Gel Crosslinking Density Research Articles

In situ composite of graphene oxide in polyacrylamide to enhance strength of hydrogel with polyethyleneimine as crosslinker

Polymer gel was widely used in the petroleum industry for water shutoffs to enhance oil recovery or for plugging to control lost circulation events. By free radical polymerization of acrylamide monomers in presence of graphene oxide (GO) nano sheets, polyacrylamide graphene oxide composite (PAMGO) was synthesized and used to prepare high strength hydrogel with polyethyleneimine (PEI) as crosslinker. The gelation time of gel was determined with viscosity test and gel strength was evaluated with rheological measurements and sand pack breakthrough pressure test. ATR-FTIR and SEM test was adopted to characterize the structure and morphology of the gel. The results showed that compared to polyacrylamide (PAM) gel, gelation time of composite gel decreased while gel elasticity modulus increased. The breakthrough pressure of composite gel is 21.71 MPa and increased by 20.71%. However, the mixing of graphene oxide in PAM gelling solution causes detrimental effects on gel strength and the breakthrough pressure of the mixture gel is only 3.72 MPa. The ATR-FTIR spectroscopy confirmed that the type of hydrogen bonding in the composite gel was different from that in the PAM GO mixture gel. The hydrogen bonding between N, O in composite and hydrogen atom on GO in the composite gel can enhance gel strength with reducing porosity and more crosslinking density of gel while the hydrogen bonding for H of NH2 in PAM with oxygen atom on GO in the mixture gel can occupy the crosslinking points, resulting in less crosslinking density and gel strength.

Microgel particle formation: Influence of mechanical properties of pectin-based gels on microgel particle size distribution

Microgel particle formation usually requires a dispersing/emulsifiying step of macrogel into pieces, or of direct-gelled droplets. Either liquid or solid-like, yields and fails under given critical conditions. In this work we studied the influence of a gel's mechanical properties on the resulting particle/droplet sizes. As the structure of the polymer influences its mechanical properties, the viscoelastic and mechanical response of pectin gels towards shear, compressive and tensile stress were analysed. Gels were prepared with different pectin types (amidated pectin and citrus pectin) and pectinic acid, at a constant stoichiometric molar ratio R = 2 x [Ca2+]/[COO−] = 1. A dependency of storage modulus G′, Young's modulus E, and critical breakup stresses and strains on the pectin type used for gelation was shown. Values of the storage modulus G′ were influenced by the degree of methyl-esterification (DM), indicating an increase on the cross-linking density of gels. Gels prepared with pectinic acid (DM = 2) showed overall the highest values of G’. Values of the Young's modulus E as well as fracture stresses and strains depended on the type of strain applied to the gel and the pectin type. Concordant with the G′ values, gels become stiffer and brittle with increasing DM, resulting in higher E values, lower fracture strains and higher fracture stresses. In addition, under compressive stress, gels investigated sustained lower stress values before structural failure. The findings of mechanical and rheological analyses were compared with the particle size distribution of microgels produced from the prepared gels at constant process conditions. Gels with high values for G′ and E, and low fracture stresses and strains were shown to be dense cross-linked gels of brittle material characteristics which resulted in smaller particles compared to more elastic gels.

Structure-property relationships of novel phosphonate-functionalized networks and gels of poly(β-amino esters)

pH sensitivity, biodegradability and high biocompatibility make poly(β-amino esters) (PBAEs) important biomaterials with many potential applications including drug and gene delivery and tissue engineering, where their degradation should be tuned to match tissue regeneration rates. Therefore, we synthesize novel phosphonate-functionalized PBAE macromers, and copolymerize them with polyethylene glycol diacrylate (PEGDA) to produce PBAE networks and gels. Degradation and mechanical properties of gels can be tuned by the chemical structure of phosphonate-functionalized macromer precursors. By changing the structure of the PBAE macromers, gels with tunable degradations of 5–97% in 2 days are obtained. Swelling of gels before/after degradation is studied, correlating with the PBAE identity. Uniaxial compression tests reveal that the extent of decrease of the gel cross-link density during degradation is much pronounced with increasing amount and hydrophilicity of the PBAE macromers. Degradation products of the gels have no significant cytotoxicity on NIH 3T3 mouse embryonic fibroblast cells.

Abstract 5779: Building a novel 3D hyaluronan-based tri-culture model of the bone metastatic microenvironment for cancer studies

Abstract The high attrition rate of potential anti-cancer drugs entering clinical trials predicates a need for more predictive pre-clinical model systems. Three-dimensional (3D) culture systems provide cells a more physiological microenvironment than traditional cell culture methods—including relevant extracellular matrix (ECM) cues and cell-cell interactions—while allowing greater experimental control and higher-throughput studies than animal models. Prevalent cancers, including breast and prostate cancer, metastasize preferentially to bone, where they become incurable. The bone marrow microenvironment is a complex network of interacting bone homeostatic and hematopoietic niches that contribute to osteotropic cancer homing, proliferation, and treatment resistance. We created a 3D in vitro tri-culture model of the bone metastatic microenvironment to serve as a new platform to study the “trialogue” between bone metastatic cancer, bone marrow stroma, and bone marrow endothelial cells. This tool can be used to inform drug discovery and to combat treatment resistance. Within the present study, the utility of peptide-functionalized hyaluronan hydrogels for supporting the self-organization of 3D bone marrow microvascular networks was explored. Gel crosslinking density was varied to examine the effect of matrix porosity on cell migration and organization over time. Tubule length and diameter were quantified and the endothelial structures were assessed for indicators of mature vasculature, including lumen formation, basement membrane production, and the recruitment of supporting stromal cells. Breast/prostate cancer cell growth, survival, and physical association with bone marrow stromal and bone marrow microvascular endothelial cell networks was examined in tri-culture models. Ongoing studies seek to identify and disrupt key cell signaling pathways that mediate cell-cell interaction and promote cancer cell proliferation and survival, providing new tools to combat bone metastatic disease. Citation Format: Lindsey K. Sablatura, Daniel A. Harrington, Mary C. Farach-Carson. Building a novel 3D hyaluronan-based tri-culture model of the bone metastatic microenvironment for cancer studies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5779. doi:10.1158/1538-7445.AM2017-5779

Enzyme-mediated stiffening hydrogels for probing activation of pancreatic stellate cells.

Hydrogels with spatial-temporal controls over crosslinking kinetics (i.e., dynamic hydrogel) are increasingly being developed for studying mechanobiology in 3D. The general principle of designing dynamic hydrogel is to perform cell encapsulation within a hydrogel network that allows for postgelation modification in gel crosslinking density. The enzyme-mediated in situ gel stiffening is innovative because of the specificity and efficiency of enzymatic reaction. Although tyrosinase has been used for hydrogel crosslinking and in situ cell encapsulation, to the best of our knowledge tyrosinase-mediated DOPA formation has not been explored for in situ stiffening of cell-laden hydrogels. Furthermore, the current work provides a gradual matrix stiffening strategy that may more closely mimic the process of tumor development.

Study of ion-exchange properties of hydrated titanium dioxide towards cesium and strontium cations

Acid-base and sorption properties of spherically granulated material based on hydrated titanium dioxide were studied. It was shown that the sorption capacity of the ion exchanger is due both to the structure (cross-linking density of gels and, consequently, their porosity) and to the number of active acid centers. The kinetic specific features of the sorption of cations were considered and it was found that the sorption kinetics is limited by the intraparticle diffusion. It was shown that the sorbent can be used to purify low-salt technological solutions of liquid radioactive wastes.

The influence of gel formulation on the diffusion of salicylic acid in polyHEMA hydrogels.

The influence of solute concentration, gel hydration, and crosslinking density on diffusion rates in polyHEMA hydrogels has been investigated using a radio-tracer technique. At hydrations above 31% water, diffusion is by pore flow, and increases in the crosslinking density of gels bring about a decrease in the diffusion coefficient, D. Below 31% hydration, diffusion is predominantly by solution diffusion and changes in crosslinking density have little effect on the diffusion coefficient. The diffusion coefficient is invariant with solute concentration within the range reported, although a very high solute concentration, for the gels of lower hydration, the diffusion coefficient is higher than expected. A high value for D may be due to saturation of the binding sites of the diffusant on the polymer chains, leaving a greater proportion of diffusant available for transport.

Presence of pores and hydrogel composition influence tensile properties of scaffolds fabricated from well‐defined sphere templates

Sphere templating is an attractive method to produce porous polymeric scaffolds with well-defined and uniform pore structures for applications in tissue engineering. While high porosity is desired to facilitate cell seeding and enhance nutrient transport, the incorporation of pores will impact gross mechanical properties of tissue scaffolds and will likely be dependent on pore size. The goals of this study were to evaluate the effect of pores, pore diameter, and polymer composition on gross mechanical properties of hydrogels prepared from crosslinked poly(ethylene glycol) (PEG) and poly(2-hydroxyethyl methacrylate) (pHEMA). Sphere templates were fabricated from uncrosslinked poly(methyl methacrylate) spheres sieved between 53-63 and 150-180 μm. Incorporating pores into hydrogels significantly decreased the quasi-static modulus and ultimate tensile stress, but increased the ultimate tensile strain. For pHEMA, decreases in gel crosslinking density and increases in pore diameters followed similar trends. Interestingly, the mechanical properties of porous PEG hydrogels were less sensitive to changes in pore diameter for a given polymer composition. Additionally, pore diameter was shown to affect skeletal myoblast adhesion whereby many cells cultured in porous hydrogels with smaller pores were seen spanning across multiple pores, but lined the inside of larger pores. In summary, incorporation of pores and changes in pore diameter significantly affect the gross mechanical properties, but in a manner that is dependent on gel chemistry, structure, and composition. Together, these findings will help to design better hydrogel scaffolds for applications where gross mechanical properties and porosity are critical.

Photo-Cross-Linkable PNIPAAm Copolymers. 1. Synthesis and Characterization of Constrained Temperature-Responsive Hydrogel Layers

Photo-cross-linkable co- and terpolymers of N-isopropylacrylamide (NIPAAm), 2-(dimethylmaleimido)-N-ethylacrylamide (DMIAAm) as the chromophore, and N,N-dimethylacrylamide (DMAAm) were prepared by free radical polymerization. Aqueous solutions of the co- and terpolymers showed lower critical solution temperature (LCST) behavior, and the corresponding phase transition temperature (Tc) was detected by differential scanning calorimetry (DSC). Tc decreased with increasing amount of DMIAAm, as low as 24.7 °C for 9.2 mol % DMIAAm, and increased with increasing DMAAm content, as high as 59.5 °C for 52.6 mol % DMAAm. The resulting polymers were shown to be photo-cross-linkable, and the sensitivity of the polymers toward UV light was studied by monitoring the photo-cross-linking reaction with ATR−FTIR. With 2 wt % thioxanthone as the photosensitizer, nearly full conversion could be achieved with 10 min of irradiation even though the photo-cross-linking was performed in the glassy state. Surface plasmon resonance (SPR) spectroscopy and optical microscopy were used to obtain information about the swelling behavior of thin hydrogel films. In the SPR scans the plasmon resonance minimum and the first waveguide mode were fit to Fresnel calculations to determine the refractive index (n) and the layer thickness (d) of the hydrogel. The volume degree of swelling was calculated from the refractive index, and the swelling ratio was calculated from the layer thickness. Changes in the degree of swelling, Tc, and the width of the transition (ΔTc) were observed by changing the chromophore content and, as a result, the gel cross-linking density. For a hydrogel film with a dry thickness of 200 nm, the collapsed film thickness above Tc was around 220 nm and only weakly dependent on chromophore content. However, at temperatures below Tc, the swollen film thickness was strongly dependent on the chromophore content and ranged from 1200 nm for 2.4 mol % DMIAAm to 800 nm for 9.2 mol % DMIAAm. The reverse is true for the refractive index, which increases as the film thickness decreases. A comparison of volume degree of swelling and swelling ratio was utilized to demonstrate the high anisotropy of swelling in these hydrogel layers that were physisorbed to the substrate and therefore constrained from expanding or contracting laterally. The swollen film expanded 9.5% laterally as compared to the dry film, and this value appeared to be independent of temperature. The swelling perpendicular to the substrate ranged from 6.4% for 9.2 mol % DMIAAm at temperatures above Tc to 630% for 2.4 mol % DMIAAm at temperatures below Tc.

Quantitative Characterization of Microscopic Variations in the Cross-Link Density of Gels

We report the visualization and quantitative analysis of the cross-link structure in model core/shell hydrogel polymers on the microscopic scale, in a fully swollen state, using soft X-ray microscopy. The cross-link density in these materials and their microscopic or even nanoscopic variation critically influence materials characteristics, yet the cross-link density is difficult to characterize by conventional methods. By the use of soft X-ray microscopy, one can investigate these materials in a fully swollen state and thus directly visualize and quantitatively determine the cross-link structure on a microscopic scale. Materials that were cross-linked by different methods were shown to give rise to differently shaped profiles. Abrupt and gradient cross-link density profiles have been investigated, and the spatial variation in their cross-link density has been determined quantitatively.

Random crosslinking of an ethylene vinyl acetate copolymer and a metallocene polyolefin elastomer in the presence of polyfunctional monomers

A combination of Monte Carlo simulation and gel content experiments was used to study the molecular structural evolution of two randomly crosslinked systems: ethylene vinyl acetate copolymer and metallocene polyolefin elastomer in the presence of a triallyl cyanurate polyfunctional monomer (PFM). The molecular weight distributions, crosslinking density and PFM density evolutions are simulated. There exist gaps in the crosslinking density and the bonded PFM density, between the gel and the sol fractions. The density gaps widen as the crosslinking reaction proceeds. Towards the end of the reaction, the gel crosslinking density increases at a rate higher than that for the gel PFM density. The crosslinking level for the PFM molecules in the gel fraction is much higher than that in the sol fraction. The evolved structures for the two polymer systems during the reaction are also discussed.

Structure-property relationships of lightly chemical crosslinked poly(vinyl chloride) thermoplastic elastomer

Chemical crosslinked poly(vinyl chloride) (C-PVC) was synthesized by vinyl chloride suspension polymerization in the presence of diallyl phthalate (DAP) and plasticized to prepare poly(vinyl chloride) (PVC) thermoplastic elastomer (TPE) materials. The chemical crosslinking and physical crosslinking structure in chemical crosslinked PVC-TPE were investigated. It showed that the gel fraction and the crosslinking density of gel increased as the feed concentration of DAP increased. C-PVC prepared by VC/DAP copolymerization was lightly crosslinked as compared with irradiation crosslinked PVC. Physical entanglements would greatly influence the crosslinking density of gel when the gel fraction was high. Chemical crosslinking had little influence on the recrystallization behavior of PVC. A structure model of chemical crosslinked PVC-TPE was proposed, in which chemical networks acted with physical networks cooperatively. It also showed that chemical crosslinking and physical crosslinking influenced the processability and mechanical properties of chemical crosslinked PVC-TPE cooperatively. Although the processability of PVC-TPE deteriorated with chemical crosslinking, the dimension stability and elasticity of PVC-TPE were improved as the permanent chemical networks were introduced. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 868–874, 2000

Synthesis of chemically crosslinked poly(vinyl chloride) by vinyl chloride/divinyls suspension copolymerization

Vinyl chloride (VC)/divinyl derivative copolymerization was carried out in the suspension process for preparing chemically crosslinked poly(vinyl chloride) (PVC). Effects of the reactivity ratios and concentration of divinyl, polymerization temperature and conversion on the structure of crosslinked PVC (such as the gel fraction, polymerization degree of sol and crosslinking density of gel) were investigated. It showed that the crosslinking behavior of various divinyls was different. The crosslinking efficiency of diallyl maleate (DAM) was higher than that of diallyl phthalate (DAP) and ethyl glycol dimethyl acrylate (EGDMA). The crosslinking efficiency of EGDMA was very low due to its high probability of homopolymerization. The gel fraction and crosslinking density of gel increased as the concentration of DAP increased, and decreased with the increase of polymerization temperature for VC/DAP copolymer. Polymerization degree of sol of VC/DAP copolymer decreased with the increase of concentration of DAP and varied with polymerization temperature. The crosslinking density of gel increased gradually with the increase of the gel fraction at lower gel fraction stage, and increased rapidly at higher gel fraction stage. The concentration of double bonds of VC/DAP copolymerization system increased as polymerization proceeded, which resulted in the increase of gel fraction of VC/DAP copolymer as conversion increased.

Study on variation of structure and properties of natural rubber during accelerated storage

The dynamic variations of molecular structure and properties of natural rubber (NR) during the accelerated storage have been studied. The results showed that, with the prolonging of the storage time, the average molecular weight and the gel content increased, the width index of molecular weight distribution decreased, the gel size increased gradually, but the crosslinking density of gel fluctuated only within a narrow range. Besides, the initial plasticity increased, whereas the plasticity retention index decreased. The studies with thermal analyses and infrared spectroscopy showed that the thermal degradation temperature dropped, the thermal degradation rate increased, and the thermal stability has decreased after the storage of NR. The molecular structure and properties of NR are in a state of dynamic variation during accelerate storage.

Effects of cyclization and pendant vinyl group reactivity on the swelling behavior of polyacrylamide gels

A series of polyacrylamide (PAAm) gels were prepared by free-radical crosslinking copolymerization of acrylamide and N,N′-methylenebis(acrylamide) (BAAm) in water at various crosslinker (BAAm) and chain transfer agent (isopropyl alcohol, IPA) concentrations. It was shown that only 5 % of the crosslinker used in the feed forms effective crosslinks in the final hydrogels. At BAAm contents as high as 3 mole %, the equilibrium swelling ratio of the gels in water is independent of the crosslinker content in the feed. This is due to the prevailing multiple crosslinking reactions during the gel formation process. At a fixed crosslinker content, the onset of gelation is shifted towards higher conversions and reaction times as the amount of IPA increases. Addition of IPA in the monomer mixture also increases the equilibrium swelling ratio of PAAm gels. It was shown that the gel crosslinking density increases on rising IPA concentration in the feed due to the increasing rate of intermolecular crosslinking reactions.

Permeation of Proteins through a Biodegradable Hydrogel

The permeation of proteins of different molecular weights into a biodegradable hydrogel, a model system for controlled release of biologically active macromolecules, was investigated. The hydrogels were prepared by a crosslinking copolymerization from a biodegradable synthetic polypeptide, poly[N5-(2-hydroxyethyl)-L-glutamine] (PHEG) with methacryloylated side chains and acrylamide. The permeation of sodium azide, trypsin-kallikrein inhibitor (TKI), trypsin, and albumin into the gels with network densities between 0.04 and 0.226 mmol/mL were determined. The permeation of sodium azide was not influenced by the gel network density. The permeability of T[and trypsin was controlled by gels with permeability coefficients ranging from 2.8 x 10−9 to 4.0 x 10−10 mmol/mL. Gels with different network densities controlled the diffusion of peptides and proteins with molecular weights between 6,000 and 60,000. The combined effect of diffusion and biodegradation of the gel on the permeation rate of macromolecules was determined for poly-α,β-[N-(2-hydroxyethyl)-DL-aspartamide]. A decrease in the gel cross-linking density due to the degradation efficiently increased the permeability to the model macromolecule.

弾性ゲルと合成ゴムのラテックスブレンドに関する研究 第１報 機能性弾性ゲルの合成およびＦＫＭラテッックスとのブレンド

Elastic emulsion gels with high three dimensional cross-links were prepared by the emulsion polymerization of n-butyl acrylate and divinylbenzene as cross-linking agents. Furthermore, concidering the vulcanization of synthetic rubbers, an allyl methacrylate was used as the third component. Emulsion gels obtained showed that the avrage particle sizes are about 0.1μm and particle size distributions are very narrow. By quantitative analysis of ethylenic double bonds in the surface layer of gels, it was found that they are proportinal to molar ratio of divinylbenzene. Rubber blends were prepared by the co-precipitation of the mixture of FKM latex and these emulsions.From the results of vulcanization curves, it was found that the rate of vulcanization did not appreciably changed and the values of final torque increased with increasing the blend ratio and the molar ratio of DVB. The moduli and tensile strngths of blend vulcanizates increased with increasing the amount and the cross-linking density of gels, but the elongations decreased. On the other hand, in the case of blending gels with allyl groups, moduli were appreciably unchanged but TB s were more improved than the FKM vulcanizate as a control. The decrease of the benzene swelling resistance was a little.

Crosslinked poly(ethylene glycol) networks as reservoirs for protein delivery

Poly(ethylene glycol)s are cross-linked by tris(6-isocyanatohexyl)isocyanurate via urethane/allophanate bond formation to obtain polymeric networks capable of swelling in phosphatebuffered saline or ethanol, resulting in gels. The protein loading of the gels swollen in ethanol is enhanced by formation of hydrophobic ion-paired complexes of proteins with sodium dodecyl sulfate. The protein release profile from loaded gels through biomimetic membranes impregnated with phosphatidylcholine is governed by the gel crosslinking density. Spectroscopic data and enzymatic activity of the proteins released from the gels into aqueous solutions indicate that proteins are recovered unchanged upon their redissolution in water. © 1996 John Wiley & Sons, Inc.

Crosslinked poly(ethylene glycol) networks as reservoirs for protein delivery

Poly(ethylene glycol)s are cross-linked by tris(6-isocyanatohexyl)isocyanurate via urethane/allophanate bond formation to obtain polymeric networks capable of swelling in phosphatebuffered saline or ethanol, resulting in gels. The protein loading of the gels swollen in ethanol is enhanced by formation of hydrophobic ion-paired complexes of proteins with sodium dodecyl sulfate. The protein release profile from loaded gels through biomimetic membranes impregnated with phosphatidylcholine is governed by the gel crosslinking density. Spectroscopic data and enzymatic activity of the proteins released from the gels into aqueous solutions indicate that proteins are recovered unchanged upon their redissolution in water. © 1996 John Wiley & Sons, Inc.

Quasielastic light scattering studies of swollen and stretched polymer gels

Cooperative diffusion D of polyacrylamide gels in water was investigated by quasielastic light scattering both under the isotropically swollen state and under the uniaxially stretched and swollen state. The concentration (Ce) dependence of D for the isotropic gels having equilibrium degrees of swelling was measured by systematically changing crosslinking density of gels. The results yielded D=(3.4±0.5)×10−6 C0.76±0.03e cm2/s, in accord with a scaling prediction. For each of the gels prepared and having a given Ce, the uniaxial stretching was applied, and the values D were investigated as a function of extension ratio α and direction β with respect to the stretching direction. The results gave D (α,Ce)=(3.4±0.5)×10−6 αx Cye cm2/s, where x=2/3 for β=0° and −1/4 for β=90°, and y=0.76, independent of α and β. The results clearly indicate that upon increasing α, D increases and decreases, respectively, in the direction parallel and perpendicular to the stretching direction, implying suppression and enhancement of the concentration fluctuations as a result of the network being stiffened and softened in the respective directions.