Effect Of Crosslink Density Research Articles

Cross-linkable and water-soluble phospholipid polymer as artificial extracellular matrix

The objective of this study is to prepare an artificial extracellular matrix (ECM) for cell culture by using polymer hydrogels. The polymer used is a cytocompatible water-soluble phospholipid polymer: poly[2-methacryloyloxyethyl phosphorylcholine (MPC)-n-butyl methacrylate-p-nitrophenyloxycarbonyl poly(ethylene oxide) methacrylate (MEONP)] (PMBN). The hydrogels are prepared using a cross-linking reaction between PMBN and diamine compounds, which can easily react to the MEONP moiety under mild conditions. The most favorable diamine is the bis(3-aminopropyl) poly(ethylene oxide) (APEO). The effects of cross-linking density and the chemical structure of cross-linking molecules on the mechanical properties of the hydrogel are evaluated. The storage modulus of the hydrogel is tailored by tuning the PMBN concentration and the MEONP/amino group ratio. The porous structure of the hydrogel networks depends not only on these parameters but also on the reaction temperature. We prepare a hydrogel with 40–50

Diels-Alder Crosslinked Block-Copolymer Gate Dielectrics for Low Voltage Operated Top-Gate Organic Field-Effect Transistors

We report a cross-linkable diblock copolymer at a mild temperature by Diels-Alder click-chemistry for a gate dielectric layer in the top-gate, bottom contact organic field-effect transistors (OFETs). The Diels-Alder reaction between poly[(methyl methacrylate)-co-(9-anthracenyl methyl methacrylate)] (P(MMA-AMA)) and 1,1′-(methylenedi-4,1-phenylene)bismaleimide (MPB) enables the preparation of robust cross-linked films at 100 °C annealing for 10 min. Poly(9,9-dioctylfluorene-alt-bithiophene (F8T2) or poly(3-hexylthiophene) (P3HT) OFETs exhibited charge carrier mobilities of 3 ∼ 4 × 10−3 cm2 V−1 s−1 with the 250 nm thick crosslinked (P(MMA-AMA)) films at gate voltages of less than −20 V. Moreover, the ratio of MMA and AMA was controlled to observe the effects of crosslinking density on the properties of the gate dielectric layer. When the portion of a crosslinkable anthracenyl unit in (P(MMA-AMA)) is increased to 50 mol.% in the base polymer, the gate leakage current of OFETs decreases below 10−8 A at Vg = −20 V.

Effect of cross-link density and the healing efficiency of self-healing poly(2-hydroxyethyl methacrylate) hydrogel

AbstractIn this study, hydrogels of poly(2-hydroxyethyl methacrylate) with different cross-link density were prepared by the free-radical polymerization method. l-Cystine, which acts as a cross-linker, was prepared at different concentrations, ranging from 0.02 to 0.08 mol/l, to identify the concentration that provided the highest mechanical strength and healing efficacy. Healing of the hydrogels was achieved by heating above their glass transition temperature. Intermolecular diffusion of the dangling chain or chain slippage led to the healing of the gels. Results showed that 0.04 mol/l of l-cystine in poly(2-hydroxyethyl methacrylate) hydrogels provided the highest ultimate tensile strength (0.780 N/mm2) and healing recovery (92%). This healing capability was also observed using optical microscopy.

Effect of crosslinker feed content on catalaytic activity of silver nanoparticles fabricated in multiresponsive microgels

We investigated the effect of crosslinking density of poly(N-isopropyl acrylamide-co-acrylic acid) microgels on catalytic activity of silver nanoparticles fabricated hybrid microgels. Multiresponsive poly(N-isopropyl acrylamide-co-acrylic acid) microgels with 2, 4, 6 and 8 mole percentage of N,N-methylene-bis-acrylamide were synthesized by emulsion polymerization. These microgels were characterized by dynamic light scattering and were used as microreactors to synthesize silver nanoparticles. Hybrid system was characterized by ultraviolet-visible spectroscopy. The catalytic activity of hybrid microgels with different crosslinker content was compared by studying the reduction of pnitrophenol as a model reaction. Kinetics of reaction was monitored by spectrophotometry. The value of the apparent rate constant decreases from 0.568 to 0.313min−1, when content of crosslinker are increased from 2 to 8 mole percentage respectively. This decreases in value of apparent rate constant is due to increase in diffusional barrier offered by high crosslinking of polymer network at high mole percentages of N,N-methylene-bis-acrylamide.

Effects of alginate hydrogel cross-linking density on mechanical and biological behaviors for tissue engineering

An effective cross-linking of alginate gel was made through reaction with calcium carbonate (CaCO3). We used human chondrocytes as a model cell to study the effects of cross-linking density. Three different pore size ranges of cross-linked alginate hydrogels were fabricated. The morphological, mechanical, and rheological properties of various alginate hydrogels were characterized and responses of biosynthesis of cells encapsulated in each gel to the variation in cross-linking density were investigated. Desired outer shape of structure was maintained when the alginate solution was cross-linked with the applied method. The properties of alginate hydrogel could be tailored through applying various concentrations of CaCO3. The rate of synthesized GAGs and collagens was significantly higher in human chondrocytes encapsulated in the smaller pore structure than that in the larger pore structure. The expression of chondrogenic markers, including collagen type II and aggrecan, was enhanced in the smaller pore structure. It was found that proper structural morphology is a critical factor to enhance the performance and tissue regeneration.

Tensile behavior of heat welded CNT network structures

In this study, a computational investigation on the mechanical properties of random carbon nanotube (CNT) networks is reported. For this purpose, atomistic models of 3-D CNT network structures are generated by an automated stochastic algorithm that is capable of randomly inserting CNTs into a design space while controlling several topological parameters including cross-link density, distance between cross-links, as well as length and chirality of CNTs. During the atomistic model generation process, heat welding is applied to the close contacts to form a welded CNT network with covalently-bonded junctions. Following the generation of atomistic models, molecular dynamics (MD) simulations of uniaxial tensile loading experiments are performed to investigate the basic deformation mechanisms of CNT networks as well as to obtain mechanical properties such as the Young’s modulus, yield strength and ultimate strain values. The effects of cross-link density on the mechanical performance of CNT network materials are obtained by employing network specimens with different cross-link densities.

Antimicrobial alginate/PVA silver nanocomposite hydrogel, synthesis and characterization

Superabsorbent hydrogel-silver nanocomposite based on poly(vinyl alcohol) (PVA) and sodium alginate (Na-Alg) was prepared using free radical polymerization in the presence of acrylamide (AAm) monomer. The reactions were conducted under normal atmospheric conditions, using ammonium persulfate (APS) as an initiator and methylene bisacrylamide (MBA) as a crosslinking agent. The effect of reaction parameters such as MBA, AAm, and APS concentration as well as Na-Alg/PVA weight ratio on the water absorbency and the gel content of the hydrogels were studied. Evidence of grafting was obtained by comparing the FT-IR spectra and the TGA of the initial substrates with that of the superabsorbent hydrogel. Furthermore, Ag nanoparticles were synthesized in a green synthesis process. Highly stable silver nanoparticles were obtained with the hydrogel networks as nanoreactor via in situ reduction of silver nitrate by using sodium borohydride as a reducing agent. The hydrogel silver nanocomposite was fully characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The effect of cross-link density, and Na-Alg/PVA weight ratio on the loading and the size of nanoparticles were studied. The antibacterial activity of the silver nanocomposite hydrogel was investigated as well.

Effect of crosslinking density on resilient performance of low-resilience flexible polyurethane foams

Low-resilience flexible polyurethane foams (FPUFs) with varying crosslinking densities, were synthesized from polyols, 4, 4′-diphenyl methane diisocyanate (MDI-100), and water. The effects of crosslinking agent content, the molecular weight (Mw) of polyether diol, and the ratio of (polyether triol)/(polyether diol) on the resilience performance of FPUFs were investigated. Results indicate that higher crosslinking density was beneficial to the increasing of recovery time. The recovery time of the FPUF using polyether diol with Mw of 400 was 24.3 s. It was 3.2 times longer compared with FPUF using polyether diol with Mw of 3000. Dynamic mechanical analysis (DMA) results showed that, FPUFs with high crosslinking density displayed viscoelasticity in a wider range of temperature. However, when molecular chains were longer enough, FPUFs with low crosslinking density also demonstrated significant viscoelasticity, which is owing to the excess of physical crosslinking points. The results in stress–strain cycling and recovery time experiments were consistent with the results of DMA. POLYM. ENG. SCI., 55:308–315, 2015. © 2014 Society of Plastics Engineers

Effect of crosslinker structure and crosslinker/monomer ratio on network parameters and thermodynamic properties of Poly (N-isopropylacrylamide) hydrogels

The effect of polymer composition and polymerization parameters such as the type of crosslinker and comonomer, crosslinker/monomer ratio and polymerization temperature on the polymer-solvent interactions and mechanical properties of tetraallylammonium bromide (TAB)-crosslinked N-isopropylacrylamide (NIPAAm), N,N’-methylenebisacrylamide (BIS)-crosslinked NIPAAm and TAB-crosslinked NIPAAm - itaconic acid (IA) hydrogels were studied. It was observed that the hydrogels prepared by 0.7 mole / L concentration of NIPAAm and crosslinked with BIS exhibited mechanical weakness and, they were broken even under a pressure, being less than 5.0 N at a swelling temperature of 25 °C while the PNIPAAm hydrogels crosslinked with ionic-octafunctional crosslinker, TAB, had better compression properties. A decrease in the concentration of NIPAAm, increasing TAB content, addition of a hydrophilic/weakly acidic comonomer, IA and an increase in the functionality of crosslinker resulted in the decreasing values of the poymer-solvent interaction parameter, χ at 25 °C. The values of χ , effective crosslinking density (νe) and enthalpy changes during the shrinkage process in the ranges of 33o - 45 °C and 37o- 45 °C indicated the exothermal nature of phase transition and the importance of intermolecular interactions between PNIPAAm chains. The partial molar dilution-enthalpies (ΔH1) and -entropies (ΔS1) that were calculated from the values of enthalpic and entropic components of χ also indicated a similar trend, referring to a decrease in ordered-structuring of water molecules around hydrophobic isopropyl groups with an increase in the swelling temperature.

Mullins effect in a filled elastomer under uniaxial tension

Modulus softening and permanent set in filled polymeric materials due to cyclic loading and unloading, commonly known as the Mullins effect, can have a significant impact on their use as support cushions. A quantitative analysis of such behavior is essential to ensure the effectiveness of such materials in long-term deployment. In this work we combine existing ideas of filler-induced modulus enhancement, strain amplification, and irreversible deformation within a simple non-Gaussian constitutive model to quantitatively interpret recent measurements on a relevant PDMS-based elastomeric cushion. We find that the experimental stress-strain data is consistent with the picture that during stretching (loading) two effects take place simultaneously: (1) the physical constraints (entanglements) initially present in the polymer network get disentangled, thus leading to a gradual decrease in the effective cross-link density, and (2) the effective filler volume fraction gradually decreases with increasing strain due to the irreversible pulling out of an initially occluded volume of the soft polymer domain.

Tensile Mechanical Properties of Hydrophobic Association Hydrogels: Effect of Crosslinking Method, Synthesis Temperature, Mineral Salt and Swelling

Hydrophobic association hydrogels (HA-gels) were prepared through micellar copolymerization of acrylamide (AM) and a small amount of octylphenol polyoxyethylene acrylate (OP-10-AC) in an aqueous solution containing sodium dodecyl sulfate (SDS). For the P(AM/OP-10-AC) HA-gels, the effect of crosslinking method, synthesis temperature, mineral salt (NaCl) and swelling on their tensile mechanical properties was investigated in detail. The experimental results show that crosslinking method, synthesis temperature, NaCl and swelling powerfully influenced tensile mechanical properties of the P(AM/OP-10-AC) HA-gels. In addition, according to the tensile experimental results of the P(AM/OP-10-AC) HA-gels swollen, an effective crosslinking density change model is proposed. The effect of swelling on tensile mechanical properties of the P(AM/OP-10-AC) HA-gels was discussed on the basis of the model.

Design and synthesis of dual-responsive hydrogels based on N,N-dimethylaminoethyl methacrylate by copolymerization with N-isopropylacrylamide

Copolymeric hydrogels of N,N-dimethylaminoethyl methacrylate (DMAEMA) and N-isopropylacrylamide (NIPA) of various monomer ratios were evaluated as thermo-responsive and pH-responsive systems for the development of controlled-release and targeted-delivery devices. The swelling properties were investigated with different temperature, pH, and monomer feed ratios. The results show that the temperature-dependent and pH-dependent phase transition of poly(N,N-dimethylaminoethyl methacrylate-N-isopropylacrylamide) (P(DMAEMA-NIPA)) copolymeric hydrogels can be controlled by changing the amount of NIPA units in the network chains. In experiments to determine the temperature-dependent swelling of copolymeric hydrogels in water, it was found that the swelling ratio rapidly decreases as the temperature increases between 35 and 70°C. To characterize the network structure of the copolymeric hydrogels corresponding to effective cross-linking density and average network chain length, uniaxial compressive mechanical testing was carried both after the preparation of hydrogels and after their equilibrium swelling in water. The data obtained demonstrates that the resulting copolymeric hydrogels are promising as materials with tunable hydrophilicity-hydrophobicity and swelling behavior responsive to temperature and pH. Open image in new window

Network parameters and biocompatibility of p(2-hydroxyethyl methacrylate/itaconic acid/oligo(ethylene glycol) acrylate) dual-responsive hydrogels

A series of copolymeric hydrogels with different mole ratios of 2-hydroxyethyl methacrylate (HEMA), itaconic acid (IA) and oligo(ethylene glycol) acrylate (OEGA) was synthesised by gamma radiation. In our previous paper, the swelling properties of P(HEMA/IA/OEGA) terpolymeric hydrogels were preliminary investigated, showing dual (pH and thermo) responsiveness and large variation in the swelling capacity. In this study, the basic structural parameters of the P(HEMA/IA/OEGA) networks such as the average molecular weight between cross-links (M¯c), effective cross-linking density (νe), polymer–solvent interaction parameter (χ) and mesh size (ξ) were determined by using swelling and mechanical measurements. Cytocompatibility and hemolytic activity measurements were used to investigate the influence of hydrogel composition (i.e. the influence of different fractions of IA and OEGA) on the biocompatibility of the hydrogels obtained. The microbe penetration test against Staphylococcus aureus and Escherichia coli was performed, too. It was observed that variation in hydrogel composition strongly influenced the smart swelling behaviour and introduced some changes in biocompatibility. The results indicate that P(HEMA/IA/OEGA) terpolymers, as smart and biocompatible hydrogels, have wide diversity and great potential for different biomedical applications, but their optimisation study can give the best candidate for certain biomedical applications.

Crosslinking effect on the deformation and fracture of monodisperse polystyrene-co-divinylbenzene particles

This study focuses on the effect of crosslinking density on the mechanical response of polystyrene-co-divinyl- benzene (PS-DVB) particles under compression by means of nanoindentation-based flat punch method combined with SEM observation of particle morphologies. The monodisperse PS-DVB particles with about 5 !m in diameter are produced by the Ugelstad activated swelling method and the crosslinking density defined as the weight percentage of activated crosslinker DVB during the preparation process varies from 2.0 to 55.3%. Results show that the particle stress-strain behaviour is independent of the crosslinking density if the strain is less than 10%. With increasing strain level over 10%, a higher crosslinking leads to a stiffer behaviour of the particles. While slightly crosslinked (2.0 and 5.0 wt%) particles undergo plastic deformation with crazing and residual strain, highly crosslinked (21.3, 32.0 and 55.3 wt%) counterparts experience perfectly viscoelastic deformation. The crosslinking density significantly influences the fracture property as well as the failure morphology. Slightly crosslinked particles become permanently deformed after compression, while highly crosslinked ones are entirely fragmented once a critical strain is reached.

Effect of Cross-Link Density of the Matrix on the Damping Properties of Magnetorheological Elastomers

The effect of cross-link density of the matrix on the controllable damping properties of magnetorheological elastomers (MREs) has been investigated. MRE samples with different cross-link densities and plasticizer contents were fabricated and their microstructures were observed using an environmental scanning electron microscope (SEM). The dynamic performances of these samples were measured using a modified dynamic mechanical analyzer (DMA). The experimental results indicated the magneto-induced change of loss factor was enhanced by decreasing the cross-link density. The plasticizer and the frequency markedly influenced the magneto-induced change of loss factor when the cross-link density of the matrix was low. In addition, by reducing cross-link density, the magneto-induced modulus and the relative MR effect increased. A mechanism for the magneto-induced change of loss factor was proposed and the analysis implied that the rearrangement of particles is an important influence on controlling the damping prop...

Effect of Cross-Link Density on Photoplasticity of Epoxide Networks Containing Allylic Dithioether Moieties

A new diepoxide monomer (ADTDE) containing an allylic dithioether moiety has been synthesized to extend the suite of monomers that form networks exhibiting radical-mediated photoplasticity. These cross-linked polymers show permanent shape change under load and stress relax only when under the stimulus of visible or UV radiation. The Lewis base-catalyzed reaction of the epoxide groups in ADTDE with tri- and tetrafunctional thiols was monitored by differential scanning calorimetry and infrared spectroscopy. These curing systems were found to be more efficient than anionic, cationic, or anhydride polymerizations. The dynamic mechanical thermal analysis of ADTDE with the tetrathiol showed that the network was rubbery at room temperature which is considered ideal for the demonstration of photoplasticity. The photoinduced creep behavior was shown to be controlled by the presence of active free radicals in the matrix—on cessation of irradiation, the creep was terminated. The stress relaxation of the stretched ne...

Effect of cross-link density on optoelectronic properties of thermally cured 1,2-epoxy-5-hexene incorporated polysiloxane

The synergesic effect from copolymerising polysiloxane and epoxy functionality offer a viable encapsulant material for use in electronic devices. A series of epoxy group-containing organopolysiloxane without ether bond spacer was prepared. The cross-link density was progressively monitored by controlling the mole ratio of the epoxy monomer. Thermal, optical, dielectric and hardness properties were measured and related to the cross-link density. The crosslink density affect an increase in refractive indices in the range 1.4359–1.4655 and dielectric constants in the range 2.78–3.34 at 500MHz. Transparency were in the range 60–80% in visible region. A higher crosslink structure displays an improved transparency in the UV–visible region. These observations were related to changes in free volume and group polarizability and discussed within the scope of Lorentz–Lorenz relationship.

Investigation of mechanical and thermodynamic properties of pH-sensitive poly(N,N -dimethylaminoethyl methacrylate) hydrogels prepared with different crosslinking agents

pH-sensitive poly(N,N-dimethylaminoethyl methacrylate) hydrogels were synthesized by free-radical crosslinking polymerization using two different crosslinking agents; tetraethylene glycol dimethacrylate (TEGMA) and N,N′-methylenebis(acrylamide) (BAAm). The influence of the polymerization factors such as the type of the crosslinking agent and the gel preparation concentration on the swelling behavior, the gel strength, the effective crosslinking density and the average chain length between the crosslink points for the resulting hydrogels was investigated. The results of the equilibrium swelling measurements in water showed that the linear swelling ratio of the resulting hydrogels increases with increasing gel preparation concentration. The swelling ratio of PDMAEMA hydrogels crosslinked with BAAm is larger than those for hydrogels crosslinked with TEGMA over the entire range of the polymer network concentration. The hydrogels exhibit very sharp pH-sensitive phase transition in a very narrow range of pH between 7.7 and 8.0. From the mechanical measurements, it was also found that the linear swelling ratio of resulting hydrogels depends on the crosslinking density and also the type of the crosslinker used in the preparation. The resulting hydrogels are thought to be good candidates for pH-sensitive drug delivery systems. POLYM. ENG. SCI. 2013. © 2012 Society of Plastics Engineers

The Importance of Crosslinking and Glass Transition Temperature for the Mechanical Strength of Nanofibers Obtained by Green Electrospinning

AbstractCrosslinked particles containing butyl acrylate, methyl methacrylate, and allyl methacrylate are prepared by free‐radical emulsion polymerization. The glass transition temperatures of the polymers are varied by the crosslinking densities in the latex particles. Aqueous acrylate dispersions with polymers of different glass transition temperatures are electrospun with PVA as the matrix polymer. The effects of crosslinking density and Tg on the structure and mechanical properties of the fibers are studied. Crosslinking of unreacted allyl groups is induced by UV irradiation to stabilize the fibers by interparticle crosslinking. Both the ability to undergo interparticle crosslinking and the E modulus depend on the merging of the particles during the electrospinning process. magnified image

Synchrotron SAXS to probe cross-linked network of polyamide ‘reverse osmosis’ and ‘nanofiltration’ membranes

Abstract A typical ‘reverse osmosis’ (RO) or ‘nanofiltration’ (NF) polyamide cross-linked structure is formed by interfacial reaction between an aqueous solution of m -phenylene diamine (or piperazine) and a trimesoyl chloride solution in n -hexane. Using synchrotron SAXS, it was observed that a little change in the reactant concentrations resulted to different cross-linked networks of the polyamides. A clear difference between the cross-linking networks of RO and NF polyamides was observed even though there were some similarities in terms of the nature of polymer chain compaction and the cluster formation by aggregation. In both the polyamides, there was strong compaction of polymer chains forming globular like objects having smooth interface which was well deviated from the Gaussian polymer chain nature ( I ∞ Q − 2 ) wherein the NF polyamide had relatively more compaction with Porod scattering I ∞ Q −3.8 than I ∞ Q −3.4 of the RO polyamide. The size of the compacted globules containing about 1400 trimesamide monomeric units for the NF polyamide was slightly smaller than that of the RO polyamide. These globules were very strongly aggregated to form polymer nodules. The nodular cluster size for the NF polyamide was ∼2000–3000 A which was ∼6–7 times larger than that of RO polyamide. These differences in the nature of polymer chain compaction and the cluster size agreed with the differences in effective cross-link density measured by equilibrium swelling in dimethyl acetamide and the polymer nodule sizes observed by the AFM.