Crosslinking Density Of Network Research Articles

PEG Containing Thiol–Ene Network Membranes for CO2 Separation: Effect of Cross-Linking on Thermal, Mechanical, and Gas Transport Properties

A new family of poly(ethylene glycol) (PEG) based membranes for CO2 separation was developed using thiol–ene photopolymerization. Compared to photopolymerized PEG-containing acrylate membranes, these new thiol–ene based membranes offer improved mechanical properties and processing advantages. The starting material, a combination of a trithiol cross-linker and a PEG diene, was gradually modified with a PEG dithiol while maintaining 1:1 thiol:ene stoichiometry. This approach made it possible to decrease the network cross-link density, resulting in simultaneous increases in free volume and PEG content. Materials with high concentrations of dithiol were very stretchable, with largely, up to 500%, improved elongation at break, yet they exhibited commendable CO2/N2, O2, H2, and CH4 permeability-selectivity performance. The average molecular weight of polymer chains between cross-links, Mc, was determined experimentally by fitting the classic network affine model to stress–strain data obtained via tensile testin...

Effect of the spatial network structure and cross-link density of diene rubbers on their thermal stability and fire hazard

The paper presents the results of investigating the effect of the density and spatial network structure of diene rubbers (NBR and SBR) on their thermal properties and fire hazard. The rubbers were either conventionally cross-linked by means of sulfur and organic peroxide or nonconventionally with the use of iodoform (CH3I). Based on thermo-kinetic analysis, the destruction activation energy of the elastomers investigated and their vulcanizates was determined under air and inert gas. The analysis of particular stages of their thermal decomposition was also presented. During the combustion of the elastomeric materials obtained, it has been found that their fire hazard depends not only on the elastomer chemical structure but also on the method of its cross-linking.

Biomimetic-Engineered Poly (Ethylene Glycol) Hydrogel for Smooth Muscle Cell Migration

We report on a biomimetic scaffold as a model system to evaluate smooth muscle cell (SMC) migration in three dimensions. To accomplish this, bio-inert poly (ethylene glycol) (PEG)-based hydrogels were designed as the scaffold substrate. To mimic properties of the extracellular matrix, cell-adhesive peptide (GRGDSP) derived from fibronectin and collagenase-sensitive peptide (GPQGIAGQ) derived from collagen type I were incorporated into the PEG macromer chain. Copolymerization of the biomimetic macromers results in the formation of bioactive PEG hydrogels with cell adhesivity and biodegradability. By utilizing these biomimetic scaffolds, we studied the effect of adhesive ligand concentration, proteolysis, and network cross-linking density on cell migration. Our results showed that three-dimensional SMC migration has a biphasic dependence on adhesive ligand density, and both adhesive and collagenase-sensitive peptides were required for cell migration to occur. Furthermore, network cross-linking density was shown to dramatically influence the behavior of cell migration in the hydrogels.

Visible light cured thiol-vinyl hydrogels with tunable degradation for 3D cell culture

We report here a synthetically simple yet highly tunable and diverse visible light mediated thiol-vinyl gelation system for fabricating cell-instructive hydrogels. Gelation was achieved via a mixed-mode step-and-chain-growth photopolymerization using functionalized 4-arm poly(ethylene glycol) as backbone macromer, eosin-Y as photosensitizer, and di-thiol containing molecule as dual purpose co-initiator/cross-linker. N-vinylpyrrolidone (NVP) was used to accelerate gelation kinetics and to adjust the stiffness of the hydrogels. Visible light (wavelength: 400–700nm) was used to initiate rapid gelation (gel points: ∼20s) that reached completion within a few minutes. The major differences between current thiol-vinyl gelation and prior visible light mediated photopolymerization are that: (1) the co-initiator triethanolamine (TEA) used in the previous systems was replaced with multifunctional thiols and (2) mixed-mode polymerized gels contain less network heterogeneity. The gelation kinetics and gel properties at the same PEG macromer concentration could be tuned by changing the identity of vinyl groups and di-thiol cross-linkers, as well as concentration of cross-linker and NVP. Specifically, acrylate-modified PEG afforded the fastest gelation rate, followed by acrylamide and methacrylate-functionalized PEG. Increasing NVP concentration also accelerated gelation and led to a higher network cross-linking density. Further, increasing di-thiol peptide concentration in the gel formulation increased hydrogel swelling and decreased gel stiffness. Due to the formation of thiol-ether-ester bonds following thiol-acrylate reaction, the gels degraded hydrolytically following a pseudo first order degradation kinetics. Degradation rate was controlled by adjusting thiol or NVP content in the polymer precursor solution. The cytocompatibility and utility of this hydrogel system were evaluated using in situ encapsulation of human mesenchymal stem cells (hMSC). Encapsulated hMSCs remained alive (>90%) throughout the duration of the study and the cells were differentiated down osteogenic lineage with varying degrees by controlling the rate and mode of gel degradation.

Influence of Cross-linkers on the Cohesive and Adhesive Self-Healing Ability of Polysulfide-Based Thermosets

Synthetic systems with intrinsic self-repairing or self-healing abilities have emerged during the past decade. In this work, the influence of the cross-linker and chain rigidity on the healing ability of thermoset rubbers containing disulfide bonds have been investigated. The produced materials exhibit adhesive and cohesive self-healing properties. The recovery of these two functionalities upon the thermally triggered healing events has shown to be highly dependent on the network cross-link density and chain rigidity. As a result, depending on the rubber thermoset intrinsic physical properties, the thermal mending leading to full cohesive recovery can be achieved in 20-300 min at a modest healing temperature of 65 °C. The adhesive strength ranges from 0.2 to 0.5 MPa and is fully recovered even after multiple failure events.

Olefin Metathesis for Effective Polymer Healing via Dynamic Exchange of Strong Carbon–Carbon Double Bonds

In this article, we demonstrate transition-metal-catalyzed olefin metathesis as a simple, effective method for healing polymers via dynamic exchange of strong carbon-carbon double bonds. Upon introducing a very low level of the Grubbs' second-generation Ru metathesis catalyst into cross-linked polybutadiene (PBD) network, the material self-heals effectively at various conditions under moderate pressures. In sharp contrast, catalyst-free control samples with identical network topology and cross-linking density show minimal healing. The healing efficiency of the materials was carefully investigated under different concentrations of the Ru catalyst, compression pressures, and temperatures. It is demonstrated for the first time that a bulk polymer could effectively heal via dynamic covalent bond formation at sub-ambient temperature. The Ru-loaded PBD samples not only heal well with themselves but also with control samples without any catalyst. Furthermore, a completely Ru-free PBD network can heal effectively upon simply applying a very small amount of Ru catalyst only at the fracture surface. The simplicity and effectiveness of this self-healing approach make it potentially applicable to a wide range of olefin-containing polymers.

Making Insoluble Polymer Networks Malleable via Olefin Metathesis

Covalently cross-linked polymers have many technological applications for their excellent properties, but they suffer from the lack of processability and adaptive properties. We report a simple, efficient method of generating adaptive cross-linked polymers via olefin metathesis. By introducing a very low level of the Grubbs' second-generation Ru metathesis catalyst, a chemically cross-linked polybutadiene network becomes malleable at room temperature while retaining its insolubility. The stress relaxation capability increases with increasing level of catalyst loading. In sharp contrast, catalyst-free control samples with identical network topology and cross-linking density do not show any adaptive properties. This chemistry should offer a possibility to combine the dimensional stability and solvent resistance of cross-linked polymers and the processability/adaptibility of thermoplastics.

The development of a high-throughput/ combinatorial workflow for the study of porous polymer networks

The development of a high-throughput/ combinatorial workflow for the study of porous polymer networks Partha Majumdar1, James Bahr1, Elizabeth Crowley1, Alekhya Kallam1, Nathan Gubbins1, Kris Schiele1, Michael Weisz1, Shawn M Dirk3, Joseph L Lenhart4, Bret J Chisholm1,21Center for Nanoscale Science and Engineering, 2Department of Coatings and Polymeric Materials, North Dakota State University, Fargo, North Dakota, 3Sandia National Laboratories, Albuquerque, New Mexico, 4US Army Research Laboratory, Aberdeen, Maryland, USAAbstract: A high-throughput workflow was developed for the study of porous polymers generated using the process of chemically induced phase separation. The workflow includes automated, parallel preparation of liquid blends containing reactive, polymer network-forming precursors and a poragen, as well as a high-throughput poragen extraction process using supercritical CO2. A structure–process–property relationship study was conducted using epoxy-amine cross-linked networks. The experimental design involved variations in polymer network cross-link density, poragen composition, poragen level, and cure temperature. A total of 216 unique compositions were prepared. Changes in opacity of the blends as they cured were monitored visually. Morphology was characterized using a scanning electron microscope on a subset of the 216 samples. The results obtained allowed for the identification of compositional variables and process variables that enabled the production of porous networks.Keywords: high-throughput, chemically induced phase separation, porous polymer, poragen

Molecular-Level Engineering of Protein Physical Hydrogels for Predictive Sol–Gel Phase Behavior

Predictable tuning of bulk mechanics from the molecular level remains elusive in many physical hydrogel systems because of the reliance on nonspecific and nonstoichiometric chain interactions for network formation. We describe a mixing-induced two-component hydrogel (MITCH) system, in which network assembly is driven by specific and stoichiometric peptide-peptide binding interactions. By integrating protein science methodologies with a simple polymer physics model, we manipulate the polypeptide binding interactions and demonstrate the direct ability to predict the resulting effects on network cross-linking density, sol-gel phase behavior, and gel mechanics.

Swelling behavior and structural characteristics of polyvinyl alcohol/montmorillonite nanocomposite hydrogels

AbstractA series of nanocomposite hydrogels based on polyvinyl alcohol containing 0–10 wt % of the organically modified montmorillonite clay were prepared by freezing‐thawing cyclic method. The morphology of the nanocomposite hydrogels was observed by the scanning electron microscopy technique. The structural properties were determined by measuring the network mesh size, crosslinking density, and average molecular weight of polymer chains between crosslinks. The swelling behavior and the effect of swelling medium temperature on the swelling kinetics and characteristics of the nanocomposite hydrogels were also investigated. The results showed that two structural characteristics i.e., network mesh size and average molecular weight of polymer chains between crosslinks have inverse dependence on the clay loading level in the nanocomposite hydrogel, while crosslinking density shows completely direct dependence. Swelling measurements demonstrated a linear relation between the degree of swelling and the square root of immersion time at all swelling medium temperatures. The results indicated that the swelling characteristics of the nanocomposite hydrogels including the equilibrium degree of weight and volume swelling and the equilibrium water content were decreased by increasing the quantity of the clay incorporated into the hydrogel as well as by decreasing the temperature of swelling medium. While, the time required to reach to the equilibrium condition, as another swelling characteristic of the hydrogels, exhibited a completely opposite behavior. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.

Hydrogel Synthesis Directed Toward Tissue Engineering: Impact of Reaction Condition on Structural Parameters and Macroscopic Properties of Xerogels

The existence of correlation and functional relationships between reaction conditions (concentrations of crosslinker, monomer and initiator, and neutralization degree of monomer), primary structural parameters (crosslinking density of network, average molar mass between crosslinks, and distance between macromolecular chains), and macroscopic properties (equilibrium swelling degree and xerogel density) of the synthesized xerogels which are important for application in tissue engineering is investigated. The structurally different xerogels samples of poly(acrylic acid), poly(methacrylic acid), and poly(acrylic acid-g-gelatin) were synthesized by applying different methods of polymerization: crosslinking polymerization, crosslinking polymerization in high concentrated aqueous solution, and crosslinking graft polymerization. The values of primary structural parameters and macroscopic properties were determined for the synthesized xerogels samples. For all of the investigated methods of polymerization, an existence of empirical power function of the dependence of primary structural parameters and macroscopic properties on the reaction conditions was established. The scaling laws between primary structural parameters and macroscopic properties on average molar mass between crosslinks were established. It is shown that scaling exponent is independent from the type of monomer and other reaction conditions within the same polymerization method. The physicochemical model that could be used for xerogel synthesis with predetermined macroscopic properties was suggested.

The Effect of Lightning Strikes on Bismaleimide Composites by Dynamic Mechanical Analysis

In this paper, a new method that has shown significant potential to characterize lightning strikes damage which is couple with dynamic mechanical analysis. The composite samples used in this work are based on carbon fiber/bismaleimide system. The analysis of the dynamic mechanical data demonstrate the glass transition temperature of the composites increased as a function of increasing lightning current , simultaneity with the presence ofpotential damage, whichare result in higher network cross-link density and the incipent degradation of the polymer matrix.

Microfluidic approaches for the fabrication of gradient crosslinked networks based on poly(ethylene glycol) and hyperbranched polymers for manipulation of cell interactions

High-throughput methods allow rapid examination of parameter space to characterize materials and develop new polymeric formulations for biomaterials applications. One limitation is the difficulty of preparing libraries and performing high-throughput screening with conventional instrumentation and sample preparation. Here, we describe the fabrication of substrate materials with controlled gradients in composition by a rapid method of micromixing followed by a photopolymerization reaction. Specifically, poly(ethylene glycol) dimethacrylate was copolymerized with a hyperbranched multimethacrylate (P1000MA or H30MA) in a gradient manner. The extent of methacrylate conversion and the final network composition were determined by near-infrared spectroscopy, and mechanical properties were measured by nanoindentation. A relationship was observed between the elastic modulus and network crosslinking density. Roughness and hydrophilicity were increased on surfaces with a higher concentration of P1000MA. These results likely relate to a phase segregation process of the hyperbranched macromer that occurs during the photopolymerization reaction. On the other hand, the decrease in the final conversion in H30MA polymerization reactions was attributed to the lower termination rate as a consequence of the softening of the network. Valvular interstitial cell attachment was evaluated on these gradient substrates as a demonstration of studying cell morphology as a function of the local substrate properties. Data revealed that the presence of P1000MA affects cell–material interaction with a higher number of adhered cells and more cell spreading on gradient regions with a higher content of the multifunctional crosslinker.

Relationship between the Photomechanical Response and the Thermomechanical Properties of Azobenzene Liquid Crystalline Polymer Networks

We report on the influence of cross-link density of azobenzene-containing liquid crystal polymer networks (azo-LCNs) on the thermomechanical properties and the laser-directed bending of cantilevers consisting of these materials. Cross-link density of azo-LCN was increased by adjusting the length of photocuring from 1 to 120 min. The storage modulus (E′), loss modulus (E′′), and glass transition temperature (Tg) of the azo-LCNs increase with cross-link density. Increasing the cross-link density of the polydomain azo-LCN reduces the magnitude of the bending angle of the cantilevers. The relationship between the thermomechanical and photomechanical properties of the polydomain azo-LCN reported here is further elucidated in the examination of laser-directed bending over a wide range of temperature. The temperature dependence of the equilibrium photodriven bending angle is shown to be strongly related to the temperature dependence of the storage modulus for a given azo-LCN sample. Normalizing the temperature d...

Preparation and properties of polyurethane/benzyl amylose semi‐interpenetrating networks

AbstractA series of semi‐interpenetrating polymer networks (semi‐IPNs) films were prepared from 20 wt % of benzyl amylose (BA) of different Mw and castor oil‐based polyurethane (PU) in N,N‐dimethylformamide (DMF). The weight‐average molecular weight (Mw), and radii of gyration (<S2>z1/2) of benzyl amylose were determined by laser scattering measurement, and the results suggested BA was in a compact coil conformation in DMF. Furthermore, the properties and miscibility of the polyurethane/benzyl amylose (PUBA) films were studied by scanning electronic microscopy, differential scanning calorimetry, dynamic mechanical thermal analysis, ultraviolet–visible spectrophotometer, and tensile testing. The PUBA films possessed much higher optical transmittance and tensile strength than the pure PU film regardless of the molecular weight of BA, but lower values of elongation at break were observed. With decreasing of the BA Mw from 9.24 × 105 to 2.69 × 105, interestingly, the elongation at break of the films increased from 135 to 326%. This might be ascribed to the decrease of crosslinking density of PU networks and the enhancement in freedom of the molecular motion. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Biodegradable polyesterurethane networks for controlled release of aspirin

AbstractIn this article, poly(D,L‐lactide‐co‐glycolide)urethane (PULG) networks were prepared from hydroxyl telechelic star‐shaped oligo(D,L‐lactide‐co‐glycolide) coupled with 1,6‐diisocyanate‐2,2,4‐trimethylhexane and 1,6‐diisocyanate‐2,4,4‐trimethylhexane or isophorone diisocyanate. The release of model drug aspirin (ASP) from biodegradable polyesterurethane networks was studied in phosphate buffered saline pH = 7.0 at 37°C. PULG networks turned from transparent to opaque after ASP loading. PULG networks with lower crosslinking density always resulted in higher drug loaded content. The results of differential scanning calorimetry and scanning electron microscope measurements demonstrated that ASP was uniformly distributed in the networks. The drug release courses of ASP from PULG networks in phosphate buffered saline pH = 7.0 at 37°C could be divided into three stages. Firstly, ASP release was at approximately uniform rate from PULG networks; Secondly, the release rate obviously increased due to the degradation of the PULG networks; Thirdly, the release rate decreased gradually because most of the ASP had diffused out of the PULG networks. The crosslinking density of polyesterurethane networks also affected both degradation of the polymer networks and drug release rate. The in vitro release test revealed that ASP accelerated the degradation process of PULG, which exhibited a typical erosion‐controlled release mechanism. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Release of Aspirin from Biodegradable Polyesterurethane Networks

In this paper, the release of model drug aspirin (ASP) from biodegradable polyesterurethane networks was studied. Poly(D,L-lactide-co-glycolide)urethane (PULG) networks were prepared from hydroxyl telechelic star-shaped oligo(D,L-lactide-co-glycolide) coupled with 1,6-diisocyanate-2,2,4-trimethylhexane and 1,6-diisocyanate-2,4,4-trimethylhexane or isophorone diisocyanate. PULG networks turned from transparent to opaque after ASP loading. PULG networks with lower crosslinking density always resulted in higher drug loaded content. The results of differential scanning calorimetry (DSC) and scanning electron microscope (SEM) measurements demonstrated that ASP was uniformly distributed in the networks. The drug release courses of ASP from PULG networks in phosphate buffered saline pH = 7.0 at 37 °C could be divided into three stages. Firstly, ASP release was at approximately uniform rate from PULG networks; Secondly, the release rate obviously increased for the degradation of the PULG networks; Thirdly, the release rate decreased gradually because most of the ASP had diffused out of the PULG networks. The crosslinking density of polyesterurethane networks also affected drug release rate. The in vitro release test revealed that ASP accelerated the degradation process of PULG, which exhibited a typical erosion-controlled release mechanism.

Lateral and Rotational Mobility of Some Drug Molecules in a Poly(Ethylene Glycol) Diacrylate Hydrogel and the Effect of Drug-Cyclodextrin Complexation

In relation to drug release properties the lateral and rotational mobility of two drugs and one drug mimic in photopolymerized poly(ethylene glycol) diacrylate (PEGDA) networks were investigated by nuclear magnetic resonance as a function of the network cross-link density and temperature. The network mesh size affects the lateral diffusivity for all drugs, even if the mesh size is an order of magnitude larger than the drug molecular size. The rotational diffusional motion is only appreciably affected when the drug size and network mesh size are of the same order of magnitude. By complexing the drug to cyclodextrin (CD) it is found that in some cases, depending on network mesh size and complex size, the complex is absorbed by the PEGDA gel, but that the diffusion of the drug in the gel is not necessarily slower than in the absence of the CD. This is explained by a theoretical model.

Uncertainties in the Determination of Cross-Link Density by Equilibrium Swelling Experiments in Natural Rubber

Equilibrium swelling is a feasible and simple experiment to determine the cross-link density of networks. It is the most popular and useful approach; however, in most of the cases, the given values are highly uncertain if not erroneous. The description of the complex thermodynamics of swollen polymer networks is usually based on the Flory−Rehner model. However, experimental evidence has shown that both the mixing term described by the Flory−Huggins expression and the elastic component derived from the affine model are only approximations that fail in the description and prediction of the rubber network behavior. This means that the Flory−Rehner treatment can only give a qualitative evaluation of cross-link density because of its strong dependence on the thermodynamic model. In this work, the uncertainties in the determination of the cross-link density in rubber materials by swelling experiments based on this model are reviewed. The implications and the validity of some of the used approximations as well as their influence in the relationship of the cross-link densities derived from swelling experiments are discussed. Importantly, swelling results are compared with results of a completely independent determination of the cross-link density by proton multiple-quantum NMR, and the correlation observed between the two methods can help to validate the thermodynamic model.

Review: Photopolymerizable and Degradable Biomaterials for Tissue Engineering Applications

Photopolymerizable and degradable biomaterials are finding widespread application in the field of tissue engineering for the engineering of tissues such as bone, cartilage, and liver. The spatial and temporal control afforded by photoinitiated polymerizations has allowed for the development of injectable materials that can deliver cells and growth factors, as well as for the fabrication of scaffolding with complex structures. The materials developed for these applications range from entirely synthetic polymers (e.g., poly(ethylene glycol)) to purely natural polymers (e.g., hyaluronic acid) that are modified with photoreactive groups, with degradation based on the hydrolytic or enzymatic degradation of bonds in the polymer backbone or crosslinks. The degradation behavior also ranges from purely bulk to entirely surface degrading, based on the nature of the backbone chemistry and type of degradable units. The mechanical properties of these polymers are primarily based on factors such as the network crosslinking density and polymer concentration. As we better understand biological features necessary to control cellular behavior, smarter materials are being developed that can incorporate and mimic many of these factors.