director and the network. We also present data on a poly-domain sample and a non-mesomorphic one which complement these results.

Polymer‐Supported Thioanisole: A Versatile Platform for Organic Synthesis Reagents.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis and properties of polysiloxane side chain cholesteric elastomers

The synthesis of new side chain cholesteric liquid crystalline elastomers containing the flexible non-mesomorphic crosslinking agent M-1 and the cholesteric monomer M-2 by a one-step hydrosilylation reaction is described. The chemical structures of the obtained monomers and network polymers were confirmed by 1H NMR and FTIR spectroscopy. The mesomorphic properties and phase behavior were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. The glass transition temperatures and isotropic temperatures of the mesomorphic elastomers decreased as the concentration of crosslinking units increased; in the mesomorphic region the liquid crystalline elastomers showed elasticity, reversible phase transitions and Grandjean texture. The flexible crosslinking agent did not disturb the cholesteric structure; moreover, it was beneficial for adjusting the helix of the cholesteric liquid crystalline polymers, and cholesteric elastomers P-6, P-7, show reversible selective reflection of visible light.

Synthesis and properties of cholesteric liquid crystalline elastomers

The synthesis of new side-chain cholesteric liquid crystalline elastomers, containing the flexible non-mesogenic crosslinking agent M-1 and the cholesteric monomer M-2, is described by a one-step hydrosilication reaction. The chemical structures of the monomers and network polymers obtained were confirmed by FT-IR spectroscopy. Their mesogenic properties and phase behavior were investigated by differential scanning calorimetry, polarizing optical microscopy, and x-ray diffraction measurements. The influence of the crosslinking units on the phase behavior is discussed. The network polymers showed elasticity, reversible phase transitions, and cholesteric Grandjean texture. The experimental results demonstrated that the glass transition temperatures and isotropization temperatures of network elastomers decreased as the concentration of crosslinking units was increased, but the cholesteric phase was not disturbed.

Use of JandaJel Resins for Copper Removal in Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) ligands were supported onto JandaJel resins to allow for easy removal of the catalyst complex from the reaction mixture. Compared to divinylbenzene cross-linked resins, JandaJels have increased organic solvent compatibility and site accessibility due to the flexible cross-linker. These resins were chosen as the support in an attempt to obtain fast reaction times, good molecular weight control, and narrow polydispersity. The JandaJel ligands were used in ATRP of methyl methacrylate (MMA), styrene, and 2-(dimethylamino)ethyl methacrylate (DMAEMA). The MMA and DMAEMA polymerizations proceeded quickly to high conversion (>90%) and were well-controlled. The styrene polymerization was sluggish and proceeded only to 63% conversion. Recyclability of the catalyst was demonstrated by reusing the catalyst for a second ATRP reaction. A PMMA-b-PDMAEMA copolymer was also synthesized using JandaJel ligands. After polymerization was complete, the catalyst/ligand complex was eas...